Influence of calcium concentration on formation of tetravalent chromium doped Y₃Al₅O₁₂ ceramics

Abstract

Yttrium Aluminium Garnet (YAG) ceramics doped with chromium were prepared by solid-state reactive sintering in a vacuum. The influence of the charge compensator Ca²⁺ concentration on microstructure, optical properties and efficacy of Cr³⁺ oxidation to Cr⁴⁺ under air annealing was investigated. A non-monotonic dependence of these features on the amount of CaO as an additive was found. The changes in ceramic transparency and microstructure were explained considering the interaction between CaO and Cr₂O₃ at the ceramic grain boundaries, which leads to a different pore evolution in distinct samples during sintering. The efficacy of the oxidation of Cr³⁺ to Cr⁴⁺ strongly depends on the concentration of Ca dissolved in the YAG. The calcium solubility decreases due to the higher oxygen partial pressure of the extra phases on the grain boundaries that decreases the amount of generated Cr⁴⁺ ions. Such phenomenon explains the lower concentration of Cr⁴⁺ ions in the sample with 0.8% of Ca against the one with 0.5%. The experiment shows that the ceramic with 0.5% of Ca has a better in-line transmission and a higher concentration of Cr⁴⁺ ions in comparison with samples with a different Ca concentration.

Introduction

Ceramics of yttrium aluminium garnet (Y₃Al₅O₁₂) doped with tetravalent chromium (Cr⁴⁺:YAG) are solid-state laser materials used as saturable absorbers for the 1.06 mm laser emission [1], [2], [3], [4], [5], [6], moreover, they are employed as active elements of tunable mid-IR lasers in the spectral range of 1.3–1.6 µm [7], [8], [9], [10].

The YAG crystal belongs to the cubic space group Ia3d, with eight unit formula in each unit cell. The YAG formula can be written as [C₃][A₂][D₃]O₁₂, where “C”, “A”, and “D” denote cation sites which are coordinated by oxygen atoms in a dodecahedral, octahedral, and tetrahedral fashion respectively. The “A” and “D” sites are occupied by Al ions whereas “C” sites are taken by Y ions [11].

Cr doped YAG ceramics originally include Cr atoms in its trivalent state, which can populate “A” sites only, [11], [12], [13] because the ionic radius of Cr³⁺ is too large to occupy the tetrahedrally coordinated sites. To be used in the above-mentioned applications, part of the Cr³⁺ has to be oxidized to Cr⁴⁺, which can fit into the tetrahedral “D” sites of the YAG crystal lattice, substituting part of the Al³⁺ ions. The extra charge generated in the YAG lattice after the chromium oxidation is usually compensated through the addition of co-dopants such as Ca²⁺ or Mg²⁺.

A considerable excess of Ca²⁺ ions is used for charge compensation in crystals [14], [15], [16]. However, the optical properties can be significantly affected even in the case of minor variations in the concentration of functional additives used during ceramics manufacturing. For example, Ikesue et al. [17], [18] have found that in the case of Nd:YAG ceramics, the amount of silicon added and the cooling rate adopted during the sample preparation change the thickness of the grain boundary phase, which affects the optical scattering loss.

So far, only a few reports have focused on the divalent dopants’ influence on the properties of Cr⁴⁺:YAG ceramics. Zhou et al. [19] have investigated the effect of the concentration of CaO and MgO on the properties of Cr⁴⁺:YAG ceramics but the simultaneous addition of the two dopants does not allow to determine the influence of Ca²⁺ itself on the optical properties of the ceramic. The sintering aid tetraethyl orthosilicate (TEOS), which is traditionally employed to obtain highly transparent YAG ceramics, is not acceptable to create Cr⁴⁺-doped YAG ceramics [19], [20] because Si⁴⁺ ions substitute part of Al³⁺ in the tetrahedral sites and suppress the Cr³⁺ to Cr⁴⁺ oxidation due to charge compensation.

To produce high optical quality Cr⁴⁺:YAG ceramics, the amount of Ca²⁺ ions used as additive has to be finely tuned assuring that there is (i) an adequate quantity to fully compensate the charge, (ii) the concentration is low enough to avoid undesirable influence on the ceramic formation or the appearance of extra phases on the grain boundaries, which causes deterioration of the optical properties.

This study is aimed at determining the influence of CaO used as a source of Ca²⁺ ions, both on the optical properties of Cr⁴⁺:YAG ceramics and on the generation of Cr⁴⁺ ions in the four-fold coordinated sites of the YAG crystal lattice.