Chemical Bonding Center | Research Theme VI: Proof of concept design problem

Based on the combined theoretical and experimental understanding that we develop, we will choose the most promising candidates, and then calculate their properties and assess their stability using accurate density functional methods.Here our initial ideas are to choose magnetic materials with transition metal cations that are most resistant to oxidation or reduction. Examples include high spin Fe³⁺ (the (t_2g³, e_g²) configuration is quite stable), or Cr³⁺ (likewise t_2g³ is a stable arrangement). Arrangement of the magnetic ions so that there is a net magnetic moment is most likely to be achieved by choosing a ferrimagnetic arrangement (in which adjacent cations are antiferromagnetically coupled to each other, but have different magnetic moments so that they do not cancel out). Ordering of the cations can be controlled by varying the radius, charge, and bonding preferences of the octahedral site cations[3,4,5]. Another approach is to use layer by layer film growth to obtain ordered cation distributions in systems where cation order cannot easily be obtained in bulk samples[6]. For calculating the properties of our proposed candidates, we will first use Woodward's SPuDS[2] to quickly obtain reasonable structures, then use Spaldin's newly developed self-interaction corrected pseudopotential method[7], which is capable of predictive accuracy for the complex materials described here.

Current research activities:

Spaldin's group has used density functional theory to predict the occurence of simultaneous ferrimagnetism and ferroelectricity in the layered perovsite, Bi(Fe,Cr)O₃. A manuscript describing the prediction has been published in Applied Physics Letters. Although this first prediction of a ferrimagnetic ferroelectric is encouraging, her work also showed that the magnetic Curie temperature would likely be below room temperature, and in addition, that deviations from ideal ordering would result in deterioration of the properties. Her group is now exploring other related multiferroics that might have higher ordering temperatures and, guided by Woodward's work decribed below, might be easier to grow!

Simultaneously, Woodward's group has been exploring synthesis of new perovskites and pyrochlores con taining Bi³⁺ with a focus on the synergy between ordering of B-site cations and ordering of A-site cations in complex perovskites. In particular, graduate student Meghan Knapp's exploratory synthetic efforts to find complex perovskites and pyrochlores have yielded a number of new and signifcant results. Meghan has prepared a number of quintinary perovskites with general composition AA'MM'O₆ (A = Li⁺, Na⁺, K⁺; A' = La³⁺, Y³⁺; M = Mg²⁺, Sc³⁺; M' = W⁶⁺, Nb⁵⁺, Sb⁵⁺). Meghan has been able to substitute a Bi³⁺ ion for the lanthanide ion into some of these compounds to prepare new compounds, such as NaBiScSbO₆ and NaBiGaSbO₆. The former compound can be prepared either as a perovskite or a pyrochlore depending upon synthesis conditions, while the latter compound is a pyrochlore. This is signifficant because there are relatively few compounds in either structural family containing Bi³⁺ ions. Yet among these few examples there are a number of compounds with attractive dielectric properties. We are currently in the process of characterizing the dielectric properties of these new phases.

Encouraged by Woodward's success in synthesizing new Bi-based perovskites, and Woodward and Seshadri's improved modeling of stereochemically active lone pairs, Spaldin has also pursued a new design project (not suggested in the original proposal); the design of new Pb-free piezoelectrics. The most widely used piezoelectric material today is the substitutional ceramic PbZr_1-xTi_xo₃ (PZT); applications include medical ultrasound devices, smart structures in automobiles, naval sonar and micromachines, to name a few. The market for such devices is huge, estimated to be tens of billions of dollars worldwide for sensors alone. The large piezoelectric response of PZT (~ 2700 µC/cm²) results from two factors. First, the stereochemical activity of the 6s₂ lone pair on the lead ion causes large structural distortions from the prototypical cubic perovskite phase, and in turn strong coupling between the electronic and structural degrees of freedom. And second, high sensitivity is caused by the competition between the different structures of the end point compounds PbTiO₃ and PbZrO₃ at the so-called morphotropic phase boundary in the alloy. However PZT poses significant environmental problems because of its high lead content. Graduate student Pio Baettig, in the Spaldin group, has shown that BiAlO₃ and BiGaO₃ have similar structural and electrical properties to the PZT end-point compounds PbZrO₃ and PbTiO₃, and therefore that Bi(Al,Ga)O₃ might be a suitable replacement for PZT. This work has been accepted for publication in Chemistry of Materials, and a number of groups are pursuing synthesis.

