The effect of hydrostatic pressure on the ferroelectric phase transition in [NH2(CH3)2]3[Sb2Cl9]

The effect of hydrostatic pressure on the ferroelectric phase transition temperature in [NH₂(CH₃)₂]₃[Sb₂Cl₉] (DMACA) has been studied by electric permittivity measurements at pressures up to 400 MPa. The pressure-temperature phase diagram is given. The phase transition temperature (T_c) increases with increasing pressure up to 150 MPa, passes through a maximum and then decreases with a further increase of pressure. The unexpected nonlinear decrease in T_c with pressure increasing above 150 MPa suggests that the mechanism of ferroelectric phase transition in DMACA is different from hitherto assumed.     

1H NMR study of [N(CH3)4]2ZnCI4 at high pressures and low temperatures

Wide-line proton NMR studies on polycrystalline tetramethylammonium tetrachlorozincate have been carried out at high hydrostatic pressures up to 15 kbar in the temperature range 77-300 K and at ambient pressure down to 4.2 K. A second-moment transition is observed to occur starting around 161 K, the temperature for the V-VI phase transition. This transition temperature is seen to have a negative pressure coefficient up to 2 kbar, beyond which it changes sign. At 77 K the second moment decreases to 4 kbar and then increases again as a function of pressure. The results are explained in terms of the dynamics of the N(CH₃)₄ groups.     

The thermoelastic properties of [N(CH3)4]2ZnBr4 and [N(CH3)4]2MnBr4

The ferrodistortive phase transition in the bis-tetramethylammonium tetrabromide crystals below room temperature is studied within the framework of the Landau theory. The specific heats of [N(CH₃)₄]₂MnBr₄ and [N(CH₃)₄]₂ZnBr₄ are correctly described down to 40°C below the transition temperature. The phenomenological parameters are determined from calorimetric results, elastic constants and thermal expansion data. Using these coefficients, the monoclinic angle in the ferrodistortive phases is obtained. The anharmonic quantities, such as the isothermal compressibility, calculated from the specific heat data, are in good agreement with the values derived from the elastic measurements.     

EPR studies of phase transitions in perchlorates [M 2+(ClO4)2 · 6H2O] at high pressures

The EPR spectra of the Mn²⁺ ion in crystals of the perchlorate hexahydrates Zn(ClO₄)₂ · 6H₂O, Mg(ClO₄)₂ · 6H₂O, and Cd(ClO₄)₂ · 6H₂O were studied in the temperature range 77–320 K under hydrostatic pressure. It is shown that the octahedron of six molecules H₂O surrounding this paramagnetic ion is contracted along the c axis and that pressure decreases this distortion. The second-order phase transition that occurs near 200 K in the perchlorates and in other crystal hydrates is shown to be associated with changes in the bonds in the nearest ligand environment. As the pressure is increased, the phase-transition temperatures shift and the perchlorate crystals tend to a single-phase state. The low-temperature phase is assumed to disappear as the pressure increases, and this phase exists in a closed T-P region in the phase diagram. As the pressure increases, the character of the high-temperature transition in the Cd(ClO₄)₂ · 6H₂O changes: the jumplike transition at T ₁ with a 1-K hysteresis changes into a smooth transition and then disappears as the pressure increases further.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

High-pressure dielectric studies of a novel hydrogen-bonded ferroelectric (NH4)2H2P2O6

The hydrostatic pressure effect on the dielectric properties of (NH₄)₂H₂P₂O₆ ferroelectric crystal was studied for pressures from 0.1 MPa to 360 MPa and for temperatures from 100 to 190 K. The pressure–temperature phase diagram obtained is linear with increasing pressure. The paraelectric–ferroelectric phase transition temperature decreases with increasing pressure with the pressure coefficient dT_c/dp=?5.16×10^?2 K MPa^?1. Additionally, the pressure dependences of Curie–Weiss constants for the crystal in paraelectric (C₊) and ferroelectric (C_?) phases are evaluated and discussed. The possible mechanism of paraelectric–ferroelectric phase transition is also discussed.     

