Drastic Connectivity Change in High Refractive Index Lanthanum

Article pubs.acs.org/cm

Drastic Connectivity Change in High Refractive Index Lanthanum Niobate Glasses Atsunobu Masuno,*,† Shinji Kohara,‡ Alex C. Hannon,§ Eugene Bychkov,∥ and Hiroyuki Inoue† †

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan § ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom ∥ Université du Littoral, 59140 Dunkerque, France ‡

S Supporting Information *

ABSTRACT: The highly ionic, high refractive index La2O3−Nb2O5 system has a La-rich glass forming region and another Nb-rich glass forming region. The La-rich and Nb-rich regions have markedly different structural and physical properties. Structural analyses using diffraction and spectroscopic measurements combined with structural modeling show that the Nb-rich glass, which has unusually high oxygen packing density, is a network of distorted NbOn polyhedra with mainly corner-sharing, and LaOx polyhedra with both corner-sharing and edgesharing. Contrastingly, in the La-rich glass, small-sized symmetrical NbOn polyhedra with a large amount of edge-sharing are inhomogenously distributed in the network of LaOx polyhedra. The drastic connectivity change of cation−oxygen polyhedra and the dense oxygen packing due to edge-sharing polyhedra contravene long-established rules of oxide glass formation. These results raise the possibility that novel higher refractive index with lower wavelength dispersion glasses, which contain highly ionic heavy elements at the lower left in the periodic table, may be synthesized. KEYWORDS: glass structure, X-ray diffraction, neutron diffraction, reverse Monte Carlo simulation, containerless processing

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INTRODUCTION According to classic glass-forming rules, glass formation requires network former oxides (NWFs) such as SiO2, P2O5, and B2O3.1,2 However, it has recently been reported that some binary oxides without any NWFs can be vitrified by containerless processing, such as MOx−Al2O3,3−7 MOx−TiO2 (M = rare earth, alkaline earth metal, or Y),8−11 La2O3− Nb2O5,12 La2O3−WO3.13 In particular, La2O3−Nb2O5 glasses, which are composed only of essential elements for high refractive index such as La and Nb, have attracted much attention from the viewpoint of optical application. The refractive indices of conventional optical glasses containing La or Nb are limited to n ≈ 2.0, due to a limitation on their concentrations arising from the low glass forming ability of La2O3 and Nb2O5. In contrast, NWF-free La2O3−Nb2O5 glasses have high refractive index n ≈ 2.3 and a wavelength dispersion, which is rather low in the visible range. Owing to these excellent optical properties, this type of glass is expected to be used as a key optical component, such as thinner lenses for digital cameras in smart phones, solid immersion lens for next-generation optical disk systems, and a fiber collimator for optical transmission systems. In this paper, we report that different types of glass formation are found on either side of the equimolar composition in La2O3−Nb2O5 binary system. The origin of the difference, which cannot be explained by the conventional glass forming model, is investigated by structural analyses combined X-ray/ © 2013 American Chemical Society

neutron diffraction with the aid of reverse Monte Carlo (RMC) simulation14 and Raman scattering.

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EXPERIMENTAL SECTION

La2O3−Nb2O5 spherical glasses with a diameter of around 2 mm were prepared by containerless processing with an aerodynamic levitation (ADL) furnace. In the ADL furnace, high-purity La2O3 and Nb 2 O 5 powders were mixed in the stoichiometric ratio of (1 − x)La2O3·xNb2O5. The mixture was pelletized under a hydrostatic pressure of 200 MPa and then sintered at 1273 K for 12 h. Specimens of approximately 40 mg were taken from the pieces of broken pellets. A specimen was then placed on the nozzle of the ADL furnace and levitated by an oxygen gas flow. The flow rate was monitored and controlled using a mass-flow meter. A high-resolution, charge-coupled device (CCD) video camera equipped with a telephoto objective lens was used to obtain a magnified view of the samples. Two 100 W CO2 lasers were used to melt the levitated sample, and a pyrometer with a sampling rate of 100 Hz was used to measure the sample temperature. The temperature of the sample was maintained 200 K above the melting point for a few seconds to ensure homogenization of the melt. Then, the laser power was turned off and the sample was rapidly cooled to room temperature at a rate of hundreds of kelvins per second. The solidified samples had high sphericity, were colorless, and transparent. Glass composition analyses were carried out by an energy dispersive X-ray fluorescence spectrometer (JEOL JSX-3100RII). It Received: April 16, 2013 Revised: June 28, 2013 Published: August 2, 2013 3056

