- Email: [email protected]
Synthesis and electrical properties of Sr4Mn2CuO9 ceramics
Materials Letters 58 (2004) 3645 – 3647 www.elsevier.com/locate/matlet
Synthesis and electrical properties of Sr4Mn2CuO9 ceramics Seymen Aygqn, Sarah Shiley, David P. Cann* Materials Science and Engineering Department, Iowa State University, 2220 Hoover Hall, Ames, IA 50011, USA Received 16 February 2004; accepted 19 July 2004 Available online 7 August 2004
Abstract The crystal structure of the compound Sr4Mn2CuO9 is closely related to the hexagonal perovskite structure. In this work, Sr3.95Mn2CuO9 was synthesized by solid state methods and the electrical properties of the polycrystalline ceramics were measured. The conductivity was observed to increase four orders of magnitude over the temperature range 100 to 700 K, with a room temperature conductivity of 5105 S/ cm. The conductivity closely followed an Arrhenius equation with an activation energy of approximately 0.14 eV. D 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Electrical properties; Cuprates; Electroceramics; Semiconductors
1. Introduction In a recent paper by El Abed et al. [1], crystal structure data on flux-grown single crystals of Sr4Mn2.09Cu0.91O9 were presented. The structure is closely related to the hexagonal perovskite 2H within the general perovskite formula unit A1+x (AxVB1x )O3 where 0VV1/2. In previous work, El Abed et al. [2,3] described the magnetic properties of a related material, Sr4Mn2NiO9. Structures in this family are characterized by chains that are built from structural units such as trigonal prisms and face-sharing octahedra. The electronic and magnetic properties of these materials can be anisotropic and are strongly influenced by the composition of the AV and B sites in the structure. The structure of Sr4Mn2CuO9 has the space group P321 with lattice parameters a=9.5817 2 and c=7.8290 2. The overall structure can be characterized as an assemblage of two separate chains oriented along the c-axis containing two structural units; MnO6 octahedra and CuO6 trigonal prisms (Fig. 1). The Sr atoms lie in between the chains. El Abed attributed a stacking fault to one of the chains whereby a
* Corresponding author. Tel.: +1 515 294 3202; fax: +1 515 294 5444. E-mail address: [email protected] (D.P. Cann). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.07.012
MnO6 octahedra was substituted for one of the CuO6 trigonal prisms [1]. Recently, there has been a great deal of interest in Cucontaining perovskite materials because of anomalous dielectric properties, including CaCu3Ti4O12 [4,5]. In contrast to many Cu-containing perovskites, the Cu atoms in this structure are randomly distributed on the square faces of the trigonal prisms in an irregular fourfold coordination. Approximately one-third of these sites is occupied by Cu atoms. In this work, the electrical properties of Sr3.95Mn2CuO9 ceramics prepared by solid state synthesis are presented.
2. Experimental The compound Sr3.95Mn2CuO9 was prepared via solid state reaction from powders of SrCO3 (Aldrich, 98+%), CuO (Alfa Aesar, ACS), and MnO2 (Alfa Aesar, 99.9%). Thermogravimetric analysis was used to determine the total carbonate and hydrate concentration for the SrCO3 powder. After vibratory milling in ethanol with zirconia media, the powders were reacted in air in the appropriate stoichiometric ratios at temperatures ranging from 1000 to 1200 8C for 24 h. The powders were then mixed with approximately 5 wt.% organic binder and 12.5 mm diameter pellets were prepared
3646
S. Aygu¨n et al. / Materials Letters 58 (2004) 3645–3647
Fig. 3. SEM micrograph of the cross-section of Sr3.95Mn2CuO9 sample.
cooling at a rate of 3 8C/min. Impedance measurements were made by an HP 4194A impedance analyzer, at frequencies ranging from 100 Hz to 1 MHz at room temperature. Fig. 1. Crystal structure of Sr4Mn2CuO9 viewed along the c-axis. The Cu atoms are black, the O atoms are white, the Sr atoms are the large gray spheres, while the Mn atoms are the small gray spheres.
