Formation peculiarities and optical properties of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics

Abstract

Formation peculiarities of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been studied by X-ray diffraction and electron microscopy methods. The phase composition evolution of 1.81Y₂O₃∙0.18La₂O₃∙0.01Yb₂O₃ powder mixtures annealed at the temperatures of 1100, 1200, 1300, and 1400 °C has been studied by XRD. It has been shown that Yb₂O₃ phase dissolves in Y₂O₃ matrix in the calcination temperature range of 1300–1400 °C. Complete dissolution of La₂O₃ in Y₂O₃ matrix occurs at temperatures above 1400 °C. La³⁺ ions enter in Y₂O₃ and Yb₂O₃ crystal structures simultaneously in the 1200–1300 °C range, which leads to a remarkable increase in the volume of the corresponding crystal lattices. The possible reasons for suppressing the crystalline growth of Y₂O₃ and Yb₂O₃ cubic phases have been discussed. Finally, (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been obtained by solid-state vacuum sintering at 1650–1750 °C. Ceramics synthesized at a temperature of 1750 °C have been characterized by an in-line optical transmittance of 60% and a homogeneous distribution of constituent components within the volume and along the grain boundaries.

Introduction

Lanthanum oxide is widely used as a sintering aid to obtain highly-transparent Y₂O₃:Yb³⁺ ceramics. La₂O₃ activates the diffusion mass transfer processes at the intermediate and final sintering stages, which allows one to reduce the sintering temperatures significantly. Moreover, doping of Y₂O₃:Yb³⁺ by lanthanum ions causes broadening of the absorption bands of Yb³⁺ ions due to local changes of the crystal field. Doping yttria with lanthanum ions increases the grain boundaries mobility several times [1], which could lead to a significant increase in grain size compared to undoped Y₂O₃ ceramics [2], as well as to abnormal grain growth [3,4].

Fabrication of yttria ceramics requires lanthanum concentrations of about 8–13 at.% [[5], [6], [7], [8], [9], [10], [11]]. The utilization of such a high La³⁺ concentration is accompanied by a number of problems associated with the fact that lanthanum oxide is an unstable and highly hygroscopic substance. Even a freshly calcined powder quickly changes its phase composition after cooling. When the lanthanum oxide does not completely dissolve in the ceramic structure, a decrease in crack resistance and degradation of optical properties of Y₂O₃ ceramics have been observed [12]. Cracking of (Y_0.94La_0.06)₂O₃ ceramics was explained by the fact that the lanthanum oxide in the uncalcined sample transforms into lanthanum hydroxide. During calcination, the reverse reaction occurs which is accompanied by specific volume change due to differences in lattice parameters per formula units [13]. Recently it was shown that more stable sources of lanthanum ions – La(OH)₃ and La₂O₂CO₃ – could be used as the lanthanum precursor. They shift the densification to lower temperatures [14].

Despite the extensive use of lanthanum oxide as a sintering aid, there are practically no published data on the mechanisms of the entry of lanthanum into the yttrium oxide lattice, though yttrium and lanthanum ionic radii differ considerably (0.090 and 0.103 nm, respectively). Moreover, La₂O₃ and Y₂O₃ crystal lattices are not isostructural (La₂O₃ has a hexagonal structure and Y₂O₃ is cubic at atmospheric pressure). Therefore, the study of the phase formation processes in the Y₂O₃–La₂O₃–Yb₂O₃ system at various stages of the synthesis of transparent ceramics is an actual problem. It is known that the dissolution processes of La₂O₃ and Yb₂O₃ in yttrium oxide, as well as significant ceramics densification occur in the temperature range from 1100 to 1400 °C [12]. The purpose of this work is to study the phase composition evolution of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ ceramics during the interaction of yttrium, ytterbium and lanthanum oxides in the temperature range of 1100–1400 °C. The high concentration of lanthanum and ytterbium ions makes it possible to fix the change in the concentration of lanthanum-containing and ytterbium-containing phases with sufficient accuracy.