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

Abstract

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.

Insights into Cation Ordering of Double Perovskite Oxides from Machine Learning and Causal Relations

“…For formation of clear layered ordering, the charge separation at A- and B-sites plays an important role, as derived from ML models. These insights can be further validated by experimental studies on ordered double perovskites NaLnNiWO 6 , where Ln= La, Pr, Nd, Sm, Eu, Gd, and Tb . In this study, the authors have investigated theeffect of cationic charges on cation ordering.…”

“…At its current stage, these data on different A-site cation ordering arrangements are not available in literature. Most of the experimental observations have reported compounds where A-site layered ordering is favorable. − Thus, our computational data can become a good addition for further explorations. The study is intended to aid into interpretability as well as discovery of cause–effect relationships (going beyond the standard practices of ML to uncover structure–property relationships) between features representing cation ordering in double perovskites.…”

“…These insights can be further validated by experimental studies on ordered double perovskites NaLnNi-WO 6 , where Ln= La, Pr, Nd, Sm, Eu, Gd, and Tb. 42 In this study, the authors have investigated theeffect of cationic charges on cation ordering. The rock-salt ordering of B-site cations helps to keep highly charged cations farther apart from each other enabling ordering in AA'BB'O 6 .…”

“…In this case, a cutoff energy difference of −32 meV is set. This choice is driven by the fact that double perovskite systems most likely exhibit clear layered ordering as validated by experimental observations. − More compounds may show clear layered ordered phases if the cutoff energy is lowered and vice versa. We specify the cutoff at −32 meV for the following reasons.…”

Switchable and Nonswitchable Polarization in Doubly Ordered Perovskites NaLnCoWO6 (Ln = Er, Tm, Yb, and Lu)

“…The neutron diffraction studies of NaYNiWO 6 revealed a spin density wave structure just below T N and transforms to a collinear structure below 18 K. 26 On the other hand, the doubly ordered perovskites NaLnMnWO 6 (Ln = La, Nd, and Tb), NaLnFeWO 6 (Ln = La, Nd, Pr, and Sm), and NaLnNiWO 6 (Ln = La, Pr, Nd, and Sm−Tb) did not exhibit any change in polarization below magnetic ordering temperature. 28,36,37 It could be due to a small polarization that is below the instrument's resolution limits. Theoretically, it has been studied that the compounds with lower size rare earth cations with increasing size difference at the A-site exhibit larger octahedral distortion, and hence, higher polarization is expected.…”