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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 μB 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 Ti1+xOs2-xRuB2 structure type (space group P6̅2m, No. 189, Pearson symbol hP18). The structure contains trigonal planar B4 boron fragments with B-B distances of 1.76(3) Å alternating along the c-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.

In metal‒rich borides, numerous boron fragments Bn (n = 2, 3, 4, 5, 6) have been discovered, the B4 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–δIr3+δB3 (x = 0.5 for M = V–Mn, x = 0 for M = Mn–Ni and δ 6 ‾ 2m, no. 189) to the Ti1+xRh2‒x+yIr3‒yB3‒type (Pbam, no. 55) leads to a change of B4 shape from trigonal planar B4 (M = V–Mn) to zigzag B4 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.

The electronic and magnetic structures of tetragonal, Cu2 Sb-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 CrCr and CrMn magnetic interaction, as well as the same indirect Mn⋅⋅⋅Mn 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. © 2018 Wiley Periodicals, Inc.