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Synthesis and structure of a new mixed cuprate-titanate: La2Sr4Cu2Ti2O13
Pergamon
Materials Research Bulletin, Vol. 31, No. 5, pp. 539-543, 1996 Copyright O i 996 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/96 $15.00 + .00
PII S0025-5408(96)00020-7
SYNTHESIS AND STRUCTURE OF A NEW MIXED CUPRATE-TITANATE: La2Sr4Cu2Ti2Ot3
R. Li Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China (Received October 18, 1995; Refereed)
ABSTRACT A new mixed cuprate-titanate compound with nominal composition of La2Sr4Cu2Ti20~3 has been prepared. Cell parameters of a = 3.8818(1) A and c = 20.3118(6) A have been obtained from indexing the X-ray diffraction (XRD) patterns of the samples. The structure refined from the XRD pattern showed that the compound is related to Ruddlesden-Popper phase with Cu and Ti statistically occupying the Ti position in Sr3Ti207. The oxygen vacancies were found at apical sites between the double Cu/Ti-O layers. We were unable to make the samples superconducting or more conductive due to the disordered nature of Ti/Cu occupation and oxygen vacancies in the samples. KEYWORDS: A. oxides, C. X-ray diffraction, D. crystal structure, D. electrical properties INTRODUCTION In developing new high Tc cuprate superconductors, a well-established rule is that the new compound must contain complete copper-oxygen planes in its structure. Some of the copper ions must occupy a unique site in the crystal lattice and this site should exclude the occupancy of any other ions presented in the chemical formula. Thus, studying the order or disorder arrangements of various ions with Cu in different structure types is an important route in the search for new superconductors. Recently, investigations on Ti-Cu ordering in perovskite-related phases have led to the discovery of several new layered compounds, e.g., Ti2Ba2Ln2Cu2Oy [1,2], (BaTiO3)m(Gd,Ce)3Cu2Oy (m = 1,2) [3,4], Ti2Ba2CaLn2Cu2Oy[5], and Ti2Ba2Ca(Gd,Ce)3Cu2Oy[6]. In all these compounds, the Ti ion 539
540
R. LI
Vol. 31, No. 5
is excluded from occupying Cu site because the pyramidal environment of Cu is not favorable for Ti. We extended the study to the mixed Sr3Ti207-LazSrCu2Oysystem, in order to find a new phase with ordered arrangement of CuO pyramidal and TiO octahedral coordinations. A new layered compound with the nominal composition of La2Sr4Cu2Ti20~3 was thus found. The synthesis and structure of the new compound will be reported here. EXPERIMENTAL The samples with nominal composition of LaSr2CuTiOy were prepared by solid-state reaction from the starting compounds La203 (99.99%), SrCO3 (99%), TiO2 (99.99%), and CuO (99%). All the starting materials were preheated at appropriate temperatures to evaporate traces of H20 or other gases absorbed. The well-ground mixtures of the starting materials were first heated at 900°C for 48 hours and then at 950 °C for another 48 hours with an intermittent grinding. After cooling down to room temperature, the samples were reground, pelletized, and calcined at 950°C for 72 hours and followed by furnace cooling. The powder X-ray diffraction (XRD) patterns were collected on a Rigaku rB diffractometer equipped with rotating anode source and CuK, radiation. The XRD pattern for the Rietveld refinement was recorded in a 20 range of 18 ° to 106" with a step width of 0.03* (20) and counting time of 3 seconds at each step.
