Why can large single-crystal halide windows deform under their own weight?

Large single-crystal halide windows are susceptible to deformation under their own weight because they lack the structural reinforcement provided by grain boundaries. While these halide materials are valued for their ability to transmit infrared radiation, they are inherently soft and lack sufficient mechanical strength in their single-crystal form to support their own mass over large areas.1

In materials science, permanent deformation—or plastic deformation—typically occurs through a process known as slip, where layers of the crystal structure slide relative to one another under stress. In polycrystalline materials, grain boundaries act as barriers that impede the movement of dislocations, thereby hardening the material and resisting such deformation. Because single crystals lack these internal boundaries, they are more prone to this type of structural failure. To mitigate this, manufacturers often use techniques like hot-forging or fusion-casting to create large-grained or polycrystalline structures that maintain optical clarity while providing the necessary mechanical resistance to deformation.213