Paramagnetic Mn Antisite Defects in Nanoceramics of Aluminum–Magnesium Spinel

Abstract

The effect of structure and size parameters on the formation of intrinsic and impurity paramagnetic centers in nanoceramics of aluminum–magnesium spinel is studied. The studied samples (grain size ~30 nm) are obtained by thermobaric synthesis. Microcrystalline ceramics and MgAl₂O₄ single crystal are used as the reference samples. Characteristic paramagnetic centers of Mn²⁺ (hyperfine structure constant (HFS) A = 82 G) are present in both single crystal and microceramics. In the studied samples of nanoceramics in the initial state, both impurity Mn²⁺ and intrinsic F⁺ centers exist. Unlike the nanoceramics, the centers of F⁺ type in the reference sample appear only after the irradiation with accelerated electrons (130 keV). The parameters of Mn²⁺ centers in nanoceramics significantly differ on that in microceramics and single crystal. EPR signal of Mn²⁺ centers in nanoceramics is characterized by two anomalous constant HFS (A₁ = 91.21 G, A₂ = 87.83 G) caused by two types of octahedrally coordinated manganese ions ([Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ antisite defects). The features of spectral parameters of manganese centers correlate with a decrease in the cell parameter of MgAl₂O₄ in the nanostructural state. The observed effects are interpreted based on the assumed scheme of [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ charge compensation by the aluminum antisite defect and F⁺ center.