Ultrasonic relaxation spectra in CoO-Co₂O₃-P₂O₅ glasses in the temperature range 6 to 300 K

Abstract

Formulae attributing acoustic relaxation in glasses to two-well systems with a distribution of barrier heights and asymmetries are related to the phenomenological theory of a standard linear solid. Acoustic wave loss measurements at frequencies ranging from 15 to 135 MHz and temperatures ranging from 6 to 300 K have been used to compute relaxation spectra for the CoO-P₂O₅ glass system. For each glass two alternative sets of spectra have been deduced on the contrary assumptions that the range of asymmetries is very broad, or very narrow. From the quality of the fit of theory with experiment, it is concluded that the former description is more likely to be applicable to the glasses. From the systematic nature of the composition dependence of several of the acoustic parameters it is concluded that a broad relaxation spectrum is to be associated with P205 structural groupings, whilst a much narrower spectrum obtains for metaphosphate and pyrophosphate groupings. The spectral analysis also showed that in all the glasses there is evidence of the existence of two-level systems which produce a pronounced upturn in the acoustic relaxation loss at temperatures below 40 K. Since accurate absolute attenuation data were required for the purpose of evaluating alternative relaxation spectra, attenuation measurement errors are analysed in some detail. As the present work is part of a long-term programme of study on the composition dependence of low-temperature ultrasonic properties of glasses, the methods by which slow sweeps of sample temperature from 4 to 300 K are achieved with maximum economy of helium consumption are discussed in some detail.