Displacive ordering in the hydrogen sublattice of yttrium trihydride

Powder samples of YH3 and YD3 have been studied by neutron diffraction (ND) with a much higher statistical accuracy than obtained previously. The profile analysis of the obtained ND patterns confirmed the high-symmetry HoH3-type structure of YH3 and ruled out the 'broken symmetry' structures proposed recently to explain the insulating properties and lattice dynamics of this compound. At the same time, it was demonstrated that the HoH3-type structure is only the structure of the mean lattice of YH3. Large static displacements of H atoms from the symmetrical positions in this structure do occur, and ordering of these displacements on a short-range scale can settle the controversies between the crystal structure and physical properties of YH3.