Interplay between Oxo and Fluoro in Vanadium Oxyfluorides for Centrosymmetric and Non-Centrosymmetric Structure Formation

Abstract

Herein, we report the syntheses of two lithium-vanadium oxide-fluoride compounds crystallized from the same reaction mixture through a time variation experiment. A low temperature hydrothermal route employing a viscous paste of V₂O₅, oxalic acid, LiF, and HF allowed the crystallization of one metastable phase initially, Li₂VO_0.55(H₂O)_0.45F₅⋅2H₂O (I), which on prolonged heating transforms to a chemically similar yet structurally different phase, Li₃VOF₅ (II). Compound I crystallizes in centrosymmetric space group, I2/a with a = 6.052(3), b = 7.928(4), c = 12.461(6) Å, and β = 103.99(2)°, while compound II crystallizes in a non-centrosymmetric (NCS) space group, Pna2₁ with a = 5.1173(2), b = 8.612(3), c = 9.346(3) Å. Synthesis of NCS crystals are highly sought after in solid-state chemistry for their second-harmonic-generation (SHG) response and compound II exhibits SHG activity albeit non-phase-matchable. In this article, we also describe their magnetic properties which helped in unambiguous assignment of mixed valency of V (+4/+5) for Li₂VO_0.55(H₂O)_0.45F₅⋅2H₂O (I) and +4 valency of V for Li₃VOF₅ (II).