RbCu1.2Ag3.8Se3 and Cs2Cu2Sb2Se5: Novel Quaternary Intermetallics Synthesized from Superheated Organic Media

Reactions in superheated ethylenediamine (en) solutions at 160°C resulted in two novel quaternary intermetallic copper selenides, RbCu1.2Ag3.8Se3(I) and Cs2Cu2Sb2Se5(II). Both I and II are metal rich and represent new layered structure types. Compound I crystallizes in the tetragonal crystal system, space group P4/nbm (No. 125) with a=5.991(1) ?, c= 10.918(2) ?, Z=2, V=391.9(1) ?3, R1/wR2=0.0373/0.0458 for all reflections. Compound II belongs to the triclinic crystal system, space group P (No. 2), a=7.645(1) ?, b=8.768(2) ?, c=10.264(1) ?, alpha=91.97(2)°, beta=92.07(2)°, gamma=103.05(1)°, Z=2, V=669.2(3) ?3, R1/wR2=0.0685/0.0740 for all reflections. I consists of 2infinity [(Cu1.2Ag3.8Se3)-] layers and Rb+ counterions located between these layers. There are two types of metal-to-selenium coordination, a square planar (Ag) and a trigonal pyramidal (Cu/Ag). The Se(1) atom displays an unusual eight coordination with Ag and Cu. II contains alternating 2infinity[(Cu2Sb2Se5)2-] anionic and Cs+ cationic layers. Each copper atom has a distorted tetrahedral coordination to four Se atoms, and each antimony atom bonds to three Se atoms to result in a trigonal pyramidal geometry. Both I and II are semiconductors with estimated band gaps of 0.7–0.8 and 1.2–1.3 eV, respectively.