IT-5-P-2151 Non-stoichiometry and order-disorder in the Sb_xV_1-xO₂ (0<x<0.5) solid solution

~SbVO₄ plays a key role in the catalyst for the ammoxidation of propane to acrylonitrile. Besides, it exhibits an amazing structural flexibility involving cation vacancies, changes in oxidation states and different degrees of order-disorder, ranging from Short Range Order (SRO) to periodic superstructures and structural modulations [1,2]. In this work we have studied the solid solution that ranges from VO₂ to SbVO₄ according to the following reaction stoichiometry:

[Sb₂O₃ + V₂O₅] + VO₂ ---> Sb_xV_1-xO₂

 During the reaction all Sb³⁺ cations are oxidized to Sb⁵⁺ while all V⁵⁺ cations are reduced to V³⁺. This implies the following substitution in the basic rutile-type VO₂ matrix: Sb⁵⁺ + V³⁺ <---> 2V⁴⁺_. In this way we have been able to synthesize a whole solid solution Sb_xV_1-xO₂ ranging from SbVO₄ (x=0.5) to Sb_0.1V_0.9O₂ (x=0.1) by heating the stoichiometric amounts of Sb₂O₃, V₂O₅ and VO₂ at 800ºC under argon atmosphere. In the Sb-richest phase, SbVO₄, most of the vanadium is V³⁺, as confirmed by EELS spectroscopy, magnetic susceptibility and neutron diffraction, showing the latter magnetic ordering at T_N < 50K. Electron diffraction shows the presence of intense SRO in the form of wavy two-dimensional sheets of diffuse intensity in the reciprocal space, see Fig. 1. HRTEM demonstrates that the SRO is due to low correlation between ...Sb-V-Sb-V... chains running along c. This SRO disappears at Sb_0.33V_0.67O₂ and magnetic ordering happens at lower temperatures (T_N~6K). For Sb_0.25V_0.75O₂ compositions, a different type of SRO appears and its intensity increases as the amount of Sb decreases, Fig. 2. At Sb_0.1V_0.9O₂ this SRO can also be observed by electron diffraction as very intense two-dimensional sheets of diffuse intensity forming a three-dimensional net of edge-sharing octahedra in reciprocal space. This phase presents a structural transition similar to that of VO₂ but at lower temperature (51ºC). A phase transition can be observed by electron diffraction (Fig. 3) when heating with the electron beam from Short Range Order (SRO, diffuse lines) (a) to a two-fold superlattice (b) and back to SRO (c) as the temperature is lowered. During the whole series we range from SbVO₄ with V³⁺ to VO₂ with V<⁴⁺ and that big change in stoichiometry has been accommodated in a "soft way" through SRO mechanisms.

[1] A.R. Landa-Cánovas, J. Nilsson, S. Hansen, K. Staahl and A. Andersson. J. Solid State Chem.116, 369-377 (1995); [2] A. R. Landa-Cánovas, F. J. García-García, S. Hansen. Catalysis Today 158, (2010) 156.