Low-Temperature Defect Healing in the Layered Zintl Phase Li₂ZnSi

Li₂ZnSi is a layered Zintl phase composed of heterographene-like Zn-Si sheets separated by Li atoms. Although the intrinsic crystal structure is fully ordered, mechanical handling readily introduces stacking faults of the Zn-Si layers. These defects significantly broaden the ⁷Li and ²⁹Si NMR signals and are described by statistically disordered structure models in single-crystal X-ray diffraction. Upon moderate heating to only 310-370 K, the ⁷Li NMR spectra sharpen, while single-crystal X-ray diffraction reveals a fully ordered structure model. The heat-capacity data exhibit a broad endothermic feature during heating, characteristic of a stress-relief annealing process rather than a thermodynamic phase transition. Mechanical treatment strongly affects physical properties, and the transport response in impedance measurements is dominated by grain-boundary effects. Density-functional calculations show that the stacking-fault formation is energetically unfavorable but localized, explaining why the defects are readily introduced mechanically and can be healed at unexpectedly low temperatures.