Elastic anharmonicity of InP: Its relationship to the high pressure transition

Ultrasonic wave transit times have been measured in n-type InP at room temperature using hydrostatic pressures up to 4 kbar. Linear pressure dependences are found for the elastic stiffness moduli implying that at the high pressure structural-electrical transition the shear-to-bulk modulus ratio $\text{(C}{}_{11}\text{?C}{}_{12}\text{)}\text{2B}$ has a (fractional) value which fits the modified Born criterion for stability developed by Demarest $\text{et al}$. The anharmonic force constants and some of the third order elastic constants are found to be smaller the higher the transition pressure for indium III-V compounds.     

Elastic constants of noble-gas crystals under pressure and the cauchy relations

Physics of the Solid State - The equations of state are solved and the elastic constants responsible for the propagation of sound in strongly compressed crystals of noble gases are calculated in...     

Photoluminescence of LLGG:Cr crystals under high pressure

We present photoluminescence spectra of La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ doped with Cr³⁺ obtained at high hydrostatic pressure up to 220 kbar, applied in a diamond anvil cell at 20 K and room temperature. In both materials we have obtained a pressure-induced ⁴T₂–²E electronic cross-over. On the basis of the low-temperature R line luminescence at pressures above 100 kbar we have distinguished two dominant Cr³⁺ sites: and β, existing in both materials, and one minor site δ, that exists only in La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂. The pressure-induced shifts of the R₁, R_1β and R_1δ lines as well as the pressure shift of the broad band related to the ⁴T₂→⁴A₂ transition in both materials have been estimated.     

Elastic properties of nanocrystalline forsterite under high pressure and high temperature

A simple theoretical model is developed to study the pressure–volume–temperature relationship and applied for nanocrystalline forsterite in the temperature range 300–1573 K and pressure range 0–9.6 GPa. The results obtained with the present model are in quite close agreement to the experimental values. The model is therefore extended to study the variation of bulk modulus and the coefficient of volume thermal expansion under high pressure and high temperature. The present study also reveals that the quasi-harmonic approximation, i.e., the product of bulk modulus and the coefficient of volume thermal expansion as constant, is valid at least up to the temperature 1573 K and pressure 9.6 GPa in case of nanocrystalline forsterite.     

Determination of the phase boundary of GaP using in situ high pressure and high-temperature X-ray diffraction

The high pressure behavior of gallium phosphide, GaP, has been examined using the synchrotron X-ray diffraction technique in a diamond anvil cell up to 27?GPa and 900?K. The transition from a semiconducting to a metallic phase was observed. This transition occurred at 22.2?GPa and room temperature, and a negative dependence of temperature of this transition was found. The transition boundary was determined to be P (GPa)?=?22.6???0.0014?×?T (K).     

Phase transition of NaGaO2 at high pressure and high temperature

This work presents a comprehensive study on phase transition in NaGaO₂ under high pressure and temperature (P–T) conditions. X-ray diffraction (XRD) was used to identify the phase composition of the recovered samples. Our results revealed that NaGaO₂ could undergo a structural phase transition from orthorhombic $\beta$- NaGaO₂ to hexagonal $\alpha$- NaGaO₂ under high pressure and temperature. The pressure–temperature phase boundaries of NaGaO₂ were determined up to 5 GPa and 2073 K. The hexagonal $\alpha$- NaGaO₂ sample recovered from higher pressure and temperature exhibited a highly (003) preferred orientation. Besides, a reversible transition from $\alpha$- to $\beta$- NaGaO₂ was observed above 473 K under normal-pressure condition. The high-pressure melting curve of NaGaO₂, for the first time, was measured by using in-situ thermal measurement.     

Blue phase of liquid crystals: Light scattering and elastic moduli

Summary The theory of the optical, electro-optical and elastic properties of liquid-crystal blue phases is presented. The efficiency of the optical methods of study of blue phase is underlined. The effects of the many-wave diffraction optics in blue phases are discussed in detail. The universal temperature dependence of the blue-phase order parameter is found for the experimentally observed blue phases. Work presented the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30–October 2, 1987.     

Shifting of the slip mode transition temperature range of AgCl crystals doped with NaCl to higher temperatures

The slip mode transition temperature range of AgCl crystals doped with 1·24 mol.% of NaCl is shifted to higher temperatures (compared with the transition range of relatively pure AgCl crystals), i.e. from the values of approximately 110–130 K to the values of approximately 145–165 K.     

氢化铒高温高压下的弹性及热力学性质

利用第一性原理平面波赝势密度泛函理论, 并结合准谐德拜模型, 计算了立方萤石结构ErH2在不同温度和压强下的体积、热膨胀系数、体弹模量和等体热容等弹性性质及热力学性质。在温度高于1 100 K的条件下,计算出的等体热容趋近于Dulong-Petit极限。得到了绝对零度、零压强下ErH2的该结构的晶格常数为0.523 2 nm,与实验值0.523 0 nm非常接近。由不同的原胞体积得出了该体系的单点能与原胞体积的关系的数据;从计算出的高压下的弹性常数,根据立方晶系的力学稳定性条件,推断出立方萤石结构ErH2的相变压力约为20 GPa。     

Elastic and thermophysical properties of BAs under high pressure and temperature

The pseudopotential plane-wave approach in the framework of the density functional theory, and the density functional perturbation theory with the generalized gradient approach for the exchange-correlation functional has been used to calculate the structural phase stability, elastic constants and thermodynamic properties of boron-arsenide (BAs) compound. The BAs compound transforms from the zincblende phase to rock-salt structure; the phase transition pressure was found to be 141.2 GPa with a volume contraction of around 8.2%. The thermodynamic properties under high pressure and temperature up to 125 GPa and 1200 K respectively were also determined, analyzed and discussed in comparison with other data of the literature. The systematic errors in the static energy were corrected using the bpscal EEC method. Our results agree well with those reported in the literature, where for example, our calculated melting temperature (2116 K) deviates from the theoretical one (2132.83 K) with only 0.8%, and the deviation between our result (1.86) of the Grüneisen parameter and the theoretical one (1.921) is only around 3.2%.     

