The Synthesis and Characterisation of Ti3+ s=1/2 Mixed-Metal Fluorides

Compounds containing d1 transition metal ions, such as Ti3+ and Cu2+ where the electron spin S = 1/2, show interesting magnetic properties such as the potential to exhibit quantum spin behaviour. The greater redox stability of Cu2+ and other S = ½ systems has resulted less attention to the Ti3+ oxidation state [1-3]. The search to synthesise new Ti3+ magnetic materials is therefore a key area of interest for applications in quantum computing, energy, magnetism and superconductivity [4]. We have explored a variety of reducing agents with the goal of synthesising Ti3+ mixed-metal fluorides via the hydrothermal method. Ti3+ is typically a difficult oxidation state to reach through conventional solid-state techniques so developing new routes to reduce Ti4+ to Ti3+ is important [3]. The reducing agents explored were dimethylformamide (DMF) [5], polyvinylpyrrolidone (PVP) [6, 7] and triethylamine (TEA) [8], which previously have been successfully applied in reduced nanoparticle synthesis. The compounds Rb2TiF5.H2O and Cs2TiF5.H2O, as well as their deuterated counterparts, were successfully synthesised using hydrothermal methods with DMF as the sole reducing agent. PVP and TEA proved to be effective reducing agents in the synthesis of Rb2TiF5.H2O but not Cs2TiF5.H2O. Rb2TiF5.H2O was successfully dehydrated and FTIR analysis combined with X-ray diffraction techniques showed a loss of water molecules with no changes to the rest of the structure. A synthetic route to form the analogous dehydrate Cs2TiF5.H2O was not successful. This work shows that a number of reducing agents can be used effectively to reduce Ti4+ to Ti3+ in solid-state hydrothermal synthesis. Rb2TiF5.H2O (Cmcm; a = 9.64164, b = 8.47313 and c = 7.86328 Å) and Cs2TiF5.H2O (Cmcm; a = 10.32487, b = 8.79496 and c = 8.01500 Å) are alkali metal titanium fluorides with one dimensional (1D) chain structures made up of Ti3+F6 corner sharing octahedra. These octahedra experience Jahn-Teller distortion, consisting of varied F -Ti - F bond angles and Ti - F bond lengths, with the Cs2TiF5.H2O compound 4 exhibiting the most distortion [9]. The Ti - F bond lengths in Rb2TiF5.H2O (Cmcm) were between 1.898 - 1.9601 Å (0.0621 Å range) and in Cs2TiF5.H2O (Cmcm) 1.89726 - 2.00023 Å (0.10297 Å range). Bond angles in Rb2TiF5.H2O showed no distortion, whereas in Cs2TiF5.H2O some angles were distorted by up to 0.01578°. Structure determination was performed using single-crystal and powder X-ray diffraction techniques. The analogous deuterated compounds were analysed using neutron powder diffraction.