Ferromagnetism-induced reentrant structural transition and phase diagram of the lightly doped insulator La_1-xSr_xMnO₃ (x<~0.17)

The crystal structure of the lightly doped insulating system La_1-xSr_xMnO₃ with x<~0.17 has been studied in detail using neutron diffraction. It has been established that this system has two orthorhombic phases. One is the O^′ phase with b/√2<c<a and with static Jahn-Teller (JT) distortions, while the other is a pseudocubic phase with b/√2∼a∼c, which we call the O^* phase. In particular, the x=0.125 sample exhibits O^*→O^′→O^* reentrant structural transitions. For 0.10<~x<~0.17 a ferromagnetic ordering strongly suppresses lattice distortions at and below the ferromagnetic transition temperature. These results demonstrate a strong coupling between structural distortion, ferromagnetic ordering, and transport properties in the insulating La_1-xSr_xMnO₃ system.