Supercomputing Institute Research Bulletin Summer 1997

Phase Transformations of Alumina: Implications for the Ruby Scale

Alumina is an important technological material due to its presumed stability over a wide range of pressures. This stability lends itself to several important applications in high-pressure physics, including use in the diamond-anvil cell. Over the last couple of decades, the diamond-anvil cell has emerged as a work horse of high-pressure research, enabling researchers, for the first time, to measure solid properties at pressures corresponding to the earth’s mantle and below. The diamond-anvil cell, an apparatus used to generate large pressures, is a chamber less than one hundredth of a millimeter in diameter in which scientists place ruby, or chromium-doped alumina, along with the material to be studied. Ruby plays an important role in such experiments, serving as the basis for a secondary pressure calibrant, known as the ruby scale. Ruby is obtained by substituting a few aluminum atoms in alumina with chromium (color centers), which provides a source of fluorescence when excited by light. The wavelength of this fluorescence depends directly on the pressure in the ruby. By measuring the wavelengths of the fluorescence emissions, researchers can determine the pressure inside the diamond-anvil cell. The reliability of the ruby scale, then, depends on the stability of the alumina structure throughout the pressures studied in a particular experiment. If alumina became unstable, it would transform into another phase at higher pressures. That phase would, in turn, have a different structure from the low-pressure phase, and this structural change could affect the reliability of the ruby scale. As yet, no phase transformation of alumina has been experimentally observed. However, calculations have predicted two pressure-induced transformations of alumina from its low-pressure phase, known as corundum, to high pressure polymorphs. These predicted polymorphs are the Rh2O3 (II) phase and an orthorhombic perovskite phase. The figures below depict the different molecular structures of these three phases. Dr. Renata M. Wentzcovitch, research fellow Kendall T. Thomson, and post-doc Wenhui Duan, of the University’s Department of Chemical Engineering and Materials Science, are using a technique called first principle-molecular dynamics (FPMD) to predict such transformations. Their procedure calculates the electronic energy, forces, and stress in a solid-state configuration quantum mechanically (by first principles) and solves for the fully-relaxed ground-state configuration (in which the atomic positions in the solid are allowed to “relax” into positions that give the lowest energy) at arbitrary pressures. By comparing the calculated enthalpies of the corundum phase and other possible high-pressure phases, accurate predictions of transformation pressures have been obtained. The work is providing, for the first time, predicted phase stabilities of alumina using fully relaxed geometries. By Wentzcovitch, Thomson and Duan’s method, the corundum phase, Al2O3, is predicted to undergo a transformation to another structure, called Rh2O3 (II), at pressures corresponding to those found midway through the earth’s lower mantle. A further transformation to an orthorhombic perovskite phase is also predicted at even greater pressures of approximately 223 GPa. Because pressures of well over 500 GPa have been obtained in diamond-anvil cell experiments recently, the predicted phase behavior of alumina could be important in pinning down the exact pressure in these experiments. It is possible that, due to the nature of reconstructive phase transformations, the observed alumina phase in a particular experiment depends strongly on the thermal history of the ruby chips (the ruby scale is therefore presumed to be sensitive to the thermal history as well). Confirmation of whether or not the Rh2O3 (II) phase is even observable is hampered by limitations in high-pressure x-ray technology. The structural similarity between corundum and Rh2O3 (II) results in very similar X-ray spectra. It is therefore quite possible that small amounts of the Rh2O3 (II) phase could go undetected in high-pressure X-ray measurements These figures show the predicted phase tranformations of alumina. Al2O3, as its molecular structure changes due to pressure. Figure 1 Corundum, the low-pressure structure of alumina   Figure 2 Rh2O3(II), a predicted high-pressure polymorph of alumina. The corundum phase is predicted to transform tothis phase at approximately 78 GPa   Figure 3 Orthorhombic perovskite, another high-pressure polymorph of alumina. Rh2O3(II) phase is predicted to transform to this phase at approximately 223 GPa The specific effect of these phase transformations on the ruby scale is the current focus of this research. By simulating a “supercell” of 80 atoms, and including chromium defects, Duan is evaluating the response of ruby fluorescence lines to changing pressure and phase composition.

In This Issue: 1997 Summer Undergraduate Intern Program Relational Drug Design Workshop Rayleigh-Taylor Instability > How Alumina Phases Impact the Ruby Scale Turbulent Flow and Hypersonic Vehicles Origin 2000 Arrives Seminar Synopses Research Reports