XRD investigations in the Nd2O3-Y2O3 system and structural studies of a stabilized monoclinic phase

The phase relation studies in the Y2O3-Nd2O3 system has been established after slow cooling of the samples from 1400 deg C. Three phase regions, namely a C-type cubic solid solution, a monoclinic solid solution and a biphasic mixture of these two phase fields were observed in this system. An important observation of this investigation is the stabilization of B-type monoclinic phase of Nd2O3 on substitution of about 10mol% Y3+. A nominal Nd0.90Y0.10O1.5 composition from the monoclinic type solid solution phase region was characterized by the Rietveld refinement of the observed powder XRD data. The typical refined unit cell parameters for this nominal composition are: 14.2761(3), 3.6449(1), 8.9022(2)A and beta=100.391(2) deg , V=455.62(2)A3. The refined results confirm the monoclinic rare-earth oxide structure with Nd3+ with seven coordinated (capped distorted trigonal prism type) polyhedra and six coordinated (trigonal prism) polyhedra. The crystal structure is built from the edge sharing of these polyhedra. The striking observation of the refinement is that the Y3+ prefers six coordinated polyhedral site instead of randomly distributed over all the Nd3+ sites, which is accounted by the smaller ionic radii of Y3+ compared to the Nd3+ ions. The further details of the trend in phase relation in Nd2O3-Y2O3 system has been explained on the basis of the crystal structure and ionic radii of the metal ions.