Why is polonium simple cubic and so highly anisotropic?

Area of Science:

• Solid-state physics
• Computational materials science
• Quantum chemistry

Background:

• The structural and mechanical properties of alpha-Polonium (α-Po) are not fully understood.
• Previous studies have not adequately explained the preference for the simple cubic (SC) structure in α-Po.
• The elastic anisotropy of α-Po requires a fundamental explanation rooted in its electronic structure.

Purpose of the Study:

• To elucidate the fundamental reasons behind α-Po's preference for the simple cubic structure.
• To explain the origin of the extreme elastic anisotropy observed in α-Po.
• To predict the pressure-induced structural phase transitions in α-Po.

Main Methods:

• State-of-the-art ab initio electronic structure calculations.
• Density Functional Theory (DFT) based methods.
• Analysis of relativistic effects (mass-velocity and Darwin terms).

Main Results:

• Relativistic effects, specifically the mass-velocity and Darwin terms, are identified as the primary cause for the stability of the SC structure in α-Po.
• The intrinsic nature of the SC crystal structure is responsible for the extreme elastic anisotropy of α-Po.
• A phase transformation to a mixture of two trigonal structures is predicted at pressures between 1 and 3 GPa.

Conclusions:

• The stability and unique properties of α-Po are intrinsically linked to relativistic quantum mechanical effects.
• The simple cubic structure inherently leads to high elastic anisotropy.
• High-pressure experiments are expected to confirm the predicted trigonal phase transformation.