Boniface P T Fokwa
University of California-Riverside, USA
Boniface P T Fokwa completed his PhD in 2003 from the Dresden University of Technology (Germany) and Post-doctoral studies from the RWTH Aachen University (Germany). He is an Associate Editor for Encyclopedia of Inorganic and Bioinorganic Chemistry (Wiley). He has published more than 70 papers in reputed journals and has won several awards including the prestigious Heisenberg Fellowship from the German Research Foundation and was Visiting Scientist at University of Auckland (New Zealand) and Cornell University as well as Visiting Professor at UCLA.
A new series of compounds TiCrIr2‑xOsxB2 (x=0-2) was successfully synthesized and characterized using X-ray diffraction as well as energy-dispersive X-ray analysis. All members of the series crystallize in the hexagonal non-centrosymmetric Ti1+xOs2-xRuB2 structure type (space group P2m, no. 189, Pearson symbol hP18). The structure contains trigonal planar B4 units strongly interacting with triangles of magnetically active Cr atoms, which are stacked on each other to form isolated Cr3-chains along the c-axis. Magnetization measurements of TiCrIr2B2 (34 valence electrons, VE) reveal ferrimagnetic behavior below TC=275 K with a large, negative Weiss constant of -750 K. Density functional theory calculations demonstrate magnetic frustration due to indirect antiferromagnetic interactions within the Cr3 triangles competing with direct ferromagnetic interactions. Tuning the valence electron count by replacing Ir with Os changes the magnetic behavior in the series. Magnetization measurements of TiCrIrOsB2 (33 VE) and TiCrOs2B2 (32 VE) exhibit paramagnetic behavior with features reminiscent of the spin liquid state. Interestingly, the crystal orbital Hamilton population (COHP) of TiCrIr2B2 indicates Cr-Cr antibonding interactions above 32.9 VE, but part of the Cr-Cr antibonding region is already occupied in TiCrOs2B2 even though it has 32 valence electrons. Therefore, the TiCrIr2‑xOsxB2 (x=0-2) series do not follow rigid band approximation.