Ruby's Optical Transitions: Effects of Pressure-Induced Phase Transformation
- PDF / 478,347 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 74 Downloads / 204 Views
2
RENATA M. WENTZCOVITCH,"'2 and A. FAZZIO 2
'Department of Chemical Engineering and Materials Science, and Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN 55455, USA 2 Instituto de Fisica, Univ. de Sio Paulo, CP 66318, 0 5389-970, Sio Paulo, SP, Brazil ABSTRACT Here we summarize our investigation of the effect of a recently observed phase transformation on ruby's optical transitions. This study involved a first principles calculation of the electronic and structural properties of a chromium impurity in alumina host lattices and a subsequent calculation of the multiple structure using eigenvalues and eigenvectors derived from the first principles calculation. This investigation is relevant to clarify the behavior of the fluorescent optical transitions which are used as pressure sensor in diamond anvil experiments across the structural transformation. INTRODUCTION Ruby (CrS+-doped alumina) is a material of central importance to high pressure science and technology. The pressure-dependence of its fluorescence transitions, the R lines, provides a convenient and accurate technique to determine pressure in diamond-anvil cell experiments[l]. The shift of R-lines has been calibrated with pressure (ruby scale) up to 180 GPa[2, 3], and extrapolations of the ruby scale have been used to determine pressures as high as 5 Mbar[4]. At low pressures ruby is in the corundum structure (see Fig. 1). This phase has been observed to be exceptionally stable within a large pressure range in various ambient temperature experiments[5]. Although calibration of the pressure-dependence of the luminescent spectrum does not require knowledge of the crystal structure or the microscopic environment of the Cr+S ions, such knowledge is complementary. Several theoretical studies have predicted that alternative phases might occur at elevated pressures[6, 7]. The latest first principles study predicts two high pressure transformations (see Fig. 2): the first from the corundum to the Rh 2 0 3 (II) phase at 78 ± 4 GPa, and a second transformation to the orthorhombic perovskite (Pbnm) structure at 223±15 GPa[7, 8]. The first of this transitions, which had also been predicted by previous calculations[6], was recebtly confirmed experimentally: a corundum-to-Rh20 3 (1I) phase transformation was observed at approximately 100 GPa and 1000 K[9]. Temperature seems to have been a key factor in overcoming kinetic hindrances in this latest experiment. The observation of this phase transformation suggests a possible need to investigate carefully the fluorescence spectrum of ruby in the multimegabar regime, especially in the pressure range beyond that of the scale calibration. In this paper we summarize our investigation of the effects of pressure and of the newly found phase transformation (corundum-to-Rh20 3 (II)) on ruby's optical transitions[10]. We have combined a first principles calculation of the chromium impurity, a color center, 275 Mat. Rem. Soc. Symp. Proc. Vol. 499 01998 Materials Research Society
(b)
(a)
Figure 1: Exp
Data Loading...
