Specific features of the formation of the ferroelectric phase in polytypes of TlGaSe2 crystals

The polytypism of layered crystals of thallium gallium diselenide TlGaSe₂ has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe₂ polytype is an improper ferroelectric phase transition occurring at a temperature T _c ≈ 108 K, whereas the phase transition observed in the 2C-TlGaSe₂ polytype is a proper ferroelectric phase transition occurring at a higher temperature T _c ≈ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe₂ crystals.     

Influence of polytypism on structural phase transformations in TlGaSe2 crystals

The polytypism is found to have a significant influence on the structural transformation of the crystal lattice of the TlGaSe₂ ferroelectric with variations in temperature. In the 2C-polytype, unlike the C-polytype, a first-order structural phase transition, which leads to a change in the translational symmetry along the C axis, is not observed in the temperature range T = 90–300 K and a second-order phase transition due to the formation of an incommensurate phase occurs at a higher temperature.     

TDPAC characterization of Ni2HfF8·12H2O and its decomposition products

The hyperfine interaction in Ni₂HfF₈·12H₂O has been determined between 77 K and 1100 K by means of the time-differential perturbed angular correlation technique. From 200 K on, the one-site phase existing at lower temperatures undergoes a gradual phase transition until, at room temperature, the populations of both phases attain a 2:1 ratio. While the quadrupole frequencies characterizing them exhibit aT ^3/2 thermal dependence, their population ratio seems to obey a Boltzmann distribution. At 350 K, when the η-value of the high temperature phase electric field gradient approaches its maximum value, the starting compound decomposes to NiHfF₆·6H₂O. A kinetics study of the Ni₂HfF₈·12H₂O recovery at room temperature seems to indicate that a tri-dimensional diffusion mechanism is responsible for the corresponding reaction process. The first decomposition product of NiHfF₆·6H₂O left to atmospheric pressure is found to be NiHfF₆·4H₂O at 368 K and, between 414 K and 590 K, the high temperature cubic phase of NiHfF₆ and Hf₂OF₆ can be simultaneously observed. Finally, monoclinic HfO₂ appears from 1020 K on, having been preceded by an interaction which can be though of as depicting a preliminary stage in hafnia formation.     

Dielectric permittivity and AC conductivity investigations of the structural phase transitions in (NH3)2(CH2)7CdCl2Br2

The AC dielectric permittivity as a function of temperature (100–400 K) at different frequencies (between 80 Hz and 20 kHz) for the layered alkylene diammonium insulator containing Cd, namely, [(NH₃)₂(CH2)₇CdCl₂Br₂] has been measured. The formation of the compound was confirmed by microchemical analysis and IR absorption spectrometry. X-ray powder diffraction indicates an orthorhombic unit cell of dimensions: a = 10.219(2) Å, b = 9.168(2) Å and c = 38.694(4) Å. The AC conductivity ([sgrave]) is presented as a function of temperature and frequency. The conductivity and permittivity results indicate the presence of first-order phase transitions at 317 and 345 K. This has been confirmed by thermal analysis techniques. The activation energies were of values ranging between 0.15 and 0.62 eV depending upon the temperature range and the applied frequencies.     

Structural transformations in a single‐crystal Rb2NaYF6: Raman scattering study

This paper reports Raman spectroscopy investigation of phase transitions in Rb₂NaYF₆ crystal. The experimental spectra were compared with the calculated one. The spectra were obtained in temperature range from 8 to 300 K. The Raman spectra shows anomalous temperature‐dependent behavior at T₁ = 154 and T₂ = 122 K. Soft mode restoration has been found, which allows us to attribute first transition at 154 K to displacive type. Detailed analysis temperature dependencies of the line positions and widths have been performed. We found no effects of possible lattice disorder anywhere, except narrow (about 20 K) range above the T₁ temperature. The Raman spectra of Rb₂NaYF₆ crystal have been obtained and analyzed under hydrostatic pressure up to 4.33 GPa (at T = 295 K). The high pressure experiment up to 4.33 GPa did not disclose any effects associated with phase transitions. The lattice vibration spectra were calculated up to 10 GPa. The calculation has been demonstrated that the Rb₂NaYF₆ does not undergo high pressure phase transition. Copyright © 2013 John Wiley & Sons, Ltd.     

Structural and dielectric properties of Pb(Li1/4Dy1/4W1/2)O3

The polycrystalline samples of Pb(Li_1/4Dy_1/4W_1/2)O₃ have been synthesized by high-temperature solid-state reaction techniques. Room temperature X-ray diffraction (XRD) studies of the compound provided preliminary structural data, and hence formation of a single phase desired material was confirmed. Detailed studies of dielectric constant (ε) and loss (tanδ) as a function of frequency (100 Hz to 10 kHz) at room temperature (298 K) and also as a function of temperature (liquid nitrogen to 403 K) at 10 kHz suggest that the compound undergoes a ferroelectric phase transition of diffuse type.     

