Octahedral tilting in the polar hexagonal tungsten bronzes RbNbW₂O₉ and KNbW₂O₉

Abstract

The ambient‐temperature structures (orthorhombic, space group Cmc21) of the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9 have been determined by high‐resolution powder neutron diffraction. Displacement of the A‐site cation along the polar c axis with concomitant octahedral tilting occurs to optimize the A cation bonding environment, hence reducing the coordination from 18 to 16. This effect is more evident in KNbW2O9 due to decreased A cation size. The octahedral tilting in both compositions results in a doubling of the c axis that has not previously been reported, highlighting the importance of neutron diffraction as a complementary technique for structural determination of such systems.

Kagomé networks of octahedrally coordinated metal atoms in minerals: Relating different mineral structures through octahedral tilting

“…The HTB oxides, ANbW 2 O 9 , A = K and Rb, also have the octahedral tilt system shown in Fig. 2c, with a lowering of space group symmetry to Cmc 2 1 to take account of polar displacements of the B -site cations (McNulty et al , 2019).…”

Corrigendum to: “Phase transitions in the hexagonal tungsten bronze RbNbW2O9” [J. Solid State Chem. 286 (2020) 121275]

“…Samples were prepared via traditional solid-state synthesis using the methodology presented in our previous work. [10] Sacrificial powder, comprised of the unreacted reagents, was used to cover the pellets to minimise alkali metal loss. Pellets for electrical measurements were mixed with a small amount of polyvinyl alcohol (PVA) solution to act as a binding agent and pressed under 200 MPa using an isostatic oil press.…”

“…In our previous work we have shown that the ambient temperature structure is orthorhombic, space group Cmc21 (a ~7.3, b ~12.7, c ~7.8 Å). [10] A minor pyrochlore impurity phase (RbNbWO6, ~1 wt. %) is present, however this was not included in refinements due to the minimal contribution to the observed diffraction pattern.…”

“…This leads to a doubling of the c-axis, resulting in orthorhombic space group Cmc21 (aO ~ a0, bO ~ √3a0, cO ~ 2c0). [10] We now report the first variable temperature study of RbNbW2O9 using high-resolution PND.…”

“…The most recent examples of these materials have focused on the structures of the ANbW2O9 series with A = K + , Rb + and Cs + . [8][9][10] Investigations into the structure-property relationships of CsNbW2O9 have shown this material to be an electronically-driven improper ferroelectric with an interesting domain microstructure that may potentially be useful in domain wall nanoelectronics. [9] Although neither RbNbW2O9 nor KNbW2O9 appear to be improper ferroelectrics, recent work involving detailed crystallographic characterisation of the ambient temperature structures utilising high-resolution powder neutron diffraction (PND) has shown that both compositions feature octahedral tilting at low temperature, as also observed in CsNbW2O9.…”