Bonding and Electronic Nature of the Anionic Framework in LaPd₃S₄

Double Dirac materials are a topological phase of matter in which a non-symmorphic symmetry enforces greater electronic degeneracy than normally expected – up to eightfold. The cubic palladium bronzes NaPd3O4 and LaPd3S4 are built of Pd3X4 (X = O, S) anionic frameworks that are ionically bonded to A cations (A = Na, La). These materials were recently identified computationally as harboring eightfold fermions. Here we report the preparation of single crystals and electronic properties of LaPd3S4. Measurements down to T = 0.45 K and in magnetic fields up to μ0H = 65 T are consistent with normal Fermi liquid physics of a Dirac metal in the presence of dilute magnetic impurities. This interpretation is further confirmed by analysis of specific heat, magnetization measurements and comparison to density functional theory (DFT) calculations. Through a bonding analysis of the DFT electronic structure of NaPd3O4 and LaPd3S4, we identify the origin of the stability of the anionic Pd3X4 framework at higher electron counts for X = S than X = O, and propose chemical tuning strategies to enable shifting the 8-fold fermion points to the Fermi level

Dumbbells of Five-Connected Silicon Atoms and Superconductivity in the Binary Silicides MSi₃ (M = Ca, Y, Lu)

The new metastable binary silicides MSi₃ (M = Ca, Y, Lu) have been synthesized by high-pressure, high-temperature reactions at pressures between 12(2) and 15(2) GPa and temperatures from 900(100) to 1400(150) K. The atomic patterns comprise intricate silicon layers of condensed molecule-like Si₂ dimers. The alkaline-earth element adopts the oxidation state +2, while the rare-earth and transition metals realize +3. All of the compounds exhibit BCS-type superconductivity with weak electron–phonon coupling below critical temperatures of up to 7 K