Última modificación: 15-06-2009
Resumen
Inelastic neutron scattering (INS) and X-ray inelastic scattering (IXS) experiments are two of the main sources of information on the lattice dynamics of solids. This type of experiments (particularly with neutrons) have been used since many decades for the determination of the phonon branches of crystals. However, only in the late nineties the phonon symmetry-extinction rules present in these experiments were demonstrated and formulated in a general rigorous form [Perez-Mato, J. M., Aroyo, M. I., Hlinka, J., Quilichini, M. & Currat, R. (1998). Phys. Rev. Lett. 81, 2462.]. These phonon absences depend only on the mode symmetry and the symmetry of the scattering vector, and not on the specific atomic positions of the compounds.
Both the design and interpretation of INS and IXS experiments can be improved with the knowledge of the phonon extinction rules applicable to each specific case. Previous to the actual measurements they can help to optimize the design of the experiment, while afterwards, they can be a powerful tool for the identification of the symmetries of the measured phonons. They can also be employed to interpret symmetrical patterns of diffuse scattering that are often observed in materials with static or dynamic disorder, caused by specific unstable modes.[Aroyo, M. I., Boysen, H. & Perez-Mato, J. M. (2002a). Physica B 316-317, 154; Appl. Phys. A 74, S1043.]
NEUTRON [A. Kirov, M.I. Aroyo, J.M.. P,erez-Mato, J. Appl. Cryst. (2003) 36, 1085.] is a computer program in the Bilbao Crystallographic Server that calculates the phonon extinction rules for inelastic neutron scattering experiments for any specified case. Given the space group of the system and the wave vector q of the phonon, the program examines the INS/IXS activity of the q phonons, depending on their symmetry (their irreducible representation), for all different types of scattering vectors or Brillouin zones.
In this lecture, after a brief presentation of the basic features of a phonon inelastic scattering experiment, some examples of the application of NEUTRON will be presented.