Magnetic ordering and metal‐atom site preference in tetragonal CrMnAs: Electronic correlation effects

The electronic and magnetic structures of tetragonal, Cu2Sb-type CrMnAs were examined using density functional theory. To obtain reasonable agreement with reported atomic and low-temperature magnetic ordering in this compound, the intra-atomic electron-electron correlation in term of Hubbard U on Mn atoms are necessary. Using GGA+U, calculations identify four low-energy antiferromagnetically ordered structures, all of which adopt a magnetic unit cell that contains the same direct CrCr and CrMn magnetic interaction, as well as the same indirect MnMn magnetic interaction across the Cr planes. One of these low-energy configurations corresponds to the reported case. Effective exchange parameters for metal-metal contacts obtained from SPRKKR calculations indicate both direct and indirect exchange couplings play important roles in tetragonal CrMnAs

Electronically Induced Ferromagnetic Transitions in Sm5Ge4-Type Magnetoresponsive Phases

The correlation between magnetic and structural transitions in Gd5SixGe4−x hampers the studies of valence electron concentration (VEC) effects on magnetism. Such studies require decoupling of the VEC-driven changes in the magnetic behavior and crystal structure. The designed compounds, Gd5GaSb3 and Gd5GaBi3, adopt the same Sm5Ge4-type structure as Gd5Ge4 while the VEC increases from 31  e−/formula in Gd5Ge4 to 33  e−/formula in Gd5GaPn3 (Pn: pnictide atoms). As a result, the antiferromagnetic ground state in Gd5Ge4 is tuned into the ferromagnetic one in Gd5GaPn3. First-principles calculations reveal that the nature of interslab magnetic interactions is changed by introducing extra p electrons into the conduction band, forming a ferromagnetic bridge between the adjacent [∝2Gd5T4] slabs

Electronic structure studies of complex intermetallic arsenides and borides

Electronic structure methods were used to investigate bonding, metal-atom site preference, magnetic ordering, and crystal structure in ternary and quaternary intermetallic compounds that include arsenic or boron. Computational methods based on density functional theory were used to investigate the electronic structure and properties of hypothetical compounds closely related to the compounds of interest, in order to investigate the origins of properties that have been observed experimentally. The Ti-M-Ir-B (M = transition metal) system was investigated through density functional theory (DFT) calculations in collaboration with experimental researchers. Compounds with approximate compositions (TixM1–x)3Ir3B3 were identified in two structures: a hexagonal structure for M = V, Cr, Mn, with Ti:M ratios near 1:1, and an orthorhombic structure for M = Mn and heavier transition metals, with Ti-M ratios near 2:1. Calculated lattice parameters for hypothetical “(Ti1/2M1/2)3Ir3B3” and “(Ti2/3M1/3)3Ir3B3” also showed a shift in stability of the hexagonal compounds for M heavier than Mn. Second-moment scaling with the Huckel method showed that the zigzag B4 subunit found in the orthorhombic structure would be, in isolation, more energetically favorable than the trigonal-planar B4 subunit of in the hexagonal structure. Crystal orbital Hamilton population (COHP) analysis suggested that the hexagonal TiCrIr2B2 structure was instead stabilized by Cr–Cr bonding, while the Ti:M ratio of the orthorhombic structures serves to maximize heteroatomic Ti:M bonds. DFT with a Hubbard U term (DFT+U) was used to investigate the importance of electron-electron correlation in CrMnAs and TmAlB4. In CrMnAs, DFT+U results give a better match for experimental magnetic ordering and metal-atom site preference than results from DFT alone. In TmAlB4, the electronic structure depends significantly on the choice of U, suggesting that previous results using DFT without U might not be accurate. COHP analysis was used to examine a possible Stone-Wales-like transformation mechanism between two related phases in TmAlB4. The strongest B–B bonds in β-TmAlB4 were found to be isolated by weaker B–B bonds, but the strongest B–B bonds in α-TmAlB4 formed chains oriented along the structure’s a axis.</p

Magnetic ordering and metal‐atom site preference in tetragonal CrMnAs: Electronic correlation effects

