Pressure-Softening Of Zone-Edge Ta Phonons And The Fourfold To Sixfold Phase Change

The observed tendency in tetrahedral solids for the pressure-induced softening of zone-edge TA phonons to correlate with the threshold pressure P-T of the fourfold to sixfold phase change is examined for 25 materials. A strong correlation is found for an effective pressure shift obtained by using a simple tight-binding model to exclude the frequency-stiffening non-central-force contribution to the zone-edge TA modes. The correlation is further improved by accounting for the material's shear rigidity. A parameter P-T(correl) is defined that correlates linearly to P-T with near-unity slope and high quality factor up to the transition of BP at similar to 1.6 Mbar. The pressure-softening of the zone-edge TA modes is driven by the negative tension produced in the bonds by compression. This tension originates from increased electron-overlap repulsion, which modifies the metallic and ionic character of the bonding, leading eventually to the phase change. This is the underlying mechanism that links the pressure-softening of the zone-edge TA phonons to the phase transition.