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

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, Li2VO0.55(H2O)(0.45)F-5.2H(2)O (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) angstrom, and beta = 103.99(2)degrees, while compound II crystallizes in a non-centrosymmetric (NCS) space group, Pna2(1) with a = 5.1173(2), b = 8.612(3), c = 9.346(3) angstrom. 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 Li2VO0.55(H2O)(0.45)F-5.2H(2)O (I) and +4 valency of V for Li3VOF5 (II).

Automated summary

In this study, the syntheses of two lithium-vanadium oxide-fluoride compounds were reported, crystallized from the same reaction mixture through a time variation experiment. A low temperature hydrothermal route allowed the formation of a metastable phase, which transformed to a structurally different phase upon extended heating. Non-centrosymmetric crystals are highly desirable in solid-state chemistry for their second-harmonic-generation response.