RESULTS AND DISCUSSION The main peaks in the XRD pattern of La2Sr4Cu2Ti20~3 (Fig. 1) can be indexed with a tetragonal unit cell with lattice constants of a = 3.8818(1) A and c = 20.3118(6) A. Remaining weak peaks at 20 of 31.20 °, 33.34 °, and 47.74 ° might be due to an impurity phase corresponding to (La, Sr)2(Cu,Ti)O4 with the K2NiF4structure. Since superstructure reflections were not observed both in the XRD pattern and selected electron diffraction patterns, we decided to take a Roddlesden-Popper phase (Sr3Ti207) [7] related structure based on statistical occupation of Ti/Cu and LaJSr ions as a starting model for the Rietveld refinement (DBWS-9411 package [8]). The total of 34 refinable variables include 13 overall parameters for background, sample displacement, scale factor, peak width, peak asymmetry, preferred orientation, and the variation of peak width with angle and 21 structural parameters of cell constants, site occupation, atomic coordinates, and temperature factors. The above mentioned impurity peaks were excluded during the refinement. The Pearson VII function was found to be a better simulation of the diffraction peak profile. The final refinement converged to the following agreement indices of R,~, = 4.70, Ra = 3.78, S = 1.63, and d = 0.74 (Fig. 1). In the beginning of refinement, isotropic temperature factor and occupation factors for La/Sr, Cu/Ti, and oxygen atoms were successfully refined. During the refinement it became clear that the La/Sr, Cu/Ti are basically statistically occupied; the related ratios did not significantly deviate from their nominal compositions and therefore are subsequently fixed. When the site occupancies of all the oxygen atoms were refined, only the O1 (apical) site between two Ti(Cu)O6 planes showed strong deficiency (70% occupied), which led to partial pyramidal coordination of Cu/Ti-O. Considering the strong tendency of Ti to be coordinated with oxygen in an octahedral environment, we concluded that the oxygen
Vol. 31, No. 5
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ZO(CuK=) FIG. 1 The X-ray diffraction pattern of La2Sr4Cu:Ti2013 (dots represent the observed pattern; stars indicate the diffraction peaks due to impurity phase.)
FIG. 2 Structure model of La~Sr4Cu2Ti2Oi3(lightly and heavily shaded polyhedra are TiO6 and CuOs, respectively; open circle represents O1 with 30% occupation).
542
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Vol. 31, No. 5
TABLE 1 Refined Atomic Coordinates of La2Sr4Cu2Ti2Ol3 (with space group I4/mmmand cell parameters a = 3.8818(1) A and c = 20.3118(6) A, Rwp= 4.70, Rp = 3.55, RB = 3.78, R~xp= 2.86 Atom La/Sr Sr/La Cu/Ti Ol 03 04
Site 2a 4e 4e 2b 8g 4e
x 0 0.5 0.5 0 0.5 0.5
y 0 0.5 0.5 0 0 0.5
z 0 0.3179(1) 0.0959(1) 0.5 0.4070(1) 0.1978(2)
B(A2) 1.9 2.2 1.9 2.1(3) 3.0 2.4
Occupation 0.33/0.67 0.67/0.33 0.5/0.5 0.70 1.0 1.0
vacancies are most probably located between two Cu sites and that two opposite CuO5 pyramids are thus formed in the double Cu(Ti)O layers. Therefore, a structure model of Figure 2 can be built up. The atomic parameters and bond lengths obtained from the refinement are listed in Tables 1 and 2. The structure of the compound is derived from SraTi207 with a statistical Cu/Ti occupation in the TiO6 planes with 30% apical oxygen removed from the CuO6 octahedral. It is related to La2SrCu206 by adding 70% oxygen in the O1 site among the CuO5 pyramidal double layers. La2Sr4Cu:Ti20~3also has some resemblance to Ln~+xSr2.xCu206[9,10], but, unlike the latter, its oxygen deficiency does not occur in an ordered fashion. The as-prepared samples were highly resistive (in the Kfi.cm range) and showed semiconductive behavior down to 200 K. This behavior is consistent with the disordering arrangements of the Cu/Ti ions and the oxygen deficiencies. Doping of Sr site with Ba and Ca are possible; however, it remains highly resistive. CONCLUSION The structure of the compound is related to S r 3 T i 2 0 7 and L a 2 S r C u 2 0 6 with statistical occupations of Cu(Ti) ions and O vacancies. The lack of ordering in the Cu/Ti-O plane ruled out the possibility to made this compound superconducting or more conductive. New ion substitutions may be needed to form an ordered phase with structure composed by alternating Sr3Ti207 and La2SrCu206 units.
