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 KH2PO4 (KDP). Doping KDP with 5 ppm of Fe introduced a broad c
harge-transfer absorption band near 270 nm. At the same time, room-temperat
ure EPR showed that two Fe3+ centers were present. Although their intensiti
es are different, the two EPR centers have similar zero-field splittings. S
pin-Hamiltonian parameters, including fourth order in crystal field, were d
etermined for the dominant center. We suggest that these EPR-active Felt io
ns substitute for KI ions, presumably with charge-compensating cation vacan
cies (i.e., potassium or hydrogen) nearby. Crystals containing both the 270
-nm absorption band and the Fe3+ EPR spectra were X-ray irradiated at room
temperature and then subjected to a series of thermal anneals up to 150 deg
reesC. These treatments caused changes in both the absorption band and the
EPR spectra, but the behaviors of the optical and EPR features did not corr
elate. This can be explained if a portion of the Fe ions occupy a site in t
he KDP crystals other than the K+ site. We suggest that most of the optical
absorption in the 200-to-300nm region is due to Fe ions substituting for p
hosphorus; i.e., (FeO4)(2-) molecular units are present in the KDP crystals
and give rise to ultraviolet absorption. (C) 2001 Elsevier Science B.V. Al
l rights reserved.