NaCeP2Se6, Cu0.4Ce1.2P2Se6, Ce4(P2Se6)3, and the Incommensurately Modulated AgCeP2Se6: New Selenophosphates Featuring the Ethane-Like [P2Se6]4- Anion

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Abstract
NaCeP2Se6, Cu0.4Ce1.2P2Se6, and AgCeP2Se6 were prepared from nearly stoichiometric proportions of the starting materials plus extra selenium at 750−850 °C. Ce1.33P2Se6 (or Ce4(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, P21/c, with a = 12.1422(2), b = 7.6982(1), c = 11.7399(2) Å, β = 111.545(1)° and Z = 4 for NaCeP2Se6; a = 12.040(1), b = 7.6418(8), c = 11.700(1) Å, β = 111.269(2)° and Z = 4 for Cu0.4Ce1.2P2Se6; and a = 6.8057(5), b = 22.969(2), c = 11.7226(8) Å, β = 124.096(1)° and Z = 6 for Ce1.33P2Se6. NaCeP2Se6 has a two-dimensional character and is isostructural to the KLnP2Q6 family of compounds, where Ln = La, Ce, and Pr for Q = Se and Ln = La for Q = S. The structure consists of [CeP2Se6]nn- layers which are separated by Na+ cations. Each layer contains CeSe9 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 P21/c(α0γ) and represents an incommensurately modulated version of the KLnP2Q6 structure type, with a = 9.971(5), b = 7.482(3), c = 11.757(4) Å, β = 145.630(9)°, q = 0.3121(18)a* + 0.4116(19)c*, and Z = 2. The structures of the compounds reported here and those of the KLnP2Q6 family are highly related to the monoclinic-II, MIIPQ3 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.