In the past decades, much attention has been paid to aluminate glasses, such as aluminoborate, aluminosilicate, or aluminoborosilicate glass, because the addition of α-alumina to oxide glasses results in high chemical stability [1] and/or ideal mechanical properties [2] . In such oxide glasses, three kinds of Al coordination exist: 4-coordinated ^[4]Al, 5-coordinated ^[5]Al, and 6-coordinated ^[6]Al, which can all be quantitatively measured by magic angle spinning nuclear magnetic resonance (MAS NMR) [3] [4] . Using structural analysis of aluminate glasses using MAS NMR, it has been clarified that their physical properties, such as fictive temperature, microhardness, elastic modulus, and refractive index, are closely related to their Al coordination [5] [6] [7] [8] . Therefore, it is important to control Al coordination in oxide glasses for adjustments of those physical properties.

In earlier reports, the relationship between the addition of network modifiers and generation of aluminum species has been reported. In particular, rare-earth ions (RE³⁺) such as Sc³⁺, Y³⁺_, or La³⁺ could produce higher-coordinated Al species (^[5]Al and ^[6]Al) because they exhibit a sufficiently large cation field strength of CFS = z/R²_, where z is the ionic valence and R is the ionic radius [9] [10] [11] [12] [13] . Despite this, the glass-forming compositions of rare-earth-containing glasses were limited by high melting points of starting materials such as Y₂O₃ or Al₂O₃ [14] [15] [16] . Hence, control of ^[5]Al and ^[6]Al formation was still difficult, and one of the few ways to control Al speciation was by using different ions (e.g., the CFS of Mg²⁺ is lower than that of rare-earth ions, which suppressed the formation of ^[5]Al and ^[6]Al). Considering the development of functional glasses, it is important to investigate the relation between more complex compositions, Al coordination, and physical properties, because commercially used glass for optical or building applications contains several elements. Therefore, it would be interesting if, even in glasses that contain four or five elements, rare-earth ions can produce higher coordinated Al species that affect the physical properties of the glass.

We considered that the addition of Na⁺ to aluminoborate glasses that contain several RE³⁺ species could expand the glass formation region and generate the intended amounts of ^[5]Al and ^[6]Al, proportional to the ratio of RE³⁺ to Al₂O₃. Although NMR analysis of various aluminoborate glasses has been performed (e.g., Na₂O-B₂O₃-Al₂O₃, Y₂O₃-B₂O₃-Al₂O₃, or La₂O₃-B₂O₃-Al₂O₃) [17] - [24] , most of these analyses have been done with mono-network modifiers. Herein, we structurally analyzed yttrium sodium aluminoborate and lanthanum sodium aluminoborate glasses in order to observe the relationship between the addition of RE³⁺ and the formation of ^[5]Al and ^[6]Al. Simultaneously, we investigated the local structure of Na⁺ in order to understand its role in these glasses. Finally, we measured the molar volumes in these glasses in order to understand the direct relationship between the formed amounts of ^[5]Al and ^[6]Al and the physical properties of the glasses. As a result of the above experiment, the structural roles of RE³⁺, Na⁺ and Al³⁺ in quaternary aluminoborate glass have been clarified.

The ²⁷Al MAS NMR spectra clearly indicated that the fractions of ^[5]Al and ^[6]Al increased in response to an increase in the ratio of Y₂O₃ to Al₂O₃ (Figure 1 and Figure 16). This result strongly suggests that Y³⁺ can produce higher-coordi- nated Al species despite the presence of Na⁺. It was thus considered that the CFS of Y³⁺ is sufficiently larger than that of Na⁺ to result in formation of ^[5]Al and ^[6]Al.

The relative amounts of ^[5]Al and ^[6]Al in Y³⁺-containing glasses were larger than those in La³⁺-containing glasses when the ratio of La₂O₃ to Al₂O₃ increased (Figure 2 and Figure 17). It therefore seems that the difference in CFS between Y³⁺ and La³⁺ affected the formation of ^[5]Al and ^[6]Al. In previous studies [9] [10] [11] [12] [13] , a greater CFS value resulted in a greater fraction of higher-coordinated Al in oxide glasses that contain RE³⁺ such as yttrium aluminosilicate or aluminoborate. As such, the above results further clarify that rare-earth ions with a large CFS can result in ^[5]Al and ^[6]Al even with co-existing Na⁺. It was considered that most of RE³⁺ produced higher coordinated Al regardless of its composition because the combination of large CFS cation and higher coordinated Al could be energetically preferable [13] .

