Growth, Morphology and Optical Properties of γ-BiB3O6Single Crystals

Article pubs.acs.org/crystal

Growth, Morphology and Optical Properties of γ-BiB3O6 Single Crystals Pavel N. Gavryushkin,*,† Ludmila I. Isaenko, Alexander P. Yelisseyev, Viktor A. Gets, and Olga S. Il’ina V.S. Sobolev Institute of Geology and Mineralogy SB RAS, 3 ac. Koptyug Avenue, Novosibirsk 630090, Russia † Novosibirsk State University, 2 Pirogova Street, Novosibirsk 630090, Russia ABSTRACT: High quality γ-BiB3O6 single crystals up to 16 × 18 × 32 mm3 in size were grown by the Kyropoulos technique. Indexes of the faces for as grown γ-BiB3O6 single crystals were determined, and transmission spectra were recorded in polarized light. The band gap was found to be Eg = 3.982, 3.968, and 4.008 eV at 300 K for E∥a, b, and c, respectively. Eg decreases linearly versus temperature with a dEg/dT = 0.00127 eV/deg rate for E∥a as temperature grows from 80 to 300 K. Photoluminescence in the broad 2.5 eV band, which is excited in a band near the fundamental absorption edge and quenches completely at 250 K, is associated with self-trapped excitons.

1. INTRODUCTION The monoclinic α-BiB3O6 (C2) exhibits particularly large nonlinear optical coefficients, and it is now one of the most promising nonlinear optical materials.1 Thermodynamic relationships between polymorphic BiB3O6 modifications, described in ref 2, are useful when choosing the techniques for synthesis and growth of these crystals. All results were obtained on powders. According to ref 2, β-BiB3O6 is a metastable phase, which can be synthesized, but its stability field cannot be determined in the P−T phase diagram. The other polymorphic BiB3O6 modifications (α, γ, δ) are thermodynamically stable. At atmospheric pressure, the hightemperature α-BiB3O6 phase is stable in the narrow temperature range 710−715 °C. The phase γ-BiB3O6 is stable in the intermediate 680−710 °C temperature range whereas δ-BiB3O6 is the phase which is stable at low temperatures. The stability field of δ-BiB3O6 extends as pressure increases. Thus, the polymorphic α → γ → δ transition may occur at enhanced pressure. To date, only α- and δ-BiB3O6 polymorphs were grown as bulk crystals using the top-seeded method,1,3 whereas β and γ phases were synthesized as polycrystalline powders by the hydrothermal technique.2 So, in the present paper we focus our attention on γ-BiB3O6, the morphology and optical properties of which are still unexplored. Unlike the structure of α-BiB3O6, consisting of borate layers with a BO4/BO3 ratio of 1/2,4,5 the structure of γ-BiB3O6 is a 3D network that is completely constructed of BO4 groups.6 γ-BiB3O6 crystallizes in the centrosymmetric space group (P21/n; a = 8.4992 Å, b = 11.7093 Å, c = 4.2596 Å, β = 121.14°7) and cannot be used as a material for the second-order nonlinear optical properties. Nevertheless, the high density of γ-BiB3O6 at 6177 g/cm3 allows one to consider it, when doped in a right way, promising as a scintillator. Growth of large single crystals allowed us to carry out their goniometrical and spectroscopic study, which promotes a search for their effective application. © 2011 American Chemical Society

2. EXPERIMENTAL PROCEDURE 2.1. Growth of γ-BiB3O6 Single Crystals. A starting charge for growing γ-BiB3O6 was produced by solid-phase synthesis from high purity bismuth oxide and boric acid. The mixture was annealed twice during 24 h at 700 °C; the charge was powdered before the second annealing. When testing the second harmonic generation from the Nd:YAG 1.06 μm radiation, we found a positive response, which showed that there was a component with a noncentrosymmetric structure. The powder X-ray structural analysis confirmed that the α-BiB3O6 phase was the result of the solid-phase reaction. The BiB3O6 charge was filled into a platinum crucible 60 mm in diameter, which was then heated to 800−850 °C. At this temperature, a Pt stirrer was put into the melt, which was mixed at a rate of 10 rpm during 2 days. After homogenizing, the melt was cooled to 708 °C and a Pt wire, preliminary heated, was introduced slowly. Afterward, the crucible was cooled at a 0.1 °C/h rate. At insufficient mixing, the melt was split and thin transparent needle-shaped Bi2B8O15 crystals grew on a Pt wire, whereas needle-shaped Bi3B5O12 crystals crystallized on the crucible bottom. A similar phenomenon described in ref 8. When the preliminary operations are carried out thoroughly, there was no delamination; α-BiB3O6 and γBiB3O6 crystals grew on the Pt wire. All crystals were identified by powder X-ray analysis. Further, they can be used as a seed. In the first series of experiments, we used α-BiB3O6 crystals for seeding. Single crystals were grown by the Kyropoulos technique without mixing the melt. We could not obtain a large α-BiB3O6 single crystal in these experiments: only half-transparent α-BiB3O6 and γ-BiB3O6 crystals, up to 5 mm in size, grew on a seed, as in the case of Pt wire. Since the melt vitrified after its cooling to room temperature, one should anneal the crucible at 550 °C during 16 h before the beginning of the second series of growth experiments. Here, the glass lost its transparency and became milky white. In the second series of experiments, the γ-BiB3O6 single crystals, oriented along the [001] axis, were used as a seed. After heating to Received: June 24, 2011 Revised: November 9, 2011 Published: December 5, 2011 75

