Polymorphism in Na₂(Co/Zn)P₂O₇ and Na₂(Co/Fe)P₂O₇ Pyrophosphates: A Combined Diffraction and ³¹P NMR Study
2021
Ferrara, Chiara | Ritter, Clemens | Mustarelli, Piercarlo | Tealdi, Cristina
Pyrophosphates of the general formula A₂BP₂O₇ (A = alkali metal and B = transition metal) find applications in several technological fields, including rechargeable Na-ion batteries. They present a rich polymorphism vs composition, which may strongly influence their functional properties. Therefore, to selectively obtain a specific structure is crucial for applications. Here, we combine structural investigation techniques (solid-state NMR, X-ray and neutron diffraction, and DFT calculations) to investigate the crystal chemistry of the series Na₂(M/M’)P₂O₇ with couples M/M’ = Co/Zn and Co/Fe. For both the series, a phase transition vs composition is observed. A tetragonal to orthorhombic phase transition is found for the Zn/Co series, and an orthorhombic to triclinic phase transition is found for the Co/Fe series. Such changes are interpreted in view of the different electronic structures of the transition metal ion. In addition, with the support of modeling, short- and long-range structural analysis, we show that the coexistence of two polymorphs for a given composition is possible, suggesting that the final structures may be strongly dependent upon the synthesis procedure.
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