The rare earth cobaltates: localised or collective electron behaviour?

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Abstract
The electrical conductivity and differential thermal analysis (DTA) characteristics of RECoO3 (RE=La, Nd, Gd, Ho, Y) have been reexamined. In general, the data are quite different from those previously reported. In particular, first-order semiconductor-to-metal transitions of YCoO3, LaCoO3 and HoCoO3 are not observed. Instead, broad, high-order semiconductor-to-metal transitions are observed over the approximate temperature ranges La: 385-570 K; Nd: 400-590 K; Gd: 490-770 K; Ho: 540-795 K; Y: 575-850 K. The form of the transitions is similar to that observed for Ti2O3 where a thermally activated band overlap model has successfully been employed. A similar scheme for the rare earth cobaltates can be constructed in which Co 3d electron character is considered collective. This is contrary to the localised-to-collective phase transition model previously employed. In addition, a first-order semiconductor-to-semiconductor transition of NdCoO3 at 342 K is observed in electrical conductivity data. The tenfold increase in conductivity accompanying this structural transition can also be understood within a collective 3d-electron model.