SPECIFIC-HEATS OF LEAD AND CADMIUM NIOBATES AT LOW-TEMPERATURES

Specific-heat data (2-25 K) are reported on the ferroelectrics PbNb2O6 (orthorhombic), Cd2Nb2O7, and Cd1.6Pb0.4Nb2O7, and on the nonferroelectrics PbNb2O6 (rhombohedral), Pb2Nb2O7, and CdNb2O6. All materials display maxima in CT-3, and excellent fits to experimental data are obtained with a single Einstein term added to the Debye background. The Einstein frequencies vary from 9 cm-1 for PbNb2O6 (rhombohedral) to 76 cm-1 for CdNb2O6, and the Debye temperatures from 171 K for PbNb2O6 (rhombohedral) to 408 K for CdNb2O6. The ferroelectric form of PbNb2O6 varies dramatically from the nonferroelectric form: Einstein frequency, 37.6 cm-1, and Debye temperature 224 K. The 20-mole%-Pb substitution on the Cd site in Cd2Nb2O7 lowers both the Einstein frequency and Debye temperature: 53 to 43 cm-1 and 393 to 315 K, respectively. For all materials except CdNb2O6, the Debye contribution is smaller than the Einstein contribution. Below 4 K, the ferroelectric niobates and CdNb2O6 have a Schottky contribution with an energy separation ∼50 mK (assumed two level). It is suggested that this term is due to the electric-quadrupole moment of Nb in an electric-field gradient. Published data on LiNbO3 and LiTaO3 support this interpretation. © 1979 The American Physical Society.