Structure and Properties of Cs7(H4PO4)(H2PO4)8: A New Superprotonic Solid Acid Featuring the Unusual Polycation (H4PO4).

We report the discovery of a new superprotonic compound, Cs7(H4PO4)(H2PO4)8, or CPP, which forms at elevated temperatures from the reaction of CsH2PO4 and CsH5(PO4)2. The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature 31P NMR spectroscopy, crystallizes in space group Pm3̅n and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsH2PO4, but features an extraordinary chemical moiety, rotationally disordered H4PO4+ cations, which periodically occupy one of every eight cation sites. The influence of this remarkable cation on the structure, thermodynamics, and proton transport properties of the CPP phase is discussed. Notably, CPP forms at a temperature of 90 °C, much lower than the superprotonic transition temperature of 228 °C of CsH2PO4, and the compound does not appear to have an ordered, low-temperature form. Under nominally dry conditions, the material is stable against dehydration to ∼151 °C, and this results in a particularly wide region of stability of a superprotonic material in the absence of active humidification. The conductivity of Cs7(H4PO4)(H2PO4)8 is moderate, 5.8 × 10-4 S cm-1 at 140 °C, but appears nevertheless facilitated by polyanion (H2PO4-) group reorientation.

Quantitative identification of metastable magnesium carbonate minerals by solid-state 13C NMR spectroscopy.

In the conversion of CO2 to mineral carbonates for the permanent geosequestration of CO2, there are multiple magnesium carbonate phases that are potential reaction products. Solid-state (13)C NMR is demonstrated as an effective tool for distinguishing magnesium carbonate phases and quantitatively characterizing magnesium carbonate mixtures. Several of these mineral phases include magnesite, hydromagnesite, dypingite, and nesquehonite, which differ in composition by the number of waters of hydration or the number of crystallographic hydroxyl groups. These carbonates often form in mixtures with nearly overlapping (13)C NMR resonances which makes their identification and analysis difficult. In this study, these phases have been investigated with solid-state (13)C NMR spectroscopy, including both static and magic-angle spinning (MAS) experiments. Static spectra yield chemical shift anisotropy (CSA) lineshapes that are indicative of the site-symmetry variations of the carbon environments. MAS spectra yield isotropic chemical shifts for each crystallographically inequivalent carbon and spin-lattice relaxation times, T1, yield characteristic information that assist in species discrimination. These detailed parameters, and the combination of static and MAS analyses, can aid investigations of mixed carbonates by (13)C NMR.

Correction: Superprotonic conductivity in RbH2-3y(PO4)1-y: a phosphate deficient analog to cubic CsH2PO4 in the (1 - x)RbH2PO4 - xRb2HPO4 system.

Correction for 'Superprotonic conductivity in RbH2-3y(PO4)1-y: a phosphate deficient analog to cubic CsH2PO4 in the (1 - x)RbH2PO4 - xRb2HPO4 system' by Grace Xiong et al., Mater. Horiz., 2023, 10, 5555-5563, https://doi.org/10.1039/D3MH00852E.

Superprotonic conductivity in RbH2-3y(PO4)1-y: a phosphate deficient analog to cubic CsH2PO4 in the (1 - x)RbH2PO4 - xRb2HPO4 system.

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)RbH2PO4 - xRb2HPO4 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.