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Valence degeneracy in CaCu3Cr4O12
Journal of Physics and Chemistry of Solids 64 (2003) 1569–1571 www.elsevier.com/locate/jpcs
Valence degeneracy in CaCu3Cr4O12 M.A. Subramaniana,*, W.J. Marshalla, T.G. Calvaresea, A.W. Sleightb a
DuPont Central Research Department, Experimental Station, P.O. Box 80328, Wilmington, DE 19880-0328, USA b Department of Chemistry, 153 Gilbert Hall, Oregon State University, Corvallis, OR 97331-4003, USA
Abstract The synthesis of CaCu3Cr4O12 has been accomplished at a pressure of 60 kbar. Analysis of single crystal X-ray diffraction data demonstrates that this compound is isostructural with CaCu3Ti4O12. The electrical resistivity data for CaCu3Cr4O12 show metallic behavior, and the magnetic susceptibility indicates delocalized electrons for both Cr and Cu. The Cu– O and Cr – O bond distances give fractional valences of Cu2.45 and Cr3.66, thus indicating both Cu and Cr 3d states at the Fermi level. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; D. Electrical properties; D. Magnetic properties
1. Introduction The AMO3 perovskite structure can be regarded as a corner-sharing network of MO6 octahedra with A cation interstitials. This is a very flexible network because the octahedra are free to tilt in many different ways [1]. The A cation is in 12-fold coordination in the ideal cubic perovskite structure, where the octahedra are not tilted. Tilting of the octahedra can reduce the A cation coordination number to accommodate smaller A cations. In a very unusual variation of the perovskite structure, the octahedra tilt to produce square planar coordination for 3/4 of the A cations with the remaining 1/4 of the A cations maintaining regular 12-fold coordination (Fig. 1). Only Cu2þ and Mn3þ have been found to occupy the square planar site (M) for these AM3M40 O12 perovskites. Many cations have been found to occupy fully or partially at the A site, but only Ti, Ta, Mn, Ru and Ge have been known to occupy the octahedral site (M0 ) in this family [2,3]. The primary interest in the ACu3M4O12 family has been the dielectric constant of those where M ¼ Ti, especially when A ¼ Ca. The dielectric constant of CaCu3Ti4O12 at low frequencies has been found to approach 105 in crystals, thin films, and ceramic samples [4 – 6]. The temperature independence of this exceptionally large dielectric constant * Corresponding author. Tel.: þ1-302-695-2966; fax: þ 1-302695-9799. E-mail address: [email protected] (M.A. Subramanian).
is unique and currently lacks a complete explanation. Other properties of interest for ACu3M4O12 perovskites include large magnetoresistance for CaCu3Mn4O12 and valence degeneracy for ACa3Ru4O12 compounds [7,2]. 2. Experimental CaCu3Cr4O12 was synthesized by heating stoichiometric quantities of CaO, CuO and CrO2 at 1100 8C in sealed Pt tubes under 60 kbar pressure in a tetrahedral anvil apparatus. Synthesis attempts at lower pressures were unsuccessful. The sample was rapidly cooled to room temperature before releasing the pressure. Powder X-ray diffraction pattern of the sample showed the phase formed is cubic and could be indexed in a body-centered cubic unit cell (space group: Examination of the sample under optical microscope Im3). revealed the formation of small cube shaped single crystals suitable for single-crystal structure refinements. Electrical resistivity was measured on a dense pellet by standard four-probe technique using Quantum Design Physical Property Measurement System and magnetic measurements were done using Quantum Design SQIUD magnetometer. 3. Results Analysis of single crystal X-ray diffraction data showed that CaCu3Cr4O12 is isostructural with CaCu3Ti4O12. Crystallographic results are summarized in Table 1.
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00095-7
1570
M.A. Subramanian et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1569–1571
The twinning, which has been present in all crystals of CaCu3Ti4O12, was not present in our crystal of CaCu3Cr4O12. Refined parameters and some bond distances and angles are in Table 2. Microprobe data indicated that this crystal contained a very small amount of Pt, a consequence of the Pt container. The crystallographic analysis confirmed that this Pt was on the Cr site. Electrical measurements show that CaCu3Cr4O12 is metallic (Fig. 2), consistent with magnetic measurements showing Pauli paramagnetic behavior.