3. Results
by cold uniaxial pressing. Finally, the pellets were sintered in air at 1300 8C. X-ray diffraction (XRD) was utilized to verify the phase purity of the powders and the sintered pellets. The pattern was indexed and the lattice parameters were calculated using software packages HKLGEN and UNITCELL. The microstructure and the morphology were examined using a Hitachi S-2460N VP-SEM. For electrical measurements, Heraeus (C1000) silver electrodes were painted on the surfaces and the pellets were then fired at 550 8C for 30 min. Electrical measurements were also carried out using In–Ga electrodes to assure that contact resistance effects could be neglected. Resistivity as a function of temperature was measured using a Keithley 6517A electrometer. Measurements were performed upon
Shown in Fig. 2 is the XRD data for a sintered polycrystalline pellet of the composition Sr3.95Mn2CuO9 indicating the presence of a single phase. Initial solid state reactions for the exact stoichiometry Sr4Mn2CuO9 yielded the presence of a Sr-rich phase. Modification of the stoichiometry to Sr3.95Mn2CuO9 yielded a diffraction pattern in which all of the peaks could be indexed to single phase Sr4Mn2CuO9. The pattern was refined in the trigonal crystal system, minimizing the sum of squares of residuals in two-theta. The fit yielded lattice parameters a=9.612 2 and c=7.903 2, which are slightly different than the values in the literature possibly due to impurities the difference in composition from the different synthesis techniques. The flux-grown crystals in El Abed’s work had the composition Sr4Mn2.09CuO9 [1]. Fig. 3 shows an SEM micrograph of the cross-section of a Sr3.95Mn2CuO9 sample. The densities of the samples
Fig. 2. X-ray diffraction pattern of Sr3.95Mn2CuO9.
Fig. 4. Conductivity as a function of temperature.
S. Aygu¨n et al. / Materials Letters 58 (2004) 3645–3647
3647
and two constant phase elements connected in parallel to two of the resistances. The resistance values were calculated as 356.5, 1107 and 1886 V.
4. Conclusions
Fig. 5. Complex impedance plot and the fit for Sr3.95Mn2CuO9 at room temperature.
measured by Archimedes method were more than 90% of the theoretical density. The SEM images are consistent with this data with some porosity near the triple points. The primary grain size was observed to approximately 10 Am. Although it was not observed in the XRD data, SEM analyses showed the presence of small amount of a Cu-rich second phase formed at the grain boundaries. The conductivity as a function of temperature is depicted in Fig. 4. The data shows a dramatic increase in the conductivity over the temperature range 100 to 700 K, with a room temperature conductivity of 5105 S/cm. The conductivity closely followed an Arrhenius law given by the equation (inset Fig. 4): r ¼ r0 expðEa =kB T Þ where E a is the activation energy for conduction, k B is the Boltzmann constant, and T is the absolute temperature. The linear fit to the plot yielded an activation energy, E a=0.136 eV. Fig. 5 shows the complex impedance spectrum of Sr3.95Mn2CuO9 measured at room temperature with zero bias. The data points obtained over the frequency range from 100 Hz to 1 MHz are fitted to a single semicircle, which is depressed relative to the R-axis. The data were assigned to an equivalent circuit with three resistances connected in series
Ceramics of the composition Sr3.95Mn2CuO9 were prepared by solid state synthesis. Calcination at temperatures between 1000 and 1200 8C and sintering at 1300 8C resulted in single-phase Sr3.95Mn2CuO9 ceramics with a density above 90% theoretical. The room temperature conductivity was 5105 S/cm, and over the temperature range 100 to 700 K the conductivity increased over four orders of magnitude. The conductivity versus temperature followed an Arrhenius law with an activation energy of 0.14 eV.
Acknowledgements This work was supported by the National Science Foundation through award number DMR 0093616 and through the Research Experience for Undergraduates (REU) program.
References [1] A. El Abed, E. Gaudin, J. Darriet, Acta Crystallogr. C58 (2002) i138 – i140. [2] A. El Abed, E. Gaudin, S. Lemaux, J. Darriet, Solid State Sci. 3 (2001) 887 – 897. [3] A. El Abed, E. Gaudin, J. Darriet, M.-H. Whangbo, J. Solid State Chem. 163 (2002) 513 – 518. [4] M.A. Subramanian, A.W. Sleight, Solid State Sci. 4 (2002) 347. [5] S. Aygqn, X. Tan, J.P. Maria, D.P. Cann, Proceedings of the 204th Electrochemical Society Meeting, Orlando, Florida, October, 2003.
