Intermetallic charge transfer between A-site Cu and B-site Fe in A-site-ordered double perovskites

In this review article, we describe in detail the temperature-induced intermetallic charge transfer between A-site Cu and B-site Fe ions in the A-site-ordered double perovskites RCu3Fe4O12 (R=La, Bi). In these compounds, a very rare Cu3+ valence state at the square-planar-coordinated A sites was stabilized by high-pressure synthesis. By increasing the temperature, a Cu–Fe intermetallic charge transfer producing a high Fe3.75+ valence state occurred. This charge transfer gave rise to a first-order isostructural phase transition with unusual volume contraction, as well as to antiferromagnetism-to-paramagnetism and insulator-to-metal transitions. The substitution of Bi for La stabilized the low-temperature phase containing Cu3+ and increased the charge transfer transition temperature from 393 K for LaCu3Fe4O12 to 428 K for BiCu3Fe4O12.


Introduction
Transition metal (TM) oxides with perovskite and perovskite-like structures have attracted much attention owing to their wide varieties of physical properties, some of which are useful for technological applications. The octahedrally coordinated B sites in a simple ABO 3 perovskite are usually occupied by TM ions, and the physical properties of the perovskite are determined in large part by strong interactions between the TM ions as well as by the hybridization of TM 3d and oxygen 2p orbitals. The 12-fold-coordinated A sites, on the other hand, often accommodate relatively large ions, such as alkaline metal, alkaline-earth metal and rare earth ions. When three-quarters of the A-site cations are replaced by another element, an A-site-ordered double perovskite with a general chemical formula A A 3 B 4 O 12 can be obtained. The crystal structure of such a double perovskite with cubic Im-3 symmetry is shown in figure 1. Because the BO 6 octahedra in this structure are significantly tilted (a typical B-O-B bond angle is about 140 • ), the A-site-ordered double perovskite has square-planar-coordinated AO 4 units rather than 12-fold-coordinated A sites like those in the simple ABO 3 perovskites.
Another interesting feature of oxides with this structure is that the square-coordinated A sites can accommodate TM ions like Jahn-Teller active Cu 2+ and Mn 3+ . Because there are TM ions at both the A and B sites in the structure, A-A and A-B interactions in addition to the B-B interaction seen in many simple perovskite oxides give the A-siteordered double perovskites interesting physical properties [1,2]. For example, giant dielectric constants (≈10 5 ) in a broad temperature region were found in CaCu 3 Ti 4 O 12 [3]- [5]. Large magnetoresistance under low applied fields was observed in a CaCu 3 Mn 4 O 12 ferrimagnetic semiconductor and ACu 3 Mn 4 O 12 (A = La, Bi) half metals, where A-site Cu and B-site Mn spins couple antiferromagnetically [6]- [8]. Either ferromagnetism or antiferromagnetism appears from the A-A magnetic interaction in CaCu 3 B 4 O 12 (B = Ge, Ti, Sn) [9,10]. Progressive transitions due to charge, orbital and spin orderings occurred in NaMn 3 Mn 4 O 12 [11]. Also reported in the literature are antiferromagnetic interactions between Mn 3+ (S = 2) spins at the A site in YMn 3 Al 4 O 12 [12] and magnetic-dielectric coupling in BiMn 3 Mn 4 O 12 [13,14]. A recent experiment on LaMn 3 Ti 4 O 12 revealed that the A site can also be occupied by Mn with an unusually low valence state rather than by Jahn-Teller Mn 3+ [15]. Very recently, a new phenomenon concerning the A-B intersite interaction was discovered in an A-site-ordered LaCu 3 Fe 4 O 12 double perovskite: Cu-Fe intermetallic charge transfer [16]. Different from chemical doping, which is a conventional way of modulating the valence states in TM oxides, the valence changes of the A-site Cu and the B-site Fe in the compound were realized by the temperature-induced charge transfer. As a result, a very rare Cu 3+ state was stabilized at the square-coordinated A sites in the ground state [16,17]. A similar charge transfer was also found in the isostructural BiCu 3 Fe 4 O 12 but at a higher transition temperature [18]. This paper reviews the newly discovered intermetallic charge transfer phenomenon found in A-site-ordered double perovskites.

