ON THE MEAN-SQUARE RELATIVE ATOMIC DISPLACEMENTS IN STRONGLY ANHARMONIC CRYSTALS

The correlative method of unsymmetrized self-consistent field is used to study the mean square relative atomic displacements in crystals tak ing into account the strong anharmonicity and interatomic correlations . General expressions for these quantities in arbitrary crystal lattic es are obtained. As an illustration, the mean square relative displace ments between nearest, second, and third neighbours in a strongly anha rmonic linear chain are calculated. The results are compared with thos e obtained with more rough approximations (harmonic, quasi-harmonic, e tc.). The influence of anharmonic effects is discussed.