A model for elastic anomalies in K₂Pt(CN)₄BR _0.33H₂O

A "spin-lattice" model is proposed for K₂Pt(CN) ₄Br_o.3·3H₂O (KCP) to solve the puzzling problem of the apparent complete softening of shear modulus C₆₆ without any structural phase transition at low temperatures. Our model emphasizes the importance of the site II water molecules lying between bromine ions and Pt(CN)₄ complexes. Each water molecule is assumed to have three configurations, which are represented by an S=1 Ising spin. An almost complete agreement is obtained between calculation and experimental data. A marked "hardening" of C₆₆ at low temperatures (T<90 K) is predicted. A structural phase transition is possible if the spin-lattice coupling can be enhanced by e.g. external pressure. Ultrasonic attenuation coefficient α₆₆, and also C₄₄, α₄₄, are calculated.