title: Ordered structure and thermal expansion in tungsten bronze Pb₂K(0.5)Li(0.5)Nb₅O₁₅.
abstract: The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb2K(0.5)Li(0.5)Nb5O15, were systematically investigated by selected-area electron diffraction (SAED), neutron powder diffraction, synchrotron X-ray diffraction (XRD), and high-temperature XRD. SAED and Rietveld refinement reveal that Pb2K(0.5)Li(0.5)Nb5O15 displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group Bb2₁m. The transition to a paraelectric phase (P4/mbm) occurs at 500 °C. Compared with Pb2KNb5O15 (PKN), the substitution of 0.5K(+) with small 0.5Li(+) into PKN causes the tilting of NbO6 octahedra away from the c axis with Δθ ≈ 10° and raises the Curie temperature by 40 °C, and the negative thermal expansion coefficient along the polar b axis increases more than 50% in the temperature range 25-500 °C. We present that, by introduction of Li(+), the enhanced spontaneous polarization is responsible for the enhanced negative thermal expansion along the b axis, which may be caused by more Pb(2+) in the pentagonal caves.
authors.affiliation: Department of Physical Chemistry and ∥State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing , Beijing 100083, China.
authors.forename: Kun
authors.initials: K
authors.lastname: Lin
authors.forename: Yangchun
authors.initials: Y
authors.lastname: Rong
authors.forename: Hui
authors.initials: H
authors.lastname: Wu
authors.forename: Qingzhen
authors.initials: Q
authors.lastname: Huang
authors.forename: Li
authors.initials: L
authors.lastname: You
authors.forename: Yang
authors.initials: Y
authors.lastname: Ren
authors.forename: Longlong
authors.initials: L
authors.lastname: Fan
authors.forename: Jun
authors.initials: J
authors.lastname: Chen
authors.forename: Xianran
authors.initials: X
authors.lastname: Xing