Superlattice reflections

Fig. 33. Selected area diffraction patterns of Ti-Al alloys taken on the [001] zone axis which show the increasing relative intensity of the superlattice reflections with increasing Ti content (a) 3 a/o Ti, (b) 5 a/o Ti, (c) 16 a/o Ti, and (d) 24 a/o Ti. A labeled schematic of the diffraction pattern is shown [188],...

All of the I/S produced by WD are randomly interstratified, with the possible exception of the Black Jack sample (Figure 3) and the most illitic Kinney sample (Figure 4), both of which show signs of partial R1 ordering between illite and smectite layers. For these samples, the 001 XRD reflections are displaced towards larger angles, and a very weak superlattice reflection is visible at small angles. 

It has been suggested that powder XRD patterns of some mineral samples of LDH minerals show evidence of superlattice reflections [7,113] but there is no clear consensus [100]. It should also be borne is mind that, as discussed in Sect. 3.5, superlattice reflections may be due to anion, rather than cation, ordering although the latter may be an indication of the former. 

Fig. 5 Diffraction profiles collected from the same powdered sample of InSb at 2 GPa, using (top) energy- and (bottom) angle-dispersive diffraction. The angle-dispersive data clearly have higher angular resolution, and are not contaminated by X-ray fluorescence peaks. The tick marks below the angle-dispersive data mark the positions of some of the weak superlattice reflections that were essential to determining the structure of the InSb-IV phase [165]...

Zone 111 is defined by the presence of an ordered mixed layered dioctahedral mineral which has an obvious superlattice reflection. Mixed layered proportions vary from 50% to 25% expandable material. The mixed layered phase is called here "allevardite-Iike". Indications from studies on deeply buried and shallow rocks suggest that as pressure increases, the mixed layer superlattice reflection appears at lower temperature. 

The crystal symmetry changes that accompany order-disorder transitions, discussed in Section 17.1.2, give rise to diffraction phenomena that allow the transitions to be studied quantitatively. In particular, the loss of symmetry is accompanied by the appearance of additional Bragg peaks, called superlattice reflections, and their intensities can be used to measure the evolution of order parameters. 

A large number of partially oxidized divalent cation salts of the bis(oxalato)platinates have been reported (see Table 2).63 Only the series Mx[Pt(C204)]-6H20 (MOP where M = Fe, Co, Ni, Zn, Mg and 0.80 < x < 0.85) has been extensively studied.68 83 For most of these salts detailed studies have been made of their crystal structures and optical reflectivity at room temperature, and the variation of superlattice reflections, diffuse X-ray scattering, thermopower and DC electrical conductivity with temperature. 

The Na ZK-4 and Y zeolites have Si to A1 ratios of 1.65 and 2.61, respectively, but although the ZK-4 zeolite is a variant of zeolite A, powder neutron diffraction data61 showed that no superlattice reflection could be found at the angle where it occurs in the T1 zeolite A.59 60 The small unit cell implied that in this zeolite the Si and A1 atoms were no longer preferentially located on alternate tetrahedral sites. A similar result was found for the Y zeolite. 

Transmitted Beam Fundamental Reflection Superlattice Reflection... 

Single-crystal X-ray results (9) point to strict alternation of Si and Al (the 4 0 ordering scheme) in accordance with Loewenstein s rules (10), but this model was recently challenged on the basis of 29si NMR measurements (1, 2) and our discovery that the sodium derivative can be rhombohedral (11,12). However, the controversy has now been resolved and the correctness of the X-ray model reaffirmed (13,14). A consequence of the Si, Al ordering is that the lattice parameter of the cubic cell is ca. 24.6 A, not 12.3 A as reported in earlier work, a feature reflected in weak superlattice reflections. The space group is Fm3c. 

The 531 superlattice reflection on the low angle side of the 600 sub-structure reflection in T1 zeolite-A (observed and calculated models, as in Figure 2). 

Of course, this may not be true when the compound contains heavy atoms [e.g., (TMTSF)2Re04 undergoes an AO phase transition leading to relatively strong superlattice reflections]. In fact, Re is a very strong x-ray scatterer compared to other atoms in the material [121,122]. 

Exposure times up to 30 s with high-speed film plates were necessary to observe very weak superlattice reflections. 

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