Structure, magnetic and optical properties of Sr3RE2(Ge3O9)2 cyclogermanates (RE = La–Gd)

Abstract

A new series of Sr₃RE₂(Ge₃O₉)₂ (RE = La, Pr, Nd, Sm, Eu, Gd) cyclogermanates has been prepared using an ethylenediaminetetraacetic acid assisted route. The powder XRD study reveals that the compounds crystallize in the monoclinic space group C2/c, Z = 4. A decrease in the crystal radius of RE³⁺ ions in Sr₃RE₂(Ge₃O₉)₂ (RE = La–Gd) causes a morphotropic phase transition followed by a step-like change of the lattice parameters. The transition is accompanied by internal reorganization, including the redistribution of rare earth and strontium ions among three symmetry independent Sr/RE sites, a conformation change of [Ge₃O₉]⁶⁻ rings and a variation of Sr/RE(2) atom coordination from seven-fold to nine-fold. Density functional theory calculations reveal that Sr₃RE₂(Ge₃O₉)₂ compounds are indirect bandgap materials with a band gap around 3.0 eV. The magnetic property measurements indicate that Sr₃RE₂(Ge₃O₉)₂ (RE = Pr, Sm, Gd) are Curie–Weiss paramagnets up to 2 K. For Sr₃Eu₂(Ge₃O₉)₂ an anomaly of magnetic susceptibility is found at about 60 K. A photoluminescence study of the Sr₃Sm₂(Ge₃O₉)₂ and Sr₃Eu₂(Ge₃O₉)₂ samples reveals an emission in the visible spectral range, corresponding to the characteristic 4f–4f transitions in rare earth ions.