Towards novel multiferroic and magnetoelectric materials: dipole stability in tetragonal tungsten bronzes

Abstract

We discuss the strategy for development of novel functional materials with the tetragonal tungsten bronze structure. From the starting composition Ba 6 GaNb 9 O 30 , the effect of A- and B-site substitutions on the dielectric properties is used to develop an understanding of the origin and stability of the dipolar response in these compounds. Both tetragonal strain induced by large B-site cations and local strain variations created by isovalent A-site substitutions enhance dipole stability but result in a dilute, weakly correlated dipolar response and canonical relaxor behaviour. Decreasing cation size at the perovskite A2-site increases the dipolar displacements in the surrounding octahedra, but insufficiently to result in dipole ordering. Mechanisms introducing small A-site lanthanide cations and incorporation of A-site vacancies to induce ferroelectricity and magnetism are presented.

Elastic and anelastic relaxations accompanying relaxor ferroelectric behaviour of Ba6GaNb9O30 tetragonal tungsten bronze from resonant ultrasound spectroscopy

“…b Comparison of f p 2 and Q -1 from fitting of two resonance peaks at the roomtemperature starting frequencies of *1 MHz (blue) and *930 kHz (black) (cooling only). c e 00 as function of temperature at 1 MHz, during cooling [6]. (Color figure online) such, the continued stiffening at low temperatures is also anomalous.…”

“…4c for comparison (see Ref. [6]). This overall pattern is closely similar to that seen for the classic relaxor ferroelectric Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) described by Carpenter et al [52] and is consistent with freezing of PNRs in a temperature interval below *150 K. The onset of elastic softening for a relaxor ferroelectric appears to coincide with the Burns temperature (i.e., see Ref.…”

“…In this paper, we present results for the elastic properties of the relaxor dielectric Ba 6 GaNb 9 O 30 (BGNO) ceramic material, with the TTB structure, obtained using resonant ultrasound spectroscopy (RUS), and relate the temperaturedependent elastic behaviour to the structural phase transitions previously observed by low-temperature dielectric spectroscopy [5,6]. Resonant ultrasound spectroscopy is a powerful thermal analysis technique-belonging to the mechanical subgroup together with the thermomechanical analysis (TMA) and dynamical mechanical analysis (DMA)-dedicated to the study of fundamental properties involving elasticity [44][45][46][47][48][49].…”

“…Nevertheless, some of these problems have been ameliorated either by finding new lead-free materials such as K 1-x Na x NbO 3 (piezoelectric) [3], doping by single site substitutions, or by solid solution formation between ferroelectric, antiferro-and ferromagnetic perovskites including BiFeO 3 -PbTiO 3 and BiFeO 3 -BaTiO 3 [4].gathered the attention of the research community. The TTB structure: (A1) 2 (A2) 4 (C) 4 (B1) 2 (B2) 8 O 30 , due to the presence of crystallographically nonequivalent A-and B-sites and an extra C-site, provides supplementary degrees of freedom for manipulation of the structure, huge compositional flexibility allowing the insertion of various metals into the five different TTB sites [5], nevertheless offering the possibility of fine-tuning both electrical and magnetic behaviour [6,7]. The TTB structure consists of a network of corner sharing BO 6 octahedra formed around the perovskitic A1 site that creates further two types of channels: pentagonal A2 channels (which can be occupied by alkali, alkaline earth and rare earth cations) and smaller triangular C channels (mostly vacant, they can be filled/ just partially filled by small low-charged cations like Li…”

“…A particular interest was developed regarding the Nb-based TTBs [15][16][17] due to their enhanced ferroelectric properties over other analogues such as Ta [18,19]. In the search for novel multiferroic and magnetoelectric materials, the effect of the A-site size in a family of unfilled ferroelectric TTBs Ba 4 RE 0.67 Nb 10 O 30 (RE = La, Nd, Sm, Gd, Dy, Y) [6] and of the A-site strain on dipole stability in fully filled TTBs family A 6 GaNb 9 O 30 (A = Ba, Sr, Ca) [7,20] was studied. In addition to their ferroelectric and/or magnetic behaviour, the majority of TTBs reported in the literature exhibit relaxor properties [21][22][23][24][25][26].…”