Zircon, structure type

The zircon structure-type (ZrSi04) [49] is also adopted occasionally by so-called ternary compositions (Table 3) [38], Zircon is isostructural with the xenotime series of phosphates, typified by xenotime-(Y), YPO4 [50, 51], In this structure-type, the eight-coordinate polyhedra (snub disphenoids) share edges to form a framework composed of cross-linked chains that extend along the [100] and [010] directions. The framework of polyhedra exhibits prominent voids along the [111] direction these are occupied by tetrahedra. The tetrahedra share edges with the polyhedra and thus serve to further connect the chains (Fig. 7). 

The structural properties of several mixed RE-orthophosphates were also investigated by Mullica et al. (1986, 1990, 1992, 1996). In the case of 1 1 Gd/Yb orthophosphate (Mullica et al. 1986), the structure was found to be of the tetragonal zircon structure type with a = 6.865(2) A, c = 6.004(2) A, and unit cell volume = 283.0(3) A. Structural refinements were also carried out for Gd/Er-, Gd/Y-, and Gd/Yb-orthophosphates with a 1 1 RE ratio and for (Gd/Yf))P04 and with a 75 25 ratio of Gd toYb (Mullica et al. 1990). The mixed systems 1 1 (Gd/Tb)P04, 3 l(Gd/Tb)P04 and 9 1 (Lu/Tb)P04 were also investigated (Mullica et al. 1992) as were seven mixed compounds in the (Ce/Tb), (Nd/Tb), and (Sm/Tb)P04 families of orthophosphates (Mullica et al. 1996). All of the mixed orthophosphates investigated by Mullica et al. were found to crystallize with the tetragonal zircon structure. At this time, a considerable body of structural and crystal chemical data exists for the synthetic RE, Y, and Sc orthophosphates as evidenced by the previously cited references of Mullica et al., Milligan et al., and Ni et al. to which the reader is referred for details. Additionally the related structural investigations of Chakoumakos et al. (1994) should be noted. In these studies, structural refinements of the zircon-type RE-, Y-, and Sc-orthovanadates are reported. 

Arsenate. — The arsenates of the rare earths crystallize [263] in two structural types, the huttonite and the zircon. The structural change from huttonite (La—Nd) to zircon (Sm—Lu) occurs at samarium. The lattice parameters of EuAsCU are a = 7.167 and c — 6.374 A. The rare earth arsenates can be prepared by reacting the nitrates with (NEU HAsCU, and heating the product to 700° C. 

Knapp et al. (144) show that for oxides containing 3d elements in spinel, perovskite, rocksalt, or zircon-type structures, the K-edge XANES spectra are quite independent of 3d electron occupation but instead nicely correlate with the crystal structure type. Various studies of Ti K edges of titanium oxides and other titanium compounds have been reported (40,158,172,177, 297). 

Zircon-type superstructures apparently occur in actinide arsenates as a result of cation ordering when both monovalent alkali metals and tetravalent actinides occupy polyhedral sites (Table 3). Cation ordering is also a factor in the orthorhombic symmetry of the low-temperature polymorph of CaU(P04)2 [41], shown in Figure 8, and which is closely related to the zircon structure. 

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. 

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... 

Xenotime, YPO4, is representative of another isomorphous series MPO4, where M = Y, Sc, Tb to Lu, which have zircon ZrSi04-type structures (Figure 5.11a). Monazite, (Ce,La,Th)P04, and xenotime, YPO4, are sources of the rare elements they contain. In monazite samples, Ce and Th are usually predominant. 

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. 

Proton conducting ceramics are commonly based upon the perovs-kite structure type with the cerate and zirconate types being most prevalent. Initial studies focused on the SrCeOs- and SrZrOs-based materials. Uchida et al. demonstrated sizeable proton conductivity in each of these materials but due to concerns over the stability of these compositions attention shifted towards the Ba analogues. In these materials the key concern is their stability in CO2 containing atmospheres. 

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. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

The authors [34] proposed to use perovskites ABO3, where A are calcium cations, or a mixture of calcium and lanthanum, and B are iron, cobalt, nickel or manganese cations, or their mixtures. Besides, aluminates, silicates, aluminium sihcates, zirconates and chromates of different types are added as structure-forming components providing strength and stability to thermal shocks [34]. 

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... 

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