Yttrium Aluminum Garnet as a Scavenger for Ca and Si Yener Kuru, w Onur Savasir, Saide Z. Nergiz, Cinar Oncel, and Mehmet A. Gulgun* Sabanci University, FENS, Orhanli-Tuzla, 34956 Istanbul, Turkey Siglinde Haug and Peter A. van Aken Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany Doped yttrium aluminum garnet, Y 3 Al 5 O 12 (YAG), has drawn considerable attention for solid-state industrial, medical, and scientific laser applications. The crystal optical activity is closely related to the type and amount of doping element. Studies on highly yttrium-doped, creep-resistant alumina ceramics with Ca and Si contamination have indicated that YAG precipitates in the ceramic had a propensity to allow the simultaneous incor- poration of Ca and Si impurities on the order of 1% into their structure. The cosolubility potential for Ca and Si in YAG crys- tals was investigated through systematic doping and codoping of YAG polycrystals with Ca 21 and/or Si 41 . It was shown via X-ray diffraction and electron probe microanalysis techniques that the ceramic can accommodate more than an order of mag- nitude amounts of Ca 21 and Si 41 when they are incorporated in equal amounts simultaneously than when they are introduced alone. The cosolubility limit for Ca and Si was determined to be between 3% and 4% of the cation amount in pure YAG. Enhanced co-solubility was discussed in terms of size and charge compensations in the lattice. Codoping with a suitable element is introduced as a possible way to increase the solubility of useful cations in this ceramic, which is the host material for lasers. I. Introduction Y TTRIUM aluminum garnet, Y 3 Al 5 O 12 (YAG), is known to be a very forgiving host material and can be doped with several lasing cations such as neodymium, erbium, ytterbium, chromium, thulium, and holmium with different sizes and valence states. 1–3 Its excellent optical, high-temperature mechan- ical properties 4 and chemical stability suggest that YAG is the most promising host material for solid-state laser applications. YAG, with a complex crystal structure consisting of 160 at- oms (8 formula units) per unit cell, belongs to the space group Ia3d.Y 31 and O 2 ions occupy 24(c) and 96(h) positions, re- spectively. Two of the Al 31 ions are situated at octahedral 16(a) positions, whereas the remaining three Al 31 ions are situated on tetrahedral 24(d) sites. 5,6 The corresponding lattice parameters are a 5 b 5 c 5 12.008 A ˚ and a 5 b 5 g 5 901. 7,8 Figure 1 illus- trates the crystal structure of YAG and the positions of the different atoms in the lattice. The concentration of the lasing material is a critical and im- perative parameter for the properties and efficiency of the lasers. For instance, Ikesue and colleagues demonstrated that an increase in the Nd concentration in YAG leads to an enhanced output power corresponding to a certain incident pumping power. 9 However, there are some factors, such as the solubility limit of the dopant in the ceramic and optical quenching 10 (due to energy transfer between different active ions and/or active ions and their surroundings (i.e., impurities, defects, and phonons)), that determine the optimum doping level. We aim to increase the amount of lasing dopant incorporated into the crys- tal structure of the host without causing optical quenching. In addition, YAG is the second-phase precipitate in highly yttrium-doped a alumina. Y-doped alumina with YAG precip- itates reveals a two orders of magnitude improvement in creep resistance over undoped alumina ceramics. 11–13 Recent studies indicated that the YAG precipitates in such alumina ceramics affect the distribution of impurity cations at the grain bound- aries of Al 2 O 3 ceramics. 11,14 YAG precipitates were also observed accommodating up to 1% of Ca and Si impurities. 15 Ca and Si impurities are known to act as a flux in the ceramic, resulting in grain boundary films that melt at relatively low temperatures. Therefore, it is important to clarify the scavenging effect of the YAG phase for the high-temperature mechanical behavior of the ceramic. Ca and Si have very low solubility in YAG. Rotman et al. 16 reported that the solubility of Ca is approximately 0.03% of the total amount of cations in YAG. Sun et al. 17 investigated the solubility limit of Si in YAG using energy dispersive X-ray spectrometry (EDS). They could not measure the amount of Si in the bulk of YAG. The detection limit of the EDS technique was probably not sufficient to probe the solubility limit of Si, which is consistent with the results obtained in the present work (see Section III). This result places the solubility limit of Si in YAG below approximately 0.5 at.%, regarding the detection capabilities of current EDS systems available. In this work, using X-ray diffraction (XRD) and electron probe microanalysis (EPMA) techniques, it has been demon- strated that YAG can be contaminated by relatively large amounts of Ca and Si when they are simultaneously incorpo- rated in equal amounts, in contrast to their low elemental solubility in the ceramic. II. Experimental Procedure Aluminum nitrate nanohydrate (Al(NO 3 ) 3  9H 2 O, purity >98%, Fluka Chemie, Buchs, Switzerland), calcium nitrate tetrahydrate (Ca(NO 3 ) 2  4H 2 O, purity >99%, Merck KgaA, Darmstadt, Germany), yttrium nitrate hexahydrate (Y(NO 3 ) 3  6H 2 O, purity >99.9%, Aldrich Chemical Company, Milwaukee, WI), and tetraethylortosilicate (TEOS, (C 2 H 5 O) 4 Si, purity >98%, Merck-Schuchardt, Hohenbrunn, Germany) were used as starting materials. Polyvinyl alcohol (PVA, aver- age molecular weight 70.000–100.000, Sigma Chemical Com- pany, St. Louis, MO) solution in distilled water (2 wt% aqueous solution) was used as the reaction medium. PVA was dissolved in distilled water by stirring for 20 min at 801C. Stoichiometric amounts (n Y /n Al 5 3/5, where n Y and S. Wiederhorn—contributing editor This work was supported from the European Union under the Framework 6 program under a contract for an Integrated Infrastructure Initiative. Reference 026019 ESTEEM, Transnational Activities (TA7) to MPI. The authors also acknowledge support of the Na- tional Center for Electron Microscopy, Lawrence Berkeley Lab, which is supported by the U.S. Department of Energy under Contract # DE-AC02-05CH11231. *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: yenerk@su.sabanciuniv. edu Manuscript No. 24342. Received February 22, 2008; approved July 24, 2008. J ournal J. Am. Ceram. Soc., 91 [11] 3663–3667 (2008) DOI: 10.1111/j.1551-2916.2008.02659.x r 2008 The American Ceramic Society 3663