Motivated by the recent experimental work,the pressure-induced structural transition of well-known two-dimensional(2D)1T-Hf Te_(2)was investigated up to 50 GPa through the advanced CALYPSO structure search technique c...Motivated by the recent experimental work,the pressure-induced structural transition of well-known two-dimensional(2D)1T-Hf Te_(2)was investigated up to 50 GPa through the advanced CALYPSO structure search technique combined with the first-principles calculations.Our calculations suggested that the 1T-Hf Te_(2)will first transform to C2/m phase at 3.6 GPa with a volume reduction of 7.6%and then to P62m phase at 9.6 GPa with a volume collapse of 4.6%.The occurrences of 3D C2/m and P62m phases mainly originated from the enhanced Te-Te interlayer coupling and the drastic distortions of Hf-Te polyhedrons in P3m1 phase under compression.Concomitantly,the coordination number of Hf atoms increased from six in P3m1 to eight in C2/m and eventually to nine in P62m at elevated pressure.The metallic and semimetallic nature of C2/m and P62m phases were characterized,and the evidence of the reinforced covalent interactions of Te-Hf and Te-Te orbitals in these two novel high-pressure phases were manifested by the atom-projected electronic DOS and Bader charge.展开更多
Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds RsM4 (R = rare earths; M = Si, Ge) are reviewed briefly. First, crystal structures, phase relations...Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds RsM4 (R = rare earths; M = Si, Ge) are reviewed briefly. First, crystal structures, phase relationships, and magnetic prop- erties of several 5:4 compounds, including Nd5 Si4-xGex, Pr5 Si4_xGex, Gds-xLaxGe4, La5 Si4, and Gd5 Sn4, are presented. In particular, the canted spin structures as well as the magnetic phase transitions in PrsSi2Ge2 and PrsGe4 investigated by neutron powder diffractions and small-angle neutron scattering are reviewed. Second, the crystal structures and magnetic properties of the most studied compounds Gds(Si,Ge)4 are summarized. The focus is on the parent compound GdsGe4, which is an amazing material exhibiting magnetic anisotropy, angular dependent spin-flop transition, metastable magnetic response, Griffiths-like phase, thermal effect under pulsed fields, antiferromagnetic and ferromagnetic resonances, pro- nounced effects of impurities, and high-field induced magnetic transitions.展开更多
We report a comprehensive study on a layered-structure compound of NaZn_(4)As_(3),which has been predicted to be an ideal topological semimetal(TSM) candidate.It is found that NaZn_(4)As_(3) undergoes a structural tra...We report a comprehensive study on a layered-structure compound of NaZn_(4)As_(3),which has been predicted to be an ideal topological semimetal(TSM) candidate.It is found that NaZn_(4)As_(3) undergoes a structural transformation from high temperature rhombohedral to a low temperature monoclinic phase.The electric resistivity exhibits a metal-to-insulatorlike transition at around 100 K,and then develops a plateau at low temperature,which might be related to the protected topologically conducting surface states.Our first-principles calculation confirms further that NaZn_(4)As_(3) is a topological insulator(TI) for both different phases rather than a previously proposed TSM.The Hall resistivity reveals that the hole carriers dominate the transport properties for the whole temperature range investigated.Furthermore,an obvious kink possibly associated to the structure transition has been detected in thermopower around ~ 170 K.The large thermopower and moderate κ indicate that NaZn_(4)As_(3) and/or its derivatives can provide a good platform for optimizing and studying the thermoelectric performance.展开更多
As a promising anode material for magnesium ion rechargeable batteries,magnesium metavanadate(MgV_(2)O_(6))has attracted considerable research interest in recent years.A MgV_(2)O_(6)sample was synthesized via a facile...As a promising anode material for magnesium ion rechargeable batteries,magnesium metavanadate(MgV_(2)O_(6))has attracted considerable research interest in recent years.A MgV_(2)O_(6)sample was synthesized via a facile solid-state reaction by multistep-firing stoichiometric mixtures of MgO and V2O5 powder under an air atmosphere.The solid-state phase transition fromα-MgV_(2)O_(6)toβ-MgV_(2)O_(6)occurred at 841 K and the enthalpy change was 4.37±0.04 kJ/mol.The endothermic effect at 1014 K and the enthalpy change was 26.54±0.26 kJ/mol,which is related to the incongruent melting ofβ-MgV_(2)O_(6).In situ XRD was performed to investigate phase transition of the as-prepared MgV_(2)O_(6)at high temperatures.The cell parameters obtained by Rietveld refinement indicated that it crystallizes in a monoclinic system with the C2/m space group,and the lattice parameters of a=9.280 A°,b=3.501 A°,c=6.731 A°,β=111.76°.The solid-state phase transition fromα-MgV_(2)O_(6)toβ-MgV_(2)O_(6)was further studied by thermal kinetics,indicating that this process is controlled first by a fibril-like mechanism and then by a spherulitic-type mechanism with an increasing heating rate.Additionally,the enthalpy change of MgV_(2)O_(6)at high temperatures was measured utilizing the drop calorimetry,heat capacity was calculated and given as:Cp=208.3+0.03583T-4809000T^(−2)(298-923 K)(J mol^(−1)K^(−1)),the high-temperature heat capacity can be used to calculate Gibbs free energy of MgV_(2)O_(6)at high temperatures.展开更多
