During laser induced bulk breakdown in sapphire using long pulse (nanosecond pulse) we observed compression of c axis of single crystal sapphire(HCP structure)
I am not an expert in laser, however, if there is rapd heating and cooling, the compression of c axis might be caused by the compressive stress.
1) Heating alone for most materials results expansion rather than contraction. Therefore before I couldn't make any rational comment if I don't have any knowledge whether there is a concurrent lateral expansions along the a, b principal axis system? İf it is so! I should suggest that there is a hooping motion of octahedral interstitial dipoles from c-sides (directed along c-axis) toward a- and b-sides (directed along basal plane) similar to the anelastic response octahedral interstitials to the applied stress system in C+ BCC iron lattice. This phenomena reduces the effective length of c-axis and increases the effective length of a- and b- axis. Cyclic stresses result energy dissipation.
2) İf you observe the same behavior (contraction) ) during the cooling by comparing c- axis change with the a- and b- axis definitely you may get the false impression since the thermal expansion coefficients for c- is 5.3 x 10-6 /K and a- and b- 4.5 x 10-6 /K. That means there is a shrinkage compare to basal directions during the cooling???
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