dx.doi.org/10.1021/cm401236s | Chem. Mater. 2013, 25, 3056−3061

Chemistry of Materials

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was found that the composition deviation from the target composition is less than 1%. Before measuring the physical properties, the glasses were annealed at a temperature slightly above Tg. The glass transition temperature Tg and the first crystallization peak temperature TP were determined using thermogravity and differential thermal analysis (TG-DTA; SII DTA6300) at a heating rate of 10 °C/min. The densities of the obtained glasses were measured by a gas pycnometer (Shimadzu AccupycII 1340). Refractive indices were measured by spectroscopic ellipsometry (J. A. Uram U2000) in the range 300−1000 nm for the glasses whose surface was optically polished. Raman scattering measurements were performed by using an Ar+ ion laser as the incident light source. Incident light with a power of 300 mW was focused through a lens to a 1 mm diameter spot on the spherical glasses. Backscattered light from the sample was then collected by using a Jovin Yvon T-64000 triple monochromator with a liquidnitrogen-cooled CCD. The high-energy X-ray diffraction experiment was carried out at the BL04B2 beamline at the SPring-8 synchrotron radiation facility, using a two-axis diffractometer dedicated to the study of disordered materials.15 The energy of the incident X-rays was 113 keV. The raw data were corrected for polarization, absorption, and background, and the contribution of Compton scattering was subtracted using standard analysis procedures.15 The measurement of the neutron total scattering was performed using the GEM time-of-flight diffractometer at the ISIS Facility.16 The measured neutron diffraction data were corrected for container scattering, attenuation, absorption, multiple scattering, and inelastic scattering using the ATLAS software suite.17 Both corrected data sets were normalized to give the Faber−Ziman total structure factor S(Q),18 and the total correlation function T(r) was obtained by a Fourier transformation of S(Q).

Figure 1. Oxygen packing density of (1 − x)La2O3·xNb2O5 glasses.

character. In covalent oxide glasses such as silica glass, large volume cavities are required to form the topological network,24,25 whereas in La-glass and Nb-glass the oxygen ions with large electronic polarizability are closely packed. The unusually dense state of the highly ionic constituent elements leads to the high refractive index of La-glass and Nb-glass. The discontinuous change in ρ between these two glasses shows that the packing states must be different, and we have analyzed the glass structure to reveal the origin of this difference. The X-ray and neutron total correlation functions, TX,N(r), for La-glass and Nb-glass are shown in Figure 2, there are clear differences depending on both the glass composition and the type of radiation. The first peak in TX,N(r) with its maximum at 1.95 Å is due to Nb−O bonds. A simple peak fit to TN(r) yields an average coordination number and average bond length as follows: NNb−O = 5.06, rNb−O = 1.957 Å for Nb-glass and NNb−O

