Cell parameters of silicon and germanium

4 CELL PARAMETERS OF SILICON AND GERMANIUM MONOCHROMATOR CRYSTALS 

These are the monochromator crystal materials that are used extensively for macromolecular crystallography at SR sources. The wavelength of the reflected beam from the monochromator can be calculated from the specific lattice plane spacing and the angle of reflection. 

Silicon and germanium both crystallise in the cubic diamond structure. Their absolute lattice parameters have been measured to an accuracy level of =S1 ppm, i.e. at 22.5°C they are (M. Hart, pers. comm., based on Becker et al (1981), Baker and Hart (1975))  

These values are obviously sensitive to temperature an increase in temperature from 22.5 °C to 25 °C increases these values by a factor of 1.000011. 

In routine data collection for macromolecular crystallography we are interested in knowing the wavelength to =0.001 A, whereas in current anomalous scattering experiments an accuracy of 0.0001 A is required. Hence, the values of 2d for the (111) and (220) planes of silicon and germanium are given in table A3.4 to four decimal places calculated 

---