  Thermal expansion of {\alpha}-rhombohedral boron ({\alpha}-B12) and two
isostructural boron-rich pnictides (B12P2 and B12As2) has been studied between
298 and 1280 K by high-temperature synchrotron X-ray diffraction. For all
studied phases no temperature-induced phase transitions have been observed. The
observed temperature dependencies of the lattice parameters and unit cell
volumes were found to be quasi-linear. Variation of the thermal expansion
coefficients in the group of boron-rich pnictides (B13N2 - B12P2 - B12As2) was
analyzed.
