Disorder in alkali lead fluorides Na2–xKxPb11F18X6 with X = Cl, Br

Abstract

Abstract The crystal structures of five new alkali-lead halide compounds of the family M′2–x M″xPb11F18X6 with M′, M″ = Na, K, Na/K mixed and X = Cl, Br were studied. The compounds crystallize in space group P63/m [176]. A disordered lead/alkali halide network around alkali-filled trigonal prismatic channels builds the structure. Both lead sites are partially replaced by alkaline ions. The trigonal prismatic channels formed by fluorides are filled with Na+ in Wyckoff site 2b (CN 6) with pop = 0.5 or K+ in Wyckoff site 2a (CN of 3 + 6) with pop = 0.5. For the potassium sample, a split position of the fluorides forming the channel was found. The structures with one alkaline ion show lattice parameters of a = b = 14.0679(9) Å, c = 3.9954(4) Å (Na/Cl), a = b = 14.2617(14) Å, c = 3.9991(4) Å (K/Cl), a = b = 14.5267(7) Å, c = 4.0580(3) Å (K/Br). In NaKPb11F18Br6 and the Na0.714(16)K1.286(16)Pb11F18Cl6 (lattice parameters a = b = 14.4377(12) Å, c = 4.0576(3) Å and a = b = 14.1780(7) Å, c = 4.0093(3) Å), the lead ions in the matrix are partially substituted only with potassium. The occupation of the channels however is different: both alkaline ions are found in the chloride, (but in different Wyckoff positions) whereas in the bromide only sodium was observed. NaKPb11F18Br6, can therefore be described as a new type in this structure family. Lattice parameters of the alkaline mixed samples show unexpected increase of the c-axis for both heavy halides. This property may be related with the occupation of Wyckoff position 2b. A similar behaviour was found for Ba7F12Cl2 (Ba2+ with CN 3 + 6) doped with Ca2+ (CN 6). The vibrational parameters of all alkaline ions in the channel are strongly anisotropic.