Study of a structure of boron–oxygen complexes in the molten and vapor states by Raman and luminescence spectroscopies

Journal of Molecular Structure 1008 (2012) 69–76

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Study of a structure of boron–oxygen complexes in the molten and vapor states by Raman and luminescence spectroscopies Yuri K. Voron’ko, Alexander A. Sobol’ ⇑, Vladislav E. Shukshin Prokhorov General Physics Institute of RAS, Vavilov Str. 38, Moscow 119991, Russia

a r t i c l e

i n f o

Article history: Received 26 September 2011 Received in revised form 18 November 2011 Accepted 18 November 2011 Available online 26 November 2011 Keywords: Boric oxide Metaborates High temperature Raman spectroscopy BO2-radicals luminescence Melts vaporization

a b s t r a c t High temperature Raman and luminescence spectroscopies were used for studying the reconstruction processes in overheated melts and vapors of alkali and alkali-earth borates and boric oxide up to 1800 K. New boron–oxygen fragments in form of the monomeric [BO2]1-anions were determined to be present in the overheated metaborates melts. Luminescence spectra of the BO2-radicals were measured in vapors over overheated borate melts at the selective laser excitation. These spectra were shown to differ from those of the BO2-radicals produced by the flash photolysis. An effect temperature and atmosphere sort on the BO2-radicals formation in the vapor state were studied. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Non-linear optical (NLO) crystals of different borates attract an attention as prospective materials for the second harmonic generation in an ultraviolet spectral region. BaB2O4 (BBO) [1], LiB3O5 (LBO) [2], CsB3O5 (CBO) [3] are well known NLO-materials. The main specialty of borates crystals is a presence of boron–oxygen fragments with strong covalent bonds. The structure of these fragments can be successively analyzed with Raman spectroscopy in crystalline [4], vitreous [5] and molten [6–9] states. Study of the construction of boron–oxygen complexes and their changes at overheating the melt is of great interest. Most NLO borates are synthesized from melts at temperatures over 1000 K. For technological reasons, before starting the synthesis, melts are overheated for the homogenization of these compositions. Such procedures can result in undesirable changes in their structure and composition, which in turn will interfere with the synthesis of high quality single crystals. Modified Raman spectroscopy was shown to supply information on structure of boron– oxygen complexes at overheating the melt [10]. Furthermore, the technique developed for Raman study of the borates melts was suite for measuring the luminescence spectra of molecular fragments in vapors over the surface of borates melts [11]. The goal of this work is to detail study the structure of molecular fragments in molten borates of series LiBO2, NaBO2, CsBO2, BaB2O4 and B2O3 and in vapors over a surface of their melts in the broad ⇑ Corresponding author. Tel.: +7 499 1350301; fax: +7 499 1350270. E-mail address: [email protected] (A.A. Sobol’). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.11.026

temperature region 800–1800 K in air and inert atmospheres by means of Raman and luminescence spectroscopies. 2. Experimental We carried out our experiments with the original technique designed earlier for Raman spectroscopy of melts at high temperatures [10]. The excitation source was a copper vapor laser operated in a pulse repetition regime of 15 kHz and the pulse duration of 10 ns. The average radiation power was about 3 W and the pick power was 30 kW. Two laser lines were used: k1 = 510.6 nm and k2 = 578.2 nm. A premonochromater ensured the excitation laser lines selection. To avoid the thermal background effect of the specimen and furnace, we used the gating registration system of 10 ns duration [10]. By means of an optical system, the scattered radiation by the specimen was directed to the entrance slit of a SPEX-Ramalog 1403 double monochromator. The resolution was 5 cm1 for all experiments. Control was provided with a SCAMP processor. A vertical resistance tubular furnace of Pt–30%Rh wire was used as the heater in air. The special chamber with the Pt– 30%Rh wire was constructed for measuring the Raman spectra in an inert atmosphere of nitrogen, which was cleaned from oxygen. The temperature was measured with a Pt–10%Rh/Pt–30%Rh thermocouple with an accuracy of 5 K. Specimens were placed into platinum crucibles of the 6 mm diameter and the 10 mm of length. The thermal zone of the furnace was 60 mm. The 180° observation geometry was used for measuring the Raman spectra through the top boundary of the studied molten compounds. It was possible to replace the inert atmosphere by air one at retaining the

70

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

temperature and the conditions of the experiment. The luminescence spectra of boron–oxygen molecules in vapors over the overheated melts were measured with the high temperature Raman technique as well. A platinum shield was placed over the melt surface to prevent the straight laser beam striking the melt volume and to discriminate the Raman signal at studying the vapors luminescence. The samples of polycrystalline a-LiBO2, NaBO2, CsBO2 were prepared by cooling the melts of corresponding compositions. Single crystals of b-BaB2O4 (studied previously in [9]), commercial KNO2 and vitreous B2O3 were used as the original samples.

