Ultralow Poisson's Ratios in Ultrahard Crystals Across Finite Strains

Abstract Materials exhibiting near-zero transverse deformation under tension, a property known as an ultralow Poisson's ratio (UPR), are highly sought for precision applications. However, UPR materials are conventionally limited to soft matters, hindering their use in extreme load-bearing environments. Here, we overturn this paradigm by discovering robust UPR and even negative Poisson's ratio behaviors in diamond and cubic boron nitride (c-BN)—two of the known hardest crystals—across finite strains. Combining finite-strain elasticity theory with first-principles calculations, we uncover highly anisotropic elastic responses hidden within their simple cubic structures. Most interestingly, Poisson's ratio of diamond and c-BN for loading along the [101] direction with transverse deformation measured along the [−101] direction (ν[101], [−101]) is ultralow (|ν| < 0.02) across a wide range of strain from −10% to +10%. These results are further supported by direct first-principles tensile simulations and analyses of in situ experimental data. The underlying mechanism is an atomic-scale cancellation of competing transverse deformation generated by bond and angle deformations. This work offers fresh insights into the nearly strain-invariant lateral dimensions in the known hardest crystals, and provides a roadmap for discovering and designing materials that combine extreme mechanical robustness with transverse dimensional stability.