Gadolinium and yttrium berates: Thermal behavior and structural considerations

The present work deals with the phase diagram of the Gd2O3-B2O3 binary system where three solid phases with respective stoichiometries Gd3BO6, GdBO3, and Gd(BO2)(3) have been evidenced. MAS NMR experiments were performed using similar yttrium compounds. Only B-IV sites exist in orthoborate at room temperature and the transition phase observed at about 900 degreesC has to be attributed to the change of the boron environment with progressive formation of B-III and fast exchange between B-III and B-IV when temperature increases. MAS NMR results confirm the existence of two yttrium sites. In Y3BO6, two threefold and one fourfold boron atoms are observed, One B-III corresponds to a planar orthoborate group; the other would be distorted. Fifteen yttrium sites have been observed which are not interpreted yet. (C) 2000 Academic Press.