High-pressure structural change in the ferroelectric layered perovskite Sr2Nb2O7

The structural changes of sintered powder samples of perovskite-slab layered polycrystalline ${\mathrm{Sr}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ (SNO) ferroelectric compound subjected to high pressures are here investigated. The samples were prepared using a solid-state reaction in the presence (SNOE) or absence (SNO) of an applied electric field. Density functional theory (DFT) calculations including hydrostatic pressure indicate that SNO's ferroelectricity remains up to 25 GPa in the structure of space group $Cmc{2}_{1}$ derived from the condensation of one zone-center soft phonon. The predicted DFT theoretical structural changes are discussed and compared with the results of the experimental Raman spectra as a function of pressure. The pressure-dependent spectra were recorded from atmospheric pressure up to 11.5 and 13.5 GPa for SNOE and SNO, respectively. Both samples underwent a pressure induced phase transition from an incommensurate to a commensurate state at room temperature, SNO at ${P}_{i\ensuremath{-}c}=6.5\ifmmode\pm\else\textpm\fi{}0.2$ GPa, and SNOE at ${P}_{i\ensuremath{-}c}=6.9\ifmmode\pm\else\textpm\fi{}0.3$ GPa. The DFT calculations enable the identification of the change in phase to the orthorhombic structure with the space group $Cmc{2}_{1}$. The experimental values for ${P}_{i\ensuremath{-}c}$ are in reasonably good agreement with the theoretical predicted value of \ensuremath{\sim}7.3 GPa. After the critical pressures, the number of observable phonons decreases, that is, when the compound adopts a higher symmetry structure, several phonons vanish abruptly in both the SNO and SNOE samples, as expected. The Raman spectra for both samples show hysteresis effects, that is, after the pressure is removed, a few extra lines remain visible, as well as many relative intensity changes and broadenings for some phonon bands. The bulk moduli of the $\mathrm{S}{\mathrm{r}}_{2}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{7}$ before and after ${P}_{i\ensuremath{-}c}$ are calculated resulting in 117.0 and 147.8 GPa, respectively. The Gr\"uneisen parameters of the phonons observed are finally calculated and discussed.