ARTICLE pubs.acs.org/JPCA
Ligand Field Modification around Cu2þ Ions in Sodium Borate Glass by Codoping Fuji Funabiki,*,† Satoru Matsuishi,† and Hideo Hosono†,‡ † ‡
Frontier Research Center, Tokyo Institute of Technology, Nagatuta-cho, Midori-ku, Yokohama 226-8503, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatuta-cho, Midori-ku, Yokohama 226-8503, Japan ABSTRACT: Understanding the effect of codoping on the properties of photonic glasses is important for improving their properties. The effect of codoping on the ligand field around Cu2þ ions in a sodium borate glass is examined using optical absorption spectroscopy, continuous-wave electron paramagnetic resonance, and three-pulse electron-spin-echo envelopemodulation. Glass with a composition of 0.1CuO 3 5Na2O 3 95 B2O3 was codoped with 2 mol % of Al3þ, Si4þ, P5þ, Zr4þ, or La3þ oxide. Three codoping effects are found: strengthening the ligand field, as observed for Zr-codoping, which induces a large blue shift of the optical absorption peak of Cu2þ; weakening the ligand field, as observed for P-codoping, which causes a red shift of the Cu2þ absorption peak; and almost no effect on the ligand field, which is observed for Al-, Si-, and La-codoping. Coordination structure models based on local charge neutrality are proposed for the codoped glasses. The mechanism of the codoping effect is revealed by elucidating the local structure around Cu2þ.
’ INTRODUCTION Recent development of the optical networking market is a result of innovation in organic and inorganic materials possessing optical functions, such as photoemission, luminescence, light guidance, and photo acceptance. Metal complexes, in which a transition metal or a rare earth ion is coordinated by organic or inorganic ligands, are important photonic materials. Their optical functions are associated with electron transitions in the d- or f-orbitals of the central metal ion. The electronic state of the ion is dependent on the surrounding ligands; their influence is called the ligand field. Because inorganic glasses of B2O3, SiO2, and P2O5 have a three-dimensional polymeric network structure, they can include metal ions in interstitial sites of the structure. That is, the structural units of triangular BO3, tetrahedral SiO4, and tetrahedral PO4 can behave as ligands. Examples of applications for glassy materials containing photoactive ions include color filters, long-lasting luminescent glasses,1 and optical fiber amplifiers.2 The relationship between the optical properties of a glass and its ligand field has been investigated using continuous wave electron paramagnetic resonance (CW-EPR).35 This method is a powerful tool for obtaining information about the neighboring environments of photoactive ions in glass that does not possess long-range order. Electron paramagnetic resonance (EPR) measurements on Cu2þ-doped B2O3 glass revealed the relationship between the optical absorption of Cu2þ and the coordination structure around Cu2þ.3,5 When a large amount of Na2O (55 mol %) was added to the glass, the structure around Cu2þ changed from a rigid network structure composed of BØ3 and BØ4 units to a structure of BØ3 and BØ2O units. Simultaneously, r 2011 American Chemical Society
the optical absorption peak shifted from 782 to 660 nm (Figure 1). The electron-spin-echo envelope-modulation (ESEEM) technique of pulsed EPR spectroscopy gives direct information about the atomic arrangement of second neighboring ions, that is, cations not adjacent to the metal center.610 For example, ESEEM spectroscopy was used to reveal the relationship between the luminescence of Ce3þ and its coordination structure in Ce3þdoped SiO2 glass.9 When a small amount (1 mol %) of P2O5 was incorporated into the glass, SiO4 units surrounding Ce3þ were exchanged with PO4 units. Simultaneously, the luminescence peak shifted from 410 to 350 nm. It is evident that the optical properties of a glass are closely related to the ligand field, and that the addition of a third component (e.g., Na2O or P2O5) changes the coordination environment around the photoactive ions either indirectly or directly. That is, codopants can be used to control the ligand field around photoactive ions, allowing further development of photonic glasses. However, few systematic studies on the effects of codoping have been reported,11 and the combination of photoactive ions with codopants has not been optimized. In this study, the effect of codoping on photonic glass is investigated by examining the influence of a range of codopants on the ligand field. The glass 0.1CuO 3 5Na2O 3 95B2O3, in which Cu2þ ions are coordinated by BØ3 and BØ4 units,1214 is codoped with 2 mol % of AlO3/2, SiO2, PO5/2, ZrO2, or LaO3/ 2. These codopant cations possess various valence charges and oxygen coordination numbers, so it is likely that they will induce Received: February 8, 2011 Revised: April 18, 2011 Published: April 28, 2011 5081
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Figure 2. Optical absorption spectra of a parent glass consisting of 0.1CuO 3 5Na2O 3 95B2O3 and glasses codoped with Al, Si, P, Zr, or La. )
