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The Si -A center is a major, radiation-damage defect produced in "pulled" silicon by a room temperature irradiation. As a result of studies described in this paper (I), and the following one (II), it is concluded that this center is a lattice vacancy with an oxygen atom impurity bridging two of the four broken bonds associated with the vacancy. Spin resonance and electrical activity arise from an electron trapped in the other two bonds. In this paper (I), the spin resonance studies are described. A molecular orbital treatment of the trapped electron wave-function satisfactorily accounts for the observed g tensor, as well as the hyperfine interaction observed with neighboring 4.7% abundant Si nuclei. Study of the changes in the spectrum of a sample subjected to uniaxial stress are also described. Under stress, the amplitudes of the individual resonance components (which correspond to different orientations of the defect in the crystal) are observed to change. This results from (1) electronic redistribution of the trapped electrons among the defects, and (2) thermally activated reorientation of the defects themselves under the applied stress. These two effects are separated and a quantitative study of their magnitudes and signs, as well as their rates, is given. The results confirm many of the important microscopic features of the model.

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The EPR spectra of Cr'+ in cubic ZnS, ZnSe, ZnTe, and CdTe, and in hexagonal ZnS and CdS are reported. For each, h, M = +4, +2, +1 transitions are observed allowing complete analysis of the S =2 spin H~~i&tonbe. A static tetragonal Jahn-Teller distortion is observed in all cases.Stress-ali~~ent studies are dcexibed which allow a direct estimate of the Jahn-Teller coupling cod5cients. From these, the magnitudes of the Jahn-Teller energies are esegnated to be~500 cm '. A ligand-field model is developed which satisfactorily accounts for the fine-structure parameters and their large variation from one crystal to another. In this treatment, ligand contributions to the spin-orbit interaction with excited d' terms are also included and found to be important. Strain~upling coefficients, describing the changes in the fine-structure terms under externally applied stress, have been experimentally determined for CdTe (Cr'+). A simple theory for this @feet is developed which includes ligand contributions to the spinwrbit interactions but relies on point-ion estimates for the strain matrix elements. Agreement with experiment is relasoliable for the tetragonal coefficients, but the wrong sign is predicted for the trigonal ca&cients. The calculated trigonal cod5cients for CdS do, however, lief lo'ly tf th Ildeparhuwf It I~y~b. A' th p' H~~i&tonian for Cr'+ in this hexagonal wiirtzite lattice.