Monazite structure

The monazite structure consists of distorted PO4 tetrahedra with each metal atom roughly equidistant from nine oxygen atoms. Minor amounts of other rare-earth elements may occur. Steady-state liuninescence under X-ray excitation of monazite revealed emission of Gd, Tb, Dy and Sm (Gorobets and Rogojine 2001). Laser-induced time-resolved liuninescence enables us to detect Sm +, Eu and Nd emission centers (Fig. 4.70). 

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of 3+p5 xv5+o compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO4) have the monoclinic Monazite structure (P2 /n) for A La, Ce-Gd, and the tetragonal Zircon structure (l4]/amd) for A Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO, and the Zircon structure for A = Ce - Lu. When the end members are iso-structural, e.g., LaPO /LaVO, Monazite, YbPC /YbVOA,... 

The monazite structure is dimorphous with xenotime (zircon stmcture-t5q)e), and is one of the major natural hosts of Th and U in cmstal rocks the tetravalent actinides can be accommodated in conjunction with a large divalent cation in a coupled substitution for lanthanide elements, e.g., complete solid solution is found between brabantite, (Cao sTho 5)P04, and monazite-(Ce), CeP04 [51]. Ternary compositions, and partial occupancy are also mechanisms of actinide substitution (Table 5). 

Table 5. Actinide phosphates and arsenates with the monazite structure ...

Bo.M2(P04)3. The thorium, uranium, and neptunium phosphates of general formula Bo 5M2(P04)3 with B = Be, Mg, Ca, Sr, Ba are characterized by extensive polymorphism [5,53,60,61,62,63,64]. As a result of thermal treatment of Mg, Ca, and Sr neptunium phosphates with unknown structures in an Ar + 5% H2 atmosphere, these compounds were established [5,53] as substances crystallizing with the monazite structure type. The authors of [35] have obtained a series of neptunium phosphates with an analogous composition by interaction between solutions of salts of divalent metals (Mg, or Ca, or Sr) with neptunium oxide Np02 and phosphoric acid followed by thermal stage-by-stage treatment... 

The Bo.5M2(P04)3 structural formula is described as belonging to the defect monazite structure of CeP04 heterovalent isomorphism takes place with the appearance of vacancies or interstitial cations according to the scheme Ce(R) <- B i/6Di/6M 2/3- Another modification has been described for Th-orthophosphates Bo,5Th2(P04)3 with B = Ca, Sr, Ba, Cd, Pb [62,64] structure type NaTh2(P04)3 (monoclinic, space group C2 c). 

BM(P04). The existence of polymorphic modifications is t5q)ical for Th and U phosphates BM(P04)2. Their crystallization led to formation of monoclinic phases with the monazite structure t5q)e. The minerals brabantite (CaTh(P04)2) and huttonite (ThSi04) belong to the monazite structure t5q)e as well. Tetragonal phases have the zircon (ZrSi04) structure t5q)e [10, 53,65-70,71],... 

The transition of monoclinic monazite-t5q)e BM(P04)2 phosphates to a different structural modification occurs at 1400°C (MgTh), 1300°C (SrTh), 1200°C (MgU), lOOO C (Call and SrU). The CaTh phosphate structure remains stable during heating up to 1600°C. The monazite structure type was not established... 

BRM(P04)3. The known Th and U compounds with 2- and 3-valent cations B and R form two series of isostructural phosphates BNdTh(P04)3, BGdU(P04)3, where B = Mg, Ca, Sr, Ba, Cd, as well as CaNdTh(P04)3 and Cao.5Nd2Tho.5(P04)3. These crystallize with the monazite structure type [5,76,77] and have similar unit cell parameters. 

The crystal chemical analysis of BRM(X04)3 compounds with M = Th and U allowed one to predict the possible existence of CaAmPu(P04)3 and make an conjecture concerning its structure. The phosphate with this composition has recently been obtained and was found with the expected monazite structure t) e, space group Pljn [81]. 

