A Complex Ternary Chromium Arsenide, Eu ₉ Cr ₁₆ As ₁₉

We report the results of extensive exploratory work in the Eu–Cr–As compositional space that led to the identification of hitherto unknown phase Eu9Cr16As19. Its tetragonal crystal structure (space group P42/mnm; a = 15.2761(6) Å and c = 14.6014(8) Å) was established via single-crystal X-ray diffraction methods and boasts a high degree of complexity. The asymmetric cell consists of 21 symmetry-independent positions, including four Eu, nine Cr, and eight As, which account for a total of 176 atoms per unit cell (Pearson symbol tP176), with all atomic sites found to be fully occupied and with no indication of crystallographic disorder. Most of the Cr atoms are coordinated to five As atoms, and the CrAs5 square-pyramids share common faces, edges, and corners to form an intricate array featuring a variety of Cr–Cr interactions. The temperature dependence of the magnetic susceptibility of polycrystalline Eu9Cr16As19 is suggestive of a multifaceted magnetic structure that is yet to be determined. There are several observed transitions, and it is unclear which ones are contributed from the ordering of the Cr or Eu moments. In the paramagnetic regime, the calculated effective magnetic moment is close to the free-ion value of Eu2+. Electronic structure calculations confirm that Eu9Cr16As19 is a metallic system with a sizable reduction of the magnetic moments on the Cr atoms in their ground state.