Dynamical properties and instability of local fluorite BaF2 structure around doped Mn2+ ions—EPR and electron spin echo studies

The electron paramagnetic resonance (EPR) and electron spin echo (ESE) were measured at the X-band for Mn2+ in a BaF2 crystal in the temperature range 4.2–300 K. In addition to the cubic symmetry centre, two other lower concentration tetragonal centres were identified. Temperature variations and computer simulation of the EPR spectrum confirm that the cubic symmetry of the MnF8 centre is deformed to two Td tetrahedra of different dimensions at around 45 K. Electron spin relaxation was measured in the temperature range 4.2–35 K, where the ESE signal was detectable. For higher temperature the Mn2+ dynamics produces homogeneously broadened EPR lines. At the lowest temperatures the spin–lattice relaxation is governed by ordinary phonon processes with 1/T1∼T5. The efficiency of these processes rapidly decreases and at about 11 K a local mode of energy 17 cm−1 becomes the relaxation mechanism. Phase relaxation observed as ESE signal dephasing indicates that after the local deformation jumps (tunnelling with frequency 4 × 108 s−1) between the two tetrahedral configurations appear, with the energy barrier being the local mode energy. This motion is directly visible as a resonance-type enhancement of the ESE dephasing rate 1/TM around 11 K. Only the cubic centre displays the above dynamics.