Anomalous reconstructions of the (001) surfaces of AgBr and NaCl

He-atom-diffraction indicates unexpected reconstructions of the (001) surfaces of AgBr and NaCl. For AgBr, reconstruction occurs only after initial thermal activation at 430 K, while for NaCl it occurred spontaneously only once in many cleavages. Both AgBr and NaCl then undergo a reversible transition to a (2 x 2) phase at temperatures below 260 and 450 K, respectively. Above these temperatures the hopping of lattice defects, which are probably responsible for the transition to the (1 x 1) phase, sets in. {[}S0031-9007(99)09271-6].