Crystal fields in PrB6 and NdB6

The electron-phonon relaxation times of the crystal-field levels of the metastable state⁵D₄ of Tb³⁺ were measured between 2 and 4.2K. The relaxation of the lowest pseudodoublet (E=1.25 cm^–1) shows an Orbach process via higher crystal-field levels. The other levels show direct relaxation on a nanosecond time scale. The higher crystal-field levels were used to look for propagation of high-energy (4THz) nonequilibrium phonons by phonon-induced fluorescence. No ballistic propagation of high-energy phonons could be found.