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The role of the Pb2+ lone electron pair for bond valence sum analysis in mullite-type PbMBO4 (M = Al, Mn and Fe) compounds

Abstract The bond valence sums (BVS) of the constituent atoms of three series of mullite-type compounds PbMnxAl1–xBO4, PbFexAl1–xBO4 and PbMnxFe1–xBO4 (x = 0 – 1) were analyzed with particular emphasis on lead. While the formal BVS of one bridging oxygen atom in the edge-sharing MO6 octahedron was found to be under-bonded, the other bridging atom is over-bonded. The lead atom showed always an over-bonded nature throughout the series and exhibited a different bonding strength than that of the expected valence. The stereoactivity of the 6s2 lone electron pair (LEP) of the Pb2+ cation was monitored in terms of the absolute value of the eccentricity parameter (|Φi|). Both calculated BVS and bond valence parameter (r0i) of Pb2+ cation showed a linear correlation with |Φi|, thus corroborated the Wang–Liebau [Z. Kristallogr. 211 (1996) 437] hypothesis concerning the influence of lone-pair electrons on bond-valence parameters.