Elastic anomalies and orientational glass transition in Na(CN)_xCl_(1 - x) mixed crystals. A Brillouin spectroscopic study

摘要

High performance Brillouin spectroscopy (BS) has been used to study the elastic properties (static and dynamic) of the orientational glassy state of Na(CN)_xCl_(1 - x) samples (x < x_c ≈ 0.65). The temperature behaviour of the elastic properties reveals a more complex scenario for the orientational glass transition than generally believed. The shear elastic constant shows the well-known c_(44)(T) anomaly, indicated by a minimum, found in other cyanide mixed crystals. The results obtained for the hypersonic attenuation 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)_xCl_(1 - x) samples shows two striking features: i) Similar to the anomalous temperature behaviour of c_(44)(T), lowering the temperature c_(11)(T) first decreases, goes through a minimum and then rises again. The minimum takes place at a temperature above the temperature, T_(min) where c_(44)(T) reaches its minimum value. ii) A kink-like anomaly 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 transition temperature T_g.