NaCeP2Se6, Cu0.4Ce1.2P2Se6, Ce4(P2Se6)3, and the incommensurately modulated AgCeP2Se6:: New selenophosphates featuring the ethane-like [P2Se6]4- anion

NaCeP2Se6, Cu0.4Ce1.2P2Se6, and AgCeP2Se6 were prepared from nearly stoichiometric proportions of the starting materials plus extra selenium at 750-850 degreesC. Ce1.33P2Se6 (or Ce-4(P2Se6)(3)) was obtained from the same reaction that produced Cu0.4Ce1.2P2Se6. The structure of all four compounds was determined by single-crystal X-ray diffraction. NaCeP2Se6, Cu0.4Ce1.2P2Se6, and Ce1.33P2Se6 crystallize in, the monoclinic space group, P2(1)/c, with a = 12.1422(2), b = 7.6982(1), c = 11.7399(2) Angstrom, beta = 111.545(1)degrees and Z= 4 for NaCeP2Se6; a = 12.040(1), b = 7.6418(8), c = 11.700(1) Angstrom, beta = 111.269(2)degrees and Z = 4 for Cu0.4Ce1.2P2Se6; and a = 6.8057(5), b = 22.969(2), c = 11.7226(8) Angstrom, beta = 124.096(1)degrees and Z = 6 for Ce1.33P2Se6. NaCeP2Se6 has a two-dimensional character and is isostructural to the KLnP(2)Q(6) family of compounds, where Ln = La, Ce, and Pr for Q = Se and Ln = La for Q = S. The structure consists of [CeP2Se6](n)(n-) layers which are separated by Na+ cations. Each layer contains CeSeg distorted, tricapped trigonal prisms and [P2Se6](4-) ethane-like anions. Cu0.4Ce1.2P2Se6 possesses a similar structure; however, substitution of the alkali metal by copper and cerium atoms renders the structure three dimensional. Ce1.33P2Se6 is the pure lanthanide member of this structure type. In this three-dimensional structure, there are three cerium metal sites which are partially occupied. The structure of AgCeP2Se6 was solved in the superspace,group P2(1)/c(alpha0gamma) and represents an incommensurately modulated version of the KLnP(2)Q(6) structure type, with a = 9.971(5), b = 7.482(3), c = 11.757(4) Angstrom, beta = 145,630(9)degrees, q = 0.3121(18)a* + 0.4116(19)c*, and Z = 2. The structures of the compounds reported here and those of the KLnP(2)Q(6) family are highly related to the monoclinic-II, M(\\)PQ(3) structure type (where M = Pb, Sn, Ca, and Sr for Q S and M Pb and Sn for Q Se). Optical and vibrational spectroscopic characterization is also reported.