The crystal structure of La1 - xRh3B2 has been determined by single-crystal X-ray analysis; La0.81Rh3B2 crystallizes in the hexagonal space group P6/mmm (D6h1) with lattice parameters a = 5.642(1) A ̊, c = 8.546(2) A ̊, Z = 3, Dx = 9.37 g cm-3, Dm = 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|Fo|, 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 La1 - xRh3B2-type structure are reported with the general formula RE1 - xRh3B2 (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.
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