Phonon spectra evolution in semiconductors under high pressure provides critical insights into structural stability, phase transitions, and lattice dynamics. High-pressure environments, often achieved using diamond anvil cells (DACs), enable the study of phonon behavior as materials undergo compression-induced transformations. Silicon (Si) and gallium nitride (GaN) serve as exemplary systems for investigating these phenomena due to their technological relevance and well-documented pressure responses.

At ambient conditions, Si crystallizes in the diamond cubic structure (Fd-3m), characterized by its covalent bonding and high symmetry. Under hydrostatic pressure, Si undergoes a series of phase transitions, beginning with the diamond-to-β-tin transition near 10-12 GPa. Phonon spectra evolution in Si reveals mode softening prior to this transition, particularly in the transverse acoustic (TA) branch near the X-point of the Brillouin zone. This softening indicates mechanical instability, as the frequency of specific phonon modes approaches zero, signaling an impending structural rearrangement. Beyond the β-tin phase, further compression leads to higher-coordinated polymorphs such as Imma and hexagonal close-packed (hcp) phases, each accompanied by distinct phonon signatures.

Raman spectroscopy is a key tool for tracking phonon behavior in Si under pressure. The triply degenerate optical phonon mode (Γ25') in the diamond phase exhibits a pressure-dependent frequency shift, typically increasing linearly at low pressures due to bond stiffening. However, as the phase transition approaches, nonlinear behavior emerges, reflecting the anharmonicity and eventual instability of the lattice. The disappearance of the diamond-phase Raman modes and the emergence of new peaks confirm the transition to the β-tin structure.

Gallium nitride, a wide-bandgap semiconductor with wurtzite (B4) structure at ambient conditions, also demonstrates rich phonon evolution under pressure. The wurtzite-to-rock salt (B1) transition in GaN occurs near 50 GPa, accompanied by significant changes in phonon dispersion. The wurtzite structure features zone-center optical modes split into A1 and E1 symmetry branches due to its anisotropic bonding. Under compression, the E2(high) mode, sensitive to in-plane strain, exhibits a pronounced frequency increase, while the A1(LO) mode shows nonlinear stiffening due to enhanced Coulombic interactions.

Phonon softening in GaN precedes the wurtzite-to-rock salt transition, particularly in shear modes associated with the hexagonal close-packed planes. This softening reflects the destabilization of the wurtzite lattice under shear stress, ultimately driving the transition to the more densely packed rock salt phase. High-pressure infrared and Raman studies confirm the disappearance of wurtzite-specific phonons and the emergence of rock salt modes, which exhibit broader linewidths due to increased disorder and reduced symmetry.

Comparative analysis of Si and GaN highlights the role of bonding character in phonon evolution. Si, with its purely covalent bonds, undergoes transitions driven by mechanical instabilities, while GaN, with mixed ionic-covalent bonding, experiences additional electrostatic contributions to phonon behavior. The pressure dependence of phonon lifetimes in both materials also reveals increased anharmonic scattering at high pressures, leading to thermal conductivity modifications.

Phase transitions in these materials are further elucidated by examining the pressure dependence of elastic constants. In Si, the shear modulus C44 decreases markedly near the diamond-to-β-tin transition, correlating with TA mode softening. Similarly, in GaN, the C33 elastic constant exhibits nonlinear behavior approaching the wurtzite-to-rock salt transition, reflecting the anisotropic compression response of the hexagonal lattice.

High-pressure phonon studies also uncover metastable states and kinetic barriers in phase transitions. For instance, Si retains local diamond-phase domains briefly after exceeding the transition pressure, detectable via transient phonon signatures. GaN, on the other hand, shows hysteresis in phonon mode recovery upon decompression, indicating incomplete reversibility of the rock salt-to-wurtzite transformation.

The influence of non-hydrostatic stress on phonon spectra cannot be overlooked. Deviatoric stresses in DAC experiments may split degenerate phonon modes or induce preferred orientation effects, complicating spectral interpretation. Careful pressure-transmitting medium selection and stress-state characterization are essential for accurate phonon analysis.

Beyond Si and GaN, other semiconductors exhibit analogous phonon evolution under pressure. For example, zinc blende-structured materials like GaAs show similar mode softening preceding phase transitions, while layered semiconductors like MoS2 display interlayer mode stiffening due to enhanced van der Waals interactions under compression.

The study of phonon spectra under high pressure extends beyond fundamental interest, with implications for materials design. Pressure-tuned phonon engineering can optimize thermal conductivity, tailor optoelectronic properties, and stabilize metastable phases for novel applications. The insights gained from Si and GaN provide a framework for understanding broader classes of semiconductors under extreme conditions.

In summary, high-pressure phonon spectroscopy reveals the dynamic response of semiconductor lattices to compression, elucidating phase transitions, mechanical instabilities, and anharmonic effects. Silicon and gallium nitride exemplify distinct pathways of phonon evolution dictated by their bonding and structural characteristics. These findings deepen the understanding of condensed matter under extreme environments and inform the development of advanced materials with pressure-tuned functionalities.