Thermal anomalies and the insulator-metal (I-M) transition in Mn3+/Mn4+ perovskites

We report a comparative study of two ferromagnets-Pr0.7Sr0.1Ca0.2MnO3 and Pr0.85K0.15MnO3-which exhibit the same formal Mn valency, similar T-c and saturated moments at 5 K. While for former sample the magnetic transition is accompanied by the insulator-metal transition at T similar to T-c, the insulator-metal (I-M) transition is not observed in the latter one despite the expectation based on the Goldschmidt's tolerance factor. The first order character of the I-M transition in Pr0.7Sr0.1Ca0.2MnO3 is demonstrated by a discontinuity of the unit cell volume, step of the specific heat and huge enhancement of the thermal conductivity. The lack of these anomalies in Pr0.85K0.15MnO3 suggests that the properties of perovskite manganites are critically influenced by both a strong phonon-electron coupling arising from a Jahn-Teller splitting of e(g) orbitals of the Mn3+ ion and the local lattice effects due to size mismatch of the large cations. (C) 1997 American Institute of Physics.