β-MoO₃ produced from a novel freeze drying route

Powdered samples of β-MoO₃ have been produced by the gentle heat treatment of freeze-dried molybdic acid at 350°C for 1 hr. The samples, yellow-green in appearance, contained varying amounts of the thermodynamically stable α-MoO₃, depending upon the time and temperature of heat treatment. Neutron diffraction data were collected at 300 K. All peaks, not attributable to α-MoO₃, were indexed on the basis of a monoclinic cell, P2₁/c, a = 7.1228(7), b = 5.3660(6), c = 5.5665(6), β = 92.01(1)°, V = 212.62(6)ų. The structure, which is related to ReO₃, contains two crystallographically independent octahedra. Both show evidence of disorder at the Mo and O sites. Two distinct orientations of a short Mo{single bond}O distance, suggestive of the type of molybdenyl bond observed in both the α and β′-forms, are primarily responsible for the observed disordering.