Crystal Structure of ma₂Mo₇O₂₂¹

A white crystalline powder, prepared by hydrothermal treatment at 200° C of a mixture of molybdic acid, H₂MoO₄, and methylammonium (ma) chloride, CH₃NH₃Cl, taken in a 1:2 molar ratio and acidified with hydrochloric acid, HCl, to pH = 3.5, resulted in a complex powder diffraction pattern, shown in Fig. 22.1. It was indexed in the monoclinic crystal system as was discussed in Sect. 14.11.2. The space group C2/c (or its acentric subgroup Cc) was established from the analysis of the systematic absences, and the unit cell dimensions were refined using 120 resolved reflections below 2Ø = 60°: a = 23.0648(6) Å, b = 5.5134(2) Å, c = 19.5609(5) Å, β = 122.931(1)°, and the sample displacement δ = −0.098(3)mm for a 250mm goniometer radius. The unit cell volume is 2087.8 ų.

The Powder Diffraction File search was unsuccessful and therefore, further analysis and a structure solution were undertaken. Thermogravimetric analysis in an oxygen atmosphere reveals sharp 7.3 wt.% weight loss at 300°C and the powder diffraction pattern, collected from a solid residue after the TGA, confirms the formation of molybdenum oxide, MoO₃. Assuming the following decomposition reaction:

$$\left( {CH_3 NH_3 } \right)_m Mo_n O_{3n} + k \to nMoO_3 + mCO_2 + m/2N_2 + 3mH_2 + 3mH_2 O$$

(22.1)

it can be shown that the observed weight loss nearly precisely corresponds to m:n ratio 2:7 and k = m/2 = 1. The latter ratio also follows from the white color of the substance under investigation, which implies that Mo is in the 6+ oxidation state. Thus, the chemical composition of the material is (CH₃NH₃)₂Mo₇O₂₂.