Elaboration, structures, and phase transitions for YFe2Dx compounds (x = 1.3, 1.75, 1.9, 2.6) studied by neutron diffraction

The structure of YFe2Dx deuterides (x = 1.3, 1.9, 1,75, and 2.6) have been studied by neutron diffraction (ND). The influence of a thermal treatment o n the synthesis of YFe2Dx deuterides has been followed by in situ neutron d iffraction for x = 1.3 and 1.9. After deuterium absorption at room temperat ure, their diffraction patterns show a mixture of YFe2 and a deuteride phas e YFe2Dy (2.3<y<2.6). Heating the samples above 400 Ii for x = 1.3 and 425 Ii for x = 1.9, leads to a deuterium diffusion from the deuterium rich phas e toward YFe2 to form a single phase deuteride with lower D content. Single phase deuterides YFe2D1.3, YFe2D1.75, and YFe2D1.9 display superstructure lines which imply a lowering of symmetry compared to the cubic C15 structur e of the parent intermetallic. YFe2D1.3 crystallizes in the tetragonal <I(4 )over bar > space group (Z = 20) with a = 11.985(2)Angstrom and 7.622(1) An gstrom, YFe2D1.75 in the cubic I(4) over bar 3 m space group (Z = 64) with a 15.336(2) Angstrom, and YFe2D1.9 in a tetragonal structure with a = 12.14 5(2) Angstrom and c = 23.064(2) Angstrom. The analysis of their ND patterns at 300 Ii shows for x = 1.3 and 1.75 an ordering of the deuterium atoms in some preferential tetrahedral Y2Fe2 sites and a ferromagnetic structure wi th 1.5 and 1.9 mu B/Fe, respectively. For YFe2D1.3 and YFe2D1.9 the superst ructure lines disappear above 460 and 444 K, respectively, and they return to a C15 structure. The origin of the structural deviation and of the order -disorder transition is discussed in relation with deuterium and magnetic o rdering. YFe2D26 deuteride crystallizes at 300 K in the cubic Fd3m space gr oup (Z = 8) with a = 7.785(2), deuterium atoms located statistically in tet rahedral Y2Fe2 sites and a ferromagnetic structure with a magnetic moment o f 1.8 mu B/Fe. (C) 1999 Academic Press.