Structural and Theoretical Study of Strontium Borophosphate Glasses

Article pubs.acs.org/JPCB

Structural and Theoretical Study of Strontium Borophosphate Glasses Using Raman Spectroscopy and ab Initio Molecular Orbital Method Maria Anastasopoulou,† Konstantinos C. Vasilopoulos,†,§ Dimitrios Anagnostopoulos,† Ioannis Koutselas,‡ Demetrios K. Papayannis,† and Michael A. Karakassides*,† †

Department of Materials Science and Engineering, University of Ioannina, GR-45110 Ioannina, Greece Materials Science Department, University of Patras, Patras GR-26504, Greece § Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, N. Plastira 100, GR-70013 Heraklion, Crete Greece ‡

S Supporting Information *

ABSTRACT: Strontium borophosphate glasses of composition xSrO·(1 − x)·[0.68B2O3·0.32P2O5], 0.40 ≤ x ≤ 0.68, have been prepared by fast quenching of high-temperature melts and studied using Raman spectroscopy. In order to comprehend and confirm the obtained spectroscopic Raman data, crystalline compounds and glassceramics of analogous compositions were also prepared and studied. Also, ab initio molecular electronic structure theory was used to predict and confirm the experimental vibrational spectra The comparison between theoretical and experimental results showed a good overall agreement. The analysis has focused on a new detailed interpretation of the P−O−B Raman bands. Also, the analysis has revealed a divergent modification of the reported glasses near the meta-stoichiometry where the dominant species in the glass network were found to be borophosphate chains [BP2O9]5−, pyrophosphate P2O74−, and orthophosphate PO43− units. NaPO3−Na2B4O7 was examined by several studies of Ducel at al12−15 using vibrational, 11B MAS NMR and XPS spectroscopies, while Villa et al. studied lithium and silver borophosphate glasses by means Raman and 31P MAS NMR.6,7 Recently, Raguenet et al. studied the local environment of lithium borophosphate glasses using 11B MAS NMR and Raman spectroscopies16 and Larink et al. investigated the short and intermediate range order structures of alkali borophosphate glasses combining different solid state NMR techniques together with Raman scattering, X-ray photoelectron spectroscopy, and charge balance considerations.17 Structural studies of glasses in the system ZnO−B2O3−P2O5 were reported initially by Brow and Tallant3,18 and then by Koudelka et al.,19,20 who also studied extensively the structure of borophosphate glasses containing two different metal cations such as Zn−K,21 Zn−Pb,4 Na−Pb,22 or Zn−Sr.23 On the other hand, to our knowledge similar data on the system SrO−B2O3−P2O5 are not available. Previous works on alkaline earth borophosphate glasses have been reported only for the systems BaO−B2O3−P2O524,25 and CaO−B2O3−P2O526

1. INTRODUCTION Borophosphate glasses belong to a class of mixed network former glasses, which is studied for numerous interesting applications. These properties are strongly dependent on the particular P2O5 or B2O3 content yet differ to the properties of pure phosphate or borate glasses. For instance, the addition of B2O3 to a hydroscopic phosphate glass leads to a significant improvement of its chemical durability and/or thermal and mechanical stability.1,2 In recent years, a large family of glasses based on a combination of both B2O3 and P2O5 with various network modifiers has been developed for widespread applications, including hermetic sealing materials3,4 and fast ion conductors in solid-state batteries.5 Such examples are alkali and silver borophosphates that were developed for fast ion conducting applications,6,7 while zinc−calcium borophosphate glasses were studied as candidates for applications as lowmelting glass solders or glass seals.8,9 Such glasses are advantageous while their properties can be effectively controlled by changing their composition. The structure of borophosphate glasses has been studied by NMR and vibrational spectroscopy. The first type of these studies was carried out on K2O−B2O3−P2O5 system by Ray10 and Yun et al.11 who revealed the reverse dependence of BØ4 formation from the B2O3/P2O5 ratio. Then, the system © XXXX American Chemical Society

Received: February 17, 2017 Revised: April 8, 2017 Published: April 11, 2017 A

DOI: 10.1021/acs.jpcb.7b01563 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Instruments), which allows excellent low energy analysis with detection starting from Be. The samples were found homogeneous also in the lateral direction. Raman spectra were obtained from samples in the form of small pieces with a confocal micro Raman (μ-Raman, Renishaw RM1000) system using the 532 nm line of a laser diode operating at 60 mW for excitation. Raman scatter was collected by means of an optical microscope equipped with lenses 50× and 100×. The probing spot was about 1 μm in diameter, using the 100× lens in the microscope. The spectrometer was calibrated by recording the spectrum from a Si sample with characteristic Raman peak at 520 cm−1. X-ray powder diffraction data were collected on a diffractometer (D8 Advance Bruker) using Cu−Kα (40 kV, 35 mA) radiation and a secondary beam graphite monochromator. Diffraction patterns were collected in the 2θ range from 2 to 80° in steps of 0.02° and 2 s counting time per step. 2.3. Computational Details. As part of the current study, ab initio quantum mechanical calculations were performed using Gaussian 03,27 based on the Hartree−Fock (HF) theory level of theory, which included electronic molecular structure optimization, allowing all atoms of the model cluster to change position in order to achieve the minimization of energy and harmonic vibration frequencies analysis of the computed Raman spectrum at the same time. Results processing was attained Gauss View.28 The analysis of the vibration frequencies (IR and Raman) were realized with the latter software package in the range 400−2000 cm−1 and compared to the experimental Raman spectra. For all calculations and all involved atoms, the standard 321G* double-ζ valence basis set29 with more accurate results on large systems was used and implemented in the selected geometrical models using the Gaussian 03 program. The simulation was performed on clusters of atoms, rather than on periodic boxes of random stoichiometry, while the composition of each computed cluster was designed to be as close as to the experimental stoichiometry of the synthesized basis glass. The clusters that have been examined have based on pyrophosphate anions P2O74− or on metaphosphate chains, to which boron addition induced modification to the structural units and the respective vibrational modes. Moreover, Sr2+ counterions have also been added as well as other counterions such Na+ or K+. Geometry optimization was performed for all of the computed clusters and as a result of the final stable configuration that was derived, the Raman spectra were calculated. In order to evaluate the effectiveness of the methodology and to obtain the scale factors needed for the metaphosphate chains and the pyrophosphate anions, the HF calculations were utilized to analyze the Raman frequencies based on the already optimized structures of other previous theoretical approaches. It is noticeable that the calculated harmonic vibration frequencies differ from the corresponding experimentally observed due to skipped nonharmonic effects, integration of the correlated electrons, and the usage of a finite base set rather a full complete set. Using the experimental vibrational frequencies of metaphosphate chains and of pyrophosphate anion, as a benchmark, a general scale factor obtained for the existence of an agreement with the theoretically obtained, was 1.014. Finally, it should be noted that the scale factor value1.014 corresponds to the frequency analysis of the HF method for all database function and appears to be better compared to the values of MP2 and B3LYP methods using the

