Magnetic Order Arising from Structural Distortion: Structure and Magnetic Properties of Ba₂LnMoO₆

The compounds Ba2LnMoO6 (Ln = Nd, Sm, Eu, Gd, Dy, Y, Er, and Yb) have been synthesized by solid-state techniques under reducing conditions at temperatures up to 1300 °C. Rietveld analyses of X-ray and neutron powder diffraction data show that these compounds adopt cation-ordered perovskite phases. At room temperature Ba2NdMoO6 and Ba2SmMoO6 adopt tetragonally distorted structures in the space groups I4/m and I4/mmm, respectively, while the data collected from all other compounds could be fitted in the cubic space group Fm3̄m. Bond valence sums show that the observed tetragonal distortions are driven by the bonding requirements of Ba2+. Neutron powder diffraction data collected below TN = 15(1) K show that Ba2NdMoO6 is triclinically distorted (I1̄: a = 5.9790(2) Å, b = 5.9840(2) Å, c = 8.6024(2) Å, α = 89.854(2)°, β = 90.056(5)°, γ = 90.003(5)°) and that Nd3+ and Mo5+ are antiferromagnetically ordered. Magnetic susceptibility data show that this compound behaves as a Curie−Weiss paramagnet above this temperature, and no other compounds in the series show evidence of magnetic order down to 2 K. Ba2YMoO6 and Ba2YbMoO6 both remain entirely paramagnetic to 2 K due to perfect geometric frustration of the cubic lattice, indicating that next-nearest-neighbor interactions between like cations dominate over nearest-neighbor Mo−O−Ln exchange. The magnetic structure of Ba2NdMoO6 is rationalized with reference to the splitting of the t2g manifold of Mo5+ by the Jahn−Teller distortion and the associated introduction of anisotropic magnetic superexchange.