Low temperature acoustic characterization of PMN single crystal

摘要

This acoustic study of the relaxor single crystal PbMg_(1/3)Nb_(2/3)O_3 is focused on the temperature evolution of longitudinal and shear ultrasonic wave velocities as well as on their attenuation within the temperature range of 4.2-300 K. Temperature dependences of all three independent elastic constants for the cubic structure (C_(11), C_(44), and C_(12)) along with the bulk modulus B, the Cauchy ratio, and the degree of lattice anisotropy were derived from the velocity data set. Important parameters such as the Debye temperature, the Grüneisen parameter H, and the Poisson ratio were determined at cryogenic temperatures as well. Deep minima were found for C_(11) and C_(44) around the temperature T_m where the dielectric constant reaches a maximum, followed by a drastic increase of C_(11) and C_(44) at decreasing temperature and ultimately by a saturation at temperatures below 25 K. The behavior of these constants at T < T_m corresponds well to relations resulting from the Einstein-oscillator model. A changeover of C_(12)(T) dependence from increasing at T < T_m to unexpected monotonic decreasing behavior upon cooling from inflection temperature T_i ≈ 220 K followed by a saturation at T < 25 K was found. Furthermore, the behavior of C_(12) (T) and interrelated parameters such as B and H was analyzed on the basis of the concept of phonon pressure due to the thermal expansion. It is essential that the electrostriction contribution was taken into account, as well.