Direct and reverse limiting series of transition metal phosphides with ordered defects and metal/non metal ratio close to 2

Two new limiting series of ternary compounds with ordered defects have been evidenced, which crystallize with hexagonal symmetry, space group P (6) ov er bar m2. The first one (direct series) shows one metal vacancy and corres ponds to the chemical formula squareR((n+l)2)T((n+3)2-3)X((n+2)2). The comp ounds alpha -UCr6P4 (n = 0) and Zr4Co13Si9 (n = 1) are the first members of the series. X-ray single crystal determination and/or electron microprobe analysis confirm the ternary phosphides Ce9Ni25P13 and Ce16Ni36P22 to be th e following members. The second family (reverse series) with chemical formu la R square (n+1)2T(n+3)2-3X(n+2)2 comprises the ternaries alpha -UCr6P4 (i n fact member n = 0 in both series) and UMo13P9 (n = 1), the structure of w hich has been determined on a single crystal. The limiting structures to wh ich the two series converge have been found to be YbPtP (direct) and WC (re verse). The structural relationships between the direct and reverse series have been discussed in terms of metal vacancies and coordination polyhedra. Moreover, a general crystal chemical rule has been established that permit s prediction of the different members for the two series and their structur al definition in terms of lattice parameters, atomic coordinates and theore tical X-ray diffraction patterns. Finally, this rule permits to give for ea ch member the number of metal vacancies as well as the distribution of the metalloid polyhedra occupied by the metal atoms (trigonal prisms, pyramids, tetrahedra, triangles).