Ultrasonic attenuation in bismuth silicon oxide and bismuth germanium oxide
The propagation of ultrasonic waves has been studied in single crystals of undoped and doped bismuth germanium oxide (Bi12GeO20) and undoped bismuth silicon oxide(Bi12SiO20). These crystals are piezoelectric (Cubic Space Group I23) and direct excitation of ultrasound was used wherever possible. Attenuation measurements were made from 10 to 190 MHz over the temperature range 4.2 to 260 K. A large attenuation peak was observed in the undoped crystals between 35 and 50 K for only those ultrasonic modes whose velocity depends on the elastic modulus C44 This peak is shown to be a single anelastic relaxation peak and is interpreted as due to point defects. Using the selection rules for anelastic relaxation in cubic crystals, it is deduced that these defects must have trigonal symmetry. The magnitude of the attenuation peak in doped BGO samples is strongly dependent on the presence of dopants. Doping with Al, Ga, Pb, P + Ga, Ga+Cr, all removed the attenuation peak, while Cr increased it and Zn reduced it. A strong correlation was observed between the attenuation peak, the colour of the crystals and the optical absorption just below the band gap measured in these samples by other workers. These results indicate that the same defect centre is responsible for both the optical and the ultrasonic absorption. Possible defects are discussed taking the optical absorption, X-ray diffraction, EPR and other available measurements into consideration. The ultrasonic attenuation was measured before and after annealing at 450°C in vacuo or in pure oxygen. No change was observed for an undoped sample. However, for the Cr doped sample the attenuation was reduced by vacuum annealing and restored by oxygen annealing. This seems to be closely related to previous observations of photo-chromic behaviour in BSO doped with Cr. Measurements were also made to study the effects of gamma irradiation on the attenuation in undoped BSO.