Exchange-driven ferric low-spin high-spin transition in iron-doped Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Mössbauer Zeeman measurements at 4.2 K on $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_{3-x}{\mathrm{Fe}}_x{\mathrm{O}}_{7-\delta }$ ($\delta \approx 0$) with $x=0.1, 0.2, \mathrm{and} 0.4$ establish that the major feature at $x=0.1\mathrm{}$ corresponds to a low-spin $\left(\mathrm{LS}\right) {3d}^5 (S=\frac{3}{2})$ configuration, but that for $x\gtrsim 0.2$ an increasing fraction of this LS spectrum transforms to a high-spin $\left(\mathrm{HS}\right) {3d}^5 (S=\frac{5}{2})$ counterpart. An explanation is given in terms of a LS-HS transition driven by $\sigma$-bonded superexchange interactions between nearest-neighbor ferric ions in their HS states.