First-principles study of structural and elastic properties of CrO2 at high pressure

The structural and elastic properties of CrO₂ in the rutile phase under high pressures have been investigated using pseudopotential plane-wave method based on density functional theory. The optimized lattice parameters and the bulk modulus at zero pressure agree well with available experimental and theoretical data. The elastic constants C ₁₁, C ₁₂, C ₄₄, C ₃₃, C ₁₃, and C ₆₆ at zero pressure are calculated to be 359.91, 264.69, 143.28, 309.45, 218.45, and 260.74 GPa, respectively. Elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio under pressures are obtained. Our results indicate that the rutile phase is mechanically stable below 11.99 GPa. The elastic anisotropy of rutile phase under pressures has also been predicted.     

High pressure behavior and structural properties of transition metal carbides

A pressure induced structural phase transition from NaCl-type (B₁) to CsCl-type (B₂) structure has been predicted in transition metal carbides, namely TiC, ZrC, NbC, HfC, and TaC by using an interionic potential theory with modified ionic charge (Z_m ), which includes Coulomb screening effect due to d-electron. The phase transition pressure (P_T ) relies on large volume discontinuity in pressure–volume relationship, and identifies the structural phase transition from B₁ phase to B₂ phase. The variation of second-order elastic constants with pressure follows a systematic trend identical to that observed in other compounds of NaCl-type structure. The Born criterion for stability is found to be valid in transition metal carbides.     

Pressure-induced phase transition and structural behavior of MgO at high temperatures: a model study

A realistic interaction potential model approach by including temperature effects is developed to study phase transition, elastic properties and thermo-physical properties at very high pressures and temperatures. This approach is effectively able to explain the inter-atomic interaction involved at high temperature and high pressure as it includes the three-body interactions. Earlier works overlooked the three-body interactions at high temperature and pressures. Moreover, the phase-transition pressures of MgO crystal at high temperatures including the three-body interaction are computed for the first time. Elastic behavior, anisotropic factor and Debye temperature of MgO at high pressures and temperatures are also reported.     

Pressure-induced phase transition and stability of EuO and EuS with NaCl structure

We have predicted the phase transition pressures and corresponding relative volume changes of EuO and EuS having NaCl-type structure under high pressure using three-body interaction potential (TBIP) approach. In addition, the conditions for relative stability in terms of modified Born criterion has been checked. Our calculated results of phase transitions, volume collapses and elastic behaviour of these compounds are found to be close to the experimental results. This shows that the inclusion of three-body interaction effects makes the present model suitable for high pressure studies.      

NbSi2奇异高压相及其热力学性质的第一性原理研究

采用基于粒子群优化算法的结构预测程序CALYPSO, 并结合第一性原理的VASP程序, 在175 GPa发现NbSi₂的奇异立方高压相. 在此结构中, Nb原子形成金刚石结构, 而Si原子则形成正四面体镶嵌在金刚石结构中. 声子谱计算结果表明该结构是动力学稳定的. 电子结构分析表明, 六角相和立方相NbSi₂均为金属, 对金属性贡献较大的是Nb原子, 而且Nb和Si原子之间存在明显的p-d杂化现象, 电荷更多地聚集在Si四面体中. 利用“应力应变”方法, 计算了NbSi₂的弹性常数, 分析了其体积模量、剪切模量、杨氏模量和德拜温度等热动力学性质随压力的变化并进行了详细的讨论. 根据剪切模量和体积模量的比值分析了NbSi₂两种相结构的脆性和延展性, 发现压力会导致六角相NbSi₂的延展性增加, 但对立方相结构的延展性影响较小; 采用经验算法计算了NbSi₂两种相结构硬度变化情况, 结合这一比值进行了详细的分析. 弹性各向异性计算结果表明, 随着压力增加, 六角结构的各向异性增强, 而立方结构的各向异性减小.     

