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.     

Pressure-induced magnetic transition in MnRhAs

The magnetic properties of a Fe₂P-type intermetallic compound MnRhAs have been investigated under high pressure up to 8.0 GPa by AC susceptibility measurement. Initially, both the antiferromagnetic (AF(I)) to the canted state magnetic transition temperature T_t and the canted state to another antiferromagnetic one (AF(II)) transition temperature T_C increase with compression. At 4.0 GPa, however, T_t decreases abruptly, while the increasing rate of T_C becomes larger above this pressure. A pressure-induced magnetic phase transition was seen at around this pressure when T_t and T_C are plotted in the pressure–temperature phase diagram. The transition from the antiferromagnetic to the ferromagnetic state observed below 160 K with increasing pressure is not frequently observed.     

High pressure effect on the crystal and magnetic structure of Pr0.1Sr0.9MnO3 manganite

The crystal and magnetic structure of Pr_0.1Sr_0.9MnO₃ manganite has been studied by the neutron diffraction at high pressures up to 5 GPa in the temperature range 10?C295 K. At normal pressure and decreasing temperature the appearance of the C-type (T _N = 220 K) and G-type (T _N = 180 K) antiferromagnetic states occurs, which is accompanied by a structural phase transition from the cubic structure (Pm $ \bar 3 $ m space group) to the tetragonal structure (I4/mcm space group). It is shown that the temperature of the transition to the C-type antiferromagnetic phase increases with pressure with the pressure coefficient dT _N/dP = 4.0(5) K/GPa and the temperature of the transition to the G-type antiferromagnetic phase weakly depends on pressure.     

Phase transition of cadmium fluoride under high pressure

We investigated the high pressure phases of CdF₂ by a joint theoretical and experimental study. The structural and electronic properties of CdF₂ were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type  (Fm-3m, Z=4) structure to the cotunnite-type (Pnma, Z=4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF₂ under high pressure.     

A raman study of hydrostatic pressure induced phase transitions in Rb2KInF6 crystals

The Raman spectra of the elpasolite (Rb₂KInF₆) crystal have been studied in the pressure range from 0 to 5.3 GPa at a temperature of 295 K. A phase transition at a pressure of approximately 0.9 GPa has been found. An analysis of the variations in the spectral parameters has led to the conclusion that the phase transition to a distorted phase is accompanied by the doubling of the volume of the primitive cell of the initial cubic phase. Numerical calculations of the lattice dynamics in the Rb₂KInF₆ crystal have been performed. The numerical simulation has established that the phase transition at a pressure of 0.9 GPa is associated with condensation of the F _lg mode. A probable high-pressure phase is the phase with space group C2/m.     

A study of the temperature and pressure dependence of the electrical resistivity of a dilute Cr-Ge system

The electrical resistivity of a series of dilute Cr-Ge alloys containing up to 1.5 atm % Ge, was measured as a function of temperature and pressure. The measurements clearly demonstrate the existence of resistivity anomalies at the incommensurate-commensurate spin density wave transition temperature (T_IC) in contrast with recently reported results. The complete magnetic phase diagram, determined for the first time from electrical resistivity measurements, contains a triple point in contrast with previous neutron diffraction results but in agreement with thermal expansion measurements. It was found that the incommensurate spin density wave state is absent in alloys with more than 1 atm % Ge. The Néel temperatures and incommensurate-commensurate transition temperatures are affected differently by pressure. Pressure decreases T_N in all the alloys while it increases T_IC for those alloys in which the incommensurate-commensurate transition occurs. The decrease of T_N with pressure is much larger for the commensurate-paramagnetic than for the incommensurate-paramagnetic transition. The electrical resistivity of the alloys at room temperature behaves anomalously with applied pressure. This anomalous behaviour is attributed to an antiferromagnetic-paramagnetic phase transition that is induced in the alloys by applied pressure.     

Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T^I_t (p = 0) = 182 K, the temperature coefficient being dT^I_t/dp = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T^I_t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm^-1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T^II_t (p) = 143 K, the pressure shift being dT^II_t/dp = 0.088 K/MPa.In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.     

Ab initio study of phase transition and thermodynamic properties of PtN

The transition phase of PtN from zincblende (ZB) structure to rocksalt (RS) structure is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The transition phase from the ZB structure to the RS structure occurs at the pressure of 18.2 GPa, which agrees well with other calculated values. Moreover, the dependences of the relative volume V/V₀ on the pressure P, the Debye temperature Θ and heat capacity C_V on the pressure P, together with the heat capacity C_V on the temperature T are also successfully obtained.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

Neutron scattering study of the phase transition in s-triazine at high pressure

The elastic transition in s-triazine (C₃N₃H₃) from a trigonal ($\text{R}\text{3}\text{c}$) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.     

