Crystal Structure of Cu2−δTe with van der Waals-like Interlayer Te–Te Interaction Revealed by Scanning Transmission Electron Microscopy

Although Cu–Te compounds have attracted increasing attention as energy materials, such as thermoelectrics, the currently accepted Cu–Te binary phase diagram and the reported crystal structures still involve uncertainties, primarily due to the complexity of the phase reactions involved. In this study, we have revisited the Cu-rich portion of the Cu–Te binary phase diagram in the composition range of Cu–33.3 to 35.0 at. %Te, and revealed the crystal structure of the low-temperature Cu2−δTe (RT) phase, lying deep within the phase diagram, through a combined use of electron diffraction, atomic-resolution scanning transmission electron microscopy, and powder X-ray diffraction. The Cu2−δTe (RT) phase forms virtually a line compound, whose composition deviates further from stoichiometry toward the Te-rich side compared to previous reports. The unit cell of Cu2−δTe (RT) belongs to the space group R3̅m with the lattice constants of a = 4.2843(7) and c = 43.251(2) Å (Pearson symbol hR36). The crystal structure comprises unit slabs of two Te layers sandwiching four Cu layers; six such slabs are stacked along the c-axis to form the unit cell. Structural vacancies of approximately four percent are present at the outer-layer Cu sites, accounting for the observed off-stoichiometry (Cu1.96Te, δ = 0.04). A high number density of interlayer Te–Te bonding via van der Waals (vdW) interaction is present in Cu2−δTe (RT) despite its Te-lean composition compared to typical vdW tellurides, such as Bi2Te3. The low lattice thermal conductivity of Cu2−δTe (RT) is attributed to the presence of the structural Cu vacancies and high-density interlayer Te–Te bonding. The electronic structure calculations indicate that Cu2−δTe (RT) is a hole-dominated semimetal, which is consistent with the positive Seebeck coefficient (p-type conduction) measured for Cu–33.3 at. %Te sample.