Environments of lead cations in oxide glasses probed by X-ray diffraction

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Abstract

The structural behavior of oxygen environments of Pb2+ cations in binary lead oxide glasses is studied by X-ray diffraction using high-energy photons from a synchrotron. Samples of different PbO content and of different type of network-forming oxide such as SiO2 and V2O5 are used. For comparison, earlier data obtained of Pb phosphate glasses are included. The oxygen environments strongly depend on the type of network-forming oxide but they do not vary with PbO content. Numbers and distances of oxygen atoms at the shortest separations are three, four and five with lengths of 0.230, 0.252 and 0.247 nm for glasses with SiO2, V2O5 and P2O5, respectively. The numbers of further oxygen atoms at distances of ∼0.275 nm are about one, two and three. The excentric positions of Pb2+ cations in the oxygen environments of silicate and vanadate networks are attributed to non-uniform distributions of negative charge on the surrounding oxygen atoms. In case of lead vanadate glasses the Pb2+ cations stabilize VO5 units which is not known of vanadate glasses of other network-modifying oxides MeO.

Introduction

Oxide glasses containing PbO possess high refractive index, high resistance against devitrification and low melting temperatures [1]. Also, for example, Pb silicate glasses are obtained in a wide compositional range. Glasses with excess of PbO form even networks of invert character [1]. To explain the stability of invert networks, sometimes, PbO is called a network modifier at low lead content and an intermediate oxide at high lead content [1], [2]. Other authors call PbO a main glass former for silicate glasses rich in PbO [3]. The question whether this change is visible in the environments of Pb2+ cations was studied by Pb-LIII edge XAFS and 207Pb NMR on a series of Pb silicate glasses [4]. No change of the Pb–O coordination was detected [4] while the mean number of links per SiO4 group was found to decrease from about three to less than one [5]. For all glass compositions studied the Pb2+ cations have three to four oxygen neighbors [4], [6] where it is suggested that the Pb atoms are sitting on the apices of PbOn pyramids by analogy with the groups in related crystals [7], [8]. When studying the Pb2+ environments in phosphate glasses [9], [10], [11] the Pb–O distances appear by 0.015 nm longer than in silicate networks with a Pb–O coordination number of about five. It reveals that these environments also do not change if the PbO content increases [11]. So it seems that Pb–O environments depend rather on the type of network-forming oxide than on the PbO content. Moreover, for Pb phosphate glasses it was shown by X-ray diffraction that about three additional oxygen neighbors exist at larger distances (∼0.28 nm) [9], [11].

The use of high-energy photons (E0>50 keV) from synchrotrons in X-ray diffraction experiments [12] allows one to investigate highly absorbing materials such as oxide glasses containing PbO also in transmission mode [11], [13]. Concerning high count rates and large measuring range the scattering intensities are of similar quality when compared with those of other materials. Due to high scattering power of Pb atoms only those pair distributions possess considerable weight which involve Pb. Combinations with neutron diffraction are useful to resolve contributions of the other pair correlations [11], [13].

In this work the structure of a series of (PbO)x(V2O5)1−x glasses with 0.1⩽x⩽0.5 is studied by X-ray diffraction using high-energy photons of a synchrotron. Except of the Pb–O environments also a possible change of the V–O coordination number is of interest. In binary vanadate glasses with other bivalent modifier oxides (MeO=ZnO [14], SrO [15], CaO [16]) a continuous decrease of NVO from ∼4.4 to ∼4.0 is found when the MeO content increases from 0 to 50 mol%. In an earlier X-ray diffraction study of a Pb metavanadate glass (50 mol% PbO) [17] a V–O coordination number of ∼4.8 was found and a Pb–O coordination number of six could be estimated with Pb–O distances of 0.254 nm. This behavior will be analyzed more precisely and studied in dependence on the PbO content. The diffraction results will allow one to estimate if also Pb–O distances exist in the range of ∼0.28 nm such as found for lead phosphate glasses [9], [11]. For comparison with Pb2+ environments in silicate networks three samples of Pb silicate glasses are included in this work. Though similar glasses were already studied by X-ray diffraction [9] here the use of high-energy photons allows one to obtain precise information about the Pb–O coordination. Distances in the vicinity of ∼0.28 nm can be extracted if exist. The same samples as used have been investigated earlier by Pb-LIII XAFS experiments [4].

Despite the more convenient conditions of absorption of Pb containing glasses in neutron diffraction experiments such studies do not give more accurate results when performed on Pb oxide glasses [10], [18]. The Pb–O distances form a broad distribution which overlaps with the O–O first-neighbor peak. Since the O–O pair information dominates total neutron scattering data the assumptions about O–O coordination numbers and O–O distances have to be made very carefully for extracting reliable parameters of the Pb–O distances. Little information is obtained for Pb–O distances at ∼0.28 nm.

Section snippets

Sample preparation

The (PbO)x(V2O5)1−x glasses with PbO fractions x=0.1, 0.2, 0.3, 0.4, and 0.5 were prepared as described in [19]. The samples were made by melting the mixtures of reagent grade V2O5 and PbO in alumina crucibles at temperatures in the range 900–1200 °C depending on composition. Vitreous state of samples was obtained by rapid cooling using a twin-roller technique. The mass densities of the samples studied are estimated from data given in [19]. The corresponding number densities of atoms used in

Results

All structure factors obtained are plotted in Fig. 1 where, additionally, also earlier results of two Pb phosphate glasses [11] are shown. Due to some unphysical features in the intensities of the Pb vanadate glasses the S(Q) data are limited to a range 9 nm−1 < Q < 156 nm−1. The sample with x=0.2 shows indications of a small crystalline fraction which is neglected below. Due to a strong dominance of those pair correlations which involve Pb atoms a clear decrease of the oscillation amplitudes is

Discussion

The Pb–O first-neighbor distances of the glasses studied show broad distributions of highly asymmetric shape which are approximated by two Gaussian functions. Their two mean distances should not be overestimated as characteristic lengths of definite PbOn polyhedra because the peaks are broad forming continuous distributions. The parameters of the first peak component are well determined. They are obtained when fitting the left flank of the asymmetric peak. The second peak component is more

Conclusions

The geometries of the oxygen environments of Pb2+ cations in binary lead oxide glasses do not depend on the PbO content but on the network-forming oxide such as SiO2, V2O5, and P2O5. The numbers and distances of oxygen atoms on the shortest separations are three, four and five at 0.230, 0.252 and 0.247 nm for glasses with SiO2, V2O5 and P2O5, respectively. The broad peaks in the correlation functions, T(r), from 0.23 to 0.27 nm indicate the existence of asymmetric distributions of Pb–O

Acknowledgements

Financial support of Deutsche Forschungsgemeinschaft (Grant KR 1372/7-1) is gratefully acknowledged.

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    Present address: Argonne National Laboratory, Chemistry Division, 9700 S Cass Ave, Argonne, IL 60439, USA.

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