Phase-Controlled Synthesis and Low Thermal Conductivity in Cu 4 TiTe 4

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Abstract
The synthesis of phase-pure Cu-based multinary tellurides remains challenging due to the competitive formation of stable Cu–Te binary phases, which frequently intercept the reaction pathway as secondary products. In the Cu 4 TiTe 4 system, rapid synthesis temperature mapping reveals the synthesis window, whereas precise compositional control suppresses secondary Cu–Te binaries to unlock non-equilibrium phase selection. We demonstrate that addition of 2 wt% excess Ti shifts the reaction chemistry toward Cu 4 TiTe 4 , suppressing detectable Cu–Te secondary phases and yielding phase-pure Cu 4 TiTe 4 over the identified 623–873 K synthesis window. Cu 4 TiTe 4 crystallises in a ‘stuffed sulvanite’ type structure, similar to Cu 4 TiSe 4 , and exhibits an order–disorder phase transition, in which partial ordering of the Cu sublattice induces structural transformation primarily governed by Cu sublattice ordering. Bulk Cu 4 TiTe 4 is a p-type narrow bandgap semiconductor that achieves a low lattice thermal conductivity of 0.63 to 0.68 W m -1 K -1 between 300 and 423 K. Transport calculations attribute the low observed thermal conductivity to Cu-sublattice disorder, local symmetry breaking, and localized Ti-dominated vibrations that collectively suppress phonon transport.