Phase transitions in II-VI semiconductors under high pressure have been extensively studied due to their fundamental scientific importance and potential technological applications. Among these transitions, the transformation from the wurtzite to the rocksalt phase is particularly significant, as it involves a change in coordination number and bonding geometry, leading to altered electronic and mechanical properties. This article explores the mechanisms, experimental techniques, and computational approaches used to investigate this transition, with a focus on diamond anvil cell (DAC) methods and density functional theory (DFT) modeling.
II-VI semiconductors, such as ZnO and CdTe, typically crystallize in the wurtzite structure at ambient conditions. The wurtzite phase is characterized by a hexagonal lattice with tetrahedral coordination, where each cation is bonded to four anions and vice versa. Under high pressure, these materials often undergo a phase transition to the rocksalt structure, which has a cubic lattice with octahedral coordination. This transition is driven by the increased stability of higher-coordination structures under compression, as the reduced interatomic distances favor more closely packed arrangements.
The diamond anvil cell is a primary tool for studying high-pressure phase transitions. In a DAC, a small sample is compressed between two diamond anvils, allowing pressures exceeding 100 GPa to be achieved. The transparency of diamonds enables in-situ characterization using techniques such as X-ray diffraction (XRD) and optical spectroscopy. For II-VI materials, XRD is particularly useful for identifying structural changes, as it provides direct information about lattice parameters and symmetry. For example, in ZnO, the wurtzite-to-rocksalt transition occurs at approximately 9-10 GPa, marked by the disappearance of hexagonal diffraction peaks and the emergence of cubic ones. Similar transitions have been observed in CdTe at around 3-4 GPa, though the exact pressure can vary depending on experimental conditions.
Optical spectroscopy complements XRD by probing electronic and vibrational changes during the transition. Raman spectroscopy, for instance, detects shifts in phonon modes as the crystal structure changes. In the wurtzite phase, II-VI materials exhibit distinct Raman-active modes corresponding to their hexagonal symmetry. As the transition proceeds, these modes weaken, and new modes associated with the rocksalt phase appear. Photoluminescence (PL) spectroscopy can also reveal changes in bandgap energy, as the transition often leads to a significant reduction in the bandgap due to the altered bonding environment.
Computational modeling, particularly DFT, plays a crucial role in understanding the energetics and kinetics of the phase transition. DFT calculations can predict the pressure at which the wurtzite phase becomes unstable relative to the rocksalt phase by comparing their Gibbs free energies. These calculations typically account for zero-point energy and thermal effects to improve accuracy. For ZnO, DFT studies have confirmed the experimental transition pressure of 9-10 GPa, with the rocksalt phase being more stable above this threshold. The calculations also provide insights into the transition pathway, suggesting that it may involve an intermediate tetragonal or orthorhombic structure in some cases.
The transition mechanism itself is often reconstructive, meaning it requires breaking and reforming bonds rather than a simple distortion of the lattice. This makes the transition kinetically hindered, leading to hysteresis in experimental observations. For example, upon decompression, the rocksalt phase may persist below the equilibrium transition pressure before reverting to wurtzite. This hysteresis is influenced by factors such as defects, impurities, and sample morphology, which can act as nucleation sites for the new phase.
The elastic properties of II-VI materials also play a role in the transition. The bulk modulus, which measures resistance to compression, differs between the wurtzite and rocksalt phases. Generally, the rocksalt phase has a higher bulk modulus, reflecting its greater density and coordination number. DFT calculations can predict these moduli, and experimental measurements using XRD or Brillouin scattering provide validation. For CdTe, the bulk modulus of the rocksalt phase is approximately 50-60 GPa, compared to 40-50 GPa for the wurtzite phase.
High-pressure studies have also revealed metastable phases and unusual behavior in some II-VI materials. For instance, under non-hydrostatic conditions or with specific doping, intermediate phases with distorted structures may appear. These findings highlight the complexity of phase transitions and the importance of controlling experimental parameters. Computational studies can help identify these metastable phases by exploring the energy landscape beyond the equilibrium structures.
The practical implications of understanding these transitions extend to materials design and optimization. For example, the rocksalt phase of ZnO has been explored for its potential in high-pressure applications, though challenges remain in stabilizing it at ambient conditions. Similarly, the pressure-induced changes in bandgap and mechanical properties could inform the development of novel optoelectronic or piezoelectric materials. However, this article excludes device-specific scenarios, focusing instead on the fundamental aspects of the transition.
In summary, the wurtzite-to-rocksalt transition in II-VI semiconductors under high pressure is a well-studied phenomenon with rich physics and broad implications. Diamond anvil cell experiments provide critical data on the structural and optical changes, while DFT modeling offers a theoretical framework for understanding the energetics and mechanisms. Together, these approaches deepen our knowledge of phase transitions in solids and pave the way for further exploration of II-VI materials under extreme conditions. Future research may delve into the effects of strain, defects, and interfaces on the transition, as well as the potential for stabilizing high-pressure phases at ambient conditions through innovative synthesis techniques.