4. Discussion
Fig. 1. Structure of CaCu3M4O12 compounds showing the tilting of MO6 octahedral network. The copper atoms (black spheres) located at the edge centers are surrounded by four oxygen atoms in a square planar configuration.
˚ in CaCu3Cr4O12 is The Cu – O distance of 1.92 A ˚ value expected for a significantly shorter than the 1.98 A Cu(II) – O distance as in fact found in insulating CaCu3Ti4O12. The Cr-O distance is also somewhat longer than that expected for a Cr(III)– O distance. These observations coupled with the metallic behavior indicates valence degeneracy, which we also found in isostructural ACu3Ru4O12 phases [2]. In the case of metallic NaCu3Ru4O12,
Table 1 Crystal data and intensity collection for CaCu3Cr4O12 Color, habit Size (mm3) Diffractometer Radiation Temperature 2u range Total frames Scans Absorption correction Transmission factors Crystal system Space group ˚) Unit cell dimensions (A ˚) 3 Volume (A Units/cell Z Formula weight Calculated density (g/cm3) Absorption coefficient, m (mm21) Total reflections Unique reflections Rint Refinement method No. of parameters refined Goodness of fit on F 2 Weight, w wR2 (all data) R1 (all data) wR2 ðI . 4ðI sÞ) R1 (I . 4(Is)) Extinction coefficient Largest difference peak and hole
Black, Cube 0.08 £ 0.08 £ 0.08 Bruker Smart 1K CCD Mo Ka, Graphite Mono. ðl ¼ 0:71069 AÞ 23 8C 7.94–55.408 6000 v; 0.38, 10 s Analytical (XPREP) and SADABS 0.59–0.64 Cubic Im3 7.253 (3) 381.52 (2) 2 630.70 5.490 14.44 2848 96 0.027 Full-matrix least-squares on F 2 17 1.27 1=½s2 ðF02 Þ þ ð0:0048 £ PÞ2 þ 3:80 £ P where P ¼ ðmaxðF02 ; 0Þ þ 2 £ Fc2 Þ=3 0.0375 0.0218 0.0367 0.0186 0.0027(5) 0.49, 20.36
M.A. Subramanian et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1569–1571 Table 2 CaCu3Cr4O12 refinement results (Ca: 0 0 0; Cu: 0 1/2 1/2; Cr: 1/4 1/4 1/4; O: x y 0) x(O) y(O) U 11(Ca)a U 11(Cu)b U 22(Cu) U 33(Cu) U 11(Cr)c U 12(Cr) U 11(O)d U 22(O) U 33(O) U 12(O) ˚) Ca–O (A ˚) Cu –O (A ˚) Cr–O (A O –Cr–O (8) O –Cr–O (8) O –Cu –O (8) O –Cu –O (8) Cr–O–Cr (8)
0.3055(4) 0.1797(4) 0.003 0.002 0.002 0.005 0.001 0.000 0.007 0.005 0.005 0.005 2.571(3) 1.921(3) 1.926(1) 89.8(1) 90.2(1) 85.5(2) 94.5(2) 140.6(2)
a Ca: U11 ¼ U22 ¼ U33 ; U12 ¼ U13 ¼ U23 ¼ 0 : standard deviations for Us are less than 0.001. b Cu: U12 ¼ U13 ¼ U23 ¼ 0: c Cr: U11 ¼ U22 ¼ U33 ; U12 ¼ U13 ¼ U23 : d O: U13 ¼ U23 ¼ 0:
Fig. 2. Electrical resistivity vs. temperature for CaCu3Cr4O12.
˚ . Bond the Cu –O distance has also dropped to 1.92 A valence calculations [8,9] offer a way to quantify the
1571
oxidation states of Cu and Cr based on the observed interatomic distances. These calculations for CaCu3Cr4O12 using the distances in Table 2 give Cu2.45 and Cr3.66. Thus, we have both Cu 3d and Cr 3d states at the Fermi level in CaCu3Cr4O12 just as we have Cu 3d and Ru 4d states at the Fermi level in the ACu3Ru4O12 compounds where A could be Na, Ca, or a rare earth. This valence degeneracy results in metallic properties and a loss of a local moment for Cu, which is present in the case of CaCu3Ti4O12.
