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Novel two-dimensional perovskites
PHYSICA@ ELSEVIER
Physiea C 282-287 (1997) 37-40
Novel Two-Dimensional Perovskites J. Georg Bednorz IBM Research Division, Zurich Research Laboratory, 8803 Rfischlikon, Switzerland Using the example of rare earth titanium oxides, a new route for tailoring superlattices in bulk oxides will be discussed. Starting with well-known ABOs compounds, series of artificial structures can be created in a systematic way by oxygen intercalation, i.e. by an ordered incorporation of excess oxygen to form ABOs+~ compositions. This intercalation process slices the three-dimensional ABOs structure into subtmits of various thickness, each representing a defined oxygen composition. These basic units can be combined in multiple ways to form new superlattices whose physical character varies between semiconductor and ferroelectric insulator. These variations can be achieved without changing the chemical character of the compound, in contrast to the usual procedure of extrinsic doping, which creates disorder.
With the discovery of high-T¢ superconductivity [1], perovskites and related layered oxides have experienced an unexpected revival in physics, chemistry, and materials science. The numerous two-dimensional (2D) compounds under investigation in the past were created by applying either the well-known Ruddlesden-Popper scheme [2] or modifications thereof. The latter result in the formation of structures in which single or multiple transition-metal (TM) oxygen octahedra layers-in contrast to the f o r m e r - - m a y consist only of fragments (ordering of oxygen defects) and are separated by one or more block layers [3]. The c o m m o n feature, however, is that the transformation from a 3D to a 2D structure is achieved by separating the TM polyhedra within a (100) perovskite plane. Only a few pure oxide compounds indicated that an ABO3 structure can be layered by cutting it along (110) planes (Fig. 1) and inserting an additional oxygen layer such as in rare earth (RE) titanates and alkaline earth (AE) niobates or tantalates of the general composition A2B2Oz [4-6]. La2Wi207 [7], Ca2Nb207 [8], and Sr2Nb2Ov [9], to mention but a few, are isostrucrural and exhibit ferroelectricity with transition temperatures >1300 K. The drastic change in electronic properties occurring when transforming the 3D perovskite base compound into a layered structure is demonstrated by the example of a LaTiO= phase diagram (Fig. 2), which has been studied in detail for the range from z -- 3.0 to 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. PII S0921-4534(97)00201-3
3.5 [10]. The single crystals used throughout our studies are obtained by applying the floating zone melting process in an argon atmosphere to ceramics in which the respective oxygen stoichiometry
(100) -cut and insert AO
A(n+ 2) B(n+ 1) O(3n+ 4)
110) - cut and insert O
A(n+ 1) B(n+ 1) O(3n+ 5) Fig. 1. Process to create layered structures from a 3D perovskite: Homologous series A(n+~)B(,~+l)O(s,~+4) with AO = RE or AE oxide and changing the anion and cation ratio according to the Ruddlesden-Popper scheme (top). New homologous series A(,~+I)B(,~+I)O(sn+5) whose structure and electronic properties change only with oxygen stoichiometry at a constant cation ratio (bottom).
38
JG. Bednorz/Physica C 282-287 (1997) 37-40
Periodic
3D Orthorhombic
2D- SL
300-
e~
E 100-
3.0
3.1
3.2
3.3
3.4
3.5
OxygenStoichiometry x
Fig. 2. Phase diagram of LaTiO= for oxygen stoichiometry z between 3.0 and 3.5 indicating structural changes from 3D orthorhombic to 2D periodic superlattices (SL) and electronic changes from antiferromagnetic (AF) insulator to metal, semiconductor, and ferroelectric insulator.
