Structural, thermal and electrical studies of a novel rubidium phosphite tellurate compound

Structural, thermal and electrical properties studies of rubidium phosphite tellurate, RbH(PO3H)·Te(OH)6, were performed. An endothermic peak, which reached a completion at about 315 °C accompanied with a weight loss of 4.6 wt.%, was attributed to dehydration. Four types of pellets were produced, namely pellets A, B, C and D. Pellet A was tested with platinum–carbon paper electrode, and pellets B, C and D were tested with gold electrodes. Both pellets A and B were studied from 113 °C to 317 °C for 135 h. Pellet C was first investigated from room temperature to 176 °C for 360 h. After cooling down to room temperature, a second measurement with pellet C was carried out under the same conditions as used for pellets A and B. Pellet D, on the other hand, was heated up to 450 °C, kept at that temperature for 2 h and then cooled down to room temperature prior to the conductivity measurements. It was observed that the conductivities of pellets A and B decreased to values of 5.2 × 10−8 S cm−1 and 6.6 × 10−7 S cm−1 at 317 °C, respectively, and an unexpected rise in the conductivity (9.89 × 10−6 S cm−1 at 317 °C) was seen with pellet C. Dehydration of RbH(PO3H)·Te(OH)6 might be responsible for this unexpected rise in the conductivity of pellet C. The monoprotic part RbH(PO3H) of RbH(PO3H)·Te(OH)6 apparently became diprotic (Rb2H2P2O5) part of Rb2H2P2O5·[Te(OH)6]2 after dehydration. The measured conductivity of pellet D, which was dehydrated prior to the measurement, reached a value of 5.41 × 10−5 S cm−1 at 317 °C and showed a good stability over-each-run time and temperatures measurement up to 317 °C. The dehydrated compound, Rb2H2P2O5·[Te(OH)6]2, has also a higher hydrogen density relative to the starting compound, RbH(PO3H)·Te(OH)6. It is deduced that completion of the dehydration can be responsible for the unexpected rise in the conductivity of RbH(PO3H)·Te(OH)6. This unusual case is important for studies in solid acid proton conductors.