THE DIBORIDE COMPOUNDS OF MOLYBDENUM - MOB2-X AND MO2B5-Y

The phase relations of the molybdenum diboride compounds MoB2-x and Mo 2B5-y and the structure of Mo2B5-y have been re-examined with samples prepared between 1400 and 1800 degrees C. Two types of six-membered bo ron ring, nearly planar and puckered, occur within the Mo2B5-y structu re. The sites at the centers of the puckered rings may be unoccupied ( K' layers) or occupied by boron atoms (K layers). Neutron diffraction refinements show that these sites are completely unoccupied (K' layers ) in Mo2B5-y samples of several batch compositions. This result, combi ned with the insensitivity of the unit cell dimensions of Mo2B5-y to b atch composition, shows that this phase has a narrow single phase regi on, with y close to unity, between 1400 and 1800 degrees C. MoB2-x als o has a narrow single-phase region centered at about 61 at.% boron ove r this temperature range.