Exothermic relaxation and endothermic recrystallization of amorphous zirconium tungstate

In a previous work, we have shown, by means of x-ray diffraction and differential thermal analysis measurements, that amorphous zirconium tungstate (a-ZrW${}_2$O${}_8$) undergoes endothermic recrystallization upon heating to above 900 K. Later on, the anomalous endothermic recrystallization did not become evident in results from drop calorimetry experiments conducted with zirconium tungstate. In this work, we provide evidences in support of the endothermic recrystallization of this compound and show that an exothermic structural relaxation prior to recrystallization may be the cause of the apparent controversy on the nature of the a-ZrW${}_2$O${}_8$ recrystallization. The net enthalpy of recrystallization was estimated as $+1.8$ kJ/mol. We also demonstrate, by means of in situ synchrotron x-ray diffraction measurements at high temperature, that a-ZrW${}_2$O${}_8$ recrystallizes into $\beta$-ZrW${}_2$O${}_8$ which, upon cooling, converts to $\alpha$-ZrW${}_2$O${}_8$. The residual configurational entropy of (nonrelaxed) a-ZrW${}_2$O${}_8$ at 0 K was estimated as 8 Jgat${}^{-1}$K${}^{-1}$.

Figure 1

In situ XRD measurements of an a-ZrW${}_2$O${}_8$ sample at 538 K (A), 874 K (B), and 883 K (C). The upper XRD pattern (D) corresponds to the sample previously heated to 883 K and then cooled to 350 K. The asterisks mark the most intense Bragg peaks from WO${}_3$.

Figure 2

Results from DSC and MDSC measurements with a-ZrW${}_2$O${}_8$. In the DSC signal (curve A), the solid and dotted lines correspond to the heating and cooling segments of the temperature program, respectively. The curve shown in the plot was obtained by subtracting from the DSC signal the baseline composed of a polynomial fit to the signal at the initial part of each heating segment. The gray area represents the estimate for the relaxation enthalpy of a-ZrW${}_2$O${}_8$ prior to recrystallization. In the MDSC result (curve B), the nonreversing signal from the second run was subtracted from the first run, and the result was then multiplied by 10/3 to account for the different heating rates in the DSC and MDSC experiments. The points a–d mark the temperatures up to which another sample of a-ZrW${}_2$O${}_8$ was heated in the differential scanning calorimeter and then subjected to ex situ x-ray diffraction at room temperature (see text for details).

Figure 3

Room temperature XRD patterns of ZrW${}_2$O${}_8$: a-ZrW${}_2$O${}_8$ as prepared and after previously heated to 870 K (A), 902 K (B), 909 K (C), and 928 K (D). The XRD patterns A, B, C, and D correspond to the points labeled a, b, c, and d, respectively, in Fig. 2.

Figure 4

STA measurement with a-ZrW${}_2$O${}_8$. The solid and the dotted lines correspond to the heating and cooling segments of the temperature program, respectively.

Figure 5

Room-temperature weighted average strain (squares) and volume-weighted crystallite size (circles) for $\alpha$-ZrW${}_2$O${}_8$ as a function of heat treatment temperature (points b, c, and d in Fig. 2). The dashed lines are just guides to the eyes.