Text this: CONCENTRATION AND TEMPERATURE DEPENDENCES IN EPR SPECTRA OD GD<sup>3+</sup> PROBE CENTERS IN THE Pb<sub>1-x</sub> Ag<sub>x</sub>S и Pb<sub>1-x</sub>Cu<sub>x</sub>S (0 ≤ x ≤ 0,011) THERMOELECTRIC CRYSTALS