A New Family of Misfit Layered Oxides with Double Rock Salt Layers Bi(A0.75±Bi0.25±O)(3+3x)/2MO2(A=Ca, Sr andM=Co, Cr)

A series of Bi-based new misfit layered oxides––Bi0.35(Sr0.74Bi0.26O)1.72CoO2, Bi0.35(Ca0.75Bi0.25O)1.8CoO2, Bi0.08Pb0.35 (Sr0.68Bi0.32O)1.62CoO2, Bi0.18Pb0.4(Sr0.76Bi0.24O)1.74CrO2, and Bi0.44(Sr0.7Bi0.3O)1.88CrO2––has been synthesized and characterized, using electron microscopy and powder X-ray diffraction. Two sets of reflections are evidenced in the ED patterns, associated to the coexistence of two monoclinicS1andS2subsystems, with [1 0 0]*and [0 0 1]*as common axes and a parameter misfit along [0 1 0]*: S1 {a,b1,c,} witha5 ?,b15 ?,c29.8 ?, and93° andS2{a,b2,c,} witha5 ?,b22.9 ?,c29.8 ?, and93°. High-resolution electron microscopy showed that it is the first misfit layered oxides involving double rock salt-type (RS) layers, i.e. triple (AO)-type layers (A=Ca and Sr), with a single (H) hexagonal (CoO2) or (CrO2) layer. As a result, these oxides can be described as the members "n=3/m=1" of a large family with the general formula Bi(A0.75±Bi0.25±O)(n+nx)/2 (MO2)m. In those structures, the bismuth cations are supposed to be distributed over two sorts of sites: (i) theA-sites, filling partly the RS layers and (ii) the sites located between the RS and the H layers, ensuring the cohesion of the structure. The(T) measurements show a paramagnetic behavior above 100 K and are in agreement with a mixed valence Co2+(high spin) and Co3+(low spin) for all the cobalt oxides. They confirm the presence of Cr2+(high spin) in the chromium phases and evidence strong antiferromagnetic interactions at low temperature and paramagnetism with a Curie–Weiss law above 100 K. The Cr oxides are insulators, whereas the pure Bi–cobalt phases are semiconductors or "dirty" metal when the samples have been lead doped.