Dynamic diffuse scattering of X-rays in fcc 4He

A first measurement of the dynamic diffuse scattering (DDS) of X-rays was made in a fcc He-4 crystal. The generally preferred method of studying lattice dynamics is through inelastic neutron scattering (INS). INS experiments, however, require large crystals which, in case of the high pressure fcc phase of helium are difficult to achieve and suffer from some poorly resolved phonon signals. An alternate promising technique uses a beryllium sample cell at cryogenic temperatures and excellent quality single crystals. Diffuse X-ray scattering measurements on such fcc samples yield information about the elastic constants in solid He-4. Present test results on a crystal at 241(3) MPa and 23.8(1) K are compared with the results from neutron experiments and several theoretical calculations, including a present Hartree-Fock LCAO model. Various disagreements are found and discussed. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.