Synthesis and electrical properties of Sr4Mn2CuO9 ceramics Seymen Aygqn, Sarah Shiley, David P. Cann* Materials Science and Engineering Department, Iowa State University, 2220 Hoover Hall, Ames, IA 50011, USA Received 16 February 2004; accepted 19 July 2004 Available online 7 August 2004
Abstract The crystal structure of the compound Sr4Mn2CuO9 is closely related to the hexagonal perovskite structure. In this work, Sr3.95Mn2CuO9 was synthesized by solid state methods and the electrical properties of the polycrystalline ceramics were measured. The conductivity was observed to increase four orders of magnitude over the temperature range 100 to 700 K, with a room temperature conductivity of 5105 S/ cm. The conductivity closely followed an Arrhenius equation with an activation energy of approximately 0.14 eV. D 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Electrical properties; Cuprates; Electroceramics; Semiconductors
1. Introduction In a recent paper by El Abed et al. [1], crystal structure data on flux-grown single crystals of Sr4Mn2.09Cu0.91O9 were presented. The structure is closely related to the hexagonal perovskite 2H within the general perovskite formula unit A1+x (AxVB1x )O3 where 0VV1/2. In previous work, El Abed et al. [2,3] described the magnetic properties of a related material, Sr4Mn2NiO9. Structures in this family are characterized by chains that are built from structural units such as trigonal prisms and face-sharing octahedra. The electronic and magnetic properties of these materials can be anisotropic and are strongly influenced by the composition of the AV and B sites in the structure. The structure of Sr4Mn2CuO9 has the space group P321 with lattice parameters a=9.5817 2 and c=7.8290 2. The overall structure can be characterized as an assemblage of two separate chains oriented along the c-axis containing two structural units; MnO6 octahedra and CuO6 trigonal prisms (Fig. 1). The Sr atoms lie in between the chains. El Abed attributed a stacking fault to one of the chains whereby a
* Corresponding author. Tel.: +1 515 294 3202; fax: +1 515 294 5444. E-mail address: [email protected] (D.P. Cann). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.07.012
MnO6 octahedra was substituted for one of the CuO6 trigonal prisms [1]. Recently, there has been a great deal of interest in Cucontaining perovskite materials because of anomalous dielectric properties, including CaCu3Ti4O12 [4,5]. In contrast to many Cu-containing perovskites, the Cu atoms in this structure are randomly distributed on the square faces of the trigonal prisms in an irregular fourfold coordination. Approximately one-third of these sites is occupied by Cu atoms. In this work, the electrical properties of Sr3.95Mn2CuO9 ceramics prepared by solid state synthesis are presented.
2. Experimental The compound Sr3.95Mn2CuO9 was prepared via solid state reaction from powders of SrCO3 (Aldrich, 98+%), CuO (Alfa Aesar, ACS), and MnO2 (Alfa Aesar, 99.9%). Thermogravimetric analysis was used to determine the total carbonate and hydrate concentration for the SrCO3 powder. After vibratory milling in ethanol with zirconia media, the powders were reacted in air in the appropriate stoichiometric ratios at temperatures ranging from 1000 to 1200 8C for 24 h. The powders were then mixed with approximately 5 wt.% organic binder and 12.5 mm diameter pellets were prepared
3646
S. Aygu¨n et al. / Materials Letters 58 (2004) 3645–3647
Fig. 3. SEM micrograph of the cross-section of Sr3.95Mn2CuO9 sample.
cooling at a rate of 3 8C/min. Impedance measurements were made by an HP 4194A impedance analyzer, at frequencies ranging from 100 Hz to 1 MHz at room temperature. Fig. 1. Crystal structure of Sr4Mn2CuO9 viewed along the c-axis. The Cu atoms are black, the O atoms are white, the Sr atoms are the large gray spheres, while the Mn atoms are the small gray spheres.
