Synthesis and electrical conductivity of nanocrystalline tetragonal FeS

A convenient method for synthesis of tetragonal FeS using iron powder as iron source, is reported. Nanocrystalline tetragonal FeS samples were successfully synthesized by reacting metallic iron powder with sodium sulfide in acetate buffer solution. The obtained sample is single-phase tetragonal FeS with lattice parameters a = 0.3767 nm and c = 0.5037 nm, as revealed by X-ray diffraction. The sample consists of fiat nanosheets with lateral dimensions from 20 nm up to 200 nm and average thickness of about 20 nm. We found that tetragonal FeS is a fairly good conductor from the electrical resistivity measurement on a pellet of the nanosheets. The temperature dependence of conductivity of the pellet was well fitted using an empirical equation wherein the effect of different grain boundaries was taken into consideration. This study provides a convenient, economic way to synthesize tetragonal FeS in a large scale and reports the first electrical conductivity data for tetragonal FeS down to liquid helium temperature.

Young’s modulus surface and Poisson’s ratio curve for tetragonal crystals

This paper gives the general expressions for the compliance s′ijkl, Young＇s modulus E（hkl） and Poisson＇s ratio v（hkl, θ） along arbitrary loading direction [hkl] for tetragonal crystals. The representation surface for which the length of the radius vector in the [hkl] direction equals E（hkl） and representation curve for which the length of the radius vector with angle θ deviated from the reference directions [001^-], [100], [001^-], [101^-] and [112^-] equals v（100, θ）, v（001, θ）, v（110,θ）, v（101,θ） and v（111, θ） respectively, are constructed for nine tetragonal crystals （ammonium dihydrogen arsenate, ammonium dihydrogen phosphate, barium titanate, indium, nickel sulfate, potassium dihydrogen arsenate, potassium dihydrogen phosphate, tin and zircon）. The characteristics of them are analysed in detail.

Dielectric Properties of Unfilled Tetragonal Tungsten Bronze Ba4PrFe(0.5)Nb(9.5)O(30) Ceramics

In order to found new dielectrics ceramics in tungsten bronze structure, unfilled tungsten bronze（TB） ceramics with nominal formula Ba4PrFe（0.5）Nb（9.5）O（30） were prepared by the solid state reaction method. The microstructure and dielectric properties were studied using powder X-ray diffraction, field emission scanning electron microscope, and variable temperature dielectric test system. The results show that the ceramics have a single phase and belong to the space group of P4bm with the cell of a = b = 12.4839（3） ？, c = 3.9409（5） ？, V = 614.192（2） ？3. The frequency dependent dielectrics properties show that the ceramics have a Debye-like relaxation at room temperature, while the temperature dependent dielectrics properties indicate that the ceramics are a relaxor and the relaxation is due to the nanopolars and oxygen vacancies. The ceramics have a potential application in electronic ceramics as temperature-stable multilayer ceramic capacitors.

Theoretical Study of Carrier Mobility in Two-Dimensional Tetragonal Carbon Allotrope from Porous Graphene

The carrier mobility of two-dimensional tetragonal carbon allotrope （T-CA） from porous graphene is investigated by first-principles calculations. T-CA can be constructed from divacancy and Stone-Thrower--Wales defects from graphene. T-CA is a direct semiconductor with a band gap of 0.4 eV at F point. T-CA possesses a high carrier mobility of the order of 104 cm2V-ls-1. As our study demonstrates, T-CA has potential applications for next-generation electronic materials.

Coulombic interaction in the colloidal oriented-attachment growth of tetragonal nanorods

In this report, the analytical expression of Coulombic interaction between a spherical nanoparticle and a tetragonal nanorod is derived. To evaluate the Coulombic interaction in the oriented attachment growth of tetragonal nanorods, we analyze the correlation between the Coulombic interaction and the important growth parameters, including： nanoparticle- nanorod separation, aspect ratio of the nanorods, and surface charge density. Our work opens up the opportunity to investi-gate interparticle interactions in the oriented attachment growth of tetragonal nanorods.

