Anelastic relaxation effects and elastic instabilities in cgg-type compounds

Abstract

We studied the elastic and anelastic proper- ties of La3Ga5SiO14 ,L a3Ga5.5Ta0.5O14 ,C a3Ga2Ge4O14 and Sr3Ga2Ge4O14 between room temperature and 1800 K using resonant ultrasound spectroscopy. In these crystal species strong attenuation effects develop in the ul- trasonic frequency regime at elevated temperatures. The temperature evolution of the inverse quality factors of vi- brational modes show a peak-like behavior which is char- acteristic for anelastic relaxation processes. Depending on composition and frequency the loss maximum occurs between 900 K and 1300 K. The strength of the attenua- tion effects suggests a large number of relaxation centers, i.e. they are probably connected with an inherent struc- tural feature like the off-center position of an ion. In lan- gatate an additional less pronounced loss maximum with an activation energy of the relaxation process of 1.05 eV occurs at about 800 K. At temperatures below the onset of the relaxation phenomena the evolution of the shear resistances of the investigated compounds is strongly non- linear. In general the corresponding temperature coeffi- cients increase with decreasing temperature. As a con- sequence some shear stiffnesses in LGS, LGT and CGG already possess positive temperature derivatives above room temperature. These anomalies indicate a structural instability that might lead to a phase transition at low temperatures or high pressures.