(A^ and A ) were determined using the simulation program EasySpin.10 Three-pulse ESEEM was performed to obtain quantitative information about the type and number of ligands around Cu2þ using a pulsed EPR spectrometer (BRUKER-E580, Bruker, Germany) equipped with a dielectric resonator in a liquid helium cryostat. The sample was sealed in a silica glass tube, inserted into the resonator, and irradiated with pulsed microwaves with a frequency of 9.7 GHz. The strength of the static magnetic field applied to the samples was set at around 275 mT. To obtain a three-pulse ESEEM spectrum, the pulse sequence was set to be (π/2)-τ-(π/2)-T-(π/2)-τ-(echo). The electron spin echo was recorded at 30 K as a function of pulse interval T with a fixed pulse interval τ. Obtained ESEEM spectra were simulated using EasySpin. In the simulation, the isotropic hyperfine coupling constant (Aihf), anisotropic hyperfine coupling constant (Aahf), nuclear quadrupole coupling constant (e2qQ/h), and asymmetry parameter (η) were used as fitting parameters.
Figure 1. Optical absorption peaks of Cu2þ contained in binary oxide glasses taken from previous studies.35 Arrows indicate the codoping effects found in this work.
new local structures in the glass, and provide new environments for Cu2þ ions. Such an intermediate range structure around Cu2þ is detected employing pulsed EPR spectroscopy. It is known that Na2OB2O3 glass system has subliquidus immiscibility. We thus need to pay attention to the change in the immiscibility dome by codopants. The electronic state and coordination structure around Cu2þ ions in the codoped glasses are analyzed using optical absorption, EPR, and ESEEM spectroscopies, and the mechanism of the codoping effect is discussed based on the information obtained.
’ EXPERIMENTAL METHODS 2þ
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Sample Preparation. A sodium borate glass containing Cu ions (parent glass) and glasses codoped with Al3þ, Si4þ, P5þ, Zr4þ, or La3þ were prepared using a melt-quenching technique. The target composition of each glass was 0.1CuO 3 5Na2O 3 95 B2O3 þ 2MOx in mol %. Each batch of 30 g consisting of reagent grade CuO, Na2CO3, and H3BO3, and Al2O3, SiO2, P2O5, ZrO2, or La2O3 was mixed, and then melted in a platinum crucible under ambient atmosphere at 1373 K for 2 h. The melts were poured onto a metal plate, rapidly cooled to room temperature, and then stored under paraffin oil to prevent hydration. Plates with a thickness of 2 mm were cut from the obtained glasses and mechanically polished. Their optical absorption spectra were acquired on a Hitachi U-4000 UVvisNIR spectrometer (Hitachi, Japan) at room temperature. Rods with a diameter of 3 mm and length of 7 mm were also prepared for EPR experiments. EPR and Electron Spin Echo Measurements. Information about the percentage of Cu2þ out of the total copper present, the bonding character between Cu2þ and oxygen, and the experimental conditions for ESEEM spectroscopy was obtained using EPR spectroscopy. The EPR signal at 77 K was recorded on a BRUKER-EMX CW-EPR spectrometer (Bruker, Germany) at 9.6 GHz. The concentration of Cu2þ was estimated by comparing the integrated intensity with that of a standard sample of hydrated copper sulfate CuSO4 3 5H2O. The g-factors (g^ and g ) and hyperfine coupling constants
’ RESULTS Figure 2 shows the optical absorption spectra of the parent and codoped glasses. The glasses were either transparent green or transparent blue in color. Opaque glass, distinctly phaseseparated, was not observed. Intense, broad absorption bands observed at around 800 nm are caused by a Cu2þ 3d3d transition, and smaller bands observed at 1420 nm result from molecular vibration of hydroxyl impurities present in the glasses. The hydroxyl bond in BØ2OH unit decreases the viscosity of glass melt, and is difficult to remove. In contrast, CuOH is unlikely to exist in the melt because the thermal stability of Cu(OH)2 is lower than that of H3BO3. Therefore, influence of hydroxyl impurities on the ligand field around Cu2þ is not taken into consideration. The wavelength of the absorption peak of Cu2þ decreased slightly from 803 to 779 nm when the glass was codoped with Al, and significantly to 693 nm when Zr was the codopant. In contrast, it increased to 837 nm upon codoping with P, but remained unchanged when the glass was codoped with Si or La. The intensity of the absorption band decreased significantly upon codoping with Zr, but increased upon codoping with P or La. Figure 3 shows CW-EPR spectra obtained for the parent and codoped glasses. Parallel peaks with hyperfine structure from Cu2þ (S = 1/2, I = 3/2) were clearly observed at lower magnetic field. Perpendicular peaks observed at higher magnetic field were difficult to resolve, because of their small splitting and broadening, except for the spectrum of the glass codoped with 5082
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Figure 4. Measured and simulated EPR spectra of P- and Zr-codoped glasses.
