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amorphous configuration change in Ca Pressure induced amorphous to Al metallic glassesH. B. Lou1, Y. K. Fang1, Q. S. Zeng1, Y. H. Lu1, X. D. Wang1, Q. P. Cao1, K. Yang2, X. H. Yu2, L. Zheng3, Y. D. Zhao3, W. S. Chu3, 4, T. D. Hu3, Z. Y. Wu3, 4, R. Ahuja1, 5, 6 J. Z. Jiang1Scientific Reports 2, Article number: 376 (2012)doi:10.1038/srep00376Download CitationApplied physicsCondensed matter physicsMaterials scienceTheory and computationAbstractPressure induced amorphous to amorphous configuration changes in Ca Al metallic glasses (MGs) were studied by performing in situ room temperature high pressure x ray diffraction up to about 40GPa. Changes in compressibility at about 18GPa, 15.5GPa and 7.5GPa during compression are detected in Ca80Al20, Ca72.7Al27.3, and Ca66.4Al33.6 MGs, respectively, whereas no clear change has been detected in the Ca50Al50 MG. The transfer of s electrons into d orbitals under pressure, reported for the pressure induced phase transformations in pure polycrystalline Ca, is suggested to explain the observation of an amorphous to amorphous configuration change in this Ca Al MG system. Results presented here show that the pressure induced amorphous to amorphous configuration is not limited to f electron containing MGs. Structural polyamorphic transitions from a low density amorphous state to high density amorphous state often result in an increase in atomic coordination. Very recently, AACCs were surprisingly observed in Ce containing MG systems11,12,13,14,15,16,17, in which the nature of AACCs is revealed due to 4f electron delocalization in Ce under high pressure14. Up to now no experimental evidence for AACCs has only been reported in non f electron containing MGs. Changes in compressibility at about 18GPa, 15.5GPa and 7.5GPa during compression have been detected in Ca80Al20, Ca72.7Al27.3, and Ca66.4Al33.6 MGs, respectively, whereas no clear change is detected in the Ca50Al50 MG. Possible mechanism for the AACC is presented and discussed. These results obtained here point out that the amorphous to amorphous configuration changes induced by pressure are not limited to f electron containing MGs. ResultsIn situ high pressure XRD patterns for all samples studied were recorded at room temperature up to about 40GPa. No crystallization reaction was detected in these samples in the studied pressure range whereas it was recently reported in the Ce Al system18,19. Figure 1 shows selected high pressure x ray diffraction patterns during compression for the Ca80Al20 MG alloy at room temperature. With increasing pressure, the main amorphous diffraction peak together with sharp Au peaks shift to high, as expected for the densification effect of pressure. The sample retains fully amorphous structure up to about 40GPa by judging from the smooth broad patterns. The reverse main amorphous diffraction peak position,, correlates with the volume of Ray Ban RB3362 Sunglasses Gunmetal Frame Crystal Green Gradient
Ray Ban RB3362 Sunglasses Gunmetal Frame Crystal Green Gradient glass having a power law function20,21,22, which can be conveniently used to reflect the relative volume (density) change as a function of pressure. Figure 2 shows the inverse main amorphous diffraction peak position,, of the Ca100xAlx (x = 20, 27.3, and 33.6 at.%) MGs as a function of pressure during compression, which were estimated from the diffraction peak fitting using a Voigt line profile after subtracting baseline. It is found that at about 18GPa, 15.5GPa and 7.5GPa, clear changes were detected for Ca80Al20, Ca72.7Al27.3, and Ca66.4Al33.6 MGs, respectively, whereas no clear change was detected for the Ca50Al50 MG. Amorphous to amorphous configuration changes occur in Ca80Al20, Ca72.7Al27.3, and Ca66.4Al33.6 MGs during compression. During pressure releases, it is found that the transformation is reversible with hysteresis. To further support this scenario, we carried out the following considerations. Au peaks as pressure calibrant are marked. DiscussionTo uncover the change detected in Fig. 2, it might be useful to revisit the pure polycrystalline Ca. It was reported that for a pure polycrystalline Ca at ambient temperature, a face center cubic (fcc) to body center cubic (bcc) phase transition