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 distortion of the MoO6 units stabilises the 4d1 electron in the dxz and dyz orbitals whilst the dxy 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 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. - Graphical Abstract: The distortion in the molybdenum crystal field is continuously adjusted by chemical composition of the perovskite. Highlights: ► Introducing Y3+ into Ba2NdMoO6 stabilises tetragonal symmetry to 2 K. ► A distortion of the ligand field around the Mo5+ 4d1 cation confers electronic stabilisation. ► The size of the distortion is progressively reduced with increasing Y3+ content. ► Distortion gives negative thermal expansion along z and antiferromagnetic order at T≤15 K