Synthesis,structure and preliminary results on electrical and magnetic properties of (Perylene)2 [Pt(mnt)2]

In this paper we report the preparation, crystal structure and some physical properties of the conducting molecular (Perylene)₂ [Pt(S₄C₄(CN)₄)].The crystal structure consists of segregated stacks of perylene and Pt S₄C₄ (CN)₄ units, with a spacing of 3.3 Å between the carbon atoms in the perylenes. The temperature dependence of the electrical conductivity is metallic with a maximum at approximately 15 K. At this temperature a sharp transition to an insulating state occurs. Magnetic suceptibility measurements confirm the existence of a transition in this temperature range.     

Anomalous pressure dependence of the Lamb-Mössbauerf-factor in the spin crossover system [Fe(2-pic-ND2)3]Cl2·EtOD

The pressure dependence of the Lamb-Mössbauer factor of the spin crossover compound [Fe(2-pic-ND₂)₃]Cl₂·EtOD(2-pic-ND₂=2-picolylamine, deuterated at the^?NH₂ group) has been measured at pressures up to 1500 bar and at three temperatures (115.7, 146.7, 185.6 K) around the transition temperatureT _t=135 K. The temperature dependence of the unit cell volume has been determined by X-ray diffraction. In the transition region (T=146.7 K), the pressure dependence of thef-factor shows an anomalous increase as compared to the dependence below and above the transition. The behaviour off(p, T), its anomaly and absolute value, can be consistently explained within a theoretical model which treats the compound as an isotropic elastic medium. The Poisson ratio of σ=0.4, the bulk modulusK (185.6 K)=1.26·10¹⁰ Nm^?2, and the Grüneisen constant γ^G=3.1 were determined.     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

On the isotope effect of Tc in squaric acid (H2C4O4)

The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on T_c is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H₂SQ) is stretched to 2R(D₂SQ) the phase transition temperature 7^H _c of H₂SQ coincides with T^D _c of D₂SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.     

X-ray diffraction investigation of the thermochromic phase transition in an [NH2(C2H5)2]2CuCl4 crystal

The unit cell parameters of an [NH₂(C₂H₅)₂]₂CuCl₄ crystal are determined using x-ray diffraction analysis, and the thermal expansion coefficients along the principal crystallographic directions are calculated in the temperature range 100–330 K. The behavior of the intensities of the diffraction reflections from the (100), (010), and (001) crystallographic planes is studied in the vicinity of the thermochromic phase transition temperature. The occurrence of a first-order phase transition in the [NH₂(C₂H₅)₂]₂CuCl₄ crystal at T ≈ 324 K is confirmed experimentally.     

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.     

Critical behaviour of Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals under high hydrostatic pressures

Basing on the temperature dependences of optical birefringence for Sn₂P₂S₆ and Sn₂P₂(Se_0.28S_0.72)₆ crystals subjected to hydrostatic pressures, we prove unambiguously that Sn₂P₂S₆ reveals a tricritical point on its (p, T)-phase diagram with the coordinates (p, T) = (4.3 kbar, 259 K), so that the second-order phase transition transforms into the first-order one whenever the pressure increases above 4.3 kbar. We also find that increasing hydrostatic pressure applied to Sn₂P₂(Se_0.28S_0.72)₆ leads to the change in the phase transition character from tricritical to first order. Further increase in the pressure up to ~2.5 kbar imposes splitting of the first-order paraelectric-to-ferroelectric phase transition into two phase transitions, a second-order paraelectric-to-incommensurate one and a first-order incommensurate-to-ferroelectric transition.     

Thermochromic Phase Transition in [NH2(C2H5)2]2CuCl4 Crystals

The absorption spectra of [NH₂(C₂H₅)₂]₂CuCl₄ crystals in the visible spectral region in the vicinity of the thermochromic phase transition at T ₁ = 311 K are investigated. It is shown that in these crystalline compounds the phenomenon of thermochromism is primarily associated with the change of the plane-quadratic geometry of the coordination environment of Cu²⁺ to the tetrahedral form. The influence of ionizing irradiation on the phase-transition temperature and on the thermochromic properties of this crystal is studied.     

