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 magne tometry, and Mossbauer spectroscopy. The system was found to be divided int o 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 orthorhombi c region, a well-defined metal-semiconductor transition took place and the transition temperature decreased with increasing x from 290 K for x = 0.0 t o 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 t his composition and CaFeO3 both indicated an orthorhombic (Pnma) to monocli nic (P2(1)/n) transition accompanying the electronic transition. The two cr ystallographically different Fe-O octahedra created in the LT phase were bo th 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 s tructural and electronic changes indicated that the CD and the breathing ph onon mode were intimately coupled in such a way that the CD proceeded (delt a: 0 --> 1) as the size difference between the two kinds of Fe-O octahedra increased. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All r ights reserved.