dx.doi.org/10.1021/cg200799n | Cryst. Growth Des. 2012, 12, 75−78

Crystal Growth & Design

Article

718 °C, the crucible was kept at this temperature up to full homogenizing of the melt. The seed was dipped in the melt at 708 °C. Visually one could see the beginning of crystal growth as temperature decreased by 2 to 3 °C. Crystals were grown without any rotation and pulling. The temperature gradient and the cooling rate were 1−2 °C/cm and 0.1 °C/day, respectively. After growth, the crystal was cooled to room temperature in air at a rate of approximately 10 °C/h. As a result of the growth cycle, we obtained a well-shaped, transparent γ-BiB3O6 crystal (Figures 1 and 2). The size of the largest γ-BiB3O6 single crystal was 16 × 18 × 32 mm3, and its mass was 42.3 g.

are ∼0.18: this corresponds to a refraction index n of about 1.8−1.9, which is close to values for α-BiB3O6.10

3. RESULTS AND DISCUSSION 3.1. Morphology of γ-BiB3O6 Crystals. The results of goniometric measurements and orientation determination for grown and spontaneous γ-BiB3O6 crystals are shown in Figures 2 and 3, respectively. The morphology investigation of the (10̅ 1),

Figure 1. Photo of a γ-BiB3O6 single crystal.

Figure 3. Morphology of spontaneous γ-BiB3O6 crystals.

(3̅01), and (1̅2̅1) faces shows some signs of destruction of the flat growth front: one can see a deepening in the face center relative to its periphery (Figure 4).

Figure 2. Scheme of a γ-BiB3O6 single crystal with indicated face indexes. 2.2. Goniometry of γ-BiB3O6 Crystals. Goniometric measurements were carried out by a standard technique using a one-circle G-5 goniometer. We studied both large crystals grown on a seed and little crystals grown on a Pt wire. Similar information concerning α-BiB3O6 single crystals is given in ref 1. 2.3. Optical Spectroscopy of γ-BiB3O6 Single Crystals. Transmission spectra were recorded along the γ-BiB3O6 transparency range using a Fourier-transform spectrometer Infralum 801 in the midIR and a UV-25013C Shimadzu spectrometer in the UV to near-IR spectral range. The photoluminescence (PL) spectra were obtained using a luminescence spectrometer SDL-1 at excitation from a 1 kW Xe lamp through a diffraction MDR2 monochromator, with color glass filters used for additional monochromatization. The photoluminescence excitation spectra (PLE) were corrected for a constant number of incident quanta using the Rhodamin 640 dye; PL spectra were not corrected. Measurements were carried out on two polished 2.2 mm thick plates with aperture ∼4 cm2 ; their edges were parallel to crystallographic axes. Absorption coefficients α were calculated from transmission T following the well-known formula:9

T = (1 − R )2 e−αd /(1 − R2e−2αd)

Figure 4. Morphology of the faces (a) (1̅01) and (b) (1̅2̅1).