Experimental
Polycrystalline samples of LaCu 3 Fe 4 O 12 and BiCu 3 Fe 4 O 12 were prepared by solid-state reaction under high pressure (needed to stabilize the special ordered structure). La 2 O 3 /Bi 2 O 3 , CuO and Fe 2 O 3 were finely mixed at a mole ratio of 1 : 6 : 4 with an appropriate amount of the oxidizing agent KClO 4 , and then held at 1300-1400 K for 1 h under 10 GPa in a cubic-anvil-type highpressure apparatus. Using a dilute acid solution to wash away the residual KCl and unreacted starting materials yielded almost single-phase samples.
Phase identification and structure analysis were carried out by x-ray and neutron diffraction. High-resolution synchrotron x-ray diffraction (SXRD) was performed at BL02B2 in SPring-8 with powder samples that were put into glass capillary tubes and rotated during the measurements. A large Debye-Scherrer camera with an imaging plate detector was used, and the 2θ angle range from 1 • to 75 • was measured with 0.01 • resolution. The wavelength used for the measurements on LaCu 3 Fe 4 O 12 was 0.77747 Å, and the diffraction data for this compound were collected at 100, 300 and 450 K. The wavelength used for the measurements on BiCu 3 Fe 4 O 12 was 0.77785 Å, and the data for this material were collected at 300, 350, 375, 400, 420, 440, 460 and 480 K. Conventional XRD was also performed using a Rigaku x-ray diffractometer with Cu-Kα radiation at temperatures from 173 to 523 K. Neutron powder diffraction (NPD) was carried out at the beamline Super-D2B in Institut Laue-Langevin with powder samples in a vanadium can of 5 mm diameter. The 2θ angle range from 5 • to 155 • was measured using a wavelength of 1.594 Å, and diffraction data were collected at 50, 125, 200, 300, 325, 350, 375, 400 and 425 K. GSAS software was used to refine both the crystal and the magnetic structural parameters [19].
The oxygen content was estimated from the change in the sample weight during the decomposition of the compound. Thermogravimetry (TG) and differential thermal analysis (DTA) measurements from room temperature (RT) to 1200 K were made, at a heating rate of 10 K min −1 and in an ambient atmosphere, using a Rigaku TG-DTA 8120 system.
The Fe oxidation states were studied by 57 Fe Mössbauer spectroscopy in transmission geometry with a 57 Co/Rh γ -ray source in combination with a constant-acceleration spectrometer. The source velocity was calibrated by pure α-Fe. The obtained spectra were fitted by using a least-square method with Lorentzian functions.
Magnetic susceptibility (χ ) and magnetization were measured using a Quantum Design superconducting quantum interference device magnetometer in the zero-field cooling mode, and a conventional method was used to measure the electrical resistance.

Crystal structure of LaCu 3 Fe 4 O 12 at 300 K
The SXRD and NPD patterns of LaCu 3 Fe 4 O 12 at 300 K (figures 2(a) and 3(a), respectively) can be well fitted based on an A-site-ordered double perovskite structure model with a cubic Im-3 space group. The NPD data include a magnetic contribution that we discuss in section 3.3. In the structure, A -site La and A-site Cu are 1 : 3 ordered at special sites 2a (0, 0, 0) and 6b (0, 0.5, 0.5), respectively. B-site Fe occupies special site 8c (0.25, 0.25, 0.25) and O occupies site 24g (x, y, 0). No anomalies in occupation factors at any of the sites were observed in the refinements, suggesting the stoichiometric composition of the material. No oxygen deficiency was observed, even in the refinement with NPD data, which are highly sensitive to oxygen.
The oxygen stoichiometry was further confirmed by the TG measurement results shown in figure 4. The sample decomposed at ≈820 K, losing 3.25% of its mass. The residual products were LaFeO 3 , CuFe 2 O 4 and CuO, so the decomposition reaction was LaCu 3 Fe 4 O 12 → LaFeO 3 + 3/2CuFe 2 O 4 + 3/2CuO + 3/4O 2 . The observed 3.25% weight loss agrees well with the change expected from this reaction (3.22%). All the results obtained in the structure analysis 3.364 (6) 3.307 (7) 3.377 (2) 3.316 (6) 3.379 (5)   The results strongly suggest that an unusual Cu 3+ with square planar coordination is stabilized in the A-site-ordered structure and that at 300 K the charge combination is LaCu 3+ 3 Fe 3+ 4 O 12 .

