The cleavage system and slip system have been determined in isostructural single crystals Li6GdB3O9 (LGBO), Li6EuB3O9 (LEBO) and Li6YB3O9 (LYBO). It has been shown that a characteristic feature of those crystals is a weak plasticity at room temperature that is due to hindered mutual shift of BO3 groups parallel to (1 0 (2) over bar) and ((3) over bar 0 1) planes. The necessity of the high-temperature annealing of the crystals has been substantiated. (c) 2005 Elsevier Ltd. All rights reserved.