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

Adetailed optical and impedance study was performed on the structural evolution and phase transition of [NH4](6)Mo7O24 center dot 4(H2O) (ammonium heptamolybdate tetrahydrate, AHM). The 1 h calcination ofAHMfrom 50 degrees C to 675 degrees C in an ambient air atmosphere induced a phase transition from an unstableAHMto the most stable two-dimensional orthorhombic molybdenum oxide (alpha-MoO3-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) [NH4](6)Mo7O24 center dot 4(H2O) (30 degrees C); (ii) [NH4](4)(Mo8O24.8 (O-2)(1.2)(H2O)(2))(H2O)(4) (ca. 50 degrees C to 75 degrees C); (iii) [NH4](8)Mo10O34 (ca. 100 degrees C to 200 degrees C); (iv) [NH4](4)Mo8O26 (ca. 225 degrees C to 275 degrees C); (v) MoO2.5(OH)(0.5) (ca.325 degrees C to 475 degrees C); (v) MoO2.69(OH)(0.3) (ca. 575 degrees C) and (vi) MoO3-x (ca. 675 degrees C). The optical properties of the calcined samples were evaluated by diffuse reflectance spectroscopy measurements. Here, the optical band edge absorptionMo-Oand 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 degrees C and decrease with temperature inferring the negative temperature coefficient of resistance behavior.