The electronic and magnetic structures of tetragonal, Cu2Sb-type CrMnAs were examined using density functional theory. To obtain reasonable agreement with reported atomic and low-temperature magnetic ordering in this compound, the intra-atomic electron-electron correlation in term of Hubbard U on Mn atoms are necessary. Using GGA+U, calculations identify four low-energy antiferromagnetically ordered structures, all of which adopt a magnetic unit cell that contains the same direct CrCr and CrMn magnetic interaction, as well as the same indirect MnMn magnetic interaction across the Cr planes. One of these low-energy configurations corresponds to the reported case. Effective exchange parameters for metal-metal contacts obtained from SPRKKR calculations indicate both direct and indirect exchange couplings play important roles in tetragonal CrMnAs.This is the peer-reviewed version of the following article: Lutz‐Kappelman, Laura, Yuemei Zhang, and Gordon J. Miller. "Magnetic ordering and metal‐atom site preference in tetragonal CrMnAs: Electronic correlation effects." Journal of Computational Chemistry 39, no. 21 (2018): 1585-1593, which has been published in final form at DOI: 10.1002/jcc.25230. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. Posted with permission.</p

Electronically Induced Ferromagnetic Transitions in Sm5Ge4-Type Magnetoresponsive Phases

The correlation between magnetic and structural transitions in Gd5SixGe4−x hampers the studies of valence electron concentration (VEC) effects on magnetism. Such studies require decoupling of the VEC-driven changes in the magnetic behavior and crystal structure. The designed compounds, Gd5GaSb3 and Gd5GaBi3, adopt the same Sm5Ge4-type structure as Gd5Ge4 while the VEC increases from 31  e−/formula in Gd5Ge4 to 33  e−/formula in Gd5GaPn3 (Pn: pnictide atoms). As a result, the antiferromagnetic ground state in Gd5Ge4 is tuned into the ferromagnetic one in Gd5GaPn3. First-principles calculations reveal that the nature of interslab magnetic interactions is changed by introducing extra p electrons into the conduction band, forming a ferromagnetic bridge between the adjacent [∝2Gd5T4] slabs.This article is from Physical Review Letters 110 (2013): 1, doi:10.1103/PhysRevLett.110.077204. Posted with permission.</p

Transformation of trigonal planar B4 into zigzag B4 units within the new boride series Ti2–xM1+x–δIr3+δB3 (x = 0.5 for M = V–Mn, x = 0 for M = Mn–Ni and δ < 0.2)

In metal-rich borides, numerous boron fragments B-n (n = 2, 3, 4, 5, 6) have been discovered, the B-4 units being the most versatile with four different shapes (bent, zigzag, trigonal planar and tetrahedral). We report on the new boride series Ti2-xM1+x-delta Ir3+delta B3 (x = 0.5 for M = V-Mn, x = 0 for M = Mn-Ni and delta < 0.2), in which a structural change occurs by successive substitution of the 3d transition metal M = V, Cr, Mn, Fe, Co and Ni. It is found that the change in structure from the Ti1+xOs2-xRuB2-type structure (P (6) over bar 2m, no. 189) to the Ti1+xRh2-x+yIr3-yB3-type (Pbam, no. 55) leads to a change of B-4 shape from trigonal planar B-4 (M = V-Mn) to zigzag B-4 fragment (M = Mn-Ni). Even though there is no group-subgroup relationship between the two structures, we present how the Ti1+xOs2-xRuB2-type structure can easily be geometrically derived from the Ti1+xRh2-x+yIr3-yB3-type.This article is published as Scheifers, Jan P., Michael Küpers, Yuemei Zhang, Laura Lutz‒Kappelman, Gordon J. Miller, and Boniface PT Fokwa. "Transformation of trigonal planar B4 into zigzag B4 units within the new boride series Ti2–xM1+ x–δIr3+ δB3 (x= 0.5 for M= V–Mn, x= 0 for M= Mn–Ni and δ< 0.2)." Solid State Sciences 107 (2020): 106294. DOI:10.1016/j.solidstatesciences.2020.106294. Copyright 2020 Published by Elsevier Masson SAS. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission

Transformation of trigonal planar B4 into zigzag B4 units within the new boride series Ti2–xM1+x–δIr3+δB3 (x = 0.5 for M = V–Mn, x = 0 for M = Mn–Ni and δ < 0.2)

