Finally for BN the transfor-mation comes into view in the range of 850 GPa
high-pressure investigations stems from the fact that group-III-nitride layers are commonly subjected to large built-in strain since they are often grown on different substrates having considerable lattice mismatch within a difference in the thermal expansion coefficients between epitaxial layer and substrate.
In the case of the heterostructures andsuperlattices, this situation becomes more complex and mutual influence between different material layers may appears [11,12]. Whereas investigations of strain effects have been widely published for the electronic and optical properties [11–15], much less is known about their influence on the elastic constants
are based on the all-electron full potential augmented plane waves plus local orbitals method
One should also notice that the experimental values of the bulk moduli are somehow uncertain due to the difficulty of growing high-quality single crystals of III–V nitrides [29]
clearly observe a linear dependence in all curves of the nitrides
observe that the elastic constants C 11 and C 12 and bulk modulus increase when pressure is enhanced. Moreover the shown shear wave modulus C S and the shear mode modulus C 44 which represent the extrema of the transverse moduli in cubic crystal [39], decrease linearly with the increasing of pressure for AlN, GaN, and InN, except for that of BN, their behavior is quite different, C 44 and C S increase. This behavior is due to the elastic stability of BN.