3. Results and discussion 3.1. The Raman spectra of LiBO2, NaBO2, CsBO2 and BaB2O4 at the crystal-melt transitions and at different degrees overheating melts The a-LiBO2, NaBO2, CsBO2 and (a-, b-) BaB2O4 crystal structures were studied previously by X-ray analysis [12–16]. The main specialty of NaBO2, CsBO2 and (a-, b-) BaB2O4 is the presence of the isolated [B3O6]3 molecular groups (metaborate rings) as the structure units. Opposite it, the a-LiBO2 structure consists of infinite chains constructed from share corner boron–oxygen triangles. According to [4] the free [B3O6]3-rings have a D3h symmetry and its total internal vibrational representation can be written as follows:

C ¼ 3A01 þ 2A02 þ 5E0 þ 2A002 þ 2E00 : The Raman active modes are 3A01 þ 5E0 þ 2E00 in this case. The part of calculated displacement configurations of the internal free metaborate ring vibrations are illustrated in Fig. 1a [9]. It should be noted the R1 – symmetric ‘‘breathing oscillation’’ of metaborate ring, the R2 – symmetric ring bending mode, the R3 – symmetric stretching of the terminal B–O groups and the R4 – out-of-plain vibration. The latest mode is forbidden in the Raman spectra of a D3h symmetry unit but it can appear at reducing the symmetry. It is difficult to divide the total internal normal mode vibrations spectrum for the long-chain ½BO2 11 -anion in the molten LiBO2. It is convenient to consider the symmetric B–O–B bridge-bond vibrations (LC1) and the symmetric stretching vibrations of the terminal B–O groups (LC2) in this case [7]. It should be noted that the LC1and LC2-modes in the first approximation have analogs in forms of the R2- and R3-metaborate ring vibrations in Fig. 1a.

The polarized Raman spectra of LiBO2, NaBO2, CsBO2 and BaB2O4 before and after melting points (at 10–15 K overheating) are displayed in Fig. 2. The main features of the Raman spectra for the crystalline LiBO2, NaBO2, CsBO2 and BaB2O4 at 300 K retained up to the melting points. The R1–3-vibrations were identified reliably in the Raman spectra of all ring metabotate crystalline structures. The R4-out-of-plane vibration was absent in the Raman spectra of the NaBO2 and CsBO2 crystals (Fig. 2a and b). A D3h symmetry of the free [B3O6]3 metaborate rings is reduced to D3 in the NaBO2 and CsBO2 crystal sites. This symmetry was insufficiently low to activate the R4-vibration in the Raman spectra of these structures [8,9]. Opposite it, the R4-vibration was present in the Raman spectra of the crystalline (a-, b-) BaB2O4 due to reducing the free [B3O6]3-ring point symmetry to a C3 in these structures (Fig. 2d). The Raman spectra of boron–oxygen units constructions demonstrated the different changes at melting metaborates studied in dependence of the cation sort. The Raman spectra of the molten NaBO2, CsBO2 consisted of the narrow lines with frequencies, which corresponded to the Raman lines of the [B3O6]3-fragments in the crystalline states (Fig. 2a and b). It should be emphasized an appearance of the R4-line in the Raman spectra of the molten NaBO2 and CsBO2. Such fact testified to essential reducing the crystal site D3-symmetry of the [B3O6]3 metaborate rings at melting NaBO2 and CsBO2. The Raman spectrum of the molten LiBO2 demonstrates practically only two LC1- and LC2-broad bands in Fig. 2c. These lines were assigned to oscillations of the B–O–B bridge-bond and the B–O-terminal vibrations of the ½BO2 11 -long chains in [7]. The broad bands Raman spectrum of the molten LiBO2 is a result of the infinite ½BO2 11 anion form variation. Such spectrum significantly differed from the narrow lines Raman spectra of the [B3O6]3-rigid form anions in the molten NaBO2 and CsBO2. The melting process of BaB2O4 essentially differed from NaBO2, CsBO2 and LiBO2. It was seen from Fig. 2d, that the characteristic [B3O6]3-ring Raman spectrum of the crystalline BaB2O4 transferred into the broad band’s one at melting, and this spectrum was similar to that of molten LiBO2 metaborate. Hence, the boron oxygen groups constructions in a crystalline barium metaborate undergoes the n½B3 O6 3 ! ½BO2 1transformation at melting 1 BaB2O4. The Raman spectra of the molten LiBO2, NaBO2, CsBO2 and BaB2O4 showed two polarized bands in the 1000–1200 cm1 range, which are labeled by Z1 and Z2 in Fig. 2 and Table 1. These bands

(a)

(b) O

O R1 A1'

O

R2 A1'

B

B

O

O

–4(1)

O

B

(c) –O –B

B

O

O

1(–6)

R3 A1'

4(3)

R4 A2"

1(–6)

4(3)

(d)

O

B

O

1(–6)

4(3) –4(1)

–4(1) 3

Fig. 1. Calculated R1–R4 – internal normal mode vibrations of the free [B3O6] and the triatomic [BO2]1 anions of ‘‘V’’ (c) and linear constructions (d).

