Structural phase transition and transport properties in topological material candidate NaZn_(4)As_(3)

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.

Tunable Electronic and Magnetic Properties from Structure Phase Transition of Layered Vanadium Diselenide

The atomic geometry, structure stability, electronic and magnetic properties of VSe2 were systematically investigated based on the density functional theory（DFT）. Varying from 3D to 2D four VSe2 structures, bulk 2H-VSe2 and 1T-VSe2, monolayer H-VSe2 and T-VSe2 are all demonstrated as thermodynamically stable by lattice dynamic calculations. More interestingly, the phase transition temperature is dramatically different due to the lattice size. Finally, owing to different crystal structures, H-VSe2 is semimetallic whereas T-VSe2 is totally metallic and also they have different magnetic moments. Our main argument is that being exfoliated from bulk to monolayer, 2H-VSe2 transforms to T-VSe2, accompanied by both semimetallic-metallic transition and dramatic magnetic moment variation. Our calculations provide a novel structure phase transition and an efficient way to modulate the electronic structure and magnetic moment of layered VSe2, which suggests potential applications as high-performance functional nanomaterial.

Shape-induced phase transition of vortex domain structures in ferroelectric nanodots and their controllability by electrical and mechanical loads

Shape-induced phase transition of vortex domain structures （VDSs） in BaTiO3 （BT） nanodots under open circuit boundary condition have been investigated using an effective Hamiltonian method. Our calculation indicates the tetragonal VDS missing in cubic BT nanodots can be induced by varying the shape of a nanodot from cube to platelet. Interestingly, a novel VDS is found in BT nanoplatelets in our simulations. Further investigation shows that it is a result of compromise between the ground state and the symmetry of the shape of the nanodot. Furthermore, based on the novel VDS, routes of controlling VDSs governed by homogeneous electric field and uniform stress are discussed. In particular, our results show the possibility of designing multi-states devices based on a single VDS. ~ 2017 The Authors. Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics.

Structural Phase Transition in Nanostructured TiO_2

Using DTA (difFerential thermal analysis) measurement on nanostructured TiO2, we find two endothermic peaks on the DTA curve. From XRD (X-ray diffraction) analysis of the original nanostructured TiO2 and its heat-treated samples, we obtain the following results: the first endothermic peak corresponds to the desorption of physical or chemical absorption, the second one is related to the structural phase transition from brookite to anatase then to rutile, and this structural phase transition is beneficial to the grain growth of nanocrystal

Effect of f–c hybridization on theγ→αphase transition of cerium studied by lanthanum doping

The hybridization between the localized 4f level(f) with conduction(c) electrons in γ-Ce upon cooling has been previously revealed in single crystalline thin films experimentally and theoretically, whereas its influence on the γ → α phase transition was not explicitly verified, due to the fact that the phase transition happened in the bulk-layer, leaving the surface in the γ phase. Here in our work, we circumvent this issue by investigating the effect of alloying addition of La on Ce, by means of crystal structure, electronic transport and angle resolved photoemission spectroscopy measurements, together with a phenomenological periodic Anderson model and a modified Anderson impurity model. Our current researches indicate that the weakening of f–c hybridization is the major factor in the suppression of γ → α phase transition by La doping. The consistency of our results with the effects of other rare earth and actinide alloying additions on the γ → α phase transition of Ce is also discussed. Our work demonstrates the importance of the interaction between f and c electrons in understanding the unconventional phase transition in Ce, which is intuitive for further researches on other rare earth and actinide metals and alloys with similar phase transition behaviors.

