1 : 1 Ca2+:Cu2+ A‐site Order in a Ferrimagnetic Double Double Perovskite

Abstract Cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we report discovery of CaCuFeReO6 with the tetragonal AA′BB′O6 double double perovskite structure that was previously only reported for A′=Mn compositions. CaCuFeReO6 occurs in the same phase field as CaCu3Fe2Re2O12 demonstrating that different A‐cation ordered peroskites may be obtained in the same chemical system. CaCuFeReO6 has ferrimagnetic order of Fe, Re and Cu spins below T C=567 K, in contrast to Mn analogues where the Mn spins order separately at much lower temperatures. The magnetoresistance of CaCuFeReO6 displays low‐field “butterfly” hysteresis with an unusual change from negative to positive values as field increases. Many more AA′BB′O6 double double perovskites may be accessible for A′=Cu and other divalent transition metals at high pressure, so the presently known phases likely represent only the “tip of the iceberg” for this family.

leads to ferrimagnetism and large magnetoresistance in Sr 2 FeMoO 6 and related materials. [7][8][9] More complex perovskites featuring order of alkali metal, alkaline earth or rare earth A cations with A' transition metal ions in addition to the latter B/B' order enable effects of the magnetic A' transition metal ions on the B/B' ferrimagnetism to be explored. Two such classes of compounds have been reported, both requiring high pressure synthesis to stabilise cation order based on size difference between the large A and small transition metal A' cations as substantial tilts of the B/B'O 6 octahedra reduce the A' coordination number from 12 in the ideal perovskite structure to 4 in the ordered derivatives. AA' 3 B 2 B' 2 O 12 (referred to as 1322-type) materials have a cubic (space group Pn-3) structure with 1 : 3 order of 12-coordinate A and 4-coordinate square-planar A' cations, and are formed predominantly for A' = Cu. Ferrimagnets with Curie temperatures (T C ) > 300 K are found for CaCu 3 B 2 B' 2 O 12 (B/B' = Fe/Re, Fe/Os, Mn/Os) [9][10][11] and ACu 3 Fe 2 Os 2 O 12 (A = Na, La) [12,13] materials, with lower temperature ferri-or antiferro-magnetic orders in other analogues; CaCu 3 B 2 B' 2 O 12 (B/B' = Ga/Sb, Cr/Sb, Fe/Sb, Fe/ Nb), [14][15][16][17] LaCu 3 Co 2 Re 2 O 12 [18] and LaMn 3 Ni 2 Mn 2 O 12 . [19] The latter is the only A' = Mn (and non-Cu) 1322 phase reported to date.
The second group of A/A' and B/B' ordered perovskites have AA'BB'O 6 stoichiometry and are labelled double double (or doubly ordered) perovskites (DDPv's) following the "double perovskite" description for AA'B 2 O 6 or A 2 BB'O 6 materials with cation order at only one type of site. [3] They have tetragonal P4 2 /n symmetry with columnar ordering of 10-coordinate A and 4-coordinate A' cations, and all previously reported materials are based on A' = Mn. RMnMnB'O 6 (R = rare earth) series are reported for B' = Sb [20,21] and Ta, [22] and many CaMnBB'O 6 materials (B = Mn, Fe, Co, Ni for B' = Re, [23,24,25] and B/B' = Fe/Ta, [26] Cr/Sb and Fe/Sb [27] ) were also synthesised at high temperatures and pressures. High temperature ferrimagnetism was reported in CaMnFeReO 6 (T C = 500 K) and also a Cu-doped derivative of nominal composition Ca(Mn 0.5 Cu 0.5 )FeReO 6 (T C = 560 K), with an unusual switch between negative and positive magnetoresistances in the latter at low temperatures. [23] The latter material suggests that DDPv's stabilised by A/Cu ordering, without any Mn content, may be accessible. This offers the exciting possibility of being able to stabilise AA' 3 B 2 B' 2 O 12 (1322) and AA'BB'O 6 (DDPv) cation ordered perovskites in the same AÀ A'(=Cu)-BÀ B' chemical system for the first time, and hence compare effects of the 1 : 3 and 1 : 1 A : Cu orders on the magnetic and electronic properties of the B/B' sublattice. Here we report synthesis of a Mnfree, high-T C DDPv material CaCuFeReO 6 and comparison of properties to the previously reported CaCu 3 Fe 2 Re 2 O 12 . [9] Samples of nominal composition CaCuFeReO 6 were heated under pressures of 10-15.5 GPa using stoichiometric mixtures of Ca 2 Fe 2 O 5 , ReO 2 and CuO in a Walker-type module as described in Supporting Information. A component with the P4 2 /n DDPv type structure was identified by powder X-ray diffraction within multiphase products. Optimum samples were synthesised at 15.5 GPa and 1400°C and were characterised by powder synchrotron X-ray diffraction at 300 K, neutron diffraction patterns from 5 to 550 K, and magnetic and electronic transport measurements (details in Supporting Information).
