An expression for the internal energy of a multicomponent ordered many-body system has been obtained by means of the correlation expansion technique. This expression has proved to be useful for strongly anharmonic crystals in wide temperature and pressure ranges. The relation between the fundamental characteristics of a solid that are used in the correlation expansion and in the phonon approximation has been found. This is, in particular, the relation between the Debye and Einstein temperatures and the average speed of sound.
Department of Quantum Statistics and fIeld Theory, Faculty of Physics, Moscow State University, Moscow, 119992, Russia
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