Single-crystal thermoelastic behavior of magnetoplumbite-type (Mg,Zr):SrGa12O19 and Pr:SrAl12O19 between 103 and 1573 K

The elastic behavior of Sr1.03Ga10.81Mg0.58Zr0.58O19 and Pr-doped SrAl12O19 single crystals grown by the top-seeded solution growth technique was investigated by resonant ultrasound spectroscopy at temperatures between 103 and 1573 K. The coefficients of thermal expansion were obtained by high-resolution dilatometry in the same temperature range. Values for bulk moduli at ambient temperatures are 206.5 and 181.0 GPa for Pr-doped SrAl12O19 and Sr1.03Ga10.81Mg0.58Zr0.58O19, respectively. The aluminate possesses higher values of the mean elastic stiffness compared to the gallate, which can be related to different oxygen packing densities. Apart from these differences, their elastic anisotropy is very similar, as expressed by the ratio of the maximum and minimum of the longitudinal elastic stiffness of 1.34 and 1.35, respectively. In the case of Pr-doped SrAl12O19, an unexpected, fully reversible softening of the stiffness coefficient c33 is noteworthy below 500 K, which is accompanied by a negative thermal expansion. In addition, Pr-doped SrAl12O19 possesses a well-reproducible strong ultrasound attenuation between 700 and 900 K, which can be interpreted as anelastic relaxation of point defects with an activation energy of 0.65 eV.