Role of Lone Pair Cations in Ferroelectric Tungsten Bronzes Gerhard Henning Olsen, †,§ Magnus Helgerud Sørby, ‡ Sverre Magnus Selbach, † and Tor Grande* ,† † Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, NO-7491 Trondheim, Norway ‡ Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway ABSTRACT: The role of lone pair cations in tetragonal tungsten bronze (TTB) ferroelectrics has so far not been addressed in detail despite the importance of lone pairs for the polarization mechanism in the prototype ferroelectric perovskite PbTiO 3 . We report a combined experimental and computational study of the effect of lone pairs in ferroelectric tungsten bronzes with particular emphasis on the important high-temperature piezoelectric lead metaniobate (PN). The ambient crystal structure of PN is revised based on X-ray and neutron powder diffraction. The most likely cation-vacancy configurations identified by the structural analysis were assessed by electron density functional theory (DFT) calculations. The ferroelectric transition was characterized by high-temperature X-ray diffraction, and the origin of the ferroelectric polarization was studied by DFT, emphasizing the relationship between polarization and cation-vacancy ordering. Covalency between Pb and O is identified as the driving force for the orthorhombic distortion of the unit cell of PN and the polarization in-plane with respect to the chains of corner-sharing octahedra. Finally, to further elucidate the role of lone pairs in ferroelectric TTBs polar lattice instabilities and resulting polarization in the TTB model system K 4 R 2 Nb 10 O 30 (R = La, ..., Gd, or Bi) were investigated by DFT. ■ INTRODUCTION Oxides with tetragonal tungsten-bronze (TTB) structure constitute the second largest group of oxide ferroelectrics, surpassed only by perovskites. 1,2 They are described by the general formula (A1) 2 (A2) 4 C 4 (B1) 2 (B2) 8 O 30 , where the B1 and B2 cations (typically Nb 5+ ) and oxygen anions form a network of corner-sharing BO 6 octahedra. 3 The A1 and A2 sites form channels that extend throughout the crystal structure and are fully or partially occupied by cations relatively larger than the B cations such as alkali or alkaline earth metals. The C channels are narrow and can be occupied only by smaller cations such as Li + or Nb 5+ and are often completely vacant. 4,5 The TTB structure is flexible both from a chemical and structural viewpoint, resulting in a large number of compositions that crystallize in this structure. One of the simplest is lead metaniobate, Pb 5 Nb 10 O 30 (PN), which is commercially available as a high-temperature piezoelectric material, owing to its ferroelectric properties and high T C of 570 °C. 6 Although the attractive properties of PN with TTB structure have been known for decades, the mechanism for ferroelec- tricity in PN has so far not been described in detail. It was recently shown that in the isostructural strontium barium niobate system, (Sr x Ba 1-x ) 5 Nb 10 O 30 (SBN), a second-order Jahn-Teller mechanism acting on Nb 5+ , is responsible for the ferroelectric polarization, which is oriented along [001] in the tetragonal unit cell. 7 PN, on the other hand, has an orthorhombic unit cell with the polarization oriented along [110] with respect to the aristotype. A possible explanation for this difference is the 6s 2 electron configuration of Pb 2+ , which is known to influence the ferroelectric distortion in lead titanate, PbTiO 3 , where Pb-O covalency leads to a tetragonal lattice strain, which in turn stabilizes the tetragonal polarization. 8 Insight into this “lone pair effect”, assisted by first-principles calculations, 9 has resulted in tremendous attention toward lone pair cations such as Bi 3+ and Sn 2+ in the ongoing search for new lead-free ferroelectrics. 10 The lone pair on Pb 2+ has been suggested as the origin of the in-plane polarization in the ferroelectric TTB Pb 2 KNb 5 O 15 (PKN), 11 but as PKN is a filled TTB, the effect of cation-vacancy disorder still remains unclear. A further complication originates from the fact that TTBs often possess modulated crystal structures. 12,13 The existence of modulated structures, commensurate or incom- mensurate with the underlying crystal lattice, has been empirically linked to ferroelectricity in TTBs. 14 Incommensu- rate structures have been reported for certain lone-pair- containing TTBs such as Ba 4 Bi 2 Ti 4 Nb 6 O 30 15 and intermediate compositions in the solid solution system (Pb x Ba 1-x ) 5 Nb 10 O 30 (PBN), 16 but to the best of our knowledge, not for pure, undoped PN. Here, we report on the ferroelectric mechanism in PN and on the role of lone pair cations in ferroelectric TTBs in general by combining experiments and first-principles density func- tional theory (DFT) calculations. The ambient temperature crystal structure of PN was first investigated by combining X- ray and neutron powder diffraction, and the influence on the Received: May 3, 2017 Revised: July 3, 2017 Published: July 10, 2017 Article pubs.acs.org/cm © XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.7b01817 Chem. Mater. XXXX, XXX, XXX-XXX