Multi-Step Topochemical Pathway to Metastable Mo2AlB2 and Related Two-Dimensional Nanosheet Heterostructures

The rational synthesis of metastable inorganic solids, which is a grand challenge in solid-state chemistry, requires the development of kinetically controlled reaction pathways. Topotactic strategies can achieve this goal by chemically modifying reactive components of a parent structure under mild conditions to produce a closely related analogue that has otherwise inaccessible structures and/or compositions. Refractory materials, such as transition metal borides, are difficult to structurally manipulate at low temperatures because they generally are chemically inert and held together by strong covalent bonds. Here, we report a multistep low-temperature topotactic pathway to bulk-scale Mo2AlB2, which is a metastable phase that has been predicted to be the precursor needed to access a synthetically elusive family of 2-D metal boride (MBene) nanosheets. Room-temperature chemical deintercalation of Al from the stable compound MoAlB (synthesized as a bulk powder at 1400 °C) formed highly strained and destabili...