Research findings:

The double perovskite Bi₂FeCrO₆ has been predicted to be a ferrimagnetic ferroelectric.

The structures and electrical properties of as-yet-unsynthesized BiAlO₃ and BiGaO₃ have been calculated. An alloy of BiAlO₃ and BiGaO₃ has been proposed as a lead-free alternative to the widely-used piezoelectric, PZT.

We have shown experimentally that a second order Jahn Teller displacement of a d⁰ cation on the perovskite B-site (such as W⁶⁺ or Nb⁵⁺) is needed in order to stabilize layered ordering of the A-site cations, Na⁺ and La³⁺. This relationship was not previously understood.

We have discovered that it is possible to substitute Bi³⁺ ions for La³ ions in many perovskite and pyrochlore compounds. New compounds, including NaBiScSbO₆ and NaBiGaSbO₆, have been synthesized exploiting this discovery. The former compound can be prepared either as a perovskite or a pyrochlore depending upon synthesis conditions, while the latter compound is a pyrochlore. This is significant because there are relatively few compounds in either structural family containing Bi³⁺ ions.

References

[1] N. A. Spaldin and W. Pickett. Computational design of multifunctional materials. J. Solid State Chem., 176:615-632, 2003.

[2] M. W. Lufaso and P. M. Woodward. SPuDS: A program for crystal structure prediction within the perovskite family. Acta Crystallogr. B, 57:725-738, 2001.
[3] P. K. Davies. Cation ordering in complex oxides. Curr. Opn. Solid State Mater. Sci., 4:467-471, 1999.
[4] P. M. Woodward, R.-D. Hoffmann, and A. W. Sleight. Order-disorder in A₂M³⁺M⁵⁺O₆ perovskites. J. Mater. Res., 9:2118-2127, 1994.
[5] M. T. Anderson, K. B. Greenwood, G. R. Taylor, and K. R. Poeppelmeier. B-cation arrangements in double perovskites. Prog. Solid State Chem., 22:197-235, 1993.
[6] T. Kawai K. Ueda, H. Tabata. Ferromagnetism in LaFeO₃-LaCrO₃ superlattices. Science, 280:1064-1066, 1998.
[7] A. Filippetti and N. A. Spaldin. Self-interaction corrected pseudopotential scheme for magnetic and strongly-correlated systems. Phys. Rev. B, 67:125109, 2003.
[8] A. J. Francis, A. Bagal, and P. A. Salvador. Thin film synthesis of metastable perovskites: YMnO₃. Cer. Trans., 115:565-75, 2000.
[9] P. A. Salvador, T.-D. Doan, Mercey B., and B. Raveau. Thin film synthesis of copper-based perovskites having two-dimensional cation order: (La_0.8Ba_0.2CuO_2.6Â±6)_m(AECuO₂) superlattices. pages 285-290. Materials research society, 1999.
[10] P. A. Salvador, B. Mercey, O. Perez, A.-M. Haghiri-Gosnet, T.-D. Doan, and B. Raveau. Growth and structural characterization of Sr₂TiO₄: Chemical control over the terminating SrTiO₃ surface. Mater. Res. Soc. Symp. Proc., 587:1-6, 2000.
[11] J. Lu and S. Stemmer. Low-loss, tunable bismuth zinc niobate films deposited by RF magnetron sputtering. Appl. Phys. Lett., 83:2411-2413, 2003.
[12] D. O. Klenov, W. Donner, B. Foran, and S. Stemmer. Impact of stress on oxygen vacancy ordering in epitaxial (La_0.5Sr_0.5)CoO₃ thin films. Appl. Phys. Lett., 82:3427-3429, 2003.
[13] J. Lu, Z. Q. Chen, T. R. Taylor, and S. Stemmer. Composition control and dielectric properties of bismuth zinc niobate thin films synthesized by RF magnetron sputtering. J. Vac. Sci. Tech. A, 21:1745-1751, 2003.
[14] A. J. Francis and P. A. Salvador. Synthesis, structure, and physical properties of yttrium-doped strontium manganese oxide thin films. Mater. Res. Soc. Symp. Proc., 718:163-8, 2002.
[15] P. M. Woodward and K. Z. Baba-Kishi. Crystal structures of the relaxor oxide Pb₂ScTaO₆ in the paraelectric and ferroelectric states. J. Appl. Crystallogr., 35:233-242, 2002.
[16] P. M. Woodward, D. E. Cox, T. Vogt, C. N. R. Rao, and A. K. Cheetham. Effect of compositional fluctuations on the phase transitions in (Nd_1/2Sr_1/2)MnO₃. Chem. Mater., 11:3528-3538, 1999.
[17] P. M. Woodward, E. Suard, and P. Karen. Structural tuning of charge, orbital and spin ordering in double-cell perovskite series between NdBaFe₂O₅ and HoBaFe₂O₅. J. Am. Chem. Soc., 125:8889-8899, 2003.
[18] R. Seshadri, M. Hervieu, C. Martin, A. Maignan, B. Domenges, B. Raveau, and A. Fitch. A study of the layered magnetoresistive perovskite La_1.2Sr_1.8Mn₂O₇ by high resolution electron microscopy and synchrotron x-ray powder diffraction. Chem. Mater., 9(8):1778-1787, 1997.
[19] R. Seshadri, E. Suard, C. Felser, E. W. Finckh, A. Maignan, and W. Tremel. The 63 K phase transition in ZrTe₃: A neutron diffraction study. J. Mater. Chem., 8:2869-2874, 1998.
[20] E. M. James, N. D. Browning, A. W. Nicholls, M. Kawasaki, Y. Xin, and S. Stemmer. Demonstration of atomic resolution Z-contrast imaging by a JEOL JEM-2010F scanning transmission electron microscope. J. Electron Microsc., 47:561-574, 1998.


