Assessment of hydrothermal parameters in the wet synthesis of α- and β-BiTaO₄ by in situ synchrotron X-ray diffraction

Abstract

Synchrotron X-ray diffraction was used for in situ monitoring of hydrothermal precursors calcination to obtain BiTaO₄ in the orthorhombic (α) and triclinic (β) crystal phases. Different precursors were prepared varying time (24 and 48 h) and temperature (150 and 200 °C) of hydrothermal treatment. According to the results of in situ X-ray analysis during calcination of the precursors, the α- and β-BiTaO₄ phases were achieved from the hydrothermal precursors treated at 200 °C. When the hydrothermal treatment time was carried out for 24 h, a precursor with Bi_1−xTa_xO_1.5−x (fluorite structure, like δ-Bi₂O₃) and BiOCl phases was obtained, which evolved to Bi₂₄Cl₁₀O₃₁ phase with heating. The presence of the chlorine-containing phases and the low crystallinity of the hydrothermal precursor contributed to the formation of β-BiTaO₄. When the precursor is obtained with 48 h of hydrothermal treatment, only the Bi_1−xTa_xO_1.5−x crystalline phase was identified and when heated it evolved to α-BiTaO₄ phase at 550 °C. This is the first time that the pure orthorhombic phase is obtained from a wet synthesis method. All identified phases were confirmed by Rietveld refinement.

Introduction

Ceramic materials have altered properties depending on the crystalline structure, the presence of defects, and composition. Among the constituent elements of these materials, tantalum has been attracting great interest, especially in the form of mixed oxides named tantalates, which can be classified into two groups according to the crystalline structure, perovskites and pyrochlores.

Bismuth tantalate and niobate (BiTaO₄ and BiNaO₄) are of great interest due to their ferroelectric, optoelectronic, and efficient luminescence properties [1]. In this way, these bimetallic oxides, often obtained as nanomaterials, are proper for several applications, such as photodegradation of organic and industrial pollutants [2], [3] and water splitting [4], [5], [6], [7], [8], [9]. Bismuth tantalate (BiTaO₄) belongs to the A³⁺B⁵⁺O₄ mixed oxide family and forms isostructural compounds with bismuth niobate (BiNbO₄) [1], [2], [3], [4]. Both can take on two different crystal structures, the orthorhombic phase, also called α type structure, is a low-temperature form and the triclinic phase, known as β type structure, is a high-temperature form [2], [3], [4]. The crystalline structure of these mixed oxides consists of octahedra TaO₆ or NbO₆, with tantalum/niobium as pentavalent cations (5+), which are interconnected by basic units of Bi₂O₂ from the polyhedral units BiO₇, in which bismuth is a trivalent cation (3 +) [3]. The band structure of BiTaO₄ is typical of a semiconductor, whose valence band (VB) is composed of filled orbitals from hybridization of O2p and Bi6s orbitals, while the conduction band (CB) is composed of empty orbitals with a major contribution of Ta5d and Bi6p orbitals and a minor contribution of O2p orbitals. [4], [5] According to Wiegel et al. [6], Bi6s level in the valence band is located above O2p levels in the orthorhombic phase, while there is an inversion of these levels in the triclinic phase. Thus, the optical and electronic properties are directly related to the crystalline structure of these materials [1], [2], [3], [4], [5], [6], [7], [8], which in turn will influence the performance in possible applications, for example, photocatalytic hydrogen production [2], [5], [9], in which the α phase is more active than the β phase. Several synthesis procedures have been developed to obtain the desired crystalline phase, α or β-BiTaO₄. Among these procedures, there are solution method [2], high pressure and high-temperature method [8], polymeric precursors method [9], [10], citrate method [11], flow method [12], solid reaction method [13], and co-precipitation method [14]. In general, the solid-state reaction leads to the formation of α-BiTaO₄ while wet chemical synthesis methods lead to β-BiTaO₄ [8]. Zhou et.al. [8] evaluated how α- and β-BiTaO₄ crystalline structures are formed, how the phases interconvert in HP-BiTaO₄, a novel phase obtained at high temperature and pressure, and what is the impact on the dielectric properties of these ceramics prepared via a solid-state reaction method with high sintering temperatures. In such study, an ex-situ method was performed by conventional X-ray diffraction (XRD) at room temperature. On the other hand, in situ study using X-ray from a synchrotron light source has shown to be very robust in determining/correlating the kinetics of the reactions with the structural transformations undergone by the materials [15], [16].

The present work proposes an in situ study using synchrotron radiation to control the experimental parameters and identify the ideal synthesis conditions for obtaining both α- and β-BiTaO₄ phases by hydrothermal method, a wet chemical synthesis method. This strategy is capable to monitor in real-time, with high resolution, the crystalline changes undergone by the hydrothermal precursors during the heat treatment from room temperature until obtaining the desired crystalline phase as well during cooling.