Abstract

A complex layered oxide has been synthesised as bulk material starting from the theoretical n = 2 R-P formula Sr₃(Mg_xMo_1 − x)₂O_7 − δ (x ≈ 0.25). This compound exhibits an unexpected crystal chemistry illustrated by the coexistence of several stacking modes — principally the ordered double perovskite Sr₂MgMoO_6 − δ and n = 2 R-P type phase — inside the same crystallite. Chemical EDS analyses performed by TEM techniques have lead to an average chemical formula Sr_2.68Mg_0.64Mo_1.67, which differs from the expected n = 2 R-P formula Sr₃Mg_0.5Mo_1.5O_7 − δ in particular through a significant Sr deficiency. From high resolution electron microscopy (HREM) and Z-contrast (SuperSTEM) images, this analytical result is interpreted by the formation of a complex intergrowth [(Sr,Mg)₃Mo₂O₇]_1 − x[Sr₂MgMoO₆]_x with x ≈ 0.4. The resulting structure can be interpreted as a compositional segregation between Mg²⁺ and Mo⁶⁺ species in two basic structures, namely Sr₃Mo₂O₇ and Sr₂MgMoO₆.