TABLE 2 Selected Bond Lengths (A) of La~Sr4Cu2Ti20~3 Bond Cu(Ti)-Oa Cu(Ti)-O2 Cu(Ti)-O3 Sr/La-O2 Sr/La-O3 Sr/La-O3' La/Sr-Oi La/Sr-O2
Lengths 2.070 1.942 1.948 2.654 2.439 2.763 2.744 2.708
Vol. 31, No. 5
CUPRATE TITANATE
543
ACKNOWLEDGMENTS Financial support from Fok Ying Tung Education Foundation is gratefully acknowledged.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
A. Gormezano and M.T. Weller, J. Mater. Chem. 3, 771 (1993). M.R. Palacin, A. Fuertes, N. Casan-Pastor and P. Gomez-Romero, Adv. Mater. 6, 54 (1994). R.K. Li, J. Solid State Chem. 114, 57 (1995). Li Rukang, J. Mater. Chem. 4, 773 (1994). A. Fukuoka, S. Adaehi, T. Sugano, X.J. Wu and H. Yamauchi, Physica C231}, 372 (1994). R.K. Li, C. Dong and L. Yang, Physica C247, 62 (1995). S. Roddlesden and P. Popper, Acta Crystallogr. 10, 538 (1957); S. Roddlesden and P. Popper, Acta Crystallogr. 11, 54 (1958). R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paiva-Santos, User's Guide toProgram DBWS9411 for Rietveld Analysis of X-ray and Neutron Patterns (1994). N. Nguyen, J. Choisnet and B. Raveau, Mater. Res. Bull. 17, 567 (1982), D.M. De Leeuw, C.A.H.A. Mutsaers, G.P. Geelen and C. Langereis, J. Solid State Chem. 80, 276 (1989).
Materials Research Bulletin, Vol. 31, No. 5, pp. 539-543, 1996 Copyright O i 996 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/96 $15.00 + .00
PII S0025-5408(96)00020-7
SYNTHESIS AND STRUCTURE OF A NEW MIXED CUPRATE-TITANATE: La2Sr4Cu2Ti2Ot3
R. Li Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China (Received October 18, 1995; Refereed)
ABSTRACT A new mixed cuprate-titanate compound with nominal composition of La2Sr4Cu2Ti20~3 has been prepared. Cell parameters of a = 3.8818(1) A and c = 20.3118(6) A have been obtained from indexing the X-ray diffraction (XRD) patterns of the samples. The structure refined from the XRD pattern showed that the compound is related to Ruddlesden-Popper phase with Cu and Ti statistically occupying the Ti position in Sr3Ti207. The oxygen vacancies were found at apical sites between the double Cu/Ti-O layers. We were unable to make the samples superconducting or more conductive due to the disordered nature of Ti/Cu occupation and oxygen vacancies in the samples. KEYWORDS: A. oxides, C. X-ray diffraction, D. crystal structure, D. electrical properties INTRODUCTION In developing new high Tc cuprate superconductors, a well-established rule is that the new compound must contain complete copper-oxygen planes in its structure. Some of the copper ions must occupy a unique site in the crystal lattice and this site should exclude the occupancy of any other ions presented in the chemical formula. Thus, studying the order or disorder arrangements of various ions with Cu in different structure types is an important route in the search for new superconductors. Recently, investigations on Ti-Cu ordering in perovskite-related phases have led to the discovery of several new layered compounds, e.g., Ti2Ba2Ln2Cu2Oy [1,2], (BaTiO3)m(Gd,Ce)3Cu2Oy (m = 1,2) [3,4], Ti2Ba2CaLn2Cu2Oy[5], and Ti2Ba2Ca(Gd,Ce)3Cu2Oy[6]. In all these compounds, the Ti ion 539
540
R. LI
Vol. 31, No. 5
