Growth And Characterization Of IR Nonlinear Optical Crystal Pb₁₇O₈Cl₁₈

Infrared nonlinear optical?IR-NLO?crystal can be used to output tunable far-infrared laser by laser frequency conversion.It is widely used in such fields such as oil exploitation,gas pipeline leak detection,greenhouse gas detection in the atmosphere,infrared guided missile detectors,infrared laser detection,cruise missiles and so on.The commercial infrared nonlinear optical crystals are mainly included AgGaQ₂?Q=S,and Se?,ZnGeP₂,which are still not good enough for different applications.Pb₁₇O₈Cl₁₈?POC?is a new type of infrared nonlinear optical crystal in recent years.High laser damage threshold and good second harmonic generation makes it a kind of great potential IR NLO material.However,it is difficult to grow POC crystals.At present,only crystals with millimeter size are reported,which cannot satisfy many performance tests.In this paper,the synthesis of raw materials,crystal growth and performance characterization of POC crystals are studied,and the main contents are as follows:?1?The partial area of phase diagrams near the composition of Pb₁₇O₈Cl₁₈ are studied.It is found that Pb₁₇O₈Cl₁₈ is a stable and congruent melting compound,which corrects the errors of the phase diagrams of PbCl₂-PbO.?2?POC polycrystalline material was synthesized by solid-phase sintering method.POC crystals were grown by micro pulling down method and the Bridgeman method.Because of the severe volatilization of components,only tiny crystal particles could be obtained by micro pulling down method.By means of crucible selection,temperature field optimization and rate control,several centimeter crystals were successfully obtained by using the Bridgeman method.The optimum process conditions are as follows:the temperature gradient is 20³0?/cm,the decreasing rate is 0.2⁰.3mm/h,and the PbCl₂ excess in the initial mixture is used to compensate for the deviation of the component.The surface morphologies and growth mechanisms of POC crystals are studied by polarizing microscope.?3?The crystal structure was measured by powder XRD.POC belongs to the orthorhombic system,space group Fmm2?No.42?,with the unit cell parameters a=35.4963?14??,b=5.8320?2??,c=16.0912?6??,Z=3.The crystal theoretical density is 6.413 g/cm³.The POC crystal has noncentrosymmetrical structure,which only exists Pb-O bond and Pb-Cl bond,Pb-O bond is advantageous to the UV and infrared cut-off wavelength.?4?The FT-IR spectrum,diffuse reflectance spectrum and Hall effects of the as-grown POC crystal was measured.From the FT-IR spectrum,the absorption peaks caused by the vibration of Pb-O bond appears at the wavelength of 16?m,outside the two atmospheric windows,and the crystal can be effectively used in the infrared wavelength.The diffuse reflectance spectrum indicates that the POC crystal has a bandgap is 3.39eV,which satisfies the requirement that the infrared nonlinear optical crystal has a bandgap greater than 3.0eV.It is speculated that the POC crystal may have a large laser damage threshold.The Hall effect results in the POC crystal being a P-type semiconductor.