Change in the bulk modulus at theB1−B2phase transition
| Autor: | Neil L. Allan, T. H. K. Barron, C. E. Sims |
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| Rok vydání: | 1999 |
| Předmět: | |
| Zdroj: | Physical Review B. 60:2968-2971 |
| ISSN: | 1095-3795 0163-1829 |
| DOI: | 10.1103/physrevb.60.2968 |
| Popis: | Ab initio Hartree-Fock calculations for NaCl are used to derive the changes across the $B1\ensuremath{-}B2$ phase transition in both the bulk modulus $(\ensuremath{\Delta}{K}_{T})$ and the transverse infrared frequency $(\ensuremath{\Delta}{\ensuremath{\nu}}_{\mathrm{TO}}).$ $\ensuremath{\Delta}{K}_{T}$ is found to be positive but very small, and $\ensuremath{\Delta}{\ensuremath{\nu}}_{\mathrm{TO}}$ negative and relatively large (\ensuremath{\approx}20%). In contrast, using a relationship based on lattice dynamics with rigid ions and short-range repulsion only between nearest neighbors, Hofmeister [Phys. Rev. B 56, 5835 (1997)] has recently deduced from the observed drop in ${\ensuremath{\nu}}_{\mathrm{TO}},$ itself in good agreement with the Hartree-Fock calculations, that there should be a marked drop in ${K}_{T}.$ We have, therefore, also calculated $\ensuremath{\Delta}{K}_{T}$ and $\ensuremath{\Delta}{\ensuremath{\nu}}_{\mathrm{TO}}$ for NaCl using lattice dynamics, with a shell model that incorporates short-range potentials between both first and second neighbors, obtaining good agreement with the Hartree-Fock calculations. This calls into question the use at high pressures of a semiempirical relation between infrared frequencies and ${K}_{T},$ based on a model which includes only nearest-neighbor short-range interactions. Similar calculations for CaO give a much larger increase in the bulk modulus across the phase boundary, still accompanying a dramatic drop in ${\ensuremath{\nu}}_{\mathrm{TO}}.$ |
| Databáze: | OpenAIRE |
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