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The crystal chemistry of AIBIIXO4 (AI = Alkali ion, BII = alkali-earth ion, X = P, V, As) is very rich and has been widely investigated, particularly the phosphate family [1]. In recent years, we have been investigated the crystal structures [2,3] and magnetic properties of some compositions within the AIBIIXO4 series [4]. However, despite its simple chemistry NaSrPO4 has never been reported so far. Here, we present the synthesis, crystal structure and phase transitions of this phosphate. Surprisingly, this material exhibits a complex structure (31 atoms in the asymmetric unit-cell, Z = 10) at room temperature characterized by a strongly under bonded Na atom. This under-bonded atom is responsible for the complex and rich phase diagram as function of temperature as illustrated in Figure 1. NaSrPO4 exhibits 4 phase transitions between room temperature and 750C. Besides its rich phase diagram, NaSrPO4 exhibits a re-entrant phase transition slightly below 600C before to reach a hexagonal paraelastic phase at high temperature. In addition, we show that the sequence of phase transitions is strongly driven by the history of the sample and several phases can be quenched at room temperature.
[1] V. A. Isupov, Ferroelectrics, 274(1), 203–283 (2002)
[2] G. Nénert, P. O’Meara, T. Degen, Phys Chem Minerals 44, 455–463 (2017)
[3] G. Nénert, Z. Kristallogr.; 232(10), 669–674 (2017)
[4] G. Nénert, et al., Inorg. Chem., 52, 9627−9635 (2013).
