Equilibrium relations in the system TiO2/V2O5/P2O5 and crystal structure of a NASICON-related vanadyl(V) titanium(IV) phosphate

• Published in: Journal of solid state chemistry 2012-12, Vol.196, p.565-573
• Main Authors: Titlbach, Sven, Hoffbauer, Wilfried, Glaum, Robert
• Format: Article
• Language: English
• Subjects: 31P –MAS-NMR; 51V –MAS-NMR; Catalysis; Catalysts: preparations and properties; Chemistry; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; General and physical chemistry; Inorganic compounds; Materials science; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Phase diagrams of metals and alloys; Physics; Selective oxidation catalyst; Solid state structure; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Vanadyl phosphate
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2012.07.022

Vanadyl(V)–titanium–orthophosphate (VVO)TiIV6(PO4)9 is formed by solid state reactions in the temperature range 525≤ϑ≤780°C. At higher temperature decomposition into V2O5 and the hitherto unknown solid solution Ti(P1−xVx)2O7 (0≤x≤0.23; 0.30≤x≤0.43) is observed. The process of phase formation has been monitored by MAS-NMR (31P, 51V) spectroscopy. Equilibrium phase relations in the quaternary system TiO2/VO2.5/PO2.5 have been determined. A structure analysis from X-ray single-crystal data (P63/m (No. 176), Z=2; a=8.4438(3)Å, c=22.215(1)Å, 14 independent atoms, 87 variables, 2066 unique reflections, R1=0.032, wR2=0.084) shows the relationship of (VVO)TiIV6(PO4)9 to the NASICON structure family. In marked contrast to the other members of this family [TiIV2O9] double-octahedra and strongly distorted tetrahedral [(VV=O)O3] groups are observed besides isolated [TiIVO6] octahedra and phosphate tetrahedra. The structure model is in agreement with the results from MAS-NMR (31P, 51V) spectroscopy. (VVO)TiIV6(PO4)9 belongs to the NASICON structure family. Its structure contains [TiIV2O9] double-octahedra and unprecedented, strongly distorted tetrahedral [(VV=O)O3] groups, in stark contrast to other members of this family. The structure model is in agreement with the results from MAS-NMR (31P, 51V) spectroscopy. [Display omitted] ► Equilibrium relations for the subsolidus have been established for the system TiO2/V2O5/P2O5. ► Phase formation has been monitored by XRPD as well as by 31P- and 51-MAS-NMR. ► A solid solution Ti(P1−xVx)2O7 exists up to x=0.43 with a miscibility gap at 0.23≤x≤0.30. ► The crystal structure of the new NASICON-related phosphate (VVO)TiIV6(PO4)9 is reported. ► The crystal structure contains the unprecedented [(V=O)O3] group.