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RESULTS AND DISCUSSION (1 − x)La2O3·xNb2O5 glasses were prepared by containerless processing using an aerodynamic levitation furnace. It was found that there are two glass forming regions, a La-rich region (0.38 ≤ x ≤ 0.41) and a Nb-rich region (0.6 ≤ x ≤ 0.75). The glasses in both regions were colorless and highly transparent in the visible region. The occurrence of two separate glass forming regions is of particular note, because there are very few reports of more than one glass forming region in binary oxides systems.19 The densities of 0.6La2O3·0.4Nb2O5 (La-glass) and 0.3La2O3·0.7Nb2O5 (Nb-glass) are 6.07 and 5.58 g/cm3, respectively. The refractive indices at 587 nm (nd) of La-glass and Nb-glass are high, 2.14 and 2.25 respectively. Considering that the refractive index usually increases when the density increases for many glass systems, the higher refractive index of Nb-glass than La-glass seems to be unusual. It has been found that high refractive index glasses prepared by containerless processing have higher oxygen packing density than conventional optical glasses,20 and the oxygen packing density ρ21 of (1 − x)La2O3·xNb2O5 glasses is shown in Figure 1. In comparison with optical glasses (e.g., ρ = 0.51 for silica and ρ = 0.56 for BK7 (borosilicate) crown glass), the oxygen packing densities for (1 − x)La2O3·xNb2O5 glasses are large, ρ > 0.59. Furthermore, it is notable that there is a discontinuous change in ρ between the two glass-forming regions, implying a remarkable difference in glass structure. The electronic polarizability of an oxygen ion, αO2−, may be estimated from the refractive index and density of the glasses according to the Lorentz−Lorenz equation,10 with values of 2.39 Å3 and 2.43 Å3 for La-glass and Nb-glass, respectively.22 These values are much larger than for conventional glasses (e.g., 1.467 Å3 for silica glass),23 indicating that the atoms in both glasses have a highly ionic nature, in contrast to conventional oxide glasses which have a strong covalent

Figure 2. X-ray and neutron total correlation functions, TX,N(r) of La2O3−Nb2O5 glass. Blue, Nb-glass; red, La-glass. 3057



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= 4.99, rNb−O = 1.950 Å for La-glass. The coordination number of 5 is unusual; the Nb−O coordination number in Nb containing glasses and crystals is almost always 6, or rarely 4.26 The second peak at about 2.5 Å for TX(r) and at about 2.7 Å for TN(r) is mainly due to overlapping La−O and O−O contributions. By peak separation, it was estimated that the average La−O bond length is about 2.5 Å for both glasses. The distances are slightly shorter than the average La−O bond length in crystalline phases such as LaNbO4 (2.523 Å) and LaNb3O9 (2.735 Å),27 implying that the coordination numbers in both glasses are smaller than those in crystals. The third peak at around 3.7−4.2 Å is due to overlapping La−La, La−Nb, and Nb−Nb contributions. From the averaged structural parameters, such as NNb−O and rNb−O, it is difficult to determine the origin of the packing density difference between La-glass and Nb-glass. Thus, in order to investigate the local structure at an atomic level, a reverse Monte Carlo (RMC) simulation of the experimental structure factors, S(Q), was performed. The RMC++ code28 was used to simulate an ensemble of 6400 (Nb-glass) and 5800 (La-glass) atoms with atomic number density 0.0754 Å−3 (Nbglass) and 0.0703 Å−3 (La-glass), respectively. The starting configuration was created using a hard-sphere Monte Carlo (HSMC) simulation with constraints on closest atom−atom approaches to avoid unreasonable spikes in the partial-pair correlation functions. The closest distances that two atoms were allowed to approach each other were set to be 3.3, 3.2, 3.0, 2.3, 1.7, and 2.2 Å, for La−La, La−Nb, La−O, Nb−Nb, Nb−O, and O−O, respectively. After the HSMC simulation, RMC simulation of the neutron and X-ray structure factors was performed. The minimum Nb−Nb distance was initially set to 2.9 Å, but poor agreement was obtained, especially for the Xray structure factor. It was necessary to reduce the minimum Nb−Nb distance to an unexpectedly low value of 2.3 Å in order to obtain a close simulation of the experimental data, and this allowed the formation of edge-sharing and face-sharing NbOn polyhedra. The revised RMC models show good agreement with both X-ray and neutron S(Q)s for both glasses, as shown in Figure S1, Supporting Information. The structural information relating to oxygen ions is of particular interest due to the unusual oxygen packing density of the glasses, and the oxygen−oxygen correlation shows an unconventional feature in both glasses. Figure 3 shows the O− O partial structure factor, SO−O(Q), for La-glass, Nb-glass, 0.43Na2O.0.57SiO229 and SiO230 glasses derived from RMC modeling, together with the experimental X-ray structure factors, SX(Q), for Zr70Cu30 bulk metallic glass (BMG),31 and liquid Hg.32 For SiO2 glass, SO−O(Q) exhibits a prominent peak at around Q = 1.5 Å−1, related to the presence of intermediate range ordering due to the cages formed by the topological connection of SiO4 tetrahedra,24 and also significant oscillations at high Q caused by corner-sharing linkages between regular SiO4 tetrahedra.33 These features are also apparent for 0.43Na2O.0.57SiO2 glass. However, these two features are not observed in SO−O(Q) for La- and Nb-glasses, implying a different connectivity of the cation−oxygen polyhedra. SO−O(Q) for La- and Nb-glasses are rather similar to SX(Q) of Zr70Cu30 BMG, and liquid Hg. Zr70Cu30 BMG has a nearly close packed structure due to the existence of icosahedral polyhedra, and the structure of liquid Hg is also densely packed. Thus, the La- and Nb-glass structure is highly dense and lacks topological intermediate range ordering of the LaOx and NbOn polyhedra. Furthermore, the O−O coordination number up to