Structural phase transition, precursory electronic anomaly, and strong-coupling superconductivity in quasi-skutterudite (Sr1-xCax)3Ir4Sn13 and Ca3Rh4Sn13

The interplay between superconductivity and structural phase transition has attracted enormous interest in recent years. For example, in Fe-pnictide high temperature superconductors, quantum fluctuations in association with structural phase transition have been proposed to lead to many novel physical properties and even the superconductivity itself. Here we report a finding that the quasi-skutterudite superconductors （Sr1-xCax）3Ir4Sn13 （x = 0, 0.5, 1） and Ca3Rh4Snl3 show some unusual properties similar to the Fe-pnictides, through 119Sn nuclear magnetic resonance （NMR） measurements. In （Sr1-xCax）3Ir4Sn13, the NMR linewidth increases below a temperature T＊ that is higher than the structural phase transition temperature Ts. The spin-lattice relaxation rate （1/T1 ） divided by temperature （T）, 1/TI T and the Knight shift K increase with decreasing T down to T＊, but start to decrease below T＊, and followed by more distinct changes at Ts. In contrast, none of the anomalies is observed in Ca3Rh4Sn13 that does not undergo a structural phase transition. The precursory phenomenon above the structural phase transition resembles that occurring in Fe-pnictides. In the superconducting state of Ca3Ir4Sn13, 1/T1 decays as exp（-△/kBT） with a large gap △ = 2.21kBTc, yet without a Hebel-Slichter coherence peak, which indicates strong-coupling superconductivity. Our results provide new insight into the relationship between superconductivity and the electronic-structure change associated with structural phase transition.

Structural Phase Transitions of ZnTe under High Pressure Using Experiments and Calculations

The pressure-induced structural transitions of ZnTe are investigated at pressures up to 59.2 GPa in a diamond anvil cell by using synchrotron powder x-ray diffraction method. A phase transition from the initial zinc blende （ZB, ZnTe-Ⅰ） structure to a cinnabar phase （ZnTe-Ⅱ） is observed at 9.6 GPa, followed by a high pressure orthorhombic phase （ZnTe-Ⅲ） with Cmcm symmetry at 12.1 GPa. The ZB, cinnabar （space group P3121）, Cmcm, P31 and rock salt structures of ZnTe are investigated by using density functional theory calculations. Based on the experiments and calculations, the ZnTe-Ⅱ phase is determined to have a cinnabar structure rather than a P3 1 symmetry.

Structural Phase Transition and a Mutation of Electron Mobility in Zn_xCd_(1-x)O Alloys

We investigate the electronic structures and phase stability of ZnO, CdO and the related alloys in rocksalt（B1）and wurzite（B4） crystal, using the first-principle density functional theory within the hybrid functional approximation. By varying the concentration of Zn components from 0% to 100%, we find that the Zn_xCd（1-x）O alloy undergoes a phase transition from octahedron to tetrahedron at x = 0.32, in agreement with the recent experimental findings. The phase transition leads to a mutation of the electron mobility originated from the changes of the effective mass. Our results qualify Zn O/Cd O alloy as an attractive candidate for photo-electrochemical and solar cell power applications.

Correlation between structural phase transition and physical properties of Co^(2+)/Gd^(3+)co-substituted copper ferrite

The doping of the spinel ferrites with selective cations usually improves the properties of the parent ferrite.The effect of Co^(2+)/Gd^(3+)co-substitution on the microstructure,optical,and magnetic properties of Cu1-xCoxFe2-xGdxO4 prepared by the citrate-nitrate auto-combustion synthesis was investigated.Characterization of the samples was performed with powder X-ray diffraction(XRD),Raman and Fouriertransform infrared(FTIR)spectroscopy,field-emission scanning electron microscopy,X-ray energydispersive spectroscopy,UV-Vis spectroscopy,and a vibrating sample magnetometer.The results of XRD,Raman,and FTIR analysis show a gradual structural phase transition from a tetragonal(I41/amd)structure to a cubic(Fd3m)structure.The bandgap energy of the studied samples is in a range of 1.57-1.75 eV with a minimum in sample x=0.06 and then increases.Magnetic investigations show that the presence of Co^(2+)/Gd^(3+)cations in an octahedral site of the copper ferrite structure could increase saturation magnetization and coercive field from 567.9 Oe and 23.62 emu/g to 929.4 Oe and 28.27 emu/g,respectively.

