Reversible Structural Transformation between Polar Polymorphs of Li₂GeTeO₆.

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• Lithium germanate tellurate (Li₂GeTeO₆) is synthesized at ambient pressure, adopting an ordered ilmenite structure (R3).
• This compound belongs to the family of double perovskites with the general formula A₂BB'O₆.

Purpose of the Study:

• To investigate the high-pressure phase transitions of Li₂GeTeO₆.
• To characterize the structural and polar properties of the high-pressure phase.
• To explore the potential ferroelectric behavior of Li₂GeTeO₆.

Main Methods:

• High-pressure synthesis and phase transformation studies.
• X-ray diffraction for crystal structure determination.
• Second harmonic generation (SHG) measurements.
• Polarization (P-E) and dielectric measurements.

Main Results:

• A new polar phase of Li₂GeTeO₆, derived from LiSbO₃ and crystallizing in the Pnn2 space group, is formed at 1073 K and 3-5 GPa.
• This high-pressure phase is reversible, reverting to the R3 ilmenite structure upon annealing at ambient pressure.
• Both R3 and Pnn2 phases exhibit polar characteristics, confirmed by structural analysis and SHG, but lack ferroelectric response.

Conclusions:

• The Pnn2 phase represents a novel structure within the LiSbO₃-derived double perovskite family.
• The absence of ferroelectricity in polar Li₂GeTeO₆ phases is likely due to random domain orientation and Li-ion conductivity.
• Further research is needed to understand and potentially harness the polar properties of this material.