Simulação clássica para análise de terras-raras em cristais e em vidro

Abstract

In this thesis, we present two works developed by classical computational modeling techniques. In the first part, we simulate perovskite-type orthorhombic crystals RN iO3, where R = P r, N d, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm, Y b, and Lu, in search of understand the structural and mechanical response of this family of nickelates under external (hydrostatic) and internal (chemical) pressure. We fitted the interatomic potential satisfactorily and verified the transferability of this potential by simulating precursor oxides. The calculations revealed that the behavior of the lattice parameters and the unit cell volume with increasing hydrostatic pressure is in agreement with the data found in the related literature. The same could be verified for the evolution of anisotropy as a function of chemical pressure. The influence of both pressures on bond lengths (R-O, Ni-O) and bond angles (Ni-O1-Ni, Ni-O2-Ni) were also investigated. An anomalous behavior was visualized in the average bond angle (<Ni-O-Ni>) with the hydrostatic pressure, which was related to the low sensitivity of the variation of the metal-insulating transition temperature (TMI ) for compounds with small R ions. Regarding the mechanical response, elastic constants, bulk and shear moduli were studied. The analysis of these quantities under internal and external pressure suggested the possibility of a structural phase transition. In the second part, we simulate using static computational modeling four phases of Al2O3 (R-3c, Pbcn, Pna21 and C2/m) and three phases of Al2SiO5 (silimanite, andalusite and kyanite) in order to adjust the potential for simulation satisfactory results of the aluminosilicate glass Al2O3 − SiO2. Initially, we defined the most appropriate charge for aluminum equal to 1.62e and, with that, we simulated the lattice parameters, unit cell volume, elastic constants and bulk modulus satisfactorily in the seven structures using static simulation and molecular dynamics. Now only using molecular dynamics, we compute the radial and cumulative distribution functions (gij (r) and nij (r)) to discuss bond lengths and coordination number in the structures of Al2SiO5. After validating the fitted potential in the crystal systems, we simulated the glass with 67% of Al2O3 and reproduced in good agreement the spectra of the normalized structural factor (S(Q)) and the correlation function (D(R)). Finally, we simulate four glasses with different percentages of Al2O3 and discuss the glass transition temperature Tg.