Tetragonal Domain Switching via Reversible tm Phase Transformation

Grinding-induced tm phase transformation and the resultant texture in ceria-yttria-doped tetragonal zirconia polycrystals with varied tetragonality have been studied by XRD. It is observed that the reversible tm phase transformation occurs during grinding and the intensity ratio of I(002)t/I(200)t increases with the transformability. The author proposes that the texture induced by grinding at low temperatures is due to the tetragonal variant reorientation via cyclic,reversible tm phase transformation, termed 'transformational domain switching', instead of the ferroelastic one

Predicting Physical Properties of Tetragonal,Monoclinic and Orthorhombic M_3N_4(M=C,Si,Sn) Polymorphs via First-Principles Calculations

The recently discovered tetragonal, monoclinie and orthorhombic polymorphs of M3N4 （M=C, Si, Sn） are in- vestigated by using first-principles calculations. A set of anisotropic elastic quantities, i.e., the bulk and shear moduli, Young＇s modulus, Poisson ratio, H/G ratio and rickets hardness of M3N4 （M=C, Si, Sn） are predicted. The quasi-harmonic Debye model, assuming that the solids are isotopic, may lead to large errors for the non-cubic crystals. The thermal effects are obtained by the traditional quasi-harmonic approach. The dependences of heat capacity, thermal expansion coefficient and Debye temperature on temperature and pressure are systematically discussed in the pressure range of 0 IOGPa and in the temperature range of 0-1100 K. More importantly, o- C3N4 is a negative thermal expansion material. Our results may have important consequences in shaping the understanding of the fundamental properties of these binary nitrides.

Quenched Fe moment in the collapsed tetragonal phase of Ca_(1-x)Pr_x Fe_2As_2

We report 7SAs NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2. In both cases of x = 0.075, 0.15, a large increase of Vq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75 T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3＋ moments or soin dynamics of mobile domain walls.

Molecular Beam Epitaxy Growth of Tetragonal FeS Films on SrTiO3(001) Substrates

We report the successful growth of the tetragonal FeS film with one or two unit-cell （UC） thickness on SrTiO33（001） substrates by molecular beam epitaxy. Large lattice constant mismatch with the substrate leads to high density of defects in single-UC FeS, while it has been significantly reduced in the double-UC thick film due to the lattice relaxation. The scanning tunneling spectra on the surface of the FeS thin film reveal the electronic doping effect of single-UC FeS from the substrate. In addition, at the Fermi level, the energy gaps of approximately 1.5？meV are observed in the films of both thicknesses at 4.6？K and below. The absence of coherence peaks of gap spectra may be related to the preformed Cooper-pairs without phase coherence.

Abnormal Elastic Changes for Cubic-Tetragonal Transition of Single-Crystal SrTiO_(3)

Strontium titanate(SrTiO_(3))is a typical perovskite-type ceramic oxide and studying its high-pressure phases are critical to understand the ferroelastic phase transition.SrTiO_(3)also can be used as an important analog of davemaoite(CaSiO_(3))to understand the compositional and velocity structure of the Earth’s interior.However,the high-pressure studies on the cubic-to-tetragonal phase transition pressure and elastic properties remain unclear for SrTiO_(3).Here,we investigate the phase transition and elasticity of single-crystal SrTiO_(3)by Raman and Brillouin scattering combined with diamond anvil cell.The acoustic velocities of single-crystal SrTiO_(3)and the independent elastic constants of cubic and tetragonal SrTiO_(3)are determined up to 27.5 GPa at room temperature.This study indicates that C_(11),C_(12),and C_(44)exhibit abnormal changes at 10.3 GPa,which is related to the cubicto-tetragonal phase transition.Interestingly,a significant softening on shear modulus and a large anisotropy of shear wave splitting(A_(S)^(PO))jump are observed at 10.3 GPa.Using obtained elastic constants,the coefficients of the Landau potential are calculated to understand the phase transition between cubic and tetragonal.The calculated coefficients of the Landau potential are,λ_(2)=3.12×10^(-2)GPa,λ_(4)=-2.02×10^(-2)GPa,B~*=1.34×10^(-4)GPa and B=1.66×10^(-4)GPa.The elastic results have profound implications in understanding the structure of the Earth’s interior and indicate that the presence of tetragonal Ti-bearing CaSiO_(3)helps to explain the large APO S of the Earth’s mid-mantle.

Study of depth-dependent tetragonal distortion of quaternary AlInGaN epilayer by Rutherford backscattering/channeling

A 240-nm thick Al0.4In0.02Ga0.58N layer is grown by metal organic chemical vapour deposition, with an over 1-μm thick GaN layer used as a buffer layer on a substrate of sapphire （0001）. Rutherford backscattering and channeling are used to characterize the microstructure of AlInGaN. The results show a good crystalline quality of AIInGaN （χmin = 1.5%） with GaN buffer layer. The channeling angular scan around an off-normal {1213} axis in the {1010} plane of the AlInGaN layer is used to determine tetragonal distortion eT, which is caused by the elastic strain in the AIInGaN. The resulting AlInGaN is subjected to an elastic strain at interracial layer, and the strain decreases gradually towards the near-surface layer. It is expected that an epitaxial AlInGaN thin film with a thickness of 850 nm will be fully relaxed （^eT = 0）.

