ZrSiO 4 and HfSiO 4 are of considerable interest because of their low thermal expansions, thermal conductivities, and the optical properties of HfSiO 4 . In addition, silicate phases of both are studied as model radioactive waste disposal materials. Previous first principles calculations reported near ideal mixing in the Zr 1−x Hf x SiO 4 system, with a very weak propensity for phase separation. Density functional theory (DFT)/cluster-expansion first principles calculations presented in this work indicate near ideal mixing with a very weak propensity for ordering. Zr 1−x Hf x SiO 4 samples (x = 0, 0.25, 0.5, 0.75, and 1.0) were synthesized from intimate stoichiometric mixtures of constituent-oxides and annealing at 1823 K for 20 days in a platinum crucible. Samples were characterized by X-ray diffraction (XRD; Rietveld analysis) and 29 Si MAS NMR. The XRD data exhibited a pronounced negative deviation from Vegard's law in the excess volume of mixing, and the 29 Si MAS NMR spectra also suggest nonideal mixing. Given the very weak energetics that favor cation ordering, it is clear that there must be some other cause(s) for the observed deviations from ideal mixing behavior.

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ABSTRACT. High charged swelling micas, with layer charge comprised between 2 and 4, has been found to readily swell with water and that complete cation exchange (CEC) can be achieved. Due to their high CEC, applications like radioactive cation fixation or removal of heavy metal cations from waste water were proposed. Their applicability can be controlled by the location of the interlayer cation in a confined space with a high electric field. In synthetic brittle micas, the interlayer cation has a low water coordination number; therefore their coordination sphere would be completed by the basal oxygen of the tetrahedral layer as innersphere complexes (ISC). However, no direct evidence of these complexes formation in brittle micas has been reported yet. In this contribution, we mainly focus on the understanding the mechanisms that provoke the formation of ISC in high charge swelling micas, Mica-n. A whole series of cations (X) were used to explore the influence of the charge and size of the interlayer cation. Three brittle swelling micas, Mica-n (n=4, 3 and 2), were selected in order to analyze the influence of the layer charge in the formation of ISC. The contribution of the ISC has been analyzed thorough the evolution of the 060 reflection and the changes in the short-range order of the tetrahedral cations will be followed 29 Si and 27 Al MAS-NMR. The results showed that ISC was favored in X-Mica-4 and that provoked a high distortion angle between the Si-Al tetrahedra. When the content of aluminum decreases, the electrostatic forces between the layers are relaxed, and the hydrated cations did not interact so strongly with the tetrahedral sheet, having the opportunity to complete their hydration sphere.

Synthesis of Na-Mica-4 has been achieved by a "mix and calcine" 14 method using sodalite and magnesium fluoride as the only precursors. Previous research 15 found sodalite as a key intermediate reaction product in the formation of Na-Mica-4 16 when the NaCl melt method was employed. Similarities in structure, chemical 17 composition and cation distribution in products using the proposed method and the 18 NaCl melt method are described and suggest that Na-Mica-4 is a very stable product. 19The use of sodalite as precursor provokes microporous formation in the final mica. The 20 absence of excess Na leads to a lower particle size and to the presence of less impurity 21