Boron nitride (BN) has at least four crystal modifications of BN_w (wurtzite structure), BN_cub (cubic BN, zinc blende structure), BN_hex (hexagonal BN), and rhombehedral. The properties of the first three (more common) modifications are presented.
BN_hex is stable under normal conditions. BN_hex (Hexagonal, also known as a-BN) with the structure similar to graphite is known for more than a century. Many properties of hexagonal BN are highly anisotropic and depend on the growth method. In many cases, the different values of BN_hex physical parameters given reflect the differences in material properties of hexagonal BN grown by different methods.
BN_cub is metastable under normal conditions. BN_cub (Zinc blende modification , also known as cubic or sphalerite or b-BN) was first synthesized in 1957 using the technique similar to that used for diamond growth. Now crystals with a few millimeter sizes are commercially available.
The BN_w phase is metastable under all conditions. BN_w (Wurtzite structure, also known as g-BN) was first synthesized in 1963. Typically, BN crystals with wurtzite symmetry are very small (fraction of microns), are highly defective, and contain other phases.

Basic Parameters for Zinc Blende crystal structure
Basic Parameters for Hexagonal crystal structure
Basic Parameters for Wurtzite crystal structure

Basic Parameters for Zinc Blende crystal structure

Brillouin zone of the face centered cubic lattice, the Bravais lattice of the diamond and zincblende structures.

		Remarks	Referens
Energy gaps, Eg	6.1÷6.4 eV	300 K	Rumyantsev et al. (2001)
Energy gaps, Egind G15v-X1c	6.4(5) eV	300 K, UV absorption; other data in range 6...8eV	Chrenko (1974)
	6.99 eV 8.6 eV	calculated, Band structure calculated, Band structure	Huang & Ching (1985) Prasad & Dubey (1984)
Energy gaps, Eg,dir G15v-G1c	14.5 eV 10.86 eV 9.94 eV	300 K, reflecsivity calculated, Band structure calculated, Band structure	Philipp & Taft (1962) Prasad & Dubey (1984) Huang & Ching (1985)
Effective electron mass  ml	0.752 mo	calculated from band structure data	Huang & Ching (1985)
Effective electron mass       (longitudinal) ml       (transversal) mt	0.35mo 0.24mo	1.2mo 0.26mo	Xu & Ching et al. (1991)
Effective hole masses (heavy) mh	0.375 mo 0.962 mo	|| [100] || [111]	Madelung (1991)
Effective hole masses (heavy) mlp	0.150 mo 0.108 mo	|| [100] || [111]	Madelung (1991)
Effective hole masses mh      in the direction G K	m1 ~=3.16 m2 ~=0.64 m3 ~=0.44	300 K	Xu & Ching et al. (1991)
in the direction G X	0.55mo	300 K	Xu & Ching et al. (1991)
in the direction G L	m1 ~=0.36 m2 ~=1.20	300 K	Xu & Ching et al. (1991)
Electron affinity	4.5 eV	300 K	Rumyantsev et al. (2001)

Basic Parameters for Hexagonal crystal structure

Brillouin zone of the hexagonal lattice.

		Remarks	Referens
Energy gaps, Eg	5.2(2) eV 3.2...5.8 eV	300 K, reflectance range of experimental data temperature dependence of resistivity	Hoffmann et al. (1984)
	4.0...5.8 eV	300 K	Rumyantsev et al. (2001)
Energy gaps, Eg dir	7.1 eV		Carpenter & Kirby (1982)
Effective electron mass ml      in the direction M G      in the direction M L	0.26mo 2.21mo	300 K	Xu & Ching et al. (1991)
Effective hole masses mh      in the direction K G      in the direction M G      in the direction M L	0.47mo 0.50mo 1.33mo	300 K	Xu & Ching et al. (1991)
Electron affinity	4.5 eV	300 K	Rumyantsev et al. (2001)

