The application of statistical methods to Raman spectroscopy is due to the need to analyse arrays of poorly resolved mineral spectra having low symmetry, large unit cells, and so forth. For the diagnosis of spectral changes under the influence of an external factor, as well as for the determination of critical values, statistical methods for the treatment of the spectrum profile provide more accuracy than a peak‐fitting procedure. The following algorithms for calculating statistical ξ parameters are used to parameterise I(ν,T) arrays of temperature‐dependent Raman spectra: Pearson correlation coefficient characterising the similarity/difference of signals; autocorrelation function, proposed earlier for estimating the weighted average width of spectral lines; and skewness and kurtosis, the parameters of the distribution of signal intensity. The ξ(T) relations were analysed for the synthetic and experimental spectra of natural zircon, titanite, and synthetic quartz in the range of T = 80–870 K. For zircon, the differences in the anharmonic behaviour of modes having various properties and symmetry are shown through the use of ξ(T). For quartz, at the temperatures preceding the displacive α–β phase transition, statistical analysis reveals the spectral region of soft modes, allowing their T‐behaviour to be analysed. For titanite, the critical temperature range corresponding to the displacive α–βγ phase transition is determined by differentiating ξ(T). The results show the potential of using a statistical approach for the rapid detection of spectral and temperature regions responding differently to an external effect, as well as for evaluating the critical values of an external parameter.
Using ab initio band structure calculations we show that mineral cubanite, CuFe2S3, demonstrates an orbital-selective behavior with some of the electrons occupying molecular orbitals of x 2 − y 2 symmetry and others localized at atomic orbitals. This is a rare situation for 3d transition metal compounds explains experimentally observed absence of charge disproportionation, anomalous Mössbauer data, and ferromagnetic ordering in between nearest neighbor Fe ions.
The study of the Durango fluorapatite single crystal was carried out by in situ ThermoRaman polarization spectroscopy in the temperature range of 80–870 K. The use of polarized radiation made it possible to study the temperature behavior of all vibrational modes in detail, excluding their superposition at elevated temperatures. The analysis of the anharmonicity of fluorapatite vibrations has been carried out; the most strongly interacting anharmonic modes responsible for structural rearrangements in the crystal have been determined. Those include, first of all, the ν1 symmetric stretching vibrations with Ag (964.3 cm−1) and E2g (964.8 cm−1) symmetry. The data obtained for E1g, E2g, and Ag symmetry species indicate that the PO3 chemical bonds in PO4 tetrahedra corresponding to the bands with Ag symmetry are less mobile and subject to less deformations due to greater rigidity along the c‐axis, and the gradual degeneration of the ν3 and ν4 components of the Davydov splitting with increasing temperature indicates an increase in the symmetry of the PO4 tetrahedron when exposed to high temperature. The obtained for fluorapatite are compared with chlorapatite in order to analyze the effect of substitutions in the halogen chain on the anharmonicity of the PO4 tetrahedral vibrations.
The study of a gypsum single crystal was carried out using in situ thermo‐Raman spectroscopy in the temperature range of 80–870 K. The conventional peak fitting approach was used to qualitatively determine the temperature ranges of gypsum dehydration and phase transitions. The dependence of the intermediate products of gypsum dehydration on the measurement conditions and the state of sample was confirmed. A detailed analysis of the temperature behavior of ν1(SO4) in a single crystal indicated the coexistence of CaSO4·2H2O + γCaSO4 (380–440 K) and γ‐CaSO4 + β‐CaSO4 (680–740 K) phases, which was explained by the reaction front movement in a solid. For the first time, an approach based on statistical algorithms was applied for processing datasets of experimental temperature‐dependent spectra of crystalline hydrates, including the calculation of the Pearson's r coefficient, skewness, ∆corr parameter, based on the autocorrelation function. The use of statistical methods for individual spectral regions made it possible to reveal the temperature differences in the pretransition regions for individual modes or their groups. In particular, the region of ν2(SO4) symmetric bending vibrations of sulfate ions was found sensitive to dehydration and accompanying phase transition at 410 K, beginning at lower temperatures (380 K) as compared with other regions, which is associated with the lack of the hydrogen bond influence on sulfate ions after the loss of water. Thus, statistical approaches are a source of additional information to clarify the quantitative indicators of the observed effects. The use of statistics is promising in the study of multistage processes of dehydration and phase transformations in crystalline hydrates in cases where peak fitting is inconvenient.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.