Dilatation thermique, anisotropie structurale, et polymorphisme des sulfates de plomb PbSO 4, nPbO ( n = 0, 1, 2, 4)
The thermal expansion of basic lead sulfates has been measured by X-ray diffraction in the 77-1000 K temperature range. The results are interpreted by taking into account the different structural characteristics of the four compounds which have been studied (PbSO 4, nPbO with n = 0, 1, 2, and 4). Lead sulfate PbSO 4 is built up from tetrahedral SO 4 groups which are connected by lead atoms, yielding thus a quasi-isotropic thermal expansion. The introduction of additional PbO groups corresponds to the substitution of lead chains by -Pb 2O- chains (monobasic sulfate), or by -Pb 3O 2- double chains (dibasic sulfate). Layer-type structures are thus created, and the thermal expansion becomes very anisotropic. The transition observed at 723 K for the dibasic lead sulfate is explained by taking into account the density and the different thermal behavior of the compounds before and after transition. Some information concerning the tetrabasic lead sulfate structure is deduced.