Phase transitions of YbX (X = P,As and Sb) with a NaCl-type structure at high pressures

The powder X-ray diffraction of YbX (X?=?P, As and Sb) with a NaCl-type structure has been studied with synchrotron radiation up to 63?GPa at room temperature. YbSb undergoes the first-order structural phase transition from the NaCl-type (B1) to the CsCl-type (B2) structure at around 13?GPa. The structural change to the B2 structure occurs with the volume collapse of about 1% at 13?GPa. The transition pressure of YbSb is surprisingly lower than that of any other heavier LnSb (Ln?=?Dy, Ho, Er, Tm and Lu). The pressure-induced phase transitions in YbP and YbAs are observed at around 51?GPa and 52?GPa respectively. The transition pressure of both compounds is much higher than that of YbSb. The high-pressure structural behaviour of YbX (X?=?P, As and Sb) is discussed. The volume versus pressure curve for YbX with the NaCl-type structure is fitted by a Birch equation of state. The bulk moduli of these compounds with the NaCl-type structure are 104?GPa for YbP, 85?GPa for YbAs and 52?GPa for YbSb.     

Structural phase transition of ScSb and YSb with a NaCl-type structure at high pressures

By use of synchrotron radiation, powder X-ray diffraction of ScSb and YSb with a NaCl-type structure has been studied up to 45 GPa at room temperature. A first-order phase transition from the NaCl-type (B1) to a CsCl-type structure (B2) began to occur at around 28 GPa for ScSb and at around 26 GPa for YSb. Crystal data of the high-pressure phase of both antimonides are obtained. The high-pressure structural behavior of ScSb and YSb is similar to that of heavier LnSb (Ln=Dy-Lu). The B1-B2 transition for ScSb and YSb can be understood according to the rigid sphere model. The bulk moduli of ScSb and YSb are about 58 GPa at ambient pressure.     

Pressure-induced phase transitions of lanthanide monoarsenides LaAs and LuAs with a NaCl-type structure

By use of synchrotron radiation the powder X-ray diffraction of lanthanide monoarsenides LaAs and LuAs with a NaCl-type structure has been studied up to 60 GPa at room temperature. First-order phase transitions with the crystallographic change were found at around 20 GPa for LaAs, and 57 GPa for LuAs. The high-pressure form of LaAs is a tetragonal structure and can be viewed as a distorted CsCl-type structure. The atoms in the tetragonal structure are located at La: 0, 0, 0; As: 1/2, 1/2, 1/2. The space group is P4/mmm. The structural change to the tetragonal structure occurs with the volume collapse of about 10%. The structure of these high-pressure phases of LuAs is unknown. The volume vs. pressure curves for LaAs and LuAs are fitted by a Birch equation of state. The bulk moduli of both arsenides are 92±6 GPa for LaAs and 85±3 GPa for LuAs. The high-pressure structural behavior of LaX (X=P, As and Sb) and LnAs (Ln=lanthanide) with the NaCl-type structure is discussed.     

First principles study of structural phase transition of YSb and ScSb compounds

High pressure induced phase transition of YSb and ScSb compounds have been studied using Density Functional Theory method within Generalized Gradient Approximation. It was found that the phase transition from the NaCl-type (B1) to a CsCl-type structure (B2) began to occur at around 29 GPa for YSb compound, agreeing well with available experiments and theoretical calculations. For ScSb compound it was suggested that structural phase transition from B1 to B2 will occur at about 40 GPa, differing greatly with experimental and theoretical results. The finding that the transition pressures increase with decreasing lattice constant in the NaCl-type structure for YSb and ScSb compounds was found to be similar to the phenomena observed for LnSb (Ln: lanthanide) compounds. Mulliken charge and overlap population analysis revealed that YSb and ScSb compounds in B1 structure show similar interaction between anion and cation, while in B2 structure a higher degree of covalency was found for ScSb than that in YSb. Also, DOS and band structure of these two compounds in B1 and B2 structures were presented and analyzed.     

