Ab initio SCF-MO calculations on AlO₄ centres in alpha-quartz. I.

Abstract

Ab initio SCF-MO calculations were done on nine-atom and fifteen-atom clusters designed to simulate alpha-quartz, i.e., a [SiO4]0 unit, and aluminum impurity centres [AlO4]−, [AlO4]0, and [AlO4]+ in quartz, terminated with protons or SiH3 groups. The geometry of the small model of each of these centres was optimized. Using these models and the enlarged versions thereof, calculated values for relative total energies, various static structural parameters, electron spin densities, net atomic charges, and orbital energies are compared with experimental data from spectroscopic and other techniques, especially high resolution electron paramagnetic resonance. The calculated results agree remarkably well with experiment. Thus it is evident that small clusters of atoms can be used successfully to simulate the local surroundings of a substitutional Al ion in quartz, i.e., the short-range effects are the most important.

Hole Trapping at Al impurities in Silica: A Challenge for Density Functional Theories

“…The picture emerging from these investigations is that of a hole localized on one of the four O neighbors of the Al impurity, with an accompanying distortion of the lattice structure. This view was supported by early Hartree-Fock calculations on small cluster models of the impurity, which indeed exhibited the hole localization and distortion of the AlO 4 tetrahedron [18,19]. However, recent DFT calculations in a repeated supercell geometry have provided results at variance with this picture: These calculations preserve the a-quartz symmetry, apart from an increase in the Al-O bond length compared to the Si-O bonds, and distribute the hole evenly over the four O neighbors of Al [21][22][23].…”

“…The neutral substitutional Al impurity in a quartz (hereafter denoted as ͓AlO 4 ͔ 0 ) is one of the most well-studied states of Al in silica [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The state is observed in irradiated samples of natural or manufactured quartz and gives rise to a dark coloration ("smoky quartz").…”

Theory ofAl2O3incorporation inLægsgaard1 2002Phys. Rev. B303270View full textAdd to dashboardCite

“…[15][16][17][18][19] A number of theoretical investigations of Al impurities in silica have appeared over the years, most of them being concerned with the structure and hyperfine parameters of Sisubstitutional Al with or without compensating ions. [20][21][22][23][24][25][26] Such calculations have played an important role in establishing the structural models for compensated substitutional Al impurities. In addition, small molecules and zeolite structures containing oxidized Si and Al ions have been studied, primarily with the purpose of extracting electric-field gradients ͑EFG's͒ at Si, Al, and O nuclei.…”

Ab initio calculations on tetramethoxymethane

“…Comparison of the net atomic charge data in Table 2 with the results of an ab initio study (22) of (H0)2Si(OSiH3)2 illustrates clearly the more electronegative character of carbon as compared to silicon. …”