Non-Kramers doublet ground state in a quaternary cubic compound PrRu_(2)In_(2)Zn_(18) investigated by ultrasonic measurements

We performed ultrasonic measurements on a quaternary cubic compound PrRu_(2)In_(2)Zn_(18) to explore the ground state properties derived from non-Kramers Γ_(3) doublet of Pr^(3+).PrRu_(2)In_(2)Zn_(18) is a quaternary derivative of the ternary compound PrRu_(2)Zn_(20) that exhibits a structural phase transition at T_S=138 K.In PrRu_(2)In_(2)Zn_(18),the Zn atoms at the 16c site in PrRu_(2)Zn_(20) are selectively replaced by In atoms.A monotonic increase was observed in the temperature dependence of elastic constants C_L=(C_(11)+2C_(12)+4C_(44))/3 and C_(T)=(C_(11)-C_(12)+C_(44))/3 in the temperature range around T_(S) to which an elastic softening was observed in(C_(11)-C_(12))/2 for PrRu_(2)Zn_(20).The disappearance of the softening indicates that the structural transition in PrRu_(2)Zn_(20) is suppressed by the substitution of Zn ions by In ones with a larger ionic radius.Alternatively,the C_(T) of PrRu_(2)In_(2)Zn_(18) exhibits a precursor Curie-type elastic softening toward low temperatures being responsible for the non-Kramers Γ_(3) ground state.We discuss the ground state and the evolution of the elastic properties of the different single-crystal samples of PrRu_(2)In_(2)Zn_(18) grown under different conditions.

High-pressure structures of InBi predicted by particle swarm optimization algorithm

We extensively explore the high-pressure structures of InBi by using a newly developed particle swarm optimization algorithm. An orthorhombic Imma structure is discovered to be stable from 43.7 GPa to 107.9 GPa, ruling out the previously speculated cubic structure. Further increasing the pressure, we find a tetragonal P4/nmm structure which is energetically more favourable from 107.9 CPa to 200 GPa. Especially, the tetragonal P4/nmm structure is known to occur at high pressure in the structures of ZnO and MgTe. We also predict this structure to be a high-pressure structure of ZnTe. Thus the tetragonal P4/nmm structure may be a universal high-pressure structure of the Ⅱ-Ⅵ and the Ⅲ-Ⅴ compounds.

Sintering characteristics, phase transitions, and microwave dielectric properties of low-firing [(Na_(0.5)Bi_(0.5))_(x)Bi_(1-x)](W_(x)V_(1-x))O_(4) solid solutionceramics

A series of high-k[(Na_(0.5)Bi_(0.5))_(x)Bi_(1-x)](W_(x)V_(1-x))O_(4)(abbreviated as NBWV(x value))solid solution ceramics with a scheelite-like structure are synthesized by a modified solid-state reaction method at the temperature range of 680-760 C.A monoclinic(0≤x<0.09)to tetragonal scheelite(0.09≤x≤1.0)structural phase transition is confirmed by X-ray difraction(XRD),Raman,and infrared(IR)analyses.The effect of structural deformation and order-disorder caused by Na^(+)/Bi^(3+)/W^(6+) complex substitution on microwave dielectric properties is investigated in deail.The compositional series possess a wide range of variable relative permittivity(er=24.8-80)and temperature coefficient of resonant frequency(TCF value,-271.9-188.9 ppm/℃).The maximum permittivity of 80 and a high Qxf value of~10,000 GHz are obtained near the phase boundary at x=0.09.Furthermore,the temperature-stable dielectric ceramics sintered at 680 C with excellent microwave dielectric properties of ε_(r)=80.7,Qxf=9400 GHz(at 4.1 GHz),and TCF value=-3.8 ppm/℃ are designed by mixing the components of x=0.07 and 0.08.In summary,similar sinterability and structural compatibility of scheelite-like solid solution systems make it potential for low-temperature co-fired ceramic(LTCC)applications.

Crystal structure and phase transition of 2-methoxyanilinium perchlorate-18-crown-6

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.

