Optical and ESR investigations of lanthanum aluminates LaMg 1- xMn xO 19 single crystals with magnetoplumbite-like structure

New lanthanum aluminates La MAl ₁₁O ₁₉ ( M²⁺ = Ni, Co, Mn, Mg _{1- x}Mn _x, 0 ≤ x ≤ 1), with magnetoplumbite-like structure have been obtained as single crystals. This paper is particularly devoted to the Mn ²⁺ and {Mg²⁺}/{Mn²⁺} mixed compounds, which exhibit promising luminescent properties. Several characteristics of the crystals are given. The absorption spectra of the materials, as grown, are assigned to Mn ²⁺ ions in tetrahedral sites. After annealing in air new absorptions attributed to octahedral Mn ³⁺ ions, appear. The ESR spectra of Mn ²⁺ in all these crystals exhibit axial symmetry. For x ≤ 0.25 they arise from isolated Mn ²⁺ ions in slightly distorted tetrahedral sites and reveal a strong disorder effect. For x ≥0.5 the spectra consist of a single line, attributed to clusters of magnetically interacting Mn ²⁺ ions.