We measured the pressure dependence of the room-temperature infrared absorption spectrum of the pseudocubic ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ manganite. On increasing pressure up to 10 Gpa we observe the increase of a broad band associated with polaronic absorption, which partially screens the high-frequency phonon at high pressures. We also measured absorption spectra of the same sample as a function of temperature, and of the layered ${\mathrm{Sr}}_{1.5}{\mathrm{La}}_{0.5}{\mathrm{MnO}}_4$ manganite as a function of pressure. The comparison between different sets of spectra clearly shows that the increase of the broad-band absorption observed in the ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ manganite is due to a pressure-induced charge delocalization process. However, the metallic state is not achieved at room temperature, at least in the explored pressure range.

• Received 14 December 2000

DOI:https://doi.org/10.1103/PhysRevB.63.184410