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Theme VI: Proof of concept design problem Theme Leader: Spaldin The final research theme is a beginning-to-end design, synthesis and characterization of a new multifunctional material. The design philosophy that we describe here permeates all of the previously described thematic research areas. However we include this as a separate section so that we have a specific trial problem as proof of concept for methodology in Phase II. Our motivation for testing a set of design principles stems in part from the desire to remain agile, and to ensure that our research remains broadly applicable to a range of materials systems. In addition, we envisage that a large component of Phase II of our Center will focus on applying the understanding that we gain in Phase I to the creation of designer new materials. Therefore one of our criteria for success in our performance evaluation is the completion of this sub-project. To have a realistic chance of success in the 3-year Phase I time-frame, we choose to focus on a project for which we have already developed some understanding of the materials chemistry. Our choice is the design of a robust magnetic ferroelectric, that is strongly insulating (so that it is able to sustain its ferroelectric polarization up to high temperatures) and strongly magnetic, with a ferro- or ferri-magnetic Curie temperature well above room temperature. Our design approach will adopt the following model[1]: Develop a complete understanding of the chemistry of specific structural arrangements that give rise to spontaneous and switchable polar behavior and the compatibility with strong magnetic ordering.Some progress will have been made on this in the previously-described themes; any remaining specific questions will be pursued here. In particular, we will examine how lone pair ions as drivers of polar, and particularly ferroelectric behavior, can be incorporated into ferrimagnetic structures. This should permit us to make more robust ferroelectrics with higher magnetic Curie temperatures. Our computational methods will range from modeling based on the bond valence concept[2] to lattice energy and ab-initio electronic structure calculations. We will perform detailed magnetic studies using our in-house SQUID magnetometer and between -5 T and 5 T with an absolute sensitivity of 10^-9 emu. Transport measurements will be carried out using the field and temperature control of this system and an insertion probe with external meters. In-house microscopy analyses will be complemented with time-of-flight neutron diffraction at the University of California HIPPO (High Intensity, Pressure, Preferred Orientation) diffractometer at the Los Alamos National Laboratories. Additional characterization experiments will include piezoelectric and pyroelectric measurements. Based on the combined theoretical and experimental understanding that we develop, we will choose the most promising candidates, and then calculate their properties and assess their stability using accurate density functional methods.Here our initial ideas are to choose magnetic materials with transition metal cations that are most resistant to oxidation or reduction. Examples include high spin Fe³⁺ (the (t_2g³, e_g²) configuration is quite stable), or Cr³⁺ (likewise t_2g³ is a stable arrangement). Arrangement of the magnetic ions so that there is a net magnetic moment is most likely to be achieved by choosing a ferrimagnetic arrangement (in which adjacent cations are antiferromagnetically coupled to each other, but have different magnetic moments so that they do not cancel out). Ordering of the cations can be controlled by varying the radius, charge, and bonding preferences of the octahedral site cations[3,4,5]. Another approach is to use layer by layer film growth to