I'm coming across articles on 'phys.let's and Nature that talk about an exciting new class of materials called hybrid improper ferroelectric. Yet nowhere have I found an explanation or even a hint as to why the word 'improper' is used......
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Improper ferroelectric are class of material in which the ferroelectricity (spontaneous polarization) is induced by two structural distortions and these distortions are the in-phase and anti-phase oxygen octahedral tiltings. The word improper comes as polarization is not the primary order parameter.
