Elastic anomalies and orientational glass transition in Na(CN)(x)Cl1-x mixed crystals. A Brillouin spectroscopic study

High performance Brillouin spectroscopy (BS) has been used to study the ela stic properties (static and dynamic) of the orientational glassy state of N a(CN)(x)Cl1-x samples (x < x(c) approximate to 0.65). The temperature behav iour of the elastic properties reveals a more complex scenario for the orie ntational glass transition than generally believed. The shear elastic const ant shows the well-known c(44)(T) anomaly. indicated by a minimum, found in other cyanide mixed crystals. The results obtained for the hypersonic atte nuation are in clear contradiction with the dynamic character of the c(44)( T)-minimum. The temperature behaviour of the longitudinal elastic constant c(11) of very dilute (x < x(c)) Na(CN)(x)Cl1-x samples shows two striking f eatures: i) Similar to the anomalous temperature behaviour of c(44)(T), low ering the temperature c(11)(T) first decreases, goes through a minimum and then rises again. The minimum takes place at a temperature above the temper ature, T-min, where c(44)(T) reaches its minimum value. ii) A kink-like ano maly of c(11)(T) is observed at lower temperatures. This second anomaly is similar to the classical one observed in canonical glasses at their glass t ransition temperature T-g.