Structural evolution in boron nitrides during the hexagonal-cubic phase transition under high pressure at high...

Area of Science:

• Materials Science
• Solid State Physics
• Crystallography

Background:

• Hexagonal boron nitride (h-BN) is a layered material with unique properties.
• Phase transitions in boron nitride are crucial for synthesizing advanced materials.
• Understanding the structural evolution is key to controlling BN polymorph formation.

Purpose of the Study:

• To investigate the structural evolution during the phase transition of hexagonal boron nitride (h-BN) to cubic boron nitride (c-BN).
• To characterize the intermediate phases and microstructural changes under high pressure and temperature.
• To analyze the electronic structure changes associated with the phase transition.

Main Methods:

• High-resolution transmission electron microscopy (HRTEM) for detailed structural imaging.
• Electron energy loss spectroscopy (EELS) for electronic structure analysis.
• High-pressure (6.5-7.7 GPa) and high-temperature (1,700-2,150 °C) synthesis conditions.

Main Results:

• Observed folding and bending of h-BN sheets, leading to the formation of monoclinic boron nitride (m-BN).
• Identified nano-scale twins and wurtzite-type BN within growing c-BN grains.
• EELS confirmed changes in the band structure, with a prominent increase in pi* bond density of states in m-BN compared to h-BN.

Conclusions:

• The phase transition involves intermediate m-BN formation through folding and shearing of h-BN layers.
• c-BN growth is facilitated by twinning mechanisms, influenced by temperature.
• The electronic structure significantly alters during the h-BN to m-BN transition, impacting material properties.