-metaborate ring according to [9] (a). Models of the three membered chain [BO2]33 anion (b)

71

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

(c)

(a) R1 Z1

R2

Z2

R3 HH 1010K (m) HV

LC1

Intensity, arb.un.

Intensity, arb.un.

R4

Z1

Z2

LC2 HH 1125K (m) HV

1100K (s)

950K (s)

+

2

300K (s) 300K (s)

500

1000

R1 R4

Z1

R2

1500

Δν, cm

-1

500

1000

Δν, cm

1500

(b)

Z2

-1

(d)

R3

1245K (s)

Intensity, arb.un.

Intensity, arb.un.

1265K (m)

LC1

Z1

Z2

LC2 HH 1420K (m)

R4

R1

HV

R2

R3

300K (s)

1170K (s) 300K (s)

500

1000

1500

Δν, cm

-1

500

1000

1500

Δν, cm

-1

Fig. 2. Polarized (excluding sodium metaborate) Raman spectra of liquid metaborates above the melting point (m). Nonpolarized spectra of crystalline samples at room temperature and below the melting point (s). CsBO2 (a), NaBO2 (b), LiBO2 (c) and BaB2O4 (d). R1–R4 – denotations of internal normal mode vibrations of the free [B3O6]3metaborate ring according to Fig. 1a, Z1,2 – new boron–oxygen fragments. The Raman spectra of barium metaborate crystals in Fig. 2d correspond to b-phase at 300 K and aphase at 1170 K. The 510.6 nm laser line excitation was used.

Table 1 The Raman lines frequencies (cm1) in the liquid metaborates spectra in Fig. 2. The cation sort

T (K)

R1

R2

R3

R4

LC1

LC2

Z1

Z2

Cs Na Li Ba

1010 1265 1125 1420

605 625 – –

745 760 – –

1460 1510 – –

540 550 – –

– – 765 720

– – 1485 1460

1050 1050 1090 1065

1220 1225 1190 1210

intensities increased with growing the temperature (Fig. 3) in the Raman spectra of all metaborates studied. Dependences of the relative integral intensity of the Z-lines (Iarb) from temperature are presented in Fig. 4. The values of Iarb(T) are equal:

Iarb ðTÞ ¼ IðZÞ=ðIðZÞ þ IðR3 or LC2 ÞÞ; where I(Z) and I(R3 or LC2) are the reduced integral intensities of the Z-lines and the R3- or LC2-bands in Fig. 3 respectively. The Bose-factor was taken into account at reducing the intensities. A redistribution of intensities of the Z-lines relatively to R- or LC-lines with a change of the melt temperature was reversible. The melts Raman spectra were restored to its initial views after cooling the overheated melts up to melting points. It was not detected any phases with the structures, which differed from the original metaborates phases after crystallizing these melts. The repeated melting and

overheating melts procedures were determined to demonstrate the regularities in of the Raman lines intensities redistribution, which were measured in the primaries experiments. The chemical analysis did not determine the essential compositions deviations from the initial ones as well. This allows rejecting the compositions deviation as a possible cause of the Z-fragments formations. It should be emphasized some important specialties of the Z1,2lines, which support their assignments. These specialties were especially noticeable in the Raman spectra of the Cs2OB2O3 melts in Figs. 2a and 3d. These spectra contained only the Raman lines of the metaborate rings [B3O6]3 and Z1,2-lines. A growth temperature of the melts resulted in significant increasing intensities of the Zlines doublet and decreasing the all Raman lines intensities assigned to the metaborate rings. New Raman lines did not appeared in the spectra even at strong overheating melts. In turns, there was not redistribution between the Z1- and Z2-lines intensities in the Raman spectra of the metaborate melts studied at strong increasing the temperature. Previously, an analysis of the Raman Z1,2-lines intensities behavior in Stocks and anti-Stockes spectral regions with a temperature growth proved reliably impossibility to assign Z1,2-lines to the Raman spectra of the second order [9]. An attempt to attribute Z1- and Z2-lines to the boron–oxygen fragments with two independent structures was made in the Raman spectra of the Na2OB2O3 melts [17]. There are some contradictions for such explanation. The Raman spectra of each Z-fragment would display only the sole intensive line in this case. The Raman spectrum of the

72

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

Z2

CsBO 2

(d)

LiBO 2 NaBO 2 BaB2 O4

Intensity, arb.un.