Boron coordination is also important for elucidation of the local structure in this glass. Although we have measured ¹¹B MAS NMR spectra, it was difficult to complete peak deconvolution because the broad background components derived

from instruments overlapped the corresponding spectrum of glass. Thus, we estimate the fraction of four-coordinated boron (^[4]B) in this glass system from the literatures. According to the earlier reports [24] , 2Y₂O₃-5B₂O₃-3Al₂O₃ glass similar composition to our glass contains 12% of ^[4]B. In the case of 2.5La₂O₃-5B₂O₃- 2.5Al₂O₃, the amount of ^[4]B is about 11% [15] . 4Na₂O-3B₂O₃-3Al₂O₃ and 5Na₂O- 3.5B₂O₃-1.5Al₂O₃ glass were reported to contain 8.3% and 14.6% of ^[4]B, respectively [27] . In addition, according to Chakraborty and Day [28] , it was suggested that Al/B ratio is important for the nature of ^[3]B/^[4]B ratio because ^[4]B decreases with being replaced by larger ^[4]Al sites. In this glass system, Al/B ratio is fixed at 0.6. Therefore, considering the above data and suggestion, it is assumed that amount of ^[4]B did not greatly changed in respect to the composition in all glass system. The amount of ^[4]B in the synthesized glass can be estimated as about 10% ± 5%.

The lower-field component of the ²³Na spectra of yttrium sodium aluminoborate glasses steadily decreased with respect to the increase in Y³⁺ (Figure 20). According to the ²³Na spectra of crystalline Na₂Al₂B₂O₇ (Figure 19), the peak attributed to 9-coordinated Na⁺ (2) near Al is located upfield from that attributed to 6-coordinated Na⁺ (1) near B. Usually, the peaks for higher-coordinated ions in simple glasses, such as silicate, borate, and phosphate glasses, are located in the upper-field of the NMR spectra [29] . Therefore, it is also reasonable to observe the peak for higher coordinated Na⁺ in the upfield. However, for complex crystals or glasses that contain more than two network-forming oxides, it should be considered that the electron density of coordinated oxygen varies with respect to the neighboring element, such as Al³⁺ or B³⁺. In this case, the chemical shift depends on the coordination number and the surrounding ions, such as Al³⁺ and B³⁺. Considering these assumptions and the previous studies [15] , the different chemical shifts for the Na⁺ (1) and Na⁺ (2) peaks were assumed to be affected by the coordination number and conjunctive Al³⁺ and B³⁺. This implies that a larger chemical shift in the Na⁺ spectrum corresponds to an increase in the coordination number or more Al surrounding Na⁺. In light of these considerations, the coordination number of Na⁺ in each glass may have gradually increased following the increase in the number of Y³⁺, and the elements surrounding Na⁺ might have affected the change from B to Al. As shown in Figure 1, the addition of Y³⁺ ions produced higher coordinated Al species, and these Al species locally concentrated the oxygen ions. This concentrated oxygen result in an increase in the coordination number of Na⁺. In addition, the ²³Na spectra of the La³⁺ glasses also shifted upfield as the La³⁺ content increased. This suggests that the above environmental change of Na⁺ also occurred in the La³⁺-containing glass. The difference of boron coordination is also considered to affect the oxygen concentration in the local structure and consequently the chemical shift of ²³Na NMR spectra of the glasses. However, as we discussed above, it was assumed that the amount of ^[4]B ratio was almost constant around 10%. Therefore, it is expected that the chemical shift of ²³Na NMR spectra was not affected by the boron coordination.

