Crystal Growth And Luminescence Properties Of Lithium Molybdate And Lithium Tungstate Scintillation Materials

Under the background of the development of microheater technology and its application in neutrinoless double beta double beta????decay experiments,Li₂MoO₄ and Li₂WO₄ single crystals were firstly grown by vertical Bridgman method.The growth process and luminescence properties of Li₂MoO₄ and Li₂WO₄ single crystals were studied systematically.In addition,in order to improve the quality of Li₂MoO₄ single crystal,crystal defects of Li₂MoO₄ scintillation crystals were also investigated systematically.Li₂MoO₄ polycrystalline powders were synthesized by solid-state reaction using high-purity raw materials of Li₂CO₃ and MoO₃,Li₂MoO₄ single crystals were grown by vertical Bridgman method.As-grown Li₂MoO₄ single crystal blongs to the hexagonal structure with an R-3 space group like Be₂SiO₄.The polished Li₂MoO₄ single crystal is colorless and transparent with a rather high optical transmissivity?85%?.Li₂MoO₄ crystal excited with a laser beam of 355 nm wavelength exhibits a wide luminescence peak around 551 nm as a result of intrinsic luminescence emission of tetrahedral[MoO₄]^2-.Luminescence intensity of Li₂MoO₄crystal is very weak at 300 K.As ambient temperatures cool down,the luminescence intensity increases rapidly and reaches a maximum at 20 K,which is equal to approximately 20 times in contrast with the intensity at 300 K.The luminesecence decay times decrease monotonically with the temperature increasing from 20 K to 200 K.The decay times at 20 K,75 K,and 150 K correspond to 11.5?s,1.5?s and 0.25?s,respectively.During the growth process of Li₂MoO₄ single crystals,there are some crystal defects,such as crystal cracks,crystal deliquescence and silk-like inclusions.The cracks greatly reduce the utilization rate of crystals.The cracks of the crystal are mainly determined by the distribution of the temperature field,the quality of the seeding,the diameter of crystal,and the rate of growth,etc.Under the same growth conditions,the crystal with too large growth diameter can easily lead to crystal cracking.The Silk-like inclusions embedded inside the crystal medium have great effects on the optical transmittance of crystal material.The silk-like inclusions inside the crystal medium were verified to contain the molybdenum-enriched phase.The Li₂MoO₄ crystal has serious deliquescence properties.The absorption of water vapor in the air results in crystal deliquescence,which leads to a decrease in optical transmissivity and an increase in scattering.In addition,the deliquescence of Li₂MoO₄ crystal is a process of physical dissolution and no-new phase formed in the process of deliquescence.Li₂WO₄ polycrystalline materials were obtained from the accurate stoichiometric mixture of raw materials LiOH and WO₃ via the conventional solid-state reaction.Colorless and transparent Li₂WO₄ single crystal was successfully grown by vertical Bridgman process.As-polished crystal samples have an excellent optical transmittance in the visible light region.The transmissivity of crystal samples with the thickness of 2.0 mm,4.0 mm,and 6.0 mm reached 85–89%,81–85%,and78–81%in 400–800 nm wavelength range,respectively.The optical energy band of Li₂WO₄ single crystal samples calculated by Tauc plot method is about 4.20 eV.The luminescence of Li₂WO₄crystal is based on the intrinsic luminescence emission of tetrahedral[WO₄]^2-.The luminescence peaks are at 626 nm,574 nm,and 543 nm at the ambient temperature of 30 K,50 K,and 75 K,respectively,and locate at around 543 nm from 75 K to 300 K.As the ambient temperature cool down,the luminescence intensity increases rapidly and reaches a maximum at 30 K.The luminescence decay times monotonically decrease with the temperature increasing in 30–75 K temperature range,and the decay times at 30 K,50 K,and 75 K are correspond to 97.92?s,30.16?s,and 1.97?s,respectively,while maintain a constant value around 1.80?s in 75–300 K.