We observed the linear-to-zigzag structural phase transition of a ^40Ca^+ crystal in a homemade linear Paul trap. The values of the total temperature of the ion crystals during the phase transition are derived using ...We observed the linear-to-zigzag structural phase transition of a ^40Ca^+ crystal in a homemade linear Paul trap. The values of the total temperature of the ion crystals during the phase transition are derived using the molecular-dynamics(MD) simulation method. A series of simulations revealed that the ratio of the radial to axial secular frequencies has a dependence on the total temperature that obeys different functional forms for linear and zigzag structures, and the transition point occurs where these functions intersect; thus, the critical value of the ratio of secular frequencies that drives the structure phase transition can be derived.展开更多
Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields.Herein,an environmentally-friendly,low-cost,high-throughput Cs_(2)NaFeCl_(6) single crystal with exc...Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields.Herein,an environmentally-friendly,low-cost,high-throughput Cs_(2)NaFeCl_(6) single crystal with exceedingly high thermal stability is designed and grown.It obtains a cubic lattice system in the temperature range of 80-500 K,accompanied by a completely reversible chromatic variation ranging from yellow to black.Importantly,the intriguing thermochromism is proved to own extremely high reproducibility(over 1000 cycles)without a hysteretic effect,originating from its structural flexibility that including(i)the noteworthy distortion/deformation of[NaCl_(6)]5−and[FeCl_(6)]3−octahedra;(ii)order-disorder arrangement transition of[NaCl_(6)]5−and[FeCl6]3−octahedra as the function of temperature.This study paves the way towards a new class of smart windows and camouflage coatings with an unprecedented colour range based on a Cs_(2)NaFeCl_(6) perovskite.展开更多
The interaction between C-60's in solid C-60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C-60's were discussed. In order to describe the d...The interaction between C-60's in solid C-60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C-60's were discussed. In order to describe the disordered degree of C-60 rotation, an equivalent M is introduced. The phase transitions at the similar to 260 K and at the similar to 90 K are studied from the viewpoint of C-60 rotation. The potential barriers of the ordered rotation below the similar to 260 K and the disordered rotation above the similar to 260 K have been given, and the effect of the external pressure on the temperature of phase transition has also been given.展开更多
A new compound Ca2PdWO6 has been synthesized by solid state sintering. The phase transition of this compound was investigated by means of differential thermal analysis (DTA), X-ray phase analysis, precise measurement ...A new compound Ca2PdWO6 has been synthesized by solid state sintering. The phase transition of this compound was investigated by means of differential thermal analysis (DTA), X-ray phase analysis, precise measurement of lattice parameters and other methods. It is discovered that the compound has a displacive phase transition of the first order at (806+5)C. The low temperature phase, a-Ca2PdWO6, belongs to orthorhombic system, with space group Pmm2. Its lattice parameters at room temperature are: a=0.79946nm, b=0.55404nm and c=0.58008nm. The measured density is Dm=6.26g/cm3, and each unit cell contains two formula weights. The high temperature phase, Ca2PdWO6, belongs to the cubic system, with space group fm3m and the lattice parameter is a = 0.810 3 nm at 860C; Z = 4. The calculated density is Dx=5.821g/cm3. The crystal structure of Ca2PdWO6 and Ca,PdWO6 was also determined by means of the X-ray polycrystal diffraction method. The factors influencing phase transition temperature are discussed in detail.展开更多
A new compound Ca2FeWO6 has been synthesized by solid state sintering. The phase transition of this compound was investigated by means of differential thermal analysis (DTA), X-ray powder diffraction and other methods...A new compound Ca2FeWO6 has been synthesized by solid state sintering. The phase transition of this compound was investigated by means of differential thermal analysis (DTA), X-ray powder diffraction and other methods. It is discovered that the compound has a displacive phase transition of the first order at (706±5)℃. The low temperature phase. α-Ca2FeWO6. belongs to orthorhombic system, with space group Pmm2. Its lattice parameters at room temperature are; a = 0.77051 nm, 6=0.54242nm and r = 0.551 08 nm, the measured density is Dm = 6.04g/cm3, and each unit cell contains two formula weight. The high temperature phase, β-Ca2FeWO6, belongs to the cubic system, with space group Fm3m and the lattice parameter is a = 0.780 8 nm at 750℃, z = 4. The calculated density is Dx = 5.802g/cm3, The crystal structures of α-Ca2FeWO6 and β-Ca2FeWO6 were also determined by means of the X-ray polycrystal diffraction method. The phase transition mechanism is expounded in detail.展开更多
A new phase transition compound,2-methoxyanilinium perchlorate-18-crown-6(1) {(oCH3OC6H4NH3)+(18-crown-6) ClO4 },has been synthesized and separated as crystals.Differential scanning calorimetry(DSC) measureme...A new phase transition compound,2-methoxyanilinium perchlorate-18-crown-6(1) {(oCH3OC6H4NH3)+(18-crown-6) ClO4 },has been synthesized and separated as crystals.Differential scanning calorimetry(DSC) measurements show a pair of sharp peaks at 225 K(heating) and 210 K(cooling),indicating the phase transition is first-order.Dielectric anomalies observed at 225 K(heating)and 210 K(cooling) further confirm the phase transition.The crystal structures determined at 298 K and123 K are both triclinic in P 1.The most distinct difference between room-temperature and lowtemperature structures is the order–disorder transition of the host 18-crown-6 molecule,which is the driving force of the phase transition.展开更多
