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 simi lar to T-c, the insulator-metal (I-M) transition is not observed in th e latter one despite the expectation based on the Goldschmidt's tolera nce factor. The first order character of the I-M transition in Pr0.7Sr 0.1Ca0.2MnO3 is demonstrated by a discontinuity of the unit cell volum e, step of the specific heat and huge enhancement of the thermal condu ctivity. The lack of these anomalies in Pr0.85K0.15MnO3 suggests that the properties of perovskite manganites are critically influenced by b oth a strong phonon-electron coupling arising from a Jahn-Teller split ting of e(g) orbitals of the Mn3+ ion and the local lattice effects du e to size mismatch of the large cations. (C) 1997 American Institute o f Physics.