What is the relationship between ferroelectric and pyroelectric crystals

What is the relationship between ferroelectric and pyroelectric crystals  

In a piezoelectric solid a surface electric charge develops when the solid is subjected to a mechanical stress such as pressure, even in the absence of an external electric field. This is called the piezoelectric effect, and crystals that exhibit this behaviour are called piezoelectrics. Piezoelectric solids are a subset of dielectrics. All piezoelectrics are dielectrics, but only some dielectrics are piezoelectrics. 

In the case of a pyroelectric solid a change of temperature induces a polarisation change. Pyroelectric crystals are a subset of piezoelectrics. All pyroelectric crystals are piezoelectrics, but not all piezoelectrics demonstrate pyroelectricity. A material that is a pyroelectric is found to possess a spontaneous polarisation, Ps- This means that a pyroelectric crystal shows a permanent polarisation that is present both in the absence of an electric field and in the absence of mechanical stress. 

Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than found in normal dielectrics. By analogy with magnetic behaviour, this behaviour is called ferroelec-tricity, and the materials are called ferroelectrics. Ferroelectrics also possess a spontaneous polarisation, Ps, in the absence of an electric field and a mechanical distortion. They are, therefore, a subset of pyroelectrics and, as such, all ferroelectrics are also pyroelectrics and piezoelectrics. The feature that distinguishes ferroelectrics from pyroelectrics is that the direction of the spontaneous polarisation, Ps, can be switched (changed) in an applied electric field. 

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