NEW FAMILY OF MISFIT LAYERED OXIDES WITH DOUBLEK SALT LAYERS Bi(A0.75+-Bi0.25+-O)(3+3x)/2MO2 (A=Ca, Sr AND M=Co, Cr)

摘要

A series of Bi-based new misfit layered oxides-Bi0.35(Sr0.74Bi0.26O)1.72CoO2, Bi0.35(Ca0.75Bi0.25O)1.8 CoO2, 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 XRD. Two sets of reflections are evidenced in the ED patterns, associated to the coexistence of two monoclinic S1 and S2 subsystems, with [1 0 0]* and [0 0 1]* as common axes and a parameter misfit along [0 1 0]*: S1 {a, b1, c, beta} with a approx equal to 5 angstroms, b1 approx equal to 5 angstroms, c approx equal to 29.8 angstroms, and approx equal to 93 degrees and S2 {a, b2, c, beta} with a approx equal to 5 angstroms, b2 approx equal to 2.9 angstroms, c approx equal to 29.8 angstroms, and beta approx equal to 93 degrees. HREM showed that it is the first misfit layered oxides involving doublek 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 Bi cations are supposed to be distributed over two sorts of sites: (i) the A-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 chi(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-Co phases are semiconductors or "dirty" metal when the samples have been lead doped. 15 refs.