M 2 B 5 or M 2 B 4 ? A Reinvestigation of the Mo/B and W/B System

The literature states different compositions (M/B = 1:2 vs. 2:5) and structures for diborides of molybdenum and tungsten. Using X‐ray and neutron powder diffraction as well as energy and wavelength dispersive electron microprobe analysis, the Mo/B and W/B systems were now reinvestigated. Molybdenum diboride crystallizes as a stoichiometric compound Mo 2 B 4 (formerly described as Mo 2 B 5 ) in space group $R{\bar 3}m$ (No. 166, a , b = 3.01375(2) Å, c = 20.9541(3) Å), and as a non‐stoichiometric compound MoB 2−x (formerly described as MoB 2 ) in P 6 /mmm (No. 191, a , b = 3.043(2) Å, c = 3.067(2) Å), whereas stoichiometric tungsten diboride W 2 B 4 (formerly described as W 2 B 5 ) is found to crystallize in space group P 6 3 /mmc (No. 194, a , b = 2.9864(4) Å, c = 13.896(2) Å). These results seem to be supported by DFT calculations which show the instability of a hypothetic W 2 B 5 .