The phase behaviors and structures of a triphenylene-derived discotic liquid crystal (LC) hexa-n-octoxyl- triphenylene (C8HET) were studied using the combined techniques of differential scanning calorimetry (DSC...The phase behaviors and structures of a triphenylene-derived discotic liquid crystal (LC) hexa-n-octoxyl- triphenylene (C8HET) were studied using the combined techniques of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron diffraction (SAED) and polarized light microscopy (PLM). Onedimensional (1D) powder WAXD results at different temperatures coupled with DSC and PLM observations revealed that the C8HET compound possessed an LC phase and three different crystalline (K3, K2 and K1) phases below the isotropic (I) melt. The I←→ LC phase transition was thermodynamically reversible and independent of the heating and cooling rates. The development and experimental observation of the three crystalline phases relied on different thermal histories. Among the three crystalline phases in CSHET, the K3 phase is the most stable phase, while the K2 and K1 phases are metastable. Note that the K1 phase only formed via a quenching process. Oh the basis of structure sensitive diffraction experiments such as 2D WAXD of oriented samples and SAED of single crystals, detailed structures and molecular packings of these four ordered phases were identified. The LC phase exhibited a hexagonal columnar phase with 2D lattice dimensions ofa = b = 2.38 nm and γ= 120°. All the three crystalline phases possess monoclinic unit cells, yet the y angle is not 90° in the cases of the K2 and the K3 phases, while in the case of the K1 phase the a angle is not 90°.展开更多
The electrical conductivity of a C60 single crystal around 274 K and the specific heat of C60 crystals from 150 to 340 K have been measured.The delta-like specific heat peak at about 251 K related to the first-order p...The electrical conductivity of a C60 single crystal around 274 K and the specific heat of C60 crystals from 150 to 340 K have been measured.The delta-like specific heat peak at about 251 K related to the first-order phase transition has been reported.The activation energy change around 274 K and the lambda-like specific heat peak beginning at 270 K and ending at 310 K show that there is an orientational phase transition in fcc C60 crystals above 251 K.By taking the symmetry into consideration and further analyzing lambda-like specific heat peak and the activation energy change around 274 K,the conclusion has been reached that this new phase transition is an orientational structure transition from the merohedral twinning fcc to the orientationally disordered fcc.The temperature of free rotation of C60 molecules is about 281 K.展开更多
The features of the single crystals 0.76PMN-0.24PT in dielectric, ferroelectric, pyroelectric properties and domain structures indicate that they are located between typical ferroelectrics and normal ones. The unpoled...The features of the single crystals 0.76PMN-0.24PT in dielectric, ferroelectric, pyroelectric properties and domain structures indicate that they are located between typical ferroelectrics and normal ones. The unpoled crystals present a transitional domain configuration between microdomains and typical macrodomains while the crystals on (001) cuts undergo field-induced phase transition under poling, showing two special temperature points Td and Tm during the succedent heating procedure. The dielectric constant starts to decrease drastically at Td during cooling, or the transformation from induced macrodomain to transitional domain takes place at Td during heating. Ferroelectric-paraelectric phase transition or depolarization continues within the whole temperature range of Td-Tm, where ferroelectric phase in the form of transitional macrodomains coexists with paraelectric phase. Then the crystals macroscopically transoform into paraelectric phase containing ferroelectric microdomains at a temperature展开更多
The crystallization of amorphous LiBO2 and its phase transition have been investigated by the X-ray diffraction, thermal analysis and infrared absorption spectrum. As the amorphous LiBO2 crystallizes, the metastable p...The crystallization of amorphous LiBO2 and its phase transition have been investigated by the X-ray diffraction, thermal analysis and infrared absorption spectrum. As the amorphous LiBO2 crystallizes, the metastable phase δ-LiBO2 is formed first. The boron-oxygen group in crystal of the δ-phase is similar to that in amorphous LiBO2. As the temperature rises, the δ-phase transforms to the γ-phase, which cannot be prepared at ordinary pressure, and then the γ-phase transforms to the α-phase. Structural relaxation takes place before crystallization. At an equal rising rate of temperature, the larger the particle size of amorphous LiBO2 is, the higher the crystallization temperature will be.展开更多
0.14 at%Mn-doped 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3(0.95NBT-0.05BT)lead-free single crystals were grown by a top-seeded solution growth method(TSSG).X-ray powder diffraction measurement showed that the as-grown single cr...0.14 at%Mn-doped 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3(0.95NBT-0.05BT)lead-free single crystals were grown by a top-seeded solution growth method(TSSG).X-ray powder diffraction measurement showed that the as-grown single crystals exhibit rhombohedral perovskite structure with apparent distortion.With the increase of temperature,successive structural phase transitions occur in the Mn-doped 0.95NBT-0.05BT single crystals.After poling,apparent dielectric anomaly is induced accompanied by the increase of the character of diffuse phase transition and the decrease of the value of dielectric constant.The Mn-doped0.95NBT-0.05BT single crystals exhibit complex domain structure,in which micro and macro domains coexist with statistically 4mm symmetry.The Mn-doped 0.95NBT-0.05BT lead-free single crystals exhibit excellent piezoelectric and pyroelectric properties,which will lead to promising advance in piezoelectric and pyroelectric applications.The infrared-absorption band occurred around 630.6 cm 1can be assigned to the"stretching"normal vibration of[Na1/2Bi1/2]/Ti-O group.The three diffused Raman bands centered around 300,560 and 800 cm 1can be attributed to F2g[TiO6]bending vibration,A1g[TiO6]stretching vibration and"soft mode"mixed by bending and stretching vibrations.展开更多
