Influence of chemical substitution and pressure on structure, resistivity and magnetism of La_1+xBa_2−xFe₃O_8+δ (x = 0, 0.5, 1; 0 < δ < 1)

Abstract

Among those Fe-based perovskite $R \hbox {Ba}_{2} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$ which are isomorphous to high-$T_{c} \, R \hbox {Ba}_{2} \hbox {Cu}_{3} \hbox {O}_{7-\delta }$ superconductors ($R=\hbox {rare earth}$ and Y), the cubic $\hbox {LaBa}_{2} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$ is special in that both its magnetism and resistivity are strongly modified when varying a control parameter such as oxygen concentration ($8+\delta$), La/Ba ratio ($1 + x/2 - x$), or applied pressure (p). With the aim of identifying and optimizing those conditions that induce a reduction in its magnetism and resistivity, we synthesized various samples of $\hbox {La}_{1+x} \hbox {Ba}_{2-x} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$ and studied their structure, magnetism, and resistivity as a function of $\delta$, p, and x. In this preliminary report, we show that (i) the cubic, $Pm{\bar{3}}m$, structure is stable across the studied ranges, however the lattice a-parameter is reduced on increasing x, $\delta$, or p; (ii) the resistivity is reduced on increasing $\delta$ or reducing x; the influence of pressure, on the other hand, is mostly manifested around and below the onset points of charge and magnetic order. (iii) The magnetism is much reduced as compared to that of the heavier $R \hbox {Ba}_{2} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$ members. We discuss the influence of each of x, $\delta$ and p in terms of the Fe ions disproportionation scenario.

Appendix: Procedures used for \(\hbox {La}_{1+x} \hbox {Ba}_{2-x} \hbox {Fe}_{3} \hbox {O}_{8+\delta }\) synthesis

The precursors $\hbox {La}_{2} \hbox {O}_{3} \, (99.9 \, \%)$, $\hbox {Ba}_{2} \hbox {O}_{3} \, (99.0 \, \%)$, and $\hbox {Fe}_{2} \hbox {O}_{3} \, (99.995 \, \%)$—all Sigma-Aldrich—were preheated for 8 h at, respectively, 800, 800 , and 400 °C. Then, for each $\hbox {La}_{1+x} \hbox {Ba}_{2-x} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$ sample, stoichiometric amount of each component were mixed together, pelletized, and afterwards subjected to conventional solid state reaction procedure with treatment at different temperatures and in different atmospheres so as to control the oxygen content.

With the aim of evaluating the influence of synthesis procedures on the structural and physical properties of $\hbox {La}_{1+x} \hbox {Ba}_{2-x} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$, two different sets of samples were prepared:

The first set, see Fig. 11a, was calcined at 1000 °C and synthesized at 1125 °C for 72 h within an open-to-air furnace. Intermediate grinding and pelletizing step were undertaken. The final step consisted in oxygen (or argon) treatment at 1125 °C for 72 h in oxygen flux for 72 h, afterwards cooled at rate of $3^{\,\circ } \hbox {C}/\hbox {min}$ down to 600 °C, and finally furnace-cooled.

The second set, see Fig. 11b, were calcinated at 870 °C for 10 h, pre-reacted at 1000 °C for 12 h, and sintered at 1150 °C for 18 h. These steps were followed by annealing in air at 870 °C for 18 h. Afterwards, re-pulverized, pelletized, treated at 1300 °C in oxygen (or argon) flux for 72 h, afterwards cooled at rate of $3^{\, \circ } \hbox {C}/\hbox {min}$ down to 400 °C, and finally furnace-cooled.

Fig. 11

Time-temperature profile during the synthesis of $\hbox {La}_{1+x} \hbox {Ba}_{2-x} \hbox {Fe}_{3} \hbox {O}_{8+\delta }$ samples. The profile consists of a series of thermal ramping/soaking steps with intermediate grinding and pelletizing. The final steps were usually carried out under oxygen (or argon) ambient pressure at specified temperature for 72 h and afterwards cooled down, under controlled rate, to 400 or 600 °C. Afterwards left to be furnace-cooled. a Procedures undertaken for the first set of samples (see above paragraphs). b Procedures undertaken for the synthesis of the second set of samples (see above). For the final steps, the temperature was ramped and hold, first, at 1100 °C for 8 h and afterwards at 1300 °C for 72 h