3.2. Spectroscopic Properties of γ-BiB3O6 Crystals. Transmission spectra for γ-BiB3O6 recorded in polarized light at 300 K are given in Figure 5. γ-BiB3O6 plates 2.2 mm thick are transparent in the 0.315−3.15 μm range on a 50% level transmission, at 300 K. The transparency range on a 5% level is 0.31− 6.33 μm. From spectra in Figure 5, one can see that examined γ-BiB3O6 crystals were of high optical quality: transmission increases quickly as wavelength decreases in the shortwave region, and a residual absorption is less than 0.05 cm−1 in a large part of the transparency region (0.35−2.5 μm). Analysis of the fundamental absorption edge shows that there is a rectification of the absorption spectrum in the α2(hν)2 dependence on photon energy hν. This allows a conclusion that

(1)

where α is in inverse centimeters, d is the plate thickness in centimeters, and R is the reflection coefficient. This formula takes into account multiple reflections from polished crystal faces. Total reflection losses 76

dx.doi.org/10.1021/cg200799n | Cryst. Growth Des. 2012, 12, 75−78

Crystal Growth & Design

Article

Figure 5. Transmission in polarized light for a γ-BiB3O6 plate 2.2 mm thick. T = 300 K. The light polarizations are E∥a (1), E∥b (2), and E∥c (3).

the fundamental absorption edge in γ-BiB3O6 is determined by direct allowed electronic transitions.11 The shape of the fundamental edge in coordinates α2(hν)2 = f(hν) for 80 and 300 K temperatures and different light polarizations is shown in Figure 6. It is known that the linear approximation of such

larger than usual values typical of oxides (∼0.1 eV12). Taking into account that BiB3O6 may be in different phase states, one can expect that such a large shift of the γ-BiB3O6 band gap is a result of some phase transition, and this stimulated us to examine carefully the absorption edge behavior in the range 80−300 K. The Eg dependence versus temperature for light polarization E∥a is given in Figure 7. It is approximated well enough by a

Figure 6. Fundamental absorption edge for γ-BiB3O6 shown in coordinates (αhν)2 = f(hν) at T = 80 K (1−3) and 300 K (4−6). The light polarizations are E∥a (1, 4), E∥b (2, 5), and E∥c (3, 6).

Figure 7. Points showing the temperature dependence of the γ-BiB3O6 band gap Eg for component E∥a. The solid line represents a linear approximation Eg = A + BT, where A = 4.298 eV and B = −0.00127 eV/K.

curves until crossing the abcissa axes (α2(hν)2 = 0) gives the value of the band gap Eg at a given temperature and polarization. The obtained band gap values for γ-BiB3O6 are given in Table 1. The Eg values at room temperature are 3.982, 3.968,

straight line Eg = A + BT with parameters A = 4.298 eV and B = −0.00127 eV/K. The values for the B parameter for polarizations E∥b and E∥c calculated from the data in Table 1 are 0.00101 and 0.0009 eV/K. A smooth Eg = f(T) dependence in Figure 7 shows that there are no phase transitions for the sample under examination in the range 80−300 K. At room temperature, the γ-BiB3O6 crystals demonstrate a weak white-bluish photoluminescence at UV excitation. Its intensity increases several orders as temperature is lowered to 80 K. The PL spectrum is given in Figure 8 (curve 2). There is a broad symmetric band centered at 2.532 eV (0.490 μm) with a full width at half-maximum (FWHF) 0.67 eV, without any fine structure. It is the case of strong electron−phonon interaction, and the band shape is a Gaussian. Curve 3 shows a photoluminescence excitation spectrum for the 2.25 eV (0.55 μm) PL emission, measured at 80 K. As follows from the PLE spectrum in Figure 8 (curve 3), PL is excited in a narrow 4.2 eV band

Table 1. Band Gap Values for γ-BiB3O6 at 80 and 300 K for Different Light Polarizations (Calculated from Figure 5)a light polarization

Eg at 80 K (eV)

Eg at 300 K (eV)

ΔEg (eV)

E∥a E∥b E∥c

4.226 4.191 4.212

3.982 3.968 4.008

0.244 0.223 0.204

Here ΔEg = Eg(80 K) − Eg(300 K); eV is a Eg shift as temperature changes from 80 to 300 K.

a

and 4.008 eV at 300 K and 4.226, 4.191, and 4.212 eV at 80 K for the polarizations E∥a, E∥b, and E∥c, respectively. Thus, the band gap decreases by ∼0.25 eV as temperature grows from 80 to 300 K. This shift is considerably (∼2.5 times) 77

dx.doi.org/10.1021/cg200799n | Cryst. Growth Des. 2012, 12, 75−78

Crystal Growth & Design

Article

4. CONCLUSION

Figure 8. Absorption spectrum (E∥a, curve 1) photoluminescence at 4.2 eV; excitation (2) and luminescence excitation (3) spectra for 2.25 eV emission in γ-BiB3O6 at T = 80 K.

