Additive Effects of Rare-Earth Ions in Sodium Aluminoborate Glasses Using 23 Na and 27 Al Magic Angle Spinning Nuclear Magnetic Resonance

We conducted structural analysis of xNa2O-yY2O3-5B2O3-3Al2O3 and xNa2OyLa2O3-5B2O3-3Al2O3 glasses to elucidate the additive effects of rare-earth ions in these sodium aluminoborate glasses, and investigated the local environment surrounding Na in them by using Na and Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The amount of highercoordinated Al species (Al and Al) gradually increased in response to an increase in the ratios of Y2O3 to Al2O3 and La2O3 to Al2O3 in each type of glass, respectively. Moreover, the difference in the cation field strength (CFS) between Y and La was observed to affect the generation of Al and Al, especially when the amount of these ions in the glasses increased. In addition to the above, the coordination number of Na ions increased with an increase in the number of rare earth ions, confirmed by comparing results with NMR spectra of crystalline Na2Al2B2O7. The latter possibly occurred due to the oxygen concentration on Al and Al. Finally, it was confirmed that the formation of Al and Al decreases molar volume in oxide glasses, which might be partially due to better atomic packing of Al and Al.


Introduction
In the past decades, much attention has been paid to aluminate glasses, such as How to cite this paper: Kaneko, S., Tokuda, Y. and Masai, H. (2017) Additive Effects of Rare-Earth Ions in Sodium Aluminoborate Glasses Using 23 Na and 27 Al Magic Angle Spinning Nuclear Magnetic Resonance.New Journal of Glass and Ceramics, 7, 58-76.https://doi.org/10.4236/njgc.2017.73006aluminoborate, 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 (RE 3+ ) such as Sc 3+ , Y 3+  , or La 3+ could produce higher-coordinated Al species ( [5] Al and [6] Al) because they exhibit a sufficiently large cation field strength of CFS = z/R 2  , 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 Y 2 O 3 or Al 2 O 3 [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 Mg 2+ 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 RE 3+ species could expand the glass formation region and generate the intended amounts of [5] Al and [6] Al, proportional to the ratio of RE 3+ to Al 2 O 3 .
Although NMR analysis of various aluminoborate glasses has been performed , 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 RE 3+ 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 RE 3+ , Na + and Al 3+ in quaternary aluminoborate glass have been clarified.1 and Table 2.Although we tried to analyze the composition of the synthesized glass with inductively-coupled plasma atomic emission spectrometry and X-ray Fluorescence, the detection accuracy of these instruments for boron was too low to measure the composition of all samples with high accuracy.However, T g of our glasses showed the additivity.Hence, the compositions of all glasses are given as batch compositions in this report (Table 1 and Table 2).

Preparation of Crystalline Na2Al2B2O7
Crystalline Na 2 Al 2 B 2 O 7 was synthesized to confirm the relationship between the local structure of Na + at different sites in the crystal and the chemical shifts of 23 Na NMR spectra.The crystal sample was prepared using a previously-reported solid state reaction [25].Briefly, a stoichiometric mixture of NaHCO 3 , B(OH) 3 , and Al 2 O 3 was crushed thoroughly in an agate mortar, calcined at 400˚C for 10 h, and finally heated at 950˚C for 48 h.

Characterization of Crystal Samples
Synthesized samples were analyzed using X-ray diffraction (XRD, Rigaku, RINT-2100, Tokyo, Japan).In addition, the XRD patterns were also simulated with known crystallographic information [25] using the Mercury software.

NMR Spectroscopy
Solid-state 23 Na and 27 Al MAS NMR spectra of all crystal and glass samples were acquired on an AVANCE III spectrometer (Bruker, Billerica, MA) using a commercial probe (4 mm).The rotation speed was set to 15 kHz with an accuracy of ± 1 Hz.Under an external field of 18.8 T, the resonance frequencies for 23 Na and 27 Al were near 212 and 208 MHz, respectively.Each measurement was conducted using single-pulse sequence.90˚ pulses were set to 7 μs for 23 Na.The complete relaxation was confirmed in this condition.In addition, in the case of 27 Al, 0.63 μs pulse which is corresponding to a radiofrequency tip angle 24˚ was applied to ensure quantitative measurement of Al spices.Spectra were obtained with a cycle time of 2 s for both 23 Na and 27 Al.Aqueous solutions of 1 M NaCl and 1 M Al(NO 3 ) 3 were used as references, with their chemical shifts set to 0 ppm.The 23 Na spectra were normalized, so that the total area in each spectrum is proportional to the alkali content.This way, the areas of the spectra can be compared with each other.

Density
The Archimedes method was used for density measurements of synthesized glasses and to calculate molar volumes.

27 Al MAS NMR
The 27 Al MAS NMR spectra of yttrium sodium aluminoborate glasses and lanthanum sodium aluminoborate glasses are shown in Figure 1 and Figure 2, respectively.The intensity of the [4] Al peaks in these spectra were set to be same.
The peak of [4] Al gradually shifted upfield in response to the increase of Y 3+ .In order to fit the experimental spectra, gaussian functions were used.We considered that the magnetic field of MAS NMR was strong (18.8T) enough to neglect quadrupolar shift and broadening effects.The error of the total fitting curve for [4] Al, [5] Al and [6] Al was calculated to be 1% -6% in all glasses.

