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Another binary oxide Sm Fe O that can be prepared in air by the solid state
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reaction [24,26, 27] or by the citrate-nitrate technique [25] possesses the cubic
garnet structure (Ia3d sp. gr.) with the unit cell parameter a=12.530 Å [39].
Ca-Co-O system. Three complex oxides have been described in the Co-Co-O
system, namely, CaCo O , Ca Co O , and Ca Co O [29-32].
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9+δ
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The crystal structure of CaCo O depends on the synthesis conditions. CaCo O
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obtained by the citrate-nitrate technology [32] or carbonates co-precipitation
method within the temperature range 500-650°C in oxygen or in air [40] possesses
the monoclinic cell with the unit cell parameters a = 11.490 Å, b = 5.772 Å,
c = 4.737 Å [32]. The high pressure synthesis at 1500°C and 6 GPa leads to the
formation of CaCo O with the CaFe O -type orthorhombic structure (sp.gr. Pnma,
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the unit cell parameters a = 8.789 Å, b = 2.901 Å and c = 10.282Å) [41]. The
decomposition temperature for CaCo O in air was found to be located between
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00°C and 700°C [32].
Widely studied thermoelectric oxide Ca Co O has a layered structure, formed by
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the alternating of Ca CoO layers with the rock-salt structure and CoO layers with
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the CdI -type structure [17, 18]. Ca Co O reveals the monoclinic structure (sp.
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gr. P2 /m) [18] with the unit cell parameters a = 4.832Å, b = 36.467Å,
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c = 10.819 Å, β = 98.05°. The upper temperature limit of Ca Co O in air was
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found to be equal to 926°C [31], however, Tran et al. [32] reported that the
decomposition of this phase starts at 800°C.
Ca Co O was prepared by the citrate technique [42], or by the solid phase
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synthesis [30, 43]. It has a pseudo-one-dimensional structure (sp. gr. R-3c) with the
unit cell parameters a = 9.079 Å, c = 10.381 Å [42]. The decomposition
temperature of Ca Co O in air was determined as 1026°C [31, 32].
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The afore-mentioned data on thermodynamic stability of all known phases [29, 30]
and the phase equilibria study reported in refs. [31, 32] confirm the absence of the
intermediate complex oxides at 1100°C in air inside the Ca-Co-O system. The
solubility of cobalt oxide in calcium oxide at 1100°C in air was estimated as 7
mole %, while the solubility of CaO in CoO was evaluated as 14 mole % [31].