is excluded from occupying Cu site because the pyramidal environment of Cu is not favorable for Ti. We extended the study to the mixed Sr3Ti207-LazSrCu2Oysystem, in order to find a new phase with ordered arrangement of CuO pyramidal and TiO octahedral coordinations. A new layered compound with the nominal composition of La2Sr4Cu2Ti20~3 was thus found. The synthesis and structure of the new compound will be reported here. EXPERIMENTAL The samples with nominal composition of LaSr2CuTiOy were prepared by solid-state reaction from the starting compounds La203 (99.99%), SrCO3 (99%), TiO2 (99.99%), and CuO (99%). All the starting materials were preheated at appropriate temperatures to evaporate traces of H20 or other gases absorbed. The well-ground mixtures of the starting materials were first heated at 900°C for 48 hours and then at 950 °C for another 48 hours with an intermittent grinding. After cooling down to room temperature, the samples were reground, pelletized, and calcined at 950°C for 72 hours and followed by furnace cooling. The powder X-ray diffraction (XRD) patterns were collected on a Rigaku rB diffractometer equipped with rotating anode source and CuK, radiation. The XRD pattern for the Rietveld refinement was recorded in a 20 range of 18 ° to 106" with a step width of 0.03* (20) and counting time of 3 seconds at each step.
RESULTS AND DISCUSSION The main peaks in the XRD pattern of La2Sr4Cu2Ti20~3 (Fig. 1) can be indexed with a tetragonal unit cell with lattice constants of a = 3.8818(1) A and c = 20.3118(6) A. Remaining weak peaks at 20 of 31.20 °, 33.34 °, and 47.74 ° might be due to an impurity phase corresponding to (La, Sr)2(Cu,Ti)O4 with the K2NiF4structure. Since superstructure reflections were not observed both in the XRD pattern and selected electron diffraction patterns, we decided to take a Roddlesden-Popper phase (Sr3Ti207) [7] related structure based on statistical occupation of Ti/Cu and LaJSr ions as a starting model for the Rietveld refinement (DBWS-9411 package [8]). The total of 34 refinable variables include 13 overall parameters for background, sample displacement, scale factor, peak width, peak asymmetry, preferred orientation, and the variation of peak width with angle and 21 structural parameters of cell constants, site occupation, atomic coordinates, and temperature factors. The above mentioned impurity peaks were excluded during the refinement. The Pearson VII function was found to be a better simulation of the diffraction peak profile. The final refinement converged to the following agreement indices of R,~, = 4.70, Ra = 3.78, S = 1.63, and d = 0.74 (Fig. 1). In the beginning of refinement, isotropic temperature factor and occupation factors for La/Sr, Cu/Ti, and oxygen atoms were successfully refined. During the refinement it became clear that the La/Sr, Cu/Ti are basically statistically occupied; the related ratios did not significantly deviate from their nominal compositions and therefore are subsequently fixed. When the site occupancies of all the oxygen atoms were refined, only the O1 (apical) site between two Ti(Cu)O6 planes showed strong deficiency (70% occupied), which led to partial pyramidal coordination of Cu/Ti-O. Considering the strong tendency of Ti to be coordinated with oxygen in an octahedral environment, we concluded that the oxygen
Vol. 31, No. 5
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ZO(CuK=) FIG. 1 The X-ray diffraction pattern of La2Sr4Cu:Ti2013 (dots represent the observed pattern; stars indicate the diffraction peaks due to impurity phase.)
FIG. 2 Structure model of La~Sr4Cu2Ti2Oi3(lightly and heavily shaded polyhedra are TiO6 and CuOs, respectively; open circle represents O1 with 30% occupation).
