Synthesis and characterization of two quaternary thoriurn chalcophosphates:: Cs4Th2P6S18 and Rb7Th2P6Se21

Two new thorium chalcophosphates have been synthesized by the reactive flux method and characterized by single-crystal X-ray diffraction, diffuse reflectance, and Raman spectroscopy: Cs4Th2P6S18 (I); Rb7Th2P6Se21 (II). Compound I crystallize's as colorless blocks in the triclinic space group P (1) over bar (No. 2) with a = 12.303(4) angstrom, b = 12.471(4) angstrom, c = 12.541(4) angstrom, alpha = 114.607(8)degrees, beta = 102.547(6)degrees, gamma = 99.889(7)degrees, and Z = 2. The structure consists of (Th2P6S18)(4-) layers separated by layers of cesium cations and only contains the (P2S6)4- building block. Compound II crystallizes as red blocks in the triclinic pace group P (1) over bar (No. 2) with a = 11.531(3) angstrom, b = 12.359(4) angstrom, c = 16.161(5) angstrom, alpha = 87.289(6)degrees, beta = 75.903(6), gamma = 88.041(6)degrees, and Z = 2. The structure consists of linear chains of (Th2P6Se21)(7-) separated by rubidium cations. Compound II contains both the (PSe4)(3) and (P2Se6)4- building blocks. Both structures may be derived from two known rare earth structures where a rare earth site is replaced by an alkali or actinide metal to form these novel structures. Optical band gap measurements show that compound I has a band gap of 2.8 eV and compound II has a band gap of 2.0 eV. Solid-state Raman spectroscopy of compound I shows the vibrations expected for the (P2S6)(4-) unit. Raman spectroscopy of compound II shows the vibrations expected for both (PSe4)(3-) and (P2Se6)(4-) units. Our work shows the remarkable diversity of the actinide chalcophosphate system and demonstrates the phase space is still ripe to discover new structures.