had been adjusted by employing appropriate ratios of T M oxides in various states of oxidation. La2Ti207 represents the fully oxidized end member (Ti4+:3d ° configuration) in the LaTiO= phase diagram at z -- 3.5. Its structure is obtained from the 3D perovskite by making cuts parallel to a (110) plane every four Ti-oxygen octahedra layers and refilling the free positions of the resulting defect octahedra with excess oxygen (y = 0.5). Every other perovskite slab (consisting of 4 octahedra layers) is shifted by one-half a unit in [001] direction. Although distortions of the octahedra lead to a low s y m m e t r y unit cell, i.e. either they are monoclinic or the canting within adjacent perovskite slabs in the niobates causes the unit cell to double, it will be sufficient in the remaining discussion to treat the layers as idealized structural subunits. Stoichiometric LaTiOa.0 (Ti:3d 1) has an orthorhombic distorted, perovskite structure (GdFeOs) and exhibits semiconductor-like transport properties below 300 K. At 150 K the compound undergoes a transition to an antiferromagnet, where spin canting superimposes weak ferromagnetism [11]. Approaching Tc, the system
exhibits a crossover to an insulator. The phase diagram clearly displays the system's high sensitivity to excess oxygen, with the transition temperature rapidly decreasing to zero in a narrow range of x, i.e. 3.0 < • < 3.1. Within this range the temperature dependence of the resistivity also changes gradually to metal-like with increasing excess oxygen; minima at Tc suggest a metal-insulator transition. The 3D structure, though increasingly disordered, remains stable up to z = 3.2, as revealed by transmission electron microscopy (TEM) and X-ray powder diffraction. When this threshold is exceeded, measurements of the electronic properties will be misleading because the microstructure of samples up to x = 3.4 indicates the occurrence of phase separation. As a result, a disordered intergrowth between the 3D and a new layered structure in the form of irregular lamellae is observed. Only the adjustment of an oxygen stoichiometry of 3.4 allows the growth of the new phase in bulk crystalline form. Like the case of La2Ti2Ov, its structure consists of perovskite slabs but its thickness has increased by one octahedra layer. Therefore, according to the terminology of the homologous series A(,~+l)B(,~+l)O(3,~+5), the former compound would represent the n -- 3 (La4Ti4014) and the latter the n = 4 (LasTisO17) member [12]. A major change, however, occurs in the electronic nature of the n = 4 compound with the formation of a mixed valent state (Ti3+/Ti 4+) leading to semiconducting transport properties with thermally activated behavior [13]. The activation energy at low temperature obtained from resistivity measurements is of the order of only a few millielectron-volts. Although the anisotropy of the structure is reflected by the resistivity ratio p.j_:p[[ (~102) at room temperature (RT), its decrease with decreasing temperatures (~10 at T < 12 K) can explain why no significant difference is observed in the respective activation energies. The general trends described by the LaTiO= phase diagram, with only some differences in the magnetic behavior of the 3D phases [14,15], are also found for the Nd, Pr, and Ce titanates [16]. In the case of the AE niobates, a continuous variation of oxygen, starting from the 3D host, has
J.G. Bednorz/Physica C 282-287 (1997) 37-40
RETiO3. 0
RETiO3.4
METAL OR
SEMICONDUCTOR
AF-MAGNET
RETiO3.44
RETiO3.46
39
RETiO3.5 INSULATOR FERROELECTRIC
F i g . 3. Schematic representation (idealized) of structures obtained by slicing 3D perovskites along a (110) plane and subsequent oxygen intercalation. Projection along the [001] perovskite axis. Filled and half-filled circles represent La positions at different heights (0 and 1/2), which also applies to the level of the surrounding octahedra of the respective slabs.
not yet been possible, owing to the presence of defect structures with Ca and Sr deficiencies, but the concept of oxygen intercalation is indeed applicable [13,17]. The range of z between 3.4 and 3.5 is of particular interest in all these compounds because, in addition to the members of the homologous series (n = 3 and n = 4), a combination of their respective subunits (Fig. 3) allows new superlattices to be created [18]. These subunits represent the basic building blocks of an insulator (n = 3) and a conductor (n = 4), which are combined in a bulk preparation process in a ratio of 1:1 or 2:1 (the latter of which is shown in Fig. 4) without any interfaces. The consequence of the progressive decoupling experienced by the "semiconductor" layers as the combination ratio is increased is reflected by an enhanced anisotropy over the entire temperature range between RT and 4 K. Hence, in perovskite-type oxides, it is possible to create a pure 2D confinement only a few atomic layers thick. This may eventually allow the various properties of the 3D perovskites to be studied in an ideal two-dimensionality when the method of oxygen intercalation can be applied to a larger variety of 3D compounds.