0,5

Z2

Z1

Z1

I arb

1110K

0,4

1170K

1070K

-2000

-1000

Z2

0

1000

Z2 Z1

Intensity, arb.un.

Z1

0,3

1010K

λ1

2000 Δν, cm

-1

(c)

T1 1000

1425K

1265K

λ1

-1000

0

1000

-1

2000 Δν, cm

Intensity, arb.un.

(b) Z1

Z2 Z 1

Z2

1600K

1500K

1420K

λ1

-2000

-1000

0

1000

2000 Δν, cm

-1

(a) Intensity, arb.un.

Z2

Z2 Z1

1425K

Z1 1335K

1135K

λ1

-2000

-1000

0

1000

2000 Δν, cm

T3 1200

T4 T, K

1400

1600

Fig. 4. Dependences of the Z1,2 Raman lines relative intensities (Iarb) versus the temperature for the metaborates. T1–T4 – the melting points of CsBO2, LiBO2, NaBO2 and BaB2O4 correspondently.

1285K

-2000

T2

-1

Fig. 3. Changes of the Raman spectra of metaborates at overheating the melts. LiBO2 (a), BaB2O4 (b), NaBO2 (c) and CsBO2 (d). The 510.6 nm laser line excitation was used.

isolated [BO3]3 anion corresponds to such specialty but the frequency of the sole intensive Raman line of an orthoborate fragment in melt corresponds to the 880–920 cm1 region [6]. The frequencies of the Z1-line (1050–1065 cm1) and the Z2-line (1190– 1210 cm1) did not suit to the [BO3]3 anion line frequency. The Z2-line put into the spectral interval where the Raman line of the terminal BO2 groups of the [B2O5]4 pyroborate anions was measured in the melts and glasses [6,18,19]. However, the Raman spectrum of the pyroborate anion in melts did not consisted of only the band in 1200 cm1 region and demonstrated two strong polarized lines in the 750–820 cm1 range [6,19]. These bands were absent in the Raman spectra of metaborate melts illustrated in Figs. 2 and 3. This excluded an assignment of the Z2-line to pyroborates fragments. The Z1-band position corresponded to the interval, which previously was assigned to the Raman band of the complicated fragment constructed from three and fourfold coordinated boron in glasses and melts of borates with the large (above 50 mol%) oxide modifier content [7,18]. However, the intensive polarized band 760–770 cm1 was present in such spectra in addition to the 1050–1060 cm1 band. Abovementioned 760–770 cm1 band was absent in the spectra in Figs. 2 and 3. Moreover such complicated fragments would be destructed at elevating the temperature according to data of the articles [7,20–22]. It means, that Z1-line cannot be assign to the Raman spectra of fragments contained the B–O tetrahedra because it was not registered any redistribution between the Z1- and Z2-lines intensities with a growth of the temperature. Thus, the Z1,2-lines doublet of the definite form in the Raman spectra in Figs. 2 and 3 should be considered as a result of a formation of the specific Z-boron–oxygen fragments in overheated melts. The Raman spectra in the form of Z-doublet were shown to be measured only for molten states [8,9] and these spectra have not had any analogs with the boron–oxygen Raman spectra of the different crystalline and vitreous borates previously studied [6–9]. Previously, three membered short chains ð½BO2 3 3 Þ were applied as a model for the Z-fragments [8,9] (Fig. 1b). These fragments were proposed to be a result of a breakage of the metaborate [B3O6]3-ring anions. However, such model has some contradictions. A number of freedom degrees of the three membered 3 ð½BO2 3 ring anions are the same. It implies 3 Þ and the [B3O6]

