Lecture 7 Lattice Defects, Vacancies PHYS 430/603 material Laszlo Takacs UMBC Department of Physics

Defects : for some properties as important as the atomic structure, affect most mechanical and physical properties Point defects (0-d) - vacancies, impurities diffusion; transport properties, plastic deformation, color Line defects (1-d) - dislocations plastic deformation; diffusion, magnetic hardness Surface defects (2-d) - surfaces, grain- and phase-boundaries plasticity, transport, physical properties Volume defects (3-d) - voids, second phases density, permeability, strength, physical properties

The simplest possible point defect: Vacancy Vacancies must exist due to thermodynamic constraints. It is more complicated than this: The neighboring atoms move closer to partially fill in the void; the electron wave functions change for the atoms nearby; the lattice vibrations will be influenced.

Point defects with increasing complexity Vacancy: single atom missing, environmental relaxes. Interstitial atom: an extra atom at an interstitial site; often two atoms forced into the place of one (dumbbell). Frankel defect: an interstitial atom combined with a vacancy. In ionic crystals, charge neutrality requires bi-vacancies or Frankel defects; single vacancy is not allowed. Impurities: larger or smaller than the matrix atoms. Impurities in ionic crystals: can have identical or different charge, responsible for color.

Point defects VacancyInterstitial imp.Substitutional imp. Substitutional imp.Frankel defectSchottky defect

The principle of positron annihilation If the positron is trapped by the negative effective charge at a vacancy, its lifetime increases due to the lower electron density at the vacancy. The typical positron lifetimes are a few 100 ps. (The angular correlation and Doppler broadening relate to the motion of the electrons.)

Typical fast-slow electronics for positron lifetime measurement

Annealing out of neutron irradiation-caused vacancies as reflected by positron lifetime measurements. Dlubek et al., Appl. Phys. A 42 (1987) 125. Radiation damage by neutrons (and other energetic particles) is an important subject in the field of engineering materials for nuclear reactors and space- based equipment