Notwithstanding the significant practical importance of hematite, α‐Fe2O3, the complete assignment and understanding of the Raman spectrum acquired on this crystalline solid are uncertain. Above all, only one of the two external Eg phonons arising from the Γ point has been resolved and hence assigned. It is well known that the Eg mode at 294 cm−1 has been attributed to one of the two external phonons arising from the Γ point. To this end, we have undertaken studies to examine the polarized Raman scattering on a pure single crystal to gain a better understanding and assignment of phonons arising from the Γ point in the Raman spectrum of hematite. Here, we resolve and assign the previously unidentified external Eg phonon at 245 cm−1 and, additionally, confirm that the band at 294 cm−1 is an external Eg phonon, in the first‐order Raman spectrum of hematite. Further, our polarized Raman spectra display interesting polarization behavior of the 2LO mode at 1320 cm−1, where this band is only Raman active in ei||es polarization configurations.

To date, only five natural carbonate minerals of calcite structure have been studied by Raman spectroscopy. These include calcite (CaCO3), magnesite (MgCO3), siderite (FeCO3), smithsonite (ZnCO3), and rhodochrosite (MnCO3). Thus far, only synthetic compounds of otavite (CdCO3), spherocobaltite (CoCO3), and gaspeite (NiCO3) have been investigated by Raman spectroscopy. However, the Raman spectra of natural otavite, spherocobaltite, and gaspeite have yet to be interpreted and compared with the Raman spectra of the other five natural carbonate minerals of calcite structure. This work has been undertaken to fill this gap and provide a comparison and interpretation of Raman spectra representative of all the eight natural carbonate minerals of calcite structure. The data here show that the carbonate Eg (T) phonon shifts are due to influences from the nearest neighbor distance; that is, M‐O, and different ionic radii of the divalent metal cation, as shown graphically by a strong correlation (r2 = 0.87 and 0.91, respectively). Using this graphical approach, we have developed a Raman spectroscopic model based on the equation, y = −2.067x + 356.2 (±5 pm) to calculate the ionic radii of the divalent metal cation present within the mineral and hence affording the identification and discrimination of calcite‐group minerals based on the band position of the Eg (T) mode.

Researchers have demonstrated that the Raman spectra collected on hematite nanoparticles undergo an intensity enhancement, due to a dependence on excitation energy. Especially, the bands at approximately 660 and approximately 1320 cm−1, which are assigned as the 1LO and 2LO modes, respectively, experience the strongest intensity enhancement, which has been attributed to resonance Raman scattering when excited with excitation energy at or close to the band gap. However, the resonance Raman scattering mechanism is not clear and further has not been experimentally determined. To this end, we have undertaken measurements to examine the excitation energy dependence and polarized Raman scattering on a pure single‐crystal of hematite to gain a better understanding of the resonance enhancement mechanism for the 1LO and 2LO modes. Resonance enhancement mechanisms giving rise to LO phonons in semiconductors can be attributed to either the deformation potential (DP) or Fröhlich interactions (F) scattering, or a combination of the two. The DP and F contributions that give rise to LO phonon scattering in semiconductors are distinguishable due to their contrasting selection rules. Here, employing these selection rules, we conducted the following polarization Raman scattering measurements: Z()XXtrueZ0.25em¯, Z()XYtrueZ¯, Z()YYtrueZ¯, Z()YXtrueZ¯, X()YYtrueX¯, X()ZZtrueX¯, and Y()XXtrueY¯ on single‐crystal hematite to distinguish between DP (dipole‐allowed) and F (dipole‐forbidden) scattering. Our polarization experiments strongly suggest that the Fröhlich LO phonon interaction dominates the Raman scattering over the weaker DP contribution. Furthermore, polarization experiments collected using the following geometries Z()X′X′trueZ¯ and 0.25emZ()Y′Y′trueZ¯ demonstrate that the interference between dipole‐allowed and dipole‐forbidden Raman scattering is constructive for LO phonons. Therefore, we attribute the mechanism of resonance Raman scattering of the 1LO and 2LO modes in the Raman spectrum of hematite due to contributions from F and DP processes.