LATTICE-PARAMETERS OF THE BETA-PHASE RARE-EARTH HYDRIDES RH2+X (R=Y, GD, TB, DY)

The lattice parameters, a, of the f.c.c. Beta-phase of the rare-earth dihydrides RH-2+x (R = Y, Gd, Tb, Dy) have been determined by X-ray di ffractometry in the temperature range between 90 and 295 K, for variou s values of the excess H-concentration on octahedral sites, x. The gen erally linear contraction of a with x up to the beta-phase limit is co nfirmed at room temperature; the contraction coefficients beta = (1/a) DELTAa/DELTAx are for YH2+x: -0.50 x 10(-2), for GdH2+x: -0.65 x 10(-2 ), for TbH2+x: -0.85 X 10(-2), for DyH2+x: -0.57 x 10(-2) per atom H. At 90 K, the contraction is generally stronger than at 295 K, and devi ates from linearity for higher values of x indicating an interaction b etween the octahedral H-atoms leading to eventual ordering. This order ing is seen on the thermal expansion a(T)-curves as an x-dependent ano maly, its temperature increasing with growing x. The beta-phase limit, x(beta)max, has been deduced for YH2+x: x(beta)max = 0.100(5), and fo r DyH2+x:x(beta)max = 0.23(1).