Crystals, magnetic and electronic properties of a new ThCr2Si2-type BaMn2Bi2 and K-doped compositions

Two Polymorphs of BaZn2P2: Crystal Structures, Phase Transition, and Transport Properties

The novel α-BaZn₂P₂ structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu₂S₂ structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With β-BaZn₂P₂ being previously identified as belonging to the ThCr₂Si₂ family and with the precedent of structural phase transitions between the α-BaCu₂S₂ type and the ThCr₂Si₂ type, the potential for the pattern to be extended to the two different structural forms of BaZn₂P₂ was explored. Thermal analysis suggests that a first-order phase transition occurs at ∼1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal β-BaZn₂P₂, the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn₂P₂ is a p-type, narrow-gap semiconductor with a direct bandgap of 0.5 eV, which is an order of magnitude lower than the calculated indirect bandgap for the β-BaZn₂P₂ phase. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 μV/K to 184 μV/K at 600 K. The electrical resistivity (ρ) of α-BaZn₂P₂ is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.

Quasi-One-Dimensional Structure and Possible Helical Antiferromagnetism of RbMn6Bi5

Quasi-one-dimensional materials exhibit not only unique crystal structure but also abundant physical properties such as charge density wave, Luttinger liquid, and superconductivity. Here we report the discovery, structure, and physical properties of a new manganese-based quasi-one-dimensional material RbMn₆Bi₅, which crystallizes in a monoclinic space group C2/m (No. 12) with lattice parameters a = 23.286(5) Å, b = 4.6215(9) Å, c = 13.631(3) Å, and β = 125.00(3)°. The structure features [Mn₆Bi₅]^-1 double-walled column extending along the [010] direction, together with Bi-Bi homoatomic bonds linking the columns and the countercation Rb⁺. The temperature-dependent resistivity clearly indicates a significant resistivity anisotropy for RbMn₆Bi₅, whereas the magnetic susceptibility and specific heat measurements show that RbMn₆Bi₅ is antiferromagnetic below 82 K. The density functional theory calculations indicate that RbMn₆Bi₅ is a quasi-one-dimensional metal with possible helical antiferromagnetic configuration. The discovery of RbMn₆Bi₅ confirms the viability of discovering new quasi-one-dimensional materials in manganese-based compounds.

Crystal chemistry and thermoelectric transport of layered AM2X2compounds

This review highlights the chemical diversity and transport properties of AM₂X₂Zintl compounds and strategies to achieve a high thermoelectric figure of merit.

Interplay of Dirac electrons and magnetism in CaMnBi2 and SrMnBi2

Dirac materials exhibit intriguing low-energy carrier dynamics that offer a fertile ground for novel physics discovery. Of particular interest is the interplay of Dirac carriers with other quantum phenomena such as magnetism. Here we report on a two-magnon Raman scattering study of AMnBi₂ (A=Ca, Sr), a prototypical magnetic Dirac system comprising alternating Dirac carrier and magnetic layers. We present the first accurate determination of the exchange energies in these compounds and, by comparison with the reference compound BaMn₂Bi₂, we show that the Dirac carrier layers in AMnBi₂ significantly enhance the exchange coupling between the magnetic layers, which in turn drives a charge-gap opening along the Dirac locus. Our findings break new grounds in unveiling the fundamental physics of magnetic Dirac materials, which offer a novel platform for probing a distinct type of spin–Fermion interaction. The results also hold great promise for applications in magnetic Dirac devices.

The interplay between the low-energy carriers in Dirac materials and magnetism is likely to reveal many novel physical phenomena. Here, the authors use two-magnon Raman scattering to determine the exchange energies of two prototypical magnetic Dirac systems, CaMnBi₂ and SrMnBi₂.

Robust antiferromagnetism preventing superconductivity in pressurized (Ba 0.61 K 0.39)Mn2Bi2

BaMn2Bi2 possesses an iso-structure of iron pnictide superconductors and similar antiferromagnetic (AFM) ground state to that of cuprates, therefore, it receives much more attention on its properties and is expected to be the parent compound of a new family of superconductors. When doped with potassium (K), BaMn2Bi2 undergoes a transition from an AFM insulator to an AFM metal. Consequently, it is of great interest to suppress the AFM order in the K-doped BaMn2Bi2 with the aim of exploring the potential superconductivity. Here, we report that external pressure up to 35.6 GPa cannot suppress the AFM order in the K-doped BaMn2Bi2 to develop superconductivity in the temperature range of 300 K-1.5 K, but induces a tetragonal (T) to an orthorhombic (OR) phase transition at ~20 GPa. Theoretical calculations for the T and OR phases, on basis of our high-pressure XRD data, indicate that the AFM order is robust in the pressurized Ba0.61K0.39Mn2Bi2. Both of our experimental and theoretical results suggest that the robust AFM order essentially prevents the emergence of superconductivity.

Coexistence of half-metallic itinerant ferromagnetism with local-moment antiferromagnetism in Ba0.60K0.40Mn2As2

Magnetization, nuclear magnetic resonance, high-resolution x-ray diffraction, and magnetic field-dependent neutron diffraction measurements reveal a novel magnetic ground state of Ba0.60K0.40Mn2As2 in which itinerant ferromagnetism (FM) below a Curie temperature TC≈100  K arising from the doped conduction holes coexists with collinear antiferromagnetism (AFM) of the Mn local moments that order below a Néel temperature TN=480  K. The FM ordered moments are aligned in the tetragonal ab plane and are orthogonal to the AFM ordered Mn moments that are aligned along the c axis. The magnitude and nature of the low-T FM ordered moment correspond to complete polarization of the doped-hole spins (half-metallic itinerant FM) as deduced from magnetization and ab-plane electrical resistivity measurements.

Crystal, magnetic, and electronic structures, and properties of new BaMnPnF (Pn = As, Sb, Bi)

New BaMnPnF (Pn = As, Sb, Bi) are synthesized by stoichiometric reaction of elements with BaF₂. They crystallize in the P4/nmm space group, with the ZrCuSiAs-type structure, as indicated by X-ray crystallography. Electrical resistivity results indicate that Pn = As, Sb, and Bi are semiconductors with band gaps of 0.73 eV, 0.48 eV and 0.003 eV (extrinsic value), respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below T_N = 338(1) K for Pn = As, and below T_N = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above T_N. The ordered magnetic moments are 3.65(5) μ_B/Mn for Pn = As, and 3.66(3) μ_B/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.