Recently, a new boron allotrope B52 with orthorhombic structure was theoretically predicted to be more stable than α-tetragonal boron B50. In experiments however, only tetragonal boron phases have been obtained so far. Here, we report for the first time on the preparation of orthorhombic boron phase of B52-type, space group Pnnn, a = 8.894 Å, b = 8.784 Å, c = 5.019 Å, by normal-pressure annealing of α-tetragonal boron, synthesized at high pressures by pyrolysis of decaborane, B10H14. We have investigated temperature-induced structure evolution and thermal desorption of boron samples, which allowed us to regard the structure of mother “α-tetragonal boron” as a boron-rich hydride with composition close to B51.5H7.7. In accordance with density-functional theory calculations, the most preferable sites of hydrogen placement in tetragonal unit cell are 8j and 4g; the tetragonal-to-orthorhombic transition takes place spontaneously upon complete dehydrogenation.