		Remarks	Referens
Dielectric constant (static)	=5.06 =6.85	|| to c axis to c axis	Geick et al.(1966)
Dielectric constant (high frequency)	4.10 4.95	parallel to c axis perpendicular to c axis for 300 K; see also Optical properties. Dielectric functions	Geick et al.(1966)
	= 2.2; =4.3	300 K	Rumyantsev et al. (2001)
Dielectric constant (static)	7.1	300 K, infrared reflectivity	Gielisse et al.(1967)
Dielectric constant (high frequency)	4.5 4.46	300 K, infrared reflectivity	Gielisse et al.(1967) Rumyantsev et al. (2001)
Refractive index, n	1.65 1.65 2.13	BN- film perpendicular to c axis parallel to c axis	Takahashi et al.(1981) Ishii et al. (1983) Ishii et al. (1983)
Debye temperature	400 K
Bulk modulus	36.5 GPa
Melting point		see Termal properties. Phase diagrams.
Decomposition temperature, Tdec	2600(100) K		Janaf Thermochemical Tables (1965)
Specific heat	~0.8 J g-1°C -1
Thermal conductivity    parallel to the c axis     perpendicular to the c axis	=<0.3 W cm-1 °C -1 =<6 W cm-1 °C -1		Rumyantsev et al. (2001)
Thermal expansion, linear     parallel to the c axis     perpendicular to the c axis	38·10-6 °C -1 -2.7·10-6 °C -1

Basic Parameters for Wurtzite, Zinc Blende & Hexagonal crystal structure at 300 K

Crystal structure	Wurtzite	Zinc Blende	Hexagonal
Group of symmetry	C46v-P63mc	T2d-F43m	D6c-P63mmc
Density	3.4870 g cm-3	3.450 g cm-3	2.0-2.28 g cm-3
Bulk modulus	400 GPa	400 GPa	36.5 GPa
Debye temperature	1400 K	1700 K	400 K
Melting point		see Termal properties. Phase diagrams.
Specific heat	~0.75 J g-1°C -1	~0.6 J g-1°C -1	~0.8 J g-1°C -1
Density	3.4870 g cm-3	3.450 g cm-3	2.0-2.28 g cm-3
Hardness on the Mohs scale		9.5	1.5
Surface hardness	3400 kg mm-2	4500 kg mm-2
Dielectric constant (static)	=6.8 ; =5.1	7.1	=6.85 ; =5.06
Dielectric constant (high frequency)	~= = 4.2-4.5	4.46	=4.3; = 2.2
Infrared refractive index	2.05	2.1	1.8
Lattice constant, a	2.55 A	3.615 A	2.5-2.9 A
Lattice constant, c	4.17 A		6.66
Effective electron mass       (longitudinal) ml       (transversal) mt	0.35mo 0.24mo	1.2mo 0.26mo
(in the direction M G)      (in the direction M L)			0.26mo 2.21mo
Effective hole masses mh      in the direction G K	0.88mo	m1 ~=3.16 m2 ~=0.64 m3 ~=0.44
in the direction G A      in the direction G M	1.08mo 1.02mo
in the direction G X		0.55mo
in the direction G L		m1 ~=0.36 m2 ~=1.20
in the direction K G      in the direction M G      in the direction M L			0.47mo 0.50mo 1.33mo
Effective hole massesof dencity of states mv	~=1.0mo
Electron affinity	4.5 eV	4.5 eV	4.5 eV
Thermal conductivity     experimentally achieved     theoretically estimated		7.4 W cm-1 °C -1 ~13 W cm-1 °C -1
parallel to the c axis     perpendicular to the c axis			=<0.3 W cm-1 °C -1 =<6 W cm-1 °C -1
Thermal expansion, linear		1.2·10-6 °C -1
Thermal expansion, linear     parallel to the c axis     perpendicular to the c axis	2.7·10-6 °C -1 2.3·10-6 °C -1		38·10-6 °C -1 -2.7·10-6 °C -1
Optical phonon energy	~130 meV	~130 meV