NEGATIVE THERMAL-EXPANSION AND PHASE-TRANSITIONS IN THE ZRV2-XPXO7 SERIES

Phases in the ZrP2-xVxO7 series show isotropic thermal expansion which for some members is negative over a broad temperature range up to at least 950 degrees C. One phase transition occurs for ZrP2O7 at 290 degrees C. The thermal expansion is normal below this transition but is very low and positive above this transition. Two phase transitions occur in ZrV2O7 close to 100 degrees C; each results in an abrupt increase in volume with increasing temperature. Strong negative thermal expansion commences in ZrV2O7 above 100 degrees C and continues to about 800 degrees C, where decomposition occurs. Below the phase transitions in cubic ZrP2O7 and ZrV2O7, a 3 x 3 x 3 superstructure develops with no apparent change in space group, In the high-temperature structures, the V-O-V and P-O-P angles are constrained by the Pa (3) over bar space group symmetry to be 180 degrees. After the 3 x 3 x 3 superstructure develops, this symmetry constraint is relaxed for some, but not all, V-O-V and P-O-P linkages, The 3 x 3 x 3 superstructure results in eleven crystallographic sites for V or P, Room-temperature V-51 and P-31 NMR spectra for ZrV2O7 and ZrP2O7 show complex patterns consistent with the Pa (3) over bar space group and the 3 x 3 x 3 superstructure. Modeling studies suggest that the unique thermal expansion properties are related to frustration in bending V-O-V or P-O-P angles away from 180 degrees in a cooperative manner. For the mid region of the ZrV2O7-ZrP2O7 solid solution, phase transitions above room temperature are completely suppressed. For the ZrP2-xVxO7 series, P-31 NMR studies show a preponderance of VPO74- groups, relative to a mixture of V2O74- and P2O74- groups, when x is close to 1.0, Results in the HfV2O7-HfP2O7 system are essentially identical to those in the ZrV2O7-ZrP2O7 system.