The crystal structure of CaCuFeReO 6 was refined using synchrotron X-ray [28] and neutron powder diffraction profiles. 6.8(2)% Fe/Re antisite disorder was found from the Xray fit (full results are in Supporting Information) and this value was fixed in all neutron refinements. The A' = Cu cation columns have two inequivalent sites with tetrahedral (Cu1) and square planar (Cu2) coordinations. Refinement against powder neutron data ( Figure 1) showed that 29(1) and 0(1)% of Fe was substituted at the Cu1 and Cu2 sites, respectively, so their occupancies were fixed as 70 %Cu/ 30 %Fe and 100 %Cu in subsequent variable temperature neutron fits.
Magnetic diffraction peaks from CaCuFeReO 6 (Figure 1) were observed in all neutron diffraction patterns from 5 to 550 K. Peaks from ordered moments at both A-site Cu and B/B' Fe/Re are observed. The magnetic peaks are indexed by propagation vector [0 0 0] and good fits were obtained from a ferrimagnetic model with spins parallel to the c-axis ( Figure 2). The small Re spin was found to be unstable in initial magnetic refinements and was constrained to be antiparallel to the Fe moment as done in previous refinements of Mn-based CaA'FeReO 6 materials. [23] The two Cu site spins were both found to be parallel to the Fe spin and were constrained to be equal in the final refinements. Results of the refinement of crystal and magnetic structures against powder neutron diffraction data are shown in Figures 1 and 2 and are tabulated in Supporting Information.
The crystal structure refinements demonstrate that the present CaCuFeReO 6 sample is slightly off-stoichiometric with refined composition CaCu 0.85 Fe 1.15 ReO 6 and also with 7 % Fe/Re antisite disorder. Fe shows a clear preference for substituting at the tetrahedral (Cu1) rather that the square planar (Cu2) site. Nevertheless the material shows a very high degree of order of four different metals over five cation sites, and demonstrates that the double double perovskite arrangement, previously reported only for Mn-based materi-   6 with Fe, Re and Cu magnetic moments as red, grey and blue arrows, respectively. b) Refined magnetic moments vs. temperature with critical functions fitted in the T C /2 < T < T C temperature range. als, can be stabilised at pressure by Ca/Cu ordering over A/ A' columns.
Bond valence sum calculations using the 320 K neutron bond lengths give BVS values Ca = 2.3, Cu1 = 1.6, Cu 2 = 1.5, Fe = 2.6 and Re = 5.1. These are consistent with the ideal valence distribution Ca 2 + Cu 2 + Fe 3 + Re 5 + O 6 which is analogous to those in related materials such as Ca 2 + Mn 2 + Fe 3 + Re 5 + O 6 , [23] allowing for disorder at some sites and structural strain induced by high pressure synthesis. Octahedral tilt angles calculated from FeÀ OÀ Re angles along the c axis (ϕ = 19.3(4)°) and within the ab plane (θ = 17.4(2)°) are within the range of 15 to 19.5°for Mn-based analogues [20] and thus demonstrate that these substantial tilts are essential to stabilisation of this DDPv structure type.
The same ferrimagnetically ordered spin structure (Figure 2a) was observed in CaCuFeReO 6 from 5 to 550 K, and thermal variations of the refined magnetic moments are shown in Figure 2b. The two independently refined magnetic moments (Fe/Re and Cu1/Cu2) both follow the same critical dependence � [1À (T/T C )] β with fitted T C = 566.70(3) K and β = 0.254 (2). The latter is close to the ideal mean field value of β = 0.25 for a tricritical transition, on the cusp between first and second order behaviour. This may reflect the complex network of interactions between four distinct spin sublattices. Ordered moments at 5 K of 3.0 and 1.2 μ B for Fe 3 + and Re 5 + (refined in a 5 : 2 ratio) are reduced from ideal values of 5 and 2 μ B reflecting B/B' antisite disorder and spin-orbit coupling for Re 5 + . However the average moment at 5 K for the Cu sites of 1.4 μ B exceeds the ideal 1 μ B value for Cu 2 + showing that the Fe spins substituted at the Cu sites are also aligned. Mössbauer spectroscopy would be useful for further characterisation of the Fe states at the Cu sites.
