Structural determination of atomic substitution ratio in (K_＜1-x＞Rb_x)_2SeO_4 mixed crystal system

概要

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In order to explain why Rb_2SeO_4 remains in normal phase down to 0 K while K_2SeO_4 transforms into the modulated phase at finite temperature, crystal structures of the mixed system have been refined at room temperature by X-ray single crystal analysis. Rb ion can be substituted for K ion with keeping the macroscopic symmetry of space group Pnam. Larger size of Rb ion occupies preferentially site 1, while smaller K ion prefers site 2. Here the atomic distance between an alkali ion and oxygen atoms is longer at site 1 than at site 2. The mean substitution ratio coincides with that from chemical analysis, however, the crystallographic structure analysis is only one method to determine the ratios at each site separately. A sublattice model is discussed to interpret the ion replacing effect on the phonon dispersion relation of the mixed crystal system. The hardening of the soft mode branch can be predicted by the change of interactions between the SeO_4 tetrahedrons if the alkali ions are replaced.
社団法人日本物理学会の論文
2005-10-15