Defect structures, scheelite

Figure 35 Structure of a regular tetragonal scheelite (a) compared with the distorted, defected structure in Bi2Mo30i2 (b)...

Figure 3. Ideal scheelite structure compared to ordered defect structures. Projection is down the c axis with only 1/2 the unit cell shown in this direction. Shaded circles and tetrahedra are at the top of level, unshaded 1/4 of the way down the unit cell, (a) CaWO. (b) La2 (Mo04)3. (c) Eu2(WO4)3. (d) Bi2(MoO4). ...

Scheelite oxides are a good example of how structure-property relationships have been used to create a modern catalyst. More detailed descriptions of these and other catalysts are given in Oxide Catalysts. The scheelite structure was discussed in Section 3.4.5. Scheelite based oxides are of interest as catalytic materials because of their ability to form with transition elements in high but easily reducible valence states and to form a variety of defects and defect structures that provide numerous sites for selective catalytic activity. 

Point defects in the form of cation vacancies () were introduced by Aykan et al. (93-95) into molybdates, tungstates, and vanadates with scheelite-type crystal structures. The authors studied the catalytic properties of more than 30 scheelite-structure phases represented by the formula A1 x< xM04 (M = molybdenum, tungsten, and/or vanadium and A may include Li, Na, K, Ag, Ca, Sr, Ba, Cd, Pb, Bi, and/or arare earth element in quantities appropriate to achieve charge balance for the normal oxidation states). It was found that the defects can be introduced... 

Additionally, Aykan et al. (98) reported the results for scheelite-type systems in which A sites are occupied by divalent elements and bismuth, and M sites contain vanadium and molybdenum. The tolerance for vacancies in this system was reported to be 15% of the A cation sites. Good yields of acrolein were obained when bismuth and defects were present in the scheelite-structured catalysts. 

A variety of related structures can be identified with 6,8, and 12-fold coordination of the A cation and four or sixfold coordination of the anion. In fact, the chemistry of ABO4 temarys is extremely complicated with solid solutions and phase transitions being common. Lattice defects may be introduced easily by appropriate dopings. Scheelites and its relatives have been studied intensively for their properties as heterogeneous catalysts, as host materials for impurity activated luminescent materials, and for specialized optical uses see Oxide Catalysts in Solid-state Chemistry and Section 4.4). 

A different deformed version of the scheelite structure is found for KCrOaCl. One of several structures found for molybdates and tungstates of trivalent metals is the defect scheelite structure, adopted by Eu2(W04)3 and one polymorph of Nd2(Mo04)3. In this structure M ions occupy two-thirds of the sites in the scheelite structure. 

One characteristic of the scheelite structure-type Is the number and extent of cationic oxidation states and defect (catlonr-deflclent) structures that have been found. The single guide to the formation of scheellte-type structure seems to be the ability of A cations to be eight-coordinated (I.e., rather large) agd B lons to attain tetrahedral coordination (note, however, that PO or SIO, containing scheelltes are unknown). 

The second generation Sohio catalyst was a uranium antimonate (USbsOio). This was more active and selective than the earher bismuth phos-phomolybdate and has been described as Phase I. Active sites in the layer strac-ture were also defect-Scheelite structures containing uranium-antimoity cation pairs. Catalysts containing USbOs, or Phase 2, were less selective. 

The crystal structures of this composition belong to three basically different structural families with a total of six well characterized structural types a) defect scheelite structures which can be disordered (1), or ordered of La2(Mo04)3 type (2) or Eu2(W04)3 type (3) b) /3-Gd2(Mo04)3 above (4) and below (5) its displacive phase transition and c) the- Sc2(W04)3 type (6) structure. For y-Dy2(Mo04)3 a seventh structure was tentatively identified by Brixner (1973) as... 

The main differences of the three structural families are in the oxygen coordination of the R ions which decreases from eight in the defect scheelites, over seven in j8- and 0 -Gd2(MoO4)3 to six in the Sc2(W04)3 type compounds. The Mo atom is always tetrahedrally coordinated by oxygen although octahedral coordination of Mo is known for other compositions. 

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