3. Results
by cold uniaxial pressing. Finally, the pellets were sintered in air at 1300 8C. X-ray diffraction (XRD) was utilized to verify the phase purity of the powders and the sintered pellets. The pattern was indexed and the lattice parameters were calculated using software packages HKLGEN and UNITCELL. The microstructure and the morphology were examined using a Hitachi S-2460N VP-SEM. For electrical measurements, Heraeus (C1000) silver electrodes were painted on the surfaces and the pellets were then fired at 550 8C for 30 min. Electrical measurements were also carried out using In–Ga electrodes to assure that contact resistance effects could be neglected. Resistivity as a function of temperature was measured using a Keithley 6517A electrometer. Measurements were performed upon
Shown in Fig. 2 is the XRD data for a sintered polycrystalline pellet of the composition Sr3.95Mn2CuO9 indicating the presence of a single phase. Initial solid state reactions for the exact stoichiometry Sr4Mn2CuO9 yielded the presence of a Sr-rich phase. Modification of the stoichiometry to Sr3.95Mn2CuO9 yielded a diffraction pattern in which all of the peaks could be indexed to single phase Sr4Mn2CuO9. The pattern was refined in the trigonal crystal system, minimizing the sum of squares of residuals in two-theta. The fit yielded lattice parameters a=9.612 2 and c=7.903 2, which are slightly different than the values in the literature possibly due to impurities the difference in composition from the different synthesis techniques. The flux-grown crystals in El Abed’s work had the composition Sr4Mn2.09CuO9 [1]. Fig. 3 shows an SEM micrograph of the cross-section of a Sr3.95Mn2CuO9 sample. The densities of the samples
Fig. 2. X-ray diffraction pattern of Sr3.95Mn2CuO9.
Fig. 4. Conductivity as a function of temperature.
S. Aygu¨n et al. / Materials Letters 58 (2004) 3645–3647
3647
and two constant phase elements connected in parallel to two of the resistances. The resistance values were calculated as 356.5, 1107 and 1886 V.
4. Conclusions
Fig. 5. Complex impedance plot and the fit for Sr3.95Mn2CuO9 at room temperature.
measured by Archimedes method were more than 90% of the theoretical density. The SEM images are consistent with this data with some porosity near the triple points. The primary grain size was observed to approximately 10 Am. Although it was not observed in the XRD data, SEM analyses showed the presence of small amount of a Cu-rich second phase formed at the grain boundaries. The conductivity as a function of temperature is depicted in Fig. 4. The data shows a dramatic increase in the conductivity over the temperature range 100 to 700 K, with a room temperature conductivity of 5105 S/cm. The conductivity closely followed an Arrhenius law given by the equation (inset Fig. 4): r ¼ r0 expðEa =kB T Þ where E a is the activation energy for conduction, k B is the Boltzmann constant, and T is the absolute temperature. The linear fit to the plot yielded an activation energy, E a=0.136 eV. Fig. 5 shows the complex impedance spectrum of Sr3.95Mn2CuO9 measured at room temperature with zero bias. The data points obtained over the frequency range from 100 Hz to 1 MHz are fitted to a single semicircle, which is depressed relative to the R-axis. The data were assigned to an equivalent circuit with three resistances connected in series
Ceramics of the composition Sr3.95Mn2CuO9 were prepared by solid state synthesis. Calcination at temperatures between 1000 and 1200 8C and sintering at 1300 8C resulted in single-phase Sr3.95Mn2CuO9 ceramics with a density above 90% theoretical. The room temperature conductivity was 5105 S/cm, and over the temperature range 100 to 700 K the conductivity increased over four orders of magnitude. The conductivity versus temperature followed an Arrhenius law with an activation energy of 0.14 eV.
Acknowledgements This work was supported by the National Science Foundation through award number DMR 0093616 and through the Research Experience for Undergraduates (REU) program.
References [1] A. El Abed, E. Gaudin, J. Darriet, Acta Crystallogr. C58 (2002) i138 – i140. [2] A. El Abed, E. Gaudin, S. Lemaux, J. Darriet, Solid State Sci. 3 (2001) 887 – 897. [3] A. El Abed, E. Gaudin, J. Darriet, M.-H. Whangbo, J. Solid State Chem. 163 (2002) 513 – 518. [4] M.A. Subramanian, A.W. Sleight, Solid State Sci. 4 (2002) 347. [5] S. Aygqn, X. Tan, J.P. Maria, D.P. Cann, Proceedings of the 204th Electrochemical Society Meeting, Orlando, Florida, October, 2003.