Structural phase transitions in KMnF3

The structural transitions of the perovskite KMnF₃ are studied with an energy-dispersive X-ray diffractometer. It is found that the 83 K transition is of first order, though the transition related to the condensation of a M₃-soft phonon mode is considerably affected by crystallographic domain-walls occurring below the 186 K transition. The latter transition is observed at 88.0 K (first-order), and 92.0 K (second-order) in different single crystals, respectively. The difference of the transition temperature and the transition order is interpreted in terms of inner strains appeared in the domain walls.     

Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral （R3c） phase of BiFeO3 first transforms to monoclinic （C2/m） phase at 7 GPa, then to orthorhombic （Pnma） phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic （Pbam） phase, indicating the coexistance of monoclinic phase with orthorhombic phase in low pressure range.     

Pressure-induced quantum phase transition in the spin-liquid TlCuCl3

The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl3, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.     

Pressure-induced phase transition in β-eucryptite (LiAlSiO4)

A pressure-induced phase transition was found in β-eucryptite using single crystal X-ray and Raman scattering measurements. The high pressure phase forms at a pressure of ～8 kbar at 300 K, is metastable at 1 bar and will revert to the normal low pressure phase with time at this temperature.     

Thermal expansion and spontaneous strain of KMnF3 near the 186 K-structural phase transition

Abstract

By means of high-angle double-crystal X-ray diffractometry (HADOX), the thermal expansivity of KMnF₃ has been determined. The linear thermal expansion coefficient along each crystallographic axis exhibits a divergent anomaly at the structural phase transition at 186 K. The critical exponents of these coefficients are determined: in the cubic phase α = 0.21 ± 3, and in the tetragonal phase α′ = 0.19 ± 3 for the a axis and α″ = 0.35 ± 3 for the c axis.

In addition, the critical exponent of the spontaneous strain, c/a ? 1, is determined to be 0.4248 ± 8, which is smaller than that of the order parameter 2β = 0.5714 ± 12. These values are reasonable if the transition occurs in the vicinity of a tricritical point.     

Calorimetric study of phase transitions in the perovskite BaCeO3

Differential scanning calorimetry was used to study phase transitions (PT) in the perovskite BaCeO₃. It is shown that its phase state is determined by a second-order λ transition at T _tr=520–540 K and a first-order δ transition at T _tr=600–670 K. Differences in PT parameters between ceramic and fused BaCeO₃ have been established. Fiz. Tverd. Tela (St. Petersburg) 40, 2109–2112 (November 1998)     

PbTiO3在压力下的异常相变和极强压电效应

运用第一性原理计算预言了在一纯化合物中可由压力诱导出顺序为四方晶－单斜体－菱形体－立方体的新的相变,而且存在有变形相界面.在相变区有可与在复杂的单晶固溶体压电材料,如人们期待在机电应用方面引起革命性变化的Pb(Mg1/3Nb2/3)O3-PbTiO3可比的,极大的介电和压电耦合常数.我们的结果表明变形相界面和巨压电效应并不需要内禀的无序,并打开了在简单系统中研究这一效应的可能性.     

PbTiO3在压力下的异常相变和极强压电效应

运用第一性原理计算预言了在一纯化合物中可由压力诱导出顺序为叫方晶-单斜体-菱形体-立方体的新的相变，而且存在有变形相界面。在相变区有可与在复杂的单晶固溶体压电材料，如人们期待在机电应用方面引起革命性变化的Pb（Mg1/3Nb2/3）O3-PbTiO3可比的，极大的介电和压电耦合常数。我们的结果表明变形相界面和巨压电效应并不需要内禀的无序，并打开了在简单系统中研究这一效应的可能性，     

Pressure-induced anomalous phase transitions and colossal enhancement of piezoelectricity in PbTiO3

We find an unexpected tetragonal-to-monoclinic-to-rhombohedral-to-cubic phase transition sequence induced by pressure, and a morphotropic phase boundary in a pure compound using first-principles calculations. Huge dielectric and piezoelectric coupling constants occur in the transition regions, comparable to those observed in the new complex single-crystal solid-solution piezoelectrics such as Pb(Mg(1/3)Nb(2/3))O3-PbTiO3, which are expected to revolutionize electromechanical applications. Our results show that morphotropic phase boundaries and giant piezoelectric effects do not require intrinsic disorder, and open the possibility of studying this effect in simple systems.     

