Kerr effect and birefringence from paraelectric CN − defects in nine alkali halides

Substitutional CN − defects in the sodium‐, potassium‐ and rubidium‐chlorides, ‐bromides, and iodides are investigated at low temperatures (2 to 8 K) under electric fields E of different symmetry for their field‐induced birefringence Δ n ( E ) (Kerr‐effect). In all nine crystal systems, a para‐electric Kerr‐effect is found with a pronounced anisotropy giving evidence for 〈111〉 CN − symmetry in the six potassium‐ and rubidium‐halides, and 〈100〉 symmetry in the three sodium‐halides. Under increase of the CN − concentration, anisotropic birefringence effects appear (without field) under cooling of the crystals, indicating low temperature ordering of interacting CN − dipole clusters.