Structural phase transitions in trigonal Selenium induce the formation of a disordered phase

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• The quasi-1D trigonal phase of selenium (Se) is theoretically unstable due to long wavelength perturbations, as per the Mermin-Wagner theorem.
• Trigonal Se, composed of parallel Se-Se chains, exhibits structural transitions to monoclinic and distorted trigonal phases at low temperatures and moderate pressures, respectively.

Purpose of the Study:

• To investigate the structural transitions of trigonal selenium under varying temperature and pressure conditions.
• To elucidate the mechanism and characteristics of the phase transitions in single-crystal trigonal Se.

Main Methods:

• Experimental investigation using sub-millimeter-sized single crystals of trigonal Se.
• Analysis of specific heat and resonance Raman spectroscopy data to detect phase transitions.
• Complementary phenomenological modeling to understand the role of disorder.

Main Results:

• Trigonal Se undergoes structural transitions via a reversible, intermediate disordered phase.
• The disordered state is evidenced by a 'Boson peak' in specific heat and Raman data, and a decrease in Raman line intensity.
• Nanocrystals of trigonal Se do not exhibit these structural transitions or disorder.

Conclusions:

• The structural transitions in trigonal Se are mediated by a novel disordered intermediate phase.
• Disorder plays a critical role in the first-order structural transition mechanism of fragile solids.
• Crystal size significantly influences the occurrence of structural transitions and disorder in selenium.