regarding the formation of various structural groups during synthesis as well as the glass properties. The aim of this work was to prepare durable strontium borophosphate glasses with high SrO content and to comprehensively investigate their structure. The structure of these glasses and that of crystalline strontium borophosphates prepared with specific compositions were both examined using Raman spectroscopy and theoretical calculations. The spectroscopic results have used to elucidate the connectivity of the various structural groups such as that of phosphorus−oxygen, boron−oxygen, and mixed units, especially after the depolymerization of the glass network by the addition of SrO. The electronic structure and vibrational frequencies of the prepared strontium borophosphate glasses were also theoretically studied with quantum mechanical calculations, which were intended to completely clarify the geometry and the modes of vibrational behavior of the system under study and at the same time to investigate the origin of the characteristic studied Raman band, such as the one appearing at 985 cm−1 for which in fact there has been no clear assignment in literature.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Strontium borophosphate glasses of composition xSrO·(1 − x)·[0.68B2O3·0.32P2O5] with 0.40 ≤ x ≤ 0.68 were prepared using analytical reagent grades of SrCO3, B2O3, and (NH4)2HPO4. A 5 g batch for each composition was homogenized using a mortar and pestle. The powder mixtures were heated initially at 500 °C for 1 h in porcelain crucibles to evaporate ammonia and water from the batch and minimize the tendency for subsequent phosphate loss. Then the resulting dry product was pulverized into fine powder and melted in Pt−Rh crucibles in a preheated electric furnace, keeping the batch for about 15 min in the temperature range 1500−1600 °C depending on the SrO content. The glasses were obtained by pouring the melts onto a stainless steel plate and immediately splat quenching the melt (cooling rate ∼103K/s). The amorphous character of the glasses was confirmed using X-ray diffraction analysis (XRD). Polycrystalline strontium ortho- and pyro- phosphates and borates (3SrO· P2O5, 3SrO·B2O3, and 2SrO·P2O5, 2SrO·B2O3) were also prepared by slow melt cooling of appropriate compositions. Xray diffraction patterns of these samples confirmed the development of phosphate or borate crystalline phases, respectively. 2.2. Characterization. The samples have been systematically analyzed with respect to their composition and homogeneity, both lateral and in-depth, by means of X-ray fluorescence (XRF) and energy dispersive (EDS) spectroscopic techniques. The requirement of high resolution is mandatory for accurate quantitative analysis due to energy overlap of the fluorescence strontium L X-rays (1.5−2.2 keV) with the phosphorus K X-rays (Kα of 2.0 keV). The quantitative analysis showed the linear dependence of the intensities (I) ratio I(Sr)/ I(P) with the nominal compositions, certifying the corresponding increase in SrO content in the studied glasses. The results are comparable, demonstrating that the in-depth sample’s homogeneity is extended in the range of few micrometers up to tenth of a millimeter. The lateral homogeneity was investigated by elemental mapping based on the measurement of the X-ray characteristics using electron microbeam, as excitation probe. The measurements were performed using the scanning electron microscope JSM-6510 LV, JEOL. The microscope is equipped with the energy dispersive silicon drift detector X-Act (Oxford B