Figure 3. EPR spectra of parent and codoped glasses. The three types of hyperfine structures detected are classified as I, II, and III.
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Zr. Although two sets of hyperfine structures (I and II) were detected in the spectra of the parent and Al-, Si-, and Lacodoped glasses (the proportion of structure II increases when Al is the codopant), the spectra of the glasses codoped with P or Zr both exhibited a single structure, which are designated I and III, respectively. In particular, the splitting caused by the two copper isotopes could be seen for the parallel peak at m = 3/2 in the spectrum of the Zr-codoped glass, and the spectrum closely resembled that of the [Cu(OH)4]2 complex.5 Figure 4 shows simulations of the hyperfine structures of the P- and Zrcodoped glasses. The EPR parameters of g^, g , |A^|, and |A | were estimated to be 2.062, 2.377, 0.0022 cm1, and 0.0142 cm1 for the P-codoped glass, and 2.056, 2.290, 0.0021 cm1, and 0.0188 cm1 for the Zr-codoped glass. The values of g and A changed significantly when the parent glass was codoped with Zr. The percentage of Cu2þ out of the total copper content in the parent and Al-, Si-, P-, Zr-, and La-codoped glasses were estimated to be 66%, 68%, 63%, 83%, 63%, and 78%, respectively. The intensity of the optical absorption of a glass (Figure 2) is proportional to the concentration of Cu2þ and the probability of the corresponding electronic transition. Accordingly, the optical absorption decreased in intensity when Zr was used as codopant, because the probability of electronic transition decreased as there was less reduction of Cu2þ to Cuþ. By contrast, the increased intensity of absorption from the P- and La-codoped glasses was caused by the oxidation of Cuþ to Cu2þ.
The electron spin echo emitted from a sample irradiated by pulsed microwaves is recorded under a static magnetic field. In this study, the strength of the applied magnetic field was set at the position of the parallel hyperfine peak for m = 3/2 (Figure 3), at around 270 mT. This is because other peaks do not make a contribution to it, and in addition, the echoes detected are restricted to ones from Cu complexes in which the molecular Z-axis is aligned with the vector of the magnetic field (Figure 5). Fourier transforms of the obtained ESEEM spectra of the parent and codoped glasses were performed to qualitatively analyze the ligands around Cu (Figure 6). The straight lines in the patterns represent the Larmor frequencies of the possible nuclei present around the Cu2þ ions. The presence of 10B was strongly detected in all of the patterns, and 11B and 23Na were detected in the Zrcodoped glass. Other nuclei, such as 27Al, 29Si, 31P, and 139La, could not be detected, because either they were absent around Cu2þ or only gave a small contribution to echo modulation. Although there was no obvious difference between the patterns of the parent and Al-, Si-, and La-codoped glasses, some differences could be seen in those of the P- and Zr-codoped glasses.
’ DISCUSSION Bonding Character between Cu2þ and Oxygen and Site Symmetry of Cu2þ. According to theoretical studies,15,16 the
frequency of the optical absorption peak corresponds to the transition energy between the ground state (B2g) and the first excited state (B1g). Here, B2g is assigned to the antibonding molecular orbital formed in the π bond between a dxy orbital of Cu2þ and a 2p orbital of oxygen, and B1g is assigned to the orbital formed from the σ bond between a dx2y2 orbital of Cu2þ and a 2p orbital of oxygen (Figure 5). Therefore, the ligand field around Cu2þ was strengthened by codoping with Zr, and conversely, it was weakened by codoping with P. The following relative equations describe the relationship between EPR parameters and 5083
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Figure 5. Schematic presentation and energy diagram of the molecular orbitals of a copperoxygen complex and the orientation of neighboring atoms (boron) on the molecular framework.
