Peculiarity of formation of the NASICON-related phosphates in the space group R3 2 : synthesis and crystal structures of Na 4 M II Al(PO 4 ) 3 (M II –Mg, Mn)

Crystallization of high-temperature self-flux of the pseudo-quaternary system Na 2 O–M II O–Al 2 O 3 –P 2 O 5 (M II –Mg, Mn) was studied, and two new complex orthophosphates with general composition Na 4 MgAl(PO 4 ) 3 (NaMgAlP) and Na 4 MnAl(PO 4 ) 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 R3 2 ) with cell parameters a = 8.8570(4), c = 21.2865(19) for NaMnAlP and a = 8.74631(13), c = 21.1534(3) Å for NaMgAlP. The basic building blocks of the rigid framework of both compounds are two different units [Al 2 (PO 4 ) 3 ] and [ M_2^II( PO_4)_3] . The sodium atoms are distributed over three crystallographic sites in cavities of the framework. The NASICON-type framework topology in space group R3 2 is discussed.