Recently, with the increasing use of TEM technique in mineralogy, more and more space groups of high-temperature phases and low-temperature phases and the domain structures of phase transitional minerals have been det...Recently, with the increasing use of TEM technique in mineralogy, more and more space groups of high-temperature phases and low-temperature phases and the domain structures of phase transitional minerals have been determined. Therefore, on the basis of summarizing the numerous results of individual minerals, it is necessary to discuss the relationship between the types of space groups of two phases and the nature of展开更多
The structural stabilities and crystal evolution behaviors of the hyper stoichiometric compound ZrC_(2)(carbon rich;C/Zr> 1.0) are studied under ambient and high pressure conditions using first-principles calculati...The structural stabilities and crystal evolution behaviors of the hyper stoichiometric compound ZrC_(2)(carbon rich;C/Zr> 1.0) are studied under ambient and high pressure conditions using first-principles calculations in combination with the particle-swarm optimization algorithm.Six viable structures of ZrC_(2) in P21/c,Cmmm,Cmc2_(1),P4_(2)/nmc,Immm and P6/mmm symmetries are identified.These structures are dynamically stable as their phonon spectra have no imaginary modes at zero pressure or at the selected high-pressure points.Among them,the P21/c phase represents the ground state structure,whereas P21/c,P4_(2)/nmc,Immm and P6/mmm phases are part of the phase transition series.The phase order and critical pressures of the phase transition are determined to be approximately 300 GPa according to the equation of states and enthalpy.Furthermore,the mechanical and electronic properties are investigated.The P21/c and Cmc2_(1) phases display a semi-metal nature,whereas the P4_(2)/nmc,Immm,P6/mmm and Cmmm phases exhibit a metallic nature.Moreover,the present study reveals considerable information regarding the structural,mechanical and electronic properties of ZrC_(2),thereby providing key insights into its material properties and evaluating its behavior in practical applications.展开更多
Different structure models of a long-period ordered phase in Fe-C martenstie formed during aging have been checked by computer simulation of electron diffraction(ED) patterns based on these models.The results showed t...Different structure models of a long-period ordered phase in Fe-C martenstie formed during aging have been checked by computer simulation of electron diffraction(ED) patterns based on these models.The results showed that the simulated ED pattern of γ'-FexC(Ⅱ) model proposed by the present authors is in good agreement with experimentally observed ED pattern.It was also confirmed that the incommensurate superperiod stems from the coexistence of several γ'-Fe_xC(H) phases with different superperiods.The Fe(144)C(24)(Fe6C) model proposed by Uwakweh et al.generated ED patterns remarkably different from the experimental ones.展开更多
To investigate the deformation of a molecule in a crystal field or the intra-and intermolecular interaction of a molecular solid, one should study not only the position but also the orientation of a molecule. Since th...To investigate the deformation of a molecule in a crystal field or the intra-and intermolecular interaction of a molecular solid, one should study not only the position but also the orientation of a molecule. Since the structure of a C<sub>60</sub> molecule is a truncated icosahedron composed of 20 6-MRs (six-membered rings)and 12 5-MRs, its symmetry展开更多
Properties of solid C<sub>60</sub> at atmospheric pressure have been widely studied since solid C<sub>60</sub> wasdiscovered. In recent years, the crystal structure, electrical and optical prop...Properties of solid C<sub>60</sub> at atmospheric pressure have been widely studied since solid C<sub>60</sub> wasdiscovered. In recent years, the crystal structure, electrical and optical properties of solidC<sub>60</sub> at high pressure have been studied, and some new interesting phenomena observed. Thehigh-pressure X-ray diffracion and electrical resistance measurements showed that solid C<sub>60</sub> hasa phase transition at about 15-22GPa. Recent studies still indicated that solid C<sub>60</sub> has展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.11964026)the Natural Science Basic Research Plan in Shaanxi Province of China(Grant Nos.2023-JC-YB-021,2022JM-035,and 2022JQ-008)。
文摘Motivated by the recent experimental work,the pressure-induced structural transition of well-known two-dimensional(2D)1T-Hf Te_(2)was investigated up to 50 GPa through the advanced CALYPSO structure search technique combined with the first-principles calculations.Our calculations suggested that the 1T-Hf Te_(2)will first transform to C2/m phase at 3.6 GPa with a volume reduction of 7.6%and then to P62m phase at 9.6 GPa with a volume collapse of 4.6%.The occurrences of 3D C2/m and P62m phases mainly originated from the enhanced Te-Te interlayer coupling and the drastic distortions of Hf-Te polyhedrons in P3m1 phase under compression.Concomitantly,the coordination number of Hf atoms increased from six in P3m1 to eight in C2/m and eventually to nine in P62m at elevated pressure.The metallic and semimetallic nature of C2/m and P62m phases were characterized,and the evidence of the reinforced covalent interactions of Te-Hf and Te-Te orbitals in these two novel high-pressure phases were manifested by the atom-projected electronic DOS and Bader charge.
