Phase transition and anisotropy of thermal expansion in TlInS2

Linear expansion coefficients parallel and perpendicular to the layer plane of TlInS₂ layer crystal were measured in the temperature range 20–250 K at T { 200 K a strong anomaly in α⊥ behaviour was observed—the value of α6 increased abruptly up to the 200 × 10^?6 K^?1 due to phase transition in this crystal. It was shown that there is no anomaly in the behaviour of linear expansion coefficient α_z.dfnc;. This fact allowed to conclude that the phase transition in TlInS₂ is caused by changes in interlayer distances.     

Features of low-temperature thermal expansion of n-type Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals in magnetic field

Tensometric study of n-type Bi₂Se₃ single crystals in dc magnetic fields to 6 T in a temperature range of 7–23 K detected a weak negative thermal expansion (NTE) in the basal plane. The NTE increases with the field strength and depends on its orientation with respect to the trigonal c axis. In a magnetic field of 6 T, parallel to the c axis, the linear NTE coefficient reaches ?7 · 10^?7 K^?1, and a minimum sample length is reached at a temperature of 13 K, where a Hall carrier concentration maximum is also detected. The found magnetoelastic anomaly can be associated with the topological insulator state.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

Anomalies in thermal expansion of DyVO4 induced by quadrupolar ordering

The thermal expansion of the DyVO₄ crystal has been experimentally and theoretically investigated in the range of the Jahn-Teller structural phase transition. The manifestation of totally symmetric magnetoelastic interactions upon this transition has been studied for the first time. It is found that the temperature dependences of the unit-cell and thermal expansion parameters along the nonactive Jahn-Teller direction in the basal plane for the DyVO₄ crystal exhibit characteristic magnetoelastic anomalies at T<T _c due to the ordering of quadrupole moments of Dy³⁺ ions. The magnetoelastic contributions of the totally symmetric ε^α1 and ε^α2 and symmetry-lowering ε^γ modes to the thermal expansion are calculated within the general crystal-field formalism. The total quadrupolar coefficient G ^γ and magnetoelastic coefficient B ^γ are determined from the spectroscopic and spontaneous deformation data. It is demonstrated that the thermal expansion of the DyVO₄ crystal in the tetragonal and orthorhombic phases is well described in the framework of the unified model using a common set of interaction parameters for both phases.     

A new phase transition in 2H-TaSe2 — A dilatometric study

A new first-order phase transition of 2H-TaSe₂ has been observed at 113 K in thermal expansion measurements, using a high-resolution capacitance dilatometer. The first order nature of the lock-in transition at 93 K has been confirmed. The Charge-Density-Wave ordering transition at 123 K is observable only in the basal plane thermal expansion, emphasing the two-dimensional nature of the ordering.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

High temperature x-ray powder diffractometric studies of the superconducting compound YBa2Cu3O7-x from room temperature to 1300 K in air

At room temperature, the compound YBa₂Cu₃O_7-x is orthorhombic, distorted perovskite, deficient in oxygen. X-ray powder diffractograms obtained for this compound over a range of temperatures between room temperature (298 K) and 1300 K in air reveal no change in X-ray diffraction pattern up to 1200 K except for slight shift in peak positions and variations in peak intensities because of thermal expansion and stoichiometry changes in oxygen or metal ion sub-lattices. At temperatures above ∼ 1250 K, the compund undergoes an irreversible transition to a new phase analogous to Y₂BaCuO₅ green phase which is an insulator. Temperature variation of lattice parameters shows an anisotropic expansion. Expansion along c-axis being maximum. Coefficients of linear thermal expansion along a and b axes are almost the same (δ_a≈δ_b≈7×10⁻⁶.K⁻¹) while along c-axis it is double (δ_c≈15×10^-6. K⁻¹) than that of a or b. Coefficient of volume thermal expansion of the unit cell (δ_v≈29×10⁻⁶.K⁻¹) obeys the relation δ_v≈δ_a+δ_b+δ_c for an orthorhombic symmetry in the range 298–1200K in air.     

