Defect structures, scheelite structure-type

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). 

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. 

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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