基金supported by the National Natural Science Foundation of China (Grants Nos. 50025102 and 50631040)the National Basic Research Program of China (Grant No. 2006CB601101)+1 种基金the Exchange Program between NIST and the Chinese Academy of Sciencesthe Natural Science Foundation of Guangxi Province, China (Grant No. 2012GXNSFGA060002)
文摘Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds RsM4 (R = rare earths; M = Si, Ge) are reviewed briefly. First, crystal structures, phase relationships, and magnetic prop- erties of several 5:4 compounds, including Nd5 Si4-xGex, Pr5 Si4_xGex, Gds-xLaxGe4, La5 Si4, and Gd5 Sn4, are presented. In particular, the canted spin structures as well as the magnetic phase transitions in PrsSi2Ge2 and PrsGe4 investigated by neutron powder diffractions and small-angle neutron scattering are reviewed. Second, the crystal structures and magnetic properties of the most studied compounds Gds(Si,Ge)4 are summarized. The focus is on the parent compound GdsGe4, which is an amazing material exhibiting magnetic anisotropy, angular dependent spin-flop transition, metastable magnetic response, Griffiths-like phase, thermal effect under pulsed fields, antiferromagnetic and ferromagnetic resonances, pro- nounced effects of impurities, and high-field induced magnetic transitions.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11874417 and 12274440)the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB33010100)the Fund from the Ministry of Science and Technology of China (Grant No. 2022YFA1403903)。
文摘We report a comprehensive study on a layered-structure compound of NaZn_(4)As_(3),which has been predicted to be an ideal topological semimetal(TSM) candidate.It is found that NaZn_(4)As_(3) undergoes a structural transformation from high temperature rhombohedral to a low temperature monoclinic phase.The electric resistivity exhibits a metal-to-insulatorlike transition at around 100 K,and then develops a plateau at low temperature,which might be related to the protected topologically conducting surface states.Our first-principles calculation confirms further that NaZn_(4)As_(3) is a topological insulator(TI) for both different phases rather than a previously proposed TSM.The Hall resistivity reveals that the hole carriers dominate the transport properties for the whole temperature range investigated.Furthermore,an obvious kink possibly associated to the structure transition has been detected in thermopower around ~ 170 K.The large thermopower and moderate κ indicate that NaZn_(4)As_(3) and/or its derivatives can provide a good platform for optimizing and studying the thermoelectric performance.
基金supported by the National Key R&D Program of China(2018YFC1900500)the Graduate Scientific Research and Innovation Foundation of Chongqing,China(Grant No.CYB20002).
文摘As a promising anode material for magnesium ion rechargeable batteries,magnesium metavanadate(MgV_(2)O_(6))has attracted considerable research interest in recent years.A MgV_(2)O_(6)sample was synthesized via a facile solid-state reaction by multistep-firing stoichiometric mixtures of MgO and V2O5 powder under an air atmosphere.The solid-state phase transition fromα-MgV_(2)O_(6)toβ-MgV_(2)O_(6)occurred at 841 K and the enthalpy change was 4.37±0.04 kJ/mol.The endothermic effect at 1014 K and the enthalpy change was 26.54±0.26 kJ/mol,which is related to the incongruent melting ofβ-MgV_(2)O_(6).In situ XRD was performed to investigate phase transition of the as-prepared MgV_(2)O_(6)at high temperatures.The cell parameters obtained by Rietveld refinement indicated that it crystallizes in a monoclinic system with the C2/m space group,and the lattice parameters of a=9.280 A°,b=3.501 A°,c=6.731 A°,β=111.76°.The solid-state phase transition fromα-MgV_(2)O_(6)toβ-MgV_(2)O_(6)was further studied by thermal kinetics,indicating that this process is controlled first by a fibril-like mechanism and then by a spherulitic-type mechanism with an increasing heating rate.Additionally,the enthalpy change of MgV_(2)O_(6)at high temperatures was measured utilizing the drop calorimetry,heat capacity was calculated and given as:Cp=208.3+0.03583T-4809000T^(−2)(298-923 K)(J mol^(−1)K^(−1)),the high-temperature heat capacity can be used to calculate Gibbs free energy of MgV_(2)O_(6)at high temperatures.
