The changes induced by LPHT treatment in the spectral range of 4000-5200 cm" are the reduction of intensity of sharp lines at 4671,4701,4986 and 5016 cm’ ‘. The 4338 cm” line, the strongest line of the spectrum, has also experienced some reduction in intensity. This observation deviates from that reported in [8]. where 4673 cm" line remained dominating after LPHT annealing, while the sharp feature at 4335 cm" is negligibly small. These differences in the behavior of optical centers may be the result of different regimes of LPHT treatmentper formed in hydrogen plasma (the case studied in [8]) and resistivelyheated graphite furnace filled with hydrogen (the present case). Hydrogen plasma has temperature close to 3000 “C and contains significantly higher concentration of atomic/ionized hydrogen than hydrogen gas at a temperature of 1800 “C in resistively heated graphite container.