Realizing Persistent Zero Area Compressibility over a Wide Pressure Range in Cu2GeO4 by Microscopic Orthogonal-Braiding Strategy

https://doi.org/10.1002/anie.202318401

Zero area compressibility (ZAC) is an extremely rare mechanical response that exhibits an invariant two-dimensional size under hydrostatic pressure. All known ZAC materials are constructed from units in two dimensions as a whole. Here, we propose another strategy to obtain the ZAC by microscopically orthogonal-braiding one-dimensional zero compressibility strips. Accordingly, ZAC is identified in a copper-based compound with a planar [CuO₄] unit, Cu₂GeO₄, that possesses an area compressibility as low as 1.58(26) TPa⁻¹ over a wide pressure range from ≈0 GPa to 21.22 GPa. Based on our structural analysis, the subtle counterbalance between the shrinkage of [CuO₄] and the expansion effect from the increase in the [CuO₄]-[CuO₄] dihedral angle attributes to the ZAC response. High-pressure Raman spectroscopy, in combination with first-principles calculations, shows that the electron transfer from in-plane bonding d_x²_-y² to out-of-plane nonbonding d_z² orbitals within copper atoms causes the counterintuitive extension of the [CuO₄]-[CuO₄] dihedral angle under pressure. Our study provides an understanding on the pressure-induced structural evolution of copper-based oxides at an electronic level and facilitates a new avenue for the exploration of high-dimensional anomalous mechanical materials.