High performing Y₃H₅P₆O₂₂ and Er₃H₅P₆O₂₂ proton conductors: preparation and conductivity study

Two novel trivalent rare-earth mixed-anion phosphates, Y₃H₅P₆O₂₂ (YHPO) and Er₃H₅P₆O₂₂ (EHPO), were synthesised using the thin layer technique (TLT) and processed into dense ceramic electrolytes via the cold sintering process (CSP). Structural characterisation by powder X-ray diffraction confirmed tetragonal symmetry for both compositions, with slight variations in lattice parameters consistent with ionic radius differences between Y³⁺ and Er³⁺. Chemical analysis and infrared spectroscopy verified the coexistence of orthophosphate (PO₄^3-) and pyrophosphate (P₂O₇^4-) groups in a 1 : 1 molar ratio, forming hydrogen-bonded networks relevant to proton transport. The cold-sintered pellets exhibited high relative densities (>88%) and preserved stoichiometry. Electrochemical impedance spectroscopy performed between 40 °C and 140 °C under controlled wet (p_H2O = 0.033 atm) and low humidity (p_H2O ∼ 10^-5 atm) nitrogen atmospheres revealed humidity-dependent proton conduction. Under wet conditions, two transport regimes were identified: a low-temperature vehicular mechanism associated with surface-adsorbed water and a higher-temperature thermally activated regime consistent with Grotthuss-type proton hopping. EHPO exhibited superior conductivity, reaching 1.47 × 10^-4 S cm^-1 at 140 °C, nearly one order of magnitude higher than YHPO. Under low humidity, both materials showed reduced conductivity and single Arrhenius-type behaviour with activation energies in the range of 50 kJ mol^-1-60 kJ mol^-1, indicating dominant structural proton diffusion. The enhanced performance of EHPO is attributed to its slightly more compact lattice, promoting favourable hydrogen-bond connectivity. These findings demonstrate the potential of trivalent rare-earth mixed phosphates as promising intermediate-temperature proton-conducting electrolytes.