Section 2.3.1.1.2. Use of monochromators

Many diffractometers are equipped with a curved highly-oriented pyrolytic graphite monochromator placed after the receiving slit as shown in Fig. 2.3.1.3. Although graphite has a large mosaic spread (∼0.35 to 0.6°), the diffracted beam from the specimen is defined by the receiving slit, which determines the profile shape and width rather than the monochromator. The same results are obtained whether the monochromator is set in the parallel or antiparallel position with respect to the specimen. The most important advantage of graphite is its high reflectivity, which is about 25–50% for Cu Kα. This is much higher than LiF, Si or quartz monochromators that have been used for powder diffraction. The Kβ filter and the parallel slits in the diffracted beam can be eliminated and, since each reduces the Kα intensity by about a factor of two, the use of a graphite monochromator actually increases the available intensity. The diffracted-beam monochromator eliminates specimen fluorescence and the scattered background whose wavelengths are different from that of the monochromator setting. For example, a Cu tube can be used for specimens containing Co, Fe, or other elements with absorption edges at longer wavelengths than Cu Kα to produce patterns with low background. Several monochromator geometries are described by Lang (1956).

A specimen in the reflection mode may be used with an incident-beam monochromator and θ–2θ scanning as shown in Fig. 2.3.1.1(c). One of the principal advantages is that it is possible to adjust the monochromator and slits to remove the Kα₂ component and produce patterns with only Kα₁ peaks. The profile symmetry, resolution and instrument function are thus greatly improved; see, for example, Warren (1969), Wölfel (1981), Göbel (1982) and Louër & Langford (1988). The high-quality crystal required causes a large loss of intensity and reduces specimen fluorescence but does not eliminate it. However, Soller slits in the incident beam and a β filter are no longer required and the net loss of intensity can be as low as 20%. Such monochromators can now be provided as standard by diffractometer manufacturers and their use is increasing, but they are not as widely used as the diffracted-beam monochromator.