Under high-pressure and high-temperature (HPHT) conditions (4.0–6.0 GPa, 1400–1900°C), when synthesizing cubic boron nitride (cBN) using hexagonal boron nitride (hBN) as the raw material, the type and ratio of catalysts and additives play a decisive role in the growth morphology of the crystals. By regulating lithium-based catalysts (Li3N, LiH) and additives (LiNH2), one can control the crystal growth direction to obtain various shapes ranging from plate-like and spherical to octahedral forms.
 

1. Specific Effects of Different Catalysts/Additives on Morphology
Li3N (Lithium Nitride) Catalyst: When Li3N is used alone as a catalyst, the synthesized cBN crystals appear primarily as pale yellow thick plates. The grain size is relatively small, and the distribution is uneven. Li3N promotes the development of cBN crystals into a thick plate-like morphology.
LiH (Lithium Hydride) Catalyst: When LiH is used alone, the synthesized cBN crystals are mostly semi-transparent black octahedrons or hex-octahedrons. LiH promotes the development of cBN crystals toward a complete octahedral morphology.
LiH + Li3N Mixed Catalyst: Adding a certain amount of LiH to the Li3N system (or increasing the LiH content) causes a transition in morphology. Experiments show that cBN crystals generated under this combination are mostly yellow or brownish-yellow sphere-like crystals. As the LiH content increases, the morphology transitions from thick plates to regular octahedrons or hex-octahedrons,.
Effect of LiNH2 (Lithium Amide) as an Additive: LiNH2 is a highly effective additive that improves the fluid environment by decomposing to produce gas and promotes the growth of specific crystal faces:
    LiH + Small Amount of LiNH2: Transforms the LiH system (originally octahedron-forming) into brownish-yellow flat pyramidal crystals (flat cones), often with one side smooth and flat. This occurs because LiH drives the crystal toward a sphere-like shape, while the Li3N generated from LiNH2 decomposition drives it toward a thick plate shape; their combined action results in the flat pyramidal morphology,.
    Li3N + Small Amount of LiNH2: Transforms the Li3N system (originally thick plate-forming) into pale yellow flat hexagonal platelets. These crystals have larger grain sizes, regular shapes, and are smooth on both sides. This is crucial for preparing high-quality substrates for thin-film deposition.

2. Microscopic Mechanisms of Morphology Change
Catalysts and additives affect morphology by altering the chemical environment and the growth rates of crystal faces:
Competition in Growth Direction: Crystal morphology depends on the difference in growth rates in different directions. When growth along the <111> direction is faster than the <100> direction, tetrahedrons are formed; conversely, when the <100> direction grows faster than the <111> direction, octahedrons or sphere-like crystals are formed.
Inhibitory Effect of Excess Boron (B): Under HPHT, LiH reacts with hBN to generate Li3N, NH3, and elemental Boron (B). Excess elemental B enters the {111} face of the cBN crystal (because the lattice mismatch is minimal), thereby inhibiting crystal growth along the <111> direction. Therefore, higher LiH content produces more free B, causing the crystal to tend toward an octahedral or sphere-like shape. Excess B is also the main reason for the black color of the crystals.
Flux Effect of NH3 Gas: The additive LiNH2 decomposes into Li3N and NH3 (ammonia gas) under HPHT. The presence of NH3 improves the fluidity of the liquid system, improving the growth environment for cBN crystals. Combined with the catalytic action of the newly generated Li3N, this promotes the development of regular plate-like or flat pyramidal crystals.

3. The Nature of "Distorted Crystals"
In reality, specific flat cBN single crystals (such as the hexagonal platelets) are essentially "distorted crystals." An ideal cBN crystal should be a regular octahedron, but catalysts and additives cause growth to be faster in specific directions (e.g., one pair of {111} faces grows faster while others grow slower). This results in a shape that looks like a complete octahedron cut along planes parallel to {111}, forming a seemingly flat, plate-like structure.

In summary, LiH tends to make crystals grow "round and thick" (octahedral/sphere-like), while Li3N tends to make crystals grow "flat" (plate-like). LiNH2 acts as a "lubricant" and "regulator," helping crystals grow larger and more regular.