Study of seebeck coefficient and electrical conductivity in single crystals of rare earth garnets

Electrical conductivity and Seebeck coefficient in gadolinium gallium garnet (GGG) and samarium gallium garnet (SGG) have been studied. Measurements have been made on single crystals grown from the melt by Czochralaski method and cut with larger faces parallel to (111) for GGG and (100) for SGG. Seebeck coefficient measurements have been made in the temperature range 200-450 deg C. Seebeck coefficient, α, is negative both for GGG and SGG crystals at all temperatures of measurement. In the α versus T curves, peaks are observed at approximately 270 deg C (both for SGG and GGG) and at 320 deg C (for GGG only). Activation energy values are found for the temperature range 200-600 deg C. The change in activation energy with temperature and occurence of peaks in the α-T plots at the same temperature suggest that the sources of charge carrier are different at different temperatures. The nature of charge carriers and conduction mechanism are discussed. (author)