Dielectrics- Ferroelectrics Flashcards
Types of dielectric materials
Piezoelectrics: 20 of the noncentrosymmetric space groups, exhibit polarisation only when subjected to mechanical stress.
Pyroelectrics: 10 of those noncentrosymmetric space groups, spontaneously polarise.
Ferroelectrics: subset of pyroelectrics, spontaneously polarise and polarisation can be reversed by an electric field
How do subsets of dielectric materials fit together?
Ferroelectrics are pyroelectrics which are piezoelectrics
Ferroelectrics and temperature
Only exhibit the property of polarisation being able to be switched by an applied electric field up to a certain temperature, the curie temperature (Tc). Above this, the spontaneous polarisation is lost and the materials becomes paraelectric (dipôles don’t align)
What material types show ferroelectricity?
Salts, polymers, oxides
How can hydrogen bonds lead to ferroelectricity?
Hydrogen atom sits between two electronegative atoms. It can move and bond to the alternative electronegative atom which will flip the direction of polarisation. This can be done using an electric field.
What happens with hydrogen bonds above Tc?
The H moves freely (overcomes energy barrier without need for field) so is effectively in a midway position and there is no polarisation.
Example hydrogen bond material
KH2PO4. The H atoms can hop between PO4 tetrahedra. Can lead to tetrahedron with 2nH atoms on one side (small dipole). Presence of H atoms causes movement of P atoms off centre (main cause of dipole). Z axis is polar axis (dipoles along [00-1]. Applied field can move H and P in direction of field. Above Tc the H atoms sit equally between tetrahedra and P in centre so no polarisation
Polar molecules causing ferroelectricity
Polar molecules normally point in different directions in a crystal so dipoles cancel out. Electric field will direct their orientation creating a collective dipole that can be moved. Above Tc they will all be rotating too much to maintain this organised orientation and polarisation will be lost
Example of polar molecules
NaNO2. Rocksalt structure. NO2(-) group is polar and quite rigid. All groups and dipoles point along b-axis. NO2 groups can be reversed by applied electric field. They rotate in their own plane. Above Tc NO2 groups arranged at random along +b and -b
Medium sized metal cations and ferroelectricity
In some crystals the cations don’t fully fill their site and can move around in it. By going off centre there is more positive charge in one half of cell and more negative charge in other (a dipole). Above Tc the cation rattles between all the positions so effectively sits in the middle (no dipole). When the cation can seal between positions based on direction of electric field there is ferroelectricity.
What is an example of medium sized cations?
BaTiO3
5 polymorphs of BaTiO3
Rhombohedral, orthorhombic, tetragonal, cubic, hexagonal. In order of increasing temperature
Why can BaTiO3 be ferroelectric?
It has perovskite structure ABO3. Ti in centre surrounded by 6 O atoms in octahedron. 8 corners are Ba. The Ti is too small so can move off-centre producing a dipole. Direction of movement can be controlled by applied electric field
What direction can the Ti move in BaTiO3?
Can move closer to certain O ions due to coulombic attraction.
[111] towards corner of cell
[110] towards edge of cell
[001] towards face of cell
Different structures directions and temperatures for Ti
Rhombohedral: <90C, [111] closer to 3 O ions
Orthorhombic: <0C, [110], closer to 2 O ions
Tetragonal: <130C, [001], closer to 1 O ion
Cubic: >130C, sits central above Tc
