Structural variability in M2+ 2-hydroxy-phosphonoacetate moderate proton conductors

The structural variability of two series of Mg2+- and Zn2+- 2-hydroxyphosphonoacetates have been studied in the range of 25- 80 degrees 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) center dot 5H(2)O ( 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 degrees C and 95 % RH, the 2D solids show moderate proton conductivities, between 2.1 x 10(-5) S center dot cm(-1) and 6.7 x 10(-5) S center dot cm(-1). The proton conductivity is slightly increased by ammonia adsorption up to 2.6 x 10(-4) S center dot cm(-1), although no ammonia intercalation was observed. As synthesized KZn6 -HPAA-3D exhibits a low proton conductivity, 1.6 x 10(-6) S center dot 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) center dot 2H(2)O, upon exposure to dry HCl(g), which enhances the proton conductivity with respect to the as-synthesized 2D material ( 4.5 x 10(-4) S center dot cm(-1)). On the other hand, NH4Zn-HPAA-3D exhibited a higher proton conductivity, 1.4 x 10(-4) S center dot cm(-1), than the K+ analog.