Cubic-hexagonal intergrowth

A second example of the intergrowth structure is tungsten bronze, M,. WO3 (M = H, Li, Na, K, Rb, Cs, Ca, Sr, Ba, In, Tl, Ge, Sn, Pb, Cu, Ag etc. nowadays many bronzes such as Ti, V, Mo are well known ), so named because its colour is similar to that of alloy bronze CuZn. It has been shown to have a kind of intergrowth structure in a limited composition (x) range. Generally, the stability of the structural types (hexagonal, tetragonal, and cubic) depends both on the ionic radius (rj and the composition (x) of M. For instance,... 

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

The occurrence of intergrowths of zeolite Y and ZSM-20, the cubic and hexagonal forms, is analogous to similar intergrowths in SiC and ZnS crystals. Intergrowths in zeolite Y and ZSM-20 do not block channels, as is the case in the erionite-offretite family, where rotation of cancrinite layers blocks the 12MR channels, but are more like intergrowths in the ZSM-5/ZSM-11 family, which modify the channel system. 

Ca2Fe205 and oxides of the CaMn03 v family are good examples of such vacancy-ordered structures. Complex intergrowth phases (ordered as well as disordered) involving brownmillerite and other related phases are commonly found in some of the anion-deficient oxides. Some of them also show polytypism due to different modes of stacking of the hexagonal and cubic layers. 

Third, the notion (and reality) of such structural infractions as twins and coherent intergrowths - as is seen by Yacaman et al. in a 923-atom nanoalloy of AuPd [35] - is meaningless in our molecular bimetallic nanoparticles. In the nanoaUoys of Yacaman et al. [35, 42] and others [43], one may discern directly, by aberration-corrected electronic microscopy, thin bands of hexagonal close-packed and face-centered cubic packed sheets. In a typical molecular nanoparticle of the kind that we have studied (also by aberration-corrected electron microscopy [39]), it is directly established (in line with theoretical predictions [44]) that a single bimetallic cluster of RUj Pt does indeed possess molecular character. Furthermore, when six or more such clusters coalesce into larger entities containing ca 200 atoms they adopt the regular crystalline, and faceted state of a bulk metal. 

SBA-2, intergrowth of hexagonal and Fm3n cubic divalent cationic template... 

Siliceous zeolite Y was also synthesised directly in both cubic and hexagonal forms (2) using the tenplated aqueous fluoride system and the intergrowth ZSM-20 (10) was also synthesised. SAPO-37 was synthesised using published procedures (3). 

Figure 9. Powder X-ray dif action data for four zeolites with, respectively, the cubic FAU-ftamework (a), the EMT-framewoik (a hexagonal variant - (d)), and with varying degrees of intergrowth between the two end member structure types (b and c).

According to TEM data, perovskites prepared via Pechini route consist of crystalline nano-sized (10-50 nm) particles (Figures 10-12). There are a lot of structural defects and domain boundaries in such samples. This resembles the domain structure of disordered complex Sr-Ba-Fe-Co-(Zr) -0 perovskites prepared via MA route (Fig. 13) [60]. For all these samples but those containing Zr, a cubic structure was observed, while particles were comprised of disorderly stacked microdomains. The BaFeo4Coo4Zro20y sample particles were comprised of the coherent intergrowth of domains with hexagonal BaFeOs-x [JCPDS 23-1024] and monoclinic Ba2Fc205 [JCPDS 43-0256] structmes. 

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