title: The katoite hydrogarnet Si-free Ca3Al2([OH]4)3: a periodic Hartree-Fock and B3-LYP study.
abstract: The structure of the Si-free katoite hydrogarnet (116 atoms in the unit cell) has been investigated at the periodic ab initio quantum mechanical level with the CRYSTAL program, by using a Gaussian type basis set and both the HF and the hybrid B3-LYP Hamiltonians. The structure has been fully optimized at various pressures in the 0-46 GPa range; the modifications of the structure, and in particular of the (OH)4 group, as a function of pressure are analyzed. At the B3-LYP level and P greater than 15 GPa, a O-H...O interaction of increasing strength appears, with important modifications in the local geometry of the tetrahedral site. The calculated omega01(O-H) fundamental vibrational frequency at zero pressure is in excellent agreement with experiment (3674 and 3663 cm(-1), respectively); the omega01(O-H) stretching frequency remains essentially constant in the 0-15 GPa interval, whereas it dramatically decreases at higher pressures with a corresponding anharmonicity increase, as a consequence of the formation of a strong hydrogen bond. The hydration energy of grossular and the formation energy of Si-free katoite have also been computed, and the B3-LYP results are in quite good agreement with experiment.
authors.affiliation: Laboratoire de Petrologie, Modelisation des Materiaux et Processus, Universite Pierre et Marie Curie, 4, Place Jussieu, 75232 Paris Cedex 05, France.
authors.forename: F
authors.initials: F
authors.lastname: Pascale
authors.forename: P
authors.initials: P
authors.lastname: Ugliengo
authors.forename: B
authors.initials: B
authors.lastname: Civalleri
authors.forename: R
authors.initials: R
authors.lastname: Orlando
authors.forename: P
authors.initials: P
authors.lastname: D'Arco
authors.forename: R
authors.initials: R
authors.lastname: Dovesi