Paraelectric KH2PO4 Nanocrystals in Monolithic Mesoporous Silica: Structure and Lattice Dynamics

Abstract 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 SiO2:KH2PO4 nanocomposite consisting of a monolithic, mesoporous silica glass host with KH2PO4 nanocrystals embedded in its tubular channels  ~ 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 KH2PO4 crystals and thus, along with X-ray diffraction experiments, corroborates the successful solution-based synthesis of KH2PO4 nanocrystals with a structure analogous to the bulk material. We succeeded in observing not only the high-frequency internal modes ( ~ 900-1200 cm−1), typical of internal vibrations of the PO4 tetrahedra, but, more importantly, also the lowest frequency modes typical of bulk KH2PO4 crystals. The experimental Raman spectrum was interpreted with a group theory analysis and first-principle lattice dynamics calculations. The calculated phonon spectrum reveals a lattice instability against two optical modes probably related to an intrinsic anharmonicity of the vibrational dynamics of KH2PO4 crystals originating from the uncertainty of proton localization within the network of hydrogen bonds. 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 KH2PO4 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.