Acknowledgements We thank A.P. Ramirez for technical collaboration and fruitful discussions.
References [1] P.M. Woodward, Octahedral tilting in perovskites, Acta Crystallogr., Sect. B 3 (1997) 32– 66. [2] M.A. Subramanian, A.W. Sleight, ACu3Ti4O12 and ACu3Ru4O12 perovskites: high dielectric constants and valence degeneracy, Solid State Sci. 4 (2002) 347 –351. [3] M.T. Anderson, V.E. Balbarin, D.A. Groenke, G.A. Bain, K.R. Poeppelmeier, J. Solid State Chem. 103 (1993) 216–227. [4] M.A. Subramanian, D. Li, N. Duan, B.A. Reisner, A.W. Sleight, High Dielectric Constant in ACu3Ti4O12 and ACu3Ti3FeO12 Phases, J. Solid State Chem. 151 (2000) 323–325. [5] C.C. Holmes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673 –675. [6] Y. Lin, Y.B. Chen, T. Garret, S.W. Liu, C.L. Chen, L. Chen, R.P. Bontchev, A. Jacobson, J.C. Jiang, E.I. Meletis, J. Horwitz, H.-D. Wu, Epitaxial growth of dielectric CaCu3Ti4O12 thin films on (001) LaAlO3 by pulsed laser deposition, Appl. Phys. Lett. 81 (2002) 631 –633. [7] Z. Zeng, M. Greenblatt, M.A. Subramanian, M. Croft, Large low-field magnetoresistance in perovskite-type CaCu3Mn4O12 without double exchange, Phys. Rev. Lett. 82 (1999) 3164–3167. [8] I.D. Brown, D. Altermatt, Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database, Acta Cryst., Sect. B 41 (1985) 244–247. [9] Bond valences calculated using EUTAX, a program written by M. O’Keeffe, Arizona State University.
Valence degeneracy in CaCu3Cr4O12 M.A. Subramaniana,*, W.J. Marshalla, T.G. Calvaresea, A.W. Sleightb a
DuPont Central Research Department, Experimental Station, P.O. Box 80328, Wilmington, DE 19880-0328, USA b Department of Chemistry, 153 Gilbert Hall, Oregon State University, Corvallis, OR 97331-4003, USA
Abstract The synthesis of CaCu3Cr4O12 has been accomplished at a pressure of 60 kbar. Analysis of single crystal X-ray diffraction data demonstrates that this compound is isostructural with CaCu3Ti4O12. The electrical resistivity data for CaCu3Cr4O12 show metallic behavior, and the magnetic susceptibility indicates delocalized electrons for both Cr and Cu. The Cu– O and Cr – O bond distances give fractional valences of Cu2.45 and Cr3.66, thus indicating both Cu and Cr 3d states at the Fermi level. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; D. Electrical properties; D. Magnetic properties
1. Introduction The AMO3 perovskite structure can be regarded as a corner-sharing network of MO6 octahedra with A cation interstitials. This is a very flexible network because the octahedra are free to tilt in many different ways [1]. The A cation is in 12-fold coordination in the ideal cubic perovskite structure, where the octahedra are not tilted. Tilting of the octahedra can reduce the A cation coordination number to accommodate smaller A cations. In a very unusual variation of the perovskite structure, the octahedra tilt to produce square planar coordination for 3/4 of the A cations with the remaining 1/4 of the A cations maintaining regular 12-fold coordination (Fig. 1). Only Cu2þ and Mn3þ have been found to occupy the square planar site (M) for these AM3M40 O12 perovskites. Many cations have been found to occupy fully or partially at the A site, but only Ti, Ta, Mn, Ru and Ge have been known to occupy the octahedral site (M0 ) in this family [2,3]. The primary interest in the ACu3M4O12 family has been the dielectric constant of those where M ¼ Ti, especially when A ¼ Ca. The dielectric constant of CaCu3Ti4O12 at low frequencies has been found to approach 105 in crystals, thin films, and ceramic samples [4 – 6]. The temperature independence of this exceptionally large dielectric constant * Corresponding author. Tel.: þ1-302-695-2966; fax: þ 1-302695-9799. E-mail address: [email protected] (M.A. Subramanian).
