High-resolution and high-pressure x-ray diffraction study of ScFe2Hx compounds

Using synchrotron-based x-ray diffraction (SXRD) techniques, the evolution of the crystal lattice in ScFe2Hx compounds is investigated at room temperature and under pressures up to 25 GPa. High-resolution SXRD experiments reveal that the C14 hexagonal MgZn2-type structure of the parent intermetallic compound ScFe2 is preserved upon H insertion into the crystal structure in spite of the large unit-cell expansion of 21.4 %. The lattice expansion proves to be anisotropic with a larger expansion within the basal plane of the hexagonal structure Delta a/a = 7.1 % against Delta c/c = 5.8 %. This huge increase in the cell volume is equivalent to 2.92 & Aring;3/ inserted hydrogen atom, a value much larger than that reported for several other Fe-rich alloys. Angle-dispersive SXRD measurements under external pressure indicate that the hexagonal symmetry of the crystalline lattice remains unaltered within the studied compression range up to 25 GPa. No signature of a structural phase transformation has been observed at least up to the highest applied pressure. The application of hydrostatic pressure induces a monotonous contraction of the cell dimensions. A bulk modulus of K0 = 142 GPa has been inferred from the pressure-volume relationship for the initial alloy ScFe2. It is further shown that hydrogen (deuterium) absorption leads to a decrease of the bulk modulus down to K0 = 112 and 92 GPa for ScFe2H2.8 and ScFe2D3.2, respectively.