Figure 3. (A) Partial structure factors SO−O(Q) for 0.43Na2O·0.57SiO2 glass,29 SiO2 glass,30 and La2O3−Nb2O5 glass obtained from the RMC model. (B) X-ray structure factors SX(Q) for Zr70Cu3031 and liquid Hg.32

3.5 Å, NO−O, is 9.3 for Nb-glass and 8.1 for La-glass, respectively, and these values are significantly larger than the value of 6 for SiO2 glass.32 The larger NO−O of Nb-glass is consistent with the larger oxygen packing density (Figure 1) and higher refractive index of Nb-glass.34 The larger NO−O is also in part due to the large oxygen coordination number of cations, compared to SiO4 network in silicate glasses, because much of the oxygen will be connecting two higher coordinated polyhedra. Nb-glass and La-glass also have similar arrangements of oxygen ions around Nb5+ and La3+. The dominant NbOn polyhedra in the RMC models are NbO4 (13%), NbO5 (48%), and NbO6 (38%) for Nb-glass (NNb−O = 5.3), and NbO4 (22%), NbO5 (51%), and NbO6 (27%) for La-glass (NNb−O = 5.0), respectively. The dominant LaOx polyhedra are LaO7 (21%), LaO8 (34%), LaO9 (23%) for Nb-glass (NLa−O = 8.0), and LaO7 (26%), LaO8 (33%), LaO9 (19%) for La-glass (NLa−O = 7.7), respectively. The distribution of structural units is almost the same in the two glasses (Figure S2(a,b), Supporting Information). In contrast, the arrangement of cations around oxygen is not similar in the two glasses as shown in Figure S2(c,d), Supporting Information. The oxygencentered coordination numbers, NO−Nb and NO−La are 1.68 and 1.09 for Nb-glass and 1.06 and 2.44 for La-glass, respectively. The total cation coordination number around oxygen is 2.77 for Nb-glass and 3.50 for La-glass. These values and the distribution of the coordination number indicate that there is difference in the environment around oxygen between Nb-glass and La-glass, which may be seen in the linkage between the cation-oxygen polyhedra. Distinct structural features between the two glasses emerge when the NbOn polyhedron shape and the linkage between the NbOn polyhedra is examined. The bond angle distributions and the partial pair distribution functions related to the NbOn 3058



