title: Thermal Expansion and Second Harmonic Generation Response of the Tungsten Bronze Pb2AgNb5O15.
abstract: The incorporation of transition metal element Ag was performed to explore negative thermal expansion (NTE) materials with tetragonal tungsten bronze (TTB) structures. In this study, the structure and thermal expansion behaviors of a polar TTB oxide, Pb2AgNb5O15 (PAN), were systematically investigated by high-resolution synchrotron powder diffraction, high-resolution neutron powder diffraction, transmission electron microscope (TEM), and high-temperature X-ray diffractions. The TEM and Rietveld refinements revealed that the compound PAN displays (√2a(TTB), √2b(TTB), 2c(TTB))-type superstructure. This superstructure within the a-b plane is caused by the ordering of A-site cations, while the doubling of the c axis is mainly induced by a slight tilt distortion of the NbO6 octahedra. The transition metal Ag has larger spontaneous polarization displacements than Pb, but the Pb-O covalence seems to be weakened compared to the potassium counterpart Pb2KNb5O15 (PKN), which may account for the similar Curie temperature and uniaxial NTE behavior for PAN and PKN. Powder second harmonic generation (SHG) measurement indicates that PAN displays a moderate SHG response of ∼0.2 × LiNbO3 (or ∼100 × α-SiO2) under 1064 nm laser radiation. The magnitudes of the local dipole moments in NbO6 and PbOx polyhedra were quantified using bond-valence approach. We show that the SHG response stems from the superposition of dipole moments of both the PbO(x) and NbO6 polyhedra.
authors.forename: Kun
authors.initials: K
authors.lastname: Lin
authors.affiliation: Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences , Beijing 100190, China.
authors.forename: Pifu
authors.initials: P
authors.lastname: Gong
authors.forename: Jing
authors.initials: J
authors.lastname: Sun
authors.forename: Hongqiang
authors.initials: H
authors.lastname: Ma
authors.forename: You
authors.initials: Y
authors.lastname: Wang
authors.forename: Li
authors.initials: L
authors.lastname: You
authors.forename: Jinxia
authors.initials: J
authors.lastname: Deng
authors.forename: Jun
authors.initials: J
authors.lastname: Chen
authors.affiliation: Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences , Beijing 100190, China.
authors.forename: Zheshuai
authors.initials: Z
authors.lastname: Lin
authors.affiliation: RIKEN SPring-8 Center , Hyogo 679-5148, Japan.
authors.forename: Kenichi
authors.initials: K
authors.lastname: Kato
authors.affiliation: Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20899-6102, United States.|Department of Materials Science and Engineering, University of Maryland , College Park, Maryland 20742-2115, United States.
authors.forename: Hui
authors.initials: H
authors.lastname: Wu
authors.affiliation: Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20899-6102, United States.
authors.forename: Qingzhen
authors.initials: Q
authors.lastname: Huang
authors.forename: Xianran
authors.initials: X
authors.lastname: Xing