Phase transitions in the hexagonal tungsten bronze RbNbW2O9

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Highlights

  • The phase transition sequence is driven by loss of an octahedral tilting mode, A6+.

  • Followed by loss of a second octahedral tilting mode, A3+.

  • Significantly different phase transition sequence between related Rb and Cs compositions.

Abstract

The hexagonal tungsten bronze RbNbW2O9 is shown, by variable-temperature powder neutron diffraction and symmetry-mode analysis, to display a significantly different phase transition sequence compared to the related CsNbW2O9 composition. At ambient temperature, RbNbW2O9 adopts the polar orthorhombic space group Cmc21. Upon heating, the thermal evolution of the crystal structure proceeds via two transitions. These correspond to sequential loss of two distinct octahedral tilting modes, leading to space group P63mc at around 655K, and space group P6mm near 700 ​K. The polar distortion is retained up to the highest temperature studied here. The differences in structural behaviour between the proper ferroelectric RbNbW2O9 and the improper ferroelectric CsNbW2O9 emphasises the need for careful crystallographic analyses of materials of this type.

Graphical abstract

The thermal evolution of the phase behaviour and crystal structure of the polar hexagonal tungsten bronze RbNbW2O9 has been characterised using powder neutron diffraction supported by symmetry-mode analysis and dielectric measurements.

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Introduction

Noncentrosymmetric materials have been studied extensively due to their wide range of symmetry-dependent properties and the pursuit of tuneable structural and electrical behaviour [1]. The perovskite and tetragonal tungsten bronze (TTB) families have undergone extensive studies due to their compositional flexibility [[2], [3], [4], [5]] but a related family of materials, the hexagonal tungsten bronzes (HTBs), have only recently begun to garner similar interest. This structure type is derived from the hexagonal polymorph of tungsten trioxide (h-WO3) and consists of corner-linked BO6 octahedra forming hexagonal and trigonal channels down the c-axis. The unit cell of the aristotype crystal structure has space group P6/mmm with a0 ~ 7.4, c0 ~ 4.0 ​Å. This structure can also be stabilised by intercalation of a cation into the hexagonal channel resulting in the general formula AxBO3 (0.15 ≤ x ​≤ ​0.33), where A is usually a large group 1 metal such as K+, Rb+ and Cs+ and B is usually WV/WVI [6]. These materials often exhibit metallic and conductive behaviour, however this can be reduced by replacement of WV with d0 cations, resulting in the formula ABW2O9. Although this type of compositional manipulation has been studied extensively [7], there are few examples of the crystal structure being either assigned unambiguously or related to physical property measurements. 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. 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. In particular, we use both symmetry mode analysis and physical property measurements to explore systematically the underlying mechanisms of symmetry-breaking.

Section snippets

Synthesis

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. The pellet was sintered at 1223 ​K for 1 ​h and subsequently polished using

Thermal evolution of the crystal structure

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.

Due to the relative insensitivity of lattice parameters to the exact space group, preliminary Rietveld refinements were carried out in Cmc21 over the entire temperature range

Discussion

The crystal structure of the ambient temperature phase, Cmc21, of RbNbW2O9 has been established [10], however, the two higher temperature (P63mc and P6mm) phases, have not been reported previously. The general trends observed in tilting behaviour with increasing temperature for RbNbW2O9 are similar to those observed in CsNbW2O9, but the absence of an additional cell-tripling (K3) mode in the Rb-compound indicates a fundamental difference between the two systems. The thermal evolution of the B–O

Summary and conclusions

The thermal evolution of the phase behaviour and crystal structure of the polar hexagonal tungsten bronze RbNbW2O9 has been characterised using powder neutron diffraction supported by symmetry-mode analysis and dielectric measurements.

The phase transition sequence in RbNbW2O9 has been shown to be driven by loss of an octahedral tilting mode (A6+) followed by the loss of a second octahedral tilting mode (A3+) with increasing temperature. This is expected to be followed by the loss of polarity,

Funding

We thank the Science and Technology Facilities Council (STFC) for the provision of neutron diffraction facilities at ISIS (HRPD experiment RB1710021, https://doi.org/10.5286/ISIS.E.RB1710021) and the School of Chemistry, University of St Andrews for funding of a studentship to JAM through the EPSRC doctoral training grant (grant No. EP/K503162/1). The research data supporting this publication can be accessed at (https://doi.org/10.17630/0e237288-4392-4cd0-b4d9-47409c824dac).

CRediT authorship contribution statement

Jason A. McNulty: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Alexandra S. Gibbs: Investigation, Writing - original draft, Writing - review & editing. Philip Lightfoot: Writing - original draft, Writing - review & editing. Finlay D. Morrison: Conceptualization, Writing - original draft, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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