Noncentrosymmetry in Mixed Metal Oxide-Fluorides: Can We Control It?
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1148-PP01-04
Noncentrosymmetry in Mixed Metal Oxide-Fluorides: Can We Control It? Rachelle Ann F. Pinlac1, Michael R. Marvel2, Julien J.-M. Lesage1, and Kenneth R. Poeppelmeier1 1
Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston IL 60208
2
Department of Chemistry, Aurora University, 347 S. Gladstone Ave., Aurora, IL 60506
ABSTRACT The rational design of crystal structures, in particular noncentrosymmetric materials, and how to differentiate polar, polar-chiral, and chiral structures, is an ongoing theme in crystal engineering. In KNaNbOF5, the combination of a second-order Jahn Teller active d0 transition metal oxyfluoride anionic unit and mixed K/Na cation coordination environments are shown to result in a polar structure (space group Pna21). The crystal structure analysis of the Na/K-O/F interactions reveals that the potassium cations form one of the two contacts to the under-bonded oxide ions. These interactions satisfy the expected bond valence sums and Pauling’s second crystal rule (PSCR), leading to O/F ordering and acentric packing of the [NbOF5]2- anionic unit. INTRODUCTION
Noncentrosymmetric solids exhibit important symmetry-dependent features and properties, including enantiomorphism, optical activity (circular dichroism), pyroelectricity (including ferroelectricity), piezoelectricity (including ferroelasticity) and second harmonic generation (SHG) (Figure 1) [1, 2]. Compounds that exhibit these properties constitute a growing group of noncentrosymmetric (chiral, polar, or chiral-polar) materials. Various approaches to the synthesis of noncentrosymmetric structures have been reviewed recently [3-5]. Our research has focused on how to crystallize the early d0 transition metal oxyfluoride anionic units, particularly the octahedral species [MOF5]2-, [MO2F4]3- (M = V5+, Nb5+), [M’O2F4]2- or [M’O3F3]3- (M’ = Mo6+, W6+). The inherent out-of-center primary electronic distortions that these molecular species exhibit arise from the interactions between the transition
Figure 1. Symmetry dependent properties of noncentrosymmetry crystal classes. (Adapted from Chem. Mater. 10, 2753-2769 (1998)[1].) metal dπ and oxygen pπ orbitals [6]. As shown in Figure 2, the distortion is ligand dependent and the transition metal moves from the center of the octahedron towards a corner in [MOF5]2(C4 distortion), an edge in [MO2F4]3- and [M’O2F4]2- (C2 distortion), or a face in [M’O3F3]3- (C3 distortion). Similar distortions arise in technologically important SHG materials, such as in the
Figure 2. Intraoctahedral distortions of d0 transition metal oxide fluoride anionic units, which roughly coincide with (a) the C4 rotational axis of [MOF5]2- (M = V5+, Nb5+), (b) the C2 rotational axis of [MO2F4]3- (M = V5+, Nb5+) or [M’O2F4]2- (M’ = Mo6+, W6+), and (c) the C3 rotational axis of [M’O3F3]3-(M’ = Mo6+, W6+).
[NbO6/2]- octahedra found in LiNbO3 [7]. These intraoctahedra distortions, which can be understood through the second-order Jahn-Teller theorem, do not alone guarantee noncentrosymmetry. Sev
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