Manganese Tetraboride,MnB4: High‐Temperature Crystal Structure,p–n Transition, 55Mn NMR Spectroscopy,Solid Solutions,and Mechanical Properties

The structural and electronic properties of MnB₄ were studied by high‐temperature powder X‐ray diffraction and measurements of the conductivity and Seebeck coefficient on spark‐plasma‐sintered samples. A transition from the room‐temperature monoclinic structure (space group P2₁/c) to a high‐temperature orthorhombic structure (space group Pnnm) was observed at about 650 K. The material remained semiconducting after the transition, but its behavior changed from p‐type to n‐type. ⁵⁵Mn NMR measurements revealed an isotropic chemical shift of ?1315 ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr_1?xMn_xB₄ (two phases in space groups Pnnm and P2₁/c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7) GPa for the Young’s modulus and hardness, respectively, compared to values of 530 and 37 GPa obtained by DFT calculations.     

ChemInform Abstract: Scaffolding,Ladders, Chains,and Rare Ferrimagnetism in Intermetallic Borides: Electronic Structure Calculations and Magnetic Ordering.

The electronic structures of “Ti_9‐nFe_2+nRu₁₈B₈” (n = 0, 0.5, 1, 2, 3), including the recently synthesized compounds Ti_9‐nFe_2+nRu₁₈B₈ (n = 1, 2), are determined by TB‐LMTO‐ASA computations.     

Validation of interstitial iron and consequences of nonstoichiometry in mackinawite (Fe(1+x)S)

A theoretical investigation of the relationship between chemical composition and electronic structure was performed on the nonstoichiometric iron sulfide, mackinawite (Fe(1+x)S), which is isostructural and isoelectronic with the superconducting Fe(1+x)Se and Fe(1+x)(Te(1-y)Se(y)) phases. Even though Fe(1+x)S has not been measured for superconductivity, the effects of stoichiometry on transport properties and electronic structure in all of these iron-excess chalcogenide compounds has been largely overlooked. In mackinawite, the amount of Fe that has been reported ranges from a large excess, Fe(1.15)S, to nearly stoichiometric, Fe(1.00(7))S. Here, we analyze, for the first time, the electronic structure of Fe(1+x)S to justify these nonstoichiometric phases. First principles electronic structure calculations using supercells of Fe(1+x)S yield a wide range of energetically favorable compositions (0 < x < 0.30). The incorporation of interstitial Fe atoms originates from a delicate balance between the Madelung energy and the occupation of Fe-S and Fe-Fe antibonding orbitals. A theoretical assessment of various magnetic structures for "FeS" and Fe(1.06)S indicate that striped magnetic ordering along [110] is the lowest energy structure and the interstitial Fe affects the values of moments in the square planes as a function of distance. Moreover, the formation of the magnetic moment is dependent on the unit cell volume, thus relating it to composition. Finally, changes in the composition cause a modification of the Fermi surface and ultimately the loss of a nested vector.     

Peierls‐Distorted Monoclinic MnB4 with a MnMn Bond

Tetraborides of chromium and manganese exhibit an unusual boron‐atom framework that resembles the hypothetical tetragonal diamond. They are believed to be very hard. Single crystals of MnB₄ have now been grown. The compound crystallizes in the monoclinic crystal system (space group P2₁/c) with a structure that has four crystallographically independent boron‐atom positions, as confirmed by ¹¹B MAS‐NMR spectroscopy. An unexpected short distance between the Mn atoms suggests a double Mn–Mn bond and is caused by Peierls distortion. The structure was solved using group‐subgroup‐relationships. DFT calculations indicate Mn^I centers and paramagnetism, as confirmed by magnetic measurements. The density of states shows a pseudo‐band gap at the Fermi energy and semiconducting behavior was observed for MnB₄.     

Structure, bonding, and magnetic response in two complex borides: Zr2Fe1−δRu5+δB2 and Zr2Fe1−δ(Ru1−xRhx)5+δB2

Polycrystalline samples of two complex intermetallic borides Zr₂Fe_1−δRu_5+δB₂ and Zr₂Fe_1−δ(Ru_1−xRh_x)_5+δB₂ (δ=ca. 0.10; x=0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc₂Fe(Ru_1−xRh_x)₅B₂ and crystallize in the space group P4/mbm (no. 127) with the Ti₃Co₅B₂-type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c-axis with an interatomic metal-metal distance of 3.078(1) Å, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr₂FeRu₅B₂” using LDA yielded a narrow peak at the Fermi level assigned to Fe-Fe antibonding interactions along the c-axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains.     