First-principles study of high-pressure phase transformations in LaBi

An investigation into the structural stability and the electronic properties of LaBi under high pressure was conducted using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin–orbit coupling (SOC). Our results demonstrate that there exists a structural phase transition from the NaCl-type (B1) structure to a primitive tetragonal (PT) structure at the transition pressure of 11.2 GPa (without SOC) and 12.9 GPa (with SOC). The chemical bond between La and Bi is mainly ionic. The band structure shows that B1-LaBi is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of LaBi, which may contribute to its stability.     

Structural properties and new phase transitions of ScN using FP-LMTO method

By full potential linear muffin-tin orbitals (FP-LMTO) method, we have studied the phase transitions of ScN under high pressures. The local density (LDA) approximation was used for the exchange and correlation energy density functional. The most important result is the prediction of the possibility of two phase transitions from the cubic rocksalt (NaCl) structure to the orthorhombic CaSi (Cmmc) structure above 252.5 GPa and to the tetragonal AuCu (P4/mmm) structure at 303.017 GPa, the first one (NaCl-CaSi) occurring at a lower pressure than the well known NaCl to CsCl transition (found here to be 324 GPa).     

十种稀土金属电阻的压力效应

 使用Bundy和Dunn发展起来的带有烧结金刚石砧的Drickamer型高压装置，用固定点测压法标定实验压力，在室温及0~43 GPa的压力范围内测量了稀土金属中Pr、Nd、Sm、Gd、Tb、Dy、Ho、Tm、Lu和Yb的电阻随压力的变化。在各稀土元素的电阻随压力变化的曲线上，观测到了若干“凸起”和斜率突变点，根据Jayaraman提出的三价稀土在压力作用下的相变顺序，得到了这些突（凸）变点分别对应着hcp→Sm-type→dhcp→fcc相变顺序中的某一类型的相变压力。此外还观测到了Pr、Gd、Tb的fcc相随着压力再增高而发生的相变，根据已报导的关于Pr的工作，推测Gd和Tb的这一相变应为fcc→dfcc相变，它们分别发生在22.0和24.5 GPa。在本工作所得结果基础上对Johansson的三价稀土总相图进行了修正。     

Pressure induced phase transition in rare earth mono-antimonides

Pressure induced structural phase transition of mono-antimonides of lanthanum, cerium, praseodymium and neodymium (LnSb, Ln=La, Ce, Pr and Nd) has been studied theoretically using an inter-ionic potential with modified ionic charge which parametrically includes the effect of Coulomb screening by the delocalized f electrons of rare earth (RE) ion. The anomalous structural properties of these compounds have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of Ln ion with the p orbital of neighbouring antimonide ion. All the four compounds are found to undergo from their initial NaCl (B₁) phase to body centered tetragonal (BCT) phase at high pressure and agree well with the experimental results. The body centered tetragonal phase is viewed as distorted CsCl structure and is highly anisotropic with c/a=0.82. The transition pressure of LnSb compounds is observed to increase with decreasing lattice constant in NaCl phase. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-Sb) distances at high pressure. The calculated values of elastic constants are also reported.     

First-principles study of phase transition and elastic properties of ScSb and YSb compounds

The phase transition of ScSb and YSb from the NaCl-type (B1) structure to the CsCl-type (B2) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the B1 structure to the B2 structure obtained from the equal enthalpies are 38.3 and 32.1 GPa for ScSb and YSb, respectively. From the variations of elastic constants with pressure, we find that the B1 phase of ScSb and YSb compounds are unstable when applied pressures are larger than 46.3 and 64.2 GPa, respectively. Moreover, the detailed volume changes during phase transition are analyzed.     

高压下TiN的结构转变、弹性和热力学性质的第一性原理计算

 利用基于密度泛函的第一性原理,计算了高压下TiN的结构转变、弹性和热力学性质。计算结果表明：在压力作用下,TiN经历了从NaCl型结构到CsCl型结构的转变,转变压力为348 GPa;TiN的弹性系数随着压力的增加呈线性增加规律。此外,还给出了德拜温度和热容量这两个重要热力学量与温度和（或）压力的依赖关系。     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

