Structural and unique magnetic properties of ${\mathrm{PrBa}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$

Tetragonal ${\mathrm{PrBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ exhibits a unique magnetic behavior and behaves differently from both ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and all R${\mathrm{Ba}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ materials. Magnetic-susceptibility study reveals two anomalies at 23 and 55 K which are sensitive to oxygen concentration, but no specific heat anomaly was observed at either temperature. Mössbauer studies reveal two inequivalent iron sites, which order antiferromagnetically at ≊230 K, in contrast to 670 K in R${\mathrm{Ba}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$. X-ray absorption spectra indicate that Pr is trivalent. In contrast to ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ [${\mathit{T}}_{\mathit{N}}$(Pr)=17 K] Pr in ${\mathrm{PrBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ does not order magnetically. The anomalies at 23 and 55 K are probably related to spin reorientation of Fe moments. © 1996 The American Physical Society.