is unique and currently lacks a complete explanation. Other properties of interest for ACu3M4O12 perovskites include large magnetoresistance for CaCu3Mn4O12 and valence degeneracy for ACa3Ru4O12 compounds [7,2]. 2. Experimental CaCu3Cr4O12 was synthesized by heating stoichiometric quantities of CaO, CuO and CrO2 at 1100 8C in sealed Pt tubes under 60 kbar pressure in a tetrahedral anvil apparatus. Synthesis attempts at lower pressures were unsuccessful. The sample was rapidly cooled to room temperature before releasing the pressure. Powder X-ray diffraction pattern of the sample showed the phase formed is cubic and could be indexed in a body-centered cubic unit cell (space group: Examination of the sample under optical microscope Im3). revealed the formation of small cube shaped single crystals suitable for single-crystal structure refinements. Electrical resistivity was measured on a dense pellet by standard four-probe technique using Quantum Design Physical Property Measurement System and magnetic measurements were done using Quantum Design SQIUD magnetometer. 3. Results Analysis of single crystal X-ray diffraction data showed that CaCu3Cr4O12 is isostructural with CaCu3Ti4O12. Crystallographic results are summarized in Table 1.
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00095-7
1570
M.A. Subramanian et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1569–1571
The twinning, which has been present in all crystals of CaCu3Ti4O12, was not present in our crystal of CaCu3Cr4O12. Refined parameters and some bond distances and angles are in Table 2. Microprobe data indicated that this crystal contained a very small amount of Pt, a consequence of the Pt container. The crystallographic analysis confirmed that this Pt was on the Cr site. Electrical measurements show that CaCu3Cr4O12 is metallic (Fig. 2), consistent with magnetic measurements showing Pauli paramagnetic behavior.
4. Discussion
Fig. 1. Structure of CaCu3M4O12 compounds showing the tilting of MO6 octahedral network. The copper atoms (black spheres) located at the edge centers are surrounded by four oxygen atoms in a square planar configuration.
˚ in CaCu3Cr4O12 is The Cu – O distance of 1.92 A ˚ value expected for a significantly shorter than the 1.98 A Cu(II) – O distance as in fact found in insulating CaCu3Ti4O12. The Cr-O distance is also somewhat longer than that expected for a Cr(III)– O distance. These observations coupled with the metallic behavior indicates valence degeneracy, which we also found in isostructural ACu3Ru4O12 phases [2]. In the case of metallic NaCu3Ru4O12,
Table 1 Crystal data and intensity collection for CaCu3Cr4O12 Color, habit Size (mm3) Diffractometer Radiation Temperature 2u range Total frames Scans Absorption correction Transmission factors Crystal system Space group ˚) Unit cell dimensions (A ˚) 3 Volume (A Units/cell Z Formula weight Calculated density (g/cm3) Absorption coefficient, m (mm21) Total reflections Unique reflections Rint Refinement method No. of parameters refined Goodness of fit on F 2 Weight, w wR2 (all data) R1 (all data) wR2 ðI . 4ðI sÞ) R1 (I . 4(Is)) Extinction coefficient Largest difference peak and hole
