Gadolinium and yttrium berates: Thermal behavior and structural considerations

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