Growth and characterization of Na/sub x/CoO/sub 2-δ/ crystals

Abstract

Na/sub x/CoO/sub 2-/spl delta// (0.5/spl les/x/spl les/1) crystals were grown by a flux method. The largest Na/sub x/CoO/sub 2-/spl delta// crystal size was 10/spl times/10/spl times/0.5 mm, and the crystal phase depended on the composition of the flux. Crystals grown from a flux of NaCl mixed with Na/sub 2/CO/sub 3/ and B/sub 2/O/sub 3/ and crystals grown from a flux of Na/sub 2/CO/sub 3/ mixed with B/sub 2/O/sub 3/ were identified as Na/sub 0.6/CoO/sub 2-/spl delta// (/spl gamma/ phase) and Na/sub 0.9/CoO/sub 2-/spl gamma// (/spl alpha/ phase), respectively. The in-plane (ab-axis) thermoelectric properties of each phase were measured. The electrical resistivity (/spl rho/) of the /spl gamma/ phase and the a phase showed the almost same metallic temperature dependence except for an unusual increase of /spl rho//sub /spl alpha// at 5-20 K and the different magnitude. The value of /spl rho//sub /spl gamma// and /spl rho//sub /spl alpha// were 1.3 m/spl Omega/cm and 30 m/spl Omega/cm at 300 K, respectively. The Seebeck coefficient (S) of each phase exhibited the different S(T) behavior and the different magnitude, and the values were 100 /spl mu/V/K and 150 /spl mu/V/K at 300 K, respectively.