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Structural Relaxation of the Amorphous Phase of Zirconium Tungstate G. Rech, F. Miotto, A. L. Martinotto, and C. A. Perottoni IMC - Instituto de Materiais Cerâmicos, Universidade de Caxias do Sul 95765-000, Bom Princípio – RS [email protected] , [email protected] , [email protected] , [email protected] EXPERIMENTAL RESULTS AND DISCUSSION CONCLUSION SUPPORT REFERENCES XIV Brazil MRS Meeting – September 27 to October 01, 2015 ACKNOWLEDGMENTS INTRODUCTION Zirconium tungstate (Fig. 1), ZrW 2 O 8 , is a ceramic material [1] that exhibits unusual behavior, including negative thermal expansion (NTE) [2], pressure-induced amorphization (PIA) [3], endothermic recrystallization above 923 K [3-6], and continuous structural relaxation when the amorphous phase is heated above room temperature [7, 8]. This relaxation is characterized by a continuous activation energy spectrum, in contrast with discrete values commonly observed in kinetic processes. In this work, the activation energy spectrum of the structural relaxation of amorphous ZrW 2 O 8 is obtained by solving a inverse problem expressed as Fredholm`s integral equation of the first kind [9], taking as experimental input the non-reversing signal of modulated differential scanning calorimetry (MDSC) results. Figure 1: Schematic representation of the crystalline structure of α-ZrW 2 O 8 . The labels O3 and O4 identify terminal, non-bridging oxygens. Adapted from [10]. α-ZW 2 O 8 Amorphization at 7.7 GPa and room temperature for 2 hours X-ray diffraction to confirm the amorphous nature of the sample Modulated Differential Scanning Calorimetry (MDSC) Kinetic Analysis Relaxation progress α Activation Energy Spectrum Figure 3: X-ray diffraction pattern of zirconium tungstate processed at 7.7 GPa and room temperature for 2 hours. Figure 5: Relaxation progress for three MDSC analysis under the same conditions. The kinetics of the structural relaxation of amorphous zirconium tungstate can be plausibly described by a first order model with a continuous activation energy spectrum characterized by maxima around 1.4 and 2.7 eV, thus suggesting two distinct local structural rearrangements. [1] GRAHAM, J.; WADSLEY, A. D.; WEYMOUTH, J. H.; WILLIAMS, L. S. A New Ternary Oxide, ZrW 2 O 8 . Journal of the American Ceramic Society, v. 42, p. 570, 1959. [2] MARTINEK, C.; HUMMEL, F. A. Linear Thermal Expansion of Three Tungstates. Journal of the American Ceramic Society, v. 51, p. 227, 1968. [3] PEROTTONI, C. A.; JORNADA, J. A. H. D. Pressure-Induced Amorphization and Negative Thermal Expansion in ZrW 2 O 8 . Science, v. 280, p. 886, 1998. [4] PEREIRA, A. S.; PEROTTONI, C. A.; JORNADA, J. A. H. D. Raman spectroscopy as a probe for in situ studies of pressure-induced amorphization: some illustrative examples. Journal of Raman Spectroscopy, v. 34, p. 578, 2003. [5] RAMOS, G. R. Relaxação exotérmica e recristalização endotérmica do tungstato de zircônio amorfo. Dissertação (Mestrado) – Universidade de Caxias do Sul, 2011. [6] PEROTTONI, C. A.; ZORZI, J. E.; JORNADA, J. A. H. D. Entropy increase in the amorphous-to-crystalline phase transition in zirconium tungstate. Solid State Communications, v. 134, p. 319, 2005. [7]ARORA,A. K.; SASTRY, V. S.; SAHU, P. C.; MARY, T.A. The pressure amorphized state in zirconium tungstate: a precursor to decomposition. Journal of Physics: Condensed Matter, v. 16, p. 1025, 2004. [8] CATAFESTA, J.; ZORZI, J. E.; PEROTTONI, C. A.; GALLAS, M. R.; JORNADA, J. A. H. D. Tunable Linear Thermal Expansion Coeficient of Amorphous Zirconium Tungstate. Journal of the American Ceramic Society, v. 89, p. 2341, 2006. [9] PRIMAK, W., Kinetics of Processes Distributed in Activation Energy. Physical. Review. v. 100, p. 1677, 1955. [10] RAMIREZ,A. P.; KOWACH, G. R. Large Low Temperature Specific Heat in the Negative Thermal Expansion Compound ZrW 2 O 8 . Physical Review Letters, v. 80, p. 4903, 1998. [11] HANSEN, P. C. Regularization tools: A Matlab package for analysis and solution of discrete ill-posed problems. Numerical algorithms, v. 6, p. 1, 1994. Figure 2 summarizes the experimental and computational procedures used in this work. Figure 2: Steps taken in the process of obtaining the activation energy spectrum of a-ZW 2 O 8 and NMR spectrum of 17 O in both α-ZrW 2 O 8 and a-ZW 2 O 8 . The authors would like to thank Jadna Catafesta, Águeda Turatti and Altair Soria Pereira (LAPMA/IF/UFRGS) for the high pressure processing of the sample. The activation energy spectrum exhibits a broad maximum around 1.4 eV and a sharp peak centered at 2.7 eV. First principle calculation are being performed with the cluster represented below (Fig. 7) to estimate the activation energy for W-O-W bond breaking. Figure 8: NMR spectrum of 17 O in α-ZrW 2 17 O 8 . The asterisks indicate side bands. The labels O1 to O4 identify the peaks assigned to different oxygen atoms in the α-ZrW 2 O 8 crystal structure. Isotopic enrichment with 17 O X-ray diffraction to confirm the crystalline nature of the sample Solid State Nuclear Magnetic Resonance of 17 O Figure 4: Rietveld analysis of x-ray diffraction of α- ZrW 2 O 8 after isotropic enrichment with 17 O. Solid state NMR are being performed in an attempt to elucidate the mechanism involved in the structural relaxation of amorphous zirconium tungstate (Fig. 8). Amorphization at 7.7 Gpa and room temperature for 2 hours Solid State Nuclear Magnetic Resonance of 17 O Figure 7: Two different views of the zirconium tungstate cluster created for estimating the activation energy for W-O-W bond breaking by first principles calculations. Tikhonov’s Regularization [11] = argmin − 2 2 + 2 2 2 Figure 6: Activation energy spectra of the a-ZrW 2 O 8 structural relaxation.