Fig. 4. TEM image of a periodic superlattice in the NdTiO= system composed of n = 3 and n = 4 subunits leading to a sequencing of n = 433433433433 etc. Overlay of idealized structure is a guide to the eye. In conclusion, it is shown that, by slicing the 3D perovskite along (110) and performing subsequent oxygen intercalation, the crystal structure and the electronic structure can be varied without changing the cation ratio. In addition to creating a new homologous series, new semiconductor superlattices with strong 2D confinement can be obtained for a series of RE titanates and AE niobates. REFERENCES 1. J . G . Bednorz and K. A. Miiller, Z. Phys. B 64, 189 (1986). 2. S.N. Ruddlesden and P. Popper, Acta Cryst. Zl, 54 (1958). 3. Y. Tokura and T. Arima, Jpn. J. Appl. Phys. 29, 2388 (1990). 4. M. Gasperin, Acta. Cryst. B 31, 2129 (1975); K. Scheunemann and H.-K. MiillerBuschbaum, J. Inorg. Nucl. Chem. 37, 1897 (1975). 5. K. Scheunemann and H.-K. Miiller-Buschbaum, J. Inorg. Nucl. Chem. 36, 1965 (1974). 6. N. Ishizawa, F. Marumo, T. Kawamura and M. Kimura, Acta Cryst. B 31, 1912 (1975). 7. S. Nanamatsu, M. Kimura, K. Doi, S. Matsushita and N. Yamada, Ferroelectrics 8, 511
40
JG. Bednorz/Physica C 282-287 (1997) 37-40
(1974). 8. R.D. Rosner and E.H. Turner, AppI. Opt. 7, 171 (1968). 9. S. Nanamatsu, M. Mimura, K. Doi and M. Takahashi, J. Phys. Soc. Japan 30, 300 (1971). 10. F. Lichtenberg, D. Widmer, J.G. Bednorz, T. Williams and A. Reller, Z. Phys. B 82, 211
(1991). 11. D.A. MacLean and J.E. Greedan, Inorganic Chem. 20, 1025 (1981). 12. T. Williams, H. Schmalle, A. Reller, F. Lichtenberg, D. Widmer and J.G. Bednorz, J. Solid State Chem. 93, 534 (1991). 13. F. Lichtenberg, T. Williams, A. Reller, D. Widmer and J.G. Bednorz, Z. Phys. B 84, 369 (1991).
14. D.A. MacLean, K. Seto and J.E. Greedan, J. Solid State Chem. 40, 241 (1981). 15. D.A. MacLean and J.E. Greedan, Inorg. Chem. 20, 1025 (1981). 16. J.G. Bednorz, K. Wachtmann, R. Broom and D. Ariosa, in Proc. NATO-ASI on Materials Aspects of High-To Superconductivity: 10 Years after the Discovery, ed. by E. Liarokapis and E. Kaldis (Kluwer, Dordrecht, 1997) in press. 17. H.W. Schmalle, T. Williams, A. Reller, F. Lichtenberg, D. Widmer and J.G. Bednorz, Acta Cryst. C 51, 1243 (1995). 18. T. Williams, F. Lichtenberg, D. Widmer, J.G. Bednorz and A. Reller, J. Solid State Chem. 103,375 (1993).
Physiea C 282-287 (1997) 37-40
Novel Two-Dimensional Perovskites J. Georg Bednorz IBM Research Division, Zurich Research Laboratory, 8803 Rfischlikon, Switzerland Using the example of rare earth titanium oxides, a new route for tailoring superlattices in bulk oxides will be discussed. Starting with well-known ABOs compounds, series of artificial structures can be created in a systematic way by oxygen intercalation, i.e. by an ordered incorporation of excess oxygen to form ABOs+~ compositions. This intercalation process slices the three-dimensional ABOs structure into subtmits of various thickness, each representing a defined oxygen composition. These basic units can be combined in multiple ways to form new superlattices whose physical character varies between semiconductor and ferroelectric insulator. These variations can be achieved without changing the chemical character of the compound, in contrast to the usual procedure of extrinsic doping, which creates disorder.
With the discovery of high-T¢ superconductivity [1], perovskites and related layered oxides have experienced an unexpected revival in physics, chemistry, and materials science. The numerous two-dimensional (2D) compounds under investigation in the past were created by applying either the well-known Ruddlesden-Popper scheme [2] or modifications thereof. The latter result in the formation of structures in which single or multiple transition-metal (TM) oxygen octahedra layers-in contrast to the f o r m e r - - m a y consist only of fragments (ordering of oxygen defects) and are separated by one or more block layers [3]. The c o m m o n feature, however, is that the transformation from a 3D to a 2D structure is achieved by separating the TM polyhedra within a (100) perovskite plane. Only a few pure oxide compounds indicated that an ABO3 structure can be layered by cutting it along (110) planes (Fig. 1) and inserting an additional oxygen layer such as in rare earth (RE) titanates and alkaline earth (AE) niobates or tantalates of the general composition A2B2Oz [4-6]. La2Wi207 [7], Ca2Nb207 [8], and Sr2Nb2Ov [9], to mention but a few, are isostrucrural and exhibit ferroelectricity with transition temperatures >1300 K. The drastic change in electronic properties occurring when transforming the 3D perovskite base compound into a layered structure is demonstrated by the example of a LaTiO= phase diagram (Fig. 2), which has been studied in detail for the range from z -- 3.0 to 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. PII S0921-4534(97)00201-3
3.5 [10]. The single crystals used throughout our studies are obtained by applying the floating zone melting process in an argon atmosphere to ceramics in which the respective oxygen stoichiometry
(100) -cut and insert AO
A(n+ 2) B(n+ 1) O(3n+ 4)
110) - cut and insert O
A(n+ 1) B(n+ 1) O(3n+ 5) Fig. 1. Process to create layered structures from a 3D perovskite: Homologous series A(n+~)B(,~+l)O(s,~+4) with AO = RE or AE oxide and changing the anion and cation ratio according to the Ruddlesden-Popper scheme (top). New homologous series A(,~+I)B(,~+I)O(sn+5) whose structure and electronic properties change only with oxygen stoichiometry at a constant cation ratio (bottom).