A Critical Study of the Elastic Properties and Stability of Heusler Compounds: Phase Change and Tetragonal <i>X₂YZ</i>Compounds

In the present work, the elastic constants and derived properties of tetragonal Heusler compounds were calculated using the high accuracy of the full-potential linearized augmented plane wave (FPLAPW) method. To find the criteria required for an accurate calculation, the consequences of increasing the numbers of k-points and plane waves on the convergence of the calculated elastic constants were explored. Once accurate elastic constants were calculated, elastic anisotropies, sound velocities, Debye temperatures, malleability, and other measurable physical properties were determined for the studied systems. The elastic properties suggested metallic bonding with intermediate malleability, between brittle and ductile, for the studied Heusler compounds. To address the effect of off-stoichiometry on the mechanical properties, the virtual crystal approximation (VCA) was used to calculate the elastic constants. The results indicated that an extreme correlation exists between the anisotropy ratio and the stoichiometry of the Heusler compounds, especially in the case of Ni2MnGa. Metastable cubic Ni2MnGa exhibits a very high anisotropy (≈28) and hypothetical cubic Rh2FeSn violates the Born-Huang stability criteria in the L21 structure. The bulk moduli of the investigated tetragonal compounds do not vary much (&asymp;130 ...190 GPa). The averaged values of the other elastic moduli are also rather similar, however, rather large differences are found for the elastic anisotropies of the compounds. These are reflected in very different spatial distributions of Young’s moduli when comparing the different compounds. The slowness surfaces of the compounds also differ considerably even though the average sound velocities are in the same order of magnitude (3.2 ... 3.6 km/s). The results demonstrate the importance of the elastic properties not only for purely tetragonal Heusler compounds but also for phase change materials that exhibit magnetic shape memory or magnetocaloric effects.

Crystallinity, Microstructure and Mechanical Strength of Yttria-Stabilized Tetragonal Zirconia Ceramics for Optical Ferrule

Yttria-stabilized zirconia ceramics were prepared by using different raw materials in order to compare commercially available optical ferrule. Injection-molded cylindrical green compacts were sintered in air at 1350°C, 1400°C and 1450°C for 2 hrs, followed by furnace cooling. Crystallinity, microstructure and mechanical strength of the sintered body were evaluated by using an X-ray diffraction analyses, a field emission-scanning electron microscope, a universal tester, and a micro-hardness tester, respectively. For practical usage, the sample B sintered at 1350°C was favorable because of high tetragonality and good mechanical strength.

Obtaining tetragonal FeAs layer and superconducting K_(x)Fe_(2)As_(2)by molecular beam epitaxy

Atomic characterization on tetragonal FeAs layer and engineering FeAs superlattices is highly desirable to get deep insight into the multi-band superconductivity in iron-pnictides.We fabricate the tetragonal FeAs layer by topotactic reaction of FeTe films with arsenic and then obtain KxFe_(2)As_(2)upon potassium intercalation using molecular beam epitaxy.The in-situ low-temperature√2×√2scanning tunneling microscopy/spectroscopy investigations demonstrate characteristic reconstruction of the FeAs layer and stripe pattern of KxFe_(2)As_(2),accompanied by the development of a superconducting-like gap.The ex-situ transport measurement with FeTe capping layers shows a superconducting transition with an onset temperature of 10 K.This work provides a promising way to characterize the FeAs layer directly and explore rich emergent physics with epitaxial superlattice design.

In situ atomic-scale observation of size-dependent (de) potassiation and reversible phase transformation in tetragonal FeSe anodes

Potassium-ion batteries(PIBs)are considered promising alternatives to lithium-ion batteries owing to cost-effective potassium resources and a suitable redox potential of-2.93 V(vs.-3.04 V for Li+/Li).However,the exploration of appro-priate electrode materials with the correct size for reversibly accommodating large K+ions presents a significant challenge.In addition,the reaction mecha-nisms and origins of enhanced performance remain elusive.Here,tetragonal FeSe nanoflakes of different sizes are designed to serve as an anode for PIBs,and their live and atomic-scale potassiation/depotassiation mechanisms are revealed for the first time through in situ high-resolution transmission electron micros-copy.We found that FeSe undergoes two distinct structural evolutions,sequen-tially characterized by intercalation and conversion reactions,and the initial intercalation behavior is size-dependent.Apparent expansion induced by the intercalation of K+ions is observed in small-sized FeSe nanoflakes,whereas unexpected cracks are formed along the direction of ionic diffusion in large-sized nanoflakes.The significant stress generation and crack extension originating from the combined effect of mechanical and electrochemical interactions are elucidated by geometric phase analysis and finite-element analysis.Despite the different intercalation behaviors,the formed products of Fe and K_(2)Se after full potassiation can be converted back into the original FeSe phase upon depotassiation.In particular,small-sized nanoflakes exhibit better cycling perfor-mance with well-maintained structural integrity.This article presents the first successful demonstration of atomic-scale visualization that can reveal size-dependent potassiation dynamics.Moreover,it provides valuable guidelines for optimizing the dimensions of electrode materials for advanced PIBs.

