High-Pressure Synthesis and Structure of Quasi-One-Dimensional Ba₃MSe₅ (M = Ti, Zr, and Hf)

The ternary selenides Ba₃MSe₅ (M = Ti, Zr, and Hf) were successfully synthesized through a solid-state reaction under high-pressure and high-temperature conditions. These compounds crystallize in a hexagonal structure, consisting mainly of one-dimensional (1D) face-sharing MSe₆ octahedral chains and Se chains. The intriguing superlattice along 1D chains was identified by theoretically calculating the Fermi surface and phonon spectrum of the Ba₃MSe₅ primitive cell (where M = Ti, Zr, and Hf). These superlattices exhibit average tetramerization, trimerization, and the primitive structures, respectively, as the M metal ions change from the 3d to 5d period. For the Ti- and Zr-containing compounds, the c-axis lengths are four times and three times that of the primitive structure, respectively. The space groups of P31c, P6̅c2, and P6₃/mcm were compatible with the Ti, Zr, and Hf selenides, respectively, resulting in the lattice parameters of a = 9.5304(8) Å and c = 25.3505(3) Å for Ba₁₂Ti₄Se₂₀, a = 9.5677(2) Å and c = 19.1731(6) Å for Ba₉Zr₃Se₁₅, and a = 9.5756(1) Å and c = 6.3802(7) Å for Ba₃HfSe₅. Comprehensive structural characterizations found that the vacancies on M sites increase from ∼20% to ∼40% and that the electronic hopping between interchains decreases in the Ba₃MSe₅ (M = Ti, Zr, and Hf) system as the M ions evolve from Ti to Hf, which dominates the electronic transport behaviors and results in a gradually increased band gap.