Abstract
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.
Highlights
IoPn2Os 7muanyitsbeandaccBoOmnmpoodlyatheedd.1ra−5crTeahtaenkas network to the combination of a robust inorganic open framework and the presence of the mobile A species, these materials are attracting attention for several technological applications.[6−8] In particular, AI2BIIP2O7 polyanionic oxides have been considered for the use in different compartments of sodium-ion batteries (SIBs)
Several polymorphs were found for different compositions, and more phases can be stabilized for a given composition by playing on the synthesis route and thermal treatments.[9,12−14] Since different structures can lead to different electrochemical responses, one of the main strategies to further improve the performance of the pyrophosphate family in the last years has been the introduction of mixed compositions of the general formula
Several compositions related to the general formula Na2MP2O7 with a wide range of M ions were already described and different structures reported in the literature, including tetragonal, orthorhombic, and triclinic crystal systems.[3−5,9,14,17]
Summary
The connectivity is similar in both the space groups, the main difference being the eclipsed and partially eclipsed staking of the layers along the c-axis (see Figure 1b−d) Both the structures were reported for the Zn-based composition obtained under similar synthesis conditions.[17−19] A similar structural description is obtained with the use of the orthorhombic space group P21cn,[4,14] where the partially eclipsed staking is maintained and a modulation of the layers along the a and b directions is visible (Figure 1e,f). Both tetragonal and orthorhombic cells were reported for the Cobased compound together with the triclinic phase. The layered forms were identified as “blue”, whereas the 3D triclinic phase
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