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with two nonbridging oxygen atoms, bonded to phosphorus atoms, appear at the highest frequencies of this range (Ca, 1179−1155 cm−1; Mg, ∼1210 cm−1)33 of this spectral region. In the Qn terminology, n represents the number of bridging oxygens (Ø) per PO4 tetrahedron, which symbols are used generally to classify the four main types of different phosphate structural groups encountered in the glass networks. These are specifically denoted as Q3, Q2, Q1, and Q0, which bear 1, 2, 3, or 4 NBOs, respectively. Concerning the origin of the 1084 cm−1 band in the spectrum of 0.40SrO·0.40B2O3·0.20P2O5 glass, it must be stressed that a significant factor is that that the addition of boron in a metaphosphate network (chains of Q2 units) causes a significant reduction in the frequencies of the vs(PO2−) symmetric stretching vibrations that take place in the phosphate chains.19 Initially, the presence of boron contributes to the modification of the phosphate chains through the intermediate participation of trigonal boron BØ2O− units and second through the formation of B−O−P bridges at both ends of the chains by terminal units BØO22−. In this way, the appearance of the band due to the Q2 units shifts toward lower frequencies where the vibrations of the pyrophosphate units Q1 are shown. It has been previously observed that a high intensity Raman band appears at ∼1000 cm−1 in the spectra of calcium borophosphate glasses belonging to the pyro and ortho stoichiometries,34 which is generally attributed to the presence of boron and to the creation of borophosphate units in the glass structure. According to Scagliotti et al.,6 the vs(PO2−) vibrations of intermediate PØ 2 O 2 − phosphate tetrahedra of lithium phosphate glasses appear at 1115−1145 cm−1 while the corresponding vibrations of terminal tetrahedra (PØO32−) appear at 1045 cm−1. Specifically, the addition of boron leads to the reduction of bands’ intensity at 1115−1145 cm−1 and to the relative intensity increase of the band at 1045 cm−1. Additionally, the increase of boron content in these glasses causes a parallel increase of the band intensity at ∼1010 cm−1 that is attributed to the increase of the number of borate terminal units (BØO22−). In another work, Raguenet16 and Lee35 assume that the Raman scattering close to ∼1110 cm−1 observed in lithium borophosphate glasses is due to overlapping vibrations of pyrophosphate Q1 and borophosphate (B−O−P)− units, while the band close to 1010 cm−1 is due to bands overlap caused by structural units such as (P4O13)−6, (P3O10)−5 and (B−O−P)0. On the basis of the above analysis and our literature knowledge, we can assign the band at 1084 cm−1 to vibrations of terminal tetrahedral PØO32− and/or to vibrations of PØ2O2− tetrahedra connected with boron atoms. Also, it is not expected that the band at 1084 cm−1 is due to vibration of the pure pyrophosphate units (P−O− vibrations of two phosphate tetrahedra which constitute the pure dimeric pyrophosphate unit P2O74−), Figure 1, since these specific units exhibit their main band at 1063 cm−1. However, their existence in the structure of the glass is not excluded because the broadness of the band that peaked at 1084 cm−1 as well as its asymmetry indicates a potential contribution of Raman scattering from pyrophosphate units close to 1063 cm−1. As mentioned before, the existence of Raman scattering in many borophosphate glasses with different oxide modifier close to 1000 cm−1 is mainly correlated to the existence of P−O−B chains. It is, therefore, reasonable that in the spectrum of the 0.40SrO·0.40B2O3·0.20P2O5 (x = 0.40) glass, the band at 985 cm−1 can be also attributed to vibration units containing

same base set, which were found to have scale factor values of 1.172 and 0.963, respectively. Our geometrical and spectroscopic results at the HF/3-21G* level allowed the calculation of reliable results using much larger model systems with only a small increase in computational cost. This confirms the finding of previous studies30,31 where the HF methods yield pretty similar results. In addition, in an extensive investigation the reliability of HF was found to exhibit excellent behavior and was especially recommended by its authors as a very suitable method for applications,32 as in the case of the present systems. Thus, the present study has shown the potential of the HF procedure to be an effective tool for the satisfactory description of the phosphate and borophosphate cluster structures regenerated precisely by the experimental Raman spectra.

3. RESULTS AND DISCUSSION 3.1. Raman Spectra. Figure 1 shows the Raman spectra of xSrO(1 − x)[0.68B2O3·0.32P2O5], glasses with compositions in

Figure 1. Raman spectra of the xSrO(1 − x)[0.68B2O3·0.32P2O5] glasses, for 0.40 ≤ x ≤ 0.68 and of crystalline compounds 3SrO·P2O5 (3:1) and 2SrO·P2O5(2:1).

the range 0.40 ≤ x ≤ 0.68, together with the spectra of pyrophosphate (2:1) and orthophosphate (3:1) polycrystalline samples for comparison purposes. The spectrum of the 0.40SrO·0.40B2O3·0.20P2O5 (x = 0.40) glass exhibits its stronger bands at 985 and 1084 cm−1 and a number of weaker bands in the lower frequencies in the frequency range of 800−1300 cm−1. According to previous Raman studies of phosphate glasses, bands in the frequency range 1200−1050 cm−1 can be attributed to symmetric stretching vibrations of phosphate tetrahedra bearing non bridging oxygen atoms (NBOs). Specifically, in the alkaline earth phosphate glasses, the symmetric stretching vibrations vs(PO2−) of PØ2O2− tetrahedra (Q2 units, Ø denotes a bridging oxygen, and O is a nonbridging oxygen (NBO)), that is, units C


