Phonon Anharmonicity of PdO Studied by Raman Spectrometry

Raman spectra were measured for palladium oxide (PdO) at temperatures from 298 to 973 K. The first-order scattering B-1g and E-g phonon modes show prominent frequency and line width dependencies on temperature. The phonon behaviors are described well by a model considering contributions from thermal expansion related quasiharmonicity and explicit lattice anharmonicity from both cubic and quartic phonon decay processes. Compared to the quasiharmonicity for the line shift, the explicit anharmonicity is found to be relatively smaller for the B-1g mode with the cubic anharmonicity leading the quartic term. The E-g mode has a smaller line shift which mostly follows a quartic phonon decay mechanism while the cubic contribution is canceled by thermal expansion effects. The cubic anharmonicity dominates mostly the line broadening with more prominent quartic effects responsible for the larger broadening of the E-g mode. Multiple potential asymmetric cubic and quartic anharmonic decay channels including down-conversion and up-conversion are identified with their characteristics discussed and correlated to the positive and negative anhamonicities found for the two phonon modes studied.