Physical properties of CaCuFeReO 6 were explored through magnetisation and resistivity measurements in zero and applied fields. Magnetic susceptibility, shown in Figure 3a, reveals a single Curie transition. The estimated T C = 551 K, from extrapolation of the steepest negative χ-T slope to χ = 0, is in agreement with the value of T C = 567 K from fitting the zero field neutron moments above. A Curie-Weiss fit to data at 600-625 K gives realistic results despite the narrow fitting range; an effective paramagnetic moment of μ eff = 6.52 μ B /f.u., close to the spin-only estimate of 6.78 μ B /f.u., and a Weiss temperature of θ = 522 K, comparable to T C and showing that 3D ferrimagnetic order occurs with little frustration. Magnetisation-field hysteresis loops confirm the ferrimagnetic order, with a saturated magnetic moment of 4 μ B /f.u. (Figure 3b) at 2 and 100 K identical to the value expected for order of S = 1 = 2 " Cu 2 + , S = 5/2 " Fe 3 + , and S = 1 # Re 5 + spins, confirming the arrangement derived from neutron diffraction. The saturated moment decreases to 2.7 μ B at 400 K (see Supporting Information). Small coercive fields of 120 and 97 mT are observed at 2 and 100 K respectively, reflecting the weak anisotropy of the tetragonal DDPv structure.
The resistivity of a ceramic pellet of CaCuFeReO 6 is 140 mΩ cm at room temperature increasing to 210 mΩ cm to 4 K (Figure 2a). This change corresponds to an energy gap of � 1 meV which is unrealistically small for intrinsic semi-conducting behaviour suggesting that grain boundary resistances mask an underlying metallic conductivity. Magnetoresistance (MR) values in Figure 3b are small, most likely due to the Fe/Re antisite disorder, but show interesting variations with field. The negative low field effect is a typical spin-valve-type magnetoresistance due to the intergrain tunneling of spin-polarized conduction carriers, as observed in Sr 2 FeMoO 6 and other ferromagnetic double perovskites. However a positive MR effect dominates at high fields. A similar switch from negative to positive MR was reported in the related Ca(Mn 0.5 Cu 0.5 )FeReO 6 DDPv [23] and in the DPv Mn 2 FeReO 6 [29,30] due to canting of Fe and Re spins arising from frustration of their order with respect to the Mn spins as field increases, and a similar mechanism may operate here due to their interactions with the Cu site spins. Low field "butterfly" MR hysteresis curves are observed but the hysteresis in MR differs slightly from that in the magnetization as the peak-to-peak MR separations do not coincide with the coercive fields. The behavior is also observed in Sr 2 FeMoO 6 and the 1322-type analogue to CaCuFeReO 6, CaCu 3 Fe 2 Re 2 O 12 , [9] and for epitaxial Fe 3 O 4 films and NiCo 2 O 4 [31,32]  CaCuFeReO 6 is an important discovery in the field of cation-ordered perovskites as it demonstrates that the P4 2 /n AA'BB'O 6 double double perovskite (DDPv) structure already reported for many RMnBB'O 6 and CaMnBB'O 6 materials can also be stabilised for A' = Cu. CaCuFeReO 6 is the first reported Mn-free DDPv. Synthesis of CaCuFeReO 6 has proved more challenging than for the CaMnFeReO 6 analogue. [23] The present neutron sample of CaCuFeReO 6 was recovered from 15.5 GPa and 1400°C, has 82 % phase purity, with 7 % Fe/Re antisite disorder and 30 % Fe substituted at one of the Cu sites, whereas CaMnFeReO 6 , prepared at 10 GPa and 1400°C was > 99 % phase pure with 3 % Fe/Re antisite disorder and 8 and 16 % Fe at the two Cu sites. Nevertheless this study indicates that increasing pressure significantly above 10 GPa may enable many more AA'BB'O 6 DDPvs to be synthesised for A' = Cu and perhaps other divalent A' = Fe, Co, Ni… cations, so the presently known phases likely represent only the "tip of the iceberg" of DDPv materials.