Pressure-induced phase transition in transition metal trifluorides

As a fundamental thermodynamic variable, pressure can alter the bonding patterns and drive phase transitions leading to the creation of new high-pressure phases with exotic properties that are inaccessible at ambient pressure. Using the swarm intelligence structural prediction method, the phase transition of TiF₃, from R—3c to the Pnma phase, was predicted at high pressure, accompanied by the destruction of TiF₆ octahedra and formation of TiF₈ square antiprismatic units. The Pnma phase of TiF₃, formed using the laser-heated diamond-anvil-cell technique was confirmed via high-pressure x-ray diffraction experiments. Furthermore, the in situ electrical measurements indicate that the newly found Pnma phase has a semiconducting character, which is also consistent with the electronic band structure calculations. Finally, it was shown that this pressure-induced phase transition is a general phenomenon in ScF₃, VF₃, CrF₃, and MnF₃, offering valuable insights into the high-pressure phases of transition metal trifluorides.     

Pressure-induced phase transition of wulfenite

The in-situ high-pressure structures of wulfenite have been investigated by means of angular dispersive X-ray diffraction with diamond anvil cell and synchrotron radiation. In the pressure up to 22.9 GPa, a pressure-induced scheelite-to-fergusonite transition is observed at about 10.6 GPa. The pressure dependence for the lattice parameters of wulfenite is reported, and the axial compression coefficients Ka₀=－1.36×10^－3 GPa^－1 and Kc₀= －2.78×10^－3 GPa^－1 are given. The room-temperature isothermal bulk modulus is also obtained by fitting the P-V data using the Murnaghan equation of state.

     

Pressure-induced structural transition and thermodynamic properties of RhN2 and the effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method. Three structures are chosen to investigate for RhN2, namely, simple hexagonal P6/mmm （denoted as SH）, orthorhombic Pnnm （marcasite）, and simple tetragonal P4/mbm （denoted as ST）. Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch Murnaghan equation of states, we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young＇s modulus, and Debye temperature of RhN2 are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN2 is investigated. This is a quantitative investigation on the structural properties of RhN2, and it still awaits experimental confirmation.     

Observation of insulating–insulating monoclinic structural transition in macro‐sized VO2 single crystals

VO₂ single crystals with unprecedented quality, exhibiting a first‐order metal–insulator transition (MIT) at 67.8 °C and an insulator –insulator transition (IIT) at ～49 °C, are grown using a self‐flux evaporation method. Using synchrotron‐based X‐ray microdiffraction analysis, it is shown that the IIT is related to a structural phase transition (SPT) from the monoclinic M2 phase to the M1 phase upon heating while the MIT occurs together with a SPT of M1 to the rutile R phase. All previous reports have shown that VO₂ exists in the M1 phase at room temperature in contrast to the M2 phase observed in this work. We suggest that internal strain inside single crystal VO₂ may generate the previously unobserved IIT and the unusual room temperature structure. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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 structural phase transition in RbAu

Using the first principle method based on density functional theory, the structural and elastic properties calculations of RbAu have been performed. The results demonstrate that RbAu is stable in the CsCl structure (B2) at ambient pressure, which is in well agreement with the experimental results. And there exists a structural phase transition from CsCl-type structure (B2) to NaTi-type structure (B32) at the transition pressure of approximate 6 GPa. The pressure effects on the elastic properties are discussed and the elastic property calculation indicates elastic instability maybe provide phase transition driving force according to the variations relation of the elastic constant versus pressure.     

ZnTe结构相变、电子结构和光学性质的研究

运用密度泛函理论体系下的投影缀加波方法, 对闪锌矿和朱砂相结构的ZnTe在高压下的状态方程和结构相变进行了研究, 并分析了相变前后的原胞体积、电子结构和光学性质. 结果表明: 闪锌矿结构转变为朱砂相结构的相变压力为8.6 GPa, 并没有出现类似材料高压导致的金属化现象, 而是表现出间接带隙半导体特性. 相变后, 朱砂相结构Zn和Te原子态密度分布均向低能级方向移动, 带隙变小; 轨道杂化增强, 更有利于Te 5p与Zn 3d间的电子跃迁, 介电常数虚部主峰明显增强, 但宏观介电常数不受压力的影响.     

