Two series of samples of the oxygen-deficient double-perovskite phase, Ba(Sm,Ca)Fe2O5+δ were synthesized employing a sample encapsulation technique that utilizes Fe metal as an oxygen getter. The hole-doping level on the FeO2 plane was controlled by varying the amount of excess oxygen or the Ca concentration at the Sm site. Earlier it had been found that at room temperature five-coordinated Fe2+ and Fe3+ species form pairs by sharing a d electron, leading to the formation of a Fe2.5++ fluctuating valence state [Phys. Rev. B 60 (1999) 15,251]. At TV, a Verwey-type transition occurred, signifying the charge separation of the Fe2.5+ fluctuating valence state into high-spin Fe2+ and Fe3+. Moreover, related to the Verwey-type transition a negative magnetoresistance peak with a magnitude of a few percent was observed [Appl. Phys. Lett. 77 (2000) 1683]. The position of the peak was found to correspond to the jumps seen in the susceptibility and resistivity vs. temperature curves. For the present samples, it is found that the value of TV is severely decreased upon increasing the Ca concentration, as seen by susceptibility and magnetoresistivity measurements. Introduction of excess oxygen leads to a less severe decrease of the transition temperature. The observed behavior of T(V) was reproduced by a simple model based on combinatorial-entropy calculations.
- A. Magnetically ordered materials
- D. Phase transitions
- D. Valence fluctuations