Diffraction diagrams, composite

Fig. 2. (a) Ray diagram in the electron microscope under imaging (microscopy) conditions. E electron source C condenser lens S sample O objective lens bfp back focal plane of O I intermediate lens P projector lens, (b) Structural imaging, diffraction and compositional functionalities of TEM. 

Figure 5. Composite diffraction diagrams of (a) the 11, form of poly(A) poly(U) (left,) and the 10, form of poly(C) pG (right,) and (b) the 11, form of 2 poly(U) A (left,) and the screw disordered 11, form of 2 poly(U) poly(A) (right,)...

Further substitution of niobium results in exceedingly complex structures, and a micrograph of a typical crystal of the niobium end-member of the series, BiuNbzOn, is illustrated in Fig. 11. The unit cell of this material is extremely large, approximately 115 x 80 X 5 5 A, and the x-ray powder diffraction diagram is impossible to interpret. From the micrograph shown, it would appear that the structure is based upon different principles from the one described above, but it can nevertheless be derived from it by repeated overlap of layers on (112) and (113) planes in a very complex sequence. Why such a complex sequence should be employed, and whether either of these phases are true "phases", or merely certain compositions in a quasi-continuous solid solution series, is not yet certain. What is however, demonstrated, is the remarkable ability which these simple layered structures show to variations in stoichiometry. 

Every crystalline phase in a sample has a unique powder diffraction pattern determined from the unit cell dimensions and the atomic arrangement within the unit cell. It can be considered a fingerprint of the material. Thus, powder diffraction can be used for phase identification by comparing measured data with diffraction diagrams from known phases. The most efficient computer searchable crystallographic database is the PDF-4 from the International Centre for Diffraction Data (ICDD) [3]. It is used by very efficient computer-based search-processes. In 2007 the PDF-4-i- database contains information about Bragg-positions and X-ray intensities for more than 450000 compounds, out of which there are about 107 500 data sets with atomic coordinates. New entries are added every year. The positions of the peaks in the measured pattern have to be determined. This can be done manually, but effective, fast and reliable automatic peak search methods have been developed. The method can obviously be successful only if the phases in the sample are included in the database. However, the database can also help to determine unknown phases if X-ray data exist for another isostructural compound albeit with a different composition. 

The formation enthalpy AHf of amorphous alloys is less negative than that of crystalline materials of similar composition, which means that the former alloys are metastable. As a function of temperature and time the amorphous alloys will therefore transform into the stable crystalline phases. This transformation can conveniently be studied by means of diffraction methods. As will be discussed later on, no sharp diffraction lines occur in the diffraction diagrams of amorphous alloys. The transformation into the crystalline state is generally accompanied by the occurrence of sharp diffraction peaks. In some cases the stable crystalline phases are not reached directly. First one or more metastable crystalline phases may be formed which transform into the stable end products at a later stage of the crystalUzation process. 

The phase diagram of the Li-Au system reveals a great deal of complexity, with separate intermetallic phases being formed based on thermal analysis supported by x-ray diffraction at specific compositions. Annealing, sometimes over long time periods, has been undertaken in some cases. 

Neutron diffraction studies under pressure [84] on the 70/30 composition have revealed that transitions in this copolymer are displaced towards higher temperature with increasing pressure, as can be seen in the phase diagram of Fig. 11. In addition, it is worth noting the non-linear increase of the Curie temperature with pressure. By considering the Clausius-Clapeyron relation dTc/dP = TCAVC/Ahc, this effect can be related to a decrease in the volume... 

These considerations lead us to propose the tentative phase diagram of fig. 28. The orthorhombic-rhombohedral transition has been tracked by Wold and Arnott (1959). The two-phase region 0.05 < 5 < 0.10 observed by room-temperature X-ray diffraction has been shown from the spin-glass behavior below 7 to extend over a wider compositional range below 200 K, which indicates that a spinodal phase segregation separates the O -orthorhombic (c/a < x/2) from an O -orthorhombic (c/a J2) or R-rhombohedral phase. With neutron powder diffraction, Huang et al. (1997) have shown the two-phase region at 300 Kin the inter-... 

---