Hardness and elastic moduli of high pressure synthesized MoB2 and WB2 compacts

High pressure and high temperature synthesized MoB₂ and WB₂ compacts were investigated using X-ray diffraction, energy dispersive spectroscope, scanning electron microscope, Vickers indentation test and ultrasonic measurements. Experiments showed that both MoB₂ and WB₂ compacts are phase pure and with a grain size of 100–200 nm. Vickers indentation test under a large loading force of 49 N showed that the Vickers hardness of MoB₂ and WB₂ are about 21 and 22 GPa, respectively. The bulk modulus and shear modulus are about 296 GPa, and 190 GPa for MoB₂ and 349 and 200 GPa for WB₂ through ultrasonic measurements. Our results indicate that MoB₂ and WB₂ are both hard materials with a hardness similar to that of tungsten carbide, which is widely used in industry.     

Elastic constants,acoustic mode vibrational anharmonicity and Grüneisen parameters of hexahalometallate crystals

The room-temperature elastic constants of a number of hexahalometallate A₂MX₆ single crystals [K₂SnCl₆, K₂ReCl₆, (NH₄)₂SnCl₆, (NH₄)₂SnBr₆, (NH₄)₂SiF₆, Rb₂SnBr₆, K₂SeBr₆, (NH₄)₂TeBr₆, K₂PtBr₆ and (NH₄)2PtBr₆] have been measured either by Brillouin scattering or by the ultrasonic pulse echo overlap technique. Refractive indices have also been determined. These antifluorite structure compounds contain large MX^2?₆ ions and the interionic spacings are much greater than those of the alkaline-earth fluorite structure halides: their elastic stiffnesses are correspondingly smaller. Hydrostatic pressure derivatives of the elastic stiffness constants have been measured for K₂SnCl₆, (NH₄)₂SnBr₆ and (NH₄)₂SnCl₆ and are found to be positive; there is no marked softening of the long-wavelength acoustic-phonon modes at room temperature. The vibrational anharmonicities of these long-wavelength modes are discussed in terms of the acousticmode Grüneisen parameters, which are compared with the thermal Grüneisen parameters. For K₂SnCl₆ a mean of optic- and acoustic-mode Grüneisen parameters is shown to correlate well with the thermal Grüneisen parameter.     

Growth and high pressure studies of zirconium sulphoselenide single crystals

Transition metal trichalcogenides are well suited for extreme pressure lubrication. These materials being semiconducting and of layered structure may undergo structural and electronic transition under pressure. In this paper authors reported the details about synthesis and characterization of zirconium sulphoselenide single crystals. The chemical vapour transport technique was used for the growth of zirconium sulphoselenide single crystals. The energy dispersive analysis by X-ray (EDAX) gave the confirmation about the stoichiometry of the as-grown crystals and other structural characterizations were accomplished by X-ray diffraction (XRD) study. The variation of electrical resistance was monitored in a Bridgman opposed anvil set-up up to 8 GPa pressure to identify the occurrence of any structural transition. These crystals do not possess any structural transitions upto the pressure limit examined.     

Interacting Frenkel defects at high concentration and the superionic transition in fluorite crystals

A spherical cell model is proposed to account for the explicit concentration dependence of Frenkel defects in an ionic system. In the model, the linearized Debye-Hückel equation is soluble exactly, subject to the boundary condition that the electric field is zero at the cell boundary R, related to the concentration c of defects by $\text{R ∝ c}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$This screened field is used to calculate the chemical potential, which in turn leads to a condition for the instability of the interacting defect assembly. This condition allows one to calculate the enhancement of the concentration of defects above its Arrhenius value at the point of instability in terms of (a) the critical concentration c₀, (b) $\text{a}\text{R}$, where a is the radius of defect and (c) the Debye-Hückel screening length k_c.It is clear from the cell model that this enhancement factor is reduced somewhat in the relevant range of parameters in some of the fluorites from its value in extended Debye-Hückel theory. It is anticipated that the instability discussed here should afford an upper bound to c_v, at the superionic transition, within the range of validity of the model. The excess heat capacity c_p is also discussed briefly.     

氢化铒高温高压下的弹性及热力学性质

利用第一性原理平面波赝势密度泛函理论, 并结合准谐德拜模型, 计算了立方萤石结构ErH2在不同温度和压强下的体积、热膨胀系数、体弹模量和等体热容等弹性性质及热力学性质。在温度高于1 100 K的条件下,计算出的等体热容趋近于Dulong-Petit极限。得到了绝对零度、零压强下ErH2的该结构的晶格常数为0.523 2 nm,与实验值0.523 0 nm非常接近。由不同的原胞体积得出了该体系的单点能与原胞体积的关系的数据;从计算出的高压下的弹性常数,根据立方晶系的力学稳定性条件,推断出立方萤石结构ErH2的相变压力约为20 GPa。