Pressure-induced structural transition and thermodynamic properties of RhN2 and effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN₂ under high pressure are investigated through the first principles calculation by means of the pseudopotential plane-waves method. Three structures are chosen to investigate for RhN₂, namely, simple hexagonal P6/mmm (denoted as SH), orthorhombic Pnnm (marcasite), and simple tetragonal P4/mbm (denoted as ST). Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch-Murnaghan equation of states, we find that phase transition pressures from SH to marcasite structure and from marcasite to ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young's modulus, and Debye temperature of RhN₂ are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN₂ is investigated. This is a quantitative investigation on the structural properties of RhN₂, and it still awaits experimental confirmation.     

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.     

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.     

High pressure structural phase transitions of TiO_2 nanomaterials

Recently, the high pressure study on the TiO_2 nanomaterials has attracted considerable attention due to the typical crystal structure and the fascinating properties of TiO_2 with nanoscale sizes. In this paper, we briefly review the recent progress in the high pressure phase transitions of TiO_2 nanomaterials. We discuss the size effects and morphology effects on the high pressure phase transitions of TiO_2 nanomaterials with different particle sizes, morphologies, and microstructures. Several typical pressure-induced structural phase transitions in TiO_2 nanomaterials are presented, including size-dependent phase transition selectivity in nanoparticles, morphology-tuned phase transition in nanowires, nanosheets,and nanoporous materials, and pressure-induced amorphization(PIA) and polyamorphism in ultrafine nanoparticles and TiO_2-B nanoribbons. Various TiO_2 nanostructural materials with high pressure structures are prepared successfully by high pressure treatment of the corresponding crystal nanomaterials, such as amorphous TiO_2 nanoribbons, α-PbO_2-type TiO_2 nanowires, nanosheets, and nanoporous materials. These studies suggest that the high pressure phase transitions of TiO_2 nanomaterials depend on the nanosize, morphology, interface energy, and microstructure. The diversity of high pressure behaviors of TiO_2 nanomaterials provides a new insight into the properties of nanomaterials, and paves a way for preparing new nanomaterials with novel high pressure structures and properties for various applications.     

Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures

Abstract

X-ray diffraction has been measured for crystalline silicon, crystalline germanium, amorphous silicon and amorphous germanium at temperatures down to 100 K and pressures up to 20 GPa using a diamond anvil cell and synchrotron radiation. The structural phase transitions, including amorphization, take place in the pressure-temperature range. It has been found that the structures after the phase transitions strongly depend on the path in the pressure-temperature diagram through which the system undergoes the phase transitions. For any of the aforementioned four materials, the high-pressure phase with the p-Sn structure is quenched during a release of pressure at 100 K, and transforms into an amorphous state when heated up to around 2 GPa. The path dependence of the states is discussed in relation to the pressure dependence of the heights of the energy barriers which have to be overcome when phase transitions occur. The effect of a structural disorder on the phase transition is also discussed by comparing the experimental results for the crystalline and amorphous materials.     

Equal compressibility structural phase transition of molybdenum at high pressure

We have studied the high-pressure compression behavior of molybdenum up to 60 GPa by synchrotron radial x-ray diffraction (RXRD) in a diamond anvil cell (DAC). It is found that all diffraction peaks of molybdenum undergo a split at around 27 GPa, and we believe that a phase transition from a body-centered cubic structure to a rhombohedral structure at room pressure has occurred. The slope of pressure-volume curve shows continuity before and after this phase transition, when fitting the pressure-volume curves of the body-centered cubic structure at low pressure and the rhombohedral structure at high pressure. A bulk modulus of 261.3 (2.7) GPa and a first-order derivative of the bulk modulus of 4.15 (0.14) are obtained by using the nonhydrostatic compression data at the angle ψ = 54.7° between the diffracting plane normal and stress axis.