In-situ high temperature studies of a negative thermal expansion coefficient material, ZrW₂O₈

Time resolved, in situ heating XRD experiments provide data under chosen non-ambient conditions that are vital for the research and development of many materials. Such a capability circumvents changes in material properties that are normal upon transferring the material from an oven to the diffractometer.

One piece of valuable information from a variable temperature experiment pertains to phase and structural changes. The ceramic ZrW₂O₈ is a well-known material with negative thermal expansion over a wide temperature range, 0-1050 K. In situ heating of ZrW₂O₈ was performed using the High Temperature Attachment (HTA) unit on the Ultima IV. Heating can be performed up to 1500°C on this unit in vacuum or under any inert atmosphere, with different types of heating and cooling cycles.

Data were collected using the high-speed, DTEX detector with a total of four minutes for each powder pattern. A 3D overlay of the in situ XRD patterns of ZrW₂O₈ in Figure 1 indicates a phase change at 150°C, and a shift in peak positions towards lower 2θ as a function of temperature. A Rietveld refinement (using structureless fitting module) of ZrW₂O₈ powder patterns collected from RT to 700°C relate these changes to a decrease in lattice constant and an order disorder transition in ZrW₂O₈ with a change in space group from P2₁3 to Pa3 upon heating as shown in Table 1.