Phase Relations in the Ln2O3–Cr2O3–B2O3 (Ln = Gd–Lu) Ternary Oxide Systems

In this work, isothermal sections of the Ln2O3–Cr2O3–B2O3 (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, LnCr3(BO3)4 (Ln = Gd–Er) and LnCr(BO3)2 (Ln = Ho–Lu), were observed in the investigated systems. Regions of phase stability for LnCr3(BO3)4 and LnCr(BO3)2 were determined. It was shown that the LnCr3(BO3)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 LnCr3(BO3)4 (Ln = Gd–Er) and LnCr(BO3)2 (Ln = Ho–Lu) compounds were characterized by using a powder X-ray diffraction method and thermal analysis.