基金Project supported by the National Basic Research Program of China(Grant Nos.2010CB832803 and 2012CB821301)the National Natural Science Foundation of China(Grant Nos.11004222 and 91121016)the Chinese Academy of Sciences
文摘We observed the linear-to-zigzag structural phase transition of a ^40Ca^+ crystal in a homemade linear Paul trap. The values of the total temperature of the ion crystals during the phase transition are derived using the molecular-dynamics(MD) simulation method. A series of simulations revealed that the ratio of the radial to axial secular frequencies has a dependence on the total temperature that obeys different functional forms for linear and zigzag structures, and the transition point occurs where these functions intersect; thus, the critical value of the ratio of secular frequencies that drives the structure phase transition can be derived.
基金The research was funded by the National Natural Science Foundation of China(No.51802120,51872126,22075103,51672111)Guangdong Basic and Applied Basic Research Foundation for Distinguished Young Scholar(No.2019B151502030)+7 种基金Natural Science Foundation of Guangdong Province(No.2018030310181)the Science and Technology Plan Project of Guangzhou(No.202002030159)Guangdong Basic and Applied Basic Research Foundation for Young Scholar(No.2020A1515111057)‘100 Talents Program of Hebei Province’(No.E2014100008)the Fundamental Research Funds for the Central Universities(No.21619406)X.Y.thanks for the Special Funds for the Cultivation of Guangdong College Students'Scientific and Technological Innovation("Climbing Program"Special Funds)(No.pdjh2019a0055)J.Fan also thanks for the project support for"Young Top talents"in the Pearl River Talent Project of Guangdong Province(2017GC010424)the Guangdong Provincial Innovation and Entrepreneurship Project(grant 2016ZT06D081).
文摘Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields.Herein,an environmentally-friendly,low-cost,high-throughput Cs_(2)NaFeCl_(6) single crystal with exceedingly high thermal stability is designed and grown.It obtains a cubic lattice system in the temperature range of 80-500 K,accompanied by a completely reversible chromatic variation ranging from yellow to black.Importantly,the intriguing thermochromism is proved to own extremely high reproducibility(over 1000 cycles)without a hysteretic effect,originating from its structural flexibility that including(i)the noteworthy distortion/deformation of[NaCl_(6)]5−and[FeCl_(6)]3−octahedra;(ii)order-disorder arrangement transition of[NaCl_(6)]5−and[FeCl6]3−octahedra as the function of temperature.This study paves the way towards a new class of smart windows and camouflage coatings with an unprecedented colour range based on a Cs_(2)NaFeCl_(6) perovskite.
基金Project supported by the National Natural Science Foundation of China.
文摘The interaction between C-60's in solid C-60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C-60's were discussed. In order to describe the disordered degree of C-60 rotation, an equivalent M is introduced. The phase transitions at the similar to 260 K and at the similar to 90 K are studied from the viewpoint of C-60 rotation. The potential barriers of the ordered rotation below the similar to 260 K and the disordered rotation above the similar to 260 K have been given, and the effect of the external pressure on the temperature of phase transition has also been given.
文摘A new compound Ca2PdWO6 has been synthesized by solid state sintering. The phase transition of this compound was investigated by means of differential thermal analysis (DTA), X-ray phase analysis, precise measurement of lattice parameters and other methods. It is discovered that the compound has a displacive phase transition of the first order at (806+5)C. The low temperature phase, a-Ca2PdWO6, belongs to orthorhombic system, with space group Pmm2. Its lattice parameters at room temperature are: a=0.79946nm, b=0.55404nm and c=0.58008nm. The measured density is Dm=6.26g/cm3, and each unit cell contains two formula weights. The high temperature phase, Ca2PdWO6, belongs to the cubic system, with space group fm3m and the lattice parameter is a = 0.810 3 nm at 860C; Z = 4. The calculated density is Dx=5.821g/cm3. The crystal structure of Ca2PdWO6 and Ca,PdWO6 was also determined by means of the X-ray polycrystal diffraction method. The factors influencing phase transition temperature are discussed in detail.
