Relativistic effects in complex compounds containing heavy elements: ternary Cu-Tl-X (X = S/Se/Te)

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Chemistry

Background:

• Copper-chalcogenides (Cu-chalcogenides) are versatile materials used in photovoltaics and optoelectronics.
• Heavier Cu-Tl-X compounds are explored for topological insulators and thermoelectric applications, with relativistic effects of thallium (Tl) being a key factor.

Purpose of the Study:

• Investigate the significant, yet understudied, relativistic effects in Cu-Tl-X compounds.
• Understand the distinct roles of mass-velocity, Darwin, and spin-orbit-coupling terms in determining electronic band structures.

Main Methods:

• Employed a tailored density-functional-theory (DFT) approach for first-principles calculations.
• Analyzed the impact of relativistic corrections on bandgaps and band topologies.

Main Results:

• Relativistic effects substantially reduce bandgaps; CuTlS2 has a bandgap of 0.11 eV (vs. 1.7 eV without relativity).
• Spin-orbit-coupling induces band inversion in CuTlTe2, while CuTlSe2 is near a topological phase boundary.
• Strong relativistic core contraction can favor defective structures with larger bandgaps, hindering inverted band topologies.

Conclusions:

• Relativistic effects are critical for understanding the electronic and topological properties of Cu-Tl-X materials.
• The interplay of relativistic terms dictates bandgap reduction and potential for topological phase transitions.
• Defective structures may limit the realization of exotic electronic properties in these compounds.