Structural investigation of tungsten bronze-type relaxor ferroelectrics in the Ba_xSr_3−xTiNb₄O₁₅system

Abstract

Compounds in the BaxSr3−xTiNb4O15 series with x = 0.0–3.0 were synthesised. Synchrotron X-ray diffraction data were collected for each member synthesised. Rietveld refinements were performed for all compositions. A composition-dependent phase transition between orthorhombic and tetragonal symmetries was found. Cation ordering was observed between barium and strontium atoms in the two distinct A site cavities

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)

“…The distribution of cations on the two A-sites in BCNT and BSNT is found to be consistent with what is previously reported for BNN, BKN and BRN, in terms of larger M-cations leading to a lower occupancy of M-cations on the A1-site due to cross occupation of Ba and M on the two A-subsites. 7 The A-site cross occupation observed in BSNT contrasts with previous report by Whittle and Schmid 50 but is in good agreement with a recent XPS study. 55 The thermodynamics calculations by DFT also indicate a drive for cation disorder in all three BMN compounds.…”

“…The crystal structure of SNT is orthorhombic with space group Pna 2 1 resulting from polar distortion Γ 3 − and Z 5 + (P3) octahedral tilting. 50 The instability found in SNT was also identified by first principles calculations 10 in SrNb 2 O 6 , while this instability was not found for BaNb 2 O 6 . All the A-sites are filled with Sr in SNT, which means that a specific size difference between A1- and A2-cations is not required to stabilize the orthorhombic crystal structure.…”

“…In the solid solution series (Ba,Sr) 6 Nb 8 Ti 2 O 30 , a shift from tetragonal to orthorhombic symmetry takes place at about 0.2-0.3 Ba-content. 12,50 So, an orthorhombic symmetry is realised as the A-site average ionic radius is reduced with increasing Srcontent. Yet, tetragonal Sr 4 Ba 2 Ti 2 Nb 8 O 30 has A-cations with smaller average ionic radii than orthorhombic BCNT, meaning that the mean size of the A-cations is not sufficient to rationalise whether tetragonal or orthorhombic structure is exhibited.…”

Multi-analyser detector (MAD) for high-resolution and high-energy powder X-ray diffraction

“…The structure models consisted of three phases of STN. The main phase, Sr 3 Nb 4 TiO 15 , crystallizes with space group Pna2 1 (Whittle & Schmid, 2014;Whittle et al, 2017). Additionally, two impurity phases were identified.…”