A neutron diffraction study of structural distortion and magnetic ordering in the cation-ordered perovskites Ba2Nd1−xYxMoO6

The cation ordered perovskites Ba2Nd1-xYxMoO6 (0.04 ≤ x ≤ 0.35) have been synthesised by solid-state techniques under reducing conditions at temperatures up to 1350°C. Rietveld analyses of X-ray and neutron powder diffraction data show that these compounds adopt a tetragonally-distorted perovskite structure. The tetragonal distortion is driven by the bonding requirements of the Ba2+ cation that occupies the central interstice of the perovskite; this cation would be underbonded if these compounds retained the cubic symmetry exhibited by the prototypical structure. The size and charge difference between the lanthanides and Mo5+ lead to complete ordering of the cations to give a rock-salt ordering of Nd3+/Y3+O6 and MoO6 octahedra. The I4/m space group symmetry is retained on cooling the x = 0.1, 0.2 and 0.35 samples to low temperature ca. 2 K. Ba2Nd0.90Y0.10MoO6 undergoes a gradual distortion of the MoO6 units on cooling from room temperature to give two long trans bonds (2.001(2) Å) along the z direction and four shorter apical bonds (1.9563(13) Å) in the xy plane. This extension is propagated through the structure and gives negative thermal expansion of -13×10-6 K-1 along c. With increasing Y3+ content this distortion is reduced in x=0.2 and eliminated in x=0.35 which contains largely regular MoO6 octahedra. The x=0.1 and x=0.2 show small peaks in the neutron diffraction profile due to long range antiferromagnetic order arising from ordered moments of ca. 2 μB.