Electrical conductivity of lanthanum hydroxide below 473 K

We have investigated the synthesis, preparation protocols and properties of La(OH)₃ powder and pellets. Preparation of useful pellet samples of La(OH)₃ from the synthesized powder required (i) elimination of the presence of carbonate oxides by powder calcination at 1173 K in flowing oxygen; followed by (ii) hydration of the remaining La₂O₃ in boiling, deionized water for 48 h. Room temperature compaction of these powders into solid pellet samples with 73% of theoretical density, suitable for electrical measurements, requires prolonged (72 h) dwell under 80 MPa uniaxial pressure, suggesting that cold sintering and formation of inter-grain bridges do take place. The electrical conductivity (σ) of the compacted sample is an increasing function of temperature from 363 K to 463 K: evidence is presented that σ derives primarily from proton transfer. The activation energy in this temperature range is E_a ≈ 0.60 eV, while at 363 K, σ = 3·10⁻¹¹ S/cm. Although 3·10⁻¹¹ S/cm is too low for applications in fuel cells it may be sufficient for electro-chemo-mechanical applications. The fact that the grain boundaries are apparently not blocking, makes it attractive to look for dopants that may potentially enhance the low temperature conductivity.