Higher Dimensionality in the Mg–Co–B System: Synthesis and Structure of Incommensurate Composite Mg1+εCo4B4

Abstract

Guided by high-temperature in situ X-ray diffraction, the discovery and synthesis of Mg1+εCo4B4 (ε ≈ 0.272) using a MgH2 hydride precursor is reported, along with a detailed crystal structure description and measurement of magnetic properties. The mismatch in lattice periodicities between Mg and Co–B substructures places Mg1+εCo4B4 in the family of incommensurate composite crystals and prompted structural refinement in a (3 + 1)-dimensional model. The structure of Mg1+εCo4B4 (P42/ncm(00γ)s00s, a = 6.75847(7) Å, c = 3.94007(8) Å, q = (0, 0, 1.2721(3))) was refined from neutron powder diffraction and high-resolution powder X-ray diffraction data and confirmed by scanning transmission electron microscopy and electron diffraction. Mg1+εCo4B4 is isostructural to Nd1+εFe4B4 and several related ternary borides with 0.07 ≤ ε ≤ 0.17, with Mg occupying the rare-earth site. Satellite reflections in the electron diffraction patterns hinted at positional modulation of the transition metal–boron substructure by Mg atoms, but this could not be refined from the neutron or X-ray diffraction data. Low-temperature magnetic measurements show no indications of long-range magnetic ordering or superconductivity down to 5 K. DFT calculations confirmed the absence of a magnetically ordered ground state and the stability of a 5:4 supercell (ε = 0.25) relative to the fully commensurate structure. Neutron diffraction and synthesis from elemental Mg demonstrated that Mg1+εCo4B4 is not a hydrogen-stabilized phase. Mg1+εCo4B4 represents the second compound reported in the Mg–Co–B system and the first superspace symmetry model of a Nd1+εFe4B4-type incommensurate composite compound refined from powder diffraction data.