Preparation, Crystal Structure, and Magnetotransport Properties of the

Article pubs.acs.org/JPCC

Preparation, Crystal Structure, and Magnetotransport Properties of the New CdCu3Mn4O12 Perovskite: A Comparison with Density Functional Theory Calculations Javier Sánchez-Benítez,† Paula Kayser,*,‡ Á ngel Morales-García,† María Jesús Martínez-Lope,‡ Federico J. Mompeán,‡ Jianmei Xu,§ Zhenmin Jin,§ and José Antonio Alonso‡ †

Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Madrid E-28040, Spain Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, Madrid E-28049, Spain § Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, China ‡

ABSTRACT: The A-site ordered perovskite oxide CdCu3Mn4O12 has been synthesized for the first time in polycrystalline form under high pressure (7 GPa) and high temperature (1000 °C) conditions, required to stabilize the small Cd and Cu cations at the A positions of the perovskite. The crystal structure has been studied by X-ray powder diffraction at room temperature. This oxide crystallizes in the cubic space group Im3̅ (no. 204) with the unit-cell parameter a = 7.2179(5) Å at 300 K. The MnO6 network is extremely tilted, giving rise to a square planar coordination for Cu2+ cations. The magnetic characterization shows that this compound is ferrimagnetic with an ordering temperature TC = 347 K, well above room temperature. A metallic behavior is displayed between 10 and 300 K. Negative magnetoresistance (MR) of 15% is achieved at 10 K for H = 9 T; MR is still significant at room temperature, displaying values above 7% for H = 9 T. Density functional theory calculations carried out on the density of states lead to electronic and magnetic features in good agreement with the obtained experimental results.

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INTRODUCTION

The replacement for aliovalent transition-metal ions at the three crystallographic sites and possible interactions between these ions such as A′−A′ and A′−B as well as B−B are at the origin of the rich variety of chemical and physical properties.9,10 As mentioned before, the A site accommodates monovalent alkaline metals (typically Na), divalent alkaline-earth metals (Ca or Sr), and trivalent lanthanides or Bi as observed in simple ABO3 perovskites. The use of Cd as an alternative divalent A cation has only been explored in CdCu3Ti4O122,11,12 and CdCu3Ru4O12.13 In this paper, we describe the preparation, for the first time, of CdCu 3 Mn 4 O 12 , under high-pressure conditions, and its structural, magnetic, and magnetotransport properties. A phase segregation into two perovskite phases with Cu or (Cu, Mn) at the A site is observed, characterized by different ferrimagnetic transition temperatures. The experimental data are contrasted with the results of ab initio density functional theory (DFT) calculations of the electronic and magnetic features, that is, the density of states (DOS) and magnetism, finding a good agreement between both sets of data.

The 1:3 ordered arrangement of the A-site ions in ABO3-type perovskite oxides gives rise to a wide family of compounds with the general formula AA′3B4O12.1 These complex perovskites exhibit an ample variety of intriguing and unexpected properties, from a giant dielectric constant for CaCu3Ti4O12,2−4 valence degeneracy of Cu and Ru in ACu3Ru4O12,5,6 to a large magnetoresistance (MR) effect for CaCu3Mn4O127 and ferrimagnetic ordering above room temperature for RCu3Mn4O12 series.8 The crystal structure of these compounds has the originality of containing Cu2+ at the A positions of the ABO3 perovskite; this Jahn−Teller cation and A atoms (A is generally a monovalent or divalent or rare-earth cation) are 3:1 ordered in a 2a0 × 2a0 × 2a0 cubic cell of Im3̅ symmetry (where a0 is the unit cell of the perovskite aristotype). These perovskites are strongly distorted, showing an important tilting of the BO6 oxygen octahedra, given the small size of the cations at the A positions, in such a way that they conform an effective square-planar coordination for the Cu ions (CuO4 units). The A site accommodates 12-fold coordinated ions, while B cations are within slightly distorted oxygen octahedra.7 AA′3B4O12 compounds are often synthesized under high-pressure and high-temperature conditions in order to stabilize the squarecoordinated Jahn−Teller ions in the original 12-fold coordinated positions of the perovskite structure. © 2014 American Chemical Society

