Structural evolution and phase transition of [NH4]6Mo7O24.4H2O to 2D layered MoO3−x

A detailed optical and impedance study was performed on the structural evolution and phase transition of [NH 4 ] 6 Mo 7 O 24 .4(H 2 O) (ammonium heptamolybdate tetrahydrate, AHM). The 1 h calcination of AHM from 50 °C to 675 °C in an ambient air atmosphere induced a phase transition from an unstable AHM to the most stable two-dimensional orthorhombic molybdenum oxide ( α -MoO 3− x ) through various meta-stable crystal structures. The samples calcined at different temperatures were systematically analyzed and interpreted by powder x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analysis. From the observations and results, the decomposition pathway and the sequential phase evolution was identified as; (i) [NH 4 ] 6 Mo 7 O 24 .4(H 2 O) (30 °C); (ii) [NH 4 ] 4 (Mo 8 O 24.8 (O 2 ) 1.2 (H 2 O) 2 )(H 2 O) 4 (ca. 50 °C to 75 °C); (iii) [NH 4 ] 8 Mo 10 O 34 (ca. 100 °C to 200 °C); (iv) [NH 4 ] 4 Mo 8 O 26 (ca. 225 °C to 275 °C); (v) MoO 2.5 (OH) 0.5 (ca. 325 °C to 475 °C); (v) MoO 2.69 (OH) 0.3 (ca. 575 °C) and (vi) MoO 3− x (ca. 675 °C). The optical properties of the calcined samples were evaluated by diffuse reflectance spectroscopy measurements. Here, the optical band edge absorption Mo–O and inter-band absorptions exist in the visible region of the solar spectrum. The impedance spectroscopy analysis was used further in order to characterize the electrical properties of AHM. The electrical resistance ( Z ′) value was maximum at 300 °C and decrease with temperature inferring the negative temperature co-efficient of resistance behavior.