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3.3 l Pressure-induced metallization of thallium halides (TlX: X=Cl, Br, I) (R.A. Secco)

The presence of d-electrons in thallium halides, resulting in band overlap and partially covalent cation-anion bonds, differentiates their chemical bonding from ionic bonding in alkali halides. The extended nature of the electronic charge distribution in Tl+ is characterized by its unusually high values of electronic polarizability of 5.1Å3 and 5.2Å3 in TlCl and TlBr, respectively. The cohesive, thermophysical and charge transfer properties of any material depend fundamentally on atomic interactions and therefore on bonding. High polarizability implies a high electron cloud deformability and hence a high compressibility (i.e. low bulk modulus; KoT (TlCl) = 16.3 GPa, KoT (TlBr) = 16.2 GPa) in the ranges of temperature and pressure where covalent interactions operate. Electrical conductivity measurements on TlX (X = Cl, Br, I) compounds at pressures up to 5GPa in previous studies showed a decrease in activation energy of conduction with pressure implying the possibility of metallization at pressures in the ranges of 40-50, 18-25, and 10-18 GPa, respectively. Experiments are being carried out in the multi-anvil apparatus in the lower pressure range and in the diamond anvil cell in the upper pressure range using electrical and optical techniques to investigate the possibility of metallization at room temperature. In an experiment up to 17 GPa, TlI was observed to metallize at 11.1 GPa, consistent with the prediction of band gap closure from the negative pressure-dependence of activation energy of conduction at low pressure. TlBr failed to metallize at pressures up to 23 GPa. Future experiments will focus on the higher pressure ranges to confirm the metallic character of TlBr and TlCl as a pressure tunable band gap-controlled property in these highly deformable cation compounds.

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