A NEW POLYMORPH——TETRAGONAL FeC_2O_4·2H_2O

Several different isomers of FeC2O4·2H2O can be obtained by using different preparation methods. Up to now, three polymorphs of FeC2O4·2H2O have been reported in the literature. One of them belongs to monoclinie system. The cell parameters are: a=12.05, b=5.57, c=9.76, β=124°18′, and its space group is C2/c. It is called a-FeC2O4·2H2O. Both of the other two belong to orthorhombic system. Their cell parameters and space groups are: a=12.26, b=5.57, c=15.48, Cccm, and a=11.78, b=15.46, c=5.56, Fmm2. The former is β-Fe C2O4·2H2O.

Synthesis and microstructure characterization of tetragonal Zr1–xTixO2 （x = 0–1） solid solutions

Oxide powders of Zr1_xTixO2(x=0-1)solid solutions with micron-sized particles were synthesized via a solution combustion method.The synthesis process and Zr/Ti molar ratio were optimized to produce powders with the tetragonal crystal structure.X-ray diffraction,Raman spectroscopy and transmission electron spectroscopy results confirm that a full crystallization microstructure with the single tetragonal phase is obtained after calcination at 600℃while maintaining the crystallite size<30 nm.Zr/Ti oxide mixtures with Zr>67 mol%exhibit a tetragonal crystal structure and the embedding Ti in ZrO2 improves the structure stability.The nitrogen sorption results indicate that the powders possess mesoporous morphology with medium specific surface areas(~10-50 m^2/g).Chemical stability tests show that these powders are relatively stable with negligible removal of titanium and zirconium after elution by 0.5 mol/L HC1.Density functional theory was used to calculate the most stable structure with low energy for the selected composition.

Thermal effects in a tetragonal Nd:YLF crystal laser

The thermal effects in host material are an important subject for solid state laser. Recently, tetragonal Nd:YLF crystal with the space group of 41/α has received great attention. The high efficiency, 40W cw Nd:YLF laser with large TEM00 mode has been reported by Giulio Cerullo and his colleagues. But the theory of thermal effects for tetragonal crystal has not been reported so far. For the first time, the calculated results of thermal effects during the lamp-pumped process for an α-axis Nd:YLF rod have been achieved in this note.

Lamellar thickness transition of melt-crystallized polybuten-1 tetragonal phase: configurational change in chain folding directions

Lamellar crystal thickness lc of isotactic polybutene-1(it-PB1)have been investigated for crystal-lization in the melt over a wide range of crystallization temperature T from 40°C to 90°C by small angle X-ray scattering experiments and density measurements.The crystal thickness lc demonstrates two linear dependences on inverse supercooling and a transition from one dependence to the other has been observed around T=65°C.Each of the two dependences obeys the nucleation theory in the high and low supercooling ranges,respec-tively.Chain folding free energy q determined from the low supercooling range is larger than that determined from the high supercooling range.Possible mechanisms for the transition are discussed taking account of entropy of chain folding directions.

Kinetic roughening transition and missing regime transition of melt crystallized polybutene-1 tetragonal phase: growth kinetics analysis

The morphology and lateral growth rate of isotactic polybutene-1(it-PB1)have been investigated for crystallization from the melt over a wide range of crystallization temperatures from 50 to 110℃.The morphology of it-PB1 crystals is a rounded shape at crystallization temperatures lower than 85℃,while lamellarsingle crystals possess faceted morphology at higher crystallization temperatures.The kinetic roughening transi-tion occurs around 85℃.The nucleation and growth mechanism for crystallization does not work below 85℃,since the growth face is rough.However,the growth rate shows the supercooling dependence derived from the nucleation and growth mechanism.The nucleation theory seems still to work even for rough surface growth.Possible mechanisms for the crystal growth of this polymer are discussed.