Polymorphism in Na2(Co/Zn)P2O7 and Na2(Co/Fe)P2O7 Pyrophosphates: A Combined Diffraction and 31P NMR Study,The Journal of Physical Chemistry C

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Polymorphism in Na2(Co/Zn)P2O7 and Na2(Co/Fe)P2O7 Pyrophosphates: A Combined Diffraction and 31P NMR Study
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2021-12-22 , DOI: 10.1021/acs.jpcc.1c08753
Chiara Ferrara ₁ , Clemens Ritter ₂ , Piercarlo Mustarelli ₁ , Cristina Tealdi ₃

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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.

中文翻译：

Na2(Co/Zn)P2O7 和 Na2(Co/Fe)P2O7 焦磷酸盐的多态性：联合衍射和 31P NMR 研究

通式为 A ₂ BP ₂ O ₇（A = 碱金属，B = 过渡金属）的焦磷酸盐可用于多个技术领域，包括可充电钠离子电池。它们呈现出丰富的多态性与组成，这可能会强烈影响它们的功能特性。因此，选择性地获得特定结构对于应用至关重要。在这里，我们结合结构研究技术（固态 NMR、X 射线和中子衍射以及 DFT 计算）来研究 Na ₂ (M/M')P ₂ O _{7系列的晶体化学}对 M/M' = Co/Zn 和 Co/Fe。对于这两个系列，观察到相变与组成。对于 Zn/Co 系列，发现了四方到斜方的相变，对于 Co/Fe 系列，发现了从斜方到三斜方的相变。鉴于过渡金属离子的不同电子结构来解释这种变化。此外，在建模、短期和长期结构分析的支持下，我们表明对于给定的组合物，两种多晶型物的共存是可能的，这表明最终结构可能强烈依赖于合成过程。

更新日期：2022-01-13

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