Semiconducting Properties of Ln2CuO4 (Ln=Rare earths)

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Abstract
The temperature dependence of the resistivity and the thermoelectric power of Ln2CuO4 (Ln=rare earths) have been measured. The compounds of Gd, Sm, Nd, and Pr are semiconductive, while La2CuO4 shows metallic conductivity. The tetragonal lattice of La2CuO4 shrinks in the a axis and expands in the c axis compared to Pr2CuO4, in spite of the steady expansion of these lattice parameters for the compounds between Gd and Pr. The thermoelectric power of Ln2CuO4, but not La2CuO4, is negative, suggesting an electron-conducting or n-type semiconductor. The resistivity of the Ln2CuO4 at room temperature increases essentially with an increase in the atomic number of Ln. The plots of the thermoelectric power vs. the reciprocal temperature showed a steeper slope for the compounds of the heavier rare earths. The semiconductor-metal transition observed between La2CuO4 and Pr2CuO4 was explained in relation to the structural change (the shrinkage of the a axis and the expansion of the c axis) as being due to the change in the electron configurations of Cu2+.