paraelectrics

Paraelectrics are dielectric materials that do not exhibit spontaneous polarization in zero electric field. They remain in a nonpolar, paraelectric state at temperatures above any ferroelectric transition and respond to applied fields primarily through induced polarization. The term is most often used for perovskite-structured oxides and related ceramics, in which a ferroelectric phase would occur upon cooling below a Curie temperature, Tc, but the material remains paraelectric at higher temperatures.

In the paraelectric phase, the crystal lattice is centrosymmetric and dipoles do not align spontaneously. The dielectric susceptibility χ and the permittivity ε increase with decreasing temperature toward Tc and often follow Curie-Weiss behavior: χ ≈ C/(T − Tc) for T > Tc. Below Tc many materials undergo a phase transition to a ferroelectric state with spontaneous polarization; in some materials, quantum fluctuations or other effects prevent long-range order, yielding a quantum paraelectric state.

Classic examples include SrTiO3 (strontium titanate) and KTaO3 (potassium tantalate), which display large dielectric responses and are used as substrates and tunable dielectrics. BaTiO3, in contrast, is ferroelectric at room temperature, becoming paraelectric only above its transition temperature around 120 °C. Other oxides and organic-inorganic hybrids can show paraelectric behavior under suitable conditions of temperature, pressure, or composition.

Paraelectrics are important in capacitor dielectrics and tunable microwave devices, where a high, temperature-dependent dielectric constant can be advantageous. Their study supports understanding of ferroelectric phase transitions and the design of materials with controllable dielectric properties.

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