Pressure effect on the structure and the luminescence of rare-earth ions doped glasses: an investigation by molecular dynamics simulation

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*Corresponding author. Tel.:#33-241-73-53-61; fax:#33- 241-73-53-52.

E-mail address:andre.monteil@univ-angers.fr (A. Monteil)

Journal of Luminescence 87}89 (2000) 691}693

Pressure e ! ect on the structure and the luminescence of rare-earth ions doped glasses: an investigation by

molecular dynamics simulation

A. Monteil !, * , C. Bernard ! , S. Chaussedent ! , M. Ferrari " , N. Balu ! , J. Obriot !

!Laboratoire POMA EP CNRS 130, 2 bd. Lavoisier, Universite&d'Angers, 49045 Angers Cedex, France

"CNR CeFSA, Centro Fisica Stati Aggregati,via Sommarive 14, 38050 Trento, Italy

Abstract

Molecular dynamics simulation has been used to study the e!ect of hydrostatic pressure on the structural and spectroscopic properties of Eu3`-doped Na2O)2SiO2glass. The short- and medium-range order has been investigated.

The evolution of angular and radial distribution has shown the dependence of the structure with pressure. It was found that pressure induces an increase of the coordination number of the Eu3`ion and a shortening in the Eu}O bond distance. The pressure e!ects on the crystal-"eld parameters and on the luminescent spectra are computed and discussed in comparison with experimental data. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords:Rare earth; Glasses; Pressure; Simulation

1. Introduction

In order to better understand the relationship between structure and spectroscopic properties, it has often been used controlled variation of structural parameters in crystals, whether applying hydrostatic pressure, or changing composition. In these cases a modi"cation of the luminescence spectra of rare-earth doped crystals has been observed and related to the nephelauxetic e!ect which is due to the change in the bonding of the cation with anions [1,2]. For doped glasses, it is more di$cult than for crystals to establish a relationship between the local structure that is not well known and the spectral characteristics of the optical ions. A speci"c method to compute luminescent spectra in glasses has been pro- posed for the"rst time by Weber and Brawer in the 1980s [3]. A microscopic model of glass is"rstly obtained by

the molecular dynamic (MD) technique. From the computed positions of the cations and the surrounding ligands, luminescence spectra are determined by means of crystal-"eld theories. This method has been used suc- cessfully in glasses of di!erent composition (for a review see Ref. [4]).

In crystals the action of a hydrostatic pressure essen- tially reduces the lattice dimensions and thus the atomic distances. In a glass, which is characterised by a dis- ordered structure, the pressure-induced e!ects are more complex. Only few works have been devoted to investigate the spectroscopic variations induced by an applied pressure in an optically activated glass [5,6]. In their paper, Lochhead and Bray [5] have studied an Eu3`-doped sodium-silicate glass. They have discussed the modi"cations induced by the pressure in term of modi"cations of both crystal-"eld strength and covalence.

In this paper we report on the simulation of an Eu3`- doped sodium-disilicate glass under hydrostatic pressure.

Comparison between our results and experimental ones has led us to reconsider the method used for the calculation of the luminescence spectra.

PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 6 0 - 9

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Fig. 1. Pressure dependence of the mean Si}O}Si bond angle.

The inset shows the angular distribution function (ADF) at ambient (dot line) and 20 GPa (solid line) pressures, respectively.

Fig. 3. Variation of the crystal-"eld strength parameterS2CFas a function of the5D0P7F0transition wavelength at two di!erent pressures. Simulation: (h) ambient pressure, (L) 20 GPa.

Experimental [5]: (j) ambient pressure, (v) 21 GPa.

Fig. 2. Radial distribution function (RDF). Inset: cumulative radial distribution function (CDF) for the Eu}O pair. The CDF gives directly the coordination number.

2. Method

The procedure employed in this work has been re- ported elsewhere [7]. We have simulated a set of 605 particles rendering a modelled glass of composition:

Na2O)2SiO2 doped with 1.5 mol% of Eu3`ions. The interaction potential is the three-body potential de- veloped by Newell et al. [8].