Received: January 17, 2014 Revised: April 1, 2014 Published: April 8, 2014 9652

dx.doi.org/10.1021/jp500562e | J. Phys. Chem. C 2014, 118, 9652−9658

The Journal of Physical Chemistry C

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were approximated using the method of Monkhorst and Pack,20 and the energies were converged with respect to the k-points density (6 × 6 × 6) and the plane-wave cutoff (480 eV). The geometry relaxation was considered to be complete when the total force on the atoms was less than 1 meV/Å. The DOS was calculated using the linear tetrahedron method and a significant increment in the number of k points. The application of the GGA+U method to a complex compound such as CdCu3Mn4O12 (Cd2+, Cu2+, and Mn4+) can be problematic. First of all, Cd2+ has a completely filled d shell, and thus, no special treatment is required as far as the electronic structure is concerned. Second, it is necessary to study the influence of localized d electrons in Cu2+ and Mn4+ by means of GGA+U. These calculations were performed following the simplified rotationally invariant form proposed by Dudarev et al.21 Within this approach, the on-site Coulomb term, U, and the exchange term, J, can be grouped together into a single effective parameter (U−J). J is constant at 1 eV for many ions, including Cu 2+ and Mn 4+ . One problem encountered using the GGA+U method is the determination of an appropriate U value for each compound. A suitable route consists of selecting these values so as to account for the experimental results of physical properties. Previous works found a value of U = 5 eV for Mn4+22 and Cu2+23 by fitting calculated electronic properties. In this work, we have studied simultaneously 3, 5, and 7 eV as U effective values for both Cu2+ and Mn4+. However, experimentally, our system is a ferrimagnetic material, and when we perform a calculation with GGA+U, the result is that CdCu3Mn4O12 is ferromagnetic, while performing a pure GGA calculation the ferrimagnetic behavior is successfully reproduced. Controversial results are obtained when different properties (i.e., structural, electronic, stability, or magnetic properties) are studied for the same systems.24 For these reasons, all the calculation results presented in this work correspond to a pure DFT calculation (GGA) without U contribution.

EXPERIMENTAL SECTION The high-pressure synthesis of CdCu3Cu4O12 perovskite requires the previous preparation of very reactive precursors, via the citrate−nitrate method. Stoichiometric amounts of Cd(NO3)2·4H2O, Cu(NO3)2·3H2O, and MnCO3 were dissolved in a citric acid aqueous solution with some droplets of HNO3. This solution was then slowly evaporated leading to organic resins that contain a homogeneous distribution of the involved cations. After evaporation, the resulting resins were dried at 140 °C and then heated at 600 °C for 12 h in order to decompose the organic materials and eliminate the nitrates. KClO4 (30% in weight), which acts as an oxidizing agent, was ground with the precursor powder, and the mixture was put into a gold capsule with 2.5 mm internal diameter. The highpressure treatment was carried out using a Walker-type multianvil press from Rockland Research. The capsule was introduced into a LaCrO3 furnace and set in a 12 mm edge MgO octahedron. The sample was compressed over 20 h to 7 GPa by eight truncated tungsten carbide cubes. The reaction took place at 7 GPa and 1000 °C for 1 h. After this time, the temperature was quenched, and the pressure was slowly released (during 12 h) until ambient conditions. A fraction of the raw product, obtained as a dense, homogeneous pellet, was partially ground to perform the structural and magnetic characterization; some as-grown pellets were kept for magnetotransport measurements. Phase identification and crystal structure analysis of the obtained compound were carried out by X-ray diffraction (XRD), using a Bruker-axs D8 diffractometer (40 kV, 30 mA), controlled by DIFFRACTplus software, in Bragg−Brentano reflection geometry with CuKα radiation (λ = 1.5418 Å) and a position sensitive detector (PSD). The data were collected between 10° and 90° 2θ in steps of 0.05°. The slit system was selected to ensure that the sample was completely within the Xray beam in the used angle range of 2θ. XRD data were refined by the Rietveld method,14 using the FULLPROF refinement program.15 A pseudo-Voigt function was chosen to generate the line shape of the diffraction peaks. No regions were excluded in the refinement. The following parameters were refined in the final runs: scale factors, background coefficients, zero-point error, unit-cell parameters, pseudo-Voigt corrected for asymmetry parameters, positional coordinates, and overall displacement factor. Magnetic properties of the sample were measured in a commercial superconducting quantum interference device magnetometer (Quantum Design). Field-cooled (FC) dc magnetic susceptibility data were collected in the 4 ≤ T ≤ 400 K range under an applied magnetic field of 0.1 T. The magnetization versus magnetic field isotherms were collected at 4 and 330 K under fields between −5 T and 5 T. Transport and magnetotransport measurements were performed by the conventional four-probe technique, under magnetic fields up to 9 T in a PPMS system from Quantum Design.