■ ■ ■

1 Large high quality γ-BiB3O6 single crystals were grown by the Kyropoulos technique. The morphology of γ-BiB3O6 was studied and indexes of faces determined. 2 The shape of the fundamental absorption edge was determined by direct allowed electronic transitions in γ-BiB3O6. The band gap decreases steadily as temperature rises in the range 80−300 K, which implies that there are no phase transitions in this range. 3 The 2.5 eV PL, which is excited in the shortwave band near the fundamental absorption edge and quenched completely to 250 K, is related to the recombination of self-trapped excitons.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone/Fax: 7-383-333-38-43. ACKNOWLEDGMENTS This work has been partially supported by the Siberian Branch of the Russian Academy of Science, RAS (Project II.7.5). (1) Becker, P.; Liebertz, J.; Bohaty, L. J. Cryst. Growth 1999, 203, 149−155. (2) Cong, R.; Zhu, J.; Wang, Y.; Wang, T.; Liao, F.; Jin, C.; Lin, J. CrystEngComm 2009, 11, 1971−1978. (3) Aleksandrovsky, A. S.; Vasiliev, A. D.; Zaitsev, A. I.; Zamkov, A. V. J. Cryst. Growth 2008, 310, 4027−4030. (4) Fröhlich, R.; Bohatý, L.; Liebertz. Acta Crystallogr. 1984, C40, 343−344. (5) Stein, W.-D.; Cousson, A.; Becker, P.; Bohatý, L.; Braden, M. Z. Kristallogr. 2007, 222, 680−689. (6) Knyrim, J. S.; Becker, P.; Johrendt, D.; Huppertz, H. Angew. Chem., Int. Ed. 2006, 45, 8239. (7) Li, L.; Li, G.; Wang, Y.; Liao, F.; Lin, J. Inorg. Chem. 2005, 44, 8243−8248. (8) Teng, B.; Wang, J.; Wang, Z.; Jiang, H.; Liu, H.; Hu, X.; Dong, S.; Liu, Y.; Shao, Z. Sci. Chin., Ser. E 2002, 45, 19−26. (9) Bhar, G. C.; Smith, R. C. Phys. Status Solidi A 1972, 13, 157. (10) Hellwig, H.; Liebertz, J.; Bohaty, L. Solid State Commun. 1998, 109, 249−251. (11) Moss, T. S. In Optical Properties of Semiconductors; Butterworth: London, 1961. (12) Aarik, J.; Mandar, H.; Kirm, M.; Pung, L. Thin Solid Films 2004, 466, 41. (13) Henderson, B.; Imbusch, G. F. In Optical Spectroscopy of Inorganic Solids; Clarendon: Oxford, 1989; p 183. (14) Song, K. S.; Williams, R. T. In Self-trapped excitons; Springer: Berlin, 1993; p 410.

Figure 9. PL temperature dependence for 2.5 eV PL at 4.2 eV excitation (experimental points) and its approximation by the Mott law I(T)  I0 /(1 + S exp(−Eg/kT)) with parameters S = 1.8 × 104 and ET = 0.15 eV.

quite near the fundamental absorption edge. The Stokes shift, defined as a distance between maxima of PLE/absorption and PL bands, is large in γ-BiB3O6 (1.7 eV), and there are no other bands in the PLE spectrum. The PL temperature quenching is demonstrated in Figure 9. The PL dependence versus temperature may be described well with a Mott law for the case of two, radiative and nonradiative, recombination channels:13

I(T ) = I0/(1 + S* exp( − E T /kT ))

REFERENCES

(2)

where I0 is the PL intensity in the absence of quenching, S is the dimensionless preexponential factor, k is the Boltzman factor, and ET is the thermal activation energy of quenching. In Figure 9 experimental data are shown as points whereas a solid line represents the Mott law approximation with parameter S = 1.8 × 104 and thermal activation energy 0.15 eV. A set of PL features such as a broad PL band, its excitation only near the fundamental absorption edge without any other absorption/PLE bands related to intracenter recombination, and a large Stokes shift as well as the rapid temperature PL quenching, suggest that PL in γ-BiB3O6 is due to self-trapped excitons.14 The excitonic processes are known to be the indication of the high quality of crystals under examination, and this evidences the high quality of our γ-BiB3O6 crystals. 78

dx.doi.org/10.1021/cg200799n | Cryst. Growth Des. 2012, 12, 75−78