Large ferroelectric polarization in the new double perovskite NaLaMnWO6 induced by non-polar instabilities
Abstract
Based on density functional theory calculations and group theoretical analysis, we have studied NaLaMnWO6 compound which has been recently synthesized [G. King, A. Wills and P. M. Woodward, Phys. Rev. B: Condens. Matter, 2009, 79, 224428] and belongs to the AA′BB′O6 family of double perovskites. At low temperature, the structure has monoclinic P21 symmetry, with layered ordering of the Na and La ions and rocksalt ordering of Mn and W ions. The Mn atoms show an antiferromagnetic collinear spin ordering, and the compound has been reported as a potential multiferroic. By comparing the low symmetry structure with a parent phase of P4/nmm symmetry, two distortion modes are found dominant. They correspond to MnO6 and WO6 octahedron tilt modes, often found in many simple perovskites. While in the latter these common tilting instabilities yield non-polar phases, in NaLaMnWO6 the additional presence of the A–A′