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RESULTS AND DISCUSSION Crystallographic Structure. The sample was obtained as a black, well-crystallized polycrystalline powder. Figure 1 shows the XRD pattern of the product, containing the characteristic peaks of a perovskite and showing sharp, well-defined superstructure reflections due to the 1:3 ordering of Cd and Cu cations. The structural refinement was performed from room temperature XRD data collected on the CdCu3Mn4O12 sample based on the A-site ordered perovskite structure model with a cubic Im3̅ (no. 204) space group. Cd atoms are placed at 2a (0, 0, 0) positions, Cu at 6b (0, 1/2, 1/2), Mn at 8c (1/4, 1/4, 1/4), and O atoms at 24g (0, y, z). After running the refinement with this first preliminary model, we noticed that the pattern indeed consists of the superposition of two diagrams corresponding to perovskites with slightly different compositions. The main phase is the stoichiometric CdCu3Mn4O12, while the second phase was found to exhibit the same crystallographic structure but with some Mn cations located at the Cu positions and a significantly larger unit-cell parameter. In this second phase, Mn atoms were introduced at random at 6b positions together with Cu, and the complementary occupancy factors were refined, constrained to a full occupancy. The y and z parameters for O atoms were constrained for both phases, as well as the overall displacement factor. After this refinement, the quality of the fit was notably improved, reaching a discrepancy factor of RI =

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CALCULATIONS Calculations on the electronic structure were carried out under the generalized gradient approximation (GGA) of the DFT as implemented in the Vienna ab initio simulation package (VASP).16 The projector-augmented wave (PAW) all-electron description of the electron-ion-core interaction,17,18 and the exchange and correlation functionals proposed by Perdew− Burke−Ernzerhof (PBE)19 were used. Brillouin zone integrals 9653



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cell parameter for the main stoichiometric phase CdCu3Mn4O12 is a = 7.2179(5) Å. Assuming a valence of 2+ for Cd cations and 2+ for Cu at the 6b sublattice, the nominal valence for Mn at 8c positions is 4+. The resulting crystallographic formula for the secondary phase is Cd[Cu2.40(6)Mn0.60(6)]Mn4O12, with a = 7.2794(7) Å. In this case, assuming that Mn ions at the A square-planar positions adopt a trivalent oxidation state (Jahn− Teller ions),25 the nominal valence for Mn at the octahedral 8c positions is 3.85+. This average value corresponds to 85% Mn4+ and 15% Mn3+. The inset of Figure 1 shows a view of the CdCu3Mn4O12 crystal structure. Cd2+ cations in the main perovskite phase are placed at the corners and cube center (m-3 point symmetry) and exhibit a regular environment, coordinated to 12 oxygen atoms, with 12 equal Cd−O distances of 2.569(2) Å. They are considerably shorter than those expected as sums of ionic radii26 for XIICd2+ (1.31 Å) and VIO2− (1.40 Å), of 2.71 Å, indicating that Cd−O bonds are under compressive stress in this perovskite, also accounting for the high-pressure preparation requirement of this material. The oxygen environment for Cu2+ is grossly irregular, with eight rather long distances (>2.8 Å) and an effective coordination number of four; therefore, the local arrangement for Cu is reduced to a pseudosquare-planar coordination (1.949(5) Å), well suited for Jahn−Teller cations. Mn4+ cations occupy the center of the MnO6 octahedra, with Mn−O bond lengths of 1.904(3) Å. This value is slightly shorter than that observed in CaCu3Mn4O12,27 of 1.915(1) Å. The Mn−O−Mn angle becomes 142.86(7)°, far away from 180° of the ideal cubic perovskite. The structure is fairly distorted because of the small size of the Cd2+ and Cu2+ cations, which forces the MnO6 octahedra to tilt in order to optimize Cd−O and Cu−O bond lengths. There is a slight decrease of the lattice parameter with respect to that reported for the CaCu3Mn4O12 compound, of 7.241(1) Å,27 which is concomitant with the smaller ionic radius of Cd2+ cations with respect to Ca2+ (1.34 Å)26 in 12fold coordination. As commented before, atoms located at the A′ positions of the AA′3B4O12 complex structure exhibit a very distorted oxygen atom environment, only suitable for Jahn−Teller transition-metal cations as Cu2+ or Mn3+. Despite the extremely oxidizing conditions during the synthesis, some Mn cations are able to occupy the Cu positions at random as Mn3+ but are segregated in a second phase (CdCu2.4Mn4.6O12) with a distinct composition from the main CdCu3Mn4O12 perovskite. The ratio of both phases probably depends on the preparation pressure and temperature; higher pressures would certainly lead to lower contents on the phase containing Mn at the A′ site; prior studies demonstrated that stoichiometric CaCu3Mn4O12 is not prepared at moderate pressures of 2 GPa, but a phase containing Mn at A′, Ca(Cu2.5Mn0.5)Mn4O12 is stabilized instead.28 As shown in Table 1, the unit-cell parameter for CdCu 2.4 Mn 4.6 O 12 is considerably longer than that of CdCu3Mn4O12, given the mixed valence Mn3+/Mn4+ induced at the octahedral B position by the electron doping effect associated with Cu2+ for Mn3+ substitution. Regarding the ab initio calculations, the geometry optimization was performed on CdCu3Mn4O12 for both nonspin polarization (NSP) and spin polarization (SP) cases. The results indicate that SP is lower in energy than NSP, by 5.76 eV per unit cell. This energy indicates that SP is the appropriate case for the ground state. For comparison, the calculated lattice parameters, bond distances, and bond angles