基金Sixteenth Congress of the International Union of Crystallography in Beijing, China, August. 1993
文摘A new compound Ca2FeWO6 has been synthesized by solid state sintering. The phase transition of this compound was investigated by means of differential thermal analysis (DTA), X-ray powder diffraction and other methods. It is discovered that the compound has a displacive phase transition of the first order at (706±5)℃. The low temperature phase. α-Ca2FeWO6. belongs to orthorhombic system, with space group Pmm2. Its lattice parameters at room temperature are; a = 0.77051 nm, 6=0.54242nm and r = 0.551 08 nm, the measured density is Dm = 6.04g/cm3, and each unit cell contains two formula weight. The high temperature phase, β-Ca2FeWO6, belongs to the cubic system, with space group Fm3m and the lattice parameter is a = 0.780 8 nm at 750℃, z = 4. The calculated density is Dx = 5.802g/cm3, The crystal structures of α-Ca2FeWO6 and β-Ca2FeWO6 were also determined by means of the X-ray polycrystal diffraction method. The phase transition mechanism is expounded in detail.
基金supported by the National Natural Science Foundation of China (No. 21101025)
文摘A new phase transition compound,2-methoxyanilinium perchlorate-18-crown-6(1) {(oCH3OC6H4NH3)+(18-crown-6) ClO4 },has been synthesized and separated as crystals.Differential scanning calorimetry(DSC) measurements show a pair of sharp peaks at 225 K(heating) and 210 K(cooling),indicating the phase transition is first-order.Dielectric anomalies observed at 225 K(heating)and 210 K(cooling) further confirm the phase transition.The crystal structures determined at 298 K and123 K are both triclinic in P 1.The most distinct difference between room-temperature and lowtemperature structures is the order–disorder transition of the host 18-crown-6 molecule,which is the driving force of the phase transition.
基金This work was supported by National Science Foundation of USA (DMR-0516602). The 1D X-ray diffraction research was carried out at the National Synchrotron Light Source in Brookhaven National Laboratory supported by the Department of Energy.
文摘The phase behaviors and structures of a triphenylene-derived discotic liquid crystal (LC) hexa-n-octoxyl- triphenylene (C8HET) were studied using the combined techniques of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron diffraction (SAED) and polarized light microscopy (PLM). Onedimensional (1D) powder WAXD results at different temperatures coupled with DSC and PLM observations revealed that the C8HET compound possessed an LC phase and three different crystalline (K3, K2 and K1) phases below the isotropic (I) melt. The I←→ LC phase transition was thermodynamically reversible and independent of the heating and cooling rates. The development and experimental observation of the three crystalline phases relied on different thermal histories. Among the three crystalline phases in CSHET, the K3 phase is the most stable phase, while the K2 and K1 phases are metastable. Note that the K1 phase only formed via a quenching process. Oh the basis of structure sensitive diffraction experiments such as 2D WAXD of oriented samples and SAED of single crystals, detailed structures and molecular packings of these four ordered phases were identified. The LC phase exhibited a hexagonal columnar phase with 2D lattice dimensions ofa = b = 2.38 nm and γ= 120°. All the three crystalline phases possess monoclinic unit cells, yet the y angle is not 90° in the cases of the K2 and the K3 phases, while in the case of the K1 phase the a angle is not 90°.
基金Project supported by the National Natural Science Foundation of China.
文摘The electrical conductivity of a C60 single crystal around 274 K and the specific heat of C60 crystals from 150 to 340 K have been measured.The delta-like specific heat peak at about 251 K related to the first-order phase transition has been reported.The activation energy change around 274 K and the lambda-like specific heat peak beginning at 270 K and ending at 310 K show that there is an orientational phase transition in fcc C60 crystals above 251 K.By taking the symmetry into consideration and further analyzing lambda-like specific heat peak and the activation energy change around 274 K,the conclusion has been reached that this new phase transition is an orientational structure transition from the merohedral twinning fcc to the orientationally disordered fcc.The temperature of free rotation of C60 molecules is about 281 K.
文摘The features of the single crystals 0.76PMN-0.24PT in dielectric, ferroelectric, pyroelectric properties and domain structures indicate that they are located between typical ferroelectrics and normal ones. The unpoled crystals present a transitional domain configuration between microdomains and typical macrodomains while the crystals on (001) cuts undergo field-induced phase transition under poling, showing two special temperature points Td and Tm during the succedent heating procedure. The dielectric constant starts to decrease drastically at Td during cooling, or the transformation from induced macrodomain to transitional domain takes place at Td during heating. Ferroelectric-paraelectric phase transition or depolarization continues within the whole temperature range of Td-Tm, where ferroelectric phase in the form of transitional macrodomains coexists with paraelectric phase. Then the crystals macroscopically transoform into paraelectric phase containing ferroelectric microdomains at a temperature
基金Project supported by the National Natural Science Foundation of China
文摘The crystallization of amorphous LiBO2 and its phase transition have been investigated by the X-ray diffraction, thermal analysis and infrared absorption spectrum. As the amorphous LiBO2 crystallizes, the metastable phase δ-LiBO2 is formed first. The boron-oxygen group in crystal of the δ-phase is similar to that in amorphous LiBO2. As the temperature rises, the δ-phase transforms to the γ-phase, which cannot be prepared at ordinary pressure, and then the γ-phase transforms to the α-phase. Structural relaxation takes place before crystallization. At an equal rising rate of temperature, the larger the particle size of amorphous LiBO2 is, the higher the crystallization temperature will be.
