Paraelectric KH$_2$PO$_4$ Nanocrystals in Monolithic Mesoporous Silica: Structure and Lattice Dynamics

Combining dielectric crystals with mesoporous solids allows a versatile design of functional nanomaterials, where the porous host provides a mechanical rigid scaffold structure and the molecular filling adds the functionalization. Here, we report a study of the complex lattice dynamics of a SiO$_2$:KH$_2$PO$_4$ nanocomposite consisting of a monolithic, mesoporous silica glass host with KH$_2$PO$_4$ nanocrystals embedded in its tubular channels $\sim$12 nm across. A micro-Raman investigation performed in the spectral range of 70-1600 cm$^{-1}$ reveals the complex lattice dynamics of the confined crystals. Their Raman spectrum resembles the one taken from bulk KH$_2$PO$_4$ crystals and thus, along with X-ray diffraction experiments, corroborates the successful solution-based synthesis of KH$_2$PO$_4$ nanocrystals with a structure analogous to the bulk material. We succeeded in observing not only the high-frequency internal modes ($\sim$900-1200 cm$^{-1}$), typical of internal vibrations of the PO$_4$ tetrahedra, but, more importantly, also the lowest frequency modes typical of bulk KH$_2$PO$_4$ crystals. The experimental Raman spectrum was interpreted with a group theory analysis and first-principle lattice dynamics calculations. The analysis of calculated eigen-vectors indicates the involvement of hydrogen atoms in most phonon modes corroborating the substantial significance of the hydrogen subsystem in the lattice dynamics of paraelectric bulk and of KH$_2$PO$_4$ crystals in extreme spatial confinement. A marginal redistribution of relative Raman intensities of the confined compared to unconfined crystals presumably originates in slightly changed crystal fields and interatomic interactions, in particular for the parts of the nanocrystals in close proximity to the silica pore surfaces.