Figure 6. Fourier-transformed ESEEM spectra of (a) the parent glass and glasses codoped with (b) Al, (c) Si, (d) P, (e) Zr, and (f) La. Straight lines indicate the Larmor frequencies of possible nuclei around copper.
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bonding character between Cu2þ and oxygen1517 7 jA j jA^ j 5 9 þ g g^ R2 ¼ 6 P P 14 7
)
g ¼ 2:0023
8λ 1 R2 β2 RR0 β2 S RR0 βð1 β2 Þ1=2 TðnÞ ΔExy 2
where R is the bonding coefficient of the σ bond between a dx2y2 orbital of copper and a 2p orbital of a neighboring oxygen, β is that of the π bond between a dxy orbital of copper and a 2p orbital of oxygen, P is 0.036 cm1, λ is 828 cm1, ΔExy is the bandgap energy between B2g and B1g corresponding to the optical absorption energy in Figure 2, R0 and S are the parameters for the overlap between the copper dx2y2 orbital and the oxygen 2p orbital that satisfy R2 þ R0 2 2RR0 S = 1, and T(n) is an integral over ligand functions. Kivelson and Neiman determined that S and T(n) for oxygen are 0.076 and 0.220, respectively.15 From the equations, the bonding coefficients of R and β were estimated at 0.896 and 0.948 for the P-codoped glass, and 0.925 and 0.881 for the Zr-codoped glass. Because these coefficients are indicators of bonding character, with 0.5 being purely covalent and 1.0 being purely ionic, β shows that the π bond between copper and oxygen is relatively ionic in the P-codoped glass, and
relatively covalent in the Zr-codoped glass. On the other hand, R shows the opposite trend for the σ bond. However, R has not been evaluated in this study because the variation of R is smaller than that of β, and R is insensitive to changes in the ligand field.3,4,15,16,18 The probability of electronic transition is proportional to the transition dipole moment μmn, Z ψm μψn dτ μmn ¼ where ψm is the excited-state wave function, ψn is the groundstate wave function, and μ is the electric dipole moment operator. If a transition is allowed, the direct product of the irreducible representations of the above three components, Γ(ψm)XΓ(μ)XΓ(ψn), contains the totally symmetric irreducible representation. In other words, the direct product of the two wave functions, Γ(ψm)XΓ(ψn), contains the irreducible representations to which XYZ Cartesian coordinates belong. In the case of a square-planar symmetry of D4h, the product of the wave functions, B1gXB2g, is A2g. Because A2g is not a representation of the coordinates, in which Γ(x, y) and Γ(z) are Eu and A2u respectively, the transition is forbidden. However, in the case of a lower symmetry of D4, the direct product of B1XB2 is A2. Because A2 is the representation of the Z coordinate, the transition is partially 5084
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Figure 7. Planar schematic diagrams of coordination structures around Cu in (a) the parent glass, (b) P-codoped glass, and (c) Zr-codoped glass, and measured and simulated ESEEM spectra at several pulse intervals τ.
