Intergrowth phase

The andA B jOj, phases described in the previous section, all with a stacking 

The n = 7 preparations are clearly on the borderline of stability between these two stacking types and often give disordered material with the (cc... chh) shift structure intergrown with (cc... ch) twinned stacking. It would seem that the balance between this pair of structures is very sensitive to the size and type of the B cations present. The 8H—Ba TijNb takes the (cccccchh), n=8, shift structure Ba Tl Nb  

FUq jOj adopts a mixture of (cccccchh) and (ccch) n=8, mixed twiimed and shift forms while Ba Ti Nb jjsLUoijsOj, exclusively adopts the (ccch) n=8, twinned structure. Similarly the twinned phase 8H—Ba Ga Ta g O exists with a composition (1.8 x 3.2). When the amount of Ta increases beyond this range, the =8 phase is replaced by the shift phase Ba Ta Oj.  

The packing sequence cc chhh is adopted by the oxygen-deficient phases 12H—BaCoOjgo, 12H—BaQ CoO go and 12H—BaCOggMngPj js 2 approximately BagCOgOjg y The h and c planes have approximate compositions BaOj and the c  

The packing sequence with further cubic layers (ccc cchhh), is shown by 16H—Ba CaggMnj 3 and the Cr-substituted phase 16H—Ba CaggjMn gjCrgjOj,  

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In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

Between n = 4 and 4.5, a large number of coherent intergrowth phases with long periodicities are known in this system (Portier et al, 1975). 

OTOOTOTOOT Other ordered intergrowth phases reported are Ca7Fe6TiO,8... 

Plate 13—Lattice image of MYMYe intergrowth phase in the barium hexaferrite system (from ref. 92). 

It is truly remarkable that in none of the ordered or disordered intergrowth phases do we see any evidence of vacancies or other point defects. Nonstoichiometry is however present in all systems showing disordered intergrowth of two or more phases. 

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. 

Examples of such materials have been known for a number of years in the mineralogical field, where the terms intergrowth phases , polysynthetic twinned phases , polysomatic phases and tropochemical cell twinning have been applied to describe the stmctures. In recent years, the application of electron microscopy has revealed the defect stmctnres of vast numbers of phases containing extended defects. In this section, some well-characterized examples chosen are chosen for illustration. 

Another system to be mentioned here is the pseudobinary Ga203-Ti02 system. The phase relations have been determined by Kamiya and Tilley and the crystal chemistry worked out by Bursill et alf Mixtures of Ti02 and Ga203 which are titania rich produce a series of intergrowth phases when fired at temperatures above about 1400 K. These compounds have a series formula Ga4Ti 402 -2. in which m... 

Table 4 Some intergrowth phases in the (Na,Ca) Nb 03 +2 system with n lying between 4 and 5...

This material is an ordered intergrowth of = 2 and n = 3 perovskite layers. The genera formulae of such intergrowth phases can be written as (Bi202) ... 

The first piece of information needed in such cases is to determine whether the intergrowth phase is formed from a solid-solid reaction, or whether the crystals grow from a liquid phase or from the vapour phase. Strangely enough this is not known, or... 

There are some comments to make here. First, the positions of the Sn, Pb, or Ba atoms in these phases is not known, so that for these compounds the model must be regarded as hypothetical. Secondly, it does not suit needle-shaped crystals, as the first layer laid down must already include the fault planes inherently contained within it. This point will be returned to later. Thus the alkali-metal intergrowth phases, which are needle shaped, do not fall into this class. This growth mechanism can also be applied to the barium ferrites, which are plate-like crystals that form from a molten flux, and it is possible that it could account for the sort of faulting noted in the amphibole chain minerals described on p. 135 et seq. 

There is a need for far more experimental studies on the formation and reactivity of these intergrowth phases, as to date there is a paucity of reliable information. However, the ease with which CS planes and the conduction planes in j8-alumina are able to grow or shrink suggest that they will prove to be the seat of an enhanced chemical reactivity. This aspect of these phases has been hardly studied at all to the present time and further experiments would be of considerable interest. 

With respect to these phases, Tronc, Moret, Legendre, and Huber feel strongly that they should be called polytypes. However, there seems to be a definite correlation between structure and composition and no two polytypes which have had their structures accurately refined appear to have the same composition. Hence we will refer to these compounds as intergrowth phases. 

When compounds of this type are heated, the first crystalline phase to appear in the 700-1000°C temperature range is the so-called /.-Na20 XAI2O3 phase x varies from 3 to 12) with the mullite crystal structure . This phase is the low temperature precursor to the P and P intergrowth phases, which begin to form at 1100°C . At 1200°C all the 2 phase converts to a mixture of the p and P" phases. 

Later, more extensive TEM studies11351 suggested that the SBA-2 is an intergrowth of hep structure and cep structure. This phenomenon is similar with the case of intergrowth of EMT/FAU in zeolites. The intergrowth phases such as zeolite ZSM-3 and ZSM-20 were believed to be pure hexagonal phase initially. Even so, the index of XRD pattern with Edyinmc symmetry is still the most convenient method to identify SBA-2-type materials. 

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