Thermal Evolution and Phase Transitions in Electrochemically Activated Sc2(MoO4)3.

Sc2(MoO4)3 shows negative thermal expansion (NTE) properties between -93 and +750 °C. Recently, electrochemical activation has been demonstrated to dramatically alter the phase evolution of structurally analogous Sc2(WO4)3. Electrochemical activation involves placing the material of choice in an electrochemical cell with Li, Na or K counter electrodes and discharging (or reacting Li+, Na+ and K+) which is followed by extraction of the activated electrode and subsequent thermal treatment. Here such a process is applied to Sc2(MoO4)3 and the results compared with the evolution of Sc2(WO4)3. For 12.5% lithium discharged Sc2(MoO4)3(12.5% of fully discharge capacity) the coefficient of thermal expansion (CTE) below 425 °C is -13.83(1) × 10-6/°C which is larger than parent material, and a new LixMoO2 phase forms at about 425 °C. The 25% lithium discharged Sc2(MoO4)3 shows the formation of a new Li2MoO4 phase after discharging (electrochemical-based structural change) and on subsequent heat treatment the electrode mix transforms to Li3ScMo3O12. Interestingly, a range of new phases at various temperatures in the sodium and potassium discharged samples appear during heat treatment. For example, Na0.9Mo2O4 forms during thermal treatment of the 50% sodium discharged Sc2(MoO4)3 while KMo4O6 and K2MoO4 form with thermal treatment of 100% potassium discharged Sc2(MoO4)3. This work showcases the rich diversity in the phases that can be accessed during and post thermal treatment of Li, Na, and K discharged Sc2(MoO4)3 electrodes.