Gallium lattice structure

The physical and chemical properties of the tetrahydroborates show more contrasts than the salts of nearly any other anion. The alkali metal salts are the most stable. In dry air, NaBH4 is stable at 300°C and in vacuo to 400°C with only partial decomposition. In contrast, several tetrahydroborates, including the titanium, thallium, gallium, copper, and silver salts, are unstable at or slightly above ambient temperatures. The chemical and physical properties of the tetrahydroborates are closely related to molecular structure. Sodium tetrahydroborate, which is typical of the alkali metal tetrahydroborates except for the lithium salt, has a face-centered cubic (fee) crystal lattice which is essentially ionic and contains the tetrahedral [BHJ- anion. The tetrahydroborates of the polyvalent metals are in many cases the most volatile derivatives of these metals known. Aluminum tris(tetrahydroborate)... 

A] bond distances. The restricted bite distances of the xanthate ligands [the S—M—S chelate angles are 73.50(6) and 69.77(4)°, respectively] are most the likely source for the deviations from the ideal octahedral geometries. The distortions are best seen in the twists of the triangular faces, that is, 40.27(7) and 46.43(5)° for the gallium and indium structures, respectively, compared to the ideal octahedral angle of 60°. Enantiomeric pairs associate in their respective crystal lattices via S- -S contacts of 3.598(4) and 3.592(2) A, respectively. 

Source of Activity in other Siliceous Catalysts.—Although various oxides can be combined with silica to give amorphous, acidic catalysts, the replacement of aluminium in zeolites (specially non-faujasitic zeolites) has proved to be very difficult with any element other than gallium. Materials of ZSM-5 structure with iron or boron in place of aluminium have been claimed recently, but it is not yet certain that either iron or boron is part of the zeolite lattice or that the catalytic activity observed is not due to residual lattice aluminium. 

During the course of crystallization, but especially in the subsequent recrystallization process the physico-chemical properties of the samples vary to a considerable extent with respect of the nature of the gallium, the crystal size and shape, and the perfection of the lattice, although the overall composition of the Ga-ZSM 5 samples as well as their structure remains essentially unchanged. The manufacturing of catalysts with desired properties on the basis of Ga-ZSM 5 has to take into account these circumstances. 

Of the group 13 metals, only A1 reacts directly with N2 (at 1020K) to form a nitride AIN has a wurtzite lattice and is hydrolysed to NH3 by hot dilute alkali. Gallium and indium nitrides also crystallize with the wurtzite structure, and are more reactive than their B or A1 counterparts. The importance of the group 13 metal nitrides, and of the related... 

Electron spin resonance reveals the unpaired electrons associated with impurities or structural defects and can be used to identify the lattice site positions of these features. Nitrogen is shown to substitute for carbon and acts as a shallow donor. The various ESR triplets due to nitrogen in several SiC polytypes give information on the lattice sites occupied. For the acceptor boron, ESR shows it to occupy Si sites only, in disagreement with DAP photoluminescence measurements which show only boron on carbon sites. It may be that boron substitutes on both sites and the two techniques have sensitivity for only one particular lattice site. The aluminium acceptor is not observed in ESR but gallium has been noted in one report. Transition metals, Ti and V, have been identified by ESR both isolated on Si sites and in Ti-N complexes. Several charged vacancy defects have been assigned from ESR spectra in irradiated samples. 

Besides structural variety, ehemieal diversity has also increased. Pure silicon forms of zeolite ZSM-5 and ZSM-11, designated silicalite-1 [19] and siliealite-2 [20]. have been synthesised. A number of other pure silicon analogues of zeolites, ealled porosils, are known [21]. Various chemical elements other than silicon or aluminium have been ineorporated into zeolite lattiee struetures [22, 23]. Most important among those from an applications point of view are the ineorporation of titanium, eobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reaetions. In some cases it remains questionable, however, whether incorporation into the zeolite lattice strueture has really occurred. 

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