Study on Lattice Parameter Variance and Eutectic Reaction during Crystal Growth of Nd,Cr∶GSGG by Czochralksi Method

During the crystal growth of Nd,Cr∶GSGG by Czochralski method,in some cases eutectic reaction occurred in the nether region of the crystal,and the boule was divided into two obvious different parts,which is upper Nd,Cr∶GSGG crystal and the nether coexisting Nd,Cr∶GSGG and GdScO_(3).By X-ray powder diffraction,the structure change of NdCr∶GSGG crystal ofΦ27 mm×120 mm with eutectic along its grown direction<111>was studied.By the least square method and extrapolation function f=sinθ-sinθ^(1-t)(t is an adjustable parameter),the lattice parameters of Nd,Cr∶GSGG and additional GdScO_(3)phase were computed.The results indicate that the lattice parameters of Nd,Cr∶GSGG increase along its growth direction,which changes from a=(1.25650±0.00007)nm of the top to(1.25798±0.00010)nm of the bottom.In the process of Nd,Cr∶GSGG growth,Gd^(3+)in Nd,Cr∶GSGG is partly replaced by Nd^(3+)with larger ionic radii,and the volatilization of Ga component results in its composition variance,which cause the lattice parameters increase along growth direction.In the eutectic section,there are the Nd,Cr∶GSGG and the second phase orthorhombic GdScO_(3).The lattice parameters of GdScO_(3)are a=0.5443±0.0007,b=0.5699±0.0005 and c=(0.7865±0.0009)nm,and that of Nd,Cr∶GSGG is(1.25798±0.00010)nm.In the final growth stage,excessive volatilization of Ga composition during the crystal growth causes the growth melt deflect of the Nd,Cr∶GSGG solid solution range seriously,and results in the eutectic reaction,and the outgrowth of Nd,Cr∶GSGG and GdScO_(3).So it is necessary to decrease the effect of gallium volatilization during the growth in order to avoid eutectic growth and obtain a high-quality Nd,Cr∶GSGG.

Lattice Parameter Change of Nd∶Gd_(3)Ga_(5)O_(12) Crystal with Growth Direction

The lattice parameter change of Nd∶GGG crystal was studied by X-ray powder diffraction and the least square fitting method.The results indicate that the lattice parameters of Nd∶GGG increase along the crystal growth direction.By analyzing the Nd∶GGG crystal structure and ions occupying site,the reason of lattice parameter change is mainly attributed to the volatilization of gallium ingredient and lead to the Ga vacancy sites(Oxygen octahedral center)were occupied by Gd^(3+)with large ionic radii,and that the substitute increases with the crystal growth direction.In addition,the Gd sites(oxygen dodecahedron center)were occupied by dopant Nd^(3+)with large ionic radii and the Nd3+distribution coefficient in GGG crystal is smaller than 1,therefore,the substitute also increased with the crystal growth direction,which is another reason led to the lattice parameter increase along the crystal growth direction.In order to decrease the lattice parameter change and improve the crystal quality,some methods were adopted to restrain effectively the Ga_(2)O_(3)volatilization and decrease the Nd^(3+)concentration change along the crystal growth direction.

Segregation during crystal growth from melt and absorption cross section determination by optical absorption method

Segregation during crystal growth from melt under two conditions is studied by using crystal mass,which can be measured easily,as an independent variable,and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given.When the segregated solute disperses into the whole or just a part of melt homogenously,the concentration CS in solid interface will change by different formulas.If the crystal growth interface is conical and segregated solute disperses into melt in total or part,the solute concentration at r=2/3R,where r is the distance from the growth cross section center and R the crystal radius,is independent on the shape of the crystal growth interface,and its variation at r=2/3R can be regarded as the result from crystal growth in flat interface.With CS variation formula in solid and absorption cross section σ for optical dopant,the absorption coefficients along the crystal growth direction can be calculated,and the corresponding experimental value can be obtained through the crystal optical absorption spectra.By minimizing the half sum,whose independent variables are k,ΔW or σ,of the difference square between the calculated and experimental absorp-tion coefficients from one or more absorption peaks along the crystal growth di-rection,k and σ,or k and ΔW,can be determined at the same time through the Levenberg-Marquardt iteration method.Finally,the effective segregation coefficient k,ΔW and absorption cross sections of Nd:GGG were determined,the results fitted by two formula gave more closed effective segregation coefficient,and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt.Experimental results show that the method to determine effective segregation coefficient k,ΔW and absorption cross sections σ is convenient and reliable,and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably.