Melting curve of NaCl0.5Br0.5 up to 4.5 GPa

Abstract

The melting curve of NaCl_0.5Br_0.5 has been measured under pressure up to 4.5 GPa. The melting temperatures of Ag and NaCl have been used to determine the pressure in the sample at its melting temperature.     

Lindemann criterion and the anomalous melting curve of sodium

Recent reports of the melting curve of sodium at high pressure have shown that it has a very steep descent after a maximum of around 1000 K at 31 GPa. This maximum does not occur due to a solid-solid phase transition. According to the Lindemann criterion, this behaviour should be apparent in the evolution of the Debye temperature with pressure. In this work, we have performed an “ab initio” analysis of the behaviour of both the Debye temperature and the elastic constants up to 102 GPa, and find a clear trend at high pressure that should cause a noticeable effect on the melting curve.     

High-pressure melting curve of nitrogen and the liquid-liquid phase transition

The melting curve of nitrogen was measured up to 71 GPa, a fourfold increase in pressure over previous measurements. The measurements were made using the laser-heated diamond anvil cell and melting was detected in situ by the laser speckle method. The melting temperature rises linearly up to a maximum at 50 GPa and 1920 K, and with increasing pressure suddenly decreases linearly to 1400 K at 71 GPa. This sharp drop in the melting slope (dT/dP) above 50 GPa indicates the appearance of a liquid denser than the solid and of a liquid-liquid phase transition. The sharpness of the changes suggests that the transition is first order and is a liquid-liquid polymer transition. This conclusion is consistent with earlier theoretical studies and experimental evidence that pressure transforms molecular nitrogen into a chainlike polymeric form.     

Lindemann''s criterion and the necessary condition for a melting curve maximum

Dielectric and differential microcalorimetry measurements have been realized on the $\text{Bi}{}_{\mathrm{1?x}}\text{Te}{}_{\mathrm{x}}\text{O}{}_{\mathrm{<mtext>3+x</mtext><mtext>2</mtext>}}$ (0,33 ? x ? 0,50) solid solution. A phase transition has been detected. The variation of the transition temperature with the composition has been determined. The low temperature phase, stable at 20°C, has piezoelectric and non-linear optical properties.     

First-principles simulation of Raman spectra and structural properties of quartz up to 5 GPa

The crystal structure and Raman spectra of quartz are calculated by using first-principles method in a pressure range from 0 to 5 GPa. The results show that the lattice constants(a, c, and V) decrease with increasing pressure and the a-axis is more compressible than the c axis. The Si–O bond distance decreases with increasing pressure, which is in contrast to experimental results reported by Hazen et al. [Hazen R M, Finger L W, Hemley R J and Mao H K 1989 Solid State Communications 725 507–511], and Glinnemann et al. [Glinnemann J, King H E Jr, Schulz H, Hahn T, La Placa S J and Dacol F 1992 Z. Kristallogr. 198 177–212]. The most striking changes are of inter-tetrahedral O–O distances and Si–O–Si angles. The volume of the Si O4-4tetrahedron decreased by 0.9%(from 0 to 5 GPa), which suggests that it is relatively rigid.Vibrational models of the quartz modes are identified by visualizing the associated atomic motions. Raman vibrations are mainly controlled by the deformation of the Si O4-4tetrahedron and the changes in the Si–O–Si bonds. Vibrational directions and intensities of atoms in all Raman modes just show little deviations when pressure increases from 0 to 5 GPa.The pressure derivatives(dνi/d P) of the 12 Raman frequencies are obtained at 0 GPa–5 GPa. The calculated results show that first-principles methods can well describe the high-pressure structural properties and Raman spectra of quartz. The combination of first-principles simulations of the Raman frequencies of minerals and Raman spectroscopy experiments is a useful tool for exploring the stress conditions within the Earth.     

Shifts in the 3He melting curve due to pore condensation

We have measured the ³He melting curve, first using a cell with an open geometry, then using the same cell filled with MgO powder. In the latter case, we found that below 200 mK the apparent melting curve is shifted.     

