Crystallisation of bismuth germanate glasses below their glass transition temperature

Highlights

• Dielectric properties of the bismuth germinate oxides reveals significant changes of the amorphous phase.

• An incipient glass transition which starts between 250–300°C was revealed by capacitive and losses measurements.

• The activation energy for these glass transition processes suggests a correlation with the increasing of the clusters.

• GeO₄ tetrahedra clusters mobility are responsible with the orientational dipolar effects.

Abstract

Dielectric measurements of bismuth germanate oxides reveal significant changes in the amorphous phase, between 200 and 350°C, where the crystallisation processes start below the glass transition temperature. The capacitive and loss measurements versus temperature and frequency suggest an incipient glass transition below 350°C associated with an increase in the clusters, mainly those formed by GeO₄ tetrahedra, responsible for the dipolar orientation effects. An increase in the capacitive parameter versus temperature at lower frequencies, especially over 250°C has been associated with the increase in mobility of the clusters. The physical meaning of those processes has been associated with the formation of a highly viscous layer enriched in GeO₄ which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth. A higher frequency is required in the crystallisation processes to compensate for the thermal disorder in the amorphous materials. The crystallisation process is identified by the decrease in the dielectric constant despite the increase in temperature.

Introduction

Today, oxide materials are of great interest for use in optoelectronic and even electronic devices. Among them, amorphous glasses like bismuth germanate play an important role due to their electro-optic [1], electro-mechanical [2] and scintillating properties [3]. Stoichiometric phase Bi₄Ge₃O₁₂ has been obtained in different forms such as powder, crystal, glass and sol-gel through different applications, the latter in valuable applications in medicine [4] or in upconversion processes as a host material [5].

The procedure for melt quenching of bismuth germanate glasses has already been described by Polosan et al., whereby mixtures of Bi₂O₃ and GeO₂ powders lead to the formation of different compounds, depending on the ratios of these two oxides [6].

Bismuth germanate oxide glasses can be easily converted into different crystalline structures depending on the Bi₂O₃:GeO₂ ratio and the experimental conditions including the melt pouring temperature and the cooling procedure [6]. These amorphous and subsequent crystalline phases obtained by thermal annealing have similar structures with the bulk crystals, but thus far, attention has been focused only on the crystals.

Different Bi₂O₃:GeO₂ systems of different compositions such as 6:1, 1:1, 2:3 and 1:3 have been investigated, but analysis of all aspects of the systems requires a variety of approaches [7], [8], [9], [10], [11], [12]. Depending on the composition, the crystals obtained were studied for specific properties such as electro-optic and piezoceramic behaviours, as well as elastic and thermoelastic properties at room temperature [13].

Another important crystalline structure with 1:1 composition is Bi₂GeO₅, due to its ferroelectric properties, which have been obtained and investigated. Crystallisation studies of bismuth germanate glasses were described well by Dimesso et al. who targeted several compositional structures and determined several parameters by using kinetic data from isothermal measurements for Bi₂O₃:GeO₂ (1:3) composition [14]. The thermal analysis was performed around 843 K (570 °C) where the crystallisation processes occur. However, this structure is unstable, manifesting as an intermediate phase that appears during the crystal growth heating procedure, which is very difficult to process with classic technologies [15], [16]. Under specific conditions, this composition has ferromagnetic behaviours due to the GeO₄ tetrahedra and anisotropic crystallography [17].

An alternative method for Bi₂GeO₅ crystals involves amorphous glasses when the Bi₂GeO₅ nanocrystals appear by devitrification beside other nanocrystals like Bi₄(GeO₂)₃ and β-Bi₂O₃. Nassau and Chadwick have studied different compositions of Bi₂O₃:GeO₂ and concluded that stable bulk glasses with Bi₂GeO₅ were obtained with up to 50% molar Bi₂O₃ on the phase diagram [18]. The 2:3 Bi₂GeO₅ composition was studied as a monocrystal [19], but also as a thin film for its scintillating properties [20].

The dielectric properties of BGO single crystals up to room temperature (320 K) at 1 kHz reveal an almost linear dependence of the dielectric constant for the sillenite (Bi₁₂GeO₂₀, 6:1) [21]. From the imaginary part of the dielectric constant versus temperature, up to 30 K, a strong relaxation mechanism is revealed.

Obtaining these amorphous materials requires no special equipment, compared with their similar monocrystals, which reduces the costs of the procedures.

For the BGO (2:3) amorphous compound, only the optical properties and X-ray diffraction have been investigated, mainly by comparison of the as-made glass with the devitrified structure at 1000 °C and 530 °C (between T_g and T_c) [22].

The aim of this paper is a study that compares the dielectric properties of glass and crystalline samples of bismuth germanate of the same composition 2:3, to discover those aspects of the crystallisation behaviours that cannot be observed by other techniques such as in-situ Raman or X-ray diffraction, below the glass transition temperature. These results were compared with those obtained up to 570 °C where macroscopic crystallisation occurs to find the composition of the annealed samples.