Hexalead chloride triorthoborate oxide, Pb4O[Pb2(BO3)3Cl

Abstract

M r = 1471.1, F(000)= 2414.7, orthorhombic, Pbcm, a = 7.021 (3), b = 17.218 (6), c = 10.952 (3) A, V = 1 3 2 4 ( 1 ) fi3, Z = 4 , D x=7.38, D m = 7-19 Mg m -3 (pycnometric method), 2(Mo K~t) = 0.71069 A, /~(Mo Ka) = 76.85 mm -~. The R value is 0.057 for 1210 symmetrically independent reflections. The structure of PbaO[Pb2(BO3)3CI] can be regarded as being built up of Pb40 tetrahedra, Pb-Pb dumbbells, isolated planar B O 3 groups and isolated CIions. The crystals show strong dispersion of the optic axial angle. Introduction. In the system PbO.PbC12.B203 two compounds have already been reported: Pb3B7OI3CI (Jona, 1959) which probably has a boracite-like structure and Pb2B5OgCI (Fouassier, Levasseur & Hagenmuller, 1971), the structure of which is still unknown. Our efforts to synthesize apatite-like compounds in this system yielded still another lead halogenoborate with the formula PbaO[Pb2(BO3)3C1] whose crystal structure could be determined. , Experimental. Single crystals of the new compound Pb40[Pb2(BO3)3CII were grown from a nearly stoichiometric melt of PbO, PbC12 and H3BO 3 in a platinum crucible. The crystals show a rod-shaped habit, the direction of the rods being parallel to c. The forms {100} and {010} are usually well developed; in some cases {001 } or {201 } are also found. The crystals show strong dispersion of the optic axial angle. These optic axial angles were measured on a universal stage: 2 V = 59 (I) ° at 440 nm, 44 (1) ° at 540 nm and 39 (1) ° at 650 nm. The optic axes lie in the ab plane with a being the acute bisectrix. For X-ray investigations a crystal with forms {100}, {010} and {001 } was selected; distances of faces from crystal centre: (100) 43, (010) 13 and (001) 68 ~tm. Crystal data and X-ray extinctions taken from precession photographs. Lattice constants refined from 16 Bragg reflections, measured on a Picker diffractometer. Data collection: 4470 reflections (hkl-hkl)measured in the range 3.5 _5OF, with a F from counting statistics. R(merge) = (~{N~lF(mean) -Fo]2/Z[(N--1)Z(F~o)]})'/2=O.041 (unit weight). Atomic scattering factors and dispersion coefficients for Pb, CI, O and B from International Tables for X-ray Crystallography ( 1974).* The structure was solved by Patterson methods. The three-dimensional Patterson function P(uvw) shows strong maxima (due to Pb-Pb vectors) in sections u = 0 and u = 0.5 only. Both maps can be brought into coincidence by a translation of 0.5 along w. This indicates an equal distribution of the 24 Pb atoms among the two layers. The interlayer distance is approximately 0.5x and the atomic positions in the two layers are displaced by about 0.5z. From these considerations approximate positions for four symmetrically independent Pb atoms can be derived. Since the intensity distribution statistics did not give clear information about the space group, the centrosymmetric Pbcm was chosen. Refinements and Fourier maps yielded the positional parameters of all other atoms. The structure was refined with anisotropic thermal parameters for all atoms. The calculated strong anisotropy of some temperature factors for the relatively light atoms (mainly C1) seems to indicate a possibly insufficient absorption correction (,u = 76.8mm -j) and some positional disordering. Alternative calculations in the noncentrosymmetric space group gave identical results. The refinement results are: * Lists of structure factors, anisotropic thermal parameters and bond angles have been deposited with the British Library Lending Division as Supplementary Publication No. SUP 38665 (13 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England. © 1983 International Union of Crystallography 1318 H E X A L E A D C H L O R I D E T R I O R T H O B O R A T E O X I D E R = 0.057, R,, = 0-059, with w -1/(a~ + 0-0025 F z) and an extinction-correction factor E = ~, x 10 -5 with Fc=F/[l+O.OO2EF2/sin(20)l 1/4. S = 1.132 and the slope for the normal probability plot is 1.026. (zJ/a)m. × = 0.04 and (A/O')mea n for all parameters is less than 0.01. The final Ap map showed maxima and minima of about 7 and 6 e A -3 immediately neighbouring the Pb atoms. Discussion. The final atomic parameters are listed in Table 1. Fig. 1 shows a central projection of the structure along e. Interatomic distances are given in Table 2. The structure can be regarded as being built up Table 1. Atom coordinates (x 10 4) and equivalent isotropic temperature factors (A. 2 x 10 3) Ueq is defined as one third of the trace of the orthogonalized U 0 tensor.