Figure 1. Observed (circles), calculated (solid line), and difference (bottom) XRD Rietveld profiles for CdCu3Mn4O12 at RT (Cu Kα radiation). First and second series of Bragg reflections correspond to the main perovskite CdCu3Mn4O12 and CdCu2.4Mn4.6O12, respectively. Third and fourth series of Bragg reflections belong to CuO and KCl. The inset represents the cubic crystal structure, highlighting the tilting of MnO6 octahedra and the square-planar coordination of (Cu, Mn) ions. Large spheres represent Cd ions.

2.64% for the main phase. The ratio of the main and secondary perovskite phases is 2.5:1. The occupancy factor for oxygen atoms was assumed to be stoichiometric, since XRD data do not allow its refinement. In the final runs, minor impurities of CuO and KCl (coming from the decomposition of KClO4) were detected and introduced as third and fourth phases, respectively. The good agreement between the observed and calculated patterns is illustrated in Figure 1. The refined structural parameters at room temperature as well as the reliability factors are summarized in Table 1. The obtained unitTable 1. Structural Parameters for the Perovskite CdCu3Mn4O12 and the Secondary Phase CdCu2.4Mn4.6O12, Both Defined in the Cubic Im3̅ (No. 204) Space Group, Z = 2, from XRD Data at 295 Ka a (Å) V (Å3) B (Å2) Cd Cu/Mn focc (Cu) Mn O

Cd−O (Å) (×12) Cu−O (Å) (×4) Mn−O (Å) (×6) Mn−O−Mn (deg) Reliability factors Rp (%) Rwp (%) χ2 RI (%)

CdCu3Mn4O12

CdCu2.4Mn4.6O12

7.2179(5) 376.04(5) 0.86(1)

7.2794(7) 385.73(6) 0.86(1)

1.00

0.80(2)

0.3038(6) 0.1855(7) 2.569(2) 1.949(5) 1.904(3) 142.86(7)

0.3038(6) 0.1855(7)

1.03 1.31 0.57 2.64

1.03 1.31 0.57 5.79

2a (0, 0, 0) 6b (0, 1/2, 1/2) 8c (1/4, 1/4, 1/4) 24g (0, y, z) y (O) z (O)

a

Main interatomic bond distances (Å) and selected angles (deg) are also included. 9654



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Table 2. Experimental (exp) and Theoretical-GGA (theor) Results for ACu3Mn4O12 (A = Ca and Cd) Are Shown for Comparisona A

Caexpb

Catheorc

Cdexpc

Cdtheorc

a (Å) V (Å3) O (x, y, z) M−O (Å) (×12) Cu−O (Å) (×4) Mn−O (Å) (×6) Mn−O−Mn (deg)

