Scaffolds of magnetically active 3d metals in the valence electron controlled borides Ti9−xM2+xRu18B8 (M=Cr–Ni; x=0.5–1): Structutral, electronic and magnetic properties

Abstract

Polycrystalline samples of the boride series Ti{sub 9−x}M{sub 2+x}Ru{sub 18}B{sub 8} (M=Cr, Co, Mn, Ni) including single crystals of Ti{sub 8}Co{sub 3}Ru{sub 18}B{sub 8} have been prepared by arc-melting the elements. The phases were characterized by powder X-ray diffraction (Rietveld refinement), single-crystal X-ray diffraction (for M=Co), and energy-dispersive X-ray (EDX) analysis. They are substitutional variants of the Zn{sub 11}Rh{sub 18}B{sub 8} structure type, space group P4/mbm (No. 127) and contain a “scaffold” structural unit (M-ladders interacting with M/Ti-chains) as well as isolated M/Ti-chains. According to DFT calculations, the Ru–X (X=B, Ti, Ti/M) bonding interactions are nearly constant throughout the series and responsible for the structural stability of these phases, whereas the M–M and Ru–M interactions vary significantly with varying valence electron count. Furthermore, density of states (DOS) analyses predict the phases with M=Mn and Ni to develop a total magnetic moment but not the M=Co phase. Susceptibility measurements confirm the Co phase to be paramagnetic and the Mn Phase orders ferrimagnetically below 120 K and thus develops a magnetic moment, as predicted. - Graphical abstract: The crystal structures of the new phases (M=Cr, Mn, Co, Ni) are confirmed by Rietveld refinement of powder diffraction data and single crystal X-ray diffractionmore » (for M=Co) to contain beside the M-ladder also M/Ti-chains. Similar to the series Ti{sub 9}M{sub 2}Ru{sub 18}B{sub 8}, the crystal structure of the new phases are mainly stabilized by the heteroatomic Ru–B and Ru–Ti bonds that remain nearly constant throughout the series, whereas the M-containing bonds vary significantly with varying valence electron count. An experimental finding confirmed and even extended by COHP bonding analyses. In addition, the DOS analyses of the M-elements reveal the development of magnetic moments for the M=Mn, Ni cases but not for M=Co. Indeed, Ti{sub 8}Co{sub 3}Ru{sub 18}B{sub 8} was found experimentally to be a paramagnet and ferrimagnetic ordering below 120 K is found for M=Mn. Highlights: • Synthesis of the valence electron controlled borides Ti{sub 9−x}M{sub 2+x}Ru{sub 18}B{sub 8} (M=Cr–Ni). • Prediction of magnetic properties of Ti{sub 9−x}M{sub 2+x}Ru{sub 18}B{sub 8}. • Ferrimagnetic ordering found in Ti{sub 8}M{sub 3}Ru{sub 18}B{sub 8}. • Chemical bonding studied by density functional theory.« less