Neutron-diffraction study of structural transition and magnetic order in orthorhombic and rhombohedral La7/8Sr1/8Mn1-γO3+δ

We report on a systematic neutron powder diffraction (NPD) study of polycrystalline La7/8Sr1/8Mn1-gamma O3+delta compounds. We investigated the structural and magnetic phases and transitions in the temperature range between 10 and 900 K for two different samples: an Ar-annealed sample with an orthorhombic Pbnm (Z = 4; O) structure at room temperature and an air-sintered sample with a rhombohedral Rempty set c (Z = 2; R) structure at room temperature. At higher temperatures, above 400 K, both samples exhibit a rhombohedral structure. For the Ar-annealed sample, a Jahn - Teller (JT) transition occurs in the orthorhombic phase at about 298 K with very large variations in the Mn - O2' and Mn - O2 bond lengths andMn - O1 - Mn bond angle on cooling from 298 to 180 K. For this sample the ferromagnetic moment at 10 K in the magnetic space group Pb'n'm amounts to 3.22(5) mu(B)/Mn. In contrast, the air-sintered sample undergoes on cooling a structural transition from rhombohedral (R) to orthorhombic (O) with a mixed phase of nearly equal R and O repartition at 120 K. Ferromagnetic order develops in this sample at about 240 K with a low-temperature moment of 3.35(4) mu(B)/Mn at 10 K. The coherent JT distortion in its orthorhombic phase occurs below some 170 K. In addition, we have determined the coherent JT distortion parameter Delta, the tolerance factor t and the one-electron bandwidth W of the e(g)-band.