In-Plane Cation Ordering and Sodium Displacements in Layered Honeycomb Oxides with Tetravalent Lanthanides: Na₂LnO₃ (Ln = Ce, Pr, and Tb)

Abstract

The detailed structural characterization of "213" honeycomb systems is a key concern in a wide range of fundamental areas, such as frustrated magnetism, and technical applications, such as cathode materials, catalysts, and thermoelectric materials. Na 2 LnO 3 (Ln = Ce, Pr, and Tb) are an intriguing series of "213" honeycomb systems because they host tetravalent lanthanides. "213" honeycomb materials have been reported to adopt either a cation-disordered R3̅ m subcell, a cationordered trigonal (P3 1 12), or monoclinic (C2/c or C2/m) supercell. On the basis of analysis of the average (synchrotron diffraction) and local [pair distribution function (PDF) and solid-state NMR] structure probes, cation ordering in the honeycomb layer of Na 2 LnO 3 materials has been confirmed. Through rationalization of the 23 Na chemical shifts and quadrupolar coupling constants, the local environment of Na atoms was probed with no observed evidence of cation disorder. Through these studies, it is shown that the Na 2 LnO 3 materials adopt a C2/c supercell derived from symmetry-breaking displacements of intralayered Na atoms from the ideal crystallographic position (in C2/m). The Na displacement is validated using distortion index parameters from diffraction data and atomic displacement parameters from PDF data. The C2/c supercell is faulted, as evidenced by the increased breadth of the superstructure diffraction peaks. DIFFaX simulations and structural considerations with a two-phase approach were employed to derive a suitable faulting model.

Uncovering the Structure of Heterogeneous Catalysts Using Atomic Pair Distribution Function Analysis

“…PDF has been also applied to unravel complex disorder in layered structures [16,[35][36][37][38][39][40][41][42]. For example, δ-MnO2 is a layered material which has attracted attention as catalyst for water splitting [43,44].…”

Ba9RE2(SiO4)6 (RE = Ho–Yb): A Family of Rare-Earth-Based Honeycomb-Lattice Magnets

“…Another difference is that the exchange interactions between RE 3+ local moments are relatively small, which let us safely consider the dominance of nearest-neighbor 18,31 Unfortunately, the experimental identification on Kitaev QSL states remains to be challenging since most discovered RE-based Kitaev materials form a magnetic ordered phase at low temperatures. For example, the relatively well-studied YbCl 3 exhibits a short-range magnetic order below 1.20 K and long-range AFM order with a Neél temperature T N of ∼0.60 K, 24 and Na 2 PrO 3 enters into the AFM state below a T N of ∼4.6 K. 25 From the structural viewpoint, magnetic RE ions in most RE-based honeycomb magnets including YbCl 3 , Yb 2 Si 2 O 7 , and Na 2 PrO 3 are arranged on a distorted honeycomb lattice not a perfect honeycomb lattice, 24,26,27 which may introduce additional anisotropic exchange interactions precluding the formation of the Kitaev QSL state. Thus, exploring new RE-based materials with a perfect honeycomb lattice are imperative to study the Kitaev QSL physics.…”

Ground-state phase diagram of spin-$S$ Kitaev-Heisenberg models

“…The representative examples of the candidate materials are Na 2 IrO 3 [16][17][18][19][20][21][22][23][24][25][26], α-Li 2 IrO 3 [18,20,26], and α-RuCl 3 [26][27][28][29][30]. In recent years, a number of new candidates have been proposed, such as cobalt compounds [31][32][33][34], iridium ilmenites [35][36][37], and f -electron compounds [38][39][40][41][42].…”