Crystal Structure and Chemical Characterization of $La_{0.7}Sr_{0.3}Mn_{0.7}Ti_{0.3-x}Al_{x}O_3$ (0 ≤ x ≤ 0.15) Compounds

Neutron powder diffraction and magnetization measurements have been performed for $La_{0.7}Sr_{0.3}Mn_{0.7}Ti_{0.3-x}Al_xO_3$ (0 ≤ x ≤ 0.15) stoichiometric compounds. Increase of the Al³⁺ content enlarges the Mn⁴⁺ ions fraction from 0% (x=0) up to around 20% (x=0.15). The x=0 composition around 150 K exhibits a structural transition from the rhombohedral phase to the orthorhombic one whereas the crystal structure of the compounds with x=0.1 and 0.15 remains to be rhombohedral down to 2 K. The substitution of Ti⁴⁺ by Al³⁺ ions is accompanied by a gradual increase in the bond angle Mn-O-Mn and decrease in the Mn-O bond length which lead to enhancement of the covalent component of the chemical bond. All these compounds exhibit ferromagnetic components below 100 K. Magnetic moments estimated per manganese from the neutron powder diffraction data are found to be around $1.3 μ_{B}$ (x=0) and $1.7 μ_{B}$ (x=0.1 and 0.15) at 2 K. It is suggested that ferromagnetism is originated predominantly from the Mn³⁺-O-Mn³⁺ and Mn³⁺-O-Mn⁴⁺ superexchange interactions whereas bond angles fluctuation leads to magnetic frustrations. Enhancement of covalence slightly increases ferromagnetism.