Non-linear lattice response of Sm oxypnictides to hydrostatic pressure

• Evidence for pressure-induced lattice modifications in 1111 pnictides.
• Agreement between Raman and synchrotron XRD measurements.
• Αnisotropic compression of unit cell and the superconducting FeAs4 tetrahedra.
• Effect absent in the parent non-superconducting compound.
• Effect not related with the F substitution being present in Sm(Fe1−xCox)AsO.

Hydrostatic pressure Raman measurements at room temperature have been carried out on the SmFeAsO (Sm1111) series of oxypnictides with various substitutions (F for O and Co for Fe) and transition temperature in order to investigate lattice modifications and their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with further increasing of the carrier concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs4 tetrahedra. The hydrostatic pressure Raman measurements indicate that the A1g mode of the rare earth atom for the superconducting compounds deviates from the linear pressure dependence at the same pressures where the XRD results show pressure-induced lattice anomalies. A similar anomaly is found for the As phonon of the same symmetry. As in cuprates, the effect is diminished in the non-superconducting compounds and it is not related with the F substitution being present in the Sm(Fe1−xCox)AsO as well. The calculated Grüneisen parameters indicate a more anharmonic phonon for the Fe atom compared with the Sm and As atoms.