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continuous niobate network, although 100% of La−O bonds are part of a continuous lanthana network. The connectivity analysis shows there is almost no difference in the LaOx network between the two glasses (yellow-colored units in Figure 5), even though the La content of La-glass is twice that

polyhedra are shown in Figure 4. For the dense packed structure of oxygen ions in these glasses, the packing state

Figure 4. (A) Bond angle distributions and (B) partial pair distribution functions, gij(r), relating to NbOn polyhedra, for the RMC models of La2O3−Nb2O5 glass. Blue, Nb-glass; red, La-glass.

becomes close to the oxygen close-packed structure for which there are both octahedral and tetrahedral cation sites.35 The O−N̂ b−O bond angle distributions have two prominent peaks at 82° and 174° for Nb-glass and 86° and 174° for La-glass, respectively, close to the angles for a regular octahedron (90° and 180°), which suggests that Nb occupy nearly octahedral sites. Thus, NbO4 and NbO5 polyhedra have the geometry of a NbO6 octahedron with one or two vacant vertices. The first peak of the Nb−O partial pair distribution function is symmetric and narrow for La-glass, but asymmetric and extended to longer distance for Nb-glass. The Nb−Ô −Nb bond angle distributions exhibit a sharp peak at 76° due to the occurrence of edge-sharing between NbOn polyhedra, whereas corner-sharing leads to a broad peak at about 135°. The contribution due to edge-sharing is clearly enhanced in La-glass. In the Nb−Nb partial pair distribution function, the first peak (at ∼2.5 Å) corresponds to edge-shared NbOn polyhedra, while the second peak (at ∼3.8 Å) corresponds to corner shared NbOn polyhedra. Thus gNbNb(r) also shows the predominance of edge-shared NbOn polyhedra in La-glass, but not in Nb-glass. These results demonstrate that, although the fraction of NbOn polyhedra and the bond angles within the NbOn polyhedra are almost the same in both glasses, there is a major difference in the linkage pattern of the NbOn polyhedra. This difference in the local structure may be the cause of the packing density difference. In contrast, the bond angle distributions and partial pair distribution functions related to the LaOx polyhedra are almost the same for the two glasses, as shown in Figure S3, Supporting Information. The network formation of NbOn and LaOx polyhedra at intermediate range may be investigated from a connectivity analysis24,25 of the RMC models. For Nb-glass, 100% of the Nb−O bonds are part of a continuous niobate (−O−Nb−O− Nb−O−) network, and 99% of the La−O bonds are part of a continuous lanthana (−O−La−O−La−O−) network. For Laglass, on the other hand, only 55% of Nb−O bonds are part of a

Figure 5. Three-dimensional atomic configurations derived from the RMC models. (A) Nb-glass, (B) La-glass. Polyhedra belonging to a continuous network are shown in the same color. It is notable that LaOx polyhedra (yellow) form a continuous network in both glasses, but this is not the case for NbOn polyhedra. The dark blue-colored part is not a continuous network and consists of up to 70 atoms.

of Nb-glass. This is because the ionic radius of La3+ is large and comparable to that of O2−, leading to large oxygen coordination numbers, so that it is easy for La3+ and O2− to make contact in both glasses. In contrast, the intermediate structure around NbOn polyhedra is quite different in the two glasses. In Nbglass, the NbOn polyhedra are distributed homogeneously and form a uniform network by corner-sharing connections between the NbOn polyhedra (see the bottom of Figure 5A). On the other hand, in La-glass, the network of NbOn is partly discontinuous and small-sized aggregations of edge-shared NbOn are surrounded by the LaOx network (see Figure 5B). The signature of the inhomogeneous distribution of NbOn polyhedra in Nb-glass is a prepeak at Q ∼ 0.85 Å−1 in the Nb− Nb partial structure factor derived from the RMC model (shown in Figure S4, Supporting Information). For the conventional MgO−SiO2 glass system, the connections between polyhedra are dominated by corner-sharing for all compositions, even though the topological network of SiO4 polyhedra is steadily broken down and isolated SiO4 and Si2O7 dimers are formed with increasing of MgO content.36 In contrast, the La2O3−Nb2O5 glass system is very different, with two different regions of glass formation and two very different 3059