was detected at 19.8GPa during compression accompanied by a volume change of about 2 3%23,24,25,26,27,28,29,30, which is much smaller than about 15% for pure Ce14. This transition was strongly linked with the transfer of s electrons into d orbitals of Ca under pressure26,28. We further performed both fcc and bcc Ca K edge x ray absorption near edge structure (XANES) calculations at 19.8GPa and 19.9GPa, respectively, as shown in Fig. 3a. By comparing XANES curves of fcc Ca with bcc Ca, the striking feature detected is the intensity change of the peak (4.07keV) at around 15eV above the main peak during the transition. We attempted to record XANES data for both Ca and Al edges for our Ca Al MGs under pressures. On the other hand, we do obtain good Ca K edge XANES data for our studied Ca100xAlx (x = 20, 27.3, 33.6 and 50 at.%) MGs at ambient pressure, as shown in Fig. 3b. Although the Ca K edge XANES curves for Ca100xAlx (x = 20, 27.3, 33.6 and 50 at.%) MGs differ from that for pure polycrystalline Ca, one similar feature observed is that about 15eV above main peak a hump is detected for all studied Ca Al MGs. For the Ca50Al50 MG, relatively speaking, the intensity of this hump is obvious low as compared with the other samples. This is also confirmed by further calculated Ca K edge XANES data using FEFF code for the Ca13Al14 and Ca8Al3 alloys at ambient pressure (see Supplementary Fig. S1 online). To shed light on the Al effect on the charge transfer from s and/or p orbitals to d orbitals of Ca, we performed first principles calculations based on density functional theory (DFT) for electronic structures of Ca Al alloy. We found that Ca d orbitals dominate around the Fermi level even at high Al concentration up to 50%, although the character of d orbitals is reduced when Al concentration increases. Al atoms provide more electrons occupying d orbitals of Ca in the dilute model as showed in Fig. 4. In the pure fcc Ca system, the occupation of d orbitals is only 0.25 and it increases to 0.29, 0.36 and 0.45 when the concentration of Al is 12.5 at.%, 25 at.% and 50 at.%, respectively. Figure 3: Calculated and experimental data of Ca K edge XANES for pure Calcium and CaAl metallic glasses respectively.(a) K edge XANES spectra for pure polycrystalline fcc Ca (at 19.8GPa) and bcc Ca (at 19.9GPa) as obtained by calculations in the framework of the multiple scattering (MS) theory using the FEFF 8.2 code. Lattice parameters for both fcc and bcc Ca phases are from Ref.23 (b) Ca K edge x ray absorption near edge structure experimental curves for Ca100xAlx (x = 20, 27.3, 33.6 and 50 at.%) MGs at ambient pressure. Based on all considerations mentioned above, we suggest that the role of Al in binary Ca Al alloys could be treated as "chemical pressure" to Ca, which promotes the transfer from s and p orbitals to d orbitals of Ca, explaining the experimental fact of lowering the AACC transition pressure from 19.8GPa, 18GPa, 15.5GPa to 7.5GPa as Al content increases from 0 at.%, 20 at.%, 27.3 at.% to 33.6 at.%, respectively. A similar role of "chemical pressure" was also reported in the literatures31,32,33,34,35. For the Ca50Al50 MG, following the "chemical pressure" effect, AACC already occurs at ambient pressure because of high occupation of Ca d orbitals, in agreement with the experimental observation: no change during compression in the studied pressure range. In conclusions, we reported amorphous to amorphous configuration changes induced by pressure in non f electron containing Ca100xAlx (x = 20, 27.3 and 33.6 at.%) metallic glasses, which were confirmed by in situ room temperature high pressure x ray diffraction up to about 40GPa. These results obtained clearly point out that the amorphous to amorphous configuration changes induced by pressure are not limited to f electron containing metallic glasses. This will open a new vista which will trigger more theoretical and experimental investigations in this and many other MG systems. MethodsCa100xAlx (x = 20, 27.3, 33.6 and 50 at.%) MG ribbons with a thickness of about 35 m and a width of about 3mm were prepared with the single roller melt spinning method. In situ high pressure angle dispersive XRD experiments with a wavelength of 0.6884 and a focused beam size of about 55 m2 were performed at the beamline 15U, Shanghai Synchrotron Radiation Facility (SSRF) in China. The Ca100xAlx (x = 20, 27.3, 33.6 and 50 at.