Persistence of the Jahn–Teller distortion of Mo5+ in double perovskites: a structural study of Ba2NdMoO6 and the effect of chemical doping in Ba2Nd1xYxMoO6

The cation ordered perovskites Ba2NdMoO6 and Ba2Nd1-xYxMoO6 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: ; 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 Ba2Nd1-xYxMoO6 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 for Ba2Nd0.5Y0.5MoO6 [ ;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 systems. The paramagnetic moments show good agreement with those anticipated to arise from the spin-only contribution from Mo5+ (S=1/2, µso = 1.73 µB) and the moment of 3.62 µB associated with the spin-orbit coupling of the 4I9/2 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.