The Effect of Zr-Doping and Crystallite Size on the Mechanical ...

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Summary Summary TiO2 <strong>and</strong> the System TiO2-<strong>Zr</strong>O2 TiO2 is an important technological material, used as white pigment, as wide b<strong>and</strong> gap semiconductor in electrochemical dye solar cells, for photocatalysis <strong>and</strong> in photochemical energy-conversion processes. <strong>The</strong> most abundant phases are rutile (P42/mnm, Z=2), anatase (I4/amd, Z=4) <strong>and</strong> brookite (Pcab, Z=8). In addition, there are a number <strong>of</strong> metastable low density modifications. Calorimetric measurements showed that the sequence for microscale material from the stable to the less stable phase is: rutile → brookite → anatase. Several phase stability crossovers occur with the decrease <strong>of</strong> the crystallite size while rutile is the stable phase for big crystallite sizes, brookite is stable for intermediate sizes <strong>and</strong> anatase for smallest crystals. High pressure polymorphs <strong>of</strong> TiO2 are subsequently denser <strong>and</strong> have increasing coordination number <strong>of</strong> Ti-O with increasing pressure, ranging from 6 for the rutile type over 7 for the baddeleyite type (<strong>Zr</strong>O2, P21/c, Z=4) <strong>and</strong> 8 for the cubic structure, which is fluorite type (Fm-3m, Z=4) or pyrite type (FeS2, Pa3, Z=4), up to 9 for the cotunnite type (PbCl2, Pnma, Z=4). Several high pressure polymorphs are suggested as c<strong>and</strong>idate materials for technological applications because they are very hard <strong>and</strong> have interesting optical properties. <strong>The</strong> phase diagram <strong>of</strong> the system TiO2-<strong>Zr</strong>O2 shows the following solid solutions. Baddeleyite <strong>and</strong> tetragonal <strong>Zr</strong>O2 contain up to ~9 <strong>and</strong> 20 mol% TiO2, respectively, several distinct phases (<strong>Zr</strong>,Ti)2O4 with the compositional range <strong>of</strong> 42 to 67 mol% exist <strong>and</strong> TiO2 rutile can be doped with up to ~15 mol% <strong>Zr</strong>O2 at 1600°C. Experiments at high pressure <strong>and</strong> temperatures performed here, using piston cylinder <strong>and</strong> multi anvil presses showed that quenched samples <strong>of</strong> rutile, anatase <strong>and</strong> high pressure polymorphs synthesized at up to 10 GPa adopt ≤10 mol% <strong>Zr</strong>O2, <strong>Zr</strong>-doped TiO2 starting materials therefore have the composition Ti0.9<strong>Zr</strong>0.1O2. 8
Summary Experimental Methods <strong>of</strong> this Study <strong>The</strong> compression behavior <strong>of</strong> anatase <strong>and</strong> rutile was studied for TiO2 <strong>and</strong> Ti0.9<strong>Zr</strong>0.1O2 starting materials with crystallite size in the micro- <strong>and</strong> also nanometer range. Compression experiments were carried out in the diamond anvil cell <strong>and</strong> samples were characterized by in-situ X-ray diffraction, X-ray absorption <strong>and</strong> Raman spectroscopic measurements. A sol-gel route was developed for the synthesis <strong>of</strong> starting materials Tix:<strong>Zr</strong>1-xO2 with x=0.00, 0.10, 0.25, 0.33, 0.50, 0.67, 0.75, 0.90 <strong>and</strong> 1.0. Product <strong>of</strong> the synthesis with x=0.90 <strong>and</strong> 1.0 was nanaoscale anatase, which was annealed at 1000°C to microscale rutile. In hydrothermal experiments, nanoscale anatase Ti0.9<strong>Zr</strong>0.1O2 was used as starting material for the synthesis <strong>of</strong> microscale <strong>Zr</strong>doped anatase. Compression Behavior <strong>of</strong> Anatase Experiments show that anatase becomes less compressible when the crystallite size is decreased to the nanometer scale <strong>and</strong> when the material is <strong>Zr</strong>-doped. Second order EoS fits (K0’=4) resulted in a bulk modulus <strong>of</strong> microscale anatase <strong>of</strong> K0=178(1) GPa [1] <strong>and</strong> K0=179(2) GPa [2]. <strong>The</strong> nanoscale counterpart shows much higher values <strong>of</strong> K0=237(3) GPa [3] <strong>and</strong> K0=243(3) GPa [4]. In this study, it was found that microscale anatase Ti0.90<strong>Zr</strong>0.10O2 has K0=195(38) GPa, which is comparable to undoped material. Largest values were found for nanoscale anatase Ti0.90<strong>Zr</strong>0.10O2 with K0=258(8) GPa. <strong>Zr</strong>-doping thus reduces the compressibility <strong>of</strong> nanoanatase, even though <strong>Zr</strong>O2 polymorphs are more compressible than the corresponding TiO2 forms. For the <strong>Zr</strong>-doped nanoanatase, XRD analysis showed a significant change in compression behavior at pressures >4 GPa, suggested as a consequence <strong>of</strong> deviatoric stresses during experimental compression <strong>of</strong> the nanoscale material. Computations on supercells with different distances <strong>of</strong> neighboring <strong>Zr</strong>-atoms suggested cluster formation <strong>of</strong> <strong>Zr</strong> in the (Ti,<strong>Zr</strong>)O2 anatase. <strong>The</strong> resulting structural distortions can further augment the change in compression behavior. 9
Page 1 and 2: The Effect
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Page 5 and 6: Zusammenfassung Experimentelle Meth
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Page 11 and 12: Summary generalized gradient approx
Page 13 and 14: I. Introduction Because rutile is r
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Page 17 and 18: I. Introduction enthalpy of
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Conclusions In contrast, the compre
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Acknowledgments Acknowledgments Thi
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Bibliography [16] J. Haines <strong
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Bibliography [52] M. Latroche, L. B
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Bibliography [154] G.Y. Shen, V.B.
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Bibliography [191] S. Ananta, R. Ti
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