Contributions on the Investigation of Inorganic Nonstoichiometric Compounds. XLV. New Thermal Decomposition Products of Ln_2CeMO_6Cl_3-Preparation of Structure-related (La, Tb)_(3.5)TaO_6Cl_(4-x)

摘要

The thermal decomposition (T approx= 900-1050 deg C) of Ln_2CeMO_6Cl_3 (M = Nb, Ta; Ln = La, Ce, Pr, Nd, Sm) leads to the formation of two mixed-valenced phases (Ln, Ce)_(3.25)MO_6Cl_(3.5 - x) (phase "AB") and (Ln, Ce)_(3.5)MO_6Cl-(4 - x) (phase "BB") and to the formation of chlorine according to redox-reactions between Ce~(4+) and Cl~-. Single crystals of both phases (Ln, Ce)_(3.25)MO_6Cl_(3.5 - x) ("AB") and (Ln,Ce)_(3.5)MO_6Cl_(4 - x) ("BB") were obtained by chemical transport reactions using both powder of Ln_2CeMO_6Cl_3 (phase "A") and powder of (Ln, Ce)_(3.25)MO_6Cl_(3.5 - X) (phase "AB") as starting materials and chlorine (p{Cl_2; 298 K} = 1 atm) or HCl (p{HCl; 298 K} = 1 atm) as transport agent. A crystal of (La, Ce)_(3.25)NbO_6Cl_(3.5 - x) ("AB") (space group: C2/m, a = 35.288(1) A, b = 5.418(5) A, c = 9.522(1) A, #beta# = 98.95(7) deg, Z = 4) was investigated by x-ray diffraction methods, a crystal of (Pr, Ce)_(3.5)NbO_6Cl_(4 - x) ("BB") was investigated by synchrotron radiation (#lambda# = 0.56 A) diffraction methods. The lattice constants are a = 18.863(6) A, b = 5.545(5) A, c = 9.527(6) A, #beta# = 102.44(3)deg and Z = 4. Structure determination in the space group C2/m (No.12) let to R1 = 0.0313. Main building units are NbO_(6~-) polyhedra with slightly distorted trigonally prismatic environment for Nb and chains of face-sharing Cl_6-octahedra along [010]. The rare earth ions are coordinated by chlorine and oxygen atoms. These main structure features confirmed the expected relation to the starting material Ln_2CeMO_6Cl_3 (phase "A") and to (Ln, Ce)_(3.25)MO_6Cl_(3.5 - x) (phase "AB").