In Situ Investigation of the Thermal Decomposition of Ammonium Tetrathiomolybdate Using Combined Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction

The thermal decomposition of ammonium tetrathiomolybdate, a process widely used for the preparation of molybdenum sulfides, has been studied in situ for the first time, using combined Mo K edge XAFS and X-ray powder diffraction. Isothermal decomposition at 120 °C produced pure amorphous molybdenum trisulfide ( a -MoS 3 ), after 135 min, with no further reaction observed on continued heating. XAFS spectra and diffraction patterns collected during the heating showed the disappearance of crystalline (NH 4 ) 2 MoS 4 and the formation of a -MoS 3 without the formation of an intermediate. Thermal decomposition of (NH 4 ) 2 MoS 4 by heating from 100 to 400 °C produced ultimately poorly crystalline MoS 2 . It is shown that the Mo−Mo nonbonded correlation of ∼3.2 Å found by some previous workers in a -MoS 3 is most likely due to the presence of small quantities of poorly crystalline MoS 2 . This allows certain structural models for a -MoS 3 to be ruled out. A detailed comparison between XAFS data collected from crystalline 2H−MoS 2 at a variety of temperatures and data collected from poorly crystalline MoS 2 prepared in situ shows that the low crystallinity exhibited by the powder pattern of the latter material arises from not only small particle size and stacking faults but also positional disorder of constituent atoms from the ideal 2H−MoS 2 structure.