  It has long been puzzling regarding the atomistic origin of the
pressure-induced Raman phonon stiffening that generally follows a polynomial
expression with coefficients needing physical indication. Here we show that an
extension of the bond-order-length-strength (BOLS) correlation mechanism to the
pressure domain has led to an analytical solution to connect the
pressure-induced Raman phonon stiffening directly to the bonding identities of
the specimen and the response of the bonding identities to the applied
stimulus. It is found that the pressure-induced blue-shift of Raman phonons
arises from the bond compression and energy storage exerted by the compressive
stress. Agreement between predictions and experimental measurement leads to the
detailed form for the polynomial coefficients, which offer an atomic
understanding of the physical mechanism of the external pressure induced energy
gain, thermally induced bond expansion as well as means of determining the mode
atomic cohesive energy in a specimen.