Temperature‐dependent vibrational studies of [N(C2H5)4]2MnCl4

Room‐temperature polarized Raman spectra of a single crystal and IR spectra of a polycrystalline sample were measured for [N(C₂H₅)₄]₂MnCl₄ and the assignment of the observed bands to the respective modes has been proposed. Temperature‐dependent Raman and far‐IR studies were also performed for the polycrystalline sample in order to obtain information on changes occurring in this material as a result of phase transitions at T₁ = 227 K and at T₂ = 199 K. These studies revealed that the higher‐temperature ferroelastic phase transition is associated with significant modification of vibrational properties due to ordering of tetraethylammonium groups. The lower‐temperature phase transition does not lead to any clear changes in the spectra. However, our results suggest that disorder of MnCl₄²⁻ ions decreases with decreasing temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Low temperature heat capacity of YBa2Cu3O7

We have measured the heat capacity of superconducting, single phase YBa₂Cu₃O₇ in the temperature range 2 to 18 K. An extrapolation of the data between 4 and 9 K gives aC/T (T → 0) of ∼ 25 mJ/mole K². The Debye temperature obtained from the high temperature linear portion ofC/T vsT ² plot is 325 K.     

Calorimetric investigation of (TEA)2MnCl4 crystals

Temperature dependencies of the specific heat in a temperature range of 14–260?K are presented for the crystals (TEA)₂MnCl₄. Two anomalies typical for the first order phase transition at T ₁?=?224?K and T ₂?=?231?K with entropy jumps of 15?J?mol^?1?K^?1 and 12?J?mol^?1?K^?1 were observed. The temperature dependence of the lattice heat was approximated by a linear combination of Debye and Einstein functions. Basing on the results an effective wave number of phonons with an essential contribution to the lattice oscillation energy was determined. Since phase transition sequence observed in (TEA)₂MnCl₄ and (TEA)₂CuCl₄ is similar and similar is the chemical composition one can suppose that the low temperature phase transition observed in (TEA)₂MnCl₄ crystals is an isostructural transition.     

Dielectric studies of the phase transition in {N(CH3)4}2HgCl4

Dielectric properties of single-crystal {N(CH₃)₄}₂HgCl₄ (structure at room temperature; orthorhombic Pmcn) have been measured in a temperature range from 4 K to about 370 K. The dielectric constant along the a-, b-, and c-axes shows a break at 278 K. No other anomalies are detected in the temperature range studied. The transition temperature increases linearly with increasing hydrostatic pressure at a rate of 0.20 K/MPa. Twin boundaries are observed in the low-temperature phase on b-plate specimens. The results indicate that the phase transition in {N(CH₃)₄}₂HgCl₄ is ferroelastic, as in {N(CH₃)₄}₂XBr₄ (X: Mn, Co, Zn).     

Raman scattering study of temperature and hydrostatic pressure phase transitions in Rb2KTiOF5 crystal

Raman spectra of Rb₂KTiOF₅ crystal were obtained and analyzed in the temperature range from 77 to 297 K and under hydrostatic pressure up to 4.2 GPa (at T = 295 K). The experimental results were compared with quantum‐chemical simulation of TiOF₅ pseudo‐octahedron. To interpret effects of lattice ordering, phonon spectra of several ordered phases of Rb₂KTiOF₅ were calculated within ab initio generalized Gordon–Kim model, and ordering of TiOF₅ molecular groups were simulated within Monte Carlo approach. The spectra exhibited orientation disordering in the cubic phase under ambient conditions. Cooling below the phase transition temperature (215 K) leads to partial ordering of the structure. The isotropic perovskite‐like phase was found to undergo first‐order transition into a low‐symmetry anisotropic phase at about 1 GPa. Further compression up to 4.1 GPa did not show any effects associated with phase transitions. Copyright © 2011 John Wiley & Sons, Ltd.     

Structure and phase transitions in Cu2P2O7

X-ray diffraction studies of the temperature variation of the Cu₂P₂O₇ crystal structure were performed in the vicinity of the transition from the α-phase (C2/c space group) to the β-phase (C2/m space group). These results as well as those of Raman spectroscopy studies and temperature variation of the electric capacitance point to the existence of an intermediate phase in the temperature range 347–363?K.     

X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

Superconductivity of Bi2 (Sr0.9Ca0.1)2 CuO y single crystals

Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y single crystals were grown by the self-flux method. The temperature dependence of resistance and ac susceptibility measurements showed that the as-grown crystal is semiconductive between 4.2 K and 300 K. However, after annealing in air for more than 20 days the crystal became superconducting with a T _c of 66 K obtained from the R-T curve. The ac susceptibility data also showed a strong superconducting transition at 71 K. X-ray diffraction indicated that the structure of the as-grown and the annealed crystal both are orthorhombic. The T _c of 71 K of the Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y crystal may be due to the increase of oxygen content in the crystal after the long-time annealing in air. It is suggested that the superconductivity of the Ca-doped 2201 phase of the Bi-based system, which up to now was found to have a T _c=7–22 K, is similar to the 2201 phase of the Tl-based system.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

