Structural, Magnetic, and Electrical Properties of Doubly Ordered Perovskites NaLnNiWO6 (Ln = La, Pr, Nd, Sm, Eu, Gd, and Tb)

We report the structural, magnetic, and electrical properties of a new family of doubly ordered perovskites NaLnNiWO6 (Ln = La, Pr, Nd, Sm, Eu, Gd, and Tb), synthesized at high-pressure (4.5 GPa) and high-temperature (1000 °C) conditions. These materials crystallize in the monoclinic structure with the polar space group P21 as revealed by neutron diffraction (Ln = La), X-ray diffraction, and second-harmonic-generation (SHG) studies. These compounds exhibit a combined layered ordering of A-site cations and rock-salt ordering of B-site cations with octahedral tilting, which breaks the inversion symmetry and results in a polar structure. The magnetic and heat capacity measurements reveal the antiferromagnetic order of Ni2+ ion moments in the temperature range of 23–30 K, depending on the Ln ion. Analysis of neutron powder diffraction data for the La compound reveals that the Ni2+ spins couple antiferromagnetically along the a- and c-directions and ferromagnetically along the b-axis, consistent with the propagation vector, k = (1/2, 0, 1/2). In contrast to the compounds with smaller lanthanide ions (Ln = Y, Dy, Ho, and Yb), these compounds do not exhibit measurable electric polarization at TN but show a dielectric anomaly.