The infrared spectrum of ortho-enstatite from reflectance experiments and first-principle simulations

Abstract

Infrared (IR) spectra provide a rich amount of information concerning chemical composition, lattice structure, size and shape of circumstellar dust. Accurate reference data are then required for the analysis of the various detected components. This study provides the IR characterization of one of the most frequently observed compounds, MgSiO3 ortho‐enstatite, and shows that IR experiments and ab initio techniques can be used synergically to obtain high‐quality data concerning crystalline materials. The IR reflectance spectrum of synthetic ortho‐enstatite was collected and compared to ab initio results (Gaussian‐type basis set, PBE0 hybrid density functional theory (DFT) functional). An excellent agreement is observed both for vibrational frequencies (ν) and intensities, the latter estimated through the oscillator strength. The mean absolute difference between experimental and calculated ν is of the order of 7–8 cm−1 (43 out of 65 peaks differ by less than 10 cm−1, only four peaks differ by 15–19 cm−1). The static dielectric tensor and its components (electronic and ionic contributions) were measured and compared to calculated data: differences are in the 2–5 per cent range.

Nanoscale infrared spectroscopy as a non-destructive probe of extraterrestrial samples

“…Pyroxenes constitute a solid solution series of single-chain silicates with chemical formula M 1 M 2 T 2 O 6 (M 1 , M 2 denoting cation sites and T the tetrahedral site, typically Si or Al) including Mg-rich (enstatite) and Ca-rich (augite) members common among chondritic meteorites. Established IR absorption spectra for enstatite 45,46 match vibrational bands of the first several nanoFTIR spectra ( Supplementary Fig. 3) of line scan 3 ( Fig.…”

The Raman spectrum of CaCO3 polymorphs calcite and aragonite: A combined experimental and computational study

“…28,29 The chosen computational setup has been shown to reproduce structure and vibrational properties of ionic and semi-ionic compounds in excellent agreement with experimental data. 17,[30][31][32][33][34][35][36][37][38] Supplementary calculations were performed by means of the Hartree-Fock Hamiltonian; corresponding data for single crystal calcite at 80 K are reported as Supplementary Information (Table S3). As regards intensities, critical qualitative aspects such as relative order remain unaltered when compared to B3LYP, even if quantitative agreement with experiment is slightly improved.…”

Use of ab initio methods for the interpretation of the experimental IR reflectance spectra of crystalline compounds

“…This is the attitude usually adopted by Suto and coworkers (private communication, and Refs. ). In this case, oscillators that are IR allowed, but whose IR intensity is very low, might be excluded from the analysis.…”

“…Once a model for the dielectric function has been chosen, its parameters can be obtained by best fit, through minimization of the chi square (χ 2 ) between the calculated (reconstructed) spectrum R calc (ν) [eq. ] and the experimental one R exp (ν), over a set of points on which the spectrum has been digitalized (σ exp is the experimental standard deviation): Typically, the spectra cover a range of about 1000–1500 cm −1 in steps of 0.25 to 4 cm −1 ; then they are composed of a number of points M in the order of thousands.…”

“…This best fit procedure is not straightforward at all, for various reasons: R (ν) is nonlinear with respect to the parameters; the χ 2 function presents a large number of local minima; parameters are strongly correlated ( f n and γ n in particular); the number of parameters can be very high. As regards the latter point, let us consider the case of orthoenstatite (MgSiO 3 ) as an example. There are three polarization directions (with B 1 u , B 2 u , and B 3 u symmetry), so the earlier discussion applies to three R i (ν) functions, one for each polarization direction i .…”