High pressure X-ray studies of UAs and UTe

The volumes of UAs and UTe have been determined as a function of pressure by X-ray powder diffraction in a diamond anvil cell up to 25 GPa at room temperature. Both compounds exhibit a transition from the cubic NaCl-type to CsCl-type structure under high pressure. The bulk modulus and its first pressure derivative have been determined for each phase. In uranium monopnictides the variation of the bulk modulus as the - 5/3 power of the unit cell volume and the large bulk modulus of UAs compared to NpAs give support to 5f delocalization. The bulk modulus of UTe has an exceptionally low value and the possibility of a mixed valence state as in TmSe is discussed. (author)