There has been a considerable amount of interest in the ion-exchange properties of layered zirconium phosphates [1-2]. This interest has been renewed due to potential application in the remediation of nuclear waste and in particular for the uptake of Sr, Co and Cs nucleotides. The majority of the ion-exchange work in the literature was carried out several decades ago and little structural characterisation was undertaken [1-2]. In order for these materials to have a potential use in the nuclear waste industry it is imperative that the exchanged ions are tightly bound to the framework and therefore knowledge of their locations from structure solution plays an important role.

Here we present powder diffraction studies of hydrothermally synthesised crystalline α-zirconium phosphate (Zr(HPO₄)₂·H₂O) and its subsequent ion-exchange products with various monovalent and divalent cations, including Sr²⁺, Na⁺, Cs⁺ and Co²⁺. The structure of α-zirconium phosphate consists of phosphate layers with water molecules located between [3]. When cation exchange occurs, the entering cation exchanges for a proton from the HPO₄ group. In some cases ion-exchange leads to a decrease in symmetry [4] and in other cases there is also a decrease in crystallinity. Traditional Rietveld methods have so far proved unsuccessful in the structure solution of these materials. Therefore attention has been turned to total scattering methods and the use of real space Pair distribution function analysis (PDF) [5-6]. The use of differential PDFs to aid cation location and structure solution will be discussed. Particular attention will be paid to the fully strontium exchanged material ZrSr(PO₄)₂·xH₂O, the PDF of which is shown below. The effects of ion-exchange temperature, pH etc. on the resulting PDFs will be discussed.

Figure: Pair distribution functions of α-zirconium phosphate, Sr²⁺ exchanged α-zirconium phosphate and the resulting differential.

References:

[1] A. Clearfield and U. Costantino, Comprehensive Supramolecular Chemistry, 7, 107, (1996).

[2] G. Alberti, Acc. Chem. Res., 11, 163, (1978).

[3] A. Clearfield and G. D. Smith, Inorg. Chem., 8, 431, (1969).

[4] D. M. Poojary and A. Clearfield, Inorg. Chem., 33, 3685 (1994).

[5] S. J. L. Billinge, M. G. Kanatzidis, Chem. Commun., 749, (2004).

[6] Th. Proffen and S. J. L. Billinge, J. Appl. Crystallogr,. 32, 572 (1999).

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