Page 63 - Materials Chemistry, Second Edition

50 2 Solid-State Chemistry

It should be noted that the perovskite structure is not only obtained for oxides, but also
for some nitrides (e.g.,Ca 3 GeN), halides (e.g., KMgF 3 ), hydrides (e.g., BaLiH 3 ), and
carbides (e.g., MgCNi 3 ). Recently, oxynitride perovskites (e.g., BaTaO 2 N) have
received considerable attention due to their potential applications for nontoxic inor-
ganic pigments and photocatalysts. [24] The reduced electronegativity of the nitride ion,
relative to the oxide anion, increases the covalency of the cation-anion bonds thus
affecting its overall structure and physical/optical properties. The ability of inducing
structural distortions by cationic substitution results in a diverse range of applications
for perovskites. In addition to numerous applications in catalysis, sensors, and elec-
tronics, [25] the perovskite backbone is a key component in modern high-temperature
superconductive (HTS) materials.

Superconductivity of perovskites: toward a room-temperature superconductor
By definition, a superconductor exhibits no resistance to electrical conductivity.
Further, when a superconductor is placed in a weak external magnetic field, H, and
cooled below its transition temperature, the magnetic field is repelled. This phenom-
enon is referred to as the Meissner effect, and is the most intriguing property of
superconductors – the ability to levitate on top of a magnetic surface (Figure 2.28a).

Figure 2.28. (a) Photograph of the Meissner effect for a rare-earth magnet above a sample of YBCO
immersed in liquid nitrogen (From http://www.physics.brown.edu/physics/demopages/Demo/em/demo/
5G5050.htm). The onset of strong diamagnetism (“superdiamagnetism,” as observed by the repulsion of
an external magnetic field) is the most reliable method to determine superconductive behavior. The
schematic illustrates the different behavior toward an applied external magnetic field for a perfect
conductor, (b), and a superconductor, (c).