Reversible crystal−amorphous transformation assisted by loss and adsorption of coordination water molecules and ionic conduction in two isomorphous decavanadate-type polyoxometalates

Two isomorphic decavanadate-type hydrogen-bonding polyoxometalates (POMs) M₂(NH₄)_4-x[V_10-xMo_xO₂₈]·10H₂O (M ​= ​Li, x ​= ​0.13 for 1a; M ​= ​Na, x ​= ​0.95 for 2a) and their nanocrystals 1b and 2b were prepared, respectively. 1a, 1b, 2a and 2b were heated at 408 ​K under vacuum to give the amorphously dehydrated samples 1a′, 1b′, 2a′ and 2b′ sequentially. All samples were characterized by IR spectra, SEM or TEM, TG, XPS and PXRD analyses. Reversible crystal−amorphous transformations (RCATs) appear between 1a/1b and 1a'/1b′, between 2a/2b and 2a'/2b′, which are related to the loss and adsorption of water molecules coordinated to Li⁺/Na⁺ ions. Remarkably, this is the first report on hydrogen-bonding POMs with RCATs. 1a′, 1b′, 2a′ and 2b′ show ionic conduction, moreover, the conductivity (σ) of nanoscale samples are several times higher than that of corresponding micrometric scale samples at the same condition, e.g., σ ​= ​3.32 ​× ​10⁻⁶ ​S ​cm⁻¹ for 1a′ versus 7.86 ​× ​10⁻⁶ ​S ​cm⁻¹ for 1b′, and 0.53 ​× ​10⁻⁶ ​S ​cm⁻¹ for 2a′ versus 1.42 ​× ​10⁻⁶ ​S ​cm⁻¹ for 2b′ at 403 ​K.