Structure and properties of Cs7(H4PO4)(H2PO4)8: A new superprotonic solid acid featuring the unusual polycation (H4PO4)+

Abstract

We report the discovery of a new superprotonic compound, Cs7(H4PO4)(H2PO4)8, or CPP, which forms at elevated temperatures from 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 Pmn and has lattice constant 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, resulting 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. Figure 1

Phase Behavior and Superprotonic Conductivity in the System (1–x)CsH2PO4 – xH3PO4: Discovery of Off-Stoichiometric α-[Cs1–xHx]H2PO4

“…For all samples, the ambient temperature diffraction patterns (example shown in Figure S1) revealed a mixture of monoclinic CDP [hereafter CDP­(m)] and CsH 5 (PO 4 ) 2 phases. For the material of global composition x = 2/9, the mixture of CDP­(m) and CsH 5 (PO 4 ) 2 reacted at 90 °C to form stoichiometric CPP, a result that is consistent with what we have reported previously . This reaction is revealed by an endothermic event that occurs without mass loss at 90 °C (Figure a) and in the diffraction pattern collected at 110 °C (Figure b).…”

“…While mass loss quickly followed this thermal event, the event itself was independent of mass loss. This transition was noted in our previous work; however, we had not fully established its nature . Here, we find the transition to correspond to the complete transformation of CPP to a simple cubic phase, Figure b ( T = 180 °C).…”

“…In all cases, the additives increased the conductivity at temperatures below the superprotonic transition with minimal impact on the high-temperature conductivity and, in some cases, also improved stability against dehydration. In parallel with this work, we recently showed the existence of the new compound Cs 7 (H 4 PO 4 )­(H 2 PO 4 ) 8 or CPP, which forms at 90 °C through the stoichiometric reaction of CDP and CsH 5 (PO 4 ) 2 . This compound is structurally similar to CDP with rotationally disordered phosphate anions arranged in an approximately simple cubic arrangement.…”

“…The phase transformation behavior of three compositions in the (1– x )­CsH 2 PO 4 – x H 3 PO 4 system is summarized in Figures –. The examples selected correspond to (i) CPP, x = 2/9 (Figure ), with selected data taken from our previous work, (ii) the eutectoid composition, x = 0.18 (Figures –), and (iii) a representative composition rich in Cs relative to the eutectoid, x = 0.07 (Figure ). The data reported are the simultaneous DSC/TGA profiles (with corresponding dTG profiles in Figure S2) and in situ diffraction patterns for all three compositions, along with variable-temperature NMR spectra for the eutectic composition.…”

“…Phase transformation behavior of (1– x )­CsH 2 PO 4 – x H 3 PO 4 at x = 2/9, the composition corresponding to the compound Cs 7 (H 4 PO 4 )­(H 2 PO 4 ) 8 or CPP: (a) simultaneously measured TGA/DSC profiles at a heating rate of 1 °C/min (data from ref with permission from the American Chemical Society); (b) XRD patterns measured at the temperatures and p H 2 O values indicated ( T = 110 °C, pattern from ref with permission from the American Chemical Society); and (c) lattice constant as a function of temperature (uncertainties in the refined values fall within the span of the datapoint symbols, Figure S4). In (a), p H 2 O = 0.6 atm (balance Ar) was introduced at 130 °C as part of the heating protocol, and data reflecting system equilibration to the high-humidity condition are omitted.…”