“…Two different types of structural arrangements are known for these compounds that can generally be classified into seven structure types denoted by roman numerals: cyclotetraphosphates M I REP 4 O 12 with a ring structure formed by four PO 4 tetrahedra that are joined by bridging O atoms, and polyphosphates M I RE(PO 3 ) 4 consisting of helical ribbons formed by corner-sharing PO 4 tetrahedra (Jouadi et al, 2003;Ettis et al, 2003). Previous structure analyses have shown that rare earth polyphosphates with the general formula M I RE(PO 3 ) 4 crystallize in two crystal systems: orthorhombic with space group C222 1 (type VII) (M I = NH 4 , RE = Y) (Bagieu-Beucher & Guitel, 1988) or space group Pbna (M I = Li, M III = Ga) (Palkina et al, 1981a), and monoclinic with space groups C2/c (Koizumi, 1976;Hamady et al, 1995;Mokhtar et al, 1984) (type I), P2 1 /n (Palkina et al, 1981b;Maksimova et al, 1978) (types II, IV and V), P2 1 /c (Ben Hassen et al, 1982;Dogo et al, 1980) or P2 1 (Yong-Hua et al, 1983;Palkina, Maksimova, Kuznetsova & Chibiskova, 1978). All these materials have been intensively studied for their electric and optical properties, and some of them have applications in catalysis.…”