Synthesis, crystal structures, and optical properties of two new layered quaternary tantalum thiophosphates and the thermal conversion of Cs4Ta4P4S24 into Cs2Ta2P2S12

The reaction of Ta with an in situ formed polythiophosphate melt of Cs2S3, P2S5, and S yields the two new quaternary tantalum thiophosphates Cs2Ta2P2S12 (I) and Cs4Ta4P4S24 (II). Both compounds were obtained with the same stoichiometric ratio but at different reaction temperatures. Compound I was prepared at 873 K and crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 8.862(2) A, b = 12.500(3) A, c = 17.408(4) A, beta = 99.23(3) degrees, and Z = 4. Compound II was prepared at 773 K and crystallizes in the monoclinic space group P2(1)/n (No. 14) with a = 14.298(3) A, b = 17.730(4) A, c = 16.058(3) A, beta = 106.19(3) degrees, and Z = 4. The two structures are closely related and exhibit two-dimensional anionic layers consisting of dimeric Ta2S11] units which are linked by two tetradentate and two tridentate PS4] tetrahedra. The significant difference between these two compounds is the orientation of the Ta2S11] units in infinite Ta2S4(PS4)]x chains which are subunits of both structures. The specific orientation of the Ta2S11] blocks in compound I leads to the formation of one cavity in the 2(infinity)Ta2P2S12]2- layers, whereas in compound II two types of cavities are observed in the 2(infinity)Ta4P4S24]4- layers. The Cs+ ions are located between the layers above and below the cavities. The compounds were characterized with infrared spectroscopy in the MIR region, Raman spectroscopy, and UV/Vis diffuse reflectance spectroscopy. When Cs4Ta4P4S24 (II) is heated at the synthesis temperature of compound I it is fully converted into compound I.