Metal-semiconductor transition, charge disproportionation, and low-temperature structure of Ca1-xSrxFeO3 synthesized under high-oxygen pressure

The solid solution Ca1-xSrxFeO3 was synthesized under high-oxygen pressure, and its structural and electronic properties were investigated by means of X-ray and neutron Rietveld analyses, resistivity measurements, SQUID magnetometry, and Mossbauer spectroscopy. The system was found to be divided into an orthorhombic region for 0.0 less than or equal to x less than or equal to 0.5, a cubic one for 0.8 less than or equal tox less than or equal to 1.0, and possibly a mixed region at x approximate to 0.6. In the orthorhombic region, a well-defined metal-semiconductor transition took place and the transition temperature decreased with increasing x from 290 K for x = 0.0 to 200 K for x = 0.4. The Mossbauer measurements on the low-temperature (LT) phase of Ca0.8Sr0.2FeO3 confirmed the occurrence of the well-known charge disproportionation (CD), 2Fe(4+) --> Fe(4-delta)+ + Fe(4+delta)+ with delta increasing toward unity with decreasing temperature. The neutron data on this composition and CaFeO3 both indicated an orthorhombic (Pnma) to monoclinic (P2(1)/n) transition accompanying the electronic transition. The two crystallographically different Fe-O octahedra created in the LT phase were both almost regular in shape but varied their sizes in an inverse fashion as temperature decreased: the average Fe-O bond lengths were typically 1.941 (9) Angstrom and 1.900 (9) Angstrom for CaFeO3 at 130 K. These concomitant structural and electronic changes indicated that the CD and the breathing phonon mode were intimately coupled in such a way that the CD proceeded (delta: 0 --> 1) as the size difference between the two kinds of Fe-O octahedra increased. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.