7.241 379.7 0, 0.3033, 0.1822 2.562 1.941 1.915 141.934

7.250 381.11 0, 0.3018, 0.1801 2.548 1.941 1.919 141.622

7.218 376.04 0, 0.3038, 0.1855 2.569 1.949 1.904 142.86

7.254 381.73 0, 0.3032, 0.1807 2.561 1.938 1.921 141.469

a

a, V, and O(i) are lattice parameter, volume of the unit cell, and internal coordinate of oxygen, respectively. Bond distances and bond angles are also shown in the table. bReference 27. cPresent work.

of CdCu3Mn4O12 and CaCu3Mn4O12 are listed in Table 2. We compare both systems because CaCu3Mn4O12 is a paradigmatic oxide with appealing colossal MR properties7,29 that should keep a close analogy with the new CdCu3Mn4O12 oxide. The volume obtained by first-principles calculations is slightly higher than the experimental one (GGA usually overestimates the lattice parameters) but is in good general agreement with the diffraction results. Magnetic Measurements. The susceptibility versus temperature data (Figure 2) shows saturation characteristic of

Figure 3. Magnetization versus magnetic field isotherms of CdCu3Mn4O12 at 4 and 330 K measured under a magnetic field ranging from −5 to 5 T.

hysteresis. The observed saturation magnetization at 4 K is 7.4 μB per formula unit. By comparison with CaCu3Mn4O12,29 which exhibits a perfect ferrimagnetic arrangement of the A and B substructures, for the stoichiometric CdCu3Mn4O12 (Cu2+, S = 1/2; Mn4+, S = 3/2; Cd2+ is nonmagnetic), the expected saturation magnetic moment would be 9 μB per formula unit. By taking into account the secondary phase where the spins of the Mn atoms located at Cu positions order antiparallel to the magnetic moments of the Mn cations at B positions, the total magnetization would decrease, and therefore, we would obtain a lower value for the saturation, closer to the observed value. We have also found a ferrimagnetic behavior for the 330 K isotherm, since TC is above this temperature for the main perovskite phase. Ab initio calculations of the magnetic structure were initiated with magnetic Cu and Mn ions aligned ferromagnetically. The results of the calculations strongly favor an antiparallel coupling of the Mn and the Cu moments, yielding a ferrimagnetic spin ordering. The resulting theoretical moments for CdCu3Mn4O12 are 2.53 and −0.40 μB for Mn and Cu, respectively, compared to the ideal values (S = 3/2, 3 μB for Mn; S = 1/2, 1 μB for Cu). This reduction results from hybridization with the O 2p states, as observed in other transition-metal oxides. The net magnetic moment is 9.02 μB per unit cell, which is also what would be obtained from the formal moments aligned ferrimagnetically. Similar magnetic behavior was determined for the CaCu3Mn4O12 system. In this case, the resulting moments are 2.52 and −0.41 μB for Mn and Cu, respectively, and the net magnetic moment is 9.01 μB. These results are in good agreement with previous studies.29 It seems clear that the substitution of Ca for Cd does not affect the magnetic

Figure 2. Temperature dependence of the dc magnetic susceptibility for the CdCu3Mn4O12 complex perovskite, measured under 0.1 T magnetic field. The upper red curve is the derivative of the susceptibility.

a spontaneous ferromagnetic (or ferrimagnetic) ordering. By taking into account the derivative of the magnetic susceptibility (upper curve in Figure 2), there are two different inflection points, arising from two consecutive Curie temperatures (TC). The main phase, CdCu3Mn4O12, has a magnetic ordering temperature of 347 K, while for the secondary phase (CdCu2.4Mn4.6O12), the observed TC is 264 K. It has been previously reported30−32 in related complex perovskites that TC decreases as Mn atoms are incorporated into the Cu positions. Magnetic order for the main phase occurs at a slightly lower temperature than for the parent CaCu3Mn4O12 compound (TC = 355 K).7 Magnetization versus magnetic field curves, measured at 4 and 330 K, are plotted in Figure 3. The shapes of the loop at 4 and 330 K are characteristic of a ferromagnetic (or ferrimagnetic) material, where a small magnetic field produces a sharp saturation of magnetization; there is a negligible 9655