allowed. Moreover, in the case of a distorted-octahedral symmetry of O, the representations of the wave functions are E and T2, and the product is T1 þ T2. Because T1 corresponds to Γ(x, y, z), the transition is allowed. Because how allowed a transition is depends on the site symmetry of the transition metal ion,19,20 it is supposed that the decrease in the optical absorption coefficient upon codoping with Zr is caused by a change in the coordination geometry of oxygen around Cu2þ from O to D4. Therefore, in all of the glasses except the Zr-codoped glass, Cu2þ is surrounded by six oxygen atoms with a distorted octahedral symmetry. In the Zr-codoped glass, Cu2þ is surrounded by four oxygen atoms with a squareplanar symmetry. These indications are corroborated by the results of ESEEM spectroscopy. Modeling and Simulation of the Coordination Structure around Cu2þ. At the beginning of modeling the coordination structure around Cu2þ, information about the ligands present around Cu2þ is obtained from the hyperfine structure in the CWEPR spectrum. Previously, three types of hyperfine structures (I, II, and III) have been observed in sodium borate glasses.5 In the composition xNa2O 3 (1 x)B2O3, structure I appears in the region 5 j x j 13, II appears in the region 20 j x j 37, and III appears in the region 55 j x j 75. It has been considered from the glass structure of sodium borates1214 that structure I indicates the presence of tetrahedral boron units of BØ4 in the first coordination shell of Cu2þ, II indicates an increased number of BØ4 units in the second shell, and III indicates the appearance of triangular units of BØ2O in the first shell. Because all of the glasses except the Zr-codoped glass exhibited structures I and II, it is likely that BØ4 units are present around Cu2þ in these glasses. On the other hand, the Zr-codoped glass exhibits structure III, and
accordingly, BØ2O units should be present around the Cu ions. This estimation is consistent with the observation that BØ2O units are a counteranion of Na, which was detected in the Fouriertransform of the ESEEM spectrum of the Zr-codoped glass. Next, a rule of local charge neutrality is imposed on the arrangement of ligands. This rule has been established in inorganic crystal chemistry, and used to understand the atomic arrangement in inorganic crystal structures. The quadrupole coupling constant (e2qQ/h) of boron in vitreous borates is similar to that in crystalline borates.21 Because the electric field gradient (q) arising from the distribution of positive and negative ions should be equivalent in both solid states, constituent ions in a glass must also obey local charge neutrality. Therefore, local charge neutrality can be applied to the borate glasses in this study. In addition, the coordination geometry around Cu2þ in a copper borate crystal is used as the basis of the structure model, assuming that the local structure is also present in glass. First, the coordination structure around Cu2þ in the parent glass is modeled upon a CuB2O4 crystal22 where Cu2þ is located at the center of an eight-membered ring made up of tetrahedral units of BØ4, and additionally, four triangular BØ3 units (two Ø2BOBØ2 bonds) are arranged out-of-plane in order to form an octahedral symmetry. In this model, local charge neutrality is satisfied because the valence bond sum of two BØ bonds and one CuO bond to an oxygen atom surrounding Cu is calculated to be 3/4 þ 3/4 þ 1/2 = 2, which is the same as the valence of an oxygen ion. Here, the valence bond (Z/N) is defined as the ratio of the valence charge (Z) to the coordination number (N),23 and is calculated to be 3/4 for BØ4, and 1/2 for CuO4. Figure 7a shows the schematic illustration of the candidate structure. The 5085
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Table 1. Nuclear Atoms and Their Spin-Hamiltonian Parameters nucleus
I
Aahf (MHz)
e2qQ/h (MHz)a
ηb
1O
BØ3
3
0.315
5.21
0
11
BØ3 1O BØ2O
3/2 3
0.940 0.315
2.50 5.21
0 0.6
BØ2O
0.6
11
3/2
0.940
2.50
1O
BØ4
3
0.315
1.46
0
11
BØ4
3/2
0.940
0.70
0
23
Na
3/2
0.775
1.50
1.0
PØ2O2
1/2
1.186
0
0
31
a
Quadrupole coupling constants. b Asymmetric parameters were obtained from reported NMR studies.12,14
bond lengths of CuO and BO and the bond angles of OCuO and OBO are set at 2.0 Å, 1.4 Å, 90, and 120, respectively, according to crystal data.22,24 On the other hand, the PØ2O2 unit has valence bonds of 1 and 3/2 for PØ and PO, respectively.25 Therefore, this unit can be coordinated to a fourhold Cu2þ center up to four, in which the valence bond sum is 3/2 þ 1/2 = 2. Figure 7b shows one of four candidate structures for the P-codoped glass. Cu is coordinated by four BØ4 units, two PØ2O2 units, and four BØ3 units arranged out-of-plane. Although this structure lacks two tetrahedral units, Cu is equivalently bonded to four oxygen atoms like in the parent glass. In the case of Zr-codoping, Cu is considered to be surrounded by BØ3 units, BØ2O units, and Na ions arranged out-of-plane. Unlike the first two glasses, there is no tetrahedral boron unit around Cu. Since the valence bond of the triangular boron units is 1, oxygen atoms surrounding a copper must be neutralized by Cu of Z/N = 1, that is, a two-hold copper. As a result, six triangular boron units coordinate to Cu. Figure 7c shows one of six candidates, where Cu is surrounded by four BØ3 units, two BØ2O units, and two Na ions. Zirconium ions are distant from Cu compared with B, P, and Na, which are arranged at a