Reinvestigation of the layered double hydroxide containing tetravalent cations: Unambiguous response provided by XAS and Mossbauer spectroscopies

The incorporation of tetravalent cations (Zr4+ and Sn4+) inside sheets of layered double hydroxide (LDH) was reported by Velu et al. As no local structural information was provided so far, the question here is whether the LDH structure may accommodate such a presence. From three different systems presenting the cation compositions MgAlSn, MgAlZr, and CoAlSn, we provide (by means of X-ray absorption and Mossbauer spectroscopies) clear evidence that the tetravalent cations are segregated from the LDH structure and form amorphous M-IV oxide-like particles. Variation of the cell parameters, a and c, is then explained by a substitution of Al cations by the divalent cations, M-II. It is also shown that despite the fact that the process of reconstruction occurs for MgAlM (M = Zr and Sn), the tetravalent cations do not participate in this structural recovery.