XRD Pattern and 23 Na MAS NMR Spectrum of Crystalline Na2Al2B2O7
The XRD pattern of synthesized crystalline Na 2 Al 2 B 2 O 7 and its pattern simulated by Mercury [25] are shown in Figure 18, while the 23 Na NMR spectrum of crystalline Na 2 Al 2 B 2 O 7 is shown in Figure 19.Considering the 1:1 ratio of two sodium ion sites in crystalline Na 2 Al 2 B 2 O 7 , the peak area ratio should be 1.Nonetheless, there is a different crystal peak present in the lower field.However, according to a previous detailed study [26], the peak at the lower chemical shift can also be assigned to the Na + (2) site that is 9-coordinated with oxygen in the vicinity of Al, and the peak at the higher chemical shift can be assigned to the Na + (1) site that is 6-coordinated with oxygen in the vicinity of B (Figure 19).

23 Na MAS NMR
The 23 Na MAS NMR spectra of yttrium sodium aluminoborate glasses and lanthanum sodium aluminoborate glasses are shown in Figure 20 and Figure 21, respectively.The peak of the 23 Na spectra of Y 3+ -containing glasses gradually shifted upfield in response to an increase of Y 3+ .The peak shifted from −2.3 ppm at (x, y) = (6, 0) to −9.5 ppm at (x, y) = (1.2,1.6).In the case of La 3+ -containing glasses, a similar upfield spectral shift was observed.The peak shifted from ppm

Molar Volume
The molar volumes of yttrium sodium aluminoborate and lanthanum sodium aluminoborate glasses are shown in Figure 22.As the RE 3+ to Al 2 O 3 ratio increased, the molar volume of the glasses steadily decreased.

Discussion
The 27 Al MAS NMR spectra clearly indicated that the fractions of [5] Al and [6] Al increased in response to an increase in the ratio of Y 2 O 3 to Al 2 O 3 (Figure 1 and Figure 16).This result strongly suggests that Y 3+ can produce higher-coordinated Al species despite the presence of Na + .It was thus considered that the CFS of Y 3+ 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 Y 3+ -containing glasses were larger than those in La 3+ -containing glasses when the ratio of La 2 O 3 to Al 2 O 3 increased (Figure 2 and Figure 17).It therefore seems that the difference in CFS between Y 3+ and La 3+ 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 RE 3+ 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 RE 3+ 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 11 B 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], 2Y 2 O 3 -5B 2 O 3 -3Al 2 O 3 glass similar composition to our glass contains 12% of [4] B. In the case of 2.5La 2 O 3 -5B 2 O 3 -2.5Al 2 O 3 , the amount of [4] B is about 11% [15].4Na 2 O-3B 2 O 3 -3Al 2 O 3 and 5Na 2 O-3.5B 2 O 3 -1.5Al 2 O 3 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 23 Na spectra of yttrium sodium aluminoborate glasses steadily decreased with respect to the increase in Y 3+ (Figure 20).
According to the 23 Na spectra of crystalline Na 2 Al 2 B 2 O 7 (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 Al 3+ or B 3+ .In this case, the chemical shift depends on the coordination number and the surrounding ions, such as Al 3+ and B 3+ .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 Al 3+ and B 3+ .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 Y 3+ , and the elements surrounding Na + might have affected the change from B to Al.As shown in Figure 1, the addition of Y 3+ 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 23 Na spectra of the La 3+ glasses also shifted upfield as the La 3+ content increased.This suggests that the above environmental change of Na + also occurred in the La 3+ -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 23 Na 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 23 Na 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 Y 3+ 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 Y 3+ -and La 3+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 Y 3+ -containing glasses were not very different from those in La 3+ -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 Y 3+ -containing glasses became larger than that of La 3+ -containing glass, while the molar volume of Y 3+ -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 Y 3+ is 1.04 Å and that of La 3+ 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.

Conclusions
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 (RE 3+ ) could result in highercoordinated Al species ( [5] Al and [6] Al), and that their amounts were proportional to the ratio of Y 2 O 3 to Al 2 O 3 .Furthermore, the difference in cation field strength (CFS) between Y 3+ and La 3+ was confirmed to affect the generation of highercoordinated Al.The CFS of rare-earth ions (Y 3+ and La 3+ ) 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 RE 3+ ions.Further analysis such as 11 B 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 RE 3+ and Al in simple glasses has previously been reported, in this study, it was found that the same particular properties of RE 3+ exist even in complex glasses.These results are expected to steadily lead to the development of ideal optical or building glass materials.

Figure 18 .
Figure 18.X-ray diffraction (XRD) pattern of crystalline Na 2 Al 2 B 2 O 7 .The XRD pattern of standard crystalline Na 2 Al 2 B 2 O 7 obtained from simulation using Mercury software is also provided as reference.

Table 2
a uncertainty in T g is ±1˚C.