Electrochemical Modification of Negative Thermal Expansion Materials in the Ta xNb1- xVO5 Series

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 Li_xMoO₂ 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, Na_0.9Mo₂O₄ 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.

Exploration of the high temperature phase evolution of electrochemically modified Sc2(WO4)3via potassium discharge

Electrochemical discharge followed by thermal treatment produces K₂WO₄ and other phases. K₂WO₄ features a large negative thermal expansion coefficient between 923–1023 K.

Electrochemically activated solid synthesis: an alternative solid-state synthetic method

Solid-state synthesis is one of the most common synthetic methods in chemistry and is extensively used in lab-scale syntheses of advanced functional materials to ton-scale production of chemical compounds. It generally requires at least one or several high temperature and/or high-pressure steps, which makes production of compounds via solid-state methods very energy and time intensive. Consequently, there is a persistent economic and environmental incentive to identify less energy and time consuming synthetic pathways. Here, we present an alternative solid-state synthetic method, which utilizes structural changes, induced by an electrochemical "activation" step followed by a thermal treatment step. The method has been used to synthesize a Sc0.67WO4-type phase where Sc0.67WO4 has previously only been obtained at 1400 °C and 4 GPa for 1 h. Through our method the Sc0.67WO4-type phase has been prepared at only 600 °C and ambient pressure. Experimental factors that influence phase formation from the electrochemical perspective are detailed. Overall, the method presented in this work appears to be able to generate the conditions for unusual and new phases to form and thus becomes another tool for synthetic solid-state chemists. This in turn permits the exploration of a larger synthetic parameter space.