Mercurous Halide Crystals and Their Applications as Infrared Polarization/Acousto-Optic and Nuclear Radiation Detectors

doi:10.16553/j.cnki.issn1000-985x.2025.0235

Abstract

Abstract: The detection of electromagnetic waves—including infrared rays， visible light， ultraviolet rays， X-rays， gamma rays—is one of the most essential approaches for humankind to understand the world. In the ultraviolet-visible light band， detection technologies are relatively mature due to the well-developed materials such as crystalline silicon. However， numerous challenges still persist in the detection of extremely short-wave X-rays and gamma rays as well as long-wave infrared rays. In the field of infrared detection， HgCdTe （MCT） focal plane array detectors are capable of capturing thermal imaging that is invisible to the naked eye. Nevertheless， they are unable to provide spectral information due to the lack of infrared spectroscopic devices at the front end. Therefore， developing optical crystals for infrared polarizing prisms and infrared tunable filter devices forms the foundation for advancing infrared multispectral and hyperspectral detection. In addition， for the detection of highly penetrating X-rays and gamma rays， semiconductor-based detection and scintillator-based detection are the most extensively studied approaches. However， the commercial semiconductors CdTe and CdZnTe （CZT） suffer from non-uniform defect distribution and high production costs， while traditional halide and oxide scintillators are hygroscopic， costly， or have insufficient energy resolution. Hg₂X₂ （X=Cl， Br， I） crystals have emerged as a promising multifunctional alternative， combining exceptional infrared polarizing optical properties， strong acousto-optic performance， and high-performance radiation detection capabilities. Hg₂X₂ crystals crystallize in the I4/mmm space group， featuring one-dimensional X-Hg-Hg-X chains stabilized by intra-chain covalent bonding and inter-chain van der Waals interactions. Halogen substitution from Cl to Br and I enables systematic tuning of key material parameters： lattice constants increase， band gaps decrease （from 2.9 to 2.1 eV）， infrared transparency extends up to 40 μm （in Hg₂I₂）， and both birefringence and the acousto-optic figure of merit improve significantly （2 600×1.5×10^-18 and 3 200×1.5×10^-18 s³/g for Hg₂Br₂ and Hg₂I₂， respectively）. Notably， researchers at Shandong University have successfully grown high-quality and large-sized Hg₂Cl₂ and Hg₂Br₂ crystals with diameters up to 54 mm via physical vapor transport （PVT） method by optimizing temperature gradient， growth rate， and cooling profile to suppress defects such as impurities and cloud-like inclusions. Hg₂Cl₂ crystals have been employed for fabricating various polarizing devices such as Glan-Taylor prisms and Wollaston prism， which are operable within the spectral range of 3 μm to 20 μm. They exhibit an extinction ratio of 30 000∶1 and a laser-induced damage threshold （LIDT） of 8.06 J/cm² at 3.5 μm. Moreover， the anti-reflection-coated polarization beam combiner （PBC） achieves combination efficiencies of 93.5% at 4.6 μm and 92.4% at 9.2 μm， with excellent beam quality （M²=1.23/1.17）. For acousto-optic applications， Hg₂Br₂-based acousto-optic tunable filter （AOTF） represents the only viable solution for MWIR/LWIR operation， demonstrating internal diffraction angles greater than 7° and a peak diffraction efficiency of 97.35%， achieved through engineered acoustic absorption layers. In radiation detection， Hg₂X₂-based detectors achieve an energy resolution of 1.35% for 662 keV γ-rays from ¹³⁷Cs. Furthermore， Hg₂Br₂ enables dual-mode detection of γ-rays and thermal neutrons due to the high neutron capture cross-section of ¹⁹⁹Hg， and has been applied in wearable α-particle detectors for real-time monitoring of water contamination. This review highlights the pivotal role of Hg₂X₂ crystals in the fields of infrared and nuclear radiation detection， and offers perspectives on the challenges associated with their crystal growth and device fabrication.

Key words: Hg₂X₂; crystal growth; physical vapor transport method; acousto-optic device; polarization device; nuclear radiation detection

CLC Number:

SONG Jian, YUE Zhongjie, QIAO Xiaojie, ZHAI Zhongjun, ZHANG Guodong, TAO Xutang. Mercurous Halide Crystals and Their Applications as Infrared Polarization/Acousto-Optic and Nuclear Radiation Detectors[J]. Journal of Synthetic Crystals, 2026, 55(3): 331-339.

Figures/Tables 7

Fig.1 Structure and physical properties of Hg2X2 crystals［27］. （a） Crystal structure；（b） phase transition temperature range；（c） relationship between birefringence and transparent range of different crystals；（d） relationship between acousto-optic quality factor and transparent range of different acousto-optic crystals

Table 1 Physical properties of Hg2X2 crystals

Property	Hg₂Cl₂	Hg₂Br₂	Hg₂I₂
Structural type	Tetragonal， I4/mmm space group
Lattice constant/Å	a=b=4.48， c=10.91	a=b=4.65， c=11.10	a=b=4.93， c=11.63
Density/（g·cm^-3）	7.180	7.307	7.702
Average atomic number	48.5	57.5	66.5
Band gap/eV	2.9	2.5	2.1
Resistivity/（Ω・cm）	—	—	6×10¹¹~2×10¹²
Light transmission range	Up to 80%@0.35~20 μm	Up to 75%@0.4 ~ 30 μm	Up to 55%@0.5~40 μm
Refractive index	n_o=1.898，n_e=2.444@20 μm	n_o=2.033，n_e=2.700 @30 μm	n_o=2.254，n_e=3.210@40 μm
Shear wave sound velocity along ［110］ direction/（m·s^-1）	347	273	254
Maximum acousto-optic quality factor/（s³·g^-1）	700×1.5×10^-18	2 600×1.5×10^-18	3 200×1.5×10^-18

Fig.2 Hg2X2 crystals grown by physical vapor transport method. （a） Hg2Cl2， Hg2Br2， and Hg2I2 crystals grown by Brimrose company［12］；（b） Hg2Cl2 and Hg2Br2 crystals grown by Shandong University

Fig.3 Glan-Taylor prism based on Hg2Cl2 crystals［33］. （a） Photograph of Glan-Taylor prism element；（b） transmittance of Hg2Cl2 crystal；（c） transmittance of Glan-Taylor prism based on Hg2Cl2 crystal

Fig.4 Polarization beam combiner based on Hg2Cl2 crystal［33］. （a） Photograph of PBC；（b） beam-combined output power and combining efficiency of PBC coated with infrared antireflective film for p- and s-polarized lights；（c） beam quality and spot intensity distribution of PBC coated with infrared antireflective film

Fig.5 Energy resolution of Hg2X2 crystal nuclear radiation detectors［5］. （a） Energy resolution of 3 mm quasi-spherical Hg2I2 detector for 241Am；（b） energy resolution of 2 mm planar Hg2I2 detector for 662 keV gamma rays from 137Cs；（c） energy resolution of 20 mm×10 mm×6 mm pixelated Hg2Br2 detector under bias voltage of 1 000 V

Fig.6 α-particle scintillation detector based on Hg2Br2 crystal［46］. （a） Detector structure；（b） spectral response of α-particles in water

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