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 P6m2. The first one (direct series) shows one metal vacancy and corresponds to the chemical formula □R T X . The compounds α‐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 the following members. The second family (reverse series) with chemical formula R□ T X comprises the ternaries α‐UCr₆P₄ (in fact member n = 0 in both series) and UMo₁₃P₉ (n = 1), the structure of which has been determined on a single crystal. The limiting structures to which the two series converge have been found to be YbPtP (direct) and WC (reverse). 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 permits prediction of the different members for the two series and their structural definition in terms of lattice parameters, atomic coordinates and theoretical X‐ray diffraction patterns. Finally, this rule permits to give for each 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).