The crystal structure of La_{1 - x}Rh₃B₂

The crystal structure of La_{1 - x}Rh₃B₂ has been determined by single-crystal X-ray analysis; La_0.81Rh₃B₂ crystallizes in the hexagonal space group P6/mmm (D_6h¹) with lattice parameters a = 5.642(1) A ̊, c = 8.546(2) A ̊, Z = 3, D_x = 9.37 g cm^-3, D_m = 9.49 g cm^-3. Intensity measurements were obtained using a four-circle diffractometer. The structure was refined by full-matrix least-square calculation. The final R value, R = Σ|ΔF|/Σ|F_o|, is 0.054 for an asymmetric set of 536 independent reflections. The crystal structure is a disordered superstructure (vacancy distorted) of the LaRh₃B₂ phase with the CeCo₃B₂-type structure (a = 5.480 A ̊, c = 3.137 A ̊). The coordination numbers of lanthanum are (1.6La) + 12Rh + 6B and those of rhodium atoms (3.2La) + 6Rh + 4B. The isolated boron atoms have tetrakaidekahedral metal coordination (2.4La) + 6Rh; no boron-boron contact occurs. Four new ternary rhodium borides with this disordered La_{1 - x}Rh₃B₂-type structure are reported with the general formula RE_{1 - x}Rh₃B₂ (x = 0.5; RE = La, Ce, Pr or Nd) through powder X-ray diffraction study. The relation between structural chemistry and physical properties (superconductivity, ferromagnetism, valence fluctuation) is discussed.