Bethe Approximation for X-Ray Diffuse Scattering Due to Orientational Disorder in RbNO₃

抄録

X-ray diffuse scattering due to multi-orientational disorder of NO₃⁻ ions in cubic RbNO₃ of CsCl type is analysed with Nagai’s formula, which is derived by the Bethe approximation. The probabilities of pair configurations of the neighbouring NO₃⁻ ions are determined with 12-orientation model. From these probabilities, the configurational entropy is estimated with Takagi’s method to be R ln 2.6 J/mol·deg, which is not incompatible with the transition entropy from the lower temperature phase (P3₁). It is believed that the Bethe approximation can well describe a disordered phase bearing a partial strong correlation due to a short range repulsive interaction.