ISOMORPHOUS SUBSTITUTION OF LEAD FOR CALCIUM IN THE SYSTEM PB<sub>8-x</sub>CAxNA<sub>2</sub>(PO<sub>4</sub> )<sub>6</sub>
Synthesized at 800 °C solid solutions Pb<sub>8-x</sub>Ca<sub>x</sub>Na<sub>2</sub>(PO<sub>4</sub>)<sub>6</sub> (0 ≤ х ≤ 8) have investigated by SEM and XRD with Rietveld refinement methods. It was established that calcium substitution for lead occurs up to x = 5.4 and is limited by necessity of presence Pb<sup>2+</sup> ions with 6s<sup>2</sup>-electron pairs in the channel of apatite structure. М(1)-О(2) and М(2)-М(2) distances are decreased up to x = 2 and in the range 2 < х ≤5 respectively with calcium content increasing in the crystal structure. The changings of these distances correlate with occupancy of M(1) and M(2) sites of apatite structure.