paraelectric

Paraelectric is a term used in dielectric and ferroelectric science to describe a state of certain materials in which there is no spontaneous, permanent polarization in zero external electric field, but the material can be polarized by an applied field. In practice, paraelectric behavior is the high-temperature phase of many ferroelectric crystals. Above the Curie temperature (T_C), these materials exhibit dielectric response that is linear for small fields, with a dielectric susceptibility χ or permittivity ε that increases with decreasing temperature; as T approaches T_C from above, ε typically rises and follows Curie-Weiss law χ = C/(T - T_C) for many classical ferroelectrics.

Phase relation: the paraelectric phase has higher crystal symmetry than the ferroelectric phase; when cooled below T_C, spontaneous polarization emerges and the material becomes ferroelectric.

Notable examples: SrTiO3 is a canonical paraelectric and sometimes called an incipient or quantum paraelectric because it remains paraelectric down to low temperatures due to quantum fluctuations; BaTiO3 and similar perovskites are paraelectric above their Curie temperature and ferroelectric below. Other insulating crystals may be paraelectric at room temperature but become ferroelectric upon cooling.

Applications: paraelectric materials are widely used as dielectric insulators and in capacitors; their large dielectric constants, especially near T_C, enable tunable devices and sensors.