基金supported by the National Natural Science Foundation of China(51242006)the Qing Lan Project,the Key Laboratory of Inorganic Function Material and Device,Chinese Academy of Sciences(KLIFMD-2011-02)the Priority Academic Program Development of Jiangsu Higher Education Institutions
文摘0.14 at%Mn-doped 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3(0.95NBT-0.05BT)lead-free single crystals were grown by a top-seeded solution growth method(TSSG).X-ray powder diffraction measurement showed that the as-grown single crystals exhibit rhombohedral perovskite structure with apparent distortion.With the increase of temperature,successive structural phase transitions occur in the Mn-doped 0.95NBT-0.05BT single crystals.After poling,apparent dielectric anomaly is induced accompanied by the increase of the character of diffuse phase transition and the decrease of the value of dielectric constant.The Mn-doped0.95NBT-0.05BT single crystals exhibit complex domain structure,in which micro and macro domains coexist with statistically 4mm symmetry.The Mn-doped 0.95NBT-0.05BT lead-free single crystals exhibit excellent piezoelectric and pyroelectric properties,which will lead to promising advance in piezoelectric and pyroelectric applications.The infrared-absorption band occurred around 630.6 cm 1can be assigned to the"stretching"normal vibration of[Na1/2Bi1/2]/Ti-O group.The three diffused Raman bands centered around 300,560 and 800 cm 1can be attributed to F2g[TiO6]bending vibration,A1g[TiO6]stretching vibration and"soft mode"mixed by bending and stretching vibrations.
基金Project supported by the National Natural Science Foundation of China
文摘Recently, with the increasing use of TEM technique in mineralogy, more and more space groups of high-temperature phases and low-temperature phases and the domain structures of phase transitional minerals have been determined. Therefore, on the basis of summarizing the numerous results of individual minerals, it is necessary to discuss the relationship between the types of space groups of two phases and the nature of
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11904081 and 11975100)the Basic Research Program of Education Bureau of Henan Province,China(Grant No.20A140007)Research Initiation Fund of Henan Institute of Technology(Grant No.KQ1817)。
文摘The structural stabilities and crystal evolution behaviors of the hyper stoichiometric compound ZrC_(2)(carbon rich;C/Zr> 1.0) are studied under ambient and high pressure conditions using first-principles calculations in combination with the particle-swarm optimization algorithm.Six viable structures of ZrC_(2) in P21/c,Cmmm,Cmc2_(1),P4_(2)/nmc,Immm and P6/mmm symmetries are identified.These structures are dynamically stable as their phonon spectra have no imaginary modes at zero pressure or at the selected high-pressure points.Among them,the P21/c phase represents the ground state structure,whereas P21/c,P4_(2)/nmc,Immm and P6/mmm phases are part of the phase transition series.The phase order and critical pressures of the phase transition are determined to be approximately 300 GPa according to the equation of states and enthalpy.Furthermore,the mechanical and electronic properties are investigated.The P21/c and Cmc2_(1) phases display a semi-metal nature,whereas the P4_(2)/nmc,Immm,P6/mmm and Cmmm phases exhibit a metallic nature.Moreover,the present study reveals considerable information regarding the structural,mechanical and electronic properties of ZrC_(2),thereby providing key insights into its material properties and evaluating its behavior in practical applications.
文摘Different structure models of a long-period ordered phase in Fe-C martenstie formed during aging have been checked by computer simulation of electron diffraction(ED) patterns based on these models.The results showed that the simulated ED pattern of γ'-FexC(Ⅱ) model proposed by the present authors is in good agreement with experimentally observed ED pattern.It was also confirmed that the incommensurate superperiod stems from the coexistence of several γ'-Fe_xC(H) phases with different superperiods.The Fe(144)C(24)(Fe6C) model proposed by Uwakweh et al.generated ED patterns remarkably different from the experimental ones.
基金Project supported by the National Natural Science Foundation of China
文摘To investigate the deformation of a molecule in a crystal field or the intra-and intermolecular interaction of a molecular solid, one should study not only the position but also the orientation of a molecule. Since the structure of a C<sub>60</sub> molecule is a truncated icosahedron composed of 20 6-MRs (six-membered rings)and 12 5-MRs, its symmetry
文摘Properties of solid C<sub>60</sub> at atmospheric pressure have been widely studied since solid C<sub>60</sub> wasdiscovered. In recent years, the crystal structure, electrical and optical properties of solidC<sub>60</sub> at high pressure have been studied, and some new interesting phenomena observed. Thehigh-pressure X-ray diffracion and electrical resistance measurements showed that solid C<sub>60</sub> hasa phase transition at about 15-22GPa. Recent studies still indicated that solid C<sub>60</sub> has