Growth, structure and optical properties of Tl4HgBr6 single crystals

Tl4HgBr6 single crystals were grown using solution-fusion method. The crystal structure of the ternary bromide was refined. Tl4HgBr6 crystallizes in the non-centrosymmetric space group P4nc with the lattice parameters a=8.9539(8) Å and c=8.7884(8) Å and it is isostructural to the Tl4HgI6 compound. The non-centrosymmetric structure of the Tl4HgBr6 compound was also confirmed by the existence of a modest second harmonic generation effect (0.4-0.5 pm/V) and by the value of piezoelectric coefficient (0.9 pm/V). The electronic structure of Tl4HgBr6 was explored using X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). In particular, for the Tl4HgBr6 crystal, we have measured XPS core-level and valence-band spectra for both pristine and Ar+ ion-bombarded surfaces. The XPS data reveal low hygroscopicity of Tl4HgBr6, the property that is very important when handling this material in optoelectronic devices working at ambient conditions. The present XPS data indicate that the Tl4HgBr6 single crystal surface is rather sensitive with respect to Ar+ ion-bombardment: such a treatment reduces significantly mercury content in the topmost surface layers. Comparison on a common energy scale of the XPS valence-band spectrum of Tl4HgBr6 and the XE Br Kβ2 band, representing peculiarities of the energy distribution of the Br 4 p states, reveals that the main contribution of the valence Br p states occurs in the upper portion of the valence band, with also their significant contributions in other valence band regions. The measurements of spectral distribution of the absorption coefficient indicate that the Tl4HgBr6 compound is a semiconductor with the bandgap energy value of 2.43 eV at 300 K, and the bandgap energy increases up to 2.48 eV when temperature decreases to 100 K.