Issue 12, 2023

Superprotonic conductivity in RbH2−3y(PO4)1−y: a phosphate deficient analog to cubic CsH2PO4 in the (1 − x)RbH2PO4xRb2HPO4 system

Abstract

In contrast to CsH2PO4 (cesium dihydrogen phosphate, CDP), a material with a well-established superprotonic transition to a high conductivity state at 228 °C, RbH2PO4 (rubidium dihydrogen phosphate, RDP) decomposes upon heating under ambient pressure conditions. Here we find, from study of the (1 − x)RbH2PO4xRb2HPO4 system, the remarkable occurrence of cubic, off-stoichiometric RbH2−3y(PO4)1−y, or α-RDP, with a variable Rb : PO4 ratio. Materials were characterized by simultaneous thermal analysis and in situ X-ray powder diffraction performed under high steam partial pressure, from which the phase diagram between RbH2PO4 (x = 0) and Rb5H7(PO4)4 (x = 1/4) was established. The system displays eutectoid behavior, with a eutectoid transition temperature of 242.0 ± 0.5 °C and eutectoid composition of x = 0.190 ± 0.004. Even the end-member Rb5H7(PO4)4 appears to transform to α-RDP, implying y in the chemical formula of 0.2 and a phosphate site vacancy concentration as high as 20%. Charge balance is attained by a decrease in the average number of protons on the remaining phosphate groups. The cubic lattice parameter at x = 0.180, near the eutectoid composition, and at a temperature of 249 °C is 4.7138(2) Å. This value is substantially smaller than the estimated ambient-pressure lattice parameter of stoichiometric RbH2PO4 of 4.837(12) Å, consistent with the proposal of phosphate site vacancies in the former. The superprotonic conductivity of the x = 0.180 material is 6 × 10−3 S cm−1 at 244 °C, a factor of three lower than that of CDP at the same temperature. While the engineering properties of α-RDP do not suggest immediate technological relevance, the discovery of a superprotonic solid acid with a high concentration of phosphate site vacancies opens new avenues for developing proton conducting electrolytes, and in particular, for controlling their transition behavior.

Graphical abstract: Superprotonic conductivity in RbH2−3y(PO4)1−y: a phosphate deficient analog to cubic CsH2PO4 in the (1 − x)RbH2PO4 – xRb2HPO4 system

Associated articles

Supplementary files

Article information

Article type
Communication
Submitted
04 Jun 2023
Accepted
12 Oct 2023
First published
12 Oct 2023

Mater. Horiz., 2023,10, 5555-5563

Author version available

Superprotonic conductivity in RbH2−3y(PO4)1−y: a phosphate deficient analog to cubic CsH2PO4 in the (1 − x)RbH2PO4xRb2HPO4 system

G. Xiong, L. S. Wang and S. M. Haile, Mater. Horiz., 2023, 10, 5555 DOI: 10.1039/D3MH00852E

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