Charge compensations of Eu²⁺ and O_i²⁻ co-exist in Eu³⁺:CaMoO₄ single-crystal fibers grown by the micro-pulling-down method

Jun Shu, Zhitai Jia, Eugenio Damiano, Haoyuan Wang, Yanru Yin, Na Lin, Xian Zhao, Xinguang Xu, Mauro Tonelli* and Xutang Tao *

CrystEngComm. 2018, DOI: 10.1039/c8ce01160e

Abstract

Eu³⁺-Doped CaMoO₄ single-crystal fibers with concentrations from 0.01 to 1 at.% have been grown by the micro-pulling-down method in an air atmosphere. The origin of the coloration of the crystals can be attributed to the charge compensations. In the air or H₂ annealing process, the absorption spectra combined with XPS proved the coexistence of Eu²⁺ and interstitial O (O_i²⁻). The density functional theory was simulated to display the absorption spectra of Eu2+-doped and (Eu²⁺–O_i²⁻) co-doped CaMoO₄, consistent with the experimental data. The low-temperature polarized absorption and emission spectra illustrated the charge compensations. The luminescence spectra of the Eu3+ probe were used to study the O_i²⁻ species. Luminescence decay curves of ⁵D₀ were measured. To the best of our knowledge, this is the first report on the growth and charge compensations of Eu³⁺:CaMoO₄ single-crystal fibers grown by the micro-pulling-down method and the reduction phenomenon of Eu³⁺ to Eu²⁺ in an oxidizing atmosphere at high temperature in molybdates. On excitation by near ultraviolet or blue light, the molybdates could be potential hosts for white-light-emitting diodes.