Research And Control Of HgI₂ Crystal Morphology

Mercury iodide??-HgI₂?has been widely used in the fields of national defense,nuclear medicine,high energy physics and astronomy due to its advantages of high atomic number of components,large bulk dark resistance,low current and high energy resolution.Obtaining the crystals with[001]crystal orientation is the need to commercialization and industrialization of mercury iodide crystals,because of the best electrical properties along this direction.During the past century,it had many method to predict crystal equilibrium morphology in theory.From a kinetic point of view,the crystal shape can also be effectively controlled through parameter adjustment of growth precessing.However,there are few reports on the crystal growth control technology from the perspective of thermodynamics and kinetics.The main influence of crystal morphology is crystal growth rate.In this paper,the effects of crystal structure,structural dynamics,thermodynamics and other factors on the growth rate are investigated based on the?-HgI2 crystal growth in solution.A growth method/theory aimed to controllable crystal morphology is attempted to be established in combination with the crystal growth environment?external factors?,which provides a technological reference for the morphology control of mercury iodide single crystal and polycrystals.The analysis suggests that dissolved enthalpy is one of key factors affecting crystal growth rate and crystal morphology.Therefore,the enthalpy of dissolution,which is numerically equal to the latent heat of crystallization under isothermal and isobaric conditions,was investigated.The dissolution process of crystal is inferred from the atomic deposition of some crystal plane of the?-HgI2 crystal.The lowest energy effect must be needed when constructing crystal layer on crystal plane.At least,form of three Hg-I bonds occurs for converting the liquid phase molecules to crystal phase ones on?001?plane.Hence,the dissolved enthalpy in the process is calculated to be 6.6 eV.Similarly,the dissolved enthalpy of?101?,?110?,?102?and?103?planes are 2.2 eV,4.4 eV,2.2 eV and 2.2 eV,respectively.The view that the attachment energy of the crystal plane is a key factors affecting the crystal growth rate and crystal morphology was built then.The crystal surface with the lowest attachment energy under the near equilibrium condition has the slowest growth rate and the largest morphological characteristics,which determines the final morphology of the crystal.Therefore,the attachment energy of the main crystal planes of?-HgI₂ crystal was analysized.The morphology of?-HgI₂ was simulated with the BFDH module of Material Studio?MS?software.However,the growth morphology module cannot be used to directly calculate the attachment energy of?-HgI₂ crystals with non-convergent structure due to the defect of force field in MS software.Therefore,a new method that the cross section method with the aid of end-blocking method"was proposed.As a comparison,the attachment energy of?-HgI₂ crystal was also calculated by the growth morphology method and the cross section method with the aid of end-blocking method.It shows that results are consistent,showing a feasible method to calculate the attachment energy of crystal.The attachment energyof the?001?,?101?,?110?,?102?and?103?crystal planes of?-HgI₂ crystal is-9.82,-23.04,-26.87,-25.23 and-28.18 kJ·mol^-1,respectively.The effects of solute concentration and growth temperature on morphology were analyzed further.Fitting the surface growth rate curves of?-HgI₂ crystals with different solute concentrationes and temperatures,the growth conditions of?-HgI₂ crystals with the[001]direction as the main crystal phase orientation were studied.The influence of solute concentration on the surface growth rate of the crystal was analyzed.It was found that the solute concentration of 1.1-1.3 g/mL was the critical value of crystal growth morphology change under seedless and near-equilibrium growth conditions.When the solute concentration is lower than the critical one,the crystal grows orderly and presents regular tetragonal crystal form.Beyond the critical value,the effective solute concentration of each crystal plane has different weakening rates to the growth rate,resulting in changes in crystal morphology.For the effect of temperature on growth rate,the?001?plane will change greatly or even disappear when the growth temperature is higher than 75?.Our discussions on above conclusion suggest that the growth temperature for?-HgI₂ crystal with[001]direction would be controlled at the range of 25-75?during the crystal growth process.The effect of solute concentration and growth temperature on the morphology had been confirmed based on some experiments.The crystal growth morphology was studied in the range of 1-1.3 g/mL at 40?with high purity HgI₂ as solute and DMSO?dimethyl sulfoxide?as solvent.The crystal shows a morphological change from tetragonal crystal to spherical crystal.XRD analysis shows that the preferred indices of crystals grown at 1 g/mL,1.1 g/mL and 1.2g/mL are 0.90,0.99 and 0.95,respectively.There no?001?diffraction peak occurs in the diffraction pattern of 1.3 g/mL crystal growth,and its appearance is smooth and spherical.Analysis shows that the ideal[001]preferred crystal orientation can be obtained in the growth system with a concentration of 1.1 g/mL.The crystal morphology with a concentration of 1.1g/mL in the range of 30-70?was analyzed,results showed that when the growth temperature exceeded 40?,the crystal morphology changed from tetragonal to heteromorphic.The phenomenon of multipoint nucleation occurs obviously.The analysis shows that the high temperature leads to the increase of solute molecular activity and the liquid convection affects the mass transfer process.Therefore,the optimal growth conditions for obtaining the best[001]crystal orientation seedless and near-equilibrium HgI₂ crystal solution method are 25-40?,and solute concentration is 1.1 g/mL.