Monazite, structure type

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 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. 

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. 

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... 

Of the rare earth orthovanadates only LaV04 crystallizes with monazite structure type, isomorphic with monoclinic LaP04. The structure contains approximately tetrahedral VO4 units and a very irregular coordination polyhedron around lanthanum (fig. 111). In a preliminary structure determination of LaV04... 

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). 

Apparently, formation of actinide (IV) compounds with more complex cationic and anionic compositions with the monazite or zircon (xenotime) structure types is possible. These compounds can be considered to be solid solutions. The possibility of forming this kind of compound is realized in the minerals mentioned above. These minerals are characterized by the complex cation compositions monazite - (La, Ce, other lanthanides, Y, Ca, Th)(P, Si)04, xenotime - (Y, lanthanides. Sc, Zr, Th, U)(Si, P)04 zircon - (Zr, Hf, Th, U, lanthanides, Ca, Fe, Nb, Ta)(Si,P)04 [71]. The ionic radii and cationic proportions, anionic sizes and synthesis conditions affect the formation of each type considered. 

Figure 17. The variation of the RE-to-phosphate distances is shown as a function of the RE-ion radius for both the monazite- and xenotime-structure orthophosphate compounds. As shown in this figure, the shorter RE-to-phosphorous distances in the monoclinic structure compounds vary linearly with the trivalent RE ion radius with a slope that is close to 1. A similar variation is evident for the RE-P distances in the tetragonal xenotime-structure compounds—a trend that supports the comparison of the [001] polyhedron-P04 tetrahedron chain arrangement in the two structural types (after Ni et al. 1995).

Recently, Nabar and Sakhardande (1985a,b) have studied the triple orthoarsenates with general formula MRTh(As04)3 where M = Ca, Cd. The X-ray diffraction and IR spectroscopic studies have revealed that, like the binary orthoarsenates, they have two structure types monoclinic monazite (La Nd) and tetragonal zircon (Sm - Tm, Y). Some of the compounds show dimorphism at high temperatures (>900°C) a monazite scheelite transition. 

The monazite structure of CeP04 is very similar to that of zircon. Its space group is P2i n, and the lattice parameters are a = 6.76 A, h = 7.00 A, c = 6.44 A j3 = 103.6°.. Transformations from both the zircon and monazite structures to that of the scheelite have been observed by Stubican and Roy (1%3) for a number of arsenates and vanadates of the rare earths. Stability regions of these phases at standard and high pressures are shown in table 28.13. These reactions occurred under the influence of pressures up to 80 000 atm at 600°C and are accompanied by an 11.5% volume decrease however, transformations between zircon and monazite were not effected because of their similar densities. Praseodymium chromate, prepared by Schwarz (1963a), was a mixture of the monazite and zircon types, and LaCr04 had the monazite structure (Schwarz, 1963b). Presumably other rare earth chromates have either the monazite or zircon structures. 

Rare earth orthovanadates (RVO4) t) ically include two isomorphic phase structures with rare earth orthophosphates, monoclinic (m-) monazite type, and tetragonal (f-) zircon t) e. The selectivity also relies on the... 

Some phosphates of the AM2(P04)3 family form other modifications as well neptunium phosphate NaNp2(P04)3 with monazite type structure, Pl ln (this type was obtained by heating the trigonal form in Ar + 5% H2 at 1100 °C [5,35]) neptunium and plutonium phosphates AgNp2(P04)3 and AgPu2(P04)3 with scheelite type structure, 74i/a [46], thorium phosphate NaTh2(P04)3 with... 

Mullica DF, Sappertfield EL, Boatner LA (1996) Monazite-and zircon-type structures of seven mixed (Ln/Ln)P04 compotmds. Inorganica Chimica Acta 244 247-252... 

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