INVESTIGATION OF THE PHASE-TRANSITION IN LEAD FLUORIDE BY PROTON CHANNELING

Axial-channeling measurements as a function of temperature have been carried out on single crystals of lead fluoride, PbF2. The original stimulus came from the term sublattice melting, and with it the expectation that channeling by fluorine would disppear at appreciably lower temperatures than for Pb. Angular scans centered around, respectively, the <100>, <110>, and <111> axes were measured for temperatures ranging from 280 to 800 K. A multistring continuum model was developed in an effort to simulate the data, including the effect of displaced fluorine atoms, and, based on the model, the data indicate that the displaced fluorine atoms are neither uniformly distributed nor on octahedral sites. At 800 K, a sharp fluorine channeling minimum disappears first along the <100> axis, but the measurements do not support the melting concept. The effect of the incident beam on the measurements is considered.