Interplay Between Oxo and Fluoro in Vanadium Oxyfluorides For Centrosymmetric and Non-Centrosymmetric Structure Formation

• Main Authors: Prashanth Sandineni, Hooman Yaghoobnejad Asl, Weiguo Zhang, P. Shiv Halasyamani, Kartik Ghosh, Amitava Choudhury
• Format: Article
• Language: English
• Published: MDPI AG 2021-01-01
• Series: Molecules
• Subjects: vanadium oxy-fluoride; non-centrosymmetric crystal; Second-Harmonic Generation
• Online Access: https://www.mdpi.com/1420-3049/26/3/603

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 (<b>I</b>), which on prolonged heating transforms to a chemically similar yet structurally different phase, Li₃VOF₅ (<b>II</b>). Compound <b>I</b> crystallizes in centrosymmetric space group, <i>I</i>2/<i>a</i> with <i>a</i> = 6.052(3), <i>b</i> = 7.928(4), <i>c</i> = 12.461(6) Å, and <i>β</i> = 103.99(2)°, while compound <b>II</b> crystallizes in a non-centrosymmetric (NCS) space group, <i>Pna</i>2₁ with <i>a</i> = 5.1173(2), <i>b</i> = 8.612(3), <i>c</i> = 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 <b>II</b> 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 (<b>I</b>) and +4 valency of V for Li₃VOF₅ (<b>II</b>).