Phase Separation Prevents the Synthesis of VBi₂Te₄ by Molecular Beam Epitaxy

Intrinsic magnetic topological insulators (IMTIs) have a non-trivial band topology in combination with magnetic order. This potentially leads to fascinating states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. One of the theoretically predicted IMTIs is VBi₂Te₄, but experimental evidence of this material is lacking so far. Here, we report on our attempts to synthesise VBi₂Te₄ by molecular beam epitaxy (MBE). X-ray diffraction reveals that in the thermodynamic phase space reachable by MBE, there is no region where VBi₂Te₄ is stably synthesised. Moreover, scanning transmission electron microscopy shows a clear phase separation to Bi₂Te₃ and VTe₂ instead of the formation of VBi₂Te₄. We suggest the phase instability to be due to either the large lattice mismatch between VTe₂ and Bi₂Te₃ or the unfavourable valence state of vanadium.