Synthesis and structural study of the new rare earth magnesium borates LnMgB 5O 10 ( Ln = La, …, Er)

To obtain rare earth luminescent materials with weak concentration quenching, the B ₂O ₃-rich portion of the ternary diagram Ln₂O ₃MgOB ₂O ₃ ( Ln = rare earth) has been investigated. A ternary phase of composition LnMgB ₅O ₁₀ has been found for Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er. These compounds all crystallize in the monoclinic space group {P2 ₁}/{c}. The structure has been determined on a LaMgB ₅O ₁₀ crystal. A full-matrix least-squares refinement leads to R = 0.039. The structure can be described as being made of (B ₅O ₁₀^5-) _n two-dimensional layers linked together by the lanthanum and magnesium ions. The rare earth atom coordination polyhedra form isolated chains. These borates are isostructural with some rare earth cobalt borates.