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, d=6.4Å, at 11% RH and lower, NbOPO4·2H2O, d=7.0Å, at 22–33% RH, NbOPO4·3H2O, d=7.8Å, at 43–84% RH, and NbOPO4·5H2O, d=8.0Å, at 92% RH and above; NbOAsO4·H2O, d=6.2Å, at 0–16% RH and NbOAsO4·3H2O, d=7.9Å 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=0.5–0.8y=0.5–0.8) has the cell parameters a=6.37a=6.37 and c=8.81–9.13Å 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=2–4y=2–4 at 0–75% RH) has the cell parameters a=6.43a=6.43 and c=9.56–9.70Å at RH from 0% to 5% and a=6.48a=6.48 and c=11.2Å 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=4y=4.

Layered phosphates containing water and inorganic acid molecules in their interlayer space represent a group of compounds with distinct protonic conductivity.Figure optionsDownload as PowerPoint slide