The origin of the change in type of the majority carrier in LaCo1-xTixO3 (0.05 <= x <= 0.15)

Electrical transport properties of polycrystalline ceramic specimens of the system LaCo1-xTixO3 (x = 0.05, 0.10, and 0.15) have been investigated as f unctions of temperature by means of complex-plane impedance analysis, diele ctric properties, four-probe de conductivity, Seebeck coefficients, and mag netic susceptibilities. The type of the majority carrier changes from elect rons to holes when x increases from 0.10 to 0.15, because the Seebeck coeff icient changes from negative to positive with x increasing from 0.10 to 0.1 5. The complex-plane impedance analysis distinguishes the bulk conduction f rom the conduction across grain boundaries. In a specimen with x = 0.15, th e activation energy in the bulk conduction is nearly equal to that for the dielectric relaxation. This implies that the hopping process of small polar ons of holes dominates the transport properties in this specimen above 190 K. The temperature dependencies of the magnetic susceptibilities indicate t hat the number of high-spin Co3+ ions (S = 2) decreases with increasing x. The expansion of the lattice parameters and the increment of the c(H)/a(H) ratio with increasing x suggest that Ti4+ ions are preferentially substitut ed for low-spin Co-III, which relaxes the rhombohedral structural distortio n in the LaCo1-xTixO3 system. The change in the type of the charge carrier has been discussed in terms of the electronic structures deduced from these experimental results.