%) MGs were cut into about 404035 m3 chips, and then loaded into a Mao type symmetric diamond anvil cell. The sample chamber was about 150 m in diameter drilled in a T301 stainless steel gasket. Silicone oil was used as pressure transmitting medium while for pressure calibration Au powders were dispersed inside. The pressure applied to the sample was calculated from the lattice constant of Au using the equation of state of Au36. The silicone oil as a pressure medium used in this work could remain hydrostatic up to about 15GPa. The pressures when the anomalous changes detected in this work for Ca80Al20, Ca72.7Al27.3, and Ca66.4Al33.6 MGs were at about 18GPa, 15.5GPa and 7.5GPa, respectively. The changes detected below and above 15GPa indicate that these anomalous changes detected here are not strongly affected by the non hydrostatic pressure caused by the pressure medium. Each XRD pattern was collected for about 5 seconds at a given pressure using a Mar 165 CCD detector and then integrated with the FIT2D program37. Ca K edge XANES measurements for Ca100xAlx (x = 20, 27.3, 33.6 and 50 at.%) MGs were carried out in vacuum at the beamline 4B7A, Beijing Synchrotron Radiation Facility in China. Theoretical Ca K edge XANES for fcc Ca at 19.8GPa, for bcc Ca at 19.9GPa, and for Ca13Al14 and Ca8Al3 alloys at ambient pressure were carried out in the framework of the multiple scattering (MS) theory38,39 using the FEFF 8.2 code40. The cluster for the simulations was generated by the ATOMS package41 using their respective lattice parameters23,24. In calculations, the Hedin Lundqvist exchange correlation potential was chosen42. A cluster of 43 atoms (for fcc Ca) and 27 atoms (for bcc Ca) were used in all MS calculations to obtain accurate self consistent field (SCF), and the full MS calculation converges using a cluster up to 6. 5 sites for Ca8Al3 and 16 sites for Ca13Al14. A XANES spectrum contains all local structural information of different sites of the absorbed atomic species. Therefore, in the calculation for both Ca13Al14 and Ca8Al3 alloys we calculated the XANES spectrum of each individual site and then did a weighted superposition of all occupied sites. First principles calculations based on density functional theory (DFT) in generalized gradient approximation (GGA) with the Perdew Burke Ernzerhof (PBE) functional43 were used to optimize crystal structures and obtain ground state properties. Self consistent calculations were performed using a plane wave basis set limited by a cutoff energy of 400eV implemented in the Vienna ab initio simulation package (VASP). The projector augmented wave potentials44 and Monkhorst Pack k points sampling45 were employed. The convergence for the number of k points was tested to ensure that the total energy was converged within at least 0.01eV/atom. D. Whalley, E. 'Melting Ice' I at 77K and 10kbar: a new method of making amorphous solids. Nature 310, 393 395 (1984). Reversible first order transition between two H2O amorphs at 0.2GPa and 135K. J Chem Phys 100, 5910 5912 (1994). E. Mishima, O. The relationship between liquid, supercooled and glassy water. Nature 396, 329 335 (1998). Liquid liquid critical point in heavy water. Phys Rev Lett 85, 334 336 (2000). H., Sciortino, F., Essmann, U. Stanley, H. E. Phase Behavior of Metastable Water. Nature 360, 324 328 (1992). J., Meade, C., Hemley, R. J., Mao, H. K. Veblen, D. R. Microstructural Observations of Alpha Quartz Amorphization. Science 259, 666 669 (1993). J. First order amorphous amorphous transformation in silica. Phys Rev Lett 84, 4629 4632 (2000). Angell, C. A. Liquid liquid phase transition in supercooled silicon. Nat Mater 2, 739 743 (2003). Ree, F. H. High Pressure Liquid Liquid Phase Change in Carbon. Phys Rev B 48, 3591 3599 (1993). V. Morishita, T. in Advances in Chemical Physics, Vol 143 (ed Rice, S. A.) 29 82 (Wiley Blackwell, 2009). S. et al. Anomalous compression behavior in lanthanum/cerium based metallic glass under high pressure. P Natl Acad Sci USA 104, 13565 13568 (2007). W. et al. Polyamorphism in a metallic glass. Nat Mater 6, 192 197 (2007).

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