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The structure of the first co-ordination shell of Pb atoms in lead - silicate glasses : a molecular dynamics study

100%

Rybicka A. , Rybicki J. , Witkowska A. , Feliziani S. , Mancini G.

tom Vol. 5

67-74

The structure of lead-silicate glasses, although being investigated for over sixty years, rema ins still controversial. One of the open questions is the structure of the lead subsystem: what are the basic structural units like, and how are they interconnected. In the literature the appearance of [PbO4]n, or [PbO3]n chains is reported even in the glasses of the same stoichiometry. In order to elucidate the problem of contradicting experimental findings, extensive molecular dynamics (MD) simulations in the isobaric-isoenthalpic (NpH) ensemble of the structure of lead-silicate glasses have been performed in the whole range of glassformation. The atoms were assumed to interact by a two-body Born-Mayer-Huggins interaction potential with full ionic charges. The potential parametrization was taken from Damodaran et al. [Phys. Chem. Glasses, 31, 212 (1990)]. The simulation box contained about 3000 atoms. For each composition an equilibrated melt at 10000 K was prepared, and then slowly (6 ź 1013 K/s) cooled down to 300 K, passing several equilibrium states at intermediate temperatures. The simulation results can be summarised as follows. For 0.1 = x = 0.33, about ? of all Pb atoms is either placed in the front of the faces of the SiO 4 tetrahedra, forming PbO3 pyramids with the Pb atom in the vertices or in the middle of an almost flat triangle formed from apical oxygens of three different SiO 4 tetrahedra. Another ? of all Pb atoms can be qualified as vertices of rather distorted square pyramids. The remaining ? o f lead atoms have neighbourhoods difficult to classify. For x = 0.67 only a few Pb atoms were identified as remaining in the front of a face of a SiO 4 tetrahedron. Most of the Pb atoms are placed in the vertices of edge sharing square PbO4 pyramids. Our MD results are compared with available experimental data.

Structure of rarefied PbGeO/sub 3/ glass: a molecular dynamics study.

100%

Rybicki J. , Witkowska A. , Bośko J. , Mancini G. , Feliziani S.

2000

tom Vol. 30, nr 4

709-717

The present contribution is dedicated to a molecular dynamics (MD) study of the structure of rarefied lead-germanate glasses of composition PbGeO/sub 3/. The simulations have been performed in the constant volume regime for systems with densities of 3000, 4000, 5000 and 6285 kg/m/sup 3/, using a two-body potential (Born-Mayer repulsive forces, and Coulomb forces due to full ionic charges). The information on short-range correlations was obtained in a conventional way (from radial and angular distribution functions), while the middle-range order was studied via cation-anion ring analysis. In the paper the short and medium range order in the rarefied glasses is discussed and compared with the structure of the PbGeO/sub 3/ glass in normal conditions.

The structure of rarefied and densified PbGeO3 and PbGeO2 glasses : a molecular dynamics study

86%

Rybicki J. , Witkowska A. , Bergmański G. , Bośko J. , Mancini G. , Feliziani S.

tom Vol. 7, nr 1

91--112

The paper is dedicated to a molecular dynamics (MD) study of the structure of rarefied and densified lead-germanate glasses, of compositions PbGeO3 and PbGeO2. The simulations have been performed at constant volume for systems with densities of 3000, 4000, 5000, 6285 (normal density), 7000, and 8000 kg/m3, using a two-body potential (Born-Mayer repulsive forces, and Coulomb forces due to full ionic charges). All the systems were initially prepared as well equilibrated hot melts, and then slowly cooled down to 300 K. The information on short-range correlations was obtained in a conventional way (from radial and angular distribution functions), while the middle-range order was studied via cation-anion ring analysis. In the paper, the short- and medium-range order in the rarefied and densified glasses is discussed and compared with the structure of the PbGeO3 and PbGeO2 glasses at normal conditions.