Pressure effects on EXAFS Debye-Waller factor and melting curve of solid krypton

The pressure effects on atomic mean-square displacement, extended X-ray absorption fine structure (EXAFS) Debye-Waller factor and melting temperature of solid krypton have been investigated in within the statistical moment method scheme in quantum statistical mechanics. By assuming the interaction between atoms can be described by Buckingham potential, we performed the numerical calculations for krypton up to pressure 120?GPa. Our calculations show that the atomic mean-square displacement and EXAFS Debye-Waller factor of krypton crystal depend strongly on pressure. They make the robust reduction of the EXAFS peak height. Our results are in good and reasonable agreements with available experimental data. This approach gives us a relatively simple method for qualitatively calculating high-pressure thermo-physical properties of materials. Moreover, it can be used to verify future high-pressure experimental and theoretical works.     

Anomalous influence of an external magnetic field on spin fluctuations in magnetic semiconductors with strong p-d hybridization and the colossal magnetoresistance effect

The colossal magnetoresistance effect in magnetic semiconductors based on lanthanum manganites has been investigated in terms of the model allowing for the effects of p-d hybridization and electronelectron Coulomb correlations. The influence of an external magnetic field on spin fluctuations has been considered under the conditions where the chemical potential is in a narrow heavy-fermion band formed in the hybridization gap. It has been shown that, in the vicinity of the Curie point T _C, the strong spin anharmonicity leads to an anomalously strong suppression of spin fluctuations by the external magnetic field, a phenomenon contributing significantly to the formation of colossal negative magnetoresistance.     

Triple point on the melting curve and polymorphism of nitrogen at high pressure

Raman spectra of solid and fluid nitrogen to pressures up to 120 GPa and temperatures up to 2500 K reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied, and there is no sign of a fluid-fluid transition. Solid phases obtained on quenching from the melt above 48 GPa are identical to the recently discovered iota and zeta' phases. We find that kinetics plays a major role in the experimentally observed phase changes and account for the metastability of various crystalline molecular phases and the existence of an amorphous single bonded eta-N.     

Lindemanns' criterion and the necessary condition for a melting curve maximum

Assuming that the generalized Lindemann's criterion adequately describes the melting process, it is shown that the softening of a branch of the frequency spectrum with pressure is associated with the occurrence of fusion curve maximum as well as a negative melting slope. By characterizing the number of “soft” modes through 〈γ〉 and σ^{$\text{1}\text{2}$}, the average and approximate width of the mode gamma distribution of the solid, a qualitative as well as a quantitative condition for a fusion curve maximum can be established for a model potential.     

A thermodynamic lattice theory on melting curve and eutectic point of binary alloys. Application to fcc and bcc structure

A thermodynamic lattice theory has been developed for determination of the melting curves and eutectic points of binary alloys. Analytical expressions for the melting curves of binary alloys composed of constituent elements with the same structure have been derived from expressions for the ratio of root mean square fluctuation in atomic positions on the equilibrium lattice positions and the nearest neighbor distance. This melting curve provides information on Lindemann’s melting temperatures of binary alloys with respect to any proportion of constituent elements, as well as on their eutectic points. The theory has been applied to fcc and bcc structure. Numerical results for some binary alloys provide a good correspondence between the calculated and experimental phase diagrams, where the calculated results for Cu_1−x Ni _x agree well with the measured ones, and those for the other alloys are found to be in a reasonable agreement with experiment.     

Thermal studies of the spin liquid state and analog to the He melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG) has an extraordinary low-temperature phase diagram. Although the Curie–Weiss temperature of GGG is −2 K, GGG shows no long-range order down to T0.4 K. At low temperatures GGG has a spin glass phase at low fields (0.1 T), a field-induced long-range ordered antiferromagnetic state at fields of between 0.7 and 1.3 T, and, at intermediate fields, an apparent spin-liquid state without long-range order. We have characterized the intermediate field (IF) state through heat capacity, thermal conductivity, and magnetocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T0.2 K, in analogy to the minimum in the melting curve of ⁴He. The existence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature.     

Elastic and vibrational properties of cobalt to 120 GPa

Impulsive stimulated light scattering and Raman spectroscopy measurements have been made on hcp cobalt to a static pressure of 120 GPa. We find that at pressures above 60 GPa the shear elastic modulus and the Raman frequency of the E(2g) transverse optical phonon exhibit a departure from a linear dependence on density. We relate this behavior to a collapse of the magnetic moment under pressure that has been predicted theoretically, but until now not observed experimentally.