The Hamiltonian of the system consists of two contri- butions. One part stands for the electrostatic crystal"eld that is described by a point-charge model. The other part stands for the free ion Hamiltonian, the 4f electron Coulomb repulsion, the con"guration interaction and the spin}orbit coupling. To describe the latter term, free- ion parameters are used which are deduced from the experimental spectra of luminescence. On the other hand, the crystal-"eld Hamiltonian is described in a C1 sym- metry. The site dependence of the crystal-"eld Hamil- tonian gives rise to the inhomogeneous broadening of the line widths. The crystal"eld parametersB

/.and a para- meterS2CFindicative of the crystal-"eld strength [5] are computed for each simulated con"guration.

3. Results and discussion

Fig. 1 shows the Si}O}Si bond angle distribution as a function of the hydrostatic pressure. This angle depicts the arrangement between the sharing corner tetrahedra in silicate glasses. A strong decrease of the mean value of the Si}O}Si angle is observed for applied pressure lower than 10 GPa. For higher pressure the e!ect is less important. This is indicative of two pressure regimes, as re- ported by Lochhead and Bray [5]. In silicate glasses the presence of non-bridging oxygens (NBO) is one of the main features that allows doping with the rare-earth ions and ensures the stability of the structure. In the simulated glass, we have found a high percentage of NBO, roughly

40% at ambient pressure decreasing to 35% at 20 GPa.

However, it was found that the 97% of the oxygen belonging to the Eu3`"rst coordination shell are NBO.

This percentage decreases slightly at 20 GPa. The coordination number of the Eu3`ion is 7 at ambient pressure and increases to 8 upon application of a pressure of 20 GPa (see inset of Fig. 2) The mean Eu}O distance varies only slightly with the pressure (see Fig. 2). This is due to two opposite e!ects. Indeed, while stress shortens distances, an increase in the coordination number pushes away ligands and thus lengthens distances.

As a consequence, the electrostatic crystal"eld varies only weakly with pressure. As it can be seen in the experimental results of Lochhead and Bray [5], the main variation in the spectroscopy of the rare earth, apart from the variation of the lifetimes, is a redshift of the 5D

0P7F

0transition. This is indicative of an increase in the covalency of the Eu}O bonds. In order to take account of such an e!ect in our simulations, we have set the values of the free ion parameters depending on both 692 A. Monteil et al./Journal of Luminescence 87}89 (2000) 691}693

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Fig. 4. Comparison between the reconstructed (dot line) and the experimental [7] (solid line) emission spectrum of Eu3`-doped sodium-disilicate glass.

the coordination number and the Eu}O distances. It can be seen in Fig. 3, that in such a way it is possible to reproduce the same dependence on pressure for the computed spectroscopic parameters as for the experimental ones. Discrepancies between computed and experimental values are due to poor statistics for the simulated data particularly on the wings. An improvement of this method should be the determination of Eu}O covalency by ab initio calculation. Such a work is in progress. Fig. 4 shows the comparison between the reconstructed and the experimental [7] emission spectrum of Eu3`in the investigated glass. It must be noticed that important improve- ments have been carried out in the simulation procedure with respect to previous works [7].

4. Conclusion

A Eu3`-doped sodium-disilicate glass has been simulated by MD technique. A strong decrease of the mean value of the Si}O}Si angle is observed for applied pressure in the range ambient-10 GPa. Important im-

provements have been carried out in the simulation procedure. We have shown that it is necessary to take into account the variation in the covalency by making depen- dent the "tting parameters both on the coordination number and the interatomic distances which have an opposite dependence on the pressure. The luminescence spectrum of Eu3`and the crystal-"eld strength are in well agreement with the experimental data [5,7].

Our results con"rm the interpretations of Lochhead and Bray [5] on the nature of Eu3`sites in the glass structure: high-crystal"eld of Eu3`sites associated with low coordination numbers, and short Eu}O bonds.

Acknowledgements

This research was partially supported by a MURST- COFIN 97, a CNR Special Project 97, and a French}Italian Program Galileo 98}99. A DLPOLY package from the Council for the Central Laboratory of the Research Councils, Daresbury laboratory, Nr. War- rington, has been used at the IDRIS Computing Center (Project no. 991007) to perform the simulations.

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