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The IR and Raman spectra of uvarovite (Ca3Cr2Si3O12) garnet were simulated with the periodic ab initio CRYSTAL code by adopting an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The two sets of 17 F1u Transverse-Optical (TO) and Longitudinal-Optical (LO) frequencies are generated, together with their intensities. As regards the IR experimental spectrum, only five peaks are available, that are in excellent agreement with the calculated data (mean absolute difference $\vert\overline\Delta\vert$ smaller than 5.2 cm-1). The analysis of the TO-LO eigenvalue overlaps permits to establish a correspondence between LO and TO modes. The set of experimental Raman peaks is much reacher (23 out of 25) and the agreement with our calculations excellent ($\vert\overline\Delta\vert$ smaller than 6 cm-1). Isotopic substitution is used to identify the zones of the spectrum where Cr and Ca contributions are relevant. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

The ground-state electronic structure of a number of magnetic phases of the garnets andradite (Ca3Fe2Si3O12) and uvarovite (Ca3Cr2Si3O12) has been investigated at the density functional theory level of approximation using the periodic ab initio code CRYSTAL. An all-electron Gaussian-type basis has been used in conjunction with the B3LYP hybrid functional. The exchange coupling constants between the first (J1a and J1b differentiating the two nonidentical sites), second (J2), and third (J3) nearest neighbors have been evaluated and are found to be in good agreement with the experimental data that is available for andradite. As a consequence of both the different J1a to J1b ratio and the opposite sign of J2 in the two minerals, different antiferromagnetic (AF) ground states are found for uvarovite and andradite, which is in agreement with experimental observation. Strong support for the additivity and transferability of the J constants is provided by calculations in which Cr and Fe ions are embedded in the related grossular structure. The mechanism for the stabilization of the AF states is discussed within the Anderson theory of superexchange; the kinetic energy gain in the AF states is calculated, and the spin density maps and profiles are examined. Density of states, charge density maps, and Mulliken population data complete the analysis of the electronic structure. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

IR spectra of pyrope Mg3Al2Si3O12, grossular Ca3Al2Si3O12 and andradite Ca3Fe2Si3O12 garnets were simulated with the periodic ab initio CRYSTAL code by adopting an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. Two sets of 17 F1u Transverse Optical (TO) and Longitudinal Optical (LO) frequencies were generated, together with their intensities. Because the generation of LO modes requires knowledge of the high frequency dielectric constant ℇ∞ and Born effective charges, they were preliminary evaluated by using a finite field saw-tooth model and well localized Wannier functions, respectively. As a by-product, the static dielectric constant ℇ0 was also obtained. The agreement of the present calculated wavenumbers (i.e. peak positions) with the available experimental data is excellent, in that the mean absolute difference for the full set of data smaller than 8 cm-1. Missing peaks in experimental spectra were found to correspond to modes with low calculated intensities. Correspondence between TO and LO modes was established on the basis of the overlap between the eigenvectors of the two sets and similarity of isotopic shifts; as result, the so called LO-TO splitting could be determined. Animation of the normal modes was employed to support the proposed pairing. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [ABSTRACT FROM AUTHOR]