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x = 0.40 the band at 625 cm−1 can be assigned to respective stretching vibrations of P−O−B groups while boron atoms are incorporated to phosphate glass network. Besides, the band observed at 728 cm−1 might appear due to the existence of borophosphate rings whose existence is favored at low modifier contents near meta-stoichiometry. This assignment is in agreement with Osaka and others36 who attributed the existence of the band at 720 cm−1 to breathing vibration of six-membered borophosphate rings in potassium borophosphate glasses. On the other hand, the presence of borate units is not obvious in the main bands of the spectrum. Raman studies on glasses based on B2O3 show the presence in the glass network of both BO3 and BO4 structural units.37−42 A variety of anionic borate groups such as di-, ditri-, tri-, tetra-, penta-, pyro-, ortho-, and metaborate besides structural entities like boroxol ring have been identified in these glasses. The concentration of these borate groups in the glass structure depends on the nature and the concentration of modifier oxide.31,33−47 The presence of boroxol rings should be excluded as the characteristic band at 806 cm−1 because the breathing vibration of these rings does not appear. However, the existence of tri-, di-, or pentaborate rings should not be excluded because the Raman spectra exhibit bands in the frequency region of 700− 800 cm−1. The formation of such groups suggests the existence of B−O−B among trigonal or tetragonal boron units. This is in agreement with the recent NMR study 48 in lithium borophosphate glasses. The possibility of existence of isolated borate units like pyroborate [B2O5]4− and orthoborate [BO3]3− is small, as in the glass’s spectrum the characteristic bands of these units are not observed, that is, according to the literature, the symmetric stretching vibrations of B−O−B bridges and the symmetric stretching of terminal bonds B−O− of pyroborate [B2O5]4− units appear at 830 and 1233 cm−1, respectively, whereas the symmetric stretching vibrations of B−O− bonds of orthoborate units [BO3]3− appear at 922 cm−1.49 In the spectrum of 0.40SrO·0.40B2O3·0.20P2O5 glass (Figure 2, x = 0.40), bands at 558, 488, 430, and 323 cm−1 are observed, as far as frequencies lower than 600 cm−1 are regarded. In this frequency area, generally the bending vibrations of phosphate tetrahedra are observed, which either isolate or participate in chains. Specifically, in the spectra of alkaline earth glasses the bending vibrations of tetrahedra in metaphosphate chains and pyrophosphate units appear as a broad band at 555 cm−1, which after the increment of the alkali size,26 seems to be separated into the distinct peaks of 570, 530, and 490 cm−1. The Raman spectra of the 0.45SrO·0.37B2O3·0.18P2O5 glass, Figure 1, having a higher content of SrO (x = 0.45), reveals small changes with respect to the x = 0.4 glass, such as the appearance of a shoulder at 940 cm−1, which can be assigned to symmetric stretching vibrations vs(PO43−) of P−O− bonds of isolated orthophosphate tetrahedra, Q0. The intensity of this band increases as the glass content of SrO increases from x = 0.45 to x = 0.50, suggesting that the number of created isolated orthophosphate tetrahedra inside the glass structure increases at the same time. Also, the intensity increase of the band at 940 cm−1 occurs with a parallel intensity increase of the bands at 577 and 430 cm−1, which are assigned to bending vibrations δ(PO−) of PO43− tetrahedra and O−P−O bonds of Q0 units, respectively (Figure 2). Further increase of SrO content (from x = 0.53 to x = 0.56) causes an initial decrease of the relative intensity of the band at 940 cm−1 accompanied by a re-

phosphate tetrahedra and trigonal/tetrahedral boron atoms, linked together in smaller or larger chains. Seeking a secure assignment for specific stoichiometric vibrations for the Raman band at 985 cm−1 the theoretical calculations performed here support this previous hypothesis and attribute this band symmetric stretching vibrations P−O− of phosphate terminal tetrahedra connected with tetrahedral or trigonal boron units. Furthermore, it is also observed that the two bands at 1084 and 985 cm−1 exhibit an asymmetry toward the higher (∼1200 cm−1) and the lower (940 cm−1) frequencies of the main peak, respectively, facts that indicate the existence of other bands. On the basis of the literature, the band at ∼1200 cm−1 can be attributed to stretching vibrations, vs(PO2−), of nonbridging oxygens (NBOs) of Q2 phosphate tetrahedra that connected each other. On the other hand, the band at ∼940 cm−1 can be assigned to symmetric stretching vibrations (vs) of P−O− bonds of isolated phosphate tetrahedra Q0 (PO43−). In Figure 1, the Raman spectrum of the crystalline compound 3SrO·P2O5 (3:1) is shown and it is obvious that it exhibits exactly at the same frequency as its stronger band, something which is in agreement with the above hypothesis. The magnification of the spectrum of 0.40SrO·0.40B2O3· 0.20P2O5 glass at lower frequencies reveals the existence of a number of weak intensity bands, displayed in Figure 2 (x =

Figure 2. Raman spectra of the xSrO(1 − x)[0.68B2O3·0.32P2O5] glasses for 0.40 ≤ x ≤ 0.68 and of crystalline compounds 3SrO·P2O5 (3:1) and 2SrO·P2O5(2:1) magnified at high and low frequencies.

0.40). The shoulder observed at 764 cm−1 can be assigned to symmetric stretching vibrations vs(P−O−P) of phosphate or borophosphate chains. Taking into consideration the Raman spectra of Pb and Zn borophosphate glasses, Koudelka and Mosner4 attributed the development of a weak band near 660 cm−1 to symmetric vibrations P−O−B created in borophosphate chains. Villa and Scagliotti7 concluded similar results for Li and Ag borophosphate glasses. Therefore, in the spectrum of D


