Vibrational properties of hexagonal Ge(2)Sb(2)Te(5) from first principles

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Materials Science

Background:

• Hexagonal Germanium-Antimony-Tellurium (Ge2Sb2Te5 or GST) is a crucial phase-change material.
• Understanding its structural properties and vibrational dynamics is key to optimizing its applications.
• Previous experimental studies proposed several stacking configurations for the Ge/Sb planes.

Purpose of the Study:

• To compute phonon spectra at the Γ point and theoretical Raman spectra for hexagonal GST.
• To investigate the influence of different Ge/Sb plane stackings on the vibrational properties.
• To compare theoretical predictions with experimental Raman spectroscopy data.

Main Methods:

• Density Functional Perturbation Theory (DFPT) was employed for phonon calculations.
• Three distinct stacking configurations of Ge/Sb planes were theoretically modeled.
• Calculated Raman spectra were analyzed and compared against experimental results.

Main Results:

• Theoretical Raman spectra showed similarities across the three stacking models.
• A disordered stacking model, proposed by Matsunaga et al., exhibited marginally better agreement with experimental data.
• The vibrational spectrum was largely insensitive to specific atomic arrangements within the Sb/Ge sublattices.

Conclusions:

• Energetic analysis suggests two plausible stacking configurations for GST, considering configurational entropy.
• The disordered stacking model by Matsunaga et al. accurately reproduces experimentally observed Ge/Sb-Te bond length distributions.
• This study provides insights into the structural and vibrational properties of hexagonal GST, aiding material design.