Scheelites structure

One proposed material is BiV04 in its monoclinic (scheelite structure) polymorph, which shows a good photocatalytic activity for 02 evolution in the presence of Ag+ (9% quantum yield at 450 nm) [125]. The band gap of this vanadate... 

M2[Mo04]-Np[Mo04]2 1 A scheelite structure system A scheelite structure... 

With 4o/44Ca- and 92/ioojio- data the differentiation of the translational and librational modes was obtained for the molybdates and tungstates with scheelite structure 98, 100, 101). Tables 29 and 30 reproduce these results. In contrast to early findings, the lower frequency bands were found to be librations and not translations. Furthermore, the band associated with Vi for CaMo04 and CaW04 cannot be expressed as a pure deformational vibration of the MOl group, but this mode is coupled with the translational vibrations, as indicated in Table 29. The relationship between the translational vibration, E, and the square root of the mass of the cation for compounds of the type AMO 4 (A = Ca, Sr, Ba, Pb M = Mo, W) was determined to be linear (100). 

A single crystal Raman spectrum of NaRe04 recorded at low temperatures has recently been published by Johnson et al. (200). The measured phonons of this scheelite structure and their assSinment are listed in Table 35. 

Scheelit-structure In these compounds CaZnp4 and SrZnF4 zinc exhibits tetrahedral coordination, never observed before in other compounds of this element containing fluorine. In this context it is interesting to note ESR evidence of tetrahedral MnF4 in substituted Scheelit structures 31). 

One typical way to improve the catalyst system was directed at the multi-component bismuth molybdate catalyst having scheelite structure (85), where metal cations other than molybdenum and bismuth usually have ionic radii larger than 0.9 A. It is important that the a-phase of bismuth molybdate has a distorted scheelite structure. Thus, metal molybdates of third and fourth metal elements having scheelite structure easily form mixed-metal scheelite crystals or solid solution with the a-phase of bismuth molybdates. Thus, the catalyst structure of the scheelite-type multicomponent bismuth molybdate is rather simple and composed of a single phase or double phases including many lattice vacancies. On the other hand, another type of multi-component bismuth molybdate is composed mainly of the metal cation additives having ionic radii smaller than 0.8 A. Different from the scheelite-type multicomponent bismuth molybdates, the latter catalyst system is never composed of a simple phase but is made up of many kinds of different crys-... 

Multicomponent Bismuth Molybdate Catalysts with Scheelite Structure... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

Bismuth molybdate catalysts activated by the metal cations with ionic radii smaller than 0.8 A (Ni2+, Co2+, Fe2+, Mg2+, and/or Mn2+ with Fe3+) are never composed of a single phase, such as scheelite structure, and many kinds of metal molybdate, including various phases of bismuth molybdate,... 

When the added metal cation, M(II) and/or M(III), has an ionic radius larger than 0.9 A and forms a molybdate with a scheelite structure, the catalyst system shows a performance quite different than that mentioned above for the scheelite-type catalysts. 

Bi2Mo3Oi2. The proposed model seems to be applicable for all kinds of multicomponent bismuth molybdate catalysts except those having a scheelite structure thus far. 

Improved catalytic performance, selectivity and resistance to fusion, over bismuth molybdate catalysts was reported by McClellan (90) for catalysts obtained by chemically combining bismuth, molybdenum, phosphorus, and silica. After calcination at 450°C, the bismuth phosphomolybdate-on-silica catalyst showed an X-ray pattern of mainly crystalline Bi2(Mo04)3 which subsequently was converted to a new, substantially amorphous, phase after calcination at 800°C. Substantially morphous meant that the X-ray diffraction lines were broad diffuse bands of low intensity. The pattern of lines for this novel phase indicated a scheelite structure. A special interaction of silica with bismuth molybdate was also suggested by Callahan et al. (91). 

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. 

Alkhazov et al. (99-101) compared the physicochemical properties and catalytic activities of Group II element molybdates. The calcium, strontium, cadmium, and barium molybdates have a scheelite structure... 

LoJacono et al. (108) also utilized X-ray diffraction methods to study the structural and phase transformations which occurred in the Bi-Fe-Mo oxide system. They detected two ternary compounds containing bismuth, molybdenum, and iron. One of the compounds formed when the atomic ratio Bi/Fe/Mo = 1 1 1 the other formed when the atomic ratio Bi/Fe/Mo = 3 1 2. The X-ray data indicated a close structural relationship of the bismuth iron molybdate compounds with the scheelite structure of a-phase bismuth molybdate. Moreover, their structures were similar to compound X. The structure of the Bi/Fe/Mo = 3 1 2 compound was identical to the compound reported by Sleight and Jeitschko (107). The authors proposed that the structures of both of the compounds could be viewed as resulting from the substitution of Fe3+ in the a-phase lattice. In the Bi/Fe/Mo = 1 1 1 compound, 1 Mo6+ ion is replaced by 2 Fe3+ ions one Fe3+ ion occupies a Mo6+ site the other Fe3+ ion occupies one of the vacant bismuth sites. In the Bi/Fe/Mo = 3 1 2 compound, the Fe3+ ion replaces one Mo6+ ion while the additional Bi3+ ion occupies one of the vacant bismuth sites. 

The molybdenum-oxygen bond strength has been shown to weaken in direct proportion to an increased ionization potential of the A cations in several scheelite structured molybdates. Thus, it would be reasonable to assume that the nature of the A cation will have a profound effect on the molybdenum-oxygen bond strength which, in turn, will influence the activity and the selectivity of the catalysts. 

The infra-red spectra of some Scheelite structures CaW04, CaMo04, PbW04 and PbMo04 were recorded and discussed by Khanna and Lippincott88). 

Because the mineral scheelite and the related pyrochlore (Section 6.3.9) are important laser host materials for rare earth metal ions, many such compounds and their doped crystals have been investigated intensively. Compounds with the scheelite structure are given in Table 6.8. 

AmGe04 with scheelite structure (a-form of the actinide (IV) germa-nates). Pure AmGe04 is obtained again by hydrothermal reaction of Am (OH) 4 -j- Ge02(aq) under the same experimental conditions as quoted for AmSi04. The thermal decomposition of AmGe04 starts at about 1050°G. 

Li4Np(W04)4 and pointed out their similarity to the wolframite (FeW04) structure type which in turn can be considered as a distorted scheelite structure. 

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