73

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

the complicated Raman spectrum for the ð½BO2 3 3 Þ anions, which cannot be restricted by two Z-bands. Furthermore, the Z-lines were registered not only in the Raman spectra of the ring (NaBO2, CsBO2), but in the chain (LiBO2, BaB2O4) metaborates melts. The [BO2]1 triatomic anions with a small freedom degree is more favorable as models of the Z-fragments (Fig. 1c and d). Two types of structures were known to exist for triatomic fragments. Well known [NO2]1 anion of ‘‘V’’-construction has a C2v symmetry. Similar model for the [BO2]1 anion was displayed in Fig. 1c. The internal vibrations spectrum of the [NO2]1 anion in a crystalline state consists of three modes, which are active in both Raman and IR spectra: 2A1 + B1 [23]. The diagonal scattering tensors correspond to the A1-modes. It means that two A1-Raman lines have to be polarized in the HH and HV geometry in the molten state. Indeed, the polarized Raman spectra of the KNO2-melt demonstrated such phenomenon in Fig. 5. The second model of a triatomic fragment implies a symmetric linear monomer of a D1h symmetry. Such model was shown to be characteristic of CO2 gas molecules [18]. Possible model for such form [BO2]1 anion is demonstrated in Fig. 1d. A vibrational spectrum of such fragments consists of P Pþ Pþ three þ u þ g þ Pu modes, but only one the g -line should occur in the Raman spectra. Thus, ‘‘V’’-construction model of the [BO2]1 triatomic anion (Fig. 1c) is the most reliable one for the abovementioned Z-fragments, the two strongly polarized Z-lines being present in the Raman spectra of the metaborate melts. Assuming proportionality between integrated Raman intensities and concentrations of the different boron–oxygen fragments, Iarb(T) in Fig. 4 illustrates elevating the [BO2]1 anions concentration with growth of temperature for all metaborates studied. Strong growth of the [BO2]1 anions concentration was especially noticeable for the overheated CsBO2 melt. 3.2. Luminescence spectroscopy of boron–oxygen fragments in vapors over molten borates Previously, investigations of the constitution and structure of boron–oxygen fragments of gaseous B2O3 and alkali metaborates were carried out with mass spectrometry and electron diffraction studies in vacuum and inert atmosphere [24–28]. Boric oxide was determined to evaporate principally to monomeric B2O3 free molecules of ‘‘V’’-form. The saturated vapors of alkali metaborates at high temperatures were shown to consist mainly of monomeric triatomic MBO2-molecules, the [BO2]-fragment having a linear form. The vaporization of boric oxide in air or oxygen atmospheres at high temperatures resulted in an appearance of the BO2-radicals in a gaseous state [29,30]. The reaction of such process was:

1=2 B2 O3 ðliq:Þ þ 1=4O2 ! BO2 ðgasÞ: Absorption spectra of the BO2-radicals were measured in the visible spectral region 530–570 nm in vapors over liquid boric oxide at 1660–1950 K [28]. The broad band with 545.0 nm maximum was measured in the experiment and the intensity of this band was used to study the effect of O2 on the BO2-radicals concentration in the vapors. More detail study of the BO2-radicals absorption spectra in the region 400–550 nm at 4 K was carried out in [30]. The gaseous BO2-fragments produced in a high temperature cell then were inserted into solid argon matrices in these experiments. Other method of the BO2-radicals generation was the flash photolysis of a BCl3 + O2 gas mixture [31,32]. Such technique of the BO2-radicals formation allowed detail studying the BO2-radicals absorption and luminescence spectra in the region 390–650 nm at 300 K. These spectra demonstrated the groups consisted of narrow spectral lines. We studied previously the luminescence spectra of the BO2-radicals in vapors over the molten boric oxide up to 1700 K in air [11]. Now we measured firstly these spectra in the broad range 400– 800 nm wavelengths over the overheated B2O3-melt up to 1800 K. These spectra are demonstrated in Fig. 6 at the separate excitation by the green 510.6 nm and the yellow 578.2 nm laser lines at focusing the laser beam on the melt surface. The platinum shield over the liquid boric oxide prevented the Raman spectra excitation in the experiment. The luminescence spectrum in Fig. 6b at the 578.2 nm excitation demonstrates series broad bands with the maxima approximately 472.0, 493.0, 518.0, 545.0, 580.0, 620.0, and 638.0 nm. These bands maxima satisfactory corresponded to the lines groups in the absorption spectra of the BO2radicals produced by the flash photolysis in [31]. The luminescence spectrum in Fig. 6b displays the bands, at spacing of approximately 1000 cm1, which were characteristic of the gaseous BO2-radicals. The spectrum at the 510.6 nm line excitation in Fig. 6a demonstrates some difference from the spectrum in Fig. 6b at the yellow line excitation. There is a redistribution of intensities of the main bands at changing the excitation wavelength, some bands having two-humps form in the spectrum at the 510.6 nm line excitation. Moreover, the spectrum in Fig. 6a contains the additional low intensity maxima at 530.5 and 603.0 nm in comparison with those in Fig. 6b. The specialty of luminescence spectra in Fig. 6 was the complicated structure of each separate band. These bands contained the broad foundation and series narrow lines (3–5 cm1 width) over the band contour. The frequencies of the separate

A1

A1

λ1

b 472.0

HH HV

1000

a

λ2

B1

500

603.0

Intensity, arb. un.