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replaced phosphate tetrahedra PØ2O2− in internal selected positions of the seven-member cluster. In the calculations, borophosphate structures of few members from trigonal borate BØ2O− units, tetrahedra BØ4−, and phosphate tetrahedra PØO32− were also used. Targeting to the theoretical study of the 0.50SrO·0.50P2O5 glass (metaphosphate glass), a model was initially selected (P/ cluster-33) including four Sr2+ cations and seven phosphate tetrahedra (five PØ2O2− and two PØO32− tetrahedra). The exact composition of the representative glass of P/cluster-33 (Figure 3a), containing in total 33 atoms, is stoichiometrically close to the composition of the glass 0.53SrO·0.47P2O5, whose mole ratios is close to the glass metaphosphate stoichiometry. Another cluster is of 36 atoms (P/cluster-36) representative of the glass 0.29Na2O·0.24CaO·0.47P2O5, containing a total of seven phosphate tetrahedra (five PØ2O2− and two PØO32− tetrahedra), five Na+ cations, and two Ca2+ cations (Figure 3b) having a corresponding content of P2O5 glass former with P/ cluster-33. In order to study the borophosphate glasses, we used a model that is produced from the replacement of one phosphate tetrahedron PØ2O2− from the five of P/cluster-33 by a trigonal boron in the form of the unit BØ2O− carrying a nonbridging oxygen. This cluster (BP/cluster-32) consisted of 32 atoms and the trigonal boron was placed in such a way so as to connect with one of the two tetrahedra PØO32− at the edges in order to have the greatest effect on its vibrations (Figure 3c). Also, the structure of (BP/cluster-24) that contained trigonal BO3 units is shown in Figure 3d. Subsequently, in a similar way the basic glass unit 0.67SrO· 0.33P2O5 of pyrophosphate stoichiometry is modeled. The model is comprised of two Sr2+ cations and two PO4 tetrahedra, that is, a structure containing in total 11 atoms (P/cluster-11). Targeting again to the effect that borate units will exert on the vibrations of the phosphate tetrahedra and in general in the vibrational spectrum, especially by the presence of borate units joined with phosphate tetrahedra, a model consisting of a borate BØ2O− unit between two phosphate tetrahedra PØO32− and having two Sr2+ cations as charge compensation (the charge of borate unit was neutralized by the charge of a hydrogen-cation), a total of 14 atoms (BP/cluster-14), was also examined. Finally, a model of 25 atoms, consisting of a borate tetrahedron BØ4− in the center of a cluster (BP/cluster-25) between four tetrahedra PØO32− surrounded by four Sr2+ cations, was used in order to obtain a complete picture of boron effect in the structure of borophosphate glasses. Figure 4 and Tables 2 and 3 present the final structures, the computed atom bond lengths and charges. In Figure 5, all the calculated Raman spectra of the structures of Table 2 that are used in the theoretical calculations are depicted along with two experimental Raman spectra of glasses for comparison purposes. The band that revealed the greatest interest in the analysis of Raman spectra of strontium borophosphate glasses is that which appears near 985 cm−1, which according to the literature is attributed to vibrations of borophosphate units. As it can be seen, the experimental spectrum of metaphosphate glass 0.50SrO·0.50P2O5 can be described quite well by the computed spectra of structures (P/ cluster-33 and P/cluster-36). The structures of the bands at 1160 and 1170 cm−1 (symmetric stretching vibrations vs(PO2−)) together with the bands at 680 and 700 cm−1 (symmetric vibrations of bridges vs(P−O−P)), are in a good agreement with the corresponding frequencies in the spectrum

enhancement of the intensity for x = 0.62 and finally the complete domination of this band in the spectra of 0.68SrO· 0.22B2O3·0.10P2O5 glass (x = 0.68). Furthermore, in the spectra of the glasses, the following effects are observed while increasing SrO content: (i) the increase of the band intensity at 1084 cm−1 due mainly to the corresponding stretching vibrations vs(PO32−) of the P−O− terminal bonds of phosphate tetrahedral and (ii) this particular band acquires its maximum intensity for x = 0.53, while for x > 0.53 the band has disappeared. As it is shown in Figures 1 and 2, the band at 764 cm−1 is observed only with high magnification (7×) of the spectra for x > 0.56, leading to the conclusion that with high modifier content, the number of pyrophosphate units within the glass structure is small compared to that of orthophosphate units. Generally, excluding the glass structure of 0.68SrO·0.22B2O3·0.10P2O5 sample (x = 0.68), in all Raman spectra the band at 985 cm−1 dominates followed by the second-high intensity band at 940 cm−1. In Raman spectra of Figures 1 and 2, the presence of borate units is not obvious. This is a result of the lack of boroxol rings or small concentration of rings with borate tetrahedra and also is due to a possible overlap of isolated orthoborate units by the vibrations of respective phosphate units. However, the presence of borate rings with nonbridging oxygens (C3h symmetry Kèkulè),50 can be supported in glasses with low Sr content (x = 0.40, 0.48) by the presence of scattering bands at 640 and 727 cm−1. The presence of trigonal boron atoms is detected in 1385 and 1440 cm−1 where low intensity bands are observed and which can be assigned to vibrations of boron bonds with NBOs (B−O−) (see Table 1). Table 1. Raman Bands and Their Assignments of xSrO(1 − x)[0.68B2O3·0.32P2O5] Glasses where 0.40 ≤ x ≤ 0.68 peak position (cm−1) 1385, 1440 1084 985 940 763 728 640 570−480 577, 430 323

assignment −

vibrations B−O of trigonal borate units vs(P−O−) of terminal phosphate unis and/or phosphate units connected with boron atoms vs(P−O−) of borophosphate units vs(PO43−) of orthophosphate units (Q0) vs(P−O−P) in phosphate or borophosphate chains vs breathing of borophosphate rings, vibrations of metaborate rings C3h breathing vibrations of metaborate rings C3h bending vibrations O−P−O in borophosphate and phosphate chains bending vibrations O−P−O of tetrahedra PO43− (Q0) bending vibrations of phosphate lattice

3.2. Models and Calculation Procedures of ab Initio Molecular Electronic Structure Theory. Initially, relatively large clusters were studied, composed of five phosphate tetrahedra PØ2O2− joined at the ends with two terminal phosphate tetrahedra PØO32−, as representative of phosphate chains of the metaphosphate glasses. Smaller clusters of two joined phosphate tetrahedra PØO32− were used as representative of pyrophosphate units P2O74−. The system was released to obtain a geometrical configuration with thermodynamically stable minimum energy state, optimizing the atomic distances and the bond angles, whereas in parallel the determination of the Sr2+ interaction with the phosphate and the borophosphate network was achieved. The primary borophosphate network was constructed by using trigonal borate BØ2O− units that E


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Figure 4. Calculated structures in the level HF/3-21G* for pyrophosphate structures before (a) pyro-Sr, (P/cluster-11) and after (b) the addition of boron in trigonal coordination, pyro-Sr (BP/cluster14) as well as (c) the structure (BP/cluster-25) that contains tetrahedral B. The distances between atoms are marked with dashed lines. Colors: orange, P; red, O; green, Sr; pink, B.

