Thermoelectric power and resistivity of La1.8Sr0.2CaCu2O6-δ and the effects of O2 hot-isostatic-press annealing

We have measured the resistivity and thermoelectric power of La1.8Sr0.2CaCu2O6-δ for 15≤T≤340 K. For the as-sintered sample heated under one atmosphere of oxygen, a minimum resistivity occurs at ca. 100 K, and the conductivity can be fitted to the form exp(T-1/2) or exp(T-1/4) for 30.3≤T≤86.8 K. The thermoelectric power varies as T1/2 for 20≤T≤200 K and exhibits a saturated value of ca. 100 μV/K for T≥230 K. Upon O2 HIP (hot isostatic press) treatment, the upturn of resistivity (metal- nonmetal transition) is suppressed and superconductivity ensues, and the thermoelectric power decreases in magnitude and displays a broad maximum at ca. 190 K in a manner similar to La2-xSrxCuO4-δ. It is conceivable that the O2 HIP treatment suppresses the metal-nonmetal transition as a result of increasing carrier concentration and possibly enhancing cation ordering. The possible origin of the metal-nonmetal transition is discussed