Electronic and dynamical properties of cobalt monogermanide CoGe phases under pressure

We present the pressure dependence of the electronic and dynamical properties of six different CoGe phases: orthorhombic Cmmm, hexagonal P6/mmm and P6̅2m, monoclinic C2/m, cubic P2_13, and orthorhombic Pnma. Using first-principles DFT calculations and the direct force-constants method, we study the dynamical stability of individual phases under external pressure. We show that the orthorombic Cmmm and hexagonal P6/mmm structures are unstable over a broad pressure range and most pronounced imaginary phonon soft mode in both cases leads to a stable hexagonal P6̅2m structure of the lowest ground-state energy of all studied phases at ambient and low (below ∼ 3 GPa) external pressure. Under these conditions, the cubic P2_13 phase has the highest energy, however, together with monoclinic C2/m and orthorombic Pnma it is dynamically stable and all these three structures can potentially coexist as meta-stable phases. Above ∼ 3 GPa, the cubic P2_13 phase becomes the most energetically favorable. Fitting the Birch–Murnaghan equation of state we derive bulk modulus for all mentioned phases, which indicate relatively high resistance of CoGe to compression. Such conclusions are confirmed by band structure calculations. Additionally, we show that electronic bands of the hexagonal P6̅2m phase reveal characteristic features of the kagome-like structure, while in the cubic P2_13 phase spectrum, one can locate spin-1 and double Weyl fermions. In both cases, the external pressure induces the Lifshitz transition, related to the modification of the Fermi surface topology.