The effect of binary glass composition on the Eu-ions luminescence properties

Highlights

• Synthesis of complex Ca1-xBaxO-2SiO2 disilicate glasses.

• Role of Ca/Ba ratio in the glass matrix on the europium valence state is discussed.

• Influence of the Ca and Ba on the silica network modification.

Abstract

Eu₂O₃-doped Ca_1-xBa_xO-2SiO₂ glasses were prepared using standard melt-quenching technique at 1500 °C in CO-rich atmosphere. The changes of the Eu²⁺/Eu³⁺ ratio as a function of Ba content (x) were investigated by optical absorption, photoluminescence (PL), radioluminescence (RL) and electron paramagnetic resonance (EPR) methods. The increase of x leads to a monotonic decrease of the absorption band related to Eu²⁺. On the other hand, the glass samples with a dominating Ca²⁺ content exhibit the strongest Eu²⁺ luminescence. Altogether this may point out the domination of the energy transfer from Eu²⁺ to Eu³⁺. A weak Eu²⁺- and Eu³⁺-related emission in mixed glasses with 1-x = 0.5, 0.75 can be the result of some kind of quenching due to the mutual Ca²⁺ and Ba²⁺ influence introduced to the nearest environments of Eu ions. The role of x in the glass structure modification was confirmed also by infrared spectroscopy (IR) methods. Specific Eu²⁺ signals were detected in EPR spectra measured in all the glass samples with the exception of the BaO–2SiO₂ one, so only the europium 3+ charge state is expected to be present in this glass. EPR measurements thus correlate well with optical observations.

Introduction

Luminescent materials either with intrinsic (exciton-like) or extrinsic (doping ions) emission centers are widely used in different branches of human activity: medicine, physics, chemistry, lighting, security etc. [1,2]. Each field requires certain optical properties of the material. For example, in lighting applications, the emitted light should have an optical spectrum closely resembling the sunlight. Another example is medical imaging, where the emission range of a phosphor coupled to a detector should match well with its spectral sensitivity range. In many cases, only one doping or phosphor host is not enough to provide the required optical properties. Actually, the development of novel effective materials, either directly possessing intrinsic luminescence centers or being hosts suitable for doping ions, is a challenging task. Glasses are very interesting and versatile host materials for luminescent activators. They are transparent to visible light throughout a wide span of possible compositions [3,4]; they can be synthesized in different temperature ranges (depending on the glass composition) and in large volumes.

Europium ions possess specific characteristics among other rare-earth dopants due to the existence of two stable charge states, Eu²⁺ and Eu³⁺, both emitting in the visible spectral region. In several cases they are present in a host material simultaneously. In those cases, one crucial aspect is the control of the Eu²⁺/Eu³⁺ ratio in order to tune properly the luminescence properties of the material. Eu²⁺ optical features in phosphors [1,[3], [4], [5]], scintillators [6,7], magnetic materials [8,9], lasers and optical amplifiers etc. [[10], [11], [12]] were the subject of deep studies for decades. Eu²⁺ ions display broad d-f emission whose position is governed by the local environment [13]. The Eu³⁺ ions usually yield narrow, predominantly red emission (f-f transitions) under UV excitation independently on the crystalline (ligand) surroundings. However, weak emission bands can also be observed sometimes in the green and rather rarely in the blue range [[14], [15], [16]].

The Eu ions are in general incorporated in +3 valence state during the process of glasses melting. In order to promote the Eu³⁺ to Eu²⁺ transformation, a strong reducing atmosphere is required [3,5]. Charge compensation by the introduction of specific chemicals during the synthesis, e.g., SiC or hydrides turns out to be also useful [17,18]. Sometimes, however, the Eu²⁺ withstands even preparation in pure oxygen atmosphere [19].

The present study addresses the control of the Eu²⁺/Eu³⁺ ratio as a function of composition in the Ca_1-xBa_xO-2SiO₂ (x = 0, 0.25, 0.5, 0.75, 1) glass system. The role of the alkaline-earth cations combination in the Eu³⁺ → Eu²⁺ valence transformation and on its luminescence features is specifically addressed and discussed.