Electrical conductivity of MOXO4 (M=V, Nb; X=P, As) compounds intercalated with H2O and H3XO4

Layered compounds with the general formula MOXO₄·yH₂O (M=V, Nb; X=P, As) were prepared. The content of water y was controlled by keeping the samples in an atmosphere with various relative humidities (RH). Depending on RH, the formation of several hydrates of niobyl phosphate and arsenate was observed and their basal spacings (d) were determined, namely, NbOPO₄·H₂O, , at 11% RH and lower, NbOPO₄·2H₂O, , at 22-33% RH, NbOPO₄·3H₂O, , at 43-84% RH, and NbOPO₄·5H₂O, , at 92% RH and above; NbOAsO₄·H₂O, , at 0-16% RH and NbOAsO₄·3H₂O, at 33% RH and above. As follows from ac and dc conductivity data, NbOXO₄·yH₂O compounds are practically pure protonic conductors, whereas VOXO₄·yH₂O compounds are mixed protonic-electronic conductors and the protonic component increases with y. Two intercalates of MOXO₄·yH₂O with inorganic acids were prepared. A new intercalate of H₃AsO₄ into VOAsO₄·yH₂O with the formula VOAsO₄·0.5H₃AsO₄·yH₂O (y=0.5-0.8) has the cell parameters a=6.37 and at 0-22% RH. Above 22% RH, the intercalate decomposes and the parent VOAsO₄·yH₂O with H₃AsO₄ adsorbed on the surface is formed. Another intercalate with formula NbOPO₄·H₃PO₄·yH₂O (y=2-4 at 0-75% RH) has the cell parameters a=6.43 and at RH from 0% to 5% and a=6.48 and at RH from 33% to 75%. Both intercalates are more conductive than their MOXO₄·yH₂O hosts and their conductivity increases with increasing RH of the surrounding atmosphere. Like NbOPO₄·yH₂O, also NbOPO₄·H₃PO₄·yH₂O can be considered pure proton conductor and its conductivity at 20 °C reaches 5×10⁻³ S cm⁻¹ for y=4.