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It was shown that the Clausius-Clapeyron relation (CCR) determines only the maximum possible slope of the first-order phase transition (PT-1) line in the temperature-pressure coordinates. This conclusion follows from the fact that the real PT-1 it is an irreversible process. An inequality has been obtained that generalizes the CCR to the case of irreversibility at PT-1: the Clausius-Clapeyron inequality (CCI). It is indicated that the CCI passes into equality (i.e., into the CCR) only in the case of reversible PT-1. Analysis of experimental data for various types of PT-1 showed the implementation of CCI in most cases. Herewith, the deviation from the CCR is the more noticeable, the greater the volume change in PT-1. It was found that deviations of experimental data from the CCR correlate with the presence of phase-transition radiation in PT-1. This makes it possible to use the value of the deviation from the CCR for thermodynamic indication of the probability of observing phase-transition radiation at PT-1 in various substances. A method has been proposed for estimating the irreversibility PT-1 value, based on the magnitude of the deviation from the CCR. The obtained CCI has been generalized to the case of the presence of an external homogeneous magnetic (or electric) field acting on the two phases of matter in equilibrium. • An inequality generalizing the CCR for an irreversible phase transition is obtained • The deviations from the CCR in the various experimental works were indicated • Deviation from the CCR is more, the greater of volume change in phase transition • Deviations from the CCR correlate to the presence of phase-transition radiation • The results were generalized for the case of an external magnetic or electric field [ABSTRACT FROM AUTHOR]

Based on the previously proposed model of paired covalent bond, the reasons for the brittleness for covalent crystals have been studied. It is shown that the brittleness of a covalent crystal is due to the "duplicity" of the paired interatomic interaction potential for elastic (reversible) and plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by breaking than by elastic stretching it. It is shown that the brittle-ductile transition temperature for single-component covalent crystals has an upper limit: T BDT / T m < 0.45, where T m is the melting temperature. • The paired interatomic interaction potential for a covalent bond is "two-faced". • Surface energy at plastic deformation of a covalent crystal is less than at elastic. • Create the surface in a covalent crystal is favorable by rupture than by stretch. • Brittleness of a covalent crystal is due to the interatomic potential "duplicity". • The brittle-ductile transition temperature for covalent crystals has an upper limit. [ABSTRACT FROM AUTHOR]

Based on calculations of the activation process parameters (vacancy formation and self-diffusion) for gold at the temperature range T = 10–1330 K, the correlations of the activation process parameters s i (T) ∼ h i (T) and s i (T) ∼ v i (T), along two isobars. P = 0 and 24 GPa, are studied. Here h i is the enthalpy, s i is the entropy, and v i is the volume of the activation process, i.e. for the formation of an electroneutral vacancy (i = v) or for the self-diffusion of atoms (i = d). It is shown that if, during isobaric heating, an energy less than a certain value (h i < h si) is required to create a defect (vacancy or diffusing atom), or if the resulting defect has a volume less than a certain value (v i < v si), then this defect has a negative entropy, i.e. this defect orders the crystal. However, if a larger energy value is required to create a defect (h i > h si) or this defect has a noticeable volume (v i > v si), then this defect has a positive entropy, i.e. this defect disorders the crystal. It is shown that at a low concentration of defects, they order the crystal, and only starting from a certain concentration (X si) does the defect formation entropy pass into the positive region, where defects disorder the crystal. The changes in the functions of h si , v si , and X si with increasing pressure are studied. Dependence of entropy on the logarithm of the defect formation probability for gold. The two upper curves are for the atom self-diffusion process, and the two lower curves are for the vacancy formation process. Solid curves are isobars P = 0, dashed curves are isobars P = 24 GPa. [Display omitted] • Correlations of entropy on enthalpy and on volume of activation process are studied. • If the defect creating energy is small h < h s , then such a defect orders the crystal. • If a small volume v < v s is needed for a defect, such a defect orders the crystal. • If the concentration of defects is low X < X s , they order the crystal. • The changes of hs , vs , and Xs functions with increasing pressure has been studied. [ABSTRACT FROM AUTHOR]