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 T
N but show a
dielectric anomaly.