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.

No abstract

The thermoelectric properties of the n-type semiconductor TiNiSn were optimized by partial substitution with metallic MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the TiMnNiSnSb system. The alloys were prepared by arc-melting and annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing, the majority phase was TiNiSn with some Ni-rich sites, and the minority phases were primarily TiSn, Sn and MnSn. The Ni-rich sites were caused by Frenkel defects; this led to metal-like behavior in the semiconductor specimens at low temperature. For x ≤ 0.05 the samples showed an activated conduction, whereas for x > 0.05 they showed metallic character. The figure of merit for x = 0.05 was increased by 61% (zT = 0.45) in comparison with the pure TiNiSn.

No abstract

Combining neutron diffraction with pair distribution function analysis, we have uncovered hidden reduced symmetry in the correlated metallic d 1 perovskite, SrVO3. Specifically, we show that both the local and global structures are better described using a GdFeO3 distorted (orthorhombic) model as opposed to the ideal cubic ABO3 perovskite type. Recent reports of imaginary phonon frequencies in the density functional theory (DFT)-calculated phonon dispersion for cubic SrVO3 suggest a possible origin of this observed non-cubicity. Namely, the imaginary frequencies computed could indicate that the cubic crystal structure is unstable at T = 0 K. However, our DFT calculations provide compelling evidence that point defects in the form of oxygen vacancies, and not an observable symmetry breaking associated with calculated imaginary frequencies, primarily result in the observed non-cubicity of SrVO3. These experimental and computational results are broadly impactful because they reach into the thin-film and theoretical communities who have shown that SrVO3 is a technologically viable transparent conducting oxide material and have used SrVO3 to develop theoretical methods, respectively.

Electrons in solids often adopt complex patterns of chemical bonding driven by the competition between energy gains from covalency and delocalization, and energy costs of double occupation to satisfy Pauli exclusion, with multiple intermediate states in the transition between highly localized, and magnetic, and delocalized, and nonmagnetic limits. Herein, we report a chemical pressure-driven transition from a proper Mn magnetic ordering phase transition to a Mn magnetic phase crossover in EuMn 2 P 2 the limiting end member of the EuMn 2 X 2 (X = Sb, As, P) family of layered materials. This loss of a magnetic ordering occurs despite EuMn 2 P 2 remaining an insulator at all temperatures, and with a phase transition to long-range Eu antiferromagnetic order at T N ≈ 17 K. The absence of a Mn magnetic phase transition contrasts with the formation of long-range Mn order at T ≈ 130 K in isoelectronic EuMn 2 Sb 2 and EuMn 2 As 2 . Temperature-dependent specific heat and 31 P NMR measurements provide evidence for the development of short-range Mn magnetic correlations from T ≈ 250− 100 K, interpreted as a precursor to covalent bond formation. Density functional theory calculations demonstrate an unusual sensitivity of the band structure to the details of the imposed Mn and Eu magnetic order, with an antiferromagnetic Mn arrangement required to recapitulate an insulating state. Our results imply a picture in which long-range Mn magnetic order is suppressed by chemical pressure, but that antiferromagnetic correlations persist, narrowing bands and producing an insulating state.