A neutron diffraction study of structural distortion and magnetic ordering in the cation-ordered perovskites Ba₂Nd_1-xY _xMoO₆

The cation ordered perovskites Ba₂Nd_1-xY _xMoO₆ (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 Ba ²⁺ 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. Ba ₂Nd_0.90Y_0.10MoO₆ 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 distortion of the MoO₆ units stabilises the 4d¹ electron in the d _xz and d_yz orbitals whilst the d_xy orbital is increased in energy due to the contraction of the Mo-O bonds in the xy-plane. This bond extension along z is propagated through the structure and gives a negative thermal expansion of -13×10^-6 K^-1 along c. The overall volumetric thermal expansion is positive due to conventional expansion along the other two crystallographic axes. With increasing Y ³⁺ 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 μ_B.