A review is given on structural studies under abnormal conditions such as low and high temperatures, electric and magnetic fields, high pressure and so on. The advantage of energy-dispersive diffraction technique is discussed. Use of SOR is then shown to be very effective in these studies not only from its high intensity.
The relative accuracy of lattice constants determined with the energy-dispersive method is expected to be 10^-4 at best; this is much poorer than that obtained with the established angular-dispersive methods. Among these methods, the Bond method and the double-crystal diffraction method are examined in detail. The latter is superior to the former provided that incident X-ray intensities are much higher than those available at present. The lattice constant measurements with the combination of the double-crystal method and SOR will realize the accuracy of 10^-7. Such measurements are important for the study of structural phase transitions and of influence of radiation damages on lattice constants; the latter will be a common problem in using SOR.