高压下ZnSe直流和交流电学性质的研究

采用高压原位测量技术在0–35 GPa压力范围内对ZnSe直流和交流电学性质进行了研究. 通过分析直流电学测量结果可知,在实验压力区间内ZnSe经历了由纤锌矿转变为朱砂相再转变为岩盐相的两次相结构转变. 分析温度与材料电阻率的变化关系表明ZnSe在高压下的相变为金属化相变,并通过交流阻抗谱的测量实验证实了这个结论. 进一步比较低压条件下晶粒和晶界电阻的变化,表明朱砂相结构的ZnSe更接近各向同性材料. 关键词： 高压 ZnSe 电学     

SmN晶体的电子结构和高压相变的第一性原理

利用基于密度泛函理论的第一性原理,研究了SmN晶体的电子结构和高压相变. SmN晶体的电子结构具有半金属特征,多数自旋电子显示金属导电性,少数自旋电子显示半导体导电性. 高压相变的结果显示,SmN晶体经历从NaCl型(B1)到CsCl型(B2)结构转变的压致结构相变,相变压力117 GPa. 弹性系数的结果显示,在环境压力下SmN晶体的弹性系数满足玻恩稳定条件,标志着B1相是力学稳定结构. 声子谱结果显示,在环境压力下B1相是热力学稳定结构,与弹性系数的计算结果一致.     

Pressure-induced phase transitions in CaB6 by means of XRD and resistance measurement

High pressure behavior of CaB₆ with cubic crystal structure is investigated by means of energy dispersive X-ray diffraction and by employing in situ resistance measurement in a diamond anvil cell. Two newcome high pressure phase transitions are found with pressure ranging from ambient to 26 GPa. The first one at 12 GPa is a structural phase transition from CsCl-type structure to orthogonal structure, which is reflected by both the X-ray diffraction and the resistance variation. The other one at 3.7 GPa is suggested to be an electronic transition, which is observed only in resistance measurement. The diffraction pattern recovered while the pressure is released to 0 GPa with a pressure hysteresis over 11 GPa, which implies the reversibility of the two phase transitions. Bulk moduli of the cubic and orthogonal phases are estimated by fitting the data to a Brich-Murnaghan equation of state equal to 169.9 and 48.2 GPa, respectively.     

The stability of the simple cubic phase of phosphorus

The transition from the rhombohedral phase to the simple cubic phase of phosphorus under pressure is studied within the local density-functional formalism with use of the norm-conserving pseudopotential. The calculated transition pressure is 15.8GPa, which is in good agreement with the measured value, 10GPa. The bonding in the simple cubic phase is shown to be predominantly covalent. It is found that the instability of the simple cubic structure under low pressures is understood by a Peierls' instability, which is just the same as the case of arsenic. It is also shown that the mixing of atomic 3d-orbitals in the occupied states is important for stabilizing the simple cubic structure under high pressures.     

First-principles study of structural stability, electronic and elastic properties of ZrC compounds

We theoretically study the possible pressure-induced structural phase transition, electronic and elastic properties of ZrC by using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin-orbit coupling (SOC). The calculations indicate that there exists a phase transition from the NaCl-type (B1) structure to CsCl-type (B2) structure at the transition pressure of 313.2 GPa (without SOC) and 303.5 GPa (with SOC). The detailed structural changes during the phase transition were analyzed. The band structure shows that B1-ZrC is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of ZrC, which may contribute to its structural stability.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

An analysis of pressure-induced phase transition and elastic properties of neodymium monopnictides

We have predicted the phase transition pressures and corresponding relative volume changes of two neodymium monopnictides (NdAs and NdSb) having NaCl-type structure at ambient conditions, using an improved interaction potential model (IIPM) approach. Both the compounds have been found to undergo from their initial NaCl(B1) phase to a body centered tetragonal (BCT) phase at high pressure. Our calculated results of phase transitions, volume collapses and elastic behavior of these compounds are found to be close to the experimental results. This shows that the inclusion of the three-body interaction and polarizability effect makes the present model suitable for high pressure studies.     

Elastic phase transitions in metals at high pressures

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-?to fourth-order elastic constants. The second-,third-?and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P?≈?69?GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.     

Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn(2)O(7). Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ~1.0 GPa at T < 70 K. A huge negative piezoresistance ~10(6) in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the e(g) electron between 3d(x(2)-y(2)) and 3d(z(2)-r(2)) states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data.