The development of new crystals evokes a growing interest in synthesizing crystals with nonlinear optical (NLO) properties.^{[1– 3]} Binary bismuth borates, such as BiB₃O₆, ^{[4, 5]} have been of enhanced interest due to their promising nonlinear optical properties^{[6– 9]} and large nonlinear optical susceptibilities.^{[10– 15]} A broad search for new NLO materials in the ternary ZnO– Bi₂O₃– B₂O₃ system leads to a new NLO crystal Bi₂ZnOB₂O₆ (BZB). The bismuth borate Bi₂ZnB₂O₇^{[16, 17]} was synthesized first by Barbier et al. using the solid-state reaction method in the temperature range 650– 825 ° C at 1 atm. Its structure has been determined by powder x-ray diffraction (XRD) and refined by the Rietveld method using powder neutron diffraction data.^{[18, 19]} In addition, a large single crystal of Bi₂ZnOB₂O₆ has been grown by the top-seeded growth method.^{[20, 21]} A high second harmonic generation (SHG) efficiency about 3– 4 times that of KH₂PO₄ (KDP) and a wide SHG phase matching range make the BZB crystal a promising NLO material. However, the structural stabilization at high temperature is important to the NLO crystal as an optical device. Temperature-dependent Raman scattering measurements are well suited to obtain the structural information of the sample at different temperatures because of the dependence of the phonon modes on the temperature.^{[22, 23]}

Bi₂ZnOB₂O₆ belongs to the space group with four formula units in the primitive cell. The zone-center optical phonons can be classified according to the following irreducible representations: Γ _opt = 28A₁ + 41A₂ + 36B₁ + 36B₂. The A₁, B₁, B₂ modes are both Raman and infrared active; and the A₂ mode is Raman active only. Polarized Raman scattering of BZB has been measured by Zhang et al.^[24] In this work, we present a study of the Raman spectra of BZB and their dependence on the temperature. The temperature dependence of the Raman bands is studied to investigate the structural change of the crystal at high temperature.

The Bi₂ZnOB₂O₆ polycrystalline powder was prepared by standard solid state reactions. ZnO, Bi₂O₃, and H₃BO₃ of analytic grade were mixed homogeneously with appropriate proportions in an agate mortar, and packed into a Pt crucible. The sample was heated slowly to 630 ° C and held at this temperature for 48 h with several intermediate grindings and mixings. The analysis of x-ray powder diffraction shows that the final product is Bi₂ZnOB₂O₆.

The room- and high-temperature measurements were carried out using a home-made heating furnace. The temperature was monitored by a thermocouple attached to the metal sample holder and controlled by a temperature controller within 1 ° C. The Raman spectra were collected with Horiba Jobin Y’ von Raman spectrometer (LabRam800HR) with a backscattering configuration. The ultraviolet light pulse of 355 nm with a repetition frequency of 10 kHz from a Coherent Innova Ultra laser was focused onto the sample using a Raman microprobe with a 4× objective lens. An intensified charge coupled device (CCD) was used to collect the scattering light. The spectral acquisition, under the accumulated mode, was 10 s each for 10 times, the average power of the laser operation was fixed at 800 mW and the slit width was set at 300 nm for all the measurements under different temperatures.

The structure of BZB is shown in Fig. 1. It has a three-dimensional network consisting of layers connected through O– Bi– O bands along the c axis. In this structure, through sharing the O atoms, two [BO₃]³⁻ triangles are condensed into a [B₂O₅]⁴⁻ group and two [BO₄]⁵⁻ tetrahedra are condensed into a [B₂O₇]⁸⁻ group. The [B₂O₅]⁴⁻ and [B₂O₇]⁸⁻ groups are bridged by tetrahedral Zn²⁺ centers through sharing three O vertices of each [ZnO₄]⁶⁻ tetrahedron to generate a 2D infinite [ZnB₂O₇]⁶⁺ layer parallel to the (001) plane. Figure 2 shows the crystal Raman spectra in the range from 100 cm^{− 1} to 1600 cm^{− 1} at room temperature.

Fig. 1.

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	Fig. 1. Structure of Bi2ZnOB2O6 viewed down the c axis.

Fig. 2.

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	Fig. 2. Raman spectrum of Bi2ZnOB2O6 crystal at room temperature.

In the structure of BZB, six-coordinated bismuth and the oxygens around form a distorted octahedron. Baia et al. assigned the Raman peaks located at 200– 550 cm^{− 1} and 880 cm^{− 1} in the Raman spectrum of B₂O₃– Bi₂O₃ glasses to the Bi– O vibration.^[25] The Raman peak at 350 cm^{− 1} was observed in the Raman spectrum of B₂O₃– ZnO– Bi₂O₃ glasses due to the Bi– O– Bi vibration.^[26] Therefore, the Raman peaks located at 350 cm^{− 1} and 873 cm^{− 1} are attributed to the Bi– O band of BiO₆ octahedra.