The relationship between the Al coordination state and physical properties in these glasses could be deduced because the fractions of ^[5]Al and ^[6]Al changed in response to the composition change, as shown above. In order to confirm this relationship, molar volume measurements were conducted, because the local structural change of Al coordination should directly affect spatial arrangement. The molar volumes steadily decreased in response to an increase in Y³⁺ content (Figure 22), which is caused by various effects. Firstly, rare-earth ions compensate negative charges of Al or B at a closer radius than Na⁺, because they possess a much larger CFS than Na⁺. Secondly, the production of higher-coordinated Al contributed to a decrease in molar volume because at higher coordination states of Al, atomic packing improves. This expectation can be clarified by comparing the molar volumes and fractions of various Al species in both Y³⁺- and La³⁺- containing glasses. When the compositions were (x, y) = (6, 0), (4.8, 0.4), and (3.96, 0.66), the summation of ^[5]Al and ^[6]Al and molar volume in Y³⁺-containing glasses were not very different from those in La³⁺-containing glasses (Figure 22, Table 3, and Table 4). However, when more rare-earth ions were present, as in the compositions (x, y) = (3, 1), (1.98, 1.32), (1.2, 1.6), and (0, 2), the summation of ^[5]Al and ^[6]Al in Y³⁺-containing glasses became larger than that of La³⁺-con- taining glass, while the molar volume of Y³⁺-containing glasses became smaller. This comparison further proves that ^[5]Al and ^[6]Al lowered the molar volume of the glasses. According to Shannon [30] , the ionic radius of Y³⁺ is 1.04 Å and that of La³⁺ 1.17 Å in 6-coordinated state. This difference may also affect the molar volume. However, there have been several reports concerning the relation between the molar volume and Al coordination [31] [32] . Therefore, Al coordination in this glass system must be one of the key-factor that affects the molar volume. In addition, it is expected that boron coordination number affect the molar volume. However, it was considered that boron coordination in our glass gave little influence on the molar volume, because the amount of ^[4]B in the samples were estimated to be almost constant as described.

Despite the above results, it remains challenging to elucidate the effects of ^[5]Al and ^[6]Al in oxide glass on certain physical properties, because other factors may also affect the latter. Therefore, we plan to investigate the effects of ^[5]Al and ^[6]Al on physical and optical properties, including fictive temperature, the elastic modulus, refractive index, and photoluminescence, of the abovementioned glass system.

We here performed the structural analysis of yttrium sodium aluminoborate and lanthanum sodium aluminoborate glasses using magic angle spinning NMR. We elucidated that the addition of rare-earth ions (RE³⁺) could result in higher- coordinated Al species (^[5]Al and ^[6]Al), and that their amounts were proportional to the ratio of Y₂O₃ to Al₂O₃. Furthermore, the difference in cation field strength (CFS) between Y³⁺ and La³⁺ was confirmed to affect the generation of higher- coordinated Al. The CFS of rare-earth ions (Y³⁺ and La³⁺) was sufficiently larger than that of Na⁺ to make higher-coordinated Al without being affected by Na⁺. It was also found that the coordination number of Na⁺ in each glass gradually increased following the increase in the number of RE³⁺ ions. Further analysis such as ¹¹B MAS NMR or Soft X-ray spectroscopy for Na⁺ can elucidate more detail about network structure or coordination environment, respectively.

In addition to the above results, it seems that the formation of ^[5]Al and ^[6]Al affected the physical properties (molar volume) due to better atomic packing with these higher-coordinated Al species. In order to clarify the relation between Al coordination number and physical properties in this glass system, further studies about fictive temperature or elastic module will be performed.

Although the unique affinity between RE³⁺ and Al in simple glasses has previously been reported, in this study, it was found that the same particular properties of RE³⁺ exist even in complex glasses. These results are expected to steadily lead to the development of ideal optical or building glass materials.

This study was carried out with the NMR spectrometer in the JURC at Institute for Chemical Research, Kyoto University. This work was supported by JSPS KAKENHI (grant number JP16K07640), Nippon Sheet Glass Foundation for Materials Science and Engineering, and Research Institute for Sustainable Humanosphere, Kyoto University.

Kaneko, S., Tokuda, Y. and Masai, H. (2017) Additive Effects of Rare-Earth Ions in Sodium Aluminoborate Glasses Using ²³Na and ²⁷Al Magic Angle Spinning Nuclear Magnetic Resonance. New Journal of Glass and Ceramics, 7, 58-76. https://doi.org/10.4236/njgc.2017.73006