Low-temperature investigations of the open-framework material HfMgMo3O12

The A₂Mo₃O₁₂ family, where A³⁺ is a large trivalent cation, can show interesting thermal properties such as negative thermal expansion. One member of this family, HfMgMo₃O₁₂, where the two A³⁺ cations have been replaced by Hf⁴⁺ and Mg²⁺, has been shown to have a low positive coefficient of thermal expansion above room temperature. This property makes HfMgMo₃O₁₂ an attractive candidate as a component for solid solutions with near-zero thermal expansion. However, its properties below room temperature were unexplored. In this work we report the phase transition from orthorhombic Pnma to monoclinic P2₁/a at T～175 K with an enthalpy change of 0.27 kJ mol^?1. Relaxation calorimetry, from 5 K to 300 K, show only the small anomaly associated with this transition. The thermal conductivity, determined from 2 K to 300 K, was low, but not as low as some other materials exhibiting negative thermal expansion. Analysis of the low-temperature heat capacity indicates the presence of low-energy phonon modes in HfMgMo₃O₁₂, consistent with the low thermal conductivity. The upper bound of the Young's modulus, estimated from the effective Debye temperature derived from the low-temperature heat capacity, is 20 GPa, a relatively low value due to the flexibility of the framework structure.     

Magneto-optical activity at the structural phase transition in paramagnetic KMnF3

The magneto-optical Verdet constant ø has been measured in the paramagnetic phase of KMnF₃ and particularly in the neighbourhood of the cubic-tetragonal phase transition driven by the softening of zone boundary modes. ø is positive and displays a sizeable decrease on cooling, thus indicating the presence of a temperature dependent paramagnetic contribution that, in view of the ⁶S ground state of the Mn²⁺ ion, is attributed to the role of spin-orbit interaction. In a temperature range of about 20K around T_c = 186K, a cusp-shaped anomaly of ø is observed. From the dependence on the wavelength, and by taking into account also EPR linewidth and g measurements and optical absorption spectra, an analysis of the possible mechanisms for the cusp-shaped anomaly is given. It is argued that the enhancement of the critical rotational fluctuations of the MnF₆ octahedra can be responsible for this effect.     

Thermal expansion anomaly of MnB

The thermal expansion of MnB has been measured in the temperature range from 0 to 420°C. An Invar type thermal expansion is observed. The thermal expansion coefficient is ?42×10^-6 deg.^-1 at 295°C(≈T_c).     

X-ray study of the thermal expansion anisotropy in the quarternary semiconductor CuGaSn□Se4

Accurate lattice parameters a and c of the tetragonal chalcopyrite quaternary semiconductor CuGaSn□Se₄ have been determined as a function of temperature by the X-ray powder diffraction method in the temperature range 300 K to about 900 K. The data have been used to evaluate the axial expansion coefficients α_a and α_c at various temperatures. The thermal expansion studies revealed the anisotropy between the axial expansion coefficients having a larger coefficient of expansion along the a-axis than that along the c-axis (α_a > α_c). The mean values α_a and α_c, in the temperature range 300–900 K, are found to be 14.02 × 10^-6K^-1 and 5.02 × 10^-6K^-1 respectively, and the axial ratio, c/a, changes with a coefficient of -8.96 × 10^-6K^-1. This result indicates an increase in the tetragonal distortion, δ = 2 - c/a with temperature. An attempt is made to explain the increase in tetragonal distortion with temperature and the anisotropic thermal expansion of CuGaSn□Se₄ in terms of the thermal expansion of the A>−;Se (where A is Cu and Ga randomly distributed) and B>−;Se (where B is Sn and vacancy randomly distributed) bonds. The results are also discussed in terms of the principal Grüneisen parameters of chalcopyrite structure compounds.     

Thermal expansion of (Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>x</Subscript>)<Subscript>3</Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> single crystals at low temperatures

For single-crystal samples of the (Sr_1?xLa_x)₃Ru₂O₇ ruthenates, the temperature dependence of the thermal expansion coefficient α(T)) is measured in the range 4.2–80 K. The effect of magnetic fields H ≤ 3.5 T on thermal expansion is analyzed. It is found that the (Sr_1?xLa_x)₃Ru₂O₇ ruthenates exhibit an anomalous (negative) thermal expansion coefficient in the temperature range T ≤ 18 K. The position and width of the anomaly revealed in the temperature dependence of the thermal expansion coefficient α(T)) depend substantially on the magnetic field. The origin of the thermal expansion anomaly in ruthenates, the correlation of this anomaly with the stability of the crystal lattice, and the common nature of the anomalies in the thermal properties of ruthenates and high-temperature superconductors are discussed.     

