Lattice and spin dynamics of γCe
Abstract
Inelasticneutronscattering techniques have been used to study the lattice dynamics and magnetic scattering of fcc γCe. The phonon dispersion curves at room temperature were determined along the [100], [110], [111], and [0ξ1] symmetry directions. Comparison of the measured dispersion curves with those of Th show that the spectrum of γCe is in general softer than one would expect by taking into account the differences in mass, interatomic spacing, and melting temperatures of these elements. This relative softening of the phonon frequencies is more pronounced for the phonon branches whose slopes in the elastic limit involve the elastic constants c_{11} and c_{12} which determine the bulk modulus of the element. Thus it seems that premonitory effects of the γ>α transition are present in the roomtemperature dispersion curves of γCe. The elastic constants and lattice specific heat were evaluated by a standard Bornvon Kármán analysis. We find that the values of c_{11} and c_{44} are comparable, which is extraordinary for an fcc metal. As a result the shear moduli c_{44} and 12(c_{11}c_{12}) differ by almost a factor of 3 which implies large anisotropy with regard to the propagation of elastic waves. ConstantQ scans performed at reciprocallattice points, where the onephonon contribution to the scattering vanishes, show unambiguously that there is no welldefined lowenergy crystalfield excitation in γCe. The magnetic scattering follows quite well the 4f magnetic form factor of Ce^{3+}. Assuming a relaxational form for the imaginary part of the generalized susceptibility, the spin relaxation energy ℏΓ is found to be approximately 4 THz.
 Publication:

Physical Review B
 Pub Date:
 June 1979
 DOI:
 10.1103/PhysRevB.19.5746
 Bibcode:
 1979PhRvB..19.5746S