| dc.description.abstract | V.7 CONCLUSION
Of all the models mentioned, the one that comes closest to explaining the high?pressure data on I and II is that due to Zimmermann and Konig (6). The model now proposed is an extension of a model that has become possible due to the additional results available at high pressures and on two different compounds. The model outlined here is thus essentially based on the above?mentioned one, but includes the pressure dependence of the strength of the strain interaction r?.
As explained in section V.7, the interaction strength r? is proportional to the difference in the volumes of the two states, viz. VHS?VLSV_{HS} - V_{LS}VHS??VLS?. This quantity, and hence r?, will change with pressure depending on the compressibilities of the two states.
The compressibilities of the HS and LS states of compounds I and II have not been measured experimentally. The following assumption is made about these quantities to explain the high?pressure results on I and II. It is assumed that the compressibility difference between the HS and LS states is large for compound I and much less for compound II. This point can however be settled only when the experimental data is available.
Granting the above assumption, the results can be explained as follows. In the case of compound I, a small increase in pressure changes both Z and r? due to the volume and compressibility differences respectively of the two spin states. Their combined decrease results in a rapid increase in the low?spin state population as well as a continuous transition. On the other hand, in compound II, only Z changes due to volume considerations and hence the much smaller decrease in r? and consequently the small increase in the low?spin state population. Further, the discontinuous transition persists to higher pressures, as r? is not reduced so drastically as in the case of compound I.
The above simple model seems to explain the high?pressure results. It would be interesting to know the experimentally determined compressibilities of the two spin states of compounds I and II, and it will also be a test of the above model.
The other aspect of the spin?state transitions revealed by the present studies is that the 'residual paramagnetism' in the low?temperature phase increases with pressure. This has led to the conclusion that a potential barrier separating the high and low spin states exists and this increases with pressure. | |
