A New Family of Misfit Layered Oxides with Double Rock Salt Layers Biα(A0.75±εBi0.25±εO)(3+3x)/2MO2(A=Ca, Sr andM=Co, Cr)
✍ Scribed by M. Hervieu; Ph. Boullay; C. Michel; A. Maignan; B. Raveau
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 961 KB
- Volume
- 142
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
A series of Bi-based new misfit layered oxides-Bi 0.35 (Sr 0.74 Bi 0.26 O) 1.72 CoO 2 , Bi 0.35 (Ca 0.75 Bi 0.25 O) 1.8 CoO 2 , Bi 0.08 Pb 0.35 (Sr 0.68 Bi 0.32 O) 1.62 CoO 2 , Bi 0.18 Pb 0.4 (Sr 0.76 Bi 0.24 O) 1.74 CrO 2 , and Bi 0.44 (Sr 0.7 Bi 0.3 O) 1.88 CrO 2 -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 [1 0 0]* and [0 0 1]* as common axes and a parameter misfit along and+93°. 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 (CoO 2 ) or (CrO 2 ) 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 (A 0.75؎ Bi 0.25؎ O) (n؉nx)/2 (MO 2 ) 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 (T) measurements show a paramagnetic behavior above 100 K and are in agreement with a mixed valence Co 2؉ (high spin) and Co 3؉ (low spin) for all the cobalt oxides. They confirm the presence of Cr 2؉ (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.