16 - Phosphates and phosphonates of tetravalent metals as protonic conductors

Summary

Introduction

For many years it has been known that amorphous precipitates having a composition of M(IV)(HPO₄)2_–x(OH)_2x.nH₂O (x usually ranging from 0 to 0.4) are easily formed when a salt solution of a tetravalent metal (M(IV) = Zr, Ti, Ce, Th, Sn etc.) is mixed with a phosphoric or arsenic acid solution. Due to their peculiar properties (high ion exchange capacity, thermal stability, insolubility in concentrated acids, resistance to radiation, selectivity for some cations of interest in nuclear energy etc.) these amorphous precipitates were intensely investigated between 1954–1964 especially in nuclear centres. During this period Hamlen and Alberti also discovered that amorphous zirconium phosphate exhibits a good protonic transport at room temperature (0.1–6 x 10^–3 Ω^–1 cm^–1).

After 1964, the discovery that layered crystalline compounds (general formula M(IV)(HO—PO₃)₂.H₂O) can be obtained by refluxing amorphous materials or prepared directly by slow precipitation in the presence of HF gave rise to an increased interest in these inorganic ion-exchangers.

It was found that a tetravalent metal can be bonded to the oxygens belonging to XO₄ and/or R—XO₃ groups (X = P, As; R = inorganic or organic radical such as —H, —OH, —CH₃OH, —C₆H₅ etc.) forming two different layered structures usually called α and γ. Some fibrous structures (e.g. cerium and thorium phosphates) or tridimensional structures (e.g. HZr₂(PO₄)₃ and Zr(R(PO₃)₂) can also be obtained.