A significant contribution to the Raman spectra of BZB comes from the vibration of the low-symmetric polyborate [B₂O₇] group with two BO₄ tetrahedra and the [B₂O₅] group consisting of two BO₃ triangles. A planar equilateral triangle molecule XY₃ with symmetry D_3h has four normal vibrational modes. For the [BO₃] group with symmetry D_3h, the ν _s mode represents a non-degenerate symmetric B– O stretching vibration, the δ mode represents a non-degenerate out-of-plane B– O bending vibration, the ν _as mode represents a doubly degenerate B– O stretching vibration, and the γ mode represents a doubly degenerate in-plane O– B– O bending vibration. It is known from the literature that the normal mode frequencies of the [BO₃] group in the vibrational spectra of the bismuth borate crystals are typically situated as follows:^{[27, 28]}ν _s ∼ 950 cm^{− 1}, δ ∼ 650– 800 cm^{− 1}, ν _as ∼ 1100– 1450 cm^{− 1}, and γ ∼ 500– 650 cm^{− 1}. In the Raman spectra of the BZB crystal, the peaks at 700 cm^{− 1} and 743 cm^{− 1} are assigned to the bending vibration of the B– O bands, while the peak at 1344 cm^{− 1} is assigned to the B– O stretching vibration. On the other hand, a tetrahedron molecule XY₄ with symmetry T_d has four normal vibrational modes: v₁ (symmetric stretching mode), v₂ (bending mode), v₃ (asymmetric stretching mode), and v₄ (bending mode). For the BO₄ tetrahedron, v₁ ∼ 800– 955 cm^{− 1}, v₂ ∼ 400– 600 cm^{− 1}, v₃ ∼ 1000 cm^{− 1}, and v₄ ∼ 600 cm^{− 1}.^{[29, 30]} Here the Raman peaks located at 637 cm^{− 1} and 968 cm^{− 1} are attributed to the B– O bond of the BO₄ tetrahedron. The Zn atoms as a bridge connect the boron– oxygen groups with Zn– O bonds. The Raman peaks at 380 cm^{− 1} and 583 cm^{− 1} were observed in the Raman spectrum of the ZnO crystal.^{[31, 32]} The Raman peaks at 328 cm^{− 1}, 380 cm^{− 1}, and 435 cm^{− 1} were present in the Raman spectra of the polycrystalline Zn_{1− x}Mg_xO (x = 0– 0.15) powders, which were attributed to the Zn– O vibration.^[33] Here, the Raman peaks located at 350 cm^{− 1}, 391 cm^{− 1}, and 582 cm^{− 1} are related to the Zn– O bond vibration in the BZB crystal.

The temperature dependent Raman spectra of a Bi₂ZnOB₂O₆ crystal with the wavenumber range from 100 cm^{− 1} to 1600 cm^{− 1} were recorded with increasing temperature from room temperature to 600 ° C, as shown in Fig. 3. Nearly all the vibrational modes exhibit a decrease in wavenumber. Figures 4 and 5 illustrate the shifts of the Raman peaks with temperature. The Raman peaks located at 873 cm^{− 1} and 1344 cm^{− 1} have wavenumber shifts of 21 cm^{− 1} and 15 cm^{− 1}, respectively. These phenomena also appear in the high-temperature Raman spectra of Bi₁₂SiO₂₀, KTa_{1− x}NbxO₃, Ba₃ (PO₄)₂, and BiB₃O₆ crystals.^{[34– 37]} With increasing temperature, all of the peaks decrease in frequency, which is ascribed to the increases in the inter-atomic distances in the BZB crystal when the crystal is heated.^[38] In addition, the Raman peaks become wider during the heating process, the bending vibration of the B– O bands at 743 cm^{− 1} becomes a shoulder at 700 cm^{− 1} after the temperature increases to 400 ° C, as a consequence of larger distributions of the bonding angles and distances at higher temperature. Some of the adjacent peaks located below 300 cm^{− 1} become overlapped and merge together with increasing temperature. However, the spectra of the Bi₂ZnOB₂O₆ crystal do not show any splitting or new bands, which indicates that no phase transition occurs in the present study.

Fig. 3.

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	Fig. 3. Temperature dependent Raman spectra of Bi2ZnOB2O6 crystal from room temperature to 600 ° C.

Fig. 4.

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	Fig. 4. The wavenumber shifts of Raman peaks at 1344 cm− 1, 873 cm− 1, 743 cm− 1, and 700 cm− 1 with increasing temperature.

Fig. 5.

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	Fig. 5. The wavenumber shifts of Raman peaks at 637 cm− 1, 582 cm− 1, 391 cm− 1, and 350 cm− 1 with increasing temperature.

A temperature-dependent Raman spectroscopic study on the Bi₂ZnOB₂O₆ crystal was carried out up to 600 ° C. Raman spectra of single crystal Bi₂ZnOB₂O₆ were recorded in the spectral range 10– 1600 cm^{− 1} at room temperature first. Compared with the vibrational spectra of the referred compounds, satisfactory assignment of most of the high-energy modes to vibrations of Bi– O, B– O, and Zn– O bonds was achieved. In particular, the Raman high-frequency peak located at 1344 cm^{− 1} was attributed to the B– O vibration in the [BO₃] triangle. The changes of the Raman peaks and the crystal structure were investigated at high temperature. It was found that all the Raman peaks exhibit decreases in frequency and the widths of the Raman peaks increase with increasing temperature. No phase transition was observed under 600 ° C.