Crystal, electronic structures, optical and magnetic properties of Tb₄Al₂O₉

The crystal and electronic structures, optical and magnetic properties of Tb₄Al₂O₉ have been reported. Polycrystalline Tb₄Al₂O₉ was synthesized by a solid state method at 1600 °C for 4 h under N₂ atmosphere (0.48 MPa) using AlN as an in situ reducing agent. Tb₄Al₂O₉ is crystallized in the monoclinic crystal system having space group P2₁/c with a = 0.74831(1) nm, b = 1.05569(1) nm, c = 1.11507(1) nm, β = 108.96(1)⁰. Tb₄Al₂O₉ was calculated to be a wide band gap semiconductor with indirect band gap energy of about 3.83 eV in fair agreement with the experimental value ∼4.59 eV obtained from the diffuse reflection spectrum. No photoluminescence of Tb³⁺ could be observed because of large effects of photoionization and concentration quenching of Tb³⁺ in Tb₄Al₂O₉. Tb₄Al₂O₉ exhibited weak antiferromagnetic interactions with an effective magnetic moment of near that of the Tb³⁺ free ion.