Persistence of the Jahn–Teller distortion of Mo5+ in double perovskites: a structural study of Ba2NdMoO6 and the effect of chemical doping in Ba2Nd1−xYxMoO6

Abstract

The cation-ordered perovskites Ba₂NdMoO₆ and Ba₂Nd_1−xY_xMoO₆ have been structurally characterised by a combination of neutron and X-ray powder diffraction. Ba₂NdMoO₆ retains the tetragonal room temperature structure on cooling to 150 K [I4/m; a = 5.98555(5) Å, c = 8.59510(10) Å] although the MoO₆ octahedra distort with an elongation of two trans Mo–O bonds. Neutron diffraction data collected at T ≤ 130 K show that this compound has undergone a structural distortion to a triclinic space group, although the MoO₆ octahedra do not distort any further on cooling below this temperature [at 130 K: I; 5.97625(14) Å, 5.9804(2) Å, 8.59650(13) Å, 89.876(2)°, 89.921(3)°, 89.994(2)°]. The room temperature tetragonal space group symmetry of Ba₂NdMoO₆ is preserved in the series Ba₂Nd_1−xY_xMoO₆ up to composition 0.35 ≤ x < 0.5. The lattice parameters converge as the value of x increases until cubic symmetry is reached for the composition of Ba₂Nd_0.5Y_0.5MoO₆ [Fmm; a = 8.4529(3) Å]. Magnetic susceptibility measurements show that all of these compounds display the Curie–Weiss behaviour associated with a fully localised electronic system. The paramagnetic moments show good agreement with those anticipated to arise from the spin-only contribution from Mo⁵⁺ (S = 1/2, µ_so = 1.73 µ_B) and the moment of 3.62 µ_B associated with the spin–orbit coupling of the ⁴I_9/2 ground state of Nd³⁺. For x ≤ 0.125 this series shows a magnetic transition in the range 10 to 15 K indicative of a distortion of the MoO₆ octahedra in these compounds that is similar to Ba₂NdMoO₆.