Effect of nonstoichiometry on the terahertz absorption of Y 3 Al 5 O 12 optical ceramics
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Daniel Steere and S.K. Sundaram Terahertz and Millimeter Wave Laboratory (T-Lab), Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University, Alfred, NY 14802, USA (Received 16 April 2014; accepted 11 August 2014)
We report the effect of nonstoichiometry on the terahertz absorption of fully dense optical ceramics of Y3Al5O12 and compare to that of undoped and 1 at.% Nd31 doped single crystals. Our research is motivated primarily by the necessity of having better control of stoichiometry during the preparation of transparent yttrium aluminum garnet (YAG) ceramics. A set of twenty ceramic samples was prepared by solid-state sintering of Y2O3 and Al2O3 powder mixtures with compositions ranging from 0.62 to 10.96 mol% of Y2O3 on each side of the stoichiometric garnet composition. After sintering, the samples were highly translucent in the visible range, with attenuations better than 2 cm1. These samples were characterized using time-domain terahertz spectroscopy between 0.06 and 2.8 THz. Ceramic and single-crystal samples exhibit a similar broad absorption band, which we assign to a 2-phonon difference process, and whose width and intensity depend upon composition.
I. INTRODUCTION
The demand for low optical loss transparent ceramics for applications such as laser-gain media,1,2 scintillators,3 and advanced optical components4,5 requires the ability to control the phase composition of multinary materials along with their microstructures. In this respect, the problem of stoichiometry, or more accurately the problem of deviation from stoichiometry, is one of the most critical issues pertaining to the fabrication of high-grade transparent ceramics. Within the phase stability domain of a given multinary compound, departure from the nominal composition leads to the formation of point defects until the solid-solubility limit is exceeded and secondary phases start segregating. Point defects and precipitates not only control the sintering kinetics and final densification state of the ceramics but also affect their optical properties by introducing color centers, charge-carrier trapping sites,6,7 and scattering centers.8 The ‘inexplicable’ nonreproducibility of high-quality transparent ceramic fabrication, that one comes to experience with certain materials, is closely related to investigating the nature and effect of these intrinsic defects on the sintering behavior and optical properties and measuring small excess of major elements with accuracies better than 0.1 mol%. While modern analytical techniques are commonly used to determine impurity concentrations a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.236 2338
J. Mater. Res., Vol. 29, No. 19, Oct 14, 2014
http://journals.cambridge.org
Downloaded: 21 Mar 2015
down to ppm or ppb levels, there is, at present, no simple and direct method in scientific practice for determining deviation from stoichiometry in insulator materials except for precision measurement of crystal lattice parameters by x-ra
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