A new family of misfit layered oxides with double rock salt layers Biα(A0.75±εBi0.25±εO)(3+3x/2)MO2 (A = Ca, Sr and M = Co, Cr)

A series of Bi-based new misfit layered oxides-Bi-0.35 (Sr0.74Ba0.26O)(1.72)CoO2, Bi-0.35 (Ca-0.75 Bi-0.25 O)(1.8) CoO2, Bi-0.08 Pb-0.35 (Sr0.68Bi0.32O)(1.62)CoO2, Bi0.18Pb0.4(Sr0.76Bi0.24O)(1.74) CrO2, and Bi-0.44(Sr0.7Bi0.3O)(1.88) CrO2-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 monoclinic S-1 and S-2 subsystems, with [100]* and [001]* as common axes and a parameter misfit along [010]*: S1 {a, b(1),c, beta} with a approximate to 5 Angstrom, b(1) approximate to 5 Angstrom, c approximate to 29.8 Angstrom, and beta approximate to 93 degrees and S-2 {a, b(2), c, beta} with a approximate to 5 Angstrom, b(2) approximate to 2.9 Angstrom, c approximate to 29.8 Angstrom, and beta approximate to 93 degrees. 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 alpha(A(0.75+/-epsilon) Bi0.25+/-epsilonO)((n+nx)/2) (MO2)(m). In those structures, the bismuth 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 mix;ed 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. (C) 1999 Academic Press.