Boron induced structure modifications in Pd-Cu-B system: new Ti2Ni-type derivative borides Pd3Cu3B and Pd5Cu5B2

Synthesis, Characterization, and First-Principles Analysis of the MAB-Like Ternary Transition-Metal Boride Fe(MoB)2

Novel transition-metal borides have attracted considerable attention because they exhibit high stability under extreme conditions. Compared with binary borides, ternary transition-metal borides (TTMBs) exhibit novel boron substructures and diverse properties, which result in excellent designability. In this study, we synthesized the MAB-like (where M = iron, A = molybdenum, and B = boron) phase Fe(MoB)₂ using a high-pressure and high-temperature method. Fe(MoB)₂ exhibited ferromagnetic metastable characteristics with a saturation magnetization of 8.35 emu/g at room temperature. Microhardness measurement revealed an indentation hardness of 10.72 GPa, which was higher than those of conventional magnetic materials. First-principles calculations revealed excellent mechanical properties, which mainly originated from the strong covalent short B₂ chains. Furthermore, magnetism was attributed to the Fe 3d electrons. Numerous d-d hybridizations existed between the Fe 3d e_g and Mo 4d orbitals, and the antibonding/nonbonding state difference for up/down-spin electrons in the hybridization orbitals led to the local magnetic moment of Fe(MoB)₂. The magnetic anisotropy energy analyses reveal that Fe(MoB)₂ prefers the easy magnetization axis along the z direction, and Mo atom acts as a medium to realize the exchange action between two Fe atoms. The B-B and Fe-B bonds were considerably stronger than the Fe-Mo and Mo-B bonds, and Fe(MoB)₂ exhibited a class of atomically laminate composed of FeB₂ and Mo layers. These results may provide guidance for the design of novel multifunctional TTMBs by adjusting the interactions between binary metal components.

Th7 Fe3 -Type Related Structures in Pd(Pt)-Cu-B Systems: Pd6 CuB3 -A New Structure Type for Borides

A new member of the series of Th₇ Fe₃ -type derivative structures, h-(Pd_0.86 Cu_0.14 )₇ B₃ (≡Pd_6.02 Cu_0.98 B₃ , unique structure type Pd₆ CuB₃ , space group P6₃ cm, a=12.9426(9) Å, c=4.8697(4) Å, single-crystal X-ray diffraction (XRD) data) was obtained from as cast alloys and alloys annealed at 600-650 °C. Further substitution of Cu by Pd led to formation of a Mn₇ C₃ -type structure, o-(Pd_0.93 Cu_0.07 )₇ B₃ (≡Pd_6.51 Cu_0.49 B₃ , space group Pnma, a=4.8971(2) Å, b=7.5353(3) Å, c=12.9743(6) Å, single-crystal XRD). Isotypic LT h-(Pt_0.70 Cu_0.30 )₇ B₃ (≡Pt_4.90 Cu_2.10 B₃ ) was observed in the Pt-Cu-B system as a low-temperature (LT) phase (T≤600 °C) (powder XRD), whereas the Th₇ Fe₃ -type (high-temperature (HT) h-(Pt_0.73 Cu_0.27 )₇ B₃ ≡Pt_5.11 Cu_1.89 B₃ , space group P6₃ mc, a=7.4671(1) Å, c=4.9039(1) Å, powder XRD) proved to be stable at high temperature. The three structures are built of columns of face connected metal octahedra and columns of metal tetrahedra alternatingly fused by common faces and vertices. Boron atoms are found in trigonal prisms formed by metal atoms. The volumes of the three new Th₇ Fe₃ -type derivative borides relate as 1:2:3. Superconductivity was discovered for Pt_4.9 Cu_2.1 B₃ (Pd₆ CuB₃ -type) and Pt_5.1 Cu_1.9 B₃ (Th₇ Fe₃ -type) below 0.67 and 0.66 K, respectively. Despite the close value of the transition temperature the values of the upper critical field at 0 K differ as 0.37 T and 0.27 T for the two compounds.