Structural variability in M2+ 2-hydroxyphosphonoacetate moderate proton conductors

Abstract

Abstract The structural variability of two series of Mg2+- and Zn2+- 2-hydroxyphosphonoacetates have been studied in the range of 25–80°C and 95% relative humidity in order to correlate the structure with the proton conductivity properties. In addition to selected previously reported 1D, 2D and 3D materials, a new compound, KZn6(OOCCH(OH)PO3)4(OH)·5H2O (KZn6-HPAA-3D), has been prepared and thoroughly characterized. The crystal structure of this solid, solved ab initio from synchrotron X-ray powder diffraction data, consists of a negatively charged 3D framework with K+ ions, as compensating counterions. It also contains water molecules filling the cavities in contrast to the potassium-free 3D anhydrous NH4Zn(OOCCH(OH)PO3) (NH4Zn-HPAA-3D). In the range of temperature studied, the 1D materials exhibit a 1D→2D solid-state transition. At 80°C and 95% RH, the 2D solids show moderate proton conductivities, between 2.1×10−5 S·cm−1 and 6.7×10−5 S·cm−1. The proton conductivity is slightly increased by ammonia adsorption up to 2.6×10−4 S·cm−1, although no ammonia intercalation was observed. As synthesized KZn6-HPAA-3D exhibits a low proton conductivity, 1.6×10−6 S·cm−1, attributed to the basic character of the framework and a low mobility of water molecules. However, this solid transforms to the 2D phase, Zn(OOCCH(OH)PO3H)·2H2O, upon exposure to dry HCl(g), which enhances the proton conductivity with respect to the as-synthesized 2D material (4.5×10−4 S·cm−1). On the other hand, NH4Zn-HPAA-3D exhibited a higher proton conductivity, 1.4×10−4 S·cm−1, than the K+ analog.