Structural and multiferroic properties of doubly ordered perovskites NaLnNiWO₆ (Ln = Dy-Lu) Ravi Shankar P N, and A. Sundaresan* School of Advanced Materials, and Chemistry and Physics of Materials Unit, Jakkur P.O., Bangalore 560064, India Email: ravishankar@jncasr.ac.in

Double perovskites, which exhibit simultaneous ordering in both A-site and B-site remarked doubly ordered perovskites. A family of doubly ordered perovskites NaLnNiWO₆ were synthesized under high pressure (4.5 GPa) and high temperature (1000 °C) using high-pressure apparatus. PXRD refinements of NaLnNiWO₆ compounds at RT reveal these compounds exhibit simultaneous uncommon layer ordering of A-site cations (Na, Ln) and rock-salt ordering of B-site cations (Ni, W) in the crystal structure, and they crystallize in the polar P2₁ space group.  Magnetic studies and heat capacity data show that these compounds exhibit antiferromagnetic ordering with T_N varying in the range of 19-23 K. The neutron powder diffraction data of NaYNiWO₆ has solved using propagation vectors (0.49, 0, 0.41) at 20 K and (1/2, 0, 1/2) at 18 K and below this temperature, up to 1.5 K. The magnetic structure of NaYNiWO₆  is antiferromagnetic spin density wave at 20 K which transforms to a simple collinear structure, below 18 K. These compounds exhibit switching of polarization at magnetic ordering temperature, confirming the multiferroic properties below T_N.