Negative thermal expansion

2O8 has been show to exhibit negative thermal expansion over a wide temperature range.

Materials with the Sc2(WO4)3 structure have also been shown to display negative thermal expansion over a wide temperature range.  Sc2(WO4)3 itself shows a bulk volume contraction from 10 K to at least 800 K.  A wide range of A and M cations can be substituted into this structure type, leading to materials with controllable expansion properties.

More information at

Thermoelectric materials

The thermoelectric effect is the coupling between a temperature gradient and electric field.
Thermoelectric materials convert heat flow into electrical current - and vice versa.  For example, thermoelectric elements in NASA spacecraft visiting the outer planets convert heat energy into electrical energy to power the spacecraft (when solar power is not feasible).  Similarly, thermoelements in a battery powered picnic cooler use electric current to pump heat from the inside of the cooler to the outside, or be used to cool microprocessors on computer chips. Thermoelectric sensors are widely used for the measurement of thermal quantities.

Some new materials that have been investigated include skutterudites (below), Chevrel phases (Cuw, CuxFey, or Tiz)Mo6Se8 [where w<4, x<2, y<1, and z<1] , and
solid solutions of beta-Zn4Sb3 and Cd4Sb3.

YbFe4Sb12 (skutterdite)


More information about the search for new materials

Correlated-electron physics in transition-metal oxides: spin/charge/orbital ordering
See article in Physics Today, July 2003

from R. Vidya et al., Phys. Rev. B65, 144422 (2002)

Ferroelectrics and active materials

Loosely speaking, ferroelectrics are electrical analogues to ferromagnets. Ferroelectrics have frozen-in electrical dipoles in their crystal structure in the absence of any external fields.  These dipoles can be switched to various orientations (determined by crystal symmetry) using external fields.  Regions of uniform dipoles are called domains, and two different ferroelectric domains differ in their dipole orientations, and are separated by a domain wall

Ferroelectrics bundle a number of electrical, elastic and optical properties in one material, leading to many cross-coupled phenomena such as piezoelectricity (electric fields and elastic strains),  pyroelectricity (temperature gradients and electric fields), electro-optic (refractive index to electric fields), elasto-optic (strains to index), photorefraction (light induced electric fields and optical changes), and nonlinear optical properties (such as optical frequency conversion).

                 LiNbO3 (not in the perovskite structure):