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proposed that in La-glass, the NbOn polyhedra connect to each other by both corner-sharing and edge-sharing, forming small network clusters within the LaOx network. On the other hand, in Nb-glass, the NbOn polyhedra are mainly connected to each other by corner-sharing, forming an extended, continuous NbOn network. Finally, another key finding of this study is to reveal that there are unexploited material spaces for glass science at the lower left in the periodic table, which is opposite side in the table for the network former oxide. The results indicate the possibility of making innovative functional glasses with new composition by increasing the ionicity of constituent heavy elements. Now we can make a new “ionic” glass system with superior properties in the new material spaces, which opens up a new stage in glass science.

types of structure in these two regions, with a different shape of the NbOn polyhedra, different linkage of the NbOn pairs, and different homogeneity of the distribution of NbOn. In other words, two structurally different types of glass formation make it possible that both La-rich and Nb-rich compositions vitrify. The Raman spectra of these glasses (Figure 6) depend on the connectivity of the network. These spectra show a distinct

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental results as discussed in the text (Tables S1; Figures S1−S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 6. Raman spectra of La2O3−Nb2O5 glasses. Blue, Nb-glass; red, La-glass.

Notes

The authors declare no competing financial interest.

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difference above 500 cm for spectroscopic features related to Nb−O bond vibrations. Two bands at 797 and 640 cm−1 were observed for Nb-glass, while only one sharp symmetric band at 767 cm−1 was observed for La-glass. In Nb-glass, the lowfrequency 640 cm−1 feature corresponds to vibrations of NbOn polyhedra in the three-dimensional NbOn network while the 797 cm−1 band corresponds to NbOn polyhedra, which connect not only to NbOn polyhedra but also to LaOx polyhedra. The Raman spectra of niobium phosphate glasses have been explained in a similar way, and this interpretation is supported by 93Nb NMR.37 Thus, the absence of the 640 cm−1 for Laglass means lack of the continuous network of NbOn. The 767 cm−1 feature in the La-glass has similar origin as the 797 cm−1 band in the Nb-glass. The higher wavenumber of the band of the Nb-glass is due to the more distorted NbOn polyhedra. The relationship between NbOn polyhedra distortion and the shift to higher frequencies is well-documented for glassy and crystalline niobium oxides.26,38,39 As a result, the microscopic vibration properties were consistently explained in atomic level using the RMC model structures.

ACKNOWLEDGMENTS The synchrotron radiation experiment was carried out with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2010A1316 and 2011B1233). This study was supported in part by the Nippon Sheet Glass Foundation, Iketani Science and Technology Foundation, Tokuyama Science Foundation, and Grants-in-Aid for Young Scientists (B) (19750174 and 23750236) and for Scientific Research (C) (21550185 and 25410236) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The Science and Technology Facilities Council is thanked for the provision of neutron beam time.

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REFERENCES

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CONCLUSION In summary, we found some remarkable features of network former free La2O3−Nb2O5 glass: (i) The glass forming region is divided into La rich and Nb rich regions. (ii) The oxygens are highly ionic and densely packed, and this is a factor that contributes to the high refractive index. Note that the oxygen packing density is different for La-glass and Nb-glass. (iii) There are mainly 5-fold coordinated Nb ions in both glasses. (iv) The Nb-glass is much denser than the La-glass even though the Nb-glass is formed by corner-shared NbOn polyhedra. This is in contrast with the classical picture of glass. A network, such as a silicate glass, which consists of corner-shared polyhedra, requires many volume cavities. The unusual features compared to conventional oxide glass systems will require the new kind of glass structure model. It was 3060



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Drastic Connectivity Change in High Refractive Index Lanthanum

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