Low-Temperature Defect Healing in the Layered Zintl Phase Li2ZnSi

TL;DR

This paper shows that defects in the material Li2ZnSi can be healed at low temperatures, improving its structural and physical properties.

Contribution

The study reveals that stacking faults in Li2ZnSi can be healed at low temperatures through stress-relief annealing.

Findings

Stacking faults in Li2ZnSi are introduced by mechanical handling and cause broad NMR signals.

Heating to 310–370 K sharpens NMR signals and restores structural order.

Density-functional calculations show stacking faults are energetically unfavorable but can be healed at low temperatures.

Abstract

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.

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