Contribution of neodymium optical spectroscopy to the crystal growth study of a silicate apatite in a glassy matrix

Neodymium migration from the glass to the unique crystal phase, Ca2Nd8(SiO4)6O2 apatite, was followed by absorption (4I9/2 → 2P1/2 transition) and emission (4F3/2 → 4I9/2 transition) spectroscopy in aluminoborosilicate glass–ceramics prepared at various temperatures. Comparison of these glass–ceramics showed that ordering of Ca2+ and Nd3+ ions in the 6h and 4f sites of the silicate apatite structure progressively increased when the crystallisation temperature Tc is raised from 950 to 1450 K. Taking advantage of the emission quenching in apatite crystals due to high Nd3+ ions concentration, the residual glass surrounding these crystals was probed. The highest neodymium depletion in the glassy phase was obtained after heat treatment at Tc = 1300 K. Neodymium was found to be a nucleation agent for apatite crystallization. The nucleation mechanism of this phase at low temperature begins by the migration of Nd3+ ions from glass sites to the apatite 4f sites that are structurally close. At higher temperature, due to the increasing thermal energy available during crystal growth, the proportion of Nd3+ ions incorporated in the apatite 6h site increased.