Boron Nitride (BN) exists in several polymorphs, each with a distinct crystal structure. The most important ones are Hexagonal Boron Nitride (h-BN), Cubic Boron Nitride (c-BN), Wurtzite Boron Nitride (w-BN), and Rhombohedral Boron Nitride (r-BN).
1. Hexagonal Boron Nitride (h-BN)
Most common form; structure similar to graphite.
A. Crystal Structure
Layered structure with sp²-hybridized B–N bonds
Hexagonal lattice
Space group: P6₃/mmc
Lattice constants:
- a ≈ 2.50 Å
- c ≈ 6.66 Å
B. Features
Layers held by weak van der Waals forces
Good lubricity, high thermal conductivity, electrical insulation
2. Cubic Boron Nitride (c-BN)
Second hardest material after diamond.
A. Crystal Structure
Diamond-like 3D network
sp³-hybridized B–N bonds
Zinc-blende structure
Space group: F-43m
- Lattice constant:
- a ≈ 3.615 Å
B. Features
Extremely hard; high thermal and chemical stability
3. Wurtzite Boron Nitride (w-BN)
Rare and extremely hard (possibly harder than diamond under certain conditions).
A.Crystal Structure
Wurtzite-type hexagonal structure
sp³-hybridized B–N bonds
Space group: P6₃mc
Lattice constants:
- a ≈ 2.55 Å
- c ≈ 4.22 Å
B.Features
High hardness; metastable, typically formed under high P–T
4. Rhombohedral Boron Nitride (r-BN)
A. Crystal Structure
Similar to h-BN but with ABC stacking instead of AB stacking
Space group: R3m
Lattice constants:
- a ≈ 2.50 Å
- c ≈ 10.0 Å
B. Features
Stable at high temperatures; intermediate between h-BN and c-BN in properties
Summary Table
| Polymorph | Hybridization | Structure Type | Space Group | Key Characteristics |
| Hexagonal Boron Nitride (h-BN) | sp² | Graphite-like layered | P6₃/mmc | Most common; lubricant; insulator |
| Cubic Boron Nitride (c-BN) | sp³ | Diamond-like cubic | F-43m | Superhard; thermally stable |
| Wurtzite Boron Nitride (w-BN) | sp³ | Wurtzite hexagonal | P6₃mc | Very hard; high-pressure phase |
| Rhombohedral Boron Nitride (r-BN) | sp² | Layered rhombohedral | R3m | ABC stacking; high-T stability |