previously mentioned frequencies is observed in the spectra of the structures having one (BP/cluster-32) and two boron atoms (BP/cluster-24). Both calculated spectra reveal their strongest peaks near 980 cm−1, shown as a doublet for BP/ cluster-32, in contrast to the spectra of structures without boron. On the contrary, the same calculated spectra also reveal peaks close at the frequencies of 765 and 720 cm−1, which is in agreement with the pure phosphate structures (P/cluster-33 and P/cluster-36, Figure 7), something that indicates that the vibrations of the bridges, vs(P−O−P), are not significantly affected by the presence of boron. It is, however, obvious that the presence of bonded boron with mainly terminal phosphate tetrahedra, which have been calculated in phosphate chain structures, is responsible for the observed changes in the spectra. On the basis of the observable peaks of borophosphate structures that are due either to symmetric stretching vibrations, vs(PO2−) of the PØ2O2− tetrahedral, or due to symmetric stretching vibrations, vs(PO32−) of tetrahedra PØO32−, which are bonded to boron atoms, the peak at

Figure 3. Calculated structures in the HF/3-21G* level for metaphosphate structures: (a) meta-Sr (P/cluster-33), (b) meta-Ca,Na (P/ cluster-36), (c) meta-Sr with one boron (BP/cluster-32), and (d) the structure (BP/cluster-24) that contained two boron atoms. Colors: orange, P; red, O; dark green, Sr; purple, Na; light green, Ca; pink, B. The distances between atoms are marked with dashed lines.

of the glass, while the relatively strong band at 1083 cm−1 (symmetric vibrations of terminal bonds, vs(PO32−)) which is not respectively transparent, can be justified due to the high ratio of tetrahedra PØO32−/PØ2O 2− in the calculated structures. As it can be characteristically observed by these bands, their characteristic variation in comparison to the F


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Table 2. Bond Lengths and Electronic Charges of the Structures: P/cluster-33, P/cluster-36, and BP/cluster-32, BP/cluster-24a parameter

P/cluster-33

P−Ob bond length (Å) P−Onb bond length (Å) Sr−O bond length (Å) Na−O bond length (Å) B−O bond length (Å) charge in B charge in P charge in Ob charge in Onb charge in Sr charge in Ca charge in Na a

P/cluster-36 B

1.61−1.70 1.53−1.60 2.32−2.63

1.58−1.69 1.53−1.59

P/cluster-32

BP/cluster-24

1.63−1.64 1.53−1.57 2.26−2.67

1.61−1.67 1.55−1.61

2.10−2.37

(2.09) − (2.17) (−0.88)−(−1.16) (−0.99)−(−1.17) (1.37)−(1.68)

(1.99)−(2.22) (−0.99)−(−1.13) (−1.03)−(−1.08) (1.53)−(1.55) (0.40)−(0.57)

1.25−1.41 (0.99)−(1.12) (2.07)−(2.18) (−0.79)−(−1.06) (−0.79)−(−0.98) (1.53)−(1.55)

2.17−2.31 1.28−1.49 (0.99)−(1.12) (2.08)−(2.17) (−0.97)−(−1.22) (−0.72)−(−0.95) (1.53)−(1.55) (0.40)−(0.57)

Calculations in BP/cluster-24 with Ca2+ and Na+ compensating cations.

spectrum of the glass at 980 cm−1 is due to the same type of borophosphate units as those of the theoretically modeled structures. In correspondence with the above analysis, Figure 6 presents analytically the calculated Raman spectra for pyrophosphate

Table 3. Bond Lengths and Electronic Charges of the Structures: P/cluster-11a, BP/cluster-14, and BP/cluster-25 parameter P−Ob bond length (Å) P−Onb bond length (Å) B−Ob bond length (Å) B−Onb bond length (Å) charge in P charge in Ob charge in Onb charge in Sr charge in Ca

P/cluster11(Ca)

P/cluster11(Sr)

BP/ cluster-14

BP/ cluster-25

1.601

1.614

1.617

1.58

1.598

1.593

1.591

1.61

1.362

1.47

1.404 2.060 −1.110 −0.990

1.990 −1.040 −1.010 1.560

1.960 −0.940 −1.010 1.570

2.037 −0.980 −1.042 1.425

1.470

a

Calculations in P/cluster-11 with Ca2+ and Sr2+ compensating cations.

Figure 6. Calculated Raman spectra using the level HF/3-21G* for pyrosphate clusters: P/cluster-11, before the addition of boron; BP/ cluster-14, similar cluster as P/cluster-11 after the addition of boron in trigonal coordination; BP/cluster-25, similarly to the previous cluster, when boron has been added in tetrahedral coordination bridging two P/cluster-11.

structures that were simulated before and after the addition of boron, as resulted by the theory level of HF/3-21G*. As it is shown in Figure 6, in the spectra P/cluster-11 and BP/cluster-14 there is a significant difference regarding the calculated frequency position of their respective strongest Raman peaks, 1063 and 985 cm−1, respectively. On the contrary, the frequencies of the two main peaks in the spectra of the borophosphate structures BP/cluster-14 (985 cm−1) and BP/cluster-25 (963 cm−1) are relatively close. Both structures include trigonal and tetrahedral boron atoms connected in between of PØO32− tetrahedral. It is obvious that the presence of boron in either trigonal or tetrahedral coordination is manifested with a dramatic change of the pyrophosphate glasses’ spectra.

Figure 5. Comparison of the calculated spectra, using HF/3-21G*, with the experimental spectra of the glasses 0.50SrO·0.50P2O5 and 0.40SrO·0.40B2O3·0.20P2O5.

