Lattice dynamics of xenotime: The phonon dispersion relations and density of states of LuPO4

LuPO4 is the nonmagnetic end member of a series of rare-earth phosphates with a common zircon-type crystal structure. The phonon-dispersion curves of LuPO4 along the [x,0,0], [x,x,0], and [0,0,x] symmetry directions were measured by neutron triple-axis spectroscopy using single-crystal samples. The phonon density of states was determined by time-of-flight neutron scattering using polycrystalline samples. Phonons involving mainly motions of rare-earth ions were found to be well separated in energy from those of the P and O vibrations. A large gap in the phonon-frequency-distribution function, which divides the O-P-O bending-type motions from the P-O stretches, was observed. All of the experimental results were satisfactorily accounted for by lattice-dynamic shell-model calculations. LuPO4 is a host material for the incorporation of rare-earth ions to produce activated luminescence. Information regarding the phonon and thermodynamic properties of LuPO4 is pertinent to extended investigations of additional rare-earth spin-lattice interactions in other zircon-structure rare-earth orthophosphates. [S0163-1829(97)06741-6].