Scaffolding, ladders, chains, and rare ferrimagnetism in intermetallic borides: synthesis, crystal chemistry and magnetism

Single-phase polycrystalline samples and single crystals of the complex boride phases Ti(8)Fe(3)Ru(18)B(8) and Ti(7)Fe(4)Ru(18)B(8) have been synthesized by arc melting the elements. The phases were characterized by powder and single-crystal X-ray diffraction as well as energy-dispersive X-ray analysis. They are new substitutional variants of the Zn(11)Rh(18)B(8) structure type, space group P4/mbm (no. 127). The particularity of their crystal structure lies in the simultaneous presence of dumbbells which form ladders of magnetically active iron atoms along the [001] direction and two additional mixed iron/titanium chains occupying Wyckoff sites 4h and 2b. The ladder substructure is ca. 3.0 ? from the two chains at the 4h, which creates the sequence chain-ladder-chain, establishing a new structural and magnetic motif, the scaffold. The other chain (at 2b) is separated by at least 6.5 ? from this scaffold. According to magnetization measurements, Ti(8)Fe(3)Ru(18)B(8) and Ti(7)Fe(4)Ru(18)B(8) order ferrimagnetically below 210 and 220 K, respectively, with the latter having much higher magnetic moments than the former. However, the magnetic moment observed for Ti(8)Fe(3)Ru(18)B(8) is unexpectedly smaller than the recently reported Ti(9)Fe(2)Ru(18)B(8) ferromagnet. The variation of the magnetic moments observed in these new phases can be adequately understood by assuming a ferrimagnetic ordering involving the three different iron sites. Furthermore, the recorded hysteresis loops indicate a semihard magnetic behavior for the two phases. The highest H(c) value (28.6 kA/m), measured for Ti(7)Fe(4)Ru(18)B(8), lies just at the border of those of hard magnetic materials.     

Scaffolding, ladders, chains, and rare ferrimagnetism in intermetallic borides: electronic structure calculations and magnetic ordering

The electronic structures of "Ti(9-n)Fe(2+n)Ru(18)B(8)" (n=0, 0.5, 1, 2, 3), in connection to the recently synthesized Ti(9-n)Fe(2+n)Ru(18)B(8) (n=1, 2), have been investigated and analyzed using LSDA tight-binding calculations to elucidate the distribution of Fe and Ti, to determine the maximum Fe content, and to explore possible magnetic structures to interpret experimental magnetization results. Through a combination of calculations on specific models and using the rigid band approximation, which is validated by the DOS curves for "Ti(9-n)Fe(2+n)Ru(18)B(8)" (n=0, 0.5, 1, 2, 3), mixing of Fe and Ti is anticipated at both the 2b- and 4h-chain sites. The model "Ti(8.5)Fe(2.5)Ru(18)B(8)" (n=0.5) revealed that both Brewer-type Ti-Ru interactions as well as ligand field splitting of the Fe 3d orbitals regulated the observed valence electron counts between 220 and 228 electrons/formula unit. Finally, models of magnetic structures were created using "Ti(6)Fe(5)Ru(18)B(8)" (n=3). A rigid band analysis of the LSDA DOS curves concluded preferred ferromagnetic ordering at low Fe content (n≤0.75) and ferrimagnetic ordering at higher Fe content (n>0.75). Ferrimagnetism arises from antiferromagnetic exchange coupling in the scaffold of Fe1-ladder and 4h-chain sites.     

Discovering Intermetallics Through Synthesis,Computation, and Data-Driven Analysis

Intermetallics adopt an array of crystal structures, boast diverse chemical compositions, and possess exotic physical properties that have led to a wide range of applications from the biomedical to aerospace industries. Despite a long history of intermetallic synthesis and crystal structure analysis, identifying new intermetallic phases has remained challenging due to the prolonged nature of experimental phase space searching or the need for fortuitous discovery. In this Minireview, new approaches that build on the traditional methods for materials synthesis and characterization are discussed with a specific focus on realizing novel intermetallics. Indeed, advances in the computational modeling of solids using density functional theory in combination with structure prediction algorithms have led to new high-pressure phases, functional intermetallics, and aided experimental efforts. Furthermore, the advent of data-centered methodologies has provided new opportunities to rapidly predict crystal structures, physical properties, and the existence of unknown compounds. Describing the research results for each of these examples in depth while also highlighting the numerous opportunities to merge traditional intermetallic synthesis and characterization with computation and informatics provides insight that is essential to advance the discovery of metal-rich solids.