Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe

Area of Science:

• Materials Science
• Solid-State Physics
• Crystallography

Background:

• Cubic energy materials like thermoelectrics and perovskites are often considered disordered.
• This disorder in Germanium Telluride (GeTe) and IV-VI compounds is linked to low thermal conductivity for thermoelectric applications.
• Conventional crystallography struggles to differentiate static disorder from dynamic atomic motion.

Purpose of the Study:

• To develop and apply a novel technique to probe atomic dynamics in energy materials.
• To resolve discrepancies between local and average structure measurements in GeTe.
• To investigate the structural dynamics of GeTe at various temperatures.

Main Methods:

• Development of the energy-resolved variable-shutter pair distribution function technique.
• Collecting time-resolved structural snapshots on timescales relevant to atomic motion.
• Utilizing a Ginzburg-Landau model coupled with elastic interactions to analyze polarization fluctuations.

Main Results:

• The time-averaged structure of GeTe is crystalline across all temperatures studied.
• Anisotropic anharmonic dynamics were observed at higher temperatures, mimicking static disorder.
• Correlated ferroelectric fluctuations along the <100>_c direction were identified.
• Spontaneous anisotropy was found to be a common feature in cubic IV-VI materials.

Conclusions:

• The perceived disorder in GeTe is attributed to anisotropic atomic dynamics, not static disorder.
• The developed technique provides insights into time-dependent atomic correlations, resolving structural ambiguities.
• Anisotropic dynamics and ferroelectric fluctuations are intrinsic properties of these materials.