介孔氧化硅中的顺电KH $_2$ PO $_4$纳米晶：结构和晶格动力学
Paraelectric KH$_2$PO$_4$ Nanocrystals in Monolithic Mesoporous Silica: Structure and Lattice Dynamics
摘要：
介电晶体与介孔固体的组合允许功能纳米材料的通用设计，其中多孔主体提供机械刚性支架结构，并且分子填充增加了功能化。在这里，我们报告了一个复杂的晶格动力学的SiO $_2$：KH $_2$ PO $_4$纳米复合材料组成的单片，介孔二氧化硅玻璃主机与KH $_2$ PO $_4$纳米晶体嵌入在其管状通道$\sim$ 12 nm宽的研究。在70-1600 cm $^{-1}$光谱范围内进行的显微拉曼研究揭示了受限晶体的复杂晶格动力学。它们的拉曼光谱类似于从块状KH $_2$ PO $_4$晶体中获得的拉曼光谱，因此，与X射线衍射实验一起，证实了成功的基于溶液的合成具有与块状材料类似的结构的KH $_2$ PO $_4$纳米晶体。我们不仅成功地观察到了PO $_4$四面体内部振动的典型高频内部模式（$\sim$ 900-1200 cm $^{-1}$），而且更重要的是，还观察到了KH $_2$ PO $_4$晶体的典型最低频率模式。用群论分析和第一性原理晶格动力学计算解释了实验拉曼光谱。计算的本征矢量的分析表明，在大多数声子模式的氢原子的参与证实了氢子系统的顺电体和KH $_2$ PO $_4$晶体在极端空间限制的晶格动力学的实质性意义。相对拉曼强度的边际重新分配的限制相比，无约束的晶体大概起源于略有变化的晶体场和原子间的相互作用，特别是对于部分的纳米晶体在接近的二氧化硅孔表面。
英文摘要：
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.
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