NQR Study of Phase Transition and Cationic Motion in 4-NH2C5H4NHBiBr4·H2O

Four ⁸¹Br NQR lines in 4-NH₂C₅H₄NHBiBr₄·H₂O were observed in the temperature range between 77 and ca. 380 K; with increasing temperatures the respective sets of higher and lower two resonance lines coalesced into single lines discontinuously at 274 K, showing the occurrence of a first-order type phase transition of this crystal. The transition was confirmed with heat anomaly on a DTA curve. Each higher and lower line of high-temperature phase is assignable to the terminal Br atoms and the bridging ones of one-dimensional poly anions (BiBr₄ ⁻) _n in the crystal structure (C2/c), which was investigated by a X-ray structure analysis at room temperature. The 1/T ₁ temperature dependence of ⁸¹Br NQR follows the usual T ² law in the temperature range between 77 and ca. 140 K, being explained by fluctuation of the EFG at Br nucleus due to lattice vibrations. The T ₁ vs. 1/T curve in the temperature range between about 160 and 190 K was describable by the exponential curves, allowing us the estimation of activation energies. These exponential behaviors of T ₁ of ⁸¹Br NQR are attributable to the fluctuations caused by the thermal motion of 4-NH₂C₅H₄H⁺ ions. Echo signals of the ⁸¹Br NQR could not be detected above 190 K owing to poor S/N with very short T ₂.     

The double-phase transition of ErFe4Ge2. An XRPD study

High-quality powder XRD data of the compound ErFe₄Ge₂ collected in the ESRF beam line BM16, are presented for the entire magnetically ordered regime (T_N=44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to distinguish between structural and magnetic satellites, both present in the neutron patterns, and to demonstrate the interdependence of structural and magnetic transitions. The high-temperature (HT) phase disproportionates by a first-order transition into two distinct phases: P4₂/mnm (T_c, T_N=44 K)→Cmmm (majority LT phase)+Pnnm (minority IT Phase) which coexist in proportions varying with temperature down to 4 K. The phase diagram comprises three temperature regions: (a) the HT range with T>T_N for the tetragonal P4₂/mnm phase; (b) the IT (intermediate temperature) range, 20 K<T<T_N, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration (≈31%) around 30 K and (c) the LT (low temperature) range, 1.5–20 K, where the Cmmm phase is dominating (up to 95%). We suggests that this phenomenon is the result of competing magneto-elastic mechanisms involving the Er crystal field anisotropy, the Er–Er, Er–Fe and the Fe–Fe exchange interactions and their coupling with the lattice strains.     

Inelastic neutron scattering study of the specific features of the phase transitions in (NH4)2WO2F4

Oxyfluoride (NH₄)₂WO₂F₄ has been studied by the inelastic neutron scattering method over a wide temperature range 10–300 K at two initial neutron energies of 15 and 60 meV. The role of tetrahedral ammonium groups in the mechanism of sequential phase transitions at T ₁ = 201 K and T ₂ = 160 K has been discussed.     

The studies of high-temperature and short-time annealing,phase transition process,and magnetic property for LaFe11.7Si1.3 compound

The high-temperature phase transition is analyzed according to the DSC of as-cast LaFe_11.7 Si_1.3 compound and the X-ray patterns of LaFe_11.7Si_1.3 compounds prepared by high-temperature and short-time annealing. Large amount of 1:13 phase begins to appear in LaFe_11.7Si_1.3 compound annealed near the melting point of LaFeSi phase (about 1422?K). When the annealing temperature is close to the temperature of peritectic reaction (about 1497?K), the speed of 1:13 phase formation is the fastest. The phase relation and microstructure of the LaFe_11.7Si_1.3 compounds annealed at 1523?K (5?h), 1373?K (2?h)?+?1523?K (5?h), and 1523?K (7?h) +1373?K (2?h) show that longer time annealing near peritectic reaction is helpful to decrease the impurity phases. For studying the influence of different high-temperature and short-time annealing on magnetic property, the Curie temperature, thermal, and magnetic hystereses, and the magnetocaloric effect of LaFe_11.7Si_1.3 compound annealed at three different temperatures are also investigated. Three compounds all keep the first order of magnetic transition behavior. The maximal magnetic entropy change ΔS_M (T, H) of the samples is 12.9, 16.04, and 23.8?J?kg^?1?K^?1 under a magnetic field of 0–2?T, respectively.     

Elastic properties of the Ta–V system: bcc Ta0.33V0.67 and C15 TaV2

Resonant ultrasound spectroscopy was used to measure the elastic constants of bcc Ta_0.33V_0.67 over the temperature range 3.5–300?K; the results were compared to earlier measurements on C15 TaV₂. The temperature dependence of the polycrystalline shear modulus is completely different in the two phases; that of the bcc phase decreases with temperature whereas that of the C15 phases increases in an anomalous fashion. This difference is consistent with a model involving doubly-degenerate levels at the X point of the Brillouin zone in the C15 phase with the Fermi level lying near the doubly degenerate level. This model accounted for the unusual behaviour of the C15 phase. Debye temperatures were determined from the ultrasonic measurements: 295?K for the C15 phase and 315?K for the bcc phase.