BaCl[ReO4] and BaBr[ReO4]: Synthesis, Crystal Structure and Properties of Two Mixed-Anionic Barium Meta-Perrhenates

Abstract Colorless transparent single crystals of new mixed-anionic barium meta-perrhenates with the formula BaX[ReO4] (X = Cl and Br) were obtained either from aqueous solution or by solid-state synthesis. As determined by single-crystal X-ray diffraction both compounds crystallize isotypically to orthorhombic LaCl[WO4] in the space group Pnma with lattice parameters of a = 1954.84(9) pm, b = 575.49(3) pm, c = 461.95(2) pm for BaCl[ReO4] and a = 2003.81(9) pm, b = 584.58(2) pm, c = 464.85(2) pm for BaBr[ReO4] at for Z = 4. Thus the anionic Ba2+ coordination consists of four halide anions and seven oxygen atoms from five [ReO4]−− tetrahedra. The vibrational spectra of both compounds exhibit differences between single crystals obtained from both synthesis methods. Fast solid-state reactions at 800 ∘C lead to phase pure bulk samples of BaCl[ReO4], which is stable up to its melting point at 753 ∘C. The diffuse reflectance spectrum of BaCl[ReO4] reveals an O → Re ligand-to-metal charge transfer with a maximum at about 250 nm and reveals an optical band gap of Eg = 4.14 eV.