Isostructural phase transition
When the sample was heated, DTA showed a sharp endothermic peak at 393 K, whereas TG showed no change (figure 4). Here we define this temperature as a critical phase transition temperature (T c ) in LaCu 3 Fe 4 O 12 . Figures 2(b) and 3(b), respectively, show high-temperature SXRD (450 K) and NPD (425 K) patterns of LaCu 3 Fe 4 O 12 . Note that the crystal symmetry of the phase above T c is the same as that of the phase below T c , suggesting that the phase transition is an isostructural one. The structural parameters obtained in the refinements are also listed in table 1. Above T c the Cu-O distance in the A-site square unit was significantly greater than it was below T c , whereas the Fe-O distance in the BO 6 octahedron was considerably less than it was below T c . As a result, the BVS values for the A-site Cu decreased to 2.06 (SXRD) and 2.08 (NPD), while those for the B-site Fe increased to 3.44 (SXRD) and 3.41 (NPD). The results indicate that the Cu valence is 2+, a typical value for the square planar coordination, and that the Fe valence is unusually high. Since the TG measurement showed no apparent weight loss at temperatures up to 500 K (figure 4), it is reasonable to conclude that the oxygen content of the material was stoichiometric at temperatures between 425 and 450 K. A simple ionic valence calculation indicates that the high-temperature charge combination is LaCu 2+   The temperature dependence of the unit cell volume (a 3 ) of LaCu 3 Fe 4 O 12 (figure 5) shows a sharp change at the transition temperature, indicating that the transition is a first-order one. The normal volume expansion with increasing temperature was interrupted by a 1.2% volume contraction at the phase transition temperature. This unusual volume contraction was mainly due to the significant reduction in the length of the Fe-O bond ( figure 6(a)). The length of the Cu-O bond increased correspondingly because oxygen atoms are bonded to both Cu in the square units and Fe in the octahedral units, but this bond does not make the structural framework.
The above structural change suggests that the charge combination changes from LaCu 3+ 3 Fe 3+ 4 O 12 below T c to LaCu 2+ 3 Fe 3.75+ 4 O 12 above T c . This is clear from the change in the BVS values for Cu and Fe ( figure 6(b)). The simultaneous valence variations of Cu and Fe imply a temperature-induced intermetallic charge transfer between the A-site Cu and the B-site Fe: 3Cu 3+ + 3e − → 3Cu 2+ and 4Fe 3+ − 3e − → 4Fe 3.75+ . At T c , electrons released from the B-site Fe 3+ ions move to the A-site Cu 3+ ions, reducing Cu 3+ to Cu 2+ .

Antiferromagnetism-to-paramagnetism transition
The  [21,22], further confirming the charge formula of LaCu 3+ 3 Fe 3+ 4 O 12 below the transition temperature. The observed magnetic sextuplet indicates that the Fe spins below T c were magnetically ordered and that there was no other magnetic transition than that at T c .
The NPD pattern of LaCu 3 Fe 4 O 12 at 300 K (figure 3(a)) included magnetic reflections in addition to the nuclear peaks, so we can determine the magnetic structure. As clearly seen in the 111 and 113 Bragg reflections, strong magnetic contributions were observed. The refinement revealed a magnetic superstructure with a (1/2 1/2 1/2) propagation vector of the cubic cell, which gave a G-type antiferromagnetic ordered structure. Thus, each Fe 3+ spin aligns antiparallel to the six nearest neighbors, as shown in figure 8. No magnetic contribution from the square-planar-coordinated 6c site was found. Because the square-planar-coordinated Cu gives rise to a considerable energy splitting between d x 2 −y 2 and d 3z 2 −r 2 orbitals, the 3d electrons in Cu 3+ (3d 8 ) ions fully occupy the d x z , d yz , d x y and d 3z 2 −r 2 orbitals, making the d x 2 −y 2 orbital empty with an S = 0 spin state. The refined magnetic moment of the B-site Fe at 50 K was 4.03(2) µ B , which was reduced from the ideal value of 5 µ B for high-spin Fe 3+ (S = 5/2), probably by a covalency effect. The antiferromagnetic ordering thus originates from Fe-O-Fe superexchange interactions. Linear behaviors in the field dependence of magnetization below T c (inset of figure 9(a)) were also consistent with an antiferromagnetic ordered state of the spins. The result also agrees with a recent density functional analysis, which gives the G-type antiferromagnetic spin ordering in the ground state [23].
When increasing the temperature, the compound changed from an antiferromagnet to a paramagnet at T c . The MS at 423 K (figure 7(c)) consisted of a paramagnetic singlet with an IS of 0.17 mm s −1 , which is an intermediate value between ≈0.07 mm s −1 for an Fe 4+ state and ≈0.35 mm s −1 for an Fe 3+ state [24,25]. The absence of a magnetic contribution was confirmed in the NPD pattern obtained at 425 K. The temperature dependence of the magnetic susceptibility, hyperfine field (HF) in MS and spin moment of Fe 3+ obtained from the NPD data is plotted in figure 9. The anomaly in χ-T at T c corresponds to an antiferromagnetism-toparamagnetism transition, and the small increase in χ at low temperature is probably due to the small amount of indiscernible paramagnetic impurities. The HF in MS decreased gradually with increasing temperature and, at T c , fell to zero. The obtained magnetic moment of Fe from NPD refinement, 4.03(2) µ B at 50 K, similarly decreased gradually to 3.28(3) µ B at 375 K and then fell to zero when the intermetallic charge transfer transition occurred. The magnetic transition at T c was of first order and was not a normal thermal evolution of the magnetic moment. The spin fluctuations do not result from valence fluctuations as the charge transfer transition is approached, but instead they reflect the intrinsic strength of the magnetic interaction between the Fe spins that would determine the antiferromagnetic Néel temperature if the intermetallic charge The inset shows the result obtained by fitting data in the temperature range between 100 and 300 K to a 3D VRH model.
than the apparent magnitude of ≈k B T c , even though the intermetallic charge transfer drives the actual antiferromagnetism-to-paramagnetism transition at 393 K.

