EPR spectroscopy and structural investigations, of Eu²⁺-doped chloroborate glass-ceramic

Abstract

Europium doped (0.1% and 1%) chloroborate BaO–B₂O₃–BaCl₂ glass and glass-ceramic with embedded BaCl₂ nanocrystals have been synthesised, investigated, and compared with the corresponding nanocrystalline powder. The structural analysis of the glass evidenced ionic bonds characteristic to the glass host structure and compositional changes during the melting. X-ray diffraction and transmission electron microscopy revealed the formation of orthorhombic BaCl₂ nanocrystals of about tens on nm in size and a smaller amount of BaB₂O₄ nanocrystals. Structural analysis of the Eu²⁺-doped BaCl₂ nanocrystals has shown a distortion of the crystalline cell assigned to the growth process, affected by defects and ionic impurities. Photoluminescence spectra of the glass-ceramic revealed Eu³⁺ and Eu²⁺ luminescent ion species, but only Eu²⁺ is incorporated within the BaCl₂ nanocrystalline phase. Electron paramagnetic resonance measurements in X-band sustain the presence of Eu²⁺ ions. The EPR parameters (g values and hyperfine constants) resulting from the Q-band EPR spectra simulations recorded on glass-ceramic are similar to those in Eu²⁺-doped BaCl₂ and confirmed the partial incorporation of Eu²⁺ ions within the BaCl₂:Eu²⁺ nanocrystals in the glass-ceramics.

Introduction

Glass-ceramics technology is based on the controlled nucleation and crystallisation of the precursor glass, thermally converted into a glass-ceramic formed by one or more crystalline phases evenly distributed within the glass phase. In particular, by partial and controlled crystallisation of a wanted phase by subsequent annealing below the glass melting temperature, we can get enhanced optical properties (such as absorption and emission cross-sections and energy transfer rate associated with the rare-earth ions) compared to their glassy counterparts if the ions are incorporated with the new grown crystalline phase. The improved functionalities of the dispersed crystals make transparent glass-ceramic a new generation of tailorable optical materials with a wide range of applications from optics to photonics ([1] and references therein). Furthermore, the optical transparency of the glass-ceramic can be reached by properly adjusting the temperature/time crystallisation profile to have the optimal crystal structure around the active ions and higher luminescence efficiency while still keeping the nanocrystals smallness and the material transparency.

New transparent oxyfluoride glass-ceramics, in which the optically active rare-earth (RE) ion partitions into a fluoride crystal phase, combine the optical advantage of a pure fluoride environment around the RE ion, keeping the thermal and chemical advantages of oxide glasses [2]. Among the various types of RE-doped oxyfluoride glass-ceramics, fluorozirconate glass-ceramics have attracted special interest due to their remarkable scintillating and X-ray phosphor storage properties related to the Eu²⁺-doped BaCl₂ nanocrystals [[3], [4], [5], [6]]. However, a significant drawback of the fluorozirconate glass-ceramics is their special processing requirements: controlled atmosphere and relatively high melting temperature. A solution to overcome these difficulties is represented by the borate glass-ceramics showing low melting temperatures and the possibility to be processed in an open-air atmosphere [[7], [8], [9]]. A recent study on Eu (1%)-doped chloroborate glass-ceramics have shown a strong distortion of the BaCl₂ crystalline lattice and the Eu²⁺ ions presence in the glass matrix and perturbed sites within the BaCl₂ nanocrystals [9]. By comparison, our recent investigations on BaCl₂:Eu doped nanocrystals have shown a slight distortion of the lattice parameters. Still, the electron paramagnetic resonance (EPR) parameters resulting after simulation of the acquired Q-band spectra are in good agreement with Eu²⁺ doped BaCl₂ single crystals [10]. Our previous studies [9,10] dealt with the synthesis, structural analysis and optical properties of Eu(1%)-doped glass-ceramic and Eu-doped BaCl₂ nanocrystals. The present study concerns the magnetic, optical and structural properties of the Eu-doped BaCl₂ nanocrystals within the glass-ceramic at different Eu-dopant levels and processing temperatures. The EPR spectra analysis of the isostructural BaBr:Eu²⁺ crystalline powder and fluorozirconate glass-ceramics have shown deviations of the spin Hamiltonian parameters due to the nanocrystallites inclusion into the glassy environment [12]. These studies have demonstrated the influence of the glass-ceramic environment and processing on the nanocrystals' structural and optical properties. Therefore, their investigation is relevant for developing and optimising Eu²⁺ doped BaCl₂ glass-ceramic phosphor-related applications.

The present study investigates the Eu²⁺ incorporation in the chloroborate glass-ceramic materials by comparing the corresponding nanocrystals using correlated analysis techniques: structural, morphological, magnetical and optical.