BRM(P04)3. CaAmPu(P04)3 has been synthesized recently and the monazite structure type has been attributed to it [81]. It is the only known compound among the BRM(P04)3 phosphates where tri- and tetravalent actinides are present together. Obviously, the Am and Pu" cations occupy cation sites with CN = 9 (monoclinic, space group P2 ln). The phosphate CaAmPu(P04)3 has an intermediate composition in the CaxAm3 2xPux(P04)3 series with x = 1. The first member of the row is AmP04 (x = 0) and the last one is Cai sPui 5(P04)3. CaPu(P04)2 (x = 1.5) has a monazite-type stmcture as well. 

Preparation and Characterization of Lanthanide and Actinide Solids. Crystalline / element phosphates were prepared as standards for comparison to the solids produced in the conversion of metal phytates to phosphates. The europium standard prepared was identified by X-ray powder diffiaction as hexagonal EuP04 H20 (JCPDS card number 20-1044), which was dehydrated at 204-234 °C and converted to monoclinic EUPO4 (with the monazite structure) at 500-600 °C. The standard uranyl phosphate solid prepared was the acid phosphate, U02HP04 2H20 (JCPDS card number 13-61). All attempts to prepare a crystalline thorium phosphate failed, though thorium solubility was low. In the latter case the solids were identified as amorphous Th(OH)4 with some minor crystalline inclusions of Th02. 

Furusaki et al. [49] reported that the kinetic characteristics of the conversion of LaCr04 (monazite structure) to LaCrOj (perovskite structure) depended onp 0- and that the rate decreased with increasing availability of oxygen. In N2, the reaction (908 to 938 K) fitted the contracting volume expression with , = 198 kJ mol. The rate limiting step for reaction in Oj is the diffusion of oxygen across the product layer, the Jander rate equation applies with about 506 kJ mol. ... 

The thermochemical properties of the rare-earth orthophosphates [plus Sc(P04) and Y(P04)] have recently been investigated in detail by Ushakov et al. (2001). These workers obtained the formation enthalpies of 14 orthophosphates by using calorimetric techniques and found an almost linear dependence between the enthalpies of formation and the rare-earth radius, from La(P04) (-321.4 kJ/mol) to Lu(P04) (-236.9 kJ/mol) xenotime and pretulite were found to be consistent with this behavior as well. The structural transition from the xenotime structure to the monazite structure was not manifested in a significant discontinuity in the relatively linear trend in the enthalpies of formation. The complete results of these detailed thermochemical studies are tabulated in Ushakov et al. (2001). 

Figure 8. The nature of the nine-fold coordination characteristic of the monoclinic monazite-structure P2 lri) orthophosphates is illnstrated in a view of Ce(P04) that also shows the hnkage to one of the surrounding PO4 structnral nnits (after Beall et al. 1981).

Table 3. Crystal data and results of structure refinements for monazite structure phases... 

Figure 13. (Left) The bidentate bonding to two of the PO4 tetrahedral units in the monoclinic monazite structure is illustrated. (Right) The pentagonal arrangement of the rare-earth-to-oxygen bonds on five surrounding PO4 tetrahedra is illustrated in a projected view.

Figure 14. The chain-like nature and interlocking mechanism of the monoclinic monazite structure illustrates the apical linking that forms the chains, shown in a stereo view. The large open circles represent phosphorous while the large solid circles represent the rare-earth ions. The small open circles represent oxygen (after Mullica et al. 1985a,b).

A chain-like structure similar to that exhibited by the monoclinic orthophosphates is also found for the tetragonal zircon type materials. Each of these structure types has four chains in each unit cell, with the principal difference between the xenotime and monazite structural types residing in the difference in the coordination number. The linking of the chains occurs laterally through edge-sharing of adjacent polyhedra (Ni et al. 1995). One view of how the RE ions link to the PO4 tetrahedral units in the xenotime structure is provided by the stereo view of Y(P04) looking down the c axis of the structure shown in... 

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