Growth, morphology and optical properties of γ-BiB3O6 single crystals

Gavryushkin P.N. ^{1, 2} , Isaenko L.I. ¹, Yelisseyev A.P. ¹, Getz V.A. ¹, Il’ina O.S. ¹, Zhurko Z.F. ²

¹ V.S. Sobolev Institute of Geology and Mineralogy SB RAS; ² Novosibirsk State University, Novosibirsk, Russia

[e-mail]

The monoclinic crystals of α-BiB₃O₆ (С2) exhibit exceptionally large nonlinear optical coefficients and up to now it’s one of the most promising nonlinear optical materials. The intense activity on α-BiB₃O₆ crystal growth led to the discovery of 3 new polymorphs: β-BiB₃O₆ (P2₁/n), γ-BiB₃O₆ (P2₁/n), δ-BiB₃O₆ (Pca2₁) [1]. As α- and δ-BiB₃O₆ single crystals are successfully grown using the top-seeded technique and their properties are investigated in detail [2, 3], in present paper the attention is focused on γ-BiB₃O₆ crystals which have been synthesized to date only as a polycrystalline powder by hydrothermal method [3].

Starting charge for growing γ-BiB₃O₆ was produced by the solid-phase synthesis from the high purity bismuth oxide and boric acid. The BiB₃O₆ charge was filled into a platinum crucible and heated to 800-850 ºC. After homogenizing, the melt was cooled to 708 ºC and a Pt wire was introduced slowly. Afterwards, crucible was cooled at a 0.1 ºC/h rate.

In the first series of experiments α-BiB₃O₆ crystals were used for seeding. A large α-BiB₃O₆ single crystal wasn’t obtain 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. In the second series of experiments the γ-BiB₃O₆ single crystals, oriented along the [001] axis, were used as a seed. After heating to 718 ºC the crucible was kept at this temperature up to full homogenizing of the melt. The seed was dipped in the melt at 708 ºC. Crystals were grown without any rotation and pulling. Temperature gradient and the cooling rate were 1-2 ˚C/cm and 0.1 ˚C/day, respectively. As a result of growth cycle, was obtained a well-shaped, transparent γ-BiB₃O₆ crystal (fig.(a)). The size of the largest γ-BiB₃O₆ single crystal was 16 ×18×32 mm³ and its mass was 42.3 g.

Goniometric measurements were carried out by a standard technique – the results is shown on fig.(b).

Transmission spectra were recorded along the γ-BiB₃O₆ transparency range using a Fourier-transform spectrometer Infralum 801 in the mid-IR and a UV-2501ЗС 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 diffraction MDR2 monochromator, with color glass filters used for additional monochromatization. Measurements were carried out on two polished 2.2 mm thick plates with aperture ~4 cm²; their edges were parallel to crystallographic axes.

γ-BiB₃O₆ plates 2.2 mm thick are transparent in the 0.315 to 3.15 μm range on a 50 % level transmission, at 300 K. Transparency range on a 5% level is 0.31 to 6.33 μm. 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 transmission spectra allows a conclusion that the fundamental absorption edge in γ-BiB₃O₆ is determined by direct allowed electronic transitions. The analysis of the shape of the fundamental edge gives the value of bandgap E_g at room temperature are 3.982, 3.968, 4.008 eV at 300K and 4.226, 4.191, 4.212 eV at 80 K for polarizations E║a, E║b and E║c, respectively. Thus bandgap decreases by ~0.25 eV as temperature grows from 80 to 300 K. The E_g dependence versus temperature for light polarization E║a shows that there are no phase transitions for the sample under examination in the range 80 to 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 shows 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. As follows from the PLE spectrum, PL is excited in a narrow 4.2 eV band quite near the fundamental absorption edge. The Stokes shift is large in γ-BiB₃O₆ (1.7 eV) and there are no other bands in the PLE spectrum. 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. 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.

This work has been partially supported by Siberian Branch of the Russian Academy of Science RAS (Project II.7.5.).

References:

1. Cong, R.; Zhu, J; Wang Y.; Wang, T.; Liao, F; Jin, C; Lin, J. CrystEngComm. 2009, 11, 1971-1978

2. Becker, P.; Liebertz, J.; Bohaty, L. J. Cryst. Growth. 1999, 203, 149 -155.

3. Aleksandrovsky, A.S.; Vasiliev, A.D.; Zaitsev, A.I.; Zamkov, A.V. J. Cryst. Growth. 2008, 310, 4027– 4030

Дата добавления: 2015-10-29