The crystal structure of La 1 − xRh 3B 2

Abstract

The crystal structure of La 1 − x Rh 3B 2 has been determined by single-crystal X-ray analysis; La 0.81Rh 3B 2 crystallizes in the hexagonal space group P6/ mmm ( D 6 h 1) 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 LaRh 3B 2 phase with the CeCo 3B 2-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 Rh 3B 2-type structure are reported with the general formula RE 1 − x Rh 3B 2 ( 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.