Simple variational approach to the thermal properties of ionic crystals

Nityananda, Rajaram ; Narayan, Ramesh (1982) Simple variational approach to the thermal properties of ionic crystals Pramana - Journal of Physics, 19 (4). pp. 367-379. ISSN 0304-4289

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The thermal properties of ionic crystals are analysed using the variational principle of classical statistical mechanics. The Einstein and Debye pictures of the lattice vibrations are adopted as trial Hamiltonians. No explicit calculation of the lattice spectrum is needed. The variational result for the thermal expansion in the Einstein picture is identical to that recently derived by Narayan and Ramaseshan by a physically motivated thermal force picture. The agreement with experimental values in the alkali halide family of crystals is surprisingly good, the root mean square error being about 14%. The parameters in the interionic potential used are obtained from the lattice spacings and compressibilities of the crystals and not from anharmonic properties. The Debye picture gives about equally good results for the thermal expansion, but better results for the thermal vibration amplitudes of the ions. It differs from the Einstein picture in incorporating correlated vibrations of atoms and in having an explicit Coulomb contribution to the thermal properties. It is suggested that the theory given in this paper has a useful role to play in studies of thermal expansion and phase stability for large families of ionic crystals when combined with semi-empirical theories.

Item Type:Article
Source:Copyright of this article belongs to Indian Academy of Sciences.
Keywords:Ionic Crystals; Thermal Properties; Variational Principle
ID Code:26474
Deposited On:06 Dec 2010 12:28
Last Modified:17 May 2016 09:46

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