Article pubs.acs.org/JPCC
Quantification of Short and Medium Range Order in Mixed Network Former Glasses of the System GeO2−NaPO3: A Combined NMR and X-ray Photoelectron Spectroscopy Study Jinjun Ren† and Hellmut Eckert*,†,‡ †
Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany Instituto de Física de São Carlos, Universidade de São Paulo (USP), C.P. 369, CEP 13560-970, São Carlos, São Paulo, Brazil
‡
S Supporting Information *
ABSTRACT: Glasses in the system xGeO2−(1−x)NaPO3 (0 ≤ x ≤ 0.50) were prepared by conventional melting−quenching and characterized by thermal analysis, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and 31P nuclear magnetic resonance (MAS NMR) techniques. The deconvolution of the latter spectra was aided by homonuclear J-resolved and refocused INADEQUATE techniques. The combined analyses of 31P MAS NMR and O-1s XPS lineshapes, taking charge and mass balance considerations into account, yield the detailed quantitative speciations of the phosphorus, germanium, and oxygen atoms and their respective connectivities. An internally consistent description is possible without invoking the formation of higher-coordinated germanium species in these glasses, in agreement with experimental evidence in the literature. The structure can be regarded, to a first approximation, as a network consisting of P(2) and P(3) tetrahedra linked via four-coordinate germanium. As implied by the appearance of P(3) units, there is a moderate extent of network modifier sharing between phosphate and germanate network formers, as expressed by the formal melt reaction P(2) + Ge(4) → P(3) + Ge(3). The equilibrium constant of this reaction is estimated as K = 0.52 ± 0.11, indicating a preferential attraction of network modifier by the phosphorus component. These conclusions are qualitatively supported by Raman spectroscopy as well as 31P{23Na} and 31P{23Na} rotational echo double resonance (REDOR) NMR results. The combined interpretation of O-1s XPS and 31P MAS NMR spectra shows further that there are clear deviations from a random connectivity scenario: heteroatomic P−O−Ge linkages are favored over homoatomic P−O−P and Ge−O−Ge linkages.
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spectroscopy (XANES).25 These works have suggested that the alkali ions preferentially modify the phosphate rather than the germanate component in the network, and a description has been advanced in which these glasses are considered random networks of unmodified GeO4/2 and anionic metaphosphate units. Furthermore, extended X-ray absorption fine structure (EXAFS) and neutron scattering studies of the binary GeO2−P2O5 glass system indicate the presence of six-coordinate germanium species.26,27 Six-coordinated GeO6/22− units have also been identified as a prominent structural element in binary Na2OGeO2 glasses,28−34 particularly at low Na2O/GeO2 ratios, where pronounced nonlinear trends in the compositional dependences of bulk macroscopic properties are observed (“germanate anomaly”).35,36 In view of these findings, the presence of such octahedral units might have to be considered in the ternary Na2O−P2O5−GeO2 glass system as well, even though previous experimental results suggest that such six-coordinated units are only expected in glasses with high Ge/P ratios.22−24
INTRODUCTION Germanium oxide containing glasses are characterized by their high refractive index, low phonon energies, and sensitivity to UV photon irradiation.1,2 Glasses of this kind have considerable application in optical fiber telecommunications, Bragg gratings, nonlinear optical devices, and optical waveguides.3−12 Likewise, phosphate glasses are widely used in optical communications, solid state lasers, optical switching devices, and threedimensional (3D) displays, because of the high solubility of rare earth ions in them.13−17 Furthermore, their high ion conductivity makes them interesting candidates for solid electrolyte applications.18 In analogy to the situation in germanate/silicate glasses, glasses based on mixed germanate/phosphate networks have attracted technological interest because of the possibility of fine-tuning physical properties by adjusting the chemical composition.2,19−21 To provide a structural rationale for these effects, detailed spectroscopic characterization is essential. Recently, the structures of alkali germanate/phosphate glasses, (R2O)x(GeO2:P2O5)1−x where R = Na, K, and Rb, Li2O−GeO2− P2O5, and xNa2O−0.5P2O5−(0.5−x)GeO2 (x = 0.0−0.5), have been studied using vibrational spectroscopy and solid state 31P MAS NMR,22−24 as well as X-ray absorption near edge © 2012 American Chemical Society
Received: February 10, 2012 Revised: May 15, 2012 Published: May 15, 2012 12747
dx.doi.org/10.1021/jp301383x | J. Phys. Chem. C 2012, 116, 12747−12763
The Journal of Physical Chemistry C
Article
of 130 kHz and 9 kHz, respectively. The spinning rate was 14.0 kHz. The sampling in the t1 dimension was done with a dwell time of 8.92 μs (1/8 rotor period). Depending on the sodium content, 120−360 scans per t1 increment were taken. Different phosphorus species involved in homonuclear indirect scalar coupling (“2J-coupling”) via P−O−P linkages were identified via homonuclear J resolved spectroscopy.39 In this experiment, the amplitude modulation of the rotor synchronized spin echo due to J-coupling (damped by spin− spin relaxation) is recorded. Fourier transformation of the 2D time domain data set yields a spectrum exhibiting J-multiplets in the F1 dimension. These data were measured at 162.0 MHz on a Bruker DSX 400 spectrometer, at a spinning frequency of 15.0 kHz. Z-filtered spin echoes (one rotor period) were measured, incorporating a 32-step phase cycle.40 The π/2 pulse lengths were around 2.0 μs. Depending on the sample, the rotor synchronized echoes were recorded for evolution times up to 100 ms corresponding to about 80 t1 increments. For the 2D data processing, the States method was used to obtain pure absorption phase spectra. Data were recorded at a 50 s recycle delay, employing a saturation comb to ensure reproducible steadystate conditions. The connectivity between different types of phosphorus units was further probed by one- and two-dimensional refocused INADEQUATE experiments (see Figure 1).41 This technique
In this work, we report a systematic structural study of the binary (NaPO3)1−x(GeO2)x system using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and advanced highresolution solid state nuclear magnetic resonance techniques. On the basis of these results, we develop a comprehensive and quantitative structural model describing the local environments, the polyhedral connectivity, and the distribution of sodium among the two network former species.
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EXPERIMENTAL SECTION Materials and Methods. All the xGeO2−(1−x)NaPO3 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50) and the reference glasses 0.5NaO1/2−0.5GeO2 and GeO2 were prepared by conventional melt-quenching methods using germanium oxide from Alfa (99.99%) and sodium polyphosphate from Acros (99+%). The powdered raw materials were mixed and melted in a platinum crucible at temperatures between 900 and 1650 °C depending on the composition. The liquid melt was kept at this temperature for 30−90 min to ensure homogenization before cooling it rapidly in a steel mold. The weight loss is below 0.5 wt %. Differential scanning calorimetry was done with a Netzsch DSC-200 apparatus, at a heating rate of 10 K/min. For x > 0.5 no transparent glasses were accessible. Raman spectroscopic measurements were conducted on a Jobin Yvon Horiba-HR800 instrument, operating with a Nd/ YAG laser at 532 nm. X-ray photoelectron spectra (XPS) were measured using an Axis-Ultra spectrometer (Kratos, Manchester, UK) in ultrahigh vacuum (pressure
