Discovery of a new magnesium iron boride Mg4Fe1.1B13.9 in the Mg-Fe-B-N system

Abstract

A new boron-rich ternary phase, Mg4Fe1.1B13.9, was discovered in the Mg-Fe-B-N quaternary system. This novel phase appears in the form of plate-like shaped crystals formed on the surface of Fe-substituted MgB2 during high-pressure, high-temperature (HPHT) solution growth at 3 GPa and 1960 C. Mg4Fe1.1B13.9 crystallizes in an orthorhombic structure with space group Pnam (No. 62) and lattice parameters a = 10.95(2) A, b = 7.07(1) A, and c = 8.72(1) A. Structural refinement reveals a layered architecture composed of alternating layers of Mg-Fe and boron, with boron atoms forming 5-, 6-, and 7-membered ring motifs. A structural comparison indicates that Mg4Fe1.1B13.9 is closely related to the Y2ReB6-type borides. This discovery highlights the effectiveness of the HPHT synthesis in accessing new, complex boron-rich phases. As research in the binary Mg-B system approaches saturation, the identification of Mg4Fe1.1B13.9 offers new insights into the formation of phases in the Mg-Fe-B-N system. This paves the way for the guided synthesis of novel borides with unique properties in other multicomponent systems.