Pressure-induced phase transition of B-type Y_2O_3

The synthesized monoclinic(B-type) phase of Y_2O_3 has been investigated by in situ angle-dispersive x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. A phase transition occurs from monoclinic(B-type) to hexagonal(A-type) phase at 23.5 GPa and these two phases coexist even at the highest pressure. Parameters of isothermal equation of state are V_0= 69.0(1) ~3, K_0= 159(3) GPa, K_0= 4(fixed) for the B-type phase and V_0= 67.8(2) ~3, K_0= 156(3) GPa,K'_0= 4(fixed) for the A-type phase. The structural anisotropy increases with increasing pressure for both phases.     

Pressure-induced spin transitions of iron in MgSiO3 perovskite: a GGA+U study

First-principles calculations using the GGA+U method have been made to investigate the spin transitions of iron in MgSiO₃ perovskite up to 120 GPa, with Fe²⁺ and Fe³⁺ at the A and B sites involving different substitution mechanisms and various charge compensation configurations. Our results, unlike those from previous local density approximation and generalized gradient approximation calculations, are now consistent with available experimental data for the spin states of Fe³⁺ in perovskite. In particular, our calculations show that both Fe²⁺ and Fe³⁺ at the A site do not exhibit any spin transition at the lower mantle conditions. However, Fe³⁺ at the B site in both (Mg_0.9375Al_0.0625)(Si_0.9375Fe_0.0625)O₃ and (Mg_0.9375Fe_0.0625)(Si_0.9375Fe_0.0625)O₃ undergoes a high-spin to low-spin transition at ～ 35 GPa, and this spin-transition pressure is largely independent on substitution mechanisms and charge compensation configurations.     

Pressure-induced phase transition in tysonite LaF3

We have studied the high-pressure phase stability of LaF₃ using full-potential linear augmented plane wave method. We have shown that experimentally observed orthorhombic phase is less stable compared to the theoretically predicted tetragonal structure above 25 GPa pressure. The structural transition is mainly due to the steric repulsion of ions and electrons to higher pressures.     

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.     

Weissenberg-type neutron diffraction camera and its application to the structural and magnetic phase transitions of KMnF3

A Weissenberg-type neutron diffraction camera (W-NDC) equipped with a neutron imaging-plate (NIP) has been developed. The W-NDC gives Weissenberg-type and rotational diffraction patterns at low-temperature (10 K<T<300 K) and high-temperature (300 K<T<800 K) regions. To reduce background counts of the NIP diffracted from the aluminum containers and the specimen holder, a radial slit with several polymer sheets coated with ⁶LiF powder is continuously oscillated around the specimen during measurements. A test structural analysis for a single crystal NaCl suggests that the integrated intensities of reflections obtained from the Weissenberg-type diffraction patterns adequately satisfy a structural refinement by means of a least square method, though a fading effect of the photon energy stored in the NIP should be regarded during measurements. Structural and magnetic phase transitions in KMnF₃ have been studied by using the W-NDC, to eliminate an inconsistency for temperature, and to enable an ordering of the phase transition as reported. PACS 61.12.Ld; 64.70.Kb; 61.72.-y     

Pressure-induced quenching of the Jahn-Teller distortion and insulator-to-metal transition in LaMnO(3)

LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.     

X-Ray powder diffraction study of structural phase transitions in (Ba0.5Sr0.5)PbO3 perovskite

Study of structural properties of (Ba_0.5Sr_0.5)PbO₃ has been carried out in a wide temperature range from 10 to 1223 K using the X-ray powder diffraction technique. Two temperature-induced structural phase transitions at 948 and 1198 K have been found. For polycrystallines the XPS and electrical resistivity measurements have been performed.