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properties. Both systems are ferrimagnetic, and the magnetic moment contribution on Ca and Cd to the net magnetic moment is 0 μB. Our conclusion is that, in CdCu3Mn4O12, Cu and Mn are antiferromagnetically coupled, whereas Cu−Cu and Mn−Mn are ferromagnetically (FM) coupled, which is consistent with the experimentally determined magnetic properties. This magnetic configuration is also similar to that found for LaCu3Mn4O12.33 Transport Measurements. Despite the tiny sample coming from the high-pressure synthesis (∼60 mg), we have been able to find a small pellet (1 × 2 × 0.5 mm3) suitable to carry out transport and magnetotransport measurements. As it is observed in Figure 4, CdCu3Mn4O12 oxide exhibits a metallic Figure 5. DOS near the Fermi energy for spin-up and spin-down calculated with GGA. TDOS (up panel) and PDOS (down panel). Black line at 0 eV indicates the Fermi level.

Figure 4. Electrical resistance versus temperature for CdCu3Mn4O12. The inset shows an image of the sample with the four electrical contacts during the transport measurements.

Figure 6. DOS near the Fermi energy for spin-up and spin-down close to Fermi level calculated with GGA. TDOS (up panel) and PDOS (down panel). Black line at 0 eV indicates the Fermi level.

behavior in the measured temperature range (4−300 K). It shows a room temperature resistance of about 7 Ω. The inset in Figure 4 shows a picture of the experimental setup with the four electrical contacts. This low resistivity value, compared to that of the parent CaCu3Mn4O12 compound, of 1.8 × 103 Ω·cm7 suggests an increase in the carrier density, which is an unexpected result. It may be a result of either (i) the influence of the Cd contribution to the DOS or band structure configuration or (ii) the admixture of the secondary perovskite phase where the electron doping effect accounts for a Mn3+/ Mn4+ mixed valence at the octahedral network, thus increasing the conductivity. The calculated total and partial density of states (TDOS and PDOS) within GGA near the Fermi level are presented in Figure 5. To get into the details of the electronic structure of this system, we plot the PDOS of Cd, Cu, Mn, and O (down panel in Figure 5). While for the down-spin channel, a small gap is observed, see Figure 6, the up-spin conduction bands cross the Fermi level, which mainly consist of Mn and O with little contribution from Cu. These results calculated with GGA indicate that CdCu3Mn4O12 has the electronic structure of a half-metallic compound. Weht and Pickett29 calculated, with our methodology, a gap and gave properties in reasonable agreement with experiment for CaCu3Mn4O12. We can conclude that the metallic behavior of CdCu3Mn4O12, observed in the transport measurements, is mainly due to an increment in the DOS near the Fermi level

(with respect to the Ca compound), although it can also be influenced by the Mn3+/Mn4+ mixed valence of the secondary perovskite phase. Concerning the changes in the electrical resistance under a magnetic field, we define MR(H) = 100 × [(R(H) − R(0))/ R(0)] as the MR response of the sample. Figure 7 shows the MR isotherms for the CdCu3Mn4O12 oxide at different temperatures. The MR is negative and reaches a maximum value of −15% at 10 K and 9 T. The MR is still significant at room temperature reaching a value of −7.5% at 9 T. It is noteworthy the strong component of low-field MR (defined for magnetic fields lower than 1T) especially at lower temperatures. This new perovskite is therefore comparable to other materials of the same family, as far as the magnetic and magnetotransport properties are concerned.

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CONCLUSIONS A complex perovskite oxide of nominal composition CdCu3Mn4O12 has been prepared at 7 GPa and 1000 °C in the presence of KClO4 as oxidizing agent. In fact, the preparation conditions provide the parallel stabilization of a minor perovskite phase with composition Cd[Cu2.40Mn0.60]Mn4O12, where the Jahn−Teller Mn3+ ion partly replaces Cu2+ at the square-planar A′ sites, driving an electron doping effect that induces a mixed valence Mn4+/Mn3+ at the octahedral positions. This phenomenon accounts for the increase of the 9656