∼980 cm−1 may be attributed to respective vibrations as vs(BPO2−) or vs(BPO32−). The comparison of the spectra of borophosphate structures BP/cluster-32 and BP/cluster-24 with the experimental spectrum of the 0.40SrO·0.40B2O3· 0.20P2O5 glass display clearly that the main band in the G


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985 cm−1 is attributed to vibrations PØO32− tetrahedra that are connected with borate tetrahedra and it comes in complete agreement with the results of the reported theoretical study concerning the structure BP/cluster-25. It is, therefore, clear that the connected borate units with phosphate tetrahedra, either trigonal BØ2O− or tetrahedral BØ4− units, provoke the appearance of the band close to 985 cm−1 in the spectra of the studied borophosphate glasses. Clearly, this is in agreement with the recent NMR study,48 where at high boron content the major structural borophosphate network units are found to be phosphate species connected to 1 boron unit, trigonal or tetragonal. However, in this study isolated phosphate units (Q0) and pyrophosphate units (Q1) units are found to be minimal, probably due to the different cation valence strength in the two borophosphate glass systems. The question that arises now refers to the type of vibration of this band. Our findings based on the simulation of this specific symmetrical vibration of tetrahedra PØO32− of the borophosphate chain show that the observed reduction of the specific frequency of about 100 wave numbers is the result of the P−Ø bond being affected by the Ø−B bond. In this specific symmetrical stretching vibration, three terminal oxygen atoms (NBOs) vibrate symmetrically about the center phosphorus atom while the fourth bridging oxygen vibrates in phase to the three NBOs; however, it drags along the phosphorus atom. During this breathing mode, (snapshot from the Supporting Information videos are shown in Figure 8), the three O atoms coordinating the B atom oscillate symmetrically with π-phase difference with respect to the O motion of the P atom. It is interesting that the length of the P−O bond remains unchangeable during the

In Figure 7, all the calculated Raman spectra of the clusters of Table 3 as well as two experimental spectra of the synthesized

Figure 7. Comparison of calculated Raman spectra, using by HF/321G*, with the experimental spectra of 0.50SrO·0.34B2O3·0.16P2O5 (glass), crystalline 2SrO·P2O5, and Ba3BP3O16.

materials are depicted for comparison purposes. As it is illustrated, the main band in the spectrum of the structure, P/ cluster-11, is located at 1063 cm−1 and as it can be concluded by its comparison with the experimental spectrum of the pyrophosphate crystalline compound 2SrO·P2O5, it can be attributed to vibrations vs(PO32−) of the two phosphate tetrahedra in the pyrophosphate units P2O74−. The observed frequencies of the symmetric vibrations of the bridges vs(P− O−P) of the joined tetrahedra belonging to pyrophosphate units P2O74− are relatively close (760 and 752 cm−1) as well with the bending vibrations δ(O−P−O) of the same units (510 and 493 cm−1), thus being evidence of the experimental and calculated spectra agreement. As it also shown in Figure 7 between the comparison of the spectra for structures concerning the “metaphosphate stoichiometry”, a characteristic variation in the frequencies of the main peaks can be noticed that is due to the vibrations of phosphate tetrahedra vs(PO32−). More specifically, both calculated spectra of structures BP/ cluster-14 (one trigonal boron BØ2O− connected with two PØO32−) and BP/cluster-25 (a tetrahedral boron BØ4− connected with four tetrahedral PØO32− units) present their main peak at the frequency of 985 cm−1. Moreover, when comparing the spectra of BP/cluster-14, Ba3BP3O16 and that of the 0.50SrO.0.34B2O3.0.16P2O5 it is observed that their main band is at 985 cm−1. The compound of Ba3BP3O16 can be obtained by thermal decomposition of the compound BaBPO5 (same crystal structure of stillwellite type with the compound of SrBPO5, space group P3121),51 belongs to the orthorhombic system with Ibca space group and shows a typical structure of helical borate tetrahedrons chains (BØ4−) connected by two oxygens at their corners, while in the other two corners these are connected with phosphate tetrahedra PØO 3 2− . The vibrational spectrum of this compound (Ba3BP3O16) has been previously analyzed based mainly on the unit BPO7 instead of separately each tetrahedron BØ4− or PØO32− that are contained in it.49 However, the significant information arising from this previous work is that the band at

Figure 8. Representative snapshots of the oscillation in the BP/cluster14 responsible for the Raman line at 980 cm−1, where the B-bound oxygen atoms extend farthest from B, while the P-bound oxygen atoms retract closest to P. Stretching (a) and compaction (b) of P−O bonds while B−O bonds move opposite. In both cases, the O atoms belonging to the B−O−P bridge moves with the P atom. H