Intensity, arb.un.

530.5

1500

Δν, cm

-1

Fig. 5. Polarized Raman spectra of the molten KNO2 at 700 K at the 510.6 nm laser line excitation.

493.0 20000

518.0

545.0 18000

580.0

620.0 638.0 16000

λ, nm

ν, cm-1

Fig. 6. Luminescence spectra of the BO2-radicals in vapors over liquid boric oxide at 1800 K (in air) at selective laser lines excitations: k1 = 510.6 nm (a) and k2 = 578.2 nm (b).

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

narrow lines did not coincide in the spectra excited by the different laser lines. The effect of the excitation laser line wavelength on the BO2radicals luminescence spectra at 300 K was displayed previously in the paper [33]. The 488.0 and 514.5 nm argon laser lines excitations were used in this article and the BO2-radicals were produced by the flash photolysis. In opposite to our experiment at high temperature, the different luminescence spectra of the paper [33] displayed only narrow lines rather than broad bands. The BO2-radicals produced by the flash photolysis were proposed to be only the D1h symmetry linear monomers [32]. The effect selective laser excitation on the luminescence spectra in this case would be explained due to the selection rules for the definite transitions, which were in resonance with the excitation illumination. Such explanation was insufficient to analyze the spectra demonstrated in Fig. 6. A presence of the luminescence bands with broad contours (200– 400 cm1) testifies to an inhomogeneous structure for the BO2-radicals at high temperatures in opposite to the linear BO2-monomers occurred at low temperatures. Probably the identical linear construction of the BO2-radicals at low temperature did not retain at high degrees overheating and there was the variation of their forms. An appearance of series of narrow luminescence lines over the broad contours at the different laser lines illumination was a result of a selective excitation of the BO2-radicals with the different structures. The 578.2 nm laser lines, as it is followed from Fig. 6, to put into the center of 580.0 nm band, whereas the 510.6 nm laser line wavelength corresponds to the far blue edge of the band with 518.0 nm maximum. Such significant difference in the excitation conditions resulted in the selective luminescence spectra of the BO2-radicals with essential different constructions. The luminescence band form in the region 545.0 nm at the yellow laser line excitation was the excellent indicator of the BO2-radicals presence in the vapor phase. Fig. 7 shows the luminescence spectra of overheated up to 1600–1700 K liquid B2O3, LiBO2, NaBO2, and BaB2O4 at the both lines copper vapor laser excitation in air. All these spectra contain the characteristic luminescence band at the 545.0 nm maximum of the BO2-radicals. Thus, the BO2-radicals formation occurred not only in vapors over the overheated B2O3 melt but in those of abovementioned metaborates. We have not registered this phenomenon in vapors over the CsBO2 melt. It was impossible to overheat the melt above 1400 K due to

-2000

-1000

0

1000

2000

Δν, cm-1

Intensity, arb.un.

1600K

its boiling, and this temperature was determined to be insufficient to form the gaseous BO2-radicals according to abovementioned reaction. The existence the BO2-radicals luminescence restricted an upper temperature limit for the high temperature Raman study of borates melts. Fig. 8 demonstrates the Raman spectra evolution from 300 up to 1800 K for boric oxide at the 510.6 nm laser line excitation in air. The sample spectra measured from 300 up to 1700 K consisted predominantly of the Raman lines, whereas the spectrum at 1800 K in Fig. 8a contained a mixture of the Raman spectra of the melt and the luminescence spectra of the vapors. The BO2-radicals luminescence spectrum in vapors at this temperature was selected by installing a shield over the melt surface to exclude the Raman lines excitation (Fig. 8b). The conditions of excitation and registration of the spectra in Fig. 8a and b were the same. Subtracting the normalized spectra in Fig. 8a and b allowed to restore the original Raman spectrum of the B2O3 liquid overheated up to 1800 K (Fig. 8c). This spectrum contains the 808 cm1 line, which corresponds to the Raman spectra of B3O6boroxol rings vibration in the molten and vitreous boric oxides previously studied in [34]. A phenomenon of the B3O6-boroxol rings breakage and the formation of a random BO3-triangles network at overheating molten boric oxide was determined in the paper [34] with Raman spectroscopy study. A decrease of the intensity of the 808 cm1 Raman line with elevating the temperature was the indicator of this process. The spectrum in Fig. 8c shows the presence of this line in the Raman spectra of molten B2O3 at 1800 K. Thus, this high temperature was insufficient for a whole

c b 1800K

a

Intensity, arb.un.