Synthesis and Pressure-induced Reversible Phase Transition of a Crystalline Solid Europium Germanate NaEuGeO4

The crystalline solid europium germanate NaEuGeO4 was prepared under hydrothermal conditions. The in-situ photoluminescence spectroscopic measurement and the synchrotron X-ray diffraction analysis in a diamond anvil cell （DAC） under high pressure were performed to study the pressure-induced phase transition of NaEuGeO4 as well as changes of the luminescent properties of Eu3＋ ions. Photoluminescence spectroscopic studies revealed that a phase transition occurred at a pressure range of 6.5--10 GPa and the high-pressure phase of NaEuGeO4 （NaEu- GeO4-HP） was still stable when the pressure raised up to about 20 GPa. As the pressure was released, the spectra returned to the original state, revealing that the pressure-induced phase transition is a completely reversible process. The synchrotron X-ray diffraction analysis demonstrated that the structure of NaEuGeO4 transformed to an uniden- tified phase NaEuGeO4-HP at a pressure higher than 10 GPa, and the phase transition is reversible. This result is consisted with that obtained by photoluminescence spectra analysis.

Surface depletion field in 2D perovskite microplates: Structural phase transition, quantum confinement and Stark effect

Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical,electronic and optoelectronic properties of the materials,especially low-dimensional materials.Two-dimensional(2D)organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field;nevertheless,studies on this remain elusive.Here we report on how the surface depletion field affects the structural phase transition,quantum confinement and Stark effect in 2D(BA)2PbI4 perovskite microplates by the thickness-,temperature-and power-dependent photoluminescence(PL)spectroscopy.Power dependent PL studies suggest that high-temperature phase(HTP)and low-temperature phase(LTP)can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates.With the decrease of the microplate thickness,the structural phase transition temperature first gradually decreases and then increases below 25 nm,in striking contrast to the conventional size dependent structural phase transition.Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase,the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs.This explanation was further supported by the temperature dependent PL studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes.Along with the thickness dependent phase transition,the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.

Synthesis,characterization,and phase transition of an inorganic-organic hybrid compound,[(3-nitroanilinium^＋)(18-crown-6)][IO4^-](CH3OH)

A novel inorganic-organic hybrid supramolecular compound,[（3-nitroanilinium^＋）（18-crown-6）][IO4]（CH3OH）（1）,was discovered as phase-transition materials displaying dielectric anomalous behaviors.The yellow block crystal formed by N-H…O hydrogen bonding that made contact through the cavity of 18-crown-6 was characterized by single-crystal X-ray diffraction,elemental analysis,infrared analysis,thermogravimetric analysis,differential scanning calorimetry,and potential-energy calculations.Differential scanning calorimetry measurements indicate that the compound experiences a reversible phase transition at around 220 K.Temperature-dependent dielectric measurements further confirm the phase transitions.Potential-energy calculations demonstrate that the phase transition occurs due to the molecular order-disorder rotation of CH3OH,whereas the space grouping of the crystal remains unchanged.

Structural evolution and bandgap modulation of layeredβ-GeSe_(2)single crystal under high pressure

Germanium diselenide(GeSe_(2))is a promising candidate for electronic devices because of its unique crystal structure and optoelectronic properties.However,the evolution of lattice and electronic structure ofβ-GeSe_(2)at high pressure is still uncertain.Here we prepared high-qualityβ-GeSe_(2)single crystals by chemical vapor transfer(CVT)technique and performed systematic experimental studies on the evolution of lattice structure and bandgap ofβ-GeSe_(2)under pressure.High-precision high-pressure ultra low frequency(ULF)Raman scattering and synchrotron angle-dispersive x-ray diffraction(ADXRD)measurements support that no structural phase transition exists under high pressure up to 13.80 GPa,but the structure ofβ-GeSe_(2)turns into a disordered state near 6.91 GPa and gradually becomes amorphous forming an irreversibly amorphous crystal at 13.80 GPa.Two Raman modes keep softening abnormally upon pressure.The bandgap ofβ-GeSe_(2)reduced linearly from 2.59 eV to 1.65 eV under pressure with a detectable narrowing of 36.5%,and the sample under pressure performs the piezochromism phenomenon.The bandgap after decompression is smaller than that in the atmospheric pressure environment,which is caused by incomplete recrystallization.These results enrich the insight into the structural and optical properties ofβ-GeSe_(2)and demonstrate the potential of pressure in modulating the material properties of two-dimensional(2D)Ge-based binary material.

