Enhanced negative thermal expansion in Lu2W3O12

Negative thermal expansion in the Sc2W3O12 family has been pushed to much more pronounced values by substituting a large cation for Sc. X-ray diffraction measurements from 127 to 627 degrees C give a linear thermal expansion coefficient of -6.8x10(-6)/degrees C for Lu2W3O12 compared to -2.2 x 10(-6)/degrees C for Sc2W3O12. Negative thermal expansion in this family is dependent on the rocking motions of polyhedra. However, polyhedra in this structure cannot rock without changing their shape. Larger cations expand the octahedra, reducing the oxygen-oxygen repulsions within the polyhedra. This facilitates the polyhedra shape changes necessary for the rocking motions required for negative thermal expansion. (C) 1998 Academic Press.