Heat Capacity and Features of the Phonon Spectrum of Single Crystals of Solid Solutions of Yttrium-Lutetium Alumogranates

The temperature dependences of the heat capacity were measured and the general patterns of the formation of the phonon spectrum of single crystals of solid solutions of yttrium-lutetium aluminum garnets Y3–xLuxAl5O12 at 0 x 3 were studied in the temperature range from 1.9 to 220 K. Based on the data obtained below 10 K, the Debye temperatures were calculated. Features of the phonon spectrum in the intermediate temperature region are interpreted as a superposition of optical modes for yttrium and lutetium garnets. It is shown that low values of the heat capacity due to the contribution of acoustic phonons for Y2.25Lu0.75Al5O12 correlate with anomalies in the concentration dependences of phonon transport, absorption of acoustic waves, and the NMR line shape of aluminum.