NZP Structures

Crystalline rhombohedral NZP, has an open three-dimensional channel-type structure built from corner-sharing tetrahedral PO4 and octahedral ZrOg units. It contains two possible sites for the Na cations, only one of which is occupied. The mineral kosnarite KZr2(P04)3 also has a similar type structure [40,41]. The sodium salt, which has a very low thermal expansion coefficient, can be prepared by heating together appropriate quantities of sodium metaphosphate and ZrOj at 1200°C [42]. 

Substantially the same rhombohedral structure can be maintained if sodium is replaced by H, NH4, Li, K, Cs, Mg, Ca, Sr, Ba or Cu, if zirconium is replaced by Ti, Ge, Cr, Al, Fe, Hf, Th, Sn and phosphorus by Si or S. Other combinations of cations may lead to extra vacancies in what is approximately the same type structure (Table 12.50). 

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The NZP structure t) e of the high temperature modifications of actinide phosphates AM2(P04)3 has been confirmed by the Rietveld refinement of the uranium phosphate KU2(P04)3 [50]. The test sample was obtained by means of heating of the monoclinic phosphate of similar stoichiometry at 1350 °C in Ar atmosphere. As shown in Figs. 2 and 3, the UOn polyhedra of monoclinic and rhombohedral stmctures of KU2(P04)3 are different UO9 and UOe, respectively. 

Ri/M 2(P04)3. a double americium zirconium phosphate Ami/3Zr2(P04)3 (M = Zr) was synthesized and characterized by X-ray diffraction methods [74,75], This phosphate belongs to the NZP structure type. The NH4H2PO4 solution was added to the starting solution of americium nitrate and ZrOCU thermal treatment of the precipitates was carried out stage-by-stage up to 800°C. 

Sheetz, B. E., Agrawal, D. K., Breval, E. Roy, R. 1994. Sodium zirconium phosphate (NZP) as a host structure for nuclear waste immobilization A review. Waste Management, 14, 489-505. 

FIGURE 5.14 The structure of NZP. See colour insert on previous page. Na, purple Si, blue P, brown 0, red. 

The pyrophosphate of uranium(IV) has been obtained and the structure determined to belong to the ZrP207-type structure. " Octahedral sites in the zirconium phosphates can accommodate U, as exemplified by the Na dizirconium tris(phosphate) structural family ([NZP]). An end member in this study was monoclinic KU2Zr(P04)3, which contains nine-coordinate Compounds of the general formula MU2(P04)3 have been reported for... 

Following this principle, some authors [74] have S3mthesized first europium phosphate Eui/3Zr2(P04)3 and then americium phosphate Ami/3Zr2(P04)3 [75] and classified them as members of the NZP family. The IR spectra and structure refinement of the Eu phosphate were carried out by the Rietveld method which... 

For instance, the structural chemical approach was used to elaborate such crystalline materials as ceramics for immobilization of actinides and other dangerous elements and isotopes present in radioactive waste. The approach was efficient in modelling new lanthanide phosphates of framework NZP-type structure [94, 95] and of the langbeinite structure type [97-101]. 

In applications where the temperature range of operation is between 1000 and 1400 °C, there is still a lack of heat-resistant materials. For these applications, a ceramic catalyst system, extruded and completed with support and active phase in one piece, would be the ultimate solution. A surface area-enhancing washcoat is probably not needed at these temperatures, since both mass transfer limitations and reaction rates are high. Probably, only a surface area around 1-10 m g would be sufficient, which could be achieved with fine-tuned extrusion techniques. Hence, complicated washcoat-support interactions can be avoided. Among the several materials that are reported suitable for support extrusion in this review, there is a possibility for some of them to be used as the active component. For example, promising support materials like NZP may be active, depending on the specific ionic substitution. On the other hand, metal structures probably have too low a surface area to be used without washcoat. 

FIGURE 12.48 Crystal Structure of NZP (Kosnarite)-type Salts. Structure buUt from [MOJ octahedrta liked to [PO4] tetrahedra by sharing corner O atoms in common. Structure contains an inter-connected system of channels. Monovalent M+ cations not shown. 

Some Compounds with NZP or Nasicon-Type Structures... 

The relatively poor ionic conductivity of NaZr2(P04)3 can also be greatly enhanced by substitution of other atoms for the zirconium as, for example, Nai+ r2 Jn (P04)3. Other compounds which have been studied include the series Li,+ Ti2 4.M (P04)3, where M=In, Sc, Cr, Y and Ga, and Nasicon analogues such as Na3M2(P04)3 and Li3M2(P04)3, where M=Cr, Fe, Sc. Many salts with Nasicon or NZP-related structures are now known (Table 12.50) [45-48]. They are possible absorbants for nuclear waste materials. 

Varieties of Nasicon, NZP or related structures containing mixed valence cations, for example, Na3Zro 6vFe +67Fe3o+67(P04)3 and Cu+Cu +Mg3(P04)3 exhibit both electronic and ionic conduction. 

The results of the high temperature diffraction analyses are shown in Figure 1. The a and c expansion behavior is anisotropic upon heating. Th. c parameter expands while the a parameter contracts. This behavior is consistent with many other NZP-type materials as well as with a structural mode depicting NZP thermal exoansion devised by Lenain.2 3 important to note that as the concentration of Mg increases the anisotropy. - ases. The c parameter expands less and the a parameter contracts sor ). " less, which should improve the thermal snock resistance of the materials. For compositions... 

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