A single crystal of lithium niobate is an important optoelectronic material. It can be grown from direct melt only in a lithium deficient non-stoichiometric form as its stoichiometric composition exhibits incongruent melting. As a result it contains a number of intrinsic point defects such as Li-vacancies, Nb antisites, oxygen vacancies, as well as different types of polarons and bipolarons. All these defects adversely influence its optical and ferroelectric properties and pose a deterrent to the effective use of this material. Hence, controlling the defects in lithium niobate has been an exciting topic of research and development over the years. In this article we discuss the different methods of controlling the intrinsic defects in lithium niobate and a comparison of the effect of these methods on the crystalline quality, stoichiometry, optical absorption in the UV-vis region, electronic band-gap, and refractive index.

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Sillenite structure crystals (general composition Bi 12 XO 20 , X ¼ Si, Ge, and Ti with space group I23) posses a number of properties important for practical applications. [1,2] They are characterized by XO 4 tetrahedra located at the center and corners of its cubic unit cell. These XO 4 tetrahedra are linked with stereochemically active 6s 2 lone electron pair of Bi 3þ via BiO 5 octahedra. [3][4][5] This, along with the absence of inversion symmetry, is the source of outstanding electrical and optical properties suitable for photorefractive applications. [1][2][3][4][5] Among sillenites, bismuth silicate (Bi 12 SiO 20 , BSO) is a technologically important material as it exhibits large electro-optic and photoconductive effects, which make it suitable for optical switches, optics-based electric field sensors, holographic storage, and photorefractive devices. [6][7][8][9][10][11][12][13][14] It is used for two-wave mixing, four-wave mixing, phase conjugation, real-time holography, optical data storage, optical computing, and electro-optical modulation. [7,8] It has been reported that the photorefractive property of BSO is enhanced significantly on doping with metals like Cr, Fe, Mg, Ru, etc. [8,[15][16][17][18][19][20][21][22][23][24][25][26][27] Though most of these dopants make material sensitive to photorefraction in the visible region, especially around 500 nm, that is, 2.5 eV, practical feasibility prefers the region near the infrared (1.5À2 eV), mainly because of the availability of compact laser diodes in this region. [28] Cr doping shifts the photorefractive sensitivity of BSO to the near-infrared region and therefore makes it a potential material for applications like optical image processing, holographic interferometry, optical storage, and phase conjugation. In literature, there are reports on the growth of Cr-doped BSO crystal and investigations on the photorefractive effect, dielectric property, optical activity, Faraday effect, etc. [15][16][17][18][19][20][21][22][23][24] For the growth of the crystal, Czochralski technique has been used as BSO is a congruently melting compound. [25,26] It is known that the devices mentioned earlier require crystals with a low dislocation density and high optical homogeneity. [29] One of the important aspects of growth of crystals having low dislocation density is to grow the crystal with a flat-ish crystalÀmelt interface. [26] A few studies have been reported on undoped BSO for relatively higher thermal gradient. [30,31] However, to obtain large-sized good quality crystal of BSO, a lower thermal gradient is favorable. [32] In view of this, one of the objectives of the work is investigate the effect of the growth condition on the interface shape control of the Cr:BSO crystal when grown along 〈110〉 in the lower axial