A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of 19F nuclei in four anionic spheres of cubic MeF2:Gd 3 § (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd 3 § cubic impurity center in crystals MeF 2 (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd3 § . For the first time not only radial displacements but also angular displacements of F ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.

Reasons for unsatisfactory operation of the superheterodyne spectrometers at increased microwave power are considere& Recommendations for designing the superheterodyne microwave bridge of the EPR spectrometer working at the microwave power of about htmdreds of millq are given. These recommendations aUow one to develop the spectrometer comparable to those with modulation of resonant conditions conceming reliability, stability and simplicity of operation. The experimental prototype of the coherent microwave bridge of the X-band superheterodyne EPR spectrometer with the intermediate frequency of 100 MHz is described. The microwave circuit of the bridge is assembled using microstrip hybrid integrated circuits and requires no adjustment in operation. The microwave Gunn-diode generators with the electronic tuning stabilized by dielectric resonators are used. The dynamic range of 54 dB at the maximum power of 40 mW is reached. As the microwave power increases up to the maximum value, no tendency to the degradation of sensitivity equal to 5.10 -11W at the 40 MHz band is noticed.