The Glass That Expands When Squeezed

If you squeeze a block of rubber, it shortens along the direction of your grip. This behavior is intuitive enough that we build it into the foundational equations of materials science. The ratio of lateral strain to axial strain—how much a substance bulges out when compressed—is governed by Poisson's ratio, and the physical response is almost universally a contraction in the direction of the applied force.

But a molecular glass synthesized by a research team led by Shuo-Ying Chen in 2025 does the exact opposite.

When placed under hydrostatic pressure, the glass expands along the direction of the squeeze. This phenomenon, known as negative linear compressibility, has been observed in a handful of complex crystalline structures before. However, Chen and his colleagues measured it in an isotropic, disordered molecular glass.

Measured.

To understand why this is strange, consider how glass is put together. In a crystalline material, atoms are arranged in a repeating, highly ordered lattice. If you push on a crystal with a specific geometric structure—like a network of hinged, wine-rack-like structures—the hinges can fold in a way that causes one axis to lengthen even as the overall volume shrinks. This is a geometric trick of ordered frameworks.

Glass has no such lattice. It is a disordered state of matter, a liquid frozen in time with its molecules locked in a chaotic, amorphous jumble. Without a repeating structure to act as a hinge, a glass subjected to uniform pressure is expected to shrink uniformly in all directions.

To measure what actually happens under pressure, Chen's team at the Chinese Academy of Sciences placed samples of a specific molecular glass inside a diamond anvil cell. This apparatus uses two opposing diamonds to apply immense, uniform hydrostatic pressure to a microscopic sample held in a steel gasket.

Using high-pressure synchrotron X-ray diffraction, the researchers tracked the distance between the molecules within the amorphous framework as the pressure climbed.

Instead of showing uniform compaction, the diffraction data revealed a stark asymmetry. Along one specific axis, the molecular glass did not contract. The distance between the molecular units actually increased as the external force intensified.

The molecular glass in question is made of small organic molecules that form weak, directional hydrogen bonds. When unpressurized, these molecules stack in a disordered, loosely packed network. But as the diamond anvil cell applies pressure, it forces a localized rearrangement of these bonds.

Rather than collapsing uniformly, the molecules slide past one another, buckling into a denser packing configuration perpendicular to the pressure axis. This lateral packing is so efficient that it forces the molecules to push outward along the longitudinal axis. The overall volume of the glass decreases, satisfying the laws of thermodynamics, but it does so by sacrificing its dimension in one direction to expand in another.

In a crystalline material, this behavior is locked into the fixed orientation of the crystal lattice. If you change the angle of the applied force, the effect vanishes.

But because the molecular glass is amorphous, this negative linear compressibility is not bound to a single, permanent crystalline axis. The material’s internal structure can adapt.

This behavior offers a blueprint for materials that do not simply yield to mechanical stress. In conventional protective gear, an impact compresses the material, thinning it at the point of contact and making it vulnerable to puncture. A structural glass that expands along the axis of compression could theoretically grow thicker and denser at the exact point of impact, hardening itself against the incoming force.

By mapping the atomic coordinates during compression, Chen's team demonstrated that this counterintuitive response is governed by the cooperative rotation of the hydrogen-bonded molecular clusters. The weak bonds act as temporary, flexible joints that allow the glass to dynamically reconfigure its internal geometry without fracturing.

The experiment challenges the traditional boundaries between crystalline order and amorphous chaos. It proves that the complex, cooperative mechanics once thought unique to highly ordered crystals can be engineered into the disordered depths of a glass.

Measured.

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