(TMTSF) 2F2PO2: An unusual member of the (TMTSF) 2X family of organic metals

Resistivity measurements were carried out as a function of temperature and pressure in (TMTSF) ₂F₂PO₂. A strong metal-insulator transition is observed at ～137K which is only incompletely suppressed by high pressures. The nature of the resistivity curves obtained at high pressure suggest the onset of a Fermi surface instability; however, the metallic-like transport at lower temperatures suggests that the energy gap covers only part of the Fermi surface. The surprising increase in ? (T) at temperatures below 20K (under pressure) may result from disorder induced localization arising from the strong electronic scattering from the randomly oriented dipole moments.     

Elastic properties of monoclinic telluric acid ammonium phosphate,Te(OH)6·2NH4H2PO4·(NH4)2HPO4, near the paraelectric-ferroelectric transition

The elastic constants of Te(OH)₆·2NH₄H₂PO₄·(NH₄)₂HPO₄, TAAP, point symmetrym, have been measured by ultrasonic resonance methods passing through the paraelectric-ferroelectric transition at ca. 320 K. In the range between 273 and 340 K the elasticity tensor exhibits only a slight anisotropy. No discontinuity of the elastic constants is observed. However, some temperature derivatives of the elastic constants show slight anomalies within the range 310 to 325 K. The strongest effect occurs with the longitudinal elastic resistancec ₂₂. The thermal expansion which varies only slightly between 263 and 340 K, is highly anisotropic in contrast to the thermoelastic behaviour. A strong negative thermal expansion is observed in a direction within the mirror plane, ca. 45° apart from the direction of spontaneous polarization.This effect is not accompanied by a corresponding thermoelastic anomaly. The interactions connected with the transition are of the totally symmetric type. Like many other properties the elastic and thermoelastic behaviour of TAAP resembles that of triglycine sulphate (TGS). Larger differences between TAAP and TGS are found in the pressure dependence of various properties. For example the pressure dependence of the transition temperatureT is negative for TAAP (–3.8 K/kbar) and positive for TGS (3.9 K/kbar).Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Pressure induced phase transition in the highly anisotropic compound: Y2[Pt(CN)4]3·21H2O

For the linear chain system Y₂[Pt(CN)₄]₃·21H₂O a pressure induced phase transition is observed by emission spectroscopy. At p_trans=(5±0.5) kbar and T=295 K the compound undergoes a first order phase transition, in the course of which the intra-chain Pt-Pt distance R shrinks by $\text{ΔR≈}\text{-0.03}\text{A}\text{?}$. An approximate value had already been found at standard pressure for a temperature induced phase transition (T_trans=218 K).     

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β^′-phases of Tb ₂(MoO ₄)₃ using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb ₂(MoO ₄)₃ phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β^′-Tb ₂(MoO ₄)₃ phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb ₂(MoO ₄)₃ phase at 5 GPa.     

Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

Raman and optical absorption studies under pressure have been conducted on KTb(MoO₄)₂ up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO₄)₂ is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.     

Critical behavior of heat capacity in the vicinity of phase transitions in [NH2(CH3)2]5Cd3Cl11

The heat capacity of a [NH₂(CH₃)₂]₅Cd₃Cl₁₁ crystal was studied calorimetrically in the temperature interval 100–300 K. The C _p (T) dependence indicates that, as the temperature is lowered, phase transitions occur at temperatures T ₁ = 176.5 K and T ₂ = 123.5 K. The thermodynamic characteristics of this crystal were determined. It is shown that the transition at T ₂ = 123.5 K is an incommensurate-commensurate phase transformation and that the transition at T ₁ = 176.5 K is a normal-incommensurate phase transition.