Melting curve of iron under pressure

The dislocation theory of melting is generalized by considering the effect of pressure. The Kraut-Kennedy law is derived by using this generalized approach and applied to iron. The theoretical result is in good agreement with experiment     

Melting of lithium hydride under pressure

We have computed the melting line of lithium hydride up to 200 GPa using the two-phase simulation technique coupled with first-principles molecular dynamics. Our predicted melting temperature at high pressures varies slowly with compression, ranging from 2000 to 2450 K at 50-200 GPa pressures. The compressed fluid close to the melting line retains the ionic character of the low pressure molten state, while at higher temperatures dynamical hydrogen clustering processes are observed, which are accompanied by changes in the electronic structure.     

Electronic structure of transition metals under pressure

Conclusions In this short review we have tried to demonstrate the advances made in the intensive study of the behavior of the electronic structure of metals under pressure in recent years. The alloys of transition metals, including intermetallic ordered compounds, have for the time being proved to lie outside the scope of this review, mainly because of the lack, in practice, of theoretical calculations in this area. Clearly, progress in this field is also being held up by the lack of detailed experimental investigations devoted to the behavior of alloys under pressure. This is due to the complexities involved in carrying out many traditional experimental procedures under high pressures. We should like to emphasize, however, that at the present time theoretical calculations have taken on a predictive power and this gives reason to hope that progress in calculations will in turn stimulate progress in experimentation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 50–62, December, 1982.     

Melting of B12P2 boron subphosphide under pressure

Melting of boron subphosphide (B₁₂P₂) to 26?GPa has been studied by in situ synchrotron X-ray powder diffraction in a laser-heated diamond anvil cell, and by quenching and electrical resistance measurements in a toroid-type high pressure apparatus. B₁₂P₂ melts congruently, and the melting curve has a positive slope of 23(6)?K/GPa. No solid-state phase transition was observed up to the melting in the whole pressure range under study.     

High temperature superconductivity under pressure

Abstract

This review is a summary of the work concerned with the pressure studies of high temperature superconductors (HTSC) held in the LOW Temperature Department of the IHPP since the last Conference in Paderborn (July, 1989).     

高温超导的压力效应研究

从修正的依赖时间的金兹堡-朗道方程出发研究高温超导体的压力效应，考虑序参量为一个复数. 在外界压力作用下，理论上获得了压强与高温超导温度的一些表达式.在一些特殊情况下，得到高温超导的临界温度T随外压强的增加而降低；在另外某些条件下高温超导的临界温度T_C随外压强的增加而增加.外部施加的压强，只有一部分反映在高温超导态. 关键词： 高温超导 压强 临界温度     

Phase separation of liquid binary mixtures containing metals under pressure

We have measured the pressure variations of the two-phase regions of liquid binary mixtures of metal with metal (LiNaandGaBi), metal with semiconductor (TlSe) and metal with salt (BiBiI₃andBiBiBr₃) up to 28 kbar. It was found that in the mixtures of LiNa and GaBi the two-phase regions enlarge at higher pressures, while in the mixtures of TlSe, BiBiI₃ and BiBiBr₃ the two-phase regions disappear at low pressures.     

Scaling analysis of high temperature superconductors under pressure

The scaling relation of single parameter scaling hypothesis is applied to the study of the scaling behavior of high temperature superconductors under pressure. The data of resistance and specific heat coefficient under various pressures are scaled onto a universal curve according to this scaling relation. The scaling parameters are pressure dependent while temperature independent. It is found that the controlling parameter Bi equals to the relative critical temperature tcP, which indicates that the superconducting energy gap at the zero temperature 2Δs0 is the controlling parameter in this scaling.     

General diagram of the phase remtions of actinide metals under pressure

Abstract

The proposed diagram is based on the results of high-pressure X-ray diffraction work and visualizes the general trends in phase relations of actinide metals under pressure. The transition from dhcp to ccp, which occurs under the action of pressure in heavy actinides and light lanthanides, also occurs with decreasing atomic number at ambient pressure. The same structural sequence is found in the lanthanide and the actinide metals, but its first two members, hcp and Sm-type, are missing in the actinide metals, and the phase transitions are shifted to lower pressures and higher atomic numbers in the actinides.     

Calculated Nb superconducting transition temperature under hydrostatic pressure

Abstract

A full-potential linear muffin-tin orbital method (FP-LMTO) based linear-response approach is used to calculate the electron-phonon coupling in Nb under hydrostatic pressure. The superconducting transition temperature T_c is calculated using the Eliashberg equation. The calculated T_c agrees nicely with the experimental result at ambient pressure, but the agreement is only fair at high pressures. The T_c measured anomaly at 60–70GPa is understood in terms of the 2.5 order Lifshitz transition and its origin is traced back to the qualitative changes in the Fermi surface topology.     

Low temperature electrical resistivity of α-cerium under pressure

We report a new method which is used to study the α-phase of cerium without inclusions of β-phase. A pressure independent low temperature resistivity has been observed. None of the exchange enhancement effects previously observed in high pressure susceptibility experiments have been detected on transport properties.     