obtain ordered cation distributions in systems where cation order cannot easily be obtained in bulk samples[6]. For calculating the properties of our proposed candidates, we will first use Woodward's SPuDS[2] to quickly obtain reasonable structures, then use Spaldin's newly developed self-interaction corrected pseudopotential method[7], which is capable of predictive accuracy for the complex materials described here. Develop methods for synthesizing the required materials. Experiments will focus on the chemical synthesis of complex oxides, in which Halasyamani, Seshadri and Woodward are experts. In addition, Stemmer and Salvador will synthesize thin films of novel polar materials that have the predicted functionalities but are not easily stabilized in their bulk forms[8,9,10,11,12,13,14]. Comprehensive materials characterization, and verification of predictions. Woodward, Halasyamani and Seshadri will use high-resolution, variable-temperature powder diffraction (laboratory and synchrotron X-ray, constant wavelength and time-of-flight neutron) to characterize structural, electronic and magnetic phase transitions[15,16,17,18,19]. Stemmer has extensive expertise in structural characterization of advanced polar materials, in particular using new atomic resolution imaging and spectroscopy techniques[20,21,22]. Stemmer and Woodward are experienced in comprehensive high-frequency dielectric characterization[11,15]. Halasyamani has constructed a Kurtz-laser system that enables measurement of the second harmonic generation (SHG) properties on polycrystalline samples[23,24,25]. Seshadri is an expert on structure-property correlations in magnetic materials[26,27,28,29,30] and his group at UCSB operates a SQUID magnetometer. Salvador has expertise in characterizing the structural properties of thin films and artificial structures using X-ray diffraction and electron microscopy[8,31,9,32,33,34,10,35,14], as well as characterizing their magneto-transport properties[8,9,33,34]. Deviations from the predictions will be used to refine and further develop the theoretical methods or the synthetic techniques. Although our initial focus will be on single-phase materials that exhibit the desired properties, we point out that an alternative possibility for achieving desired multifunctional behavior is the creation of nanocomposites. In fact the construction of multifunctional magnetoelectric nano-pillar structures has recently been demonstrated[36,37], in which three-dimensional pillars of ferrimagnetic cobalt ferrite, CoFe₂O₄, are embedded in a matrix of ferroelectric barium titanate, BaTiO₃. Initial measurements of the magnetoelectric properties are encouraging. The electric polarization and piezoelectric response are both robust, although only about half of their values in equivalent volumes of single-crystal BaTiO₃. Such a reduction is typical of thin film samples, and is caused by clamping of the ferroelectric material by its surroundings, and possibly also by the presence of impurities. The magnetization is also strong, at about three-quarters of the ideal bulk value. Although the magnetoelectric coefficients have not yet been determined, temperature-dependent magnetization measurements show a discontinuity in the magnetization at the ferroelectric Curie temperature, T_C. Current research activities: Spaldin's group has used density functional theory to predict the occurence of simultaneous ferrimagnetism and ferroelectricity in the layered perovsite, Bi(Fe,Cr)O₃. A manuscript describing the prediction has been published in Applied Physics Letters. Although this first prediction of a ferrimagnetic ferroelectric is encouraging, her work also showed that the magnetic Curie temperature would likely be below room temperature, and in addition, that deviations from ideal ordering would result in