The phase diagram and optical properties of La2-xSrxMnO3 for x <= 0.2

We report on detailed susceptibility, magnetization and magnetoresistance s tudies in La1-xSrxMnO3 for x less than or equal to 0.2. We arrive at a phas e diagram which shows a ferromagnetic and insulating ground state for 0.1 l ess than or equal to x less than or equal to 0.15, followed by a canted ant iferromagnetic phase at higher temperatures. The ferromagnetic transition i s strongly coupled to a structural transition from a Jahn-Teller distorted phase to a pseudocubic orthorhombic phase, and is accompanied by the appear ance of large positive magnetoresistance effects. We interpret the rich pha se diagram close to x = 1/8 in terms of orbital ordering, due to an interpl ay of superexchange interactions and Jahn-Teller distortions. In addition, we report on the temperature dependence of phonon spectra and on optical co nductivities. The temperature dependence of bending and stretching modes cl early indicate the structural and magnetic-phase transitions. The optical c onductivities are explained in terms of small polaron absorption. (C) 2000 Elsevier Science B.V. All rights reserved.