The Formation of Striped Phases by Charge Localization in Vanadium Phosphates
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The Formation of Striped Phases by Charge Localization in Vanadium Phosphates Ranko P. Bontchev, Junghwan Do and Allan J. Jacobson Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA. ABSTRACT The compound (H3O)1.5{[VO(H2O)]PO4}3·2.5H2O is a new member of a series of vanadium phosphates containing vanadium in both plus four and five oxidation states. Charge ordering occurs in these mixed vanadium phosphate layered compounds forming striped phases with different periodicities. The VOPO4 layers undulate as a consequence of differences in the O-O atom distances in the ordered V(V)O6 and V(IV)O6 octahedra. INTRODUCTION Many aspects of the chemistry of layered vanadium phosphates have been studied since Ladwig’s early work on the synthesis and properties of VOPO4·2H2O [1]. The structure of VOPO4·2H2O was determined by Tietze using single crystal X-ray diffraction [2]. Layers are formed by connecting distorted VO5(H2O) octahedra with PO4 tetrahedra. Adjacent VO5(H2O) octahedra are related by an inversion center within the layer so that the orientation of the vanadyl V=O groups alternates above and below the layer (see Figure 1). The interlayer space is occupied by an additional water molecule that is hydrogen bonded to the coordinated water molecule. Intercalation reactions of both neutral molecules and ions have been reported [3-7]. Mixed valence compounds are formed by insertion of cations into the interlayer space and reduction of V(V) to V(IV) [3,5]. A large number of compounds based on VOPO4 layers were subsequently synthesized in single crystal form by using mild hydrothermal conditions [8-12]. The crystal chemistry, including the interlayer cation and solvent molecule locations, are now well understood from the detailed structural studies. Here we note only that phases are known with a water molecule coordinated to the vanadium atom (distorted octahedral) [8,12] and without (square pyramidal) [8-10] and that only a small number of mixed valence systems have been characterized by single crystal X-ray diffraction [13-17]. The known mixed valence systems are summarized in Table I. A small number of mixed valence compounds in which the layers are cross linked by an additional phosphate group are also included [18]. Here we discuss the evidence for charge ordering in the mixed valence compounds and describe a new example with the composition (H3O)1.5{[VO(H2O)]PO4}3·2.5H2O. EXPERIMENTAL Synthesis: a) The compound Na[VO(H2O)PO4]2·2 H2O (1) was prepared by hydrothermal synthesis in a Teflon lined steel autoclave at 130°C for 3 days: NaVO3 (0.122 g, 1 mmol), BPO4 (0.317 g, 3 mmol) and 3 mL H2O. The reaction products were prismatic dark green crystals of (1), average size 0.1-0.3 mm, yield approximately 80% based on vanadium, together with some unidentified brownish powder. b) (H3O)1.5{[VO(H2O)]PO4}3·2.5H2O (2): V2O5 (1.819 g, 10 mmol), H2C2O4 (0.480 g, 5 mmol), 1.20 mL H3PO4 (85 wt% solution in H2O, 20 mmol) and 30 mL H2O were mixed in a 100 mL flask and refluxed for 10 h at 70oC in air. After coolin
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