J. Appl. Cryst. (2015). 48, 1665–1671 http://dx.doi.org/10.1107/S1600576715016040 1665
Received 13 May 2015
Accepted 27 August 2015
Edited by J. M. Garcı´a-Ruiz, Instituto Andaluz de
Ciencias de la Tierra, Granada, Spain
Keywords: -BaB2O4; Czochralski method; high-temperature top-seeded solution growth; phase diagrams; mathematical representation.
Supporting information: this article has supporting information at journals.iucr.org/j
Controlled growth of large b-BaB2O4 crystals based on theoretical guidelines
Zhihua Li,a* Ran Zhang,a Yaohuan Wu,b Bo Tanga and Guochun Zhangc* aCollege of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular and Nano Probes,
Collaborative Innovation Center of Functionalized Probe for Chemical Imaging in Universities of Shandong, Shandong
Normal University, Jinan, 250014, People’s Republic of China, bDepartment of Chemistry, Moscow University, Moscow, 119991, Russian Federation, and cTechnical Institute of Physics and Chemistry, CAS, Beijing, 100190, People’s Republic of China. *Correspondence e-mail: email@example.com, firstname.lastname@example.org
The diagram of phase equilibria in the BaB2O4–NaF system has been used to deduce the relationship between the cooling speed (T), the pulling speed (v), the crystal radius (Rs), the slope of liquidus (m), the solution component (x) and the total quantity of melt, namely T = 0.00159Rs 2vmx2/G. The theoretical curves of the crystal thickness dependence on cooling rate and pulling rate have also been drawn. Under the guidance of the deduced formulas, the controlled growth of -BaB2O4 (BBO) crystals to a desired size has been achieved. A typical as-grown BBO crystal with dimensions of Ø76 33 mm (525.25 g) has been grown successfully by using the high-temperature top-seeded solution growth method. The measured optical homogeneity indicates that the as-grown
BBO crystal has high optical quality (n ’ 6.9 106). The experimental curves of the crystal thickness versus the cooling rate and pulling rate were in line with the theoretical curves. The phenomenon of diameter shrinkage in the crystal growth has also been explained according to theory and practice. The theoretical derivation and experimental results provide the rationale for further growth of large BBO crystals with high optical quality. 1. Introduction
Up to now, -BaB2O4 (BBO) (Liebertz & Sta¨hr, 1983; Chen et al., 1984; Chen, Wu, Jiang & You, 1985; Chen, Wu, Jiang, You et al., 1989; Cheng et al., 1988; Feigelson et al., 1989; Wu, 2001),
KTiOPO4 (Masse & Grenier, 1971; Tordjman et al., 1974;
Zumsteg, 1976; Laudise et al., 1986; Jacco et al., 1984) and
LiB3O5 (Chen, Wu & Li, 1985; Chen, Wu, Jiang, Wu et al., 1989; Zhao et al., 1990; Markgraf et al., 1994) have been the most widely used nonlinear optical (NLO) crystals. In particular, borate has a high NLO efficiency resulting from its B—
O structure (Ye et al., 2010). BBO has always been the main focus of attention owing to its excellent properties, such as broad transparency range (190–3500 nm), large nonlinear coefficients, high damage threshold (2.3 GW cm2 for 14 ns pulses at 1.06 mm) and a broad phase-matchable second harmonic generation (SHG) over its entire transparent window. Furthermore, BBO is considered to be the only crystal suitable for obtaining Nd:YAG (YAG is yttrium aluminum garnet) laser pulse quadruple output (from 1064 nm to deep ultraviolet 265 nm) directly. So, BBO is one of the most useful optical frequency converters in the ultraviolet region and is widely used in applications such as Nd:YAG lasers, dye lasers, ultra-fast laser pulses, Ti:sapphire lasers, alexandrite lasers, argon ion lasers, copper vapor lasers, optical parametric oscillators, optical parametric amplification and especially laser fusion (Eckardt et al., 1990; Nakatani et al., 1988; Bekker et al., 2012; Takahashi et al., 2011). To date, BBO
ISSN 1600-5767 # 2015 International Union of Crystallography has generated tens of millions of dollars in the market. In order to meet market demands and scientific and technological requirements, the growth of large BBO crystals with consistently high optical quality is of great significance.
Barium metaborate (BaB2O4) has two structures, the phase (high-temperature phase) and the phase (lowtemperature phase). -BaB2O4 cannot generate the SHG effect because of its centrosymmetric structure. -BaB2O4 (BBO) belongs to the trigonal system and crystallizes in the point group C3v (Liebertz & Sta¨hr, 1983; Fro¨hlich, 1984). Its larger SHG coefficient is attributed to the presence of the (B3O6) 3 group in the structure. The melting point of BBO is 1368 K, and the temperature of the phase transition from the to the structure is 1198 K (Liang et al., 1982). In the past few decades, the vast majority of research has been focused on the growth technology of BBO crystals in order to avoid the formation of -BaB2O4, and a variety of growth methods have been developed. The main growth methods of BBO are summarized as follows: 1.1. Czochralski method
Compared with the flux growth method, the Czochralski (Cz) method (Czochralski, 1918) has many advantages: fast growth, lack of contamination and the growth of large crystals from a relatively small crucible. In 1990, Itoh et al. (1990) reported a BBO crystal with dimensions of Ø7 18 mm, grown from a stoichiometric melt of BaB2O4 using a radiofrequency induction heater by the Cz method in air for the first time. Since then, a vast amount of experimental work and research using this method has been reported (Kouta et al., 1991, 1993; Kouta & Kuwano, 1996; Kozuki & Itoh, 1991).
Unfortunately, this technique has severe shortcomings. (1) A steep temperature gradient is necessary in order to avoid the phase transition of BBO. This leads to substantial stress in the as-grown crystals and could result in severe crystal cleavage, optical inhomogeneity and a low value of SHG, and therefore it is difficult to grow large crystals. (2) Since the growth temperature is higher than the phase-transition temperature,