The surface plasmon resonances of gold nanospheres and nanorods have been measured as a function of hydrostatic pressure up to 17 GPa in methanol−ethanol 4:1 solvent and up to 10 GPa in paraffin. Both the sphere resonance and the longitudinal rod resonance exhibit redshifts, whereas the transverse rod mode shows an extremely weak redshift or blueshift depending on the nanorod aspect ratio. Solidification of the solvent around 11 GPa causes some aggregation of the particles, readily identified through broadening of the surface plasmon band and further redshifting. Spectra collected during loading and unloading cycles exhibit only minimal hysteresis if the pressure remains below 11 GPa. The surface plasmon shifts are the result of two competing effects. Compression of the conduction electrons in the metals increases the bulk plasma frequency, which causes a blueshift. However, the increase in the solvent density under hydrostatic load leads to an increase in the solvent refractive index, which in turn leads to a redshift. We find that after accounting for the solvent contribution, we can spectroscopically determine the bulk modulus of the gold nanoparticles with a precision of 10%. The value obtained of K 0 = 190 GPa is significantly higher than the value for bulk gold (167 GPa). Furthermore, we show that pressure-induced solidification causes a significant broadening and anomalous shift of the surface plasmon band that we attribute to aggregation and nanorod deformation.

We report a complete structural study of CoF 2 under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first-principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co K-edge experiments, Raman spectroscopy, and optical absorption in the 0-80 GPa range. We have determined the structural phase-transition sequence in CoF 2 and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF 2 but different to ZnF 2 and MgF 2 , despite that the Co 2+ size (ionic radius) is similar to Zn 2+ and Mg 2+ . We found that the complete phase-transition sequence is tetragonal rutile (P 4 2 /mnm) → CaCl 2 type (orthorhombic P nnm) → distorted PdF 2 (orthorhombic P bca) + PdF 2 (cubic P a3) in coexistence → fluorite (cubic F m3m) → cotunnite (orthorhombic P nma). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties. We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal I 4/mmm) instead. Although the first rutile → CaCl 2 -type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the E g Raman mode which is split in the CaCl 2 -type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF 2 as a function of pressure.

This work reports an X-ray diffraction, X-ray absorption, and Raman spectroscopy study of [(CH₃)₄N]₂MnX₄ (X = Cl, Br) under pressure. We show that both compounds share a similar phase diagram with pressure. A P2₁/c monoclinic structure describes precisely the [(CH₃)₄N]₂MnCl₄ crystal in the 0.1-6 GPa range, prior to crystal decomposition and amorphization, while [(CH₃)₄N]₂MnBr₄ can be described by a Pmcn orthorhombic structure in its stability pressure range of 0-3 GPa. These materials are attractive systems for pressure studies since they are readily compressible through the weak interaction between organic/inorganic [(CH₃)₄N⁺/MnX₄²⁻] tetrahedra through hydrogen bonds and contrast with the small compressibility of both tetrahedra. Here we determine the equation-of-state (EOS) of each crystal and compare it with the corresponding local EOS of the MnX₄²⁻ and (CH₃)₄N⁺ tetrahedra, the compressibility of which is an order and 2 orders of magnitude smaller than the crystal compressibility, respectively, in both chloride and bromide. The variations of the Mn-Cl bond distance obtained by extended X-ray absorption fine structure and the frequency of the totally symmetric ν₁(A₁) Raman mode of MnCl₄²⁻ with pressure in [(CH₃)₄N]₂MnCl₄ allowed us to determine the associated Grüneisen parameter (γ(loc) = 1.15) and hence an accurate local EOS. On the basis of a local compressibility model, we obtained the Grüneisen parameters and corresponding variations of the intramolecular Mn–Br and C–N bond distances of MnBr₄²⁻ (γ(loc) = 1.45) and (CH₃)₄N⁺ (γ(loc) = 3.0) in [(CH₃)₄N]₂MnBr₄.

Crystal field theory (CFT) predicts that crystal field acting on an transition-metal (TM) ion complex of cubic symmetry varies as R –5, where R is the TM-ligand distance. Yet simple and old-fashioned, CFT is used extensively since it provides excellent results in most TM ion-bearing systems, although no direct and thorough validation has been provided so far. Here we investigate the evolution of the electronic and crystal structures of two archetypal Co2+ compounds by optical absorption and X-ray diffraction under high pressure. Both the electronic excited states and crystal-field splitting, Δ = 10Dq, between 3d(e g + t 2g) orbitals of Co2+ as a function of volume, V, and Co–F bond length, R, in 6-fold octahedral (oct) and 8-fold hexahedral (cub) coordination in compressed CoF2 have been analyzed. We demonstrated that Δ scales with R in both coordinations as R –n , with n close to 5 in agreement with CFT predictions. The pressure-induced rutile to fluorite structural phase transition at 15 GPa in CoF2 is associated with an increase of R due to the 6 → 8 coordination change. The experimental Δ­(oct)/ Δ­(cub) = −1.10 for the same R-values is close to −9/8, in agreement with CFT. A similar R-dependence is observed in KCoF3 in which the CoF6 O h coordination is maintained in the 0–80 GPa pressure range.

The density and compressibility of nanoscale gold (both nanospheres and nanorods) and microscale gold (bulk) were simultaneously studied by X-ray diffraction with synchrotron radiation up to 30 GPa. Colloidal stability (aggregation state and nanoparticle shape and size) in both hydrostatic and nonhydrostatic regions was monitored by small-angle X-ray scattering. We demonstrate that nonhydrostatic effects due to solvent solidification had a negligible influence on the stability of the nanoparticles. Conversely, nonhydrostatic effects produced axial stresses on the nanoparticle up to a factor 10× higher than those on the bulk metal. Working under hydrostatic conditions (liquid solution), we determined the equation of state of individual nanoparticles. From the values of the lattice parameter and bulk modulus, we found that gold nanoparticles are slightly denser (0.3%) and stiffer (2%) than bulk gold: V 0 = 67.65(3) Å 3 , K 0 = 170(3)GPa, at zero pressure.