Neutron Diffraction Study of the Crystal Structure of BaMoO4: A Suitable Precursor for Metallic BaMoO3 Perovskite

BaMoO3, metallic and Pauli paramagnetic, has been prepared by controlled reduction of BaMoO4. This precursor, containing Mo(VI), is unusually stable against reduction, due to structural factors. The crystal structure of BaMoO4 has been refined from neutron powder diffraction data: space group I41/a (no. 88), Z=4, A=5.5479(9), and C=12.743(2) ?. A bond-valence study allowed us to detect the presence of slight tensile and compressive stresses in the crystal structure of BaMoO4, in which Ba is overbonded and Mo is underbonded. However, this effect is less pronounced than in other AMO4 oxides with a scheelite structure (A=Ca, Sr, Ba; M=Mo, W): BaMoO4 contains the M cation exhibiting the closest valence to the nominal value of 6 +, suggesting a large covalent contribution to the Mo–O bonds. This observation is coherent with the large thermal stability of this compound against reduction, taking place at temperatures above 920°C in H2 flow.