CuN₁₀: a high-energy-density pentazolate with an antiferromagnetic state

The pentazolates have attracted intensive interest because of their intrinsic green and high-energy density features. However, the physical role of spin electrons in pentazolate compounds has not yet been revealed. In this work, we use a constrained first principles structure search method to investigate the energy landscape of copper(II) pentazolate. The most stable phase (P2₁2₁2₁-CuN₁₀) adopts a typical square-layered configuration with dsp² hybridized Cu atoms. Interestingly, the four empty dsp² hybridization orbitals fully coordinate with the lone pair electrons of the pentazolate anions (cyclo-N₅^-), and the unpaired electron of the Cu²⁺ cation occupies the 4p orbital, which induces a high peak (Van Hove singularity) at the Fermi level in the density of states (DOS), further causing instability. Instead, this high peak is split into two spin channels via the electron's rearrangement, and the unpaired electrons transfer to the dsp² hybridization orbital and form an antiferromagnetic state. As a result, the spin electrons are demonstrated to effectively lower the DOS at the Fermi level and stabilize the structure by reducing the energy of 0.128 eV per formula. In addition, the energy density of P2₁2₁2₁-CuN₁₀ is 4.05 kJ g^-1, which is 1.83 times that of the reported P2₁2₁2₁-CuN₅ (2.21 kJ g^-1), and is a bit higher than that of the known Pnma-CuN₆ (3.84 kJ g^-1), indicating that P2₁2₁2₁-CuN₁₀ is a promising high-energy-density material.