Peculiarity of formation of the NASICON-related phosphates in the space group R32: synthesis and crystal structures of Na4MIIAl(PO4)3 (MII–Mg, Mn)

Crystallization of high-temperature self-flux of the pseudo-quaternary system Na2O–MIIO–Al2O3–P2O5 (MII–Mg, Mn) was studied, and two new complex orthophosphates with general composition Na4MgAl(PO4)3 (NaMgAlP) and Na4MnAl(PO4)3 (NaMnAlP) were prepared and characterized. Their structures were determined using the single-crystal (NaMnAlP) and powder (NaMgAl) X-ray diffraction data. Both compounds belong to NASICON family (rhombohedral system, space group R32) with cell parameters a = 8.8570(4), c = 21.2865(19) for NaMnAlP and a = 8.74631(13), c = 21.1534(3) A for NaMgAlP. The basic building blocks of the rigid framework of both compounds are two different units [Al2(PO4)3] and \(\left[ {{\text{M}}_{2}^{\text{II}} \left( {{\text{PO}}_{4} } \right)_{3} } \right]\). The sodium atoms are distributed over three crystallographic sites in cavities of the framework. The NASICON-type framework topology in space group R32 is discussed.