Statistical approaches in the analysis of in situ thermo‐Raman spectroscopic data for gypsum as a basis for studying dehydration and phase transformations in crystalline hydrates

Abstract

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.

Thermal behavior of modern and archeological enamel and dentin by in situ temperature‐dependent Raman spectroscopy

“…The phenomenon of the vibrational mode shift to the higher wavenumbers during heating is well‐known for crystallohydrates and has been demonstrated by the example of the H 2 O molecule in gypsum 35,88–91 . The shift of the vibrational mode corresponding to the shorter OH bond (0.944 Å) in the H 2 O molecule of gypsum to the higher wavenumbers is due to the weakening of the hydrоgen bond and the strengthening of the OH bond at higher temperatures.…”

“…The phenomenon of the vibrational mode shift to the higher wavenumbers during heating is well-known for crystallohydrates and has been demonstrated by the example of the H 2 O molecule in gypsum. 35,[88][89][90][91] The shift of the vibrational mode corresponding to the shorter O H bond (0.944 Å) in the H 2 O molecule of gypsum to the higher wavenumbers is due to the weakening of the hydrоgen bond and the strengthening of the O H bond at higher temperatures. The ν 1 (SO 4 ) vibrational mode in γ-CaSO 4 , which, in contrast to gypsum, lacks of molecular H 2 O and, correspondingly, hydrogen bond, shifts to high wavenumbers (1026 cm À1 ) during heating…”

“…is preferable for analyzing the variations of the full spectral profile of Raman-active modes without its peak fitting, providing more accurate diagnosis of spectral changes under the influence of temperature and the determination of critical values. 34,35 The present study is devoted to the investigation of the thermal behavior of modern and archeological human enamel and dentin using in situ temperaturedependent Raman spectroscopy in temperature range 83-873 K in order to assess structural changes both in organic and mineral components of a tooth and their interrelationship. Additionally, the extent of diagenetic alteration of archeological tooth tissues as well as the impact of heat treatment on their morphology and composition was estimated using the scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS).…”

“…In this case, the use of statistical methods of spectral parametrization (such as Pearson's correlation coefficient, autocorrelation function, skewness, kurtosis, etc.) is preferable for analyzing the variations of the full spectral profile of Raman‐active modes without its peak fitting, providing more accurate diagnosis of spectral changes under the influence of temperature and the determination of critical values 34,35 …”