Phase Relations in the Ln 2 O 3 -Cr 2 O 3 -B 2 O 3 ( Ln = Gd-Lu) Ternary Oxide Systems.

In this work, isothermal sections of the Ln 2 O 3 -Cr 2 O 3 -B 2 O 3 ( Ln = Gd-Lu) ternary oxide systems at 900, 1000, and 1100 °C were constructed by determining the phase relations by using a powder X-ray diffraction technique. As a result, these systems were divided into subsidiary subsystems. Two types of double borates, Ln Cr 3 (BO 3 ) 4 ( Ln = Gd-Er) and Ln Cr(BO 3 ) 2 ( Ln = Ho-Lu), were observed in the investigated systems. Regions of phase stability for Ln Cr 3 (BO 3 ) 4 and Ln Cr(BO 3 ) 2 were determined. It was shown that the Ln Cr 3 (BO 3 ) 4 compounds crystallized in rhombohedral and monoclinic polytype modifications up to 1100 °C; above this temperature and up to the melting points, the monoclinic modification was predominantly formed. The Ln Cr 3 (BO 3 ) 4 ( Ln = Gd-Er) and Ln Cr(BO 3 ) 2 ( Ln = Ho-Lu) compounds were characterized by using a powder X-ray diffraction method and thermal analysis.