Thermal expansion of the one-dimensional conductor K2Pt(CN)4Br0.3·xH2O and K2Pt(CN)4·xH2O

Measurements of the thermal expansion coefficients of K₂Pt(CN)₄Br_0.3· xH₂O and K₂Pt(CN)₄·xH₂O show a large anisotropy of the a-and c- directions. Their temperature dependence could be described by a simple Grüneisen theory. In the range from 80–330°K no anomaly indicating a Kohn-Peierls transition could be found.     

Mössbauer study of (NH4)2FeCl5·H2O

Mössbauer spectra of the compound (NH₄)₂FeCl₅·H₂O have been studied as a function of temperature. Two phase transitions are observed in the temperature range between 7 K and 9 K. The transition at 9 K is structural and presents an unusually high thermal hysteresis. AroundT=8 K the substance orders magnetically and different Fe³⁺ contributions are present.     

High-temperature structural phase transition in the LiCu2O2 multiferroic

The results of thermogravimetric, X-ray diffraction, and electrical studies of LiCu₂O₂ single crystals in the temperature range 300–1100 K are presented. A reversible first-order phase transition between the orthorhombic and tetragonal phases is found to occur in these single crystals at T = 993 K. A pronounced peak on a differential thermal analysis curve and jumps in the unit cell parameters and the electrical resistivity are detected at the phase-transition temperature. The data on the crystal structure of LiCu₂O₂ and the phase transition-induced change in the entropy determined in this work are used to conclude that the revealed phase transition is caused by the ordering-disordering of Li⁺ and Cu²⁺ cations in their structural positions.     

First-order phase transition from an antiferromagnetic ferroelectric to a cycloidal multiferroic with weak ferromagnetism during the joint action of applied magnetic and electric fields

The thermodynamics of the phase transition in a perovskite-like multiferroic, in which an antiferromagnetic ferroelectric transforms into a new magnetic state where a spiral spin structure and weak ferromagnetism can coexist in applied magnetic field H, is described. This state forms as a result of a first-order phase transition at a certain temperature (below Néel temperature T _N ), where a helicoidal magnetic structure appears due to the Dzyaloshinskii-Moriya effect. In this case, the axes of electric polarization and the helicoid of magnetic moments are mutually perpendicular and lie in the ab plane, which is normal to principal axis c. Additional electric polarization p, which decreases the total polarization of the ferroelectric P, appears in the ab plane. The effect of applied magnetic and electric fields on the properties of a multiferroic with a helicoidal magnetic structure is described. An alternating electric field is shown to cause a field-linear change in magnetic moment m, whose sign is opposite to the sign of the change of electric field E. The detected hysteretic phenomena that determine the temperature ranges of overheating and supercooling of each phase are explained. A comparison with the experimental data is performed.     

Thermal evidences for successive CDW phase transitions in 1T-TaS2

The heat capacity of 1T-TaS₂ has been measured over the temperature range including the successive phase transitions (140 K–370 K) by an adiabatic calorimeter. There are three transitions in the measured temperature range, two first-order transitions (at about 226 K (T₁) and about 353.5 K (T₃)) and one small anomaly at about 283 K (T₂) with a broad peak. The transition enthalpies are as follows; ΔH₁=52±5 cal·mol^-1, ΔH₂=7.5±2 cal· mol^-1 and ΔH₃=122±8cal·mol^-1.     

Thermal conductivity and specific heat of SrCu2(BO3)2: A quasi-two-dimensional metal oxide compound with a spin gap

The thermal conductivity and specific heat of SrCu₂(BO₃)₂, a quasi-two-dimensional metal oxide compound with a spin gap, were studied at low temperatures. In the temperature interval 0.4<T<3.2 K, the thermal conductivity of a single crystal sample in the ab plane varies according to the power law κ∝T ^2.73. As the temperature increases further, a deep minimum is observed in the region of T _min≈9.8 K. This behavior is explained by the scattering of phonons—the major heat carriers—on the fluctuations of the spin subsystem.     

Antiferromagnetic ordering in terbium dihydride. Low temperature heat capacity studies

The specific heat of terbium dihydride, TbH_2.01, was measured in the temperature range from 4–200 K. An extremely sharp λ-type specific heat anomaly due to an antiferromagnetic transition has been found at T_N = 16.06 ± 0.01 K.