In metal-rich borides, numerous boron fragments B-n (n = 2, 3, 4, 5, 6) have been discovered, the B-4 units being the most versatile with four different shapes (bent, zigzag, trigonal planar and tetrahedral). We report on the new boride series Ti2-xM1+x-delta Ir3+delta B3 (x = 0.5 for M = V-Mn, x = 0 for M = Mn-Ni and delta < 0.2), in which a structural change occurs by successive substitution of the 3d transition metal M = V, Cr, Mn, Fe, Co and Ni. It is found that the change in structure from the Ti1+xOs2-xRuB2-type structure (P (6) over bar 2m, no. 189) to the Ti1+xRh2-x+yIr3-yB3-type (Pbam, no. 55) leads to a change of B-4 shape from trigonal planar B-4 (M = V-Mn) to zigzag B-4 fragment (M = Mn-Ni). Even though there is no group-subgroup relationship between the two structures, we present how the Ti1+xOs2-xRuB2-type structure can easily be geometrically derived from the Ti1+xRh2-x+yIr3-yB3-type.This article is published as Scheifers, Jan P., Michael Küpers, Yuemei Zhang, Laura Lutz‒Kappelman, Gordon J. Miller, and Boniface PT Fokwa. "Transformation of trigonal planar B4 into zigzag B4 units within the new boride series Ti2–xM1+ x–δIr3+ δB3 (x= 0.5 for M= V–Mn, x= 0 for M= Mn–Ni and δ< 0.2)." Solid State Sciences 107 (2020): 106294. DOI:10.1016/j.solidstatesciences.2020.106294. Copyright 2020 Published by Elsevier Masson SAS. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission

Spin Frustration and Magnetic Ordering from One-Dimensional Stacking of Cr₃ Triangles in TiCrIr₂B₂

Spin-frustrated chains of Cr₃ triangles are found in the new metal boride TiCrIr₂B₂ by synergistic experimental and theoretical investigations. Although magnetic ordering is found at 275 K, competing ferro- and anti-ferromagnetic interactions coupled with spin frustration induce a rather small total magnetic moment (0.05 μ_B at 5 T), and density functional theory (DFT) calculations propose a canted, nonlinear magnetic ground-state ordering in the new phase. TiCrIr₂B₂ crystallizes in the hexagonal Ti_1+<i>x</i>Os_2–<i>x</i>RuB₂ structure type (space group <i>P</i>6̅2<i>m</i>, No. 189, Pearson symbol <i>hP</i>18). The structure contains trigonal planar B₄ boron fragments with B–B distances of 1.76(3) Å alternating along the <i>c</i>-direction with Cr₃ triangles with intra- and intertriangle Cr–Cr distances of 2.642(9) and 3.185(1) Å, respectively. Magnetization measurements of TiCrIr₂B₂ reveal ferrimagnetic behavior and a large, negative Weiss constant of −750 K. DFT calculations demonstrate a strong site preference of Cr for the triangle sites, as well as magnetic frustration due to indirect anti-ferromagnetic interactions within the Cr₃ triangles

Spin Frustration and Magnetic Ordering from One-Dimensional Stacking of Cr₃ Triangles in TiCrIr₂B₂

Spin-frustrated chains of Cr₃ triangles are found in the new metal boride TiCrIr₂B₂ by synergistic experimental and theoretical investigations. Although magnetic ordering is found at 275 K, competing ferro- and anti-ferromagnetic interactions coupled with spin frustration induce a rather small total magnetic moment (0.05 μ_B at 5 T), and density functional theory (DFT) calculations propose a canted, nonlinear magnetic ground-state ordering in the new phase. TiCrIr₂B₂ crystallizes in the hexagonal Ti_1+<i>x</i>Os_2–<i>x</i>RuB₂ structure type (space group <i>P</i>6̅2<i>m</i>, No. 189, Pearson symbol <i>hP</i>18). The structure contains trigonal planar B₄ boron fragments with B–B distances of 1.76(3) Å alternating along the <i>c</i>-direction with Cr₃ triangles with intra- and intertriangle Cr–Cr distances of 2.642(9) and 3.185(1) Å, respectively. Magnetization measurements of TiCrIr₂B₂ reveal ferrimagnetic behavior and a large, negative Weiss constant of −750 K. DFT calculations demonstrate a strong site preference of Cr for the triangle sites, as well as magnetic frustration due to indirect anti-ferromagnetic interactions within the Cr₃ triangles