A COMPARATIVE-STUDY OF 2 NEW STRUCTURE TYPES - SYNTHESIS AND STRUCTURAL AND ELECTRONIC CHARACTERIZATION OF K(RE)P2SE6 (RE=Y,LA,CE,PR,GD)

Two polytypes of potassium rare-earth-metal hexaselenodiphosphates(IV) , K(RE)P2Se6 (RE = Y, La, Ce, Pr, Gd), have been synthesized from the stoichiometric reaction of RE, P, Se, and K2Se4 at 750 degrees C. Both single-crystal and powder X-ray diffraction analyses showed that the structures of these polytypes vary with the size of the rare earth met als. For the smaller rare-earth metals, Y and Gd, K(RE)P2Se6 crystalli zed in the orthorhombic space group P2(1)2(1)2(1). The yttrium compoun d was studied by single-crystal X-ray diffraction with the cell parame ters a = 6.7366(5) Angstrom, b = 7.4286(6) Angstrom, c = 21.603(2) Ang strom, and Z = 4. This structure type comprises a layered, square netw ork of yttrium atoms that are bound to four distinct [P2Se6](4-) units through selenium bonding. Each [P2Se6](4-) unit possesses a Se atom t hat is not bound to any Y atom but is pointing out into the interlayer spacing, into an environment of potassium cations. For larger rare-ea rth metals, La, Ce, and Pr, K(RE)P2Se6 crystallized in a second, monoc linic polytype, the structure of which has been published. Both of the se two different polytypes can be related to each other and several ot her isoelectronic chalcophosphate structures based on a Parthe valence electron concentration analysis. These structures include Ag4P2S6, K2 FeP2S6, and the hexagonal M(II)PS(3) structure types. The magnetic sus ceptibilities of the title compounds have been studied, and the behavi or can been explained based on a simple set of unpaired f-electrons. T he diffuse reflectance spectroscopy also showed that these yellow plat es are moderately wide band gap (similar to 2.75 eV) semiconductors.