74

1700K

1600K

NaBO2

1500K

1750K

1400K

LiBO2

1300K

1600K

B2O3

1000K

BaB2O4

22000

21000

20000

300K

1750K

λ1

545.0 nm 19000

18000

- 808 cm

ν, cm

-1

Fig. 7. Luminescence spectra in the Stokes and anti-Stokes k1 region of the BO2radicals in vapors over overheated melts of LiBO2, NaBO2, BaB2O4 metaborates and boric oxide at simultaneous k1 = 510.6 nm and k2 = 578.2 nm laser lines excitation in air. The 545.0 nm-line denotes the maximum of the BO2-radicals characteristic band.

-2000

-1000

-1

λ1 0

808 cm -1 1000

2000

Δν, cm-1

Fig. 8. The Raman spectra of boric oxide from 300 to 1700 K at the k1 = 510.6 nm excitation in air. A mixture of the Raman spectrum of overheated liquid B2O3 and the luminescence of the BO2-radicals at 1800 K in air (a). The selected BO2-radicals luminescence spectrum at 1800 K (b). The Raman spectrum of liquid boric oxide received by subtracting a and b spectra.

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

breakage of the boroxol rings motif and a part of these rings retained at 1800 K. It should be noted that the original Raman spectra of the liquid B2O3 was previously displayed only up to 1423 K [34]. The effect of oxygen on the gaseous BO2-radicals formation at heating the molten B2O3 was studied previously only with the absorption spectra in the paper [29]. We studied this phenomenon using the BO2-radicals luminescence spectra, and included the LiBO2 and BaB2O4 metaborates in addition to boric oxide as the experimental objects. The experimental chamber with the sample was evacuated primarily. Then, it was filled by gaseous nitrogen up

T=1575 K

Intensity, arb.un.

(c)

2

1 λ1 -2000

-1000

0

T=1750K

2000 Δν, cm

-1

Intensity, arb.un.

(b)

2

1 λ1 -2000

-1000

0

T=1600K

2000 Δν, cm

-1

Intensity, arb.un.

(a)

-2000

-1000

0

1 545.0 nm

1000

2000 Δν, cm

This work was performed with financial support from the Russian Foundation for Basic Research (Project No. 10-02-00401). References

2

λ1

High temperature Raman and luminescence spectroscopies allowed registering essential transformations of boron oxygen fragments in the molten and vaporous states at high temperatures. Overheating the alkali and alkali-earth metaborates melts resulted in a formation of new Z-boron–oxygen fragments. These Z-complexes have presumably the [BO2]1 triatomic ‘‘V’’-construction. The vapors over overheated borates melts contained the BO2radicals, which were studied by a luminescence spectroscopy. The structure of these radicals at high temperature differed from that of the BO2-radicals produced by the flash photolysis. A formation of the BO2-radicals took place in vapors not only over overheated boric oxide melt but over alkali and alkali-earth liquid metaborates. Acknowledgment

545.0 nm

1000

to atmosphere pressure and the gaseous medium was cleaned from water vapors and oxygen with a getter. The spectra were measured in the Raman anti-Stokes and Stokes regions of the green laser line excitation. A use of both copper vapor laser lines supplied the simultaneous excitation the Raman spectra of melts and the luminescence spectra of vapors. After measuring the spectra at high temperature in nitrogen atmosphere, the chamber was opened on air. The heater was not switched off and the conditions of excitation and registration of the spectra were retained before and after the gaseous atmospheres changing. Fig. 9 displays the Raman and luminescence spectra of LiBO2, BaB2O4 and B2O3 molten samples at 1600–1700 K in nitrogen and then in air atmospheres. The spectra of these samples in nitrogen atmosphere contained mainly the Raman spectra of the boron oxygen fragments of molten borates (Fig. 9(1)). The small intensity characteristic luminescence band of the BO2-radicals in the 545.0 nm region shows the formation of some concentration of these radicals in our experiments due to background oxygen. This band intensity in the spectra of all samples violently grew in air atmosphere as factors of 5–10, the BO2-radicals luminescence spectra intensity exceeding essentially the Raman spectra of the borates melts (Fig. 9(2)). Thus, the oxygen presence was necessary for the BO2radicals formation in vapors not only over molten boric oxide, but over metaborates melts. 4. Conclusions

545.0 nm

1000

75

-1

Fig. 9. The mixture of the Raman spectra of liquid borates B2O3 (a), BaB2O4 (b), LiBO2 (c) and the luminescence spectra of BO2-radicals in vapors over the melts at simultaneous k1 = 510.6 nm and k2 = 578.2 nm laser lines excitation in nitrogen (1) and air (2) atmospheres: B2O3 (a), BaB2O4 (b), LiBO2 (c). The 545.0 nm-line denotes the maximum of the BO2-radicals characteristic band.

