Poster Presentation The 35th Biennial Conference of the Society of Crystallographers in Australia and New Zealand 2024 (Crystal 35)

Investigating local order in vacancy ordered double perovskites (#215)

Caleb Bennett 1 , Brendan Kennedy 1
  1. Chemistry, The University of Sydney, Camperdown, NSW, Australia

The family of antifluorite-type hexahalides A2MX6 (A = alkali metals; M = metal; X = halides) feature isolated MX6 octahedra that are separated by alkali metal cations.1 The M-type cations form an fcc lattice. An alternate description of the A2MX6 structure is as a rock salt ordered double perovskite of the type A2MM’X6 where the M’ site is vacant, leading to the nomenclature of vacancy-ordered double perovskites (VODPs). These halides were actively studied as model systems for exploring NN-NNN (next nearest neighbour) fcc magnetism, lattice dynamics, and structural phase transitions. Despite the absence of direct connectivity between the MX6 octahedra, cooperative tilting of these, like in AMX3 perovskites, can occur. This leads to symmetry lowering structural phase transitions. The extraordinary optical and electronic properties of lead halide perovskites, such as CH3NH3PbX6, have spawned countless studies of these materials over the past decade. More recently, the VODPs have emerged as a flexible scaffold that enables the lead cation to be replaced with an environmentally benign metal whilst retaining the compositional and structural flexibility of the perovskite structure that can be exploited to optimise its desirable properties.2 When M is a second or third row transition metal, such as Re, Os or Ir, antiferromagnetic ordering can occur. Although the strong spin-orbit coupling of the Ir and Os cations means these are not simple Heisenberg systems. K2IrCl6 is of particular interest in the study of Jeff = ½ systems3 and in the elucidation of the role that Kitaev interactions may play in establishing their magnetic properties. In recent work on K2OsCl6 and K2OsBr64 using high resolution synchrotron X-ray diffraction (SXRD) showed that these compounds have a cubic VODP structure but undergo different symmetry-lowering structural phase transitions upon cooling, associated with a combination of the relative size of the ions and differences in their chemical bonding. K2IrCl6 which shows long range magnetic ordering around 20 K but remains cubic down to 0.3 K. The absence of a symmetry lowering structural phase transition is incompatible with the reported splitting of the Jeff = 3/2 quartet into two doublets observed by resonant inelastic X-ray scattering (RIXS). A recent Raman study by Lee5 provided evidence for the continuous development of noncubic local distortions in K2IrCl6 upon cooling to 4 K. This conflicts with the absence of any symmetry lowering structural transition evident in earlier diffraction studies. Using a combination of TOPAS6 and RMCprofile the X-ray pair distribution function (PDF) of K2IrCl6 has been observed to exhibit lower local symmetry (in that of monoclinic P21/n) than what is observed in the long-range average structure which is cubic (Fm-3m). There is a clear transition between the average structure being cubic at temperatures above 200 K, and below, where the local structure becomes monoclinic. Other M-site metals (Pt, Sn, Os) are included in the analysis of local structure and show no difference between the long range and short-range structures, highlighting the findings of the K2IrCl6 system.

  1. 1 Fedorovskiy, A. E. et al. Small Methods 4, 1900426 (2020).
  2. 2 Maughan, A. E. et al. Chemistry of Materials 31, 1184-1195 (2019).
  3. 3 Reig-I-Plessis, D. et al. Physical Review Materials 4 (2020).
  4. 4 Saura-Múzquiz, M. et al. Inorganic Chemistry 61, 15961-15972 (2022).
  5. 5 Lee, S. et al. Physical Review B 105 (2022).