542
R. LI
Vol. 31, No. 5
TABLE 1 Refined Atomic Coordinates of La2Sr4Cu2Ti2Ol3 (with space group I4/mmmand cell parameters a = 3.8818(1) A and c = 20.3118(6) A, Rwp= 4.70, Rp = 3.55, RB = 3.78, R~xp= 2.86 Atom La/Sr Sr/La Cu/Ti Ol 03 04
Site 2a 4e 4e 2b 8g 4e
x 0 0.5 0.5 0 0.5 0.5
y 0 0.5 0.5 0 0 0.5
z 0 0.3179(1) 0.0959(1) 0.5 0.4070(1) 0.1978(2)
B(A2) 1.9 2.2 1.9 2.1(3) 3.0 2.4
Occupation 0.33/0.67 0.67/0.33 0.5/0.5 0.70 1.0 1.0
vacancies are most probably located between two Cu sites and that two opposite CuO5 pyramids are thus formed in the double Cu(Ti)O layers. Therefore, a structure model of Figure 2 can be built up. The atomic parameters and bond lengths obtained from the refinement are listed in Tables 1 and 2. The structure of the compound is derived from SraTi207 with a statistical Cu/Ti occupation in the TiO6 planes with 30% apical oxygen removed from the CuO6 octahedral. It is related to La2SrCu206 by adding 70% oxygen in the O1 site among the CuO5 pyramidal double layers. La2Sr4Cu:Ti20~3also has some resemblance to Ln~+xSr2.xCu206[9,10], but, unlike the latter, its oxygen deficiency does not occur in an ordered fashion. The as-prepared samples were highly resistive (in the Kfi.cm range) and showed semiconductive behavior down to 200 K. This behavior is consistent with the disordering arrangements of the Cu/Ti ions and the oxygen deficiencies. Doping of Sr site with Ba and Ca are possible; however, it remains highly resistive. CONCLUSION The structure of the compound is related to S r 3 T i 2 0 7 and L a 2 S r C u 2 0 6 with statistical occupations of Cu(Ti) ions and O vacancies. The lack of ordering in the Cu/Ti-O plane ruled out the possibility to made this compound superconducting or more conductive. New ion substitutions may be needed to form an ordered phase with structure composed by alternating Sr3Ti207 and La2SrCu206 units.
TABLE 2 Selected Bond Lengths (A) of La~Sr4Cu2Ti20~3 Bond Cu(Ti)-Oa Cu(Ti)-O2 Cu(Ti)-O3 Sr/La-O2 Sr/La-O3 Sr/La-O3' La/Sr-Oi La/Sr-O2
Lengths 2.070 1.942 1.948 2.654 2.439 2.763 2.744 2.708
Vol. 31, No. 5
CUPRATE TITANATE
543
ACKNOWLEDGMENTS Financial support from Fok Ying Tung Education Foundation is gratefully acknowledged.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
A. Gormezano and M.T. Weller, J. Mater. Chem. 3, 771 (1993). M.R. Palacin, A. Fuertes, N. Casan-Pastor and P. Gomez-Romero, Adv. Mater. 6, 54 (1994). R.K. Li, J. Solid State Chem. 114, 57 (1995). Li Rukang, J. Mater. Chem. 4, 773 (1994). A. Fukuoka, S. Adaehi, T. Sugano, X.J. Wu and H. Yamauchi, Physica C231}, 372 (1994). R.K. Li, C. Dong and L. Yang, Physica C247, 62 (1995). S. Roddlesden and P. Popper, Acta Crystallogr. 10, 538 (1957); S. Roddlesden and P. Popper, Acta Crystallogr. 11, 54 (1958). R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paiva-Santos, User's Guide toProgram DBWS9411 for Rietveld Analysis of X-ray and Neutron Patterns (1994). N. Nguyen, J. Choisnet and B. Raveau, Mater. Res. Bull. 17, 567 (1982), D.M. De Leeuw, C.A.H.A. Mutsaers, G.P. Geelen and C. Langereis, J. Solid State Chem. 80, 276 (1989).