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J Phys Condens Matter. 2007 May 8;19(18):186213. doi: 10.1088/0953-8984/19/18/186213. Epub 2007 Apr 11.

Modulated and unmodulated structures, and the transport mechanisms in the triangular lattice system Na(x)CoO(2) with [Formula: see text] 0.48, 0.58 and 0.65.

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1
Institute of Physics, University of Tsukuba, Tennodai, Tsukuba 305-8571, Japan.

Abstract

The crystal structures of the triangular lattice system γ-phase Na(x)CoO(2) with compositions of [Formula: see text], 0.58 and 0.65 have been determined by means of x-ray four-circle diffraction. Na(0.58)CoO(2) has lattice constants of a(h) = 2.8180(5) and c(h) = 11.005(9) Å with space group P6(3)/mmc and does not exhibit structural modulation. On the other hand, Na(0.65)CoO(2) has a P6(3)/mmc-type superlattice with a doubled unit cell of a(sh) = 5.6527(5) and c(sh) = 10.9356(10) Å, and Na(0.48)CoO(2) indicates a Pmmn-type superlattice with a(so) = 5.6261(5), b(so) = 11.1406(10) and c(so) = 4.8723(5) Å. The structural modulation for [Formula: see text] is attributed to the incomplete order of Na ions without an order for Co valence, while that for [Formula: see text] results in almost complete Na order and partial valence order for Co. For [Formula: see text], the metal-poor metal transition occurs at about 50 K. The electrical resistivities for [Formula: see text] and 0.65 follow a T(3/2) dependence below 200 and 80 K, respectively, which may be due to antiferromagnetic spin fluctuations in three dimensions. At higher temperatures, the resistivities for the CoO(2) plane have a T(1) dependence, likely due to an enhancement of two dimensionality. For [Formula: see text], the resistivity maximum appears at about 250 K, which may be attributed to the renormalization effect of the Fermi surface by the fluctuations.

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