MNiSn (M = Ti, Zr, and Hf) half-Heusler (HH) compounds are widely studied n-type thermoelectric (TE) materials for power generation. Most studies focus on Zr- and Hf-based compounds due to their high thermoelectric performance. However, these kinds of compositions are not cost-effective. Herein, the least expensive alloy in this half-Heusler family-TiNiSn-is investigated. Modulation doping of half-metallic MnNiSb in the TiNiSn system is realized by using spark plasma sintering. It is found that MnNiSb dissolves into the TiNiSn matrix and forms a heavily doped Ti1-xMnxNiSn1-xSbx phase, which leads to largely enhanced carrier concentration and also slight increase of carrier mobility. As a result, the electrical conductivity and power factor of the modulation doped compounds are greatly improved. A maximum power factor of 45 X 10(-4) W K-2 m(-1) is obtained at 750 K for the modulation doping system (TiNiSn)(1-x) + (MnNiSb)(x) with x = 0.05, which is one of the highest reported values in literature for TiNiSn systems. Furthermore, the lattice thermal conductivity is also suppressed due to the enhanced phonon scattering. Beneficial from the improved power factor and suppressed lattice thermal conductivity, a peak zT of 0.63 is obtained at 823 K for x = 0.05, which is an similar to 70% increase compared to the peak zT of TiNiSn. These results highlight the potential application of inexpensive TiNiSn-based TE materials and the effectiveness of modulation doping in enhancing the TE performance of HH compounds

Materials with nonsymmorphic symmetries have many applications and have recently come to the forefront as possibly harboring new topological states of matter, such as 8-fold fermions. Here, we report the single crystal growth of Bi 2 CuO 4 using the traveling solvent floating zone technique. Using laser heating combined with a 0.875 Bi 2 CuO 4 :0.125 Bi 2 O 3 solvent, we produce untwinned single crystal pieces. Three-dimensional X-ray microcomputed tomography is used to probe the fundamental origins of twinning, grain formation, and growth. Powder X-ray diffraction and Laue diffraction show that Bi 2 CuO 4 crystallizes in the space group P4/ncc (#130), orders antiferromagnetically with T N = 43 K, and, combined with comparisons to the literature, demonstrate the crystallinity and reproducibility of the synthesis. The entropy lost at the magnetic phase transition is ΔS mag = 0.25 ln(2); it arises from a high anisotropy in the magnetic interactions. We carry out a symmetry analysis demonstrating that Bi 2 CuO 4 's magnetic order implies a rich breaking of the parent 8-fold symmetric states. Our results provide a roadmap for the creation of future magnetic derivatives of 8-fold, double Dirac single crystals and related quantum states of matter with nonsymmorphic symmetries. This approach also offers guidance on improving the single growth of nonsymmorphic materials from cuprates to van der Waals solids.

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal, and superconducting properties. Here, we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ∼4.5°) in the metallic kagomé MgCo 6 Ge 6 at T = 100(2) K via X-ray diffraction measurements, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ∼300 to 100 K, no evidence for a phase transition was found in physical property measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge 2 ) dimers that connect adjacent layers. Further modeling of the displacive disorder in the crystal structure shows the presence of a second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits interlayer twisting, but with a different pattern, consistent with the change in electron count due to the removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.