Black, Cube 0.08 £ 0.08 £ 0.08 Bruker Smart 1K CCD Mo Ka, Graphite Mono. ðl ¼ 0:71069 AÞ 23 8C 7.94–55.408 6000 v; 0.38, 10 s Analytical (XPREP) and SADABS 0.59–0.64 Cubic Im3 7.253 (3) 381.52 (2) 2 630.70 5.490 14.44 2848 96 0.027 Full-matrix least-squares on F 2 17 1.27 1=½s2 ðF02 Þ þ ð0:0048 £ PÞ2 þ 3:80 £ P where P ¼ ðmaxðF02 ; 0Þ þ 2 £ Fc2 Þ=3 0.0375 0.0218 0.0367 0.0186 0.0027(5) 0.49, 20.36
M.A. Subramanian et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1569–1571 Table 2 CaCu3Cr4O12 refinement results (Ca: 0 0 0; Cu: 0 1/2 1/2; Cr: 1/4 1/4 1/4; O: x y 0) x(O) y(O) U 11(Ca)a U 11(Cu)b U 22(Cu) U 33(Cu) U 11(Cr)c U 12(Cr) U 11(O)d U 22(O) U 33(O) U 12(O) ˚) Ca–O (A ˚) Cu –O (A ˚) Cr–O (A O –Cr–O (8) O –Cr–O (8) O –Cu –O (8) O –Cu –O (8) Cr–O–Cr (8)
0.3055(4) 0.1797(4) 0.003 0.002 0.002 0.005 0.001 0.000 0.007 0.005 0.005 0.005 2.571(3) 1.921(3) 1.926(1) 89.8(1) 90.2(1) 85.5(2) 94.5(2) 140.6(2)
a Ca: U11 ¼ U22 ¼ U33 ; U12 ¼ U13 ¼ U23 ¼ 0 : standard deviations for Us are less than 0.001. b Cu: U12 ¼ U13 ¼ U23 ¼ 0: c Cr: U11 ¼ U22 ¼ U33 ; U12 ¼ U13 ¼ U23 : d O: U13 ¼ U23 ¼ 0:
Fig. 2. Electrical resistivity vs. temperature for CaCu3Cr4O12.
˚ . Bond the Cu –O distance has also dropped to 1.92 A valence calculations [8,9] offer a way to quantify the
1571
oxidation states of Cu and Cr based on the observed interatomic distances. These calculations for CaCu3Cr4O12 using the distances in Table 2 give Cu2.45 and Cr3.66. Thus, we have both Cu 3d and Cr 3d states at the Fermi level in CaCu3Cr4O12 just as we have Cu 3d and Ru 4d states at the Fermi level in the ACu3Ru4O12 compounds where A could be Na, Ca, or a rare earth. This valence degeneracy results in metallic properties and a loss of a local moment for Cu, which is present in the case of CaCu3Ti4O12.
Acknowledgements We thank A.P. Ramirez for technical collaboration and fruitful discussions.
References [1] P.M. Woodward, Octahedral tilting in perovskites, Acta Crystallogr., Sect. B 3 (1997) 32– 66. [2] M.A. Subramanian, A.W. Sleight, ACu3Ti4O12 and ACu3Ru4O12 perovskites: high dielectric constants and valence degeneracy, Solid State Sci. 4 (2002) 347 –351. [3] M.T. Anderson, V.E. Balbarin, D.A. Groenke, G.A. Bain, K.R. Poeppelmeier, J. Solid State Chem. 103 (1993) 216–227. [4] M.A. Subramanian, D. Li, N. Duan, B.A. Reisner, A.W. Sleight, High Dielectric Constant in ACu3Ti4O12 and ACu3Ti3FeO12 Phases, J. Solid State Chem. 151 (2000) 323–325. [5] C.C. Holmes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673 –675. [6] Y. Lin, Y.B. Chen, T. Garret, S.W. Liu, C.L. Chen, L. Chen, R.P. Bontchev, A. Jacobson, J.C. Jiang, E.I. Meletis, J. Horwitz, H.-D. Wu, Epitaxial growth of dielectric CaCu3Ti4O12 thin films on (001) LaAlO3 by pulsed laser deposition, Appl. Phys. Lett. 81 (2002) 631 –633. [7] Z. Zeng, M. Greenblatt, M.A. Subramanian, M. Croft, Large low-field magnetoresistance in perovskite-type CaCu3Mn4O12 without double exchange, Phys. Rev. Lett. 82 (1999) 3164–3167. [8] I.D. Brown, D. Altermatt, Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database, Acta Cryst., Sect. B 41 (1985) 244–247. [9] Bond valences calculated using EUTAX, a program written by M. O’Keeffe, Arizona State University.