Interesting differences between the physical properties of CaCuFeReO 6 and CaMnFeReO 6 are apparent. Both have high T C 's for ferrimagnetic Fe/Re spin order, of 567 and 500 K respectively, typical for A 2 FeReO 6 double perovskites, but the Cu spins also order at T C in CaCuFeReO 6 whereas the A-site Mn spins in CaMnFeReO 6 order at a separate transition at T A = 70 K. [9] Order of Mn 2 + spins at T A = 75 K, well below T C = 520 K, is also seen in the double perovskite Mn 2 FeReO 6 . [28,29] These differences evidence stronger Fe/ReÀ OÀ A' superexchange for A' = Cu compared to A' = Mn, and perhaps also a shift in relative values of the various exchange interactions so that A' spins in CaCuFeR-eO 6 are less magnetically frustrated. CaMnFeReO 6 has negative MR diverging to large values at low temperature and high field (À 32 % at 20 K and 7 T), typical for ferrimagnetic double perovskites like Sr 2 FeMoO 6 , [7] whereas CaCuFeReO 6 shows a switch from negative to positive MR with increasing field. A similar switch in Mn 2 FeReO 6 results from increased canting of Fe and Re spins due to frustration of their order with respect to the Mn spins as field increases. High field neutron diffraction experiments would be needed to verify the same effect in CaCuFeReO 6 . It is also interesting to compare the mixed DDPv Ca-(Mn 0.5 Cu 0.5 )FeReO 6 [23] to CaA'FeReO 6 (A' = Cu and Mn). Ca(Mn 0.5 Cu 0.5 )FeReO 6 has T C = 560 K and a separate ordering transition at T A = 150 K unlike in CaCuFeReO 6 , but the increase relative to T A = 70 K in CaMnFeReO 6 demonstrates the effect of Cu in strengthening Fe/Re-OÀ A' superexchange. However Ca(Mn 0.5 Cu 0.5 )FeReO 6 displays a switch from negative to positive MR on warming, as seen in CaCuFeReO 6 (e.g. by comparing 2 and 100 K MR values in a 3 T field in Figure 3b) but not in CaMnFeReO 6 . Such complex MR behaviour is therefore not directly linked to the presence (or absence) of a distinct ordering transition of A-site spins, but rather to field effects on competing A'À OÀ B, A'À OÀ B' and BÀ OÀ B' interactions within AA'B-B'O 6 DDPvs with ordered A', B and B' spin sublattices.
Formation of CaCuFeReO 6 is also important because a 1322 type phase CaCu 3 Fe 2 Re 2 O 12 (written here as Ca 1/2 Cu 3/ 2 FeReO 6 for comparison) is also known and so this is the first demonstration that two different A-cation ordered perovskites can be prepared within the same chemical system. Comparison of their properties is again instructive. CaCuFeReO 6 and Ca 1/2 Cu 3/2 FeReO 6 have similar T C 's of 567 and 560 K with Cu spins ordering parallel to Fe spins at T C leading to large saturated magnetisations of 4.0 and 4.35 μ B respectively, the latter value reflecting the extra 0.5 Cu 2 + spin per double perovskite unit. Both materials have apparent metallic conductivity but with resistive grain boundaries in ceramic samples. A high spin-polarization of conduction electrons was reported for Ca 1/2 Cu 3/2 FeReO 6 and both materials display low-field "butterfly" spin-valve MR effects although the magnitude of MR is small. However, Ca 1/2 Cu 3/2 FeReO 6 has negative MR while CaCuFeReO 6 shows an unusual switch to positive values at high field. Exploration and comparison of DDPv, 1322 type, and perhaps other types of A and B-site ordered perovskites in mixed transition metal oxide systems is likely to lead to discovery of spintronic and other types of interesting property.
In conclusion, discovery of CaCuFeReO 6 marks an important milestone in development of A-and B-cation ordered perovskites, as it demonstrates that the tetragonal AA'BB'O 6 double double perovskite structure, previously only reported for A'=Mn, can be stabilised by Cu and perhaps other metals at pressures above 10 GPa. CaCuFeR-eO 6 occurs in the same phase field as 1322-type CaCu 3 Fe 2 Re 2 O 12 demonstrating that different A-cation ordering types may be obtained in the same chemical system. Ferrimagnetic Fe (up), Re (down) and Cu (up) spin order occurs in CaCuFeReO 6 below T C = 567 K and magnetoresistance shows low-field spin-valve-type butterfly hysteresis with an unusual change from negative to positive values as field increases.