Contributions on the investigation of inorganic nonstoichiometric compounds. XLV. New thermal decomposition products of Ln(2)CeMO(6)Cl(3) - Preparation of structure-related (La,Tb)(3,5)TaO6Cl4-x

The thermal decomposition (T approximate to 900-1050 degrees C) of Ln(2)CeM O(6)Cl(3) (M = Nb, Ta; Ln = La, Ce, Pr, Nd, Sm) leads to the formation of t wo mixed-valenced phases (Ln, Ce)(3.25)MO6Cl3.5-x (phase "AB") and (Ln, Ce) (3.5)MO6Cl4-x (phase "BB") and to the formation of chlorine according to re dox-reactions between Ce4+ and Cl-. Single crystals of both phases (Ln, Ce) (3.25)MO6Cl3.5-x ("AB") and (Ln,Ce)(3.5)MO6Cl4-x ("BB") were obtained by ch emical transport reactions using both powder of Ln(2)CeMO(6)Cl(3) (phase "A ") and powder of (Ln, Ce)(3.25)MO6Cl3.5-x (phase "AB") as starting material s and chlorine (p{Cl-2; 298 K} = 1 atm) or HCl (p{HCl; 298 K} = 1 atm) as t ransport agent. A crystal of (La, Ce)(3.25)NbO6Cl3.5-x ("AB") (space group: C2/m, a = 35.288(1) Angstrom, b = 5.418(5) Angstrom, c = 9.522(1) Angstrom , beta = 98.95(7)degrees, Z = 4) was investigated by xray diffraction metho ds, a crystal of (Pr,Ce)(3.5)NbO6Cl4-x ("BB") was investigated by synchrotr on radiation (lambda = 0.56 Angstrom) diffraction methods. The lattice cons tants are a = 18.563(6) Angstrom, b = 5.454(5) Angstrom, c = 9.527(6) Angst rom, beta = 102.44(3)degrees and Z = 4. Structure determination in the spac e group C2/m (No. 12) let to R1 = 0.0313. Main building units are NbO6-poly hedra with slightly distorted trigonally prismatic environment for Nb and c hains of face-sharing Cl-6-octahedra along [010]. The rare earth ions are c oordinated by chlorine and oxygen atoms. These main structure features conf irmed the expected relation to the starting material Ln(2)CeMO(6)Cl(3) (pha se "A") and to (Ln, Ce)(3.25)MO6Cl3.5-x (phase "AB").