X-ray and neutron powder diffraction study of the order–disorder transition in Eu 2IrH 5 and the mixed crystal compounds Eu 2− xAxIrH 5 ( A=Ca, Sr; x=1.0, 1.5)

Abstract

The title compounds and their deuterides have been prepared by solid-state and solid–gas reactions from the elements and investigated by X-ray and neutron powder diffraction as a function of temperature. At room temperature they crystallize with an anion-deficient cubic K 2PtCl 6-type structure (space group Fm 3 ̄ m ) in which five hydrogen (deuterium) atoms surround iridium randomly on six octahedral sites with average bond distances of Ir–D=169–171 pm. At low temperature they undergo a tetragonal deformation (space group I4 /mmm) to the partially ordered Sr 2IrD 5 ( T=4.2 K )-type structure in which four hydrogen (deuterium) atoms occupy planar sites with full occupancy (Ir–D=166–170 pm) and two hydrogen (deuterium) atoms axial sites (Ir–D=174–181 pm) with ∼50% occupancy, i.e., the data are consistent with a mixture of square–pyramidal [IrD 5] 4− complexes pointing in two opposite directions. The transitions occur at ∼240 K (Eu 0.5Ca 1.5IrD 5, Eu 0.5Sr 1.5IrD 5), ∼210 K (EuSrIrD 5), ∼200 K (EuCaIrD 5, Eu 2IrD 5), and are presumably of first order.