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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]:

  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.

  2. 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 (t2g3, eg2) configuration is quite stable), or Cr3+ (likewise t2g3 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.

  3. 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].

  4. 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, TC.

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 PbZr1-xTixo3 (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: (La0.8Ba0.2CuO2.6±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 (La0.5Sr0.5)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 (Nd1/2Sr1/2)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 La1.2Sr1.8Mn2O7 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.

[21] S. Stemmer, Y. Lu, B. Foran, P. S. Lysaght, S. K. Streiffer, P. Fuoss, and S. Seifert. Grazing incidence small angle x-ray scattering studies of phase separation in hafnium silicate films. Appl. Phys. Lett., 83:3141-3143, 2003.

[22] S. Stemmer, S. K. Streiffer, F. Ernst, and M. Rühle. Atomistic structure of 90° domain walls in ferroelectric PbTiO3 thin films. Philos. Mag. A, 71:713-724, 1995.

[23] J. Goodey, J. Broussard, and P. S. Halasyamani. Synthesis, structure and characterization of a new second-harmonic-generating tellurite: Na2TeW2O9. Chem. Mater., 14:3174-3180, 2002.

[24] J. Goodey, K. M. Ok, C. Hofmann, J. Broussard, F. V. Escobedo, and P. S. Halasyamani. Syntheses, structures, and second-harmonic generating properties in new quaternary tellurites: A2TeW3O12 (A = K, Rb, or Cs). J. Solid State Chem., 175:3-12, 2003.

[25] H.-S. Ra, K. M. Ok, and P. S. Halasyamani. Combining second-order Jahn-Teller distorted cations to create highly efficient SHG materials: Synthesis, characterization and NLO properties of BaTeM2O9 (M = Mo(VI) or W(VI)). J. Am. Chem. Soc., 125:7764-7765, 2003.

[26] C. Felser and R. Seshadri. Conduction band polarization in some CMR materials: Evolving guidelines for new systems. Int. J. Inorg. Mater., 2:677-685, 2000.

[27] A. S. Risbud, N. A. Spaldin, Z. Q. Chen, S. Stemmer, and R. Seshadri. Magnetism in polycrystalline cobalt-substituted zinc oxide. Phys. Rev. B, 68:205202, 2003.

[28] R. Basu and R. Seshadri. Suppressing the charge-ordering transition in LiMn2O4 through substitution of Li by Mg. J. Mater. Chem., 10:507-510, 2000.

[29] R. Basu, C. Felser, A. Maignan, and R. Seshadri. Magnetization and magnetoresistive response of LiMn2O4 near the charge ordering transition. J. Mater. Chem., 10:1921-1924, 2000.

[30] U. K. Gautam, R. Seshadri, S. Vasudevan, and A. Maignan. Magnetic and transport properties, and electronic structure of the layered chalcogenide AgCrSe2. Solid State Commun., 122:607-612, 2002.

[31] P. A. Salvador, T.-D. Doan, B. Mercey, and B. Raveau. Stabilization of YMnO3 in a perovskite structure as a thin film. Chem. Mater., 10:2592-5, 1998.

[32] B. Mercey, P. A. Salvador, W. Prellier, T.-D. Doan, J. Wolfman, J. F. Hamet, M. Hervieu, and B. Raveau. Thin film deposition: a novel synthetic route to new materials. J. Mater. Chem., 9:233-244, 1999.

[33] B. Mercey, A. M. Haghiri-Gosnet, M. Hervieu, Ch. Simon, D. Chippaux, Ph. LeCoeur, W. Prellier, P. A. Salvador, and B. Raveau. In-situ monitoring of the growth and characterization of (PrMnO3)n(SrMnO3)n superlattices. J. Appl. Phys., 94:2716-2724, 2003.

[34] P. A. Salvador, A.-M. Haghiri-Gosnet, B. Mercey, M. Hervieu, and B. Raveau. Growth and magnetoresistive properties of (LaMnO3)m(SrMnO3