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Instability model of the NaZr2(PO4)3 Structure Albert Troole, Sergey Stefanovsky, SIA “Radon”, 7th Rostovskii per. 2/14, Moscow 119121 RUSSIA, [email protected]

ABSTRACT Incorporation of paramagnetic species in NaZr2(PO4)3 (NZP) single crystal made possible to investigate local distortion of the NZP structure from EPR data. Cr3+ and Fe3+ ions were used as paramagnetic probes. We mapped invariant sum of cubic component of the fourth rank tensor of crystal field, the indicative surface of the B04 element of this tensor as well as 2nd, 3rd, and 4th orders axles of the cubic component of the same tensor. From these data the coordination polyhedron of the ion-probe has been modeled and direction to charge compensator has been determined. Based on the coordination polyhedron model obtained it has been shown how would the local symmetry be lowered from C3 (initial symmetry of the Zr-polyhedron) to C1 (Zr substitute ion-probe). A comparison of dynamics of the structure occurring at lowering the symmetry with allowed dynamics of the NZP structure (Roy -Alamo-Agrawal model) makes easily explainable why the dynamics observed results in an instability of the NZP structure and breaking the symmetry from R3c to С2/с. INTRODUCTION Alkali and alkali earth zirconium phosphates AB2(PO4)3 (A = alkali or alkali earth cations and B = normally Zr4+) is of interest as a host phase for a number of radioactive waste elements due to very wide range of isomorphic substitutions [1-3]. However, in reality products of waste incorporation in the NZP structure are not single phase. In particular this has been demonstrated in experiments with Idaho plant waste surrogate [4]. As follows from more detailed investigations orthophosphate compounds with similar chemical compositions crystallize in different symmetries [5]. We have demonstrated that the reason is thermodynamic instability of the NZP structure being brought about trivalent cations incorporation [6,7]. In this work an instability model of the NZP structure is suggested and substantiated. Investigations were performed using electron paramagnetic resonance (EPR) technique. We recorded EPR spectra of NaZr2(PO4)3 single crystals doped with Fe3+ and Cr3+ ions as paramagnetic probes. We have determined fine structure tensors of spin-Hamiltonian from angular variations of EPR spectra of these ions, formulated a thermodynamic stability criterion using a maximum invariant component (MIC) [8] procedure and established that the NZP structure becomes instable due to trivalent cations incorporation. These investigations are described in details in our previous works [6,7]. MODELLING Application of the MIC procedure both determines local symmetry of ion-probe and restores coordination polyhedron [9]. For this, after determination of fine structure parameters of spin-Hamiltonian the following maps must be calculated • an invariant sum of cubic component of fourth rank tensor of fine structure; JJ11.16.1

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an indicative surface of B04 element of fourth rank tensor of fine structure; second-, thir