E.G. Tsvetkov

Growth of α-BaB2O4 single crystals from melts at various compositions: comparison of optical properties

Abstract The conditions for growing bulk α-BaB2O4 single crystals from stoichiometric melts and high-temperature solutions as well as some associated problems are reported. The morphology of crystals, formation of inclusions and light scattering depending on crystal growth conditions are described. Comparative estimation of spectral characteristics and of the optical quality, as well as the damage threshold of crystal samples suitable for production of polarization devices of laser systems have been performed.

Some concepts concerning twinning and cellular growth of bulk barium metaborate (BBO) crystals

Abstract Bulk single β -BaB2O4 (BBO) crystals have been grown by the TSSG method in Czochralski variant, using Na2O and NaF as the solvents. It was found that formation of twins (electric type) or cellular substructures of different scales is their specific growth feature. We believe that their formation is governed by changes in the composition and structure of the double-electric layer (DEL) at the interface of crystal growth. In Z-axis crystals only microtwins structures were revealed whose number and localization are determined by the composition of used solvent. The cellular growth of these crystals at a particular stage is a result of the impossibility of frontal formation of an antipolar structure (macrotwin) under the conditions of increasing charge density in the DEL. In the Y(X-)-axis crystals the conditions for formation of one or three (five, etc.) macrotwin boundaries and, hence, noncellular growth are always realized. Model concepts, characterizing seeding and growth of Y(X-)- and Z-axis BBO crystals are discussed.

BeAl6O10:Cr3+ (Ti3+, Ni2+) laser crystals and their spectroscopic characteristics

Abstract The EPR, optical absorption, luminescence spectra, and lasing characteristics of Cr 3+ , Ti 3+ , and Ni 2+ ions in BeAl 6 O 10 (beryllium hexaaluminate, BHA) crystals are studied. The spectroscopic data show that the BHA crystals have an orthorhombic structure with the space group Pbcm. The temperature dependence of luminescence lifetime of the excited state of Cr 3+ ions ( τ ≈13×10 −6 s at 300 K) is investigated. The energy gap between the 2 E- and 4 T 2 -levels of Cr 3+ (∼236 cm −1 ) and the nonradiative relaxation activation energy ( E a ≈1667 cm −1 ) are determined. High-efficient tunable radiation of a new BeAl 6 O 10 :Cr 3+ laser crystal is demonstrated in the region of 0.78–0.92 μm at 300 K. It is shown that the doped BHA crystals are promising active media for tunable solid state lasers in the spectral regions of 0.7–1.0 μm (Cr 3+ ) and 1.35–1.65 μm (Ni 2+ ).

The major problems of seeding and growth of barium borate crystals in terms of new data on phase relations in BaO–B2O3–Na2O system

Abstract Differential-thermal analysis indicates that phase instability occurs in the majority of melt-solutions of the BaO–B 2 O 3 –Na 2 O system, from which crystals of barium metaborate (BaB 2 O 4 , BBO) can be grown. Experiments show that the arising inhomogeneity of the crystallization medium accompanied by liquation phenomenon might result in faulty seeding and growth of BBO crystals, difficulties in controlled shaping, trapping of abundant inclusions, and other negative effects. The composition of the widely used solvent in growing BBO crystals (“eutectic” between the composition of BaB 2 O 4 and BaB 2 O 4 ·Na 2 O) is unstable and tends to polyvariant transformations and, hence, is far from optimum. The compounds Na 4 BaB 4 O 9 and NaBO 2 might be the components that form pseudo-binary systems with BBO relative to stable melt-solutions promising for further technological processing.

Specific features of seeding and growth of bulk polar crystals

Abstract Formal analysis of seeding and growth of crystals exhibiting spontaneous polarization has been attempted using lithium iodate (α-LiIO3) and barium metaborate (β-BaB2O4) as representative materials grown from aqueous and high-temperature solutions, respectively. We suggest that the specific growth features of nonlinear optical α-LiIO3 and β-BaB2O4 crystals are being determined by formation and evolutionary restructuring of a double electric layer (DEL) at the growth interface. Both composition and structure of the DEL are governed by potential-determining ions of the growth medium as well as by the nature of crystal polarization and its properties. We have found that the composition and structure of the DEL together with the magnitude and direction of spontaneous polarization of the seed (crystal) predetermine the macrotwinning boundaries during seeding and subsequent stages of crystal growth as well as the formation of microtwin structures of various sizes. Similar reasoning is applied to possible crystal asymmetry, cellular growth, extinction of growth, etc. Model concepts of seeding and growth of bulk polar crystals are discussed.

Growth and crystal structure of the BeAl6O10 single crystals

Abstract Unlike earlier published works we have established incongruent melting for the compound BeAl6O10 (BHA). The conditions of growing crystals from their own melt with a superstoichiometric excess of BeO, using the Czochralski method, have been determined. The nature of inclusions in grown BHA crystals is described. On the basis of X-ray crystal structure analysis and data of spectroscopic studies the symmetry and space group of BHA crystal structure have been refined, as well as uncertainties arising in their interpretation are discussed.

Study of voltage changes at interface during growth of β-BaB2O4 crystals and possibilities for improving their quality by an external potential

Abstract Experiments on the current monitoring of changes in the differences of electric potentials (Δ ϕ ) between solution–melts and BBO crystals during seeding and growth have been carried out. It is shown that such monitoring reflects important integral long-term crystallization processes as well as their current (short-term) features. Also Δ ϕ changes allowed us to control the weight of crystallized material and kinetics of crystallization. The yield of high-quality material from grown boules of BBO crystals can be somewhat increased by applying various external potentials on them during crystallization.