Electrical conductivity of MO XO 4 ( M=V, Nb; X=P, As) compounds intercalated with H 2O and H 3XO 4

Abstract

Layered compounds with the general formula MO XO 4· yH 2O ( 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, NbOPO 4·H 2O, d = 6.4 Å , at 11% RH and lower, NbOPO 4·2H 2O, d = 7.0 Å , at 22–33% RH, NbOPO 4·3H 2O, d = 7.8 Å , at 43–84% RH, and NbOPO 4·5H 2O, d = 8.0 Å , at 92% RH and above; NbOAsO 4·H 2O, d = 6.2 Å , at 0–16% RH and NbOAsO 4·3H 2O, d = 7.9 Å at 33% RH and above. As follows from ac and dc conductivity data, NbO XO 4· yH 2O compounds are practically pure protonic conductors, whereas VO XO 4· yH 2O compounds are mixed protonic–electronic conductors and the protonic component increases with y. Two intercalates of MO XO 4· yH 2O with inorganic acids were prepared. A new intercalate of H 3AsO 4 into VOAsO 4· yH 2O with the formula VOAsO 4·0.5H 3AsO 4· yH 2O ( y = 0.5 – 0.8 ) has the cell parameters a = 6.37 and c = 8.81 – 9.13 Å at 0–22% RH. Above 22% RH, the intercalate decomposes and the parent VOAsO 4· yH 2O with H 3AsO 4 adsorbed on the surface is formed. Another intercalate with formula NbOPO 4·H 3PO 4· yH 2O ( y = 2 – 4 at 0–75% RH) has the cell parameters a = 6.43 and c = 9.56 – 9.70 Å at RH from 0% to 5% and a = 6.48 and c = 11.2 Å at RH from 33% to 75%. Both intercalates are more conductive than their MO XO 4· yH 2O hosts and their conductivity increases with increasing RH of the surrounding atmosphere. Like NbOPO 4· yH 2O, also NbOPO 4·H 3PO 4· yH 2O can be considered pure proton conductor and its conductivity at 20 °C reaches 5×10 −3 S cm −1 for y = 4 .