[1] C. Chen, B. Wu, A. Jiang, G. You, Sci. Sin. (China) B28 (1985) 235–243. [2] C. Chen, Y. Wu, A. Jiang, B. Wu, G. You, R. Li, S. Lin, J. Opt. Soc. Am. B6 (1989) 616–621. [3] Y. Wu, T. Takatomo, S. Nakai, A. Yokotani, H. Tang, C. Chen, Appl. Phys. Lett. 62 (1993) 2614–2615. [4] T.W. Bril, Philips Res. Rep. 2 (1976) 144. [5] W.L. Konijnendijk, Philips Res. Rep. 1 (1975) 243. [6] Y.K. Voronko, A.V. Gorbachev, A.B. Kudryavtsev, V.V. Osiko, A.A. Sobol, Inorg. Mater. 28 (1992) 1361–1367. [7] Y.K. Voronko, A.V. Gorbachev, A.B. Kudryavtsev, A.A. Sobol, Inorg. Mater. 28 (1992) 1368–1372. [8] Y.K. Voronko, A.V. Gorbachev, A.B. Kudryavtsev, A.A. Sobol, Inorg. Mater. 28 (1992) 1373–1378. [9] Y.K. Voronko, A.V. Gorbachev, V.V. Osiko, A.A. Sobol, R.S. Feigelson, R.K. Route, J. Phys. Chem. Sol. 54 (1993) 1579–1585. [10] Yu.K.Voron’ko, A.B. Kudryavtsev, V.V. Osiko, A.A. Sobol’, in: Kh.S. Bagdasarov, E.L. Lube (Eds.), Growth of Crystals, vol. 16, Consultant Bureau, N.Y., 1991, pp. 178–202. [11] Yu.K. Voron’ko, A.A. Sobol’, V.E. Shukshin, Inorg. Mater. 45 (2009) 561–567. [12] W.H. Zachariasen, Acta Cryst. 17 (1964) 749–751. [13] S.M. Fang, Z. Kristallogr. 99 (1938) 1–8. [14] W. Schneider, G.B. Carpenter, Acta Cryst. B26 (1970) 1189–1191. [15] A.D. Mighell, A. Perioroff, S. Block, Acta Cryst. 20 (1966) 819–823.

76

Y.K. Voron’ko et al. / Journal of Molecular Structure 1008 (2012) 69–76

[16] J. Liebertz, S. Stahr, Z. Kristallogr. 165 (1983) 91. [17] A.A. Osipov, L.M. Osipova, Physica B 405 (2010) 4718–4732. [18] E.I. Kamitsos, M.A. Karakassides, G.D. Chryssikos, Phys. Chem. Glasses 28 (1987) 203–209. [19] E.I. Kamitsos, M.A. Karakassides, A.P. Patsis, J. Non-Cryst. Sol. 111 (1989) 252– 262. [20] T. Yano, N. Kunimine, S. Shibata, M. Yamane, J. Non-Cryst. Sol. 321 (2003) 147– 156. [21] O. Maje´rus, L. Cormier, G. Calas, B. Beuneu, Phys. Rev. B 67 (2003) 024210. [22] S. Sen, J. Non-Cryst. Sol. 253 (1999) 84. [23] M.H. Brooker, D.E. Irish, Can. J. Chem. 46 (1968) 229–233. [24] R.S. Mulliken, J. Chem. Phys. 3 (1935) 720–739.

[25] [26] [27] [28] [29] [30] [31] [32] [33] [34]

M.G. Inghram, R.F. Porter, W.A. Chupka, J. Chem. Phys. 25 (1956) 498–501. A. Buchler, J.B. Berkowitz-Mattuck, J. Chem. Phys. 39 (1963) 286–291. P.A. Akishin, V.P. Spiridonov, J. Struct. Chem. 2 (1960) 53–54. P.A. Akishin, V.P. Spiridonov, Doklady Akad. Nauk S.S.S.R. (in Russian) 131 (1960) 557. W.E. Kaskan, J.D. Mackenzie, R.C. Millikan, J. Chem. Phys. 34 (1961) 570–574. A. Sommer, D. White, M.J. Linevsky, D.E. Mann, J. Chem. Phys. 38 (1963) 87–98. J.W. Johns, Can. J. Phys. 39 (1961) 1738–1768. G. Herzberg, Spectra and Structure of Simple Free Radicals. Cornell U.P. N.Y., 1971. D.K. Russell, M. Kroll, R.A. Beaudet, J. Chem. Phys. 66 (1977) 1199–2008. G.E. Walrafen, S.R. Samanta, S.R. Krishman, J. Chem. Phys. 72 (1980) 113–120.