Phase transitions and upconversion luminescence in oxyfluoride glass ceramics containing Ba₄Gd₃F₁₇ nanocrystals

Abstract

Novel transparent Er³⁺ doped oxyfluoride glass-ceramics containing Ba₄Gd₃F₁₇ nanocrystals were prepared by melt quenching followed by heat treatment of as-prepared glasses. The phase composition and microstructure were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Intense upconversion luminescence (UCL) was detected. Longer characteristic decay times and splitting of the luminescence bands compared to the precursor glass indicated the incorporation of erbium ions in the crystalline phase. The spectroscopic properties of glass ceramics were compared with single phase cubic and rhombohedral Ba₄Gd₃F₁₇ ceramics. The unit cell parameters and atomic positions in the rhombohedral phase were calculated using Rietveld refinement. The local environment of Er³⁺ and the phonon energy of both polymorphs were analyzed using luminescence and Raman spectroscopy. In the glass ceramics, a phase transition from distorted metastable fluorite to ordered rhombohedral Ba₄Gd₃F₁₇ was observed and resulted in the enhancement of the efficiency of UCL.

Introduction

Recently considerable attention has been devoted to investigation of rare earth (RE) doped materials for upconversion luminescence (UCL) – an anti-Stokes process in which low-energy photons are converted to photons with higher energy [1]. UCL phosphors have demonstrated many potential applications such as upconversion lasers [2], [3], multifunctional biological markers and drug delivery systems [4], [5], three dimensional displays [6], [7], optical sensors [8], [9], luminescent layers for solar cells [10], [11], data storage [12] and others.

From RE ions, Er³⁺ is one of the most widely investigated activator for the conversion of near infrared to visible radiation due to favorable ladder-like energy levels and long lifetimes of the emitting states [13]. In fact Er³⁺ doped materials exhibit the highest upconversion quantum yield in comparison to other RE ion doped materials and organic dyes [14]. The shielding effect of the outer electron shells of trivalent RE ions prevents large variations of energies of the 4f states for different hosts, nevertheless, erbium has been successfully used both as an activator for UCL and a luminescent probe for the investigation of local structure of lanthanides in crystalline and amorphous materials [15].

The efficiency of the UCL processes depends on the properties of the host material such as phonon energy of the lattice, distances between activator ions and the local symmetry of the activator. Fluoride crystals are extensively investigated as hosts for the UCL due to the low phonon energy that minimizes the non-radiative relaxations in the material and therefore enables an efficient UCL [13], [16]. Among other fluorides, BaF₂ is a suitable host for the UCL due to lower phonon energy than other alkaline earth fluorides and formation of heterovalent fluorite-type solid solutions with all RE ions [17], [18]. Unfortunately, RE activators tend to form clusters in BaF₂ lattice even if the dopant content is as low as 0.05 mol%, leading to cross-relaxation (CR) between activators and reduction of the efficiency of radiative processes [19]. As the result, complex RE doped systems such as BaF₂-REF₃ (RE = La, Gd, Y, Lu) have to be considered. In these quasi-binary systems several compounds have been detected: monoclinic and orthorhombic BaRE₂F₈, two ordered phases (tetragonal and rhombohedral) with distorted fluorite structure and solid solutions with tysonite and fluorite structure [18]. RE doped single crystals of monoclinic BaRE₂F₈ are widely investigated and are proposed as excellent laser hosts [20], [21]. Considerable attention is also devoted to the investigation of fluorite solid solutions [22], [23], however there is limited information about optical properties of the other compounds.

BaF₂-REF₃ compounds have been extensively studied as nanocrystals in various oxyfluoride glass ceramics. Efficient UCL processes in glass ceramics containing fluorite type phases such as Ba₂LaF₇ [24], BaYF₅ [25], BaYbF₅ [26] and Ba_1−xLu_xF_2+x [27]. Oxyfluoride glass ceramics are excellent candidates for applications in which transparency is required. Recently we have developed new transparent glass ceramics in which nanocrystalline rhombohedral Ba₄Y₃F₁₇ can be prepared [28], however isostructural Ba₄RE₃F₁₇ are formed in other binary BaF₂-REF₃ (RE = Gd–Lu) systems [29].

In this work we report new Er³⁺ doped glass ceramics containing Ba₄Gd₃F₁₇ nanocrystals. The structure of cubic and rhombohedral Ba₄Gd₃F₁₇ is discussed. The phase transition and UCL processes in the glass ceramics are analyzed.