ISOTROPIC MATERIALS WITH LOW OR NEGATIVE THERMAL EXPANSION. Nazy Khosrovani and Arthur W. Sleight Department of Chemistry Corvallis, Oregon, 97331-4003 First author NIST address Bldg 235 Room A110. (301) 975-4570. Khosrovn@rrdjazz.nist.gov

 

 

Materials which contract on heating (negative thermal expansion materials) are of great importance since they can be mixed with other materials (positive thermal expansion materials) to make composites which have zero thermal expansion. Materials with zero or low thermal expansion have many applications in electronic devices, space telescope mirrors, stove tops, cook wares and so on. The purpose of this research was to understand why some network structures show anomalous (very low or negative) thermal expansion. Isotropic materials have isotropic thermal expansion which means that on heating, the structure will contract (or expand) the same in all dimensions. Examples of isotropic materials with low or negative thermal expansion are ZrP2O7 and ZrV2O7 respectively. We have solved the structures of ZrP2O7 and ZrV2O7, and found that ZrP2O7 shows normal thermal expansion up to about 290 °C at which the phase transition occurs,and very low and positive thermal expansion after 290 °C. However, ZrV2O7 shows positive thermal expansion up to about 100 °C, and very strong negative thermal expansion after 100 °C up to about 800 °C at which the material decomposes. There are two phase transitions observed for ZrV2O7 before 100 °C which makes the structure more challenging. The structure of both ZrP2O7 and ZrV2O7 is cubic and the space group is Pa3. A 3 x 3 x 3 superstructure is observed for both compounds at room temperature. The 3 x 3 x 3 superstructure disappears after the phase transition is achieved in ZrP2O7, but it doesn't fully disappear until the second phase transition is achieved in ZrV2O7. We believe that the unusual thermal expansion of these compounds is a result of frustration in bending some of the P-O-P or V-O-V angles away from 180°. Reference : Khosrovani, N.; Sleight A.W., Inorg. Chem. 35, 485 (1996)