Boron nitride (BN) is a synthetic material that exists in several polymorphic forms, each with distinct crystal structures and properties. Among these, hexagonal boron nitride (hBN) is the most stable and widely studied, but cubic (cBN) and wurtzite (wBN) phases also exhibit unique characteristics. Understanding the structural differences between these phases, their lattice dynamics, and the role of defects is essential for explaining their mechanical, thermal, and electronic behaviors.

The most common phase, hBN, adopts a layered structure similar to graphite, consisting of stacked hexagonal planes where boron and nitrogen atoms are arranged in a honeycomb lattice. Each layer is held together by strong in-plane covalent bonds, while weak van der Waals forces govern interlayer interactions. The lattice parameters of hBN are approximately a = b = 2.504 Å and c = 6.656 Å, with a stacking sequence often described as AA' or AB, where boron atoms in one layer sit above nitrogen atoms in the adjacent layer. This arrangement differs from graphite’s AB stacking, where carbon atoms are offset between layers. The slight ionic character of the B-N bond, compared to the purely covalent C-C bond in graphite, leads to differences in interlayer spacing and electronic properties.

In contrast, cubic boron nitride (cBN) exhibits a zinc-blende structure analogous to diamond, with each boron atom tetrahedrally coordinated by nitrogen atoms and vice versa. The lattice parameter of cBN is approximately 3.615 Å, and its dense, three-dimensional network of sp³ bonds makes it exceptionally hard, second only to diamond. The wurtzite phase (wBN) is less common and adopts a hexagonal structure similar to lonsdaleite, with alternating boron and nitrogen layers in an ABAB stacking sequence. Its lattice parameters are a = b = 2.55 Å and c = 4.22 Å. Both cBN and wBN are metastable under ambient conditions but can form under high-pressure and high-temperature synthesis, typically above 5 GPa for cBN and even higher for wBN.

The stability of these phases is dictated by thermodynamic and kinetic factors. hBN is the most stable at ambient conditions, while cBN becomes favorable at high pressures and temperatures due to its denser packing. The transition from hBN to cBN or wBN requires overcoming a significant energy barrier, often facilitated by catalysts or extreme conditions. Once formed, cBN retains its structure under normal conditions due to kinetic trapping, despite being thermodynamically metastable.

Defects and stacking faults play a crucial role in modifying the properties of hBN. Point defects, such as vacancies or substitutions, can introduce localized electronic states, while line defects like dislocations influence mechanical strength. Stacking faults, where the AA' or AB sequence is disrupted, alter interlayer interactions and thermal conductivity. For instance, turbostratic disorder—random rotations or translations between layers—reduces the anisotropy in thermal transport. Unlike graphite, hBN exhibits a wide bandgap (~6 eV), making it an excellent electrical insulator, but defects can introduce mid-gap states that affect its dielectric properties.

The structural differences between hBN, cBN, and wBN lead to significant variations in their mechanical, thermal, and electronic properties. hBN’s layered structure gives it lubricating properties similar to graphite, with a low coefficient of friction, but its thermal conductivity is highly anisotropic—high in-plane (~400 W/m·K) and much lower cross-plane (~30 W/m·K). In contrast, cBN’s isotropic sp³ bonding results in high hardness (~50 GPa) and uniform thermal conductivity (~13 W/m·K). wBN, though less studied, shows intermediate mechanical properties due to its mixed bonding character.

Thermally, hBN’s strong in-plane bonds and weak interlayer interactions make it an excellent thermal conductor in the basal plane but a poor one perpendicular to it. The absence of free electrons in hBN, unlike graphite, means its thermal transport is dominated by phonons, with Umklapp scattering limiting conductivity at high temperatures. cBN, with its stiff lattice and low anharmonicity, exhibits lower thermal expansion and higher thermal stability, making it suitable for high-temperature applications.

Electronically, hBN’s large bandgap and low dielectric constant make it ideal for insulating layers in 2D devices, while cBN’s smaller indirect bandgap (~6.4 eV) and high resistivity are exploited in high-power electronics. The wurtzite phase, with its polar structure, may exhibit piezoelectric effects, though research on this phase is still evolving. The absence of dangling bonds on hBN’s surface also makes it an excellent substrate for graphene and other 2D materials, minimizing charge scattering.

Comparisons with graphite highlight key differences. While both hBN and graphite are layered materials with similar lattice constants, hBN’s ionic character leads to a wider bandgap and higher chemical stability. Graphite’s semi-metallic behavior arises from π-electron delocalization, whereas hBN’s electrons are tightly bound. The stronger interlayer registry in hBN compared to graphite also affects exfoliation behavior and defect dynamics.

In summary, the polymorphs of boron nitride—hBN, cBN, and wBN—exhibit distinct crystal structures dictated by bonding configurations and stacking sequences. These structural variations lead to divergent mechanical, thermal, and electronic properties, each suited for specific applications. Defects and stacking faults further modulate these properties, making the study of BN polymorphs critical for advanced material design. The interplay between structure and properties in BN phases provides a foundation for understanding their behavior in extreme conditions and technological applications.