Structural Tuning of Charge, Orbital, and Spin Ordering in Double-Cell Perovskite Series between NdBaFe₂O₅ and HoBaFe₂O₅

Abstract

Charge, orbital, and magnetic ordering of NdBaFe(2)O(5) and HoBaFe(2)O(5), the two end-members of the double-cell perovskite series RBaFe(2)O(5), have been characterized over the temperature range 2-450 K, using differential scanning calorimetry, neutron thermodiffractometry and high-resolution neutron powder diffraction. Upon cooling, both compounds transform from a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions, through a "premonitory" charge ordering into a class-II MV compound, and finally to a class-I MV phase at low-temperature. The latter phase is characterized by Fe(2+)/Fe(3+) charge ordering as well as orbital ordering of the doubly occupied Fe(2+) d(xz) orbitals. The relative simplicity of the crystal and magnetic structure of the low-temperature charge-ordered state provide an unusual opportunity to fully characterize the classical Verwey transition, first observed in magnetite, Fe(3)O(4). Despite isotypism of the title compounds at high temperature, neutron diffraction analysis reveals striking differences in their phase transitions. In HoBaFe(2)O(5), the Verwey transition is accompanied by a reversal of the direct Fe-Fe magnetic coupling across the rare earth layer, from ferromagnetic in the class-II and -III MV phases to antiferromagnetic in the low-temperature class-I MV phase. In NdBaFe(2)O(5), the larger Nd(3+) ion increases the Fe-Fe distance, thereby weakening the Fe-Fe magnetic interaction. This decouples the charge and magnetic ordering so that the Fe-Fe interaction remains ferromagnetic to low temperature. Furthermore, the symmetry of the charge-ordered class-I MV phase is reduced from Pmma to P2(1)()ma and the magnitude of the orbital ordering is diminished. These changes destabilize the charge-ordered state and suppress the temperature at which the Verwey transition occurs. A comparison of the magnetic and structural features of RBaFe(2)O(5) compounds is included in order to illustrate how structural tuning, via changes in the radius of the rare-earth ion, can be used to alter the physical properties of these double-cell perovskites.

Orbital occupancy evolution across spin- and charge-ordering transitions in YBaFe 2 O 5

“…12). When averaged, their temperature evolution yields T = 433(1) K N via least-squares fit with Brillouin function (Fig 2), in agreement with 432 K for HoBaFe 2 O 5 obtained from the decay of the ordered moment seen by neutron powder diffraction [18]. An analogous fit of the center shifts yields a Debye temperature of 629 K (Fig.…”

“…Fig. 17 shows that the entropy gain upon heating through T V and T p (a measure of valence mixing) is proportional to shortening of the Fe-Fe basal-plane distance [11], which occurs despite a minor expansion along c [18]. The donation of the minority-spin electron from Fe 2+ leads to an interaction that is weaker than, but not entirely unlike, the weak bonding of a transition-metal centralatom donor to a ligand acceptor where the donor orbital removes much of the ligand-field repulsion upon morphing into a weak bond.…”

“…Almost all mixing entropy is supplied at the first step; seen by powder diffraction as a loss of the Fe 2+ -Fe 3+ order [11] at the transition temperature denoted T V . The entropy of the second step, at T p , is much smaller and no discontinuous structural change is detected by powder diffraction; only a smooth decrease of the tiny orthorhombic distortion that vanishes at the Néel temperature T N [13,18].…”

Pressure effects on the charge-ordering transition ofBaYCo2O5.0

“…9 This common feature suggests that the correlation originates in the chain-type CO along the b axis, for which the orthorhombic distortion is indispensable. 16,17 Fulfillment of this correlation between s and T CO may take priority over the volume change effect, and we see that despite a slight volume increase upon the CO transition in BaYCo 2 O 5.0 , the applied pressure increases T CO .…”

Large ferroelectric polarization in the new double perovskite NaLaMnWO6 induced by non-polar instabilities

“…[2,3] Analogously, ordering of the A-cations can also occur, showing a strong preference for a layered arrangement. [4][5][6] Finally, simultaneous Aand B-site cation ordering can also occur in compounds of type AA ′ BB ′ O 6 . [7] These compounds are interesting from a structural point of view since they exhibit rocksalt ordering of B-site cations and layered ordering of A-site cations.…”