deterioration of the properties. Her group is now exploring other related multiferroics that might have higher ordering temperatures and, guided by Woodward's work decribed below, might be easier to grow! Simultaneously, Woodward's group has been exploring synthesis of new perovskites and pyrochlores con taining Bi³⁺ with a focus on the synergy between ordering of B-site cations and ordering of A-site cations in complex perovskites. In particular, graduate student Meghan Knapp's exploratory synthetic efforts to find complex perovskites and pyrochlores have yielded a number of new and signifcant results. Meghan has prepared a number of quintinary perovskites with general composition AA'MM'O₆ (A = Li⁺, Na⁺, K⁺; A' = La³⁺, Y³⁺; M = Mg²⁺, Sc³⁺; M' = W⁶⁺, Nb⁵⁺, Sb⁵⁺). Meghan has been able to substitute a Bi³⁺ ion for the lanthanide ion into some of these compounds to prepare new compounds, such as NaBiScSbO₆ and NaBiGaSbO₆. The former compound can be prepared either as a perovskite or a pyrochlore depending upon synthesis conditions, while the latter compound is a pyrochlore. This is signifficant because there are relatively few compounds in either structural family containing Bi³⁺ ions. Yet among these few examples there are a number of compounds with attractive dielectric properties. We are currently in the process of characterizing the dielectric properties of these new phases. Encouraged by Woodward's success in synthesizing new Bi-based perovskites, and Woodward and Seshadri's improved modeling of stereochemically active lone pairs, Spaldin has also pursued a new design project (not suggested in the original proposal); the design of new Pb-free piezoelectrics. The most widely used piezoelectric material today is the substitutional ceramic PbZr_1-xTi_xo₃ (PZT); applications include medical ultrasound devices, smart structures in automobiles, naval sonar and micromachines, to name a few. The market for such devices is huge, estimated to be tens of billions of dollars worldwide for sensors alone. The large piezoelectric response of PZT (~ 2700 µC/cm²) results from two factors. First, the stereochemical activity of the 6s₂ lone pair on the lead ion causes large structural distortions from the prototypical cubic perovskite phase, and in turn strong coupling between the electronic and structural degrees of freedom. And second, high sensitivity is caused by the competition between the different structures of the end point compounds PbTiO₃ and PbZrO₃ at the so-called morphotropic phase boundary in the alloy. However PZT poses significant environmental problems because of its high lead content. Graduate student Pio Baettig, in the Spaldin group, has shown that BiAlO₃ and BiGaO₃ have similar structural and electrical properties to the PZT end-point compounds PbZrO₃ and PbTiO₃, and therefore that Bi(Al,Ga)O₃ might be a suitable replacement for PZT. This work has been accepted for publication in Chemistry of Materials, and a number of groups are pursuing synthesis. Research findings: The double perovskite Bi₂FeCrO₆ has been predicted to be a ferrimagnetic ferroelectric. The structures and electrical properties of as-yet-unsynthesized BiAlO₃ and BiGaO₃ have been calculated. An alloy of BiAlO₃ and BiGaO₃ has been proposed as a lead-free alternative to the widely-used piezoelectric, PZT. We have shown experimentally that a second order Jahn Teller displacement of a d⁰ cation on the perovskite B-site (such as W⁶⁺ or Nb⁵⁺) is needed in order to stabilize layered ordering of the A-site cations, Na⁺ and La³⁺. This relationship was not previously understood. We have discovered that it is possible to substitute Bi³⁺ ions for La³ ions in many perovskite and pyrochlore compounds. New compounds, including NaBiScSbO₆ and NaBiGaSbO₆, have been synthesized exploiting this discovery. The former compound can