Insulator-to-metal transition
In accordance with the cation valence changes due to the intermetallic charge transfer, the electronic properties of the compound also changed. In the A-site-ordered double perovskites, the AO 4 units are spatially isolated from each other, and the electrical properties are thus determined mainly by the corner-sharing BO 6 octahedral network. As shown in figure 10, the low-temperature phase with Fe 3+ showed insulating behavior. The temperature dependence of resistance between 100 and 300 K can be fitted using a Mott 3D variable-range hopping (VRH) model: R(T ) = R 0 exp(T 0 /T ) 1/4 (inset of figure 10). An insulator-to-metal transition occurred at T c , and the observed thermal hysteresis in resistance is in agreement with the first-order nature of the intermetallic charge transfer transition. Above T c the mixed valence state of the B-site Fe 3.75+ made the compound metallic.
The insulator-to-metal transition in LaCu 3 Fe 4 O 12 is different from those observed in TM oxides with a simple perovskite structure. Chemical doping by substitution is a conventional way of modulating the carrier concentration and/or the width of bands of TM ions, which often gives rise to an insulator-to-metal transition [26]. In the present A-site-ordered double perovskite LaCu 3 Fe 4 O 12 , in contrast, the insulator-to-metal transition is a result of the intermetallic charge transfer between the A-site Cu and B-site Fe changing the electronic states of these two cations simultaneously. This change in electronic structure can be regarded as an 'internal doping' between Cu and Fe without involving any external doping elements.

Intermetallic charge transfer in BiCu 3 Fe 4 O 12
BiCu 3 Fe 4 O 12 also crystallized with the A-site-ordered perovskite structure. The structural parameters of BiCu 3 Fe 4 O 12 that were refined from SXRD data taken at 300 K are listed in   [27], but Bi 3+ has a lone pair of 6s 2 electrons that often causes crystal structure distortion due to a stereochemical effect. The present A-site-ordered perovskite BiCu 3 Fe 4 O 12 , however, showed a cubic symmetry and thus Bi 3+ did not manifest a lone pair effect.
One sees in figure 11 that BiCu 3 Fe 4 O 12 , like LaCu 3 Fe 4 O 12 , showed an isostructural phase transition associated with a volume contraction. The contraction was smaller (about 0.6%), however, and occurred at a higher temperature (between 420 and 440 K). The lengths of the Cu-O bonds in the square-planar-coordinated CuO 4 unit and the Fe-O bonds in the FeO 6 octahedron changed significantly at the transition temperature ( figure 12(a)). The valence states of Cu and Fe calculated from BVS changed correspondingly ( figure 12(b)), indicating that an intermetallic charge transfer occurred and that the compound changed from BiCu 3+ 3 Fe 3+ 4 O 12 below T c to BiCu 2+ 3 Fe 3.75+ 4 O 12 above T c . The temperature dependences of magnetic susceptibility and normalized resistivity ( figure 13) show that the intermetallic charge transfer transition also causes antiferromagnetism-to-paramagnetism and insulator (semiconductor)-to-metal transitions.  The transition temperature obtained from the temperature derivative of the susceptibility was 428 K, which was higher than that of LaCu 3 Fe 4 O 12 . This implies that the Cu 3+ state in the square planar coordination at the A sites is more stable in BiCu 3 Fe 4 O 12 than in LaCu 3 Fe 4 O 12 . Because 6s and 6p orbitals in Bi often hybridize with 2p orbitals in O in oxides, the stabilization of Cu 3+ may be mediated through the orbital hybridization in Bi-O-Cu bonds. Although the Bi cations at the A sites do not take part in the charge transfer, they play a role in stabilizing the square-planar-coordinated Cu 3+ and thereby increase the intermetallic charge transfer transition temperature.

