Optical absorption and electron paramagnetic resonance (EPR) have been used to study the sites and the charge states associated with Fe ions in single crystals of KH 2 PO 4 (KDP). Doping KDP with 5 ppm of Fe introduced a broad charge-transfer absorption band near 270 nm. At the same time, room-temperature EPR showed that two Fe 3 centers were present. Although their intensities are different, the two EPR centers have similar zero-field splittings. SpinHamiltonian parameters, including fourth order in crystal field, were determined for the dominant center. We suggest that these EPR-active Fe 3 ions substitute for K ' ions, presumably with charge-compensating cation vacancies (i.e., potassium or hydrogen) nearby. Crystals containing both the 270-nm absorption band and the Fe 3 EPR spectra were X-ray irradiated at room temperature and then subjected to a series of thermal anneals up to 150°C. These treatments caused changes in both the absorption band and the EPR spectra, but the behaviors of the optical and EPR features did not correlate. This can be explained if a portion of the Fe ions occupy a site in the KDP crystals other than the K + site. We suggest that most of the optical absorption in the 200-to-300 nm region is due to Fe ions substituting for phosphorus; i.e., (FeO 4 ) 2- molecular units are present in the KDP crystals and give rise to ultraviolet absorption.
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