Pressure-induced coordination crossover in magnetite,a high pressure Mössbauer study

Temperature-dependent ⁵⁷Fe Mössbauer spectroscopy to 40 GPa shows that Fe₃O₄ magnetite undergoes a coordination crossover (CC) whereby charge-density is shifted from octahedral to tetrahedral sites and the spinel structure thus changes from inverse to normal with increasing pressure and decreasing temperature. A precursor to the CC is a d-charge decoupling within the octahedral sites at the inverse spinel phase. The CC-transition takes place almost exactly at the Verwey transition temperature (T_V=122 K) at ambient pressure. While T_V decreases with pressure, the CC-transition temperature increases with pressure, reaching 350 K at 10 GPa. The d electron localization mechanism proposed by Verwey and later by Mott for T<T_V is shown to be unrelated to the actual mechanism of the metal–insulator transition attributed to the Verwey transition. It is proposed that a first-order phase transition taking place at ∼T_V at ambient pressure opens a small gap within the oxygen p-band, resulting in the observed insulating state at T>T_V.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

Exciton condensation in intermediate valent Sm0.90La0.10S

Sm_1−xLa_xS for x more than a few percents are metals at ambient conditions. At low temperature and high pressure they develop a small gap in the order of some meV and become semiconductors or insulators. This has been interpreted as a manifestation of the excitonic insulator. In this Letter we will concentrate on Sm_0.90La_0.10S, which is the only composition showing a first order transition. Measurements of the volume change with pressure at ambient temperature show this first order volume collapse at 5 kbar with hysteresis. The resistivity is measured in function of temperature and pressure and exhibits also at 5 kbar and ambient temperature a first order phase transition to a more metallic state. At low temperatures and in function of pressure the resistivity exhibits a peak. The optical reflectivity at 300 K has been measured at low and high pressure and transforms with pressure above 5 kbar into the golden metallic phase.     

Structural,magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

At ambient pressure CaFe₂As₂ has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ～ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ～0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ～1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe₂As₂ transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.     

First-principles study of the pressure-induced phase transition in CaTiO3

An investigation into the phase stabilities of CaTiO₃ under high pressure was conducted using first-principles calculations based on density functional theory. We have identified three candidate structures of CaTiO₃, Pbnm, Pm3m and Cmcm, respectively. Our results demonstrate that a phase transition from orthorhombic (Pbnm) to cubic (Pm3m) is impossible for CaTiO₃ under high pressure at ambient temperature, and further predict that Pbnm-CaTiO₃ will transform to post-perovskite phase (Cmcm) at enough temperature and pressure.     

Pressure-induced coordination crossover in magnetite; the breakdown of the Verwey–Mott localization hypothesis

Temperature-dependent ⁵⁷Fe Mössbauer spectroscopy to 40 GPa shows that Fe₃O₄ magnetite undergoes a coordination crossover (CC), whereby charge density is shifted from octahedral to tetrahedral sites and the spinel structure thus changes from inverse to normal with increasing pressure and decreasing temperature. A precursor to the CC is a d-charge decoupling within the octahedral sites at the inverse-spinel phase. The CC transition takes place almost exactly at the Verwey transition temperature (T_V=122 K) at ambient pressure. While T_V decreases with pressure the CC-transition temperature increases with pressure, reaching 300 K at 10 GPa. The d electron localization mechanism proposed by Verwey and later by Mott for T<T_V is shown to be unrelated to the actual mechanism of the metal–insulator transition attributed to the Verwey transition. It is proposed that a first-order phase transition taking place at ∼T_V at ambient pressure opens a small gap within the oxygen p-band, resulting in the observed insulating state at T>T_V.     

Complete pentagon orientational ordering in C60 fullerite charged with NO

Solid C₆₀ was stored in NO under high pressure, and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its ¹³C NMR MAS spectra are composed of a primary resonance at 143.7 ppm accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase transition temperature at 260 K in pure C₆₀ and to substantially reduce the drop Δ?′ at the phase transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P-orientation below T _c was calculated to be larger than 11/12. These results show that a completely P-oriented phase occurred in (NO)_0.1C₆₀ and that this phase is favored by a negative pressure on the C₆₀ lattice exerted by NO, as well as by the electrostatic interaction between the two.     

High-pressure thermoelectric characteristics of Bi2Te3 semiconductor with different charge carrier densities

The measurements of the absolute values of the thermopower and of the relative electrical resistance have been performed for n type Bi₂Te₃ under hydrostatic pressure up to 9 GPa at room temperature. Under pressures exceeding 5 GPa and up to the phase transition (at 7 GPa), the samples with the charge carrier density below 10^?19 cm^?3 exhibit an anomalous growth of the thermopower. For the purest sample (n = 10^?18 cm^?3), the thermopower is as high as +150 μV/K. The pressure dependence of the electrical resistance for n-Bi₂Te₃ does not exhibit any anomalies up to the pressure corresponding to the phase transition (7 GPa). Thus, the state with the giant thermoelectric efficiency is found in Bi2Te3 under pressure before the phase transition.     

Transition phase and thermodynamic properties of GaN via first-principles calculations

The transition phase of GaN from zincblende (ZB) structure to rocksalt structure (RS) is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures are obtained through the quasi-harmonic Debye model. We find that the transition phase from the ZB structure to the RS structure occurs at the pressure of 42.2 GPa, which is in good agreement with other calculated values. Moreover, the dependences of the relative volume V/V₀ on the pressure P, the Debye temperature Θ and heat capacity C_V on the pressure P, as well as the heat capacity C_V on the temperature T are also successfully obtained.     

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.