Control of Bond-Strain-Induced Electronic Phase Transitions in Iron Perovskites

Unusual electronic phase transitions
in the A-site ordered perovskites LnCu3Fe4O12 (Ln:
trivalent lanthanide ion) are investigated. All LnCu3Fe4O12 compounds are in identical
valence states of Ln3+Cu2+3Fe3.75+4O12 at high temperature. LnCu3Fe4O12 with larger Ln ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb)
show an intersite charge transfer transition (3Cu2+ + 4Fe3.75+ → 3Cu3+ + 4Fe3+) in which
the transition temperature decreases from 360 to 240 K with decreasing Ln ion size. In contrast, LnCu3Fe4O12 with smaller Ln ions
(Ln = Dy, Ho, Er, Tm Yb, Lu) transform into a charge-disproportionated
(8Fe3.75+ → 5Fe3+ + 3Fe5+)
and charge-ordered phase below ∼250–260 K. The former
series exhibits metal-to-insulator, antiferromagnetic, and isostructural
volume expansion transitions simultaneously with intersite charge
transfer. The latter shows metal-to-semiconductor, ferrimagnetic,
and structural phase transitions simultaneously with charge disproportionation.
Bond valence calculation reveals that the metal–oxygen bond
strains in these compounds are classified into two types: overbonding
or compression stress (underbonding or tensile stress) in the Ln–O (Fe–O) bond is dominant in the former
series, while the opposite stresses or bond strains are found in the
latter. Intersite charge transfer transition temperatures are strongly
dependent upon the global instability indices that represent the structural
instability calculated from the bond valence sum, whereas the charge
disproportionation occurs at almost identical temperatures, regardless
of the magnitude of structural instability. These findings provide
a new aspect of the structure–property relationship in transition
metal oxides and enable precise control of electronic states by bond
strains.