Improving the Thermal Expansion and Capacitance Properties of MoO₃ by Introducing Oxygen Vacancies

Extrinsic Point Defects in Low-Positive Thermal Expansion Al2W3O12 and Their Effects on Thermal and Optical Properties

“…However, their unit-cell volumes were about 0.2% higher than that for the A800 sample. This is a significant difference (the difference is outside the range of uncertainty) and an indication that the calcinations in inert or reducing atmospheres affected unit-cell volume through the formation of extrinsic point defects since it is known that the presence of defects distorts the crystal structure in different manners. ,, In the case of In 0.6 (HfMg) 0.7 Mo 3 O 12 , a structural parent phase to Al 2 W 3 O 12 , a similar increase in unit-cell volume (∼0.2%) was found after the heat treatment in the He atmosphere at 500 °C.…”

“…Even more relevant is the red shift ( i.e. , shift of vibrational bands to lower wavenumbers) identified for the 190 cm –1 (ν 2 ) band and all ν 3 and ν 1 bands in the range of 800–1100 cm –1 for Al 2 W 3 O 12 in AH400, which could be an indication of oxygen vacancies. ,, These shifts are consequences of weakening of W–O bond strength within the WO 4 tetrahedra due to the formation of extrinsic vacancies and consequent enlargement of the unit cell. Hardcastle and Wachs proposed, for several tungstate systems, the following empirical relation where ω is the wavenumber and R is the interatomic distance between the W and O ions, from which we can infer that the red shift of 6.5 cm –1 observed in our experiments for the ν 1 mode close to 1040 cm –1 is associated with an average volumetric expansion of around 0.5% for the WO 4 tetrahedra in AH400, which is not far from the 0.2% calculated for the unit cell by XRPD for Al 2 W 3 O 12 in AH400.…”

“…More recently, Cheng et al 16 proposed that introducing extrinsic oxygen vacancies could provide another mechanism to tune the CTE since In 0.6 (HfMg) 0.7 Mo 3 O 12 with extrinsic oxygen vacancies, produced by heat treatment in an inert He atmosphere, presented a more negative CTE than its extrinsic-point-defect-free counterpart calcined in air. Many other binary and ternary oxides, such as TiO 2 , 19,20 WO 3 , 19,21 MoO 3 , 22 Ca 2 RuO 4 , 23 and Y 2 O 3 , 24 have been studied for extrinsic point defects formed in reducing or inert atmospheres; however, to the best of the authors' knowledge, only MoO 3 22 and Ca 2 RuO 4 23 were studied for their effect on the CTE.…”

“…Many other binary and ternary oxides, such as TiO 2 , , WO 3 , , MoO 3 , Ca 2 RuO 4 , and Y 2 O 3 , have been studied for extrinsic point defects formed in reducing or inert atmospheres; however, to the best of the authors’ knowledge, only MoO 3 and Ca 2 RuO 4 were studied for their effect on the CTE.…”

Oxygen deficient α-MoO3 with enhanced adsorption and state-quenching of H2O for gas sensing: a DFT study

“…For oxygen deficient MoO 3 , the formation of V Ot introduces a gap state at ∼0.59 eV below the CBM and reduces the bandgap from 2.92 to 2.81 eV, which is consistent with experimental results. 49 This is due to the fact that the electrons transfer from the V O to the 4d states of Mo sufficiently reduces the crystal field. 50 Interestingly, the intermediate state associated with H 2 O above perfect MoO 3 disappears for H 2 O adsorbed above Vo–MoO 3 , leaving an intermediate state at ∼0.57 eV below the CBM in Fig.…”