Europium valence control in the hydrothermal synthesis of apatites and borosilicates

• Published in: Journal of alloys and compounds 2016-01, Vol.656, p.206-212
• Main Authors: Heyward, Carla C., Kimani, Martin M., Moore, Cheryl A., McMillen, Colin D., Kolis, Joseph W.
• Format: Article
• Language: English
• Subjects: Apatite; Borosilicate; Crystal growth; Dopants; Emission analysis; Europium; Fluid dynamics; Fluid flow; Fluids; Luminescence; Rare earth compounds; X-ray diffraction
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2015.08.279

The solid-state chemistry of novel metal silicate and borosilicate crystals containing Eu2+ and Eu3+ in high temperature hydrothermal fluids was investigated. It was found that europium ions could be readily incorporated into a number of crystals using a variety of simple glass-based feedstocks. The hydrothermal growth reactions were performed at nearly 600 °C in 1–5 M NaOH mineralizer fluids, and this was found to be a versatile route to both europium ion oxidation states with Eu acting as either a fundamental structural building block or a dopant ion. Several new single crystals were identified including two new apatites as well as a new europium borosilicate. The apatites crystallize in space group P63/m with a = 9.4619(13) Å and c = 7.0054(14) Å for Eu10(SiO4)6O2, and a = 9.4413(13) Å and c = 6.9087(14) Å for NaEu9(SiO4)6O2. The new borosilicate, Eu2SiB2O8, crystallizes in space group Pbcn with a = 13.310(3) Å, b = 4.4247(9) Å and c = 9.2394(18) Å. The investigation of Eu-doped borosilicates demonstrated the strong blue emission generated from 370 nm excitation that was expected for the Eu2+ doped materials, as well as the less intense orange-red emission expected for the Eu3+ dopants. The hydrothermal reactions naturally facilitated a degree of europium reduction in Eu:Ba3Si2B6O16. It was found that the oxidation states of europium in the final product could be further controlled by using hydrazine as a reducing reagent. [Display omitted] •The synthesis of Eu2+ and Eu3+ containing silicates and borosilicates is demonstrated.•Structural and spectroscopic properties are investigated.•Hydrothermal conditions assist in partial europium reduction.•Addition of hydrazine to the hydrothermal fluid accomplishes further Eu reduction.