Revisiting the structures and phase transitions of Ba₂NaNb₅O₁₅

Abstract

The room-temperature and low-temperature structure(s) of Ba2NaNb5O15 (BNN) have been debated since the structure was proposed in the 1960s. This work revisits the structures and phase transitions of BNN, combining high-resolution X-ray and neutron powder diffraction with density functional theory calculations. Temperature-dependent high-resolution X-ray powder diffraction patterns are collected from 4 to 918 K, and sequential batch Rietveld refinement using a symmetry mode approach to describe the structure is used to extract the main structural changes as a function of temperature. The data show that the average structure of BNN is best described by the Ama2 space group, and no other structural phase transitions were observed below the ferroelastic transition. The symmetry mode analysis, combining results from diffraction and density functional theory, shows significant octahedral tilting and corrugations of both the A1 and A2 sites along the c direction. A strong correlation between the spontaneous strain and the octahedral tilting was observed, and a potential connection with emerging microstructure at low temperatures is proposed, all enabled by the symmetry mode approach used in this work.

The effect of cation size on structure and properties of Ba-based tetragonal tungsten bronzes Ba4M2Nb10O30 (M = Na, K or Rb) and Ba4M2Nb8Ti2O30 (M = Ca or Sr)

“…7 The crystallographic data, for all five compositions, including the space group, lattice parameters and occupation of M-cation on A1 site are included in Table 1. The ambient temperature lattice parameters increase systematically with the size of M. BCNT was refined towards the same orthorhombic crystal structure as reported for BNN, 19,43 while BSNT have the same tetragonal crystal structure as reported for BKN and BRN. Thus, the two compounds with the smallest M-cations have orthorhombic symmetry, while the compounds with larger M-cations have a tetragonal symmetry.…”

“…43 For BNN, it has been an ongoing discussion related to what is the true room temperature space group. The Ama2 space group, first suggested by Labbe et al, 53 has recently been confirmed most probable based on group theory and thorough literature review by Whittle et al, 54 as well as by Grendal et al 19 through their experimental work. Grendal and co-workers also demonstrate how tilting occurs with the ferroelastic transition and is not allowed at temperatures above said orthorhombic distortion.…”

“…A fundamental parameter model described the peak shape, and the background was described by a Chebychev polynomial of minimal degree. An Ama 2 19 structure model was used to refine the BCNT and BNN data, and all symmetry inequivalent ion positions were refined. For BSNT a P 4 bm structure 6,20 was used in the refinement and symmetry inequivalent ion positions for Ba, M, and B-cation positions were refined, while all the O-positions were kept constant.…”

“…The number of independent parameters that were refined was 33. Unit cell parameters: a = 12.69931(19) Å and c = 4.00135(5) Å. GOF: 2.13, R wp : 5.92.…”