title: Inversion symmetry breaking by oxygen octahedral rotations in the Ruddlesden-Popper NaRTiO4 family.
abstract: Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO4 (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P42(1)m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides.
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Akamatsu
authors.affiliation: Department of Material Chemistry, Kyoto University, Nishikyo, Kyoto 615-8510, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Fujita
authors.affiliation: Department of Material Chemistry, Kyoto University, Nishikyo, Kyoto 615-8510, Japan.
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Kuge
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Arnab
authors.initials: A
authors.lastname: Sen Gupta
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Togo
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Shiming
authors.initials: S
authors.lastname: Lei
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Fei
authors.initials: F
authors.lastname: Xue
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Greg
authors.initials: G
authors.lastname: Stone
authors.affiliation: Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA.
authors.forename: James M
authors.initials: JM
authors.lastname: Rondinelli
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Long-Qing
authors.initials: LQ
authors.lastname: Chen
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan.
authors.forename: Isao
authors.initials: I
authors.lastname: Tanaka
authors.affiliation: Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
authors.forename: Venkatraman
authors.initials: V
authors.lastname: Gopalan
authors.affiliation: Department of Material Chemistry, Kyoto University, Nishikyo, Kyoto 615-8510, Japan.
authors.forename: Katsuhisa
authors.initials: K
authors.lastname: Tanaka