Ferroelectric phase transitions a review of theory and experiment

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Abstract
We have seen in Part 1 that a resonance observed at a single atomic site in a crystal can give information about the local electric field which depends on crystal structure. To obtain a unique structural picture in this way, one must observe resonances at as many sites as possible, and therefore we are interested in a technique which can be applied to sites occupied by nucleii whose ground states have no quadrupole moments and which cannot be used to measure the EFG unless they are excited into a higher spin state. For example Fe57 has S = 1/2 in the ground state, but it can absorb a low-energy gamma ray to form an S = 3/2 excited state which will be split by quadrupole interaction with the EFG. One can thus examine the local field gradients by measuring the splitting of the resonance dip in the absorption cross section.