Synthesis, crystal structure and magnetic properties of the Sr2Al0.78Mn1.22O5.2 anion-deficient layered perovskite

A new layered perovskite Sr{sub 2}Al{sub 0.78}Mn{sub 1.22}O{sub 5.2} has been synthesized by solid state reaction in a sealed evacuated silica tube. The crystal structure has been determined using electron diffraction, high-resolution electron microscopy, and high-angle annular dark field imaging and refined from X-ray powder diffraction data (space group P4/mmm, a=3.89023(5) A, c=7.8034(1) A, R{sub I}=0.023, R{sub P}=0.015). The structure is characterized by an alternation of MnO{sub 2} and (Al{sub 0.78}Mn{sub 0.22})O{sub 1.2} layers. Oxygen atoms and vacancies, as well as the Al and Mn atoms in the (Al{sub 0.78}Mn{sub 0.22})O{sub 1.2} layers are disordered. The local atomic arrangement in these layers is suggested to consist of short fragments of brownmillerite-type tetrahedral chains of corner-sharing AlO{sub 4} tetrahedra interrupted by MnO{sub 6} octahedra, at which the chain fragments rotate over 90 deg. This results in an averaged tetragonal symmetry. This is confirmed by the valence state of Mn measured by EELS. The relationship between the Sr{sub 2}Al{sub 0.78}Mn{sub 1.22}O{sub 5.2} tetragonal perovskite and the parent Sr{sub 2}Al{sub 1.07}Mn{sub 0.93}O{sub 5} brownmillerite is discussed. Magnetic susceptibility measurements indicate spin glass behavior of Sr{sub 2}Al{sub 0.78}Mn{sub 1.22}O{sub 5.2}. The lack of long-range magnetic ordering contrasts with Mn-containing brownmillerites and is likely causedmore » by the frustration of interlayer interactions due to presence of the Mn atoms in the (Al{sub 0.78}Mn{sub 0.22})O{sub 1.2} layers. - Graphical abstract: In contrast to Sr{sub 2}Al{sub 1.07}Mn{sub 0.93}O{sub 5}, the local atomic arrangement in these layers consist of short fragments of brownmillerite-type tetrahedral chains of corner-sharing AlO{sub 4} tetrahedra interrupted by MnO{sub 6} octahedra, at which the chain fragments rotate over 90 deg. When derived by bulk structure determination techniques such as X-ray powder diffraction, the structure will be described with an averaged tetragonal symmetry.« less