38
JG. Bednorz/Physica C 282-287 (1997) 37-40
Periodic
3D Orthorhombic
2D- SL
300-
e~
E 100-
3.0
3.1
3.2
3.3
3.4
3.5
OxygenStoichiometry x
Fig. 2. Phase diagram of LaTiO= for oxygen stoichiometry z between 3.0 and 3.5 indicating structural changes from 3D orthorhombic to 2D periodic superlattices (SL) and electronic changes from antiferromagnetic (AF) insulator to metal, semiconductor, and ferroelectric insulator.
had been adjusted by employing appropriate ratios of T M oxides in various states of oxidation. La2Ti207 represents the fully oxidized end member (Ti4+:3d ° configuration) in the LaTiO= phase diagram at z -- 3.5. Its structure is obtained from the 3D perovskite by making cuts parallel to a (110) plane every four Ti-oxygen octahedra layers and refilling the free positions of the resulting defect octahedra with excess oxygen (y = 0.5). Every other perovskite slab (consisting of 4 octahedra layers) is shifted by one-half a unit in [001] direction. Although distortions of the octahedra lead to a low s y m m e t r y unit cell, i.e. either they are monoclinic or the canting within adjacent perovskite slabs in the niobates causes the unit cell to double, it will be sufficient in the remaining discussion to treat the layers as idealized structural subunits. Stoichiometric LaTiOa.0 (Ti:3d 1) has an orthorhombic distorted, perovskite structure (GdFeOs) and exhibits semiconductor-like transport properties below 300 K. At 150 K the compound undergoes a transition to an antiferromagnet, where spin canting superimposes weak ferromagnetism [11]. Approaching Tc, the system
exhibits a crossover to an insulator. The phase diagram clearly displays the system's high sensitivity to excess oxygen, with the transition temperature rapidly decreasing to zero in a narrow range of x, i.e. 3.0 < • < 3.1. Within this range the temperature dependence of the resistivity also changes gradually to metal-like with increasing excess oxygen; minima at Tc suggest a metal-insulator transition. The 3D structure, though increasingly disordered, remains stable up to z = 3.2, as revealed by transmission electron microscopy (TEM) and X-ray powder diffraction. When this threshold is exceeded, measurements of the electronic properties will be misleading because the microstructure of samples up to x = 3.4 indicates the occurrence of phase separation. As a result, a disordered intergrowth between the 3D and a new layered structure in the form of irregular lamellae is observed. Only the adjustment of an oxygen stoichiometry of 3.4 allows the growth of the new phase in bulk crystalline form. Like the case of La2Ti2Ov, its structure consists of perovskite slabs but its thickness has increased by one octahedra layer. Therefore, according to the terminology of the homologous series A(,~+l)B(,~+l)O(3,~+5), the former compound would represent the n -- 3 (La4Ti4014) and the latter the n = 4 (LasTisO17) member [12]. A major change, however, occurs in the electronic nature of the n = 4 compound with the formation of a mixed valent state (Ti3+/Ti 4+) leading to semiconducting transport properties with thermally activated behavior [13]. The activation energy at low temperature obtained from resistivity measurements is of the order of only a few millielectron-volts. Although the anisotropy of the structure is reflected by the resistivity ratio p.j_:p[[ (~102) at room temperature (RT), its decrease with decreasing temperatures (~10 at T < 12 K) can explain why no significant difference is observed in the respective activation energies. The general trends described by the LaTiO= phase diagram, with only some differences in the magnetic behavior of the 3D phases [14,15], are also found for the Nd, Pr, and Ce titanates [16]. In the case of the AE niobates, a continuous variation of oxygen, starting from the 3D host, has
J.G. Bednorz/Physica C 282-287 (1997) 37-40
RETiO3. 0
RETiO3.4
METAL OR
SEMICONDUCTOR
AF-MAGNET
RETiO3.44
RETiO3.46
39
RETiO3.5 INSULATOR FERROELECTRIC
F i g . 3. Schematic representation (idealized) of structures obtained by slicing 3D perovskites along a (110) plane and subsequent oxygen intercalation. Projection along the [001] perovskite axis. Filled and half-filled circles represent La positions at different heights (0 and 1/2), which also applies to the level of the surrounding octahedra of the respective slabs.
