ENHANCED NEGATIVE THERMAL-EXPANSION IN LU2W3O12

Negative thermal expansion in the Sc2W3O12 family has been pushed to m uch more pronounced values by substituting a large cation for Sc. X-ra y diffraction measurements from 127 to 627 degrees C give a linear the rmal expansion coefficient of -6.8x10(-6)/degrees C for Lu2W3O12 compa red to -2.2 x 10(-6)/degrees C for Sc2W3O12. Negative thermal expansio n in this family is dependent on the rocking motions of polyhedra. How ever, polyhedra in this structure cannot rock without changing their s hape. 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 therma l expansion. (C) 1998 Academic Press.