radial distance of 3.0 Å. Before describing the results of the simulations, it should be noted that the Euler angle θ between the direction vector from Cu to a ligand and Z-axis can be resolved in a simulation, but the Euler angle j between the direction vector from Cu to a ligand cast on the XY plane and X-axis cannot be resolved, because the samples examined are amorphous (Figure 5).8 However, in this study, ligands were not orientated, because the type and number of ligands around copper were too great to simulate. The ESEEM spectra did not include any clear information about the orientation of nuclei, because they showed a Larmor frequency of νI, and not electronnuclear double resonance (ENDOR) frequencies of νR(β) = |νI ( Aahf(3 cos2 θ 1)/2|. Accordingly, the parameter θ was varied from 0 to π in integration, not fixed, and j varied from 0 to 2π. It is generally recognized that such simplifications can be applied to both glassy samples and polycrystalline powders, because the error induced is negligible.6 The ESEEM spectra were simulated by the structure models and the ligand parameters listed in Table 1. Here, the isotropic hyperfine coupling constant Aihf is neglected, because there is no Fermi contact between the ligand nuclei and Cu. The anisotropic hyperfine coupling constant Aahf is calculated using the equation Aahf = geβeνI/(H0 3 r3), where νI is the Larmor frequency, H0 is the strength of the applied magnetic field, and r is the interatomic distance between nuclei and Cu. The nuclear quadrupole coupling constant (e2qQ/h) and the asymmetry parameter (η) of
boron units and sodium were obtained from nuclear magnetic resonance (NMR) studies on sodium borate glasses.12,14 The natural abundance ratio of the isotopes 10B and 11B was set to be 20% and 80%, respectively. The results of the simulation are attached to Figure 7. The simulated ESEEM spectra fit the experimental ones, which gives credence to the above structure models. The three coordination structures shown in the figure are the best fitting ones among the candidates considered. Mechanism of the Codoping Effect in Glass. The mechanism of the codoping effects observed in a sodium borate glass is considered from the viewpoint of the valence charge (Z), oxygen coordination number (N), and ionic radius (R) of the codopant cations. Al3þ, Si4þ, and P5þ ions form a tetrahedral unit, but only P5þ ions could replace B3þ ions around Cu, and change the ligand field. The reason for this is thought to be the nonbridging bond. The PØ2O2 unit is flexible compared with MØ4 units of B, Al, and Si, because of two nonbridging bonds,25 and therefore, it is capable of replacing BØ4 units despite the differences in Z and R. Additionally, PO bond has double bond character because of the dp π bond, which reduces the freedom of a lone pair of electrons on oxygen, unlike in the BO bond.3,24 Therefore, P-codoping decreases the covalent nature of the π bond between Cu2þ and oxygen. This is also the reason why the absorption of Cu2þ in P2O5 glass appears at a longer wavelength than that in B2O3 glass (Figure 1). Zr4þ ions have several possible oxygen coordination numbers ranging from 6 to 8, similar to La3þ. In the case of cubic ZrO8 observed in zirconia, the ion has eight ZrO bonds with Z/N = 1/2, which is equal to that of CuO bonds in square CuO4. That is to say, Zr4þ should be coordinated by BØ4 units like Cu2þ. Indeed, such a coordination is seen in a ZrB2O5 crystal,26 which was synthesized at high pressure. In the same way, octahedral LaO6 with Z/N = 1/2 should also be coordinated by BØ4 units, even though a crystal with such coordination has not been reported. Because the charge of Zr4þ is twice as high as that of Cu2þ, Zr4þ requires more BØ4 units to satisfy local charge neutrality. Probably, Zr4þ and Cu2þ share BØ4 units, and create local ZrCu-rich structures. However, according to the glass structure of Na2OB2O3,1214 the structure containing a high fraction of BØ4 units (>40%) is thermodynamically unstable, and BØ4 units convert into BØ2O units. The ZrO8 cube coordinated by BØ4 units must convert to a ZrO6 octahedron coordinated by six triangular boron units and eight alkali ions, as seen in a crystal structure of K2ZrB2O6,27 which was synthesized by a conventional solid state reaction. That is to say, the high field strength (Z/R) of Zr4þ ion is responsible for the formation of borate anion groups containing nonbridging oxygen ions.11 When a network structure consisting of BØ3 and BØ2O units and Naþ ions is formed in the Zr-codoped glass, copper ions should occupy the local structure to be thermodynamically stable. This is in contrast to the P5þ ion, because it possesses a higher valence than Zr4þ. However, the formation of covalent bonds of PO effectively reduces the positive charge of P5þ, decreasing its net charge. To support our conclusion, the Raman spectra of the examined glasses are shown in Figure 8, which is very sensitive to the borate anionic structure. According to literature, molecular vibrational modes of tetrahedral boron locate at around 510, 770, 970, and 1100 cm1.28,29 On the other hand, four Raman bands attributed to the stretching of triangular boron appear at a range from 1200 to 1600 cm1.13 In addition, BØ3 units in boroxol rings cause peaks at 470, 500, 610, and 808 cm1, and the units not incorporated into the rings 5086
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Figure 8. Raman spectra of parent and codoped glasses.
