  During past years, a number of reports have been published on synthesis of
tetragonal allotrope of boron, t-B52 phase. However, no unambiguous
characterization of the crystal structure has been performed to the present
time, while remarkable variation of the a/c lattice-parameter ratio raises
strong doubts about its uniqueness. Here the Rietveld refinement of the crystal
structure of the high pressure - high temperature boron phase synthesized by a
direct solid-state transformation of rhombohedral beta-B106 at 20 GPa and 2500
K has been reported for the first time. Although this boron allotrope belongs
to the t-B52 type, its structure can be considered as pseudo-cubic with the a/c
ratio of sqr(2).