Characteristics of urea under high pressure and high temperature

The properties of urea under high pressure and high temperature(HPHT) are studied using a China-type large volume cubic high-presentation apparatus(CHPA)(SPD-6 × 600).The samples are characterized by scanning electron microscopy(SEM), x-ray diffraction(XRD), and Raman spectroscopy.By directly observing the macroscopic morphology of urea with SEM, it is confirmed that the melting point of urea rises with the increase of pressure.The XRD patterns of urea residues derived under different pressures show that the thermal stability of urea also increases with the increase of pressure.The XRD pattern of the urea residue confirms the presence of C_3H_5N_5O(ammeline) in the residue.A new peak emerges at 21.80°, which is different from any peak of all urea pyrolysis products under normal pressure.A more pronounced peak appears at 708 cm~(-1) in the Raman spectrum, which is produced by C–H off-plane bending.It is determined that the urea will produce a new substance with a C–H bond under HPHT, and the assessment of this substance requires further experiments.     

Equation of state and elastic stiffness constants under pressure of noble metals

Using the total crystal energy expression with the overlap potential energy by Moriarty and with the model pseudopotential by Kulshrestha et al., the pressure-volume relations of noble metals are studied and the obtained results are in good agreement with the observed data. Then, the elastic stiffness constants B, C₄₄ and C′ under pressure are obtained by the homogeneous deformation method, and our calculated data will be useful to describe the typical pressure effect on the elastic constants of noble metals.     

Deep levels related to transition metals in Si under hydrostatic pressure

The influence of hydrostatic pressure up to 5×10⁸ Pa on deep levels related to transition metal impurities in silicon is determined by means of an isothermal capacitance method. Under pressure, donor levels of isolated Fe, V, Ti, and Mn shift towards the valence band in contrast to earlier results for deep chalcogen donors. This behavior is contrary to what is expected by considering only effects of hybridization. Quantitative differences between Fe, Ti, V, and, on the other hand, Mn suggest a different microscopic structure of these defects. The Fe-acceptor pairs FeB, FeAl, and FeGa move towards the valence band with a rate comparable to that of the ₁ conduction band. The thermal capture coefficients of isolated Fe, V, and Ti are found to be pressure independent up to 5×10⁸ Pa.On leave from Sony Corp., Research Center, Yokohama, Japan     

Low-temperature conductivity of orbitally ordered Jahn-Teller components of transition metals under pressure

A model is proposed for the metal-dielectric transition under pressure with the formation of a dielectric gap of the Mott-Hubbard type because of stabilization of an orbitally ordered state with the cooperative Jahn-Teller effect in a narrow doubly degenerate conductivity band, allowing for broadening of the latter under compression. A cluster approach is used choosing two neighboring lattice sites as the cluster. The results of the calculation are compared with experimental data on the electric conductivity of Jahn-Teller orbitally ordered crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 36–41, November, 1987.     

Melting of the rare earth metals and f-electron delocalization

Melting curves for Pr, Nd, Sm, Gd, and Y were measured in a diamond-anvil-cell to nearly 100 GPa and 4000 K. f-electron volume collapses are observed as triple points for Pr (24 GPa and 1400 K) and Gd (65 GPa and 3100 K). These pressures coincide with the volume collapses observed at room temperature. For Nd and Sm, the f-electron volume collapse has not been observed at room temperature but appears at approximately 2000-2500 K as a broad minimum in the melting curve, similar to that of Ce, near 50 GPa (Nd) and 70 GPa (Sm). The melting curve of Y goes smoothly along the entire rare earth sequence.     

Superconducting transition temperature in mercury HTSC-cuprates under hydrostatic pressure

A study is made of the properties of the homologous series of mercury HTSC-cuprates HgBa₂Ca_n−1Cu_nO_2n +2+δ with n=1–8. Experiments are conducted under pressure for samples with n=1–5. The Hg-1223 and Hg-1234 phases were synthesized using a controlled high pressure chamber. The oxygen content of an initial mixture corresponding to the Hg-1234 phase was varied by changing the composition of the initial BaO/BaO₂ oxides. The dependence of the superconducting transition temperature T _c on the lattice constant a (and, therefore, on the oxygen content) and of T _c ^max and dT _c ^max /dp on n are convex upward up to n=4, 5. The maximum values always correspond to the Hg-1223 phase. Experimental T _c ^max (n) curves for the phases with n=1–6 and dT _c ^max /dp curves for n=1–5 are compared with Anderson’s theory (the so-called RVB model). A general analysis of these results indicates that the mercury cuprates have an ideal structure for HTSC. The Hg-1223 phase is the “champion” in this ideal structure and the critical temperature corresponding to this phase (T _c =135 K) is the highest at atmospheric pressure. Zh. éksp. Teor. Fiz. 113, 1474–1483 (April 1998)     

Melting curve of sodium at high pressure

A semiempirical model equation of state is developed in terms of bulk modulus and the Grüneisen parameter to compute the melting temperature of sodium in the gigapascal range of pressure. The model successfully explains the increase and decrease of T _m as the pressure increases. Computed values of the critical pressure and temperature are in very good agreement with the experimental observations.