Structure and properties of the CaFe{sub 2}O{sub 4}-type cobalt oxide CaCo{sub 2}O{sub 4}

Journal Article · Sat Sep 15 00:00:00 EDT 2007 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2007.07.008· OSTI ID:21043840

Shizuya, Mitsuyuki ^[1]; Isobe, Masaaki ^[1]; Takayama-Muromachi, Eiji ^[1]

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1. Advanced Nano Materials Laboratory, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)

The calcium cobalt oxide CaCo{sub 2}O{sub 4} was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo{sub 2}O{sub 4} crystallizes in the CaFe{sub 2}O{sub 4} (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO{sub 6} octahedral network. The structure of CaCo{sub 2}O{sub 4} belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) A, b=2.9006(7) A and c=10.282(3) A. Curie-Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co{sup 3+}, 3dt{sub 2g}{sup 6}, S=0), semiconductor-like temperature dependence of resistivity ({rho}=3x10{sup -1} {omega} cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 {mu}V/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant ({gamma}=4.48(7) mJ/Co mol K{sup 2}), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides. - Graphical abstract: A new calcium cobalt oxide CaCo{sub 2}O{sub 4} phase, which crystallizes in the calcium-ferrite-type structure (space group: Pnma; a=8.789(2) A, b=2.9006(7) A, and c=10.282(3) A), has been synthesized for the first time under high temperature and high pressure (1500 deg. C, 6 GPa). This compound exhibits large thermoelectric power (Seebeck coefficient: S=+147 {mu}V/K at 380 K) and an unsaturated temperature dependence of S around 380 K.

• OSTI ID: 21043840
• Journal Information: Journal of Solid State Chemistry, Vol. 180, Issue 9; Other Information: DOI: 10.1016/j.jssc.2007.07.008; PII: S0022-4596(07)00285-X; Copyright (c) 2007 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CALCIUM COMPOUNDS
COBALT IONS
COBALT OXIDES
DOPED MATERIALS
ELECTRIC CONDUCTIVITY
FERMI LEVEL
LATTICE PARAMETERS
MAGNETIC SUSCEPTIBILITY
ORTHORHOMBIC LATTICES
PRESSURE RANGE GIGA PA
SEMICONDUCTOR MATERIALS
SOMMERFELD CONSTANT
SPACE GROUPS
SPECIFIC HEAT
SYNTHESIS
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE 0273-0400 K
TEMPERATURE RANGE 1000-4000 K
X-RAY DIFFRACTION