PENTACALCIUM HEXAPHOSPHAHYPODIPHOSPHATE, CA5P8, A COMPOUND WITH ISOLATED ANIONS P8(10-) IN THE STAGGERED ETHANE CONFORMATION

Ca5P8 is prepared from calcium and red phosphorus in a molar ratio 5:8 in argon atmosphere in corundum crucibles inserted in quartz ampullae (3.5 d 1000-degrees-C). It is a red-brown powder which is hydrolyzed by moisture. Single crystals are formed from powder at 1100-degrees-C within 60 d. Ca5P8 crystallizes in a new structure type (mC26) with is olated anions P-8(10-) in staggered conformation: C2/m (no. 12); a = 6 89.9(4) pm, b = 1 188,3(4) pm, c = 748.4(3) pm, beta = 108.25(2)degree s Z = 2; d4(25) = 2.56(1). Ca5P8 is the first compound containing poly phosphide anions with fourfold and single bonded phosphorus atoms (for mally P+ and P2-, resp.). The cations Ca2+ are arranged in a distorted cubic close-packing. The centers of the polyphosphide anions replace in an ordered way one third of Ca2+ in every second layer. The termina l P atoms occupy all octahedral interstices, so that P-8(10-) is coord inated by 18 Ca2+ in form of a cuboctahedron with capped squaric faces .