be prepared either as a perovskite or a pyrochlore depending upon synthesis conditions, while the latter compound is a pyrochlore. This is significant because there are relatively few compounds in either structural family containing Bi³⁺ ions. References [1] N. A. Spaldin and W. Pickett. Computational design of multifunctional materials. J. Solid State Chem., 176:615-632, 2003. [2] M. W. Lufaso and P. M. Woodward. SPuDS: A program for crystal structure prediction within the perovskite family. Acta Crystallogr. B, 57:725-738, 2001. [3] P. K. Davies. Cation ordering in complex oxides. Curr. Opn. Solid State Mater. Sci., 4:467-471, 1999. [4] P. M. Woodward, R.-D. Hoffmann, and A. W. Sleight. Order-disorder in A₂M³⁺M⁵⁺O₆ perovskites. J. Mater. Res., 9:2118-2127, 1994. [5] M. T. Anderson, K. B. Greenwood, G. R. Taylor, and K. R. Poeppelmeier. B-cation arrangements in double perovskites. Prog. Solid State Chem., 22:197-235, 1993. [6] T. Kawai K. Ueda, H. Tabata. Ferromagnetism in LaFeO₃-LaCrO₃ superlattices. Science, 280:1064-1066, 1998. [7] A. Filippetti and N. A. Spaldin. Self-interaction corrected pseudopotential scheme for magnetic and strongly-correlated systems. Phys. Rev. B, 67:125109, 2003. [8] A. J. Francis, A. Bagal, and P. A. Salvador. Thin film synthesis of metastable perovskites: YMnO₃. Cer. Trans., 115:565-75, 2000. [9] P. A. Salvador, T.-D. Doan, Mercey B., and B. Raveau. Thin film synthesis of copper-based perovskites having two-dimensional cation order: (La_0.8Ba_0.2CuO_2.6Â±6)_m(AECuO₂) superlattices. pages 285-290. Materials research society, 1999. [10] P. A. Salvador, B. Mercey, O. Perez, A.-M. Haghiri-Gosnet, T.-D. Doan, and B. Raveau. Growth and structural characterization of Sr₂TiO₄: Chemical control over the terminating SrTiO₃ surface. Mater. Res. Soc. Symp. Proc., 587:1-6, 2000. [11] J. Lu and S. Stemmer. Low-loss, tunable bismuth zinc niobate films deposited by RF magnetron sputtering. Appl. Phys. Lett., 83:2411-2413, 2003. [12] D. O. Klenov, W. Donner, B. Foran, and S. Stemmer. Impact of stress on oxygen vacancy ordering in epitaxial (La_0.5Sr_0.5)CoO₃ thin films. Appl. Phys. Lett., 82:3427-3429, 2003. [13] J. Lu, Z. Q. Chen, T. R. Taylor, and S. Stemmer. Composition control and dielectric properties of bismuth zinc niobate thin films synthesized by RF magnetron sputtering. J. Vac. Sci. Tech. A, 21:1745-1751, 2003. [14] A. J. Francis and P. A. Salvador. Synthesis, structure, and physical properties of yttrium-doped strontium manganese oxide thin films. Mater. Res. Soc. Symp. Proc., 718:163-8, 2002. [15] P. M. Woodward and K. Z. Baba-Kishi. Crystal structures of the relaxor oxide Pb₂ScTaO₆ in the paraelectric and ferroelectric states. J. Appl. Crystallogr., 35:233-242, 2002. [16] P. M. Woodward, D. E. Cox, T. Vogt, C. N. R. Rao, and A. K. Cheetham. Effect of compositional fluctuations on the phase transitions in (Nd_1/2Sr_1/2)MnO₃. Chem. Mater., 11:3528-3538, 1999. [17] P. M. Woodward, E. Suard, and P. Karen. Structural tuning of charge, orbital and spin ordering in double-cell perovskite series between NdBaFe₂O₅ and HoBaFe₂O₅. J. Am. Chem. Soc., 125:8889-8899, 2003. [18] R. Seshadri, M. Hervieu, C. Martin, A. Maignan, B. Domenges, B. Raveau, and A. Fitch. A study of the layered magnetoresistive perovskite La_1.2Sr_1.8Mn₂O₇ by high resolution electron microscopy and synchrotron x-ray powder diffraction. Chem. Mater., 9(8):1778-1787, 1997. [19] R. Seshadri, E. Suard, C. Felser, E. W. Finckh, A. Maignan, and W. Tremel. The 63 K phase transition in ZrTe₃: A neutron diffraction study. J. Mater. Chem., 8:2869-2874, 1998. [20] E. M. James, N. D. Browning, A. W. Nicholls, M. Kawasaki, Y. Xin, and S. Stemmer. Demonstration of atomic resolution Z-contrast imaging by a JEOL JEM-2010F scanning transmission electron microscope. J. Electron Microsc., 47:561-574, 1998.