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(5) Kobayashi, W.; Terasaki, I.; Takeya, J.; Tsukuda, I.; Ando, Y. A. Novel Heavy-Fermion State in CaCu3Ru4O12. J. Phys. Soc. Jpn. 2004, 73, 2373−2376. (6) Tran, T. T.; Takubo, K.; Mizokawa, T.; Kobayashi, W.; Terasaki, I. Electronic Structure of CaCu3Ru4O12 Studied by X-ray Photoemission Spectroscopy. Phys. Rev. B 2006, 73, 193105/1−4. (7) Zeng, Z.; Greenblatt, M.; Subramanian, M. A.; Croft, M. Large Low-Field Magnetoresistance in Perovskite-Type CaCu3Mn4O12 without Double Exchange. Phys. Rev. Lett. 1999, 82, 3164−3167. (8) Sánchez-Benítez, J.; Alonso, J. A.; Martínez-Lope, M. J.; de Andrés, A.; Fernández-Díaz, M. T. Enhancement of the Curie Temperature along the Perovskite Series RCu3Mn4O12 Driven by Chemical Pressure of R3+ Cations (R = Rare Earths). Inorg. Chem. 2010, 49, 5679−5685. (9) Vasil’ev, A. N.; Volkova, O. S. New Functional Materials AC3B4O12. Low Temp. Phys. 2007, 33, 895−914. (10) Shimakawa, Y. A-Site-Ordered Perovskites with Intriguing Physical Properties. Inorg. Chem. 2008, 47, 8562−8570. (11) Deschanvres, A.; Raveau, B.; Tollemer, F. Substitution of Copper for a Divalent Metal in Perovskite-Type Titanates. Bull. Soc. Chim. Fr. 1967, 4077−4078. (12) Bochu, B.; Deschizeaux, M. N.; Joubert, J. C.; Collomb, A.; Chenavas, J.; Marezio, M. Synthèse et Caractérisation d’une Série de Titanates Pérowskites Isotypes de [CaCu3](Mn4)O12. J. Solid State Chem. 1979, 29, 291−298. (13) Labeau, M.; Bochu, B.; Jourbert, J. C.; Chenavas, J. Synthèse et Caractérisation Cristallographique et Physique d’une Série de Composés ACu3Ru4O12 de Type Perovskite. J. Solid State Chem. 1980, 33, 257−261. (14) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (15) Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Phys. B 1993, 192, 55−69. (16) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (17) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (18) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (20) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1979, 13, 5188−5192. (21) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505− 1509. (22) Zu, J.; Wang, J.; Wu, Z. Pressure-Induced Half-Metallic Ferrimagnetism in La2VMnO6. J. Phys. Chem. C 2013, 117, 7231− 7235. (23) Wang, L.; Maxisch, T.; Ceder, G. Oxidation Energies of Transition Metal Oxides within the GGA+U Framework. Phys. Rev. B 2006, 73, 195107/1−6. (24) Arroyo-de-Dompablo, M. E.; Morales-García, A.; Taravillo, M. DFT+U Calculations of Crystal Lattice, Electronic Structure, and Phase Stability Under Pressure of TiO2 Polymorphs. J. Chem. Phys. 2011, 135, 054503−054511. (25) Sánchez-Benítez, J.; Prieto, C.; de Andrés, A.; Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T. Evidence of Two Different Mn States in CaCu3Mn4O12 Derivatives with Colossal Magnetoresistance. Phys. Rev. B 2004, 70, 024419/1−6. (26) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A 1976, 32, 751−767. (27) Chenavas, J.; Joubert, J. C.; Marezio, M.; Bochu, B. The Synthesis and Crystal Structure of CaCu 3 Mn 4 O 12 : A New

Figure 7. Negative MR versus magnetic field isotherms at different temperatures. MR is defined as MR(H) = 100 × [(R(H) − R(0))/ R(0)].

unit-cell parameter of this second phase. The magnetic properties are characteristic of ferrimagnets with two distinct TC values corresponding to both perovskite phases; the TC for CdCu3Mn4O12 is comparable to that of CaCu3Mn4O12 and, as it happens for this compound, the MR is also significant above room temperature. DFT calculations carried out on CdCu3Mn4O12 have well reproduced the crystallographic and magnetic properties experimentally obtained and also have confirmed that the observed metallic behavior in the bulk material is due to the changes in the DOS when introducing Cd instead of Ca to the A position of the perovskite. CdCu3Mn4O12 is a half-metal while CaCu3Mn4O12 is a semiconductor.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 913349071; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support of the Spanish Ministry of Science and Innovation to project MAT2010-16404, of the Comunidad de Madrid to project S2009PPQ-1551 (QUIMAPRES), and of the Spanish Consolider Ingenio 2010 Program (project CDS2007-00045). J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Ministerio de Economiá y Competitividad.

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