We know after application of electric field if we remove field than a polarization remains which is remnant polarization. But is there any polarization which a material posses before application of electric field particularly in case of the materials in ABX3 group of tetragonal phase? If so how.
You are talking about ferroelectric materials. Firstly, not all ABX3 compounds are ferroelectric. It could be ferroelectric (e.g BaTiO3) but most of the time it won't. The question you ask is embedded in the definition of ferroelectric materials. For a material to be ferroelectric it's crystal structure MUST have a spontaneous dipole moment (i.e polarization) and that spontaneous polarization MUST be capable of being switched by a strong enough external electric field.
So to answer your questions - yes. ferroelectric materials do have spontaneous polarization but ONLY on the microscopic scale. The macroscopic crystal breaks up into smaller regions called domains (to reduce electrostatic energy) where the direction of the polarization varies. So for example a domain may be polarized in the up direction while the adjacent domain will be polarized in the opposite direction and so on. The vector product of all the polarization of the domains adds up to zero and thats why you don't see any macroscopic polarization of a ferroelectric material before applying electric field.
When an electric field is applied the domains that are polarized along the electric field grow at the expense of other unfavorably oriented domains. This results in net polarization of the material. When electric field is sufficiently high, there is only one single domain which has its polarization aligned with the field- this is called saturation. Now when the field is removed the domains cannot get back to their initial state completely. The volume of domains aligned along the electric field vector remains larger than those of the opposite alignment. Thus the sample shows polarization even at zero electric field - that is remanent polarization.
You are talking about ferroelectric materials. Firstly, not all ABX3 compounds are ferroelectric. It could be ferroelectric (e.g BaTiO3) but most of the time it won't. The question you ask is embedded in the definition of ferroelectric materials. For a material to be ferroelectric it's crystal structure MUST have a spontaneous dipole moment (i.e polarization) and that spontaneous polarization MUST be capable of being switched by a strong enough external electric field.
So to answer your questions - yes. ferroelectric materials do have spontaneous polarization but ONLY on the microscopic scale. The macroscopic crystal breaks up into smaller regions called domains (to reduce electrostatic energy) where the direction of the polarization varies. So for example a domain may be polarized in the up direction while the adjacent domain will be polarized in the opposite direction and so on. The vector product of all the polarization of the domains adds up to zero and thats why you don't see any macroscopic polarization of a ferroelectric material before applying electric field.
When an electric field is applied the domains that are polarized along the electric field grow at the expense of other unfavorably oriented domains. This results in net polarization of the material. When electric field is sufficiently high, there is only one single domain which has its polarization aligned with the field- this is called saturation. Now when the field is removed the domains cannot get back to their initial state completely. The volume of domains aligned along the electric field vector remains larger than those of the opposite alignment. Thus the sample shows polarization even at zero electric field - that is remanent polarization.
Most of the term , i would explain is explained in answered by Hasan but spontaneous polarization is wrongly interpreted (may be true in case for some FE ceramics, but not in general), that forces me to answer.
Irrespective of material is ferroelectric or not; it is dielectric any way(degree may vary). Remember dielectricity is universal (Think why).
But ferroelectric materials always have net dipole moment in a particular direction (may be more than one e.g. Layered structure Pervoskite), and hence spontaneous polarization even at macroscopic level though value may be very small as compare to when E is applied along the same direction. Even some time multi directional spontaneous polarization creates problem to overall properties of FE materials as it is tough to remove in-spite of external E.
It also depends on whether your sample is poly-crystalline or single crystal. As far as How is concerned: The structural asymmetry (e.g. tetragonal) ) give you net dipole moment and hence polarization. Details you can find in any standard text book for SSP viz. ISSP by Kittel.
As the molecular formula indicates that this molecule as such is expected to be polar. The magnitude depends upon the the Electronegativity of the atoms constituting the molecule and the type and order of the bonds present. Even in the absence of the Electric field there is polarisation ie., the molecule is polar. In the crystalline state, as the molecules are held together strongly the crystal as such can be polar or non polar which depends upon whether the crystal belongs to a symmetric or non symmetric space group.
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