Charge transfer and charge disproportionation
Ca 2+ substitution at the A sites produced another A-site-ordered double perovskite, CaCu 3 Fe 4 O 12 , but this compound showed a completely different transition: charge disproportionation [28]. When this compound is cooled, at 210 K the unusually high valence state of the Fe 4+ ions at the B sites is resolved by a charge disproportionation from Fe 4+ to Fe 3+ and Fe 5+ in a phase transition from CaCu 3 Fe 4+ 4 O 12 to CaCu 3 (Fe 3+ 2 Fe 5+ 2 )O 12 . This charge disproportionation is similar to that seen in the distorted orthorhombic perovskite CaFeO 3 [25]. In the present (La/Bi)Cu 3 Fe 4 O 12 , on the other hand, the instability of the unusually high valence  The unusually high valence states of TM ions like Fe 4+ and Cu 3+ in oxides have very low-lying 3d levels and thus make covalent electronic states with ligand holes L [29]. The charge disproportionation in CaCu 3 Fe 4 O 12 (2Fe 4+ → Fe 3+ + Fe 5+ ) is thus expressed as (2d 5 L → d 5 + d 5 L 2 ), and the disproportionation at 210 K is thought to be due to the redistribution of the ligand holes in the Fe-O bonds. This redistribution makes the Fe-O bonds alternately shorter and longer, producing the 1 : 1 rock-salt-type ordering with Pn-3 symmetry, which appears to be a reasonable way to minimize the lattice energy. In (La/Bi)Cu 3 Fe 4 O 12 with the fractional 3.75+ state of Fe, on the other hand, charge disproportionation (8Fe 3.75+ → 5Fe 3+ + 3Fe 5+ ) may be possible but the large lattice energy makes it hard to form a simple ordered redistribution of the ligand holes. Instead, the intermetallic charge transfer between the A-site Cu ions and the B-site Fe ions (3Cu 2+ + 4Fe 3.75+ → 3Cu 3+ + 4Fe 3+ ) is mediated by the redistribution of the ligand holes from the Fe-O bonds to the Cu-O bonds (3d 9 + 4d 5 L 0.75 → 3d 9 L + 4d 5 ) while keeping the cubic Im-3 symmetry. The redistribution of the ligand holes from the Fe-O bonds to the Cu-O bonds does not need much energy because all the oxygen atoms in the A-site-ordered double perovskite structure are shared by the FeO 6 octahedra and the CuO 4 square-planar units. Thus, the intermetallic charge transfer energy overcomes the charge disproportionation energy in (La/Bi)Cu 3 Fe 4 O 12 and the unusually high valence state of Fe 3.75+ is resolved by the intermetallic charge transfer at a low temperature [30].

Summary
A temperature-induced Cu-Fe intermetallic charge transfer phenomenon found in A-siteordered double perovskites RCu 3 Fe 4 O 12 (R = La, Bi) was reviewed. The materials were prepared under high-pressure and high-temperature conditions. In the ground state a very rare Cu 3+ state was stabilized at the square-planar-coordinated A sites and the charge combination was RCu 3+  The energy levels of the unusually high Cu 3+ and Fe 3.75+ states were comparable so that changing the temperature triggered the intermetallic charge transfer. This charge transfer resulted in a first-order isostructural phase transition accompanied by a large contraction of unit cell volume and by antiferromagnetismto-paramagnetism and insulator-to-metal transitions. Substitution of Bi for La stabilized the Cu 3+ state at the square-planar-coordinated A sites and raised the intermetallic charge transfer transition temperature.