Abstract
The influence of pressure and temperature on low-frequency lines (100–300 cm–1) in Raman spectra of ZrB12 and LuB12 single crystals has been investigated. These spectral features have been identified as one-phonon and two-phonon excitations of the acoustic branches of the phonon spectrum. It has been found that the observed spectra of ZrB12 single crystals are more structured and indicate the development of phonon anomalies with a decrease in the temperature. Despite the fact that the low-frequency features in the phonon spectrum of ZrB12 are characterized by a high value of the isothermal Gruneisen parameter, their isobaric Gruneisen parameter has a negative value. This indicates large contributions from the fourth-order anharmonicity, which significantly exceed the volume effects. The appearance of narrow lines in the frequency range from 155 to 175 cm–1 at temperatures T < 100 K suggests the possibility of a structural transition with an incomplete softening of the lattice.
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