not yet been possible, owing to the presence of defect structures with Ca and Sr deficiencies, but the concept of oxygen intercalation is indeed applicable [13,17]. The range of z between 3.4 and 3.5 is of particular interest in all these compounds because, in addition to the members of the homologous series (n = 3 and n = 4), a combination of their respective subunits (Fig. 3) allows new superlattices to be created [18]. These subunits represent the basic building blocks of an insulator (n = 3) and a conductor (n = 4), which are combined in a bulk preparation process in a ratio of 1:1 or 2:1 (the latter of which is shown in Fig. 4) without any interfaces. The consequence of the progressive decoupling experienced by the "semiconductor" layers as the combination ratio is increased is reflected by an enhanced anisotropy over the entire temperature range between RT and 4 K. Hence, in perovskite-type oxides, it is possible to create a pure 2D confinement only a few atomic layers thick. This may eventually allow the various properties of the 3D perovskites to be studied in an ideal two-dimensionality when the method of oxygen intercalation can be applied to a larger variety of 3D compounds.
Fig. 4. TEM image of a periodic superlattice in the NdTiO= system composed of n = 3 and n = 4 subunits leading to a sequencing of n = 433433433433 etc. Overlay of idealized structure is a guide to the eye. In conclusion, it is shown that, by slicing the 3D perovskite along (110) and performing subsequent oxygen intercalation, the crystal structure and the electronic structure can be varied without changing the cation ratio. In addition to creating a new homologous series, new semiconductor superlattices with strong 2D confinement can be obtained for a series of RE titanates and AE niobates. REFERENCES 1. J . G . Bednorz and K. A. Miiller, Z. Phys. B 64, 189 (1986). 2. S.N. Ruddlesden and P. Popper, Acta Cryst. Zl, 54 (1958). 3. Y. Tokura and T. Arima, Jpn. J. Appl. Phys. 29, 2388 (1990). 4. M. Gasperin, Acta. Cryst. B 31, 2129 (1975); K. Scheunemann and H.-K. MiillerBuschbaum, J. Inorg. Nucl. Chem. 37, 1897 (1975). 5. K. Scheunemann and H.-K. Miiller-Buschbaum, J. Inorg. Nucl. Chem. 36, 1965 (1974). 6. N. Ishizawa, F. Marumo, T. Kawamura and M. Kimura, Acta Cryst. B 31, 1912 (1975). 7. S. Nanamatsu, M. Kimura, K. Doi, S. Matsushita and N. Yamada, Ferroelectrics 8, 511
40
JG. Bednorz/Physica C 282-287 (1997) 37-40
(1974). 8. R.D. Rosner and E.H. Turner, AppI. Opt. 7, 171 (1968). 9. S. Nanamatsu, M. Mimura, K. Doi and M. Takahashi, J. Phys. Soc. Japan 30, 300 (1971). 10. F. Lichtenberg, D. Widmer, J.G. Bednorz, T. Williams and A. Reller, Z. Phys. B 82, 211
(1991). 11. D.A. MacLean and J.E. Greedan, Inorganic Chem. 20, 1025 (1981). 12. T. Williams, H. Schmalle, A. Reller, F. Lichtenberg, D. Widmer and J.G. Bednorz, J. Solid State Chem. 93, 534 (1991). 13. F. Lichtenberg, T. Williams, A. Reller, D. Widmer and J.G. Bednorz, Z. Phys. B 84, 369 (1991).
14. D.A. MacLean, K. Seto and J.E. Greedan, J. Solid State Chem. 40, 241 (1981). 15. D.A. MacLean and J.E. Greedan, Inorg. Chem. 20, 1025 (1981). 16. J.G. Bednorz, K. Wachtmann, R. Broom and D. Ariosa, in Proc. NATO-ASI on Materials Aspects of High-To Superconductivity: 10 Years after the Discovery, ed. by E. Liarokapis and E. Kaldis (Kluwer, Dordrecht, 1997) in press. 17. H.W. Schmalle, T. Williams, A. Reller, F. Lichtenberg, D. Widmer and J.G. Bednorz, Acta Cryst. C 51, 1243 (1995). 18. T. Williams, F. Lichtenberg, D. Widmer, J.G. Bednorz and A. Reller, J. Solid State Chem. 103,375 (1993).