Temperature-dependent magnetic and local structural properties of diamagnetic α-HgI2

• α-HgI2 is considerably interesting diamagnetic material for practical applications at room temperature to x-ray, γ-ray, and particle detectors because it has a special capability of electron-hole creation, comparable to that of CdZnTe, TlBr, and PbI2.
• We successfully synthesized single crystal α-HgI2 and observed a magnetic transition from the α-HgI2, when heated. The magnetic transition near 75 °C is unusual.
• We examined the structural properties of the single crystal α-HgI2 using temperature-dependent XRD and XAFS measurements.
• No structural change was observed, however, XANES at the I K edge demonstrated a clear shift at the transition temperature.
• This is the clear evidence of a competition between diamagnetic and paramagnetic properties of outermost shell electrons in a single crystal α-HgI2.

We examined the temperature-dependent magnetic and structural properties of α-HgI2 in the temperature range of 300–400 K using temperature-dependent DC-magnetization, x-ray diffraction (XRD), and x-ray absorption fine structure (XAFS) measurements. DC-magnetization measurements of α-HgI2 single crystals show a sudden reduction of diamagnetic susceptibility for T > 350 K. XRD measurements with Rietveld refinements reveal a small but substantial amount of decrease in disorder in both Hg and I sites above 350 K, with neither a structural transition nor vacancies in the temperature range. XAFS demonstrates a shift of the I K edge for T > 350 K but no changes at the Hg L3 edge. A stronger bonding of Hg-I pairs above 350 K likely affects the electrons in the outermost shells of iodine atoms and enhances the intrinsic paramagnetic properties, thereby producing a reduction in diamagnetism.