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(5) Magistris, A.; Chiodelli, G.; Duclot, M. Silver Borophosphate Glasses: Ion Transport, Thermal Stability and Electrochemical Behaviour. Solid State Ionics 1983, 9-10, 611−615. (6) Scagliotti, M.; Villa, M.; Chiodelli, G. Short Range Order in the Network of the Borophosphate Glasses: Raman results. J. Non-Cryst. Solids 1987, 93, 350−360. (7) Scagliotti, M.; Villa, M.; Chiodelli, G. Short Range Order in the Network of the Borophosphate Glasses. J. Non-Cryst. Solids 1987, 94, 101−121. (8) Grossman, D. G.; Phillips, C. J. Zinc Borophosphate Glass. J. Am. Ceram. Soc. 1964, 47, 471−471. (9) Clinton, J. M.; Coffeen, W. W. Low Melting Glasses in the System B2O3-ZnO-CaO-P2O5. Ceram. Bull. 1984, 63, 1401−1404. (10) Ray, N. H. A Study of the Coordination of Boron in Potassium Borophosphate Glasses by Raman Spectroscopy. Phys. Chem. Glasses 1975, 16, 75−80. (11) Yun, Y. H.; Bray, P. J. Nuclear Magnetic Resonance Studies of the Glasses in the System K2O-B2O3-P2O5. J. Non-Cryst. Solids 1978, 30, 45−60. (12) Ducel, J. F.; Videau, J. J. Physical and Chemical Characterizations of Sodium Borophosphate Glasses. Mater. Lett. 1992, 13, 271−274. (13) Ducel, J. F.; Videau, J. J.; Couzi, M. Structural Study of Borophosphate Glasses by Raman and Infrared Spectroscopy. Phys. Chem. Glasses 1993, 34, 212−218. (14) Ducel, J. F.; Videau, J. J.; Suh, K. S.; Senegas, J. 31P MAS and 11B NMR Study of Sodium Rich Borophosphate Glasses. Phys. Chem. Glasses 1994, 35, 10−16. (15) Ducel, J. F.; Videau, J. J.; Gonbeau, D.; Pfilster-Guillouzo, G. XRay Photoelectron-Spectra of Sodium Borophosphate Glasses. Phys. Chem. Glasses 1995, 36, 247−272. (16) Raguenet, B.; Tricot, G.; Silly, G.; Ribes, M.; Pradel, A. The Mixed Glass Former Effect in Twin-Roller Quenched Lithium Borophosphate glasses. Solid State Ionics 2012, 208, 25−30. (17) Larink, D.; Eckert, H.; Reichert, M.; Martin, S. W. Mixed Network Former Effect in Ion-Conducting Alkali Borophosphate Glasses: Structure/Property Correlations in the System [M2O]1/3[(B2O3)x(P2O 5)1‑x]2/3 (M = Li, K, Cs). J. Phys. Chem. C 2012, 116, 26162−26176. (18) Brow, R. K. An XPS Study of Oxygen Bonding in Zinc Phosphate and Zinc Borophosphate Glasses. J. Non-Cryst. Solids 1996, 194, 267−273. (19) Koudelka, L.; Mošner, P. Borophosphate Glasses of the ZnOB2O3-P2O5 System. Mater. Lett. 2000, 42, 194−199. (20) Koudelka, L.; Mošner, P.; Zeyer, M.; Jager, C. Structural Study of PbO-B2O3-P2O5 Glasses by NMR, Raman and IR Spectroscopy. Phys. Chem. Glasses 2002, 43C, 102−107. (21) Koudelka, L.; Mošner, P.; Zeyer-Düsterer, M.; Jäger, C. Study of Potassium-Zinc Borophosphate Glasses. J. Phys. Chem. Solids 2007, 68, 638−644. (22) Koudelka, L.; Mošner, P.; Zeyer, M.; Jäger, C. Structure and Properties of Mixed Sodium-Lead Borophosphate Glasses. J. NonCryst. Solids 2005, 351, 1039−1045. (23) Koudelka, L.; Mošner, P.; Vecera, M. Thermomechanical Properties and Stability of Mixed Strontium-Zinc Borophosphate Glasses. J. Mater. Sci. 2000, 35, 5593−5596. (24) Takebe, H.; Harada, T.; Kuwabara, M. Effect of B2O3 Addition on the Thermal Properties and Structure of Bulk and Powdered Barium Phosphate Glasses. J. Am. Ceram. Soc. 2006, 89, 247−250. (25) Sedmale, G.; Vaivads, J.; Sedmalis, U.; Kabanov, V. O.; Yanush, O. V. Formation of Borophosphate Glass Structure Within the System BaO-B2O3-P2O5. J. Non-Cryst. Solids 1991, 129, 284−291. (26) Saranti, A.; Koutselas, I.; Karakassides, M. A. Bioactive Glasses in the System CaO−B2O3−P2O5: Preparation, Structural Study and in Vitro Evaluation. J. Non-Cryst. Solids 2006, 352, 390−398. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H. Z. et al. Gaussian 03, Revision C.02. Gaussian, Inc.: Wallingford, CT, 2009.

vibration, where it seems that the particular breathing mode of B attached to the bridging oxygen increases the effective mass of the oscillating system, thereby reducing its frequency. Similar behavior is observed in the results of the calculations for the BP/cluster-25.

4. CONCLUSIONS The structure of strontium borophosphate glasses with compositions xSrO·(1 − x)·[0.68B2O3·0.32P2O5], 0.40 ≤ x ≤ 0.68 have been investigated combining Raman spectroscopy and ab initio molecular electronic structure calculations. According to the experimental Raman results, the glass structure for small SrO contents (0.40 ≤ x ≤ 0.50) consists mainly of phosphate and borophosphate chains and rings, while for intermedium SrO contents (0.50 ≤ x ≤ 0.62) the glass network is depolymerized rapidly to orthophosphate PO43− units, smaller borophosphate units and trigonal borate units with NBOs. At high SrO content, x > 0.62, orthophosphate units were the dominant species in the glass indicating the maximum degree of modification of the phosphate network. The ab initio calculations, which were performed on specific phosphate and borophosphate cluster structures, regenerated precisely the experimental Raman spectra and revealed the origin of the Raman band at around 985 cm−1. This band can be assigned to vs(P−O−) vibrations of phosphorus PØO32− tetrahedral units which are connected with BØ2O− or tetrahedral BØ4− borate units. The vibration also involves the in-phase motion of the fourth bridging oxygen of the PO4 tetrahedral unit that simultaneously drags along the phosphorus atom.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b01563. Videos of the oscillation in the BP/cluster-14 responsible for the Raman line at 980 cm−1, also animation videos of the vibrations of BP/cluster-24 and BP/cluster-25 (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ioannis Koutselas: 0000-0002-8879-8816 Michael A. Karakassides: 0000-0003-4344-0375 Notes

The authors declare no competing financial interest.

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REFERENCES

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Structural and Theoretical Study of Strontium Borophosphate Glasses

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