Surface free energy of the crystal-liquid interface on the metastable extension of the melting curve

The surface free energy γ, excess surface energy e, and excess surface stress τ at the crystal-liquid interface of a Lennard-Jones system have been determined by molecular dynamics simulation. The calculations have been performed for temperatures both above and below the triple-point temperature for the region where each of the coexisting phases is metastable and is at a negative pressure. The asymptotic behavior of γ, e, and τ has been analyzed near the endpoint of the melting curve, which is a point of the contact of the metastable extension of the melting curve and the spinodal of the stretched liquid [V.G. Baidakov and S.P. Protsenko, Phys. Rev. Lett. 95, 015701 (2005)]. It has been found that γ, e, and τ at this point are finite and the excess surface entropy is zero.     

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.     

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.     

Adiabatic curve of a metastable plasma

Expressions are obtained relating the value of the thermodynamic parameters of a metastable supercooled plasma during adiabatic (isentropic) processes. The dependence of the sound velocity on the plasma parameters is determined.General Physical Institute, Russian Academy of Sciences, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 15–20, January, 1994.     

A phase diagram of ammonia close to the melting point

We calculate here the phase line equations using the mean field theory for the liquid-solid I - solid II phases in the ammonia close to the melting point. Our calculated phase line equations have been fitted to the experimental data. Our calculated phase diagram agrees very well with the experimentally obtained P-T phase diagram from the literature.     

Pseudocritical NMR frequency shift above the normal-incommensurate phase transition

The NMR satellite frequencies were measured as a function of temperature in the normal high-temperature phase for ⁸⁷Rb in Rb₂ZnBr₄ and Rb₂ZnCl₄ and for ³⁵Cl in betaine calciumchloride dihydrate. Approaching the respective normal-incommensurate phase transition an anomalous shift of the NMR frequency is observed for the first two cases. This effect is ascribed to the increasing order parameter fluctuations. The experimental data are compared to calculations which relate the observed behaviour of the NMR frequencies to the non-classical critical behaviour of the substances under investigation. Received 6 August 1998     

晶体相场法研究预变形对熔点附近六角相/正方相相变的影响

应用双模晶体相场模型计算二维相图,并模拟了在熔点附近预变形和保温温度对六角相晶界演化以及六角相/正方相相变的影响.研究发现:在相变初期,当预变形为零、保温温度离熔点很近时在晶界发生缺陷诱发预熔;增大预变形,变形与缺陷的交互作用在熔点附近诱发预熔;随着预变形的进一步增大,变形在畸变处同时诱发液相和正方相,且预变形越大、保温温度越接近熔点,液相生长越明显,反之正方相生长明显.持续保温使得畸变能释放,晶粒最终完全转变为平衡正方相.模拟结果表明:预变形六角相在熔点附近保温时,由于晶界固有缺陷和预变形双重作用使得原子无序度增加,从而在晶界或其他缺陷处产生液相,待能量释放后晶粒再转变成平衡正方相,进而延缓了六角相/正方相相变时间.     

Theoretical study of a melting curve for tin

The melting curve of Sn has been calculated using the dislocation-mediated melting model with the `zone-linking method'. The results are in good agreement with the experimental data. According to our calculation, the melting temperature of γ -Sn at zero pressure is about 436~K obtained by the extrapolation of the method from the triple point of Sn. The results show that this calculation method is better than other theoretical methods for predicting the melting curve of polymorphic material Sn.     

On a plausible explanation of the connection of point defect parameters with the melting point

Since a few decades it has been empirically found that the activation enthalpy for the self-diffusion process and the defect formation enthalpy are proportional to the melting temperature. The corresponding proportionality constants have been empirically determined for various categories of solids. An explanation of this empirical fact is suggested on a unified basis that is valid for various classes of solids. By comparing the theoretical values of the proportionality constants with the empirical ones an excellent agreement emerges. Furthermore, for first time, the physical meaning of these proportionality constants is obtained.     

The high-pressure melting curve of CaO

Shell-model molecular dynamics simulation has been performed to investigate the melting of the major Earth-forming mineral CaO at elevated temperatures and high pressures, based on thermal instability analysis. The interatomic potential is taken to be the sum of effective pair-wise additive Coulomb, van der Waals attraction, and repulsive interactions. It is shown that the simulated molar volume of CaO is successful in reproducing recent experimental data and our DFT-GGA calculations up to the core–mantle boundary pressure of 135 GPa. The pressure dependence of the simulated high pressure melting temperature of CaO is in good agreement with the results obtained from the Lindemann melting equation at a pressure of below 7 GPa. The extrapolated melting temperatures are in good agreement with the results obtained from Wang’s empirical model up to 60 GPa. The predicted high pressure melting curve, being very steep at lower pressures, rapidly flattens on increasing pressure. The thermodynamic properties of the rocksalt phase of CaO are summarized in the 0–135 GPa pressure range and for temperatures up to 9300 K.     

Pressure effect on the melting point of alkali metals

The mean-square displacement of alkali metals is studied theoretically using our local Heine-Abarenkov-type model potential in the perturbational scheme. The temperature-dependent mean-square displacement of alkali metals decreases as function of the compressed volume. Lindemann's criterion for melting x_m, which is defined as the ratio of two times the root-mean-square displacement to the nearest-neighbour distance, is found to be nearly constant for five alkali metals. The volume effect on the melting temperature of alkali metals is studied by keeping x_m constant. The obtained melting curve increases as function of the compressed volume and are qualitatively in good agreement with the observed tendency for alkali metals.     

On the structure of a metastable phase in the lead-bismuth system

A new metastable phase is detected in liquisol-quenched (splat-cooled) lead-bismuth alloys. Its crystal structure of the new phase is established as tetragonal with a maximum of 48 atoms per unit cell and with the lattice parameters $\text{a = 9.931}\text{A}\text{?}\text{, c = 14.99}\text{A}\text{?}$ and c/a = 1.510.     

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.     

Dynamic loading of solids with a negative slope of the melting curve

The results of experiments on shock-wave loading of materials distinguished by a negative slope of the melting curve are reported. An anomalous situation is possible, when the material is melted by a shock wave and then a fast transition to the solid phase takes place in the expansion wave. The newly formed phase should be a nanostructural formation of the given element. Experiments confirmed the existence of the transition from the liquid to the solid phase of the nanostructural modification.     

On the melting point of amorphous silicon

The entropy and melting point of amorphous silicon cannot be measured in a thermodynamic sense.     

Amorphization of ice near the melting point

In addition to reflections of the hexagonal phase of ice I _h, the intense diffuse scattering of X-rays mainly due to the amorphization of ice is revealed on the X-ray diffraction patterns of water ice samples prepared at liquid nitrogen (studied by the authors earlier) and samples prepared at T = ?10°C (this work). The measurements are performed in the temperature range from ?25 to 0°C. The existence of reflections of the crystalline phase and intense diffuse scattering on the X-ray diffraction patterns makes it possible make a conclusion about the coexistence of crystalline and amorphous structures of ice. Splitting of the first maximum on the electron-density radial distribution function is detected on the basis of an X-ray diffraction pattern recorded at T = ?3°C. This splitting is explained by an increase in the interatomic distances between the nearest-neighbor atoms located at different levels. Similar splitting was also detected on a radial distribution function constructed using an X-ray diffraction pattern recorded at ?10°C.