Low- and high-temperature modifications of rubidium triborate have been found. The low-temperature modification of rubidium triborate transforms to high-temperature modification in the range from 675 to 707$^{o}$C when heating at average rate 0.2 degrees per min.
The low-temperature RbB$_{3}$O$_{5}$ prepared by solid state reactions crystallizes in an orthorhombic unit cell ( a=8.217(3), b=10.095(3), c=5.391(2)Å), space group P2$_{1}2_{1}2_{1}$. The structure is isostructural to that of CsB$_{3}$O$_{5}$ [1]. An thermal expansion of low-temperature modification is anisotropical. It has negative values along axis b.
When heated above 670$^{o}$C peaks of new phase appear in HTXRD pattern, but an amorphous "halo" appears at 660$^{o}$C. The transformation of rubidium triborate starts before the new modification is forming. Low-, high-temperature modifications and amorphous phase are observed up 702$^{o}$C. Then peaks of low-temperature modification disappear, according to peaks of high-temperature modification increase. Some main peaks of high-temperature modification at 20 C (d, A/I, %) are listed below: 6.08/10, 4.35/20, 3.87/10, 3.58/40, 3.50/20, 3.02/10, 2.93/100, 2.77/10. There is no way to prepare pure high-temperature modification by quench from 710-730$^{o}$C. In this case peaks of low- and high-temperature modifications are observed in HTXRD pattern.
1. J.Krogh-Moe: Acta.Cryst. 13 (1960) 889