The Hydrothermal Growth Of Colorless Bi₁₂SiO₂₀ Crystal

With the rapid development of the information technology,the total quantity of data becomes larger and larger,while the demand that we bring forward for information processing speed gets higher and higher.In comparison with the one-dimensional and two-dimensional storage devices,the volume holographic storage provides ultra-density,high redundancy,a long life of storage,anti-electromagnetic interference,high speed of addressing,high sustained transfer rate and fast access time,so it will most likely become the next generation mainstream storage technology,which makes it has a very broad application prospect.Therefore some developed countries,such as America,Japan and Britain,have invested huge sums of money in researching it,and strive to make it commercial soon.In addition,some domestic institutes also have carried out some early research work about this technology.By comparing synthetically,the researchers found that photorefractive crystal materials are probably to be the recording material of volume holographic storage eventually,consequently the photorefractive crystal material becomes the one of hot spots researched at home and abroad.Bismuth silicate(Bi₁₂SiO₂₀,Abbreviated to BSO) crystal is a kind of non-ferroelectric cubic insulator with large band gap and high-resistivity.It has the sillenite structure with space group I23,belongs to body-centered cubic crystal system and melts congruently at about 895℃.It has the effects of linear electro-optic,photoconductivity,piezoelectric,optical activity and faraday rotation simultaneously,is a promising multi-function material for optical signal processing. Since Ballman reported having prepared BSO single crystal by the Czochralski method for the first time in 1967,BSO has about forty years of history already.At first it's researched as a piezoelectric material,with the depth of researches,the scientists found that BSO crystal showed the photorefractive effect when it's imposed external electric field,and could achieve many nonlinear optical transformation driven by a low-power laser at room temperature.Besides these, considered as a real time volume storage material,BSO was the one of few photorefractive materials which provided the highest sensitivity and the fastest response time currently.All of these make BSO a kind of nonlinear optical crystal material attracting worldwide attention.At present,the Czochralski and Bridgeman methods are the main way to get large bulk BSO crystal fast.When the researchers adopted these melt technologies to prepare BSO,they found that it's difficult to make the components uniform and that the crystals had growth stripes,dark core,dislocation and other light scattering defects,like inclusions,bubbles and so on.Especially the presence of tawny cores in crystals makes the spectral response range narrow and reduces the quality of crystals,and then restricts the practical application of BSO to optics devices.While hydrothermal method introduces a sealed system,which can prevent external impurities from affecting the process of growth,and has lower growth temperature than melt methods.These are of advantage to the acquisition of high-quality BSO crystals.So hydrothermal technique is also an important way of preparing BSO crystals.Based on these,against those problems in BSO by melt methods,our paper choose hydrothermal technique to prepare colorless BSO single crystal,with emphases on the following four respects:1.The preparation and selection of nutrientsHigh purity Bi₂O₃ and SiO₂ were used as starting materials.We weighed out each raw materials according to stoichiometric ratio,and prepared the nutrients by three ways:preparing glassy materials by sintering in platinum and corundum crucibles separately,and getting BSO power by coprecipitation.Then we adopted these materials prepared to carry out the experiment of hydrothermal spontaneous nucleation and analyzed the results.By comparison the hydrothermal experiment of glassy material by sintering in platinum crucible showed a good result.So we selected it as the nutrient of colorless BSO crystal growth by hydrothermal method.2.Hydrothermal growthThe system of hydrothermal growth is rather complex and synthetical,and there are many influence factors,such as nutrient,the kind and concentration of mineralizer,crystallization temperature,temperature gradient,the quality and orientation of seed,pressure and impurity.By analyzing systematically and referring to the basic properties of BSO and the results of related basic researches,we fixed scientifically the initial process parameters of growing colorless BSO single crystal by hydrothermal technique.Then on the base of analyzing the growth results,we adjusted the process parameters timely. We adopted the homemade materials above as the nutrient,4 mol/L NaOH solution as mineralizer,confirmed 75%(70%) for inside(outside) filling coefficient,and maintained the temperature of crystalline region at 390℃.At last,we succeeded in getting a 21.59×16.77×7.17 mm³ colorless BSO single crystal in the homemade autoclave containing gold liner after a long run of about 20 days.In view of the fact that the cycle of hydrothermal growth is too long,we also researched further to simplify the process in order to gain high quality colorless BSO faster and lower loss. This study is still in process.3.Crystallization characteristics and the analysis on colored crystalIn this paper the colorless BSO crystal prepared by hydrothermal method is short columnar. The main exposed crystal planes are(001) and(110),and the exposed area of(001) face is bigger. Considering the practical applications,the hydrothermal BSO has higher utilization rate.By observing the results of hydrothermal spontaneous nucleation,we found that the addition of SiO₂ could change the crystallization morphology obviously.Before the addition,the crystal grain developed fully is near to tetrahedral,and it's the approximate shape of ball after that.Related to the phenomenon of color changes in the course of experiment,we encourage simple discussion and analysis.We thought that the main reason of colored BSO crystals is the presence of Bi⁵⁺,and that the change of color is caused by the concentration of Bi⁵⁺.The glassy BSO sintered in Pt crucible is dark brown,which is much darker than the tawny Czochralski crystal.Regarding this point,our explanation is that fewer Bi³⁺ are oxidized to Bi⁵⁺ by air under the catalysis of Pt crucible.We got golden BSO crystal grains in our hydrothermal spontaneous nucleation experiment and colorless BSO single crystal by the further hydrothermal growth finally.On these,we thought that the course of hydrothermal crystallization led to the reduction of Bi⁵⁺ concentration continuously,further more,the change of color.4.Test and analysisWe tested and analyzed the X-ray powder diffraction,Infrared and transmission spectroscopy of colorless hydrothermal BSO crystal.The results show that the crystal is pure Bi₁₂SiO₂₀ phase and that little water present in that crystal.Besides these,the tawny absorption shoulder in the Czochralski BSO crystal has moved to below 390 nm towards short wave in our hydrothermal crystal,which is out of the visible region.So we have solved the problem of colored crystal basically.In addition,we also studied the X-ray photoelectron energy spectroscopy of glassy BSO sintered in Pt crucible,attempting to be sure the valence of Bi element in source material. According to the result,we could conclude that Bi³⁺ is the main form of existence.Because that the electron binding energy of Bi³⁺ is very near to Bi⁵⁺,we are uncertain whether Bi⁵⁺ is existent yet.Other measurements are still running.