High pressure raman study of Y-124

In this work, a micro-Raman study under high hydrostatic pressure (up to ～5.5 GPa) has been carried on YBa₂Cu₄O₈ and Y(Ba, Sr)₂Cu₄O₈ single crystals at room temperature. In both samples, seven strong modes, of A_g symmetry, and one weak, of B_3g symmetry, have been observed and examined in connection with previously published results concerning YBa₂Cu₄O₈. With the Sr substitution for Ba, the ambient pressure measurements show an upward shift in energy for all modes, except those that involve vibrations of the plane and apex oxygen atoms. With increasing hydrostatic pressure all phonons shift to higher energies. Anomalous nonlinear pressure behaviour has been observed for three phonons, which is correlated with the pressure dependence of T _c of these compounds.