Structural and Electronic Transitions in AgReO 4 Under Pressure
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STRUCTURAL AND ELECTRONIC TRANSITIONS IN AgReO 4 UNDER PRESSURE John K. Vassiliou', Jens W. Otto, Richard F. Porter and Arthur L. Ruoff, Cornell University, Materials Science Center, Ithaca, NY 14853.
ABSTRACT The behavior of AgReO 4 under pressure has been studied in a diamond anvil cell. The equation of state determined by synchrotron radiation shows considerable change in compressibility with increasing pressure. X-ray and Raman scattering show a first-order phase transition from the scheelite to an as yet undetermined structure at 13 GPa. Changes in both the internal and external Raman active modes suggest a change in Re coordination. Optical transmission experiments demonstrate that the energy gap in the scheelite phase decreases at the rate of 75 meV/GPa. There is a small jump in the energy gap at 13 GPa, probably related to the structural transition. AgReO 4 remains an insulator to at least 47 GPa. INTRODUCTION A variety of ABO 4 tungstates and molybdates crystallizing in the scheelite structure undergo structural transitions under pressure [1,2]. A series of perrhenates AReO 4 (where A is a monovalent atom) crystallize in the scheelite or pseudoscheelite structure [3]. A Raman study of TIReO 4 under pressure [4] demonstrates the existence of a series of structural transitions in the pressure range to 16 GPa. At a transition at 9.7 GPa, the energy gap of TlReO4 decreases by about 2 eV [4]. This was suggested to be due to a valence change of the TI ion to the trivalent state. AgReO 4 (silver perrhenate) crystallizes in the scheelite structure (space group 141/a) with four molecules per unit cell. Ag is known to have a tendency to disproportionate into Ag(I) and Ag(III), as for example in AgO [5]. High pressures tend to reduce ionic radii, and hence favor higher valence states. We therefore investigate the possibility that AgReO 4 will undergo structural transitions and a valence change in Ag from the mono to the trivalent state under pressure. We report here Raman and x-ray scattering and optical absorption measurements on AgReO4 under pressure. A structural transition is observed at 13 GPa, accompanied by frequency jumps in some Raman active modes. Optical absorption measurements show that there is no dramatic decrease in the energy gap at this transition. AgReO 4 is semiconducting to at least 47 GPa.
1 Current address: Dept. of Physics, Villanova University, Villanova, PA Mat. Res. Soc. Symp. Proc. Vol. 210. @1991 Materials Research Society
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EXPERIMENTAL Materials Preparation and Characterization AgReO 4 was prepared from saturated solutions of AgNO3 (Mallinckrodt Chemical Reagent) and KReO 4 (Aldrich, 99.99% purity) by an ion exchange reaction. The purity of the starting materials was checked by x-rays. Distilled hot water was used for the preparation of the solutions. An excess of the AgNO 3 solution was added to the KReO 4 solution, and AgReO 4 precipitated on cooling. The AgReO 4 crystals were removed and recrystallized again in a solution to which an excess of AgNO 3 had been added. The crystal
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