Abstract
We study hexagonal materials of MgB (h-BN) and related compounds under various compression conditions. We found the anomalous behavior of lattice constants a (b) for MgB (h-BN) and HBC. Lattice constants a of MgB (h-BN) and HBC contract under c-axis compression. This means that the Poisson ratio is negative. The lattice contraction of a-axis is 0.0017 nm for MgB (h-BN) when P z=50 GPa, 0.0002 nm for HBC when P z=20 GPa. The contraction of HBC is quite small.
Acknowledgements
The numerical calculations were performed at NIMS (Namiki, AlphaServer GS140[HP]).