CRYSTAL-STRUCTURE OF THE HIGH-TEMPERATURE AND HIGH-PRESSURE PHASE OF MAGNESIUM MONOPHOSPHATE

Solid-state synthesis and crystal growth of the high-temperature and h igh-pressure form of magnesium monophosphate, now referred as Mg-3(PO4 )(2)-III, mere achieved at 22 kbar, 1000 degrees C and followed by a c omplete crystallographic characterization. Mg-3(PO4)(2)-III belongs to the triclinic space group P (1) over bar. Unit cell contains four for mula groups, and crystal parameters are found to be a = 8.512(2) Angst rom; b = 8.982(2) Angstrom; c = 9.320(3) Angstrom; 116.34(2)degrees; b eta = 91.50(2)degrees; gamma = 114.49(2)degrees. The crystal structure may be described as nearly regular PO4 tetrahedra sharing corners and edges with distorted MgO6 and MgO5 polyhedra. The dense structure is discussed and compared with those of other polymorphs, Mg-3(PO4)(2)-I, i.e., farringtonite (cf. gamma-Zn-3(PO4)(2)) and Mg-3(PO4)(2)-II, cf. sarcopside (i.e., olivine structure with ordered vacancies). (C) 1997 Academic Press.