High-pressure low-temperature phase transition in a dopedpara-terphenyl crystal: A spectral-hole-burning study

The homogeneous width of zero-phonon lines in a chlorin-doped para-terphenyl crystal is determined by spectral hole burning at T=4. 2--14K under various hydrostatic pressures up to 7 kbar. A drastic broadening of spectral holes with pressure occurs on approaching the order-disorder (triclinic-to-monoclinic) phase transition which takes place under P5. 5kbar at 4. 2<~T<~14K. The thermal broadening of spectral holes at fixed pressures is an activated process with the activation energy depending on pressure and having a deep minimum at P5. 5kbar, i. e. , in the phase-transition region.