Tuning Order-Disorder Phase Transition through Regulating the Substituent Group of Anion

The structural phase transition compound {[（CH3）2CHCH2]2NH2}·[CF3COO] （1） has been synthesized based on the diisobutylamine and trifluoroacetic acid. The DSC data reveal that 1 undergoes a phase transition at 277 K （Tc）. When one chlorine atom replaces one fluorine atom on trifluoroacetate anion, three order-disorder phase transitions at 251 K （Ti）, 282 K （T2） and 290 K could be achieved owing to a stepwise release of rotation motions of the anion and cation in {[（CH3）2CHCH2]2NH2}·[CF2C[COO] （2）. Based on the variable temperature crystal structures, the phase transition in compound 1 is triggered by the rotation of three fluorine atoms on the trifluoroacetate anion synchronized with the motions of the methyl groups on the diisobutylammonium cation. However, in compound 2, the phase transitions can be realized by a sequence of motions of both the anion and cation. Besides, the dielectric- temperature dependences were investigated in order to prove this regulation process. All of this investigation will be beneficial to design and synthesis of the novel phase transition materials and molecular dielectric materials on pur- pose.

Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

The engineering of switchable materials with controllable stimuli-responsive multistability remains challenging in materials science.Herein,we present syntheses and structural and magnetic studies of a one-dimensional cobalt(Ⅱ)coordination polymer[(enbzp)Co(bpy)](ClO_(4))_(2)·-MeOH·H2O(1;enbzp=N,N′-(ethane-1,2-diyl)bis(1-phenyl-1-(pyridin-2-yl)methanimine,bpy=4,4′-bipyridine)and its desolvated analogue[(enbzp)Co(bpy)](ClO_(4))_(2)(2),obtained by reversible single-crystal-to-single-crystal(SCSC)transformation.

An electron-transfer[Fe_(2)Co_(2)]square complex exhibiting unprecedented wide room-temperature hysteresis

Switchable molecules with magnetic bistability that can function at standard conditions are highly desirable from the perspective of practical applications.Over the last decade,significant progress has been made in modulating thermally induced electrontransfer-coupled spin transition(ETCST)events between diamagnetic{Fe_(LS)^(Ⅱ)(μ-CN)Co_(LS)^(Ⅲ)}and paramagnetic{Fe_(LS)^(Ⅲ)(μ-CN)-Co_(HS)^(Ⅱ)}electron configurations(named LS and HS,respectively,for short)within the Fe/Co mixed-valence cyanometallate complexes.Herein,we prepared a novel[Fe_(2)Co_(2)]square complex[(pzTp)Fe(CN)_(3)Co(bnbpen)]_(2)[NO_(3)]_(2)·4CH_(3)OH·2.5H_(2)O(1·Solv)and its desolvated phase[(pzTp)Fe(CN)_(3)Co(bnbpen)]_(2)[NO_(3)]_(2)(1),obtained via single-crystal-to-single-crystal(SCSC)transformation.Complex 1·Solv remaining in the diamagnetic state below room temperature exhibits an irreversible ETCST behavior accompanied by desolvation upon heating.Notably,complex 1 displays an unprecedented rate-dependent ETCST behaviour involving with two diamagnetic LS states(a dynamically favoured LS1 state and a thermodynamically favoured LS2state),between which a net one-way transition from LS1 to LS2 is accessible likely via a structural phase transition(SPT)above255 K.Consequently,two well separated transition channels(HS?LS1 at around 255 K and HS←→LS2 in the temperature range of 317-350 K)are achieved according to either fast(>20 K/min)or slow rate(<1 K/min),respectively.At a moderate rate of 3 K/min,complex 1 shows a wide hysteresis of 66 K centered at 328 K,the largest yet reported for discrete Fe/Co cyanometallate complexes.Complex 1 represents the first ETCST example coupled with dynamic SPT,providing a new strategy for manipulating bistability within this fascinating cyanometallate family.