(loose BØ3) create a peak at 670 cm1.30 The peak intensities at 970 and 1100 cm1 are convenient to quantify BØ4 units in a glass. Compared to the spectra of the parent glass and the codoped glasses, it is evident that the number of BØ4 units is quite few in the Zr-codoped glass. This result may be understood by the fact that Zr ions convert BØ4 units into triangular ones as the proposed model. This study demonstrates that codoping can be used to control the ligand field around a transition metal ion contained in a glass. In particular, codoping with Zr had a marked effect on the coordination structure around Cu2þ, because of the specific valence and coordination number of Zr4þ. On the basis of the present findings, one would expect that codoping could also be used to modify the ligand field around rare-earth ions such as Er3þ, Nd3þ, and Tb3þ in photonic glass materials. The optical functions of such glasses will be enhanced by the proper combination of a rare-earth ion with a codopant. Systematic studies are required to achieve a comprehensive view of the codoping effect in glass toward a rational coordination designing around an optically active cation. Finally, we’d like to comment on the possibility of immiscibility on the present results. It is known that there is a subliquidus immiscibility dome in the Na2OB2O3 glass system. The end members of the immiscibility are 7Na2O 3 93B2O3 and 24Na2O 3 76B2O3.31 If the codopant ions raise the immiscibility dome to high-temperature, Cu2þ should be distributed to 24Na2O 3 76B2O3 glass phase. However, the observed spectroscopic data on Zr-codoping differs from this anticipation. The spectra corresponding to 50Na2O 3 50B2O3 is observed. Thus, it is possible that only local borate anionic structure around Cu2þ is drastically changed by codoping, i.e., ligand field modification is attained.
’ CONCLUSIONS CW-EPR spectroscopy and ESEEM were performed to reveal the effect of codoping on the ligand field around Cu2þ ions in a sodium borate glass. The results obtained are summarized below.
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)
(1) The codoping effect can be categorized into: (a) strengthening the ligand field, (b) weakening the ligand field, and (c) almost no effect on the ligand field. The wavelength of the optical absorption peak of Cu2þ decreased significantly upon codoping with Zr (a), increased by codoping with P (b), and was almost unchanged by codoping with Al, Si, or La (c). (2) Codoping with Zr caused the optical absorption intensity to decrease and large changes in the EPR parameters (g and A ) of Cu2þ. These changes are understood by considering that the coordination geometry of oxygen around Cu2þ is changed from a distorted octahedron to a square plane, and the covalent nature of the π bond between Cu2þ and oxygen is increased. (3) ESEEM revealed the coordination structures around Cu2þ in the P- and Zr-codoped glasses. While Cu2þ in the parent glass is located at the center of an eightmembered ring composed of BØ4 units, in the P-codoped glass, Cu2þ is coordinated by four BØ4 units and two PØ2O2 units, and in the Zr-codoped glass, Cu2þ is coordinated by four BØ3 units, two BØ2O units and two Naþ ions. While P5þ ions replace the BØ4 units coordinating to Cu2þ, Zr4þ ions induce the formation of BØ2O units, which interact with nearby Naþ ions, and provide a local structure composed of BØ3 and BØ2O units for Cu2þ. (4) The present results imply that metal cations with a high ionicity and high field strength are an effective codopant to induce changes in the ligand field around photoactive transition metal ions.
’ AUTHOR INFORMATION Corresponding Author
*Address: Tokyo Institute of Technology S2-13, 4259 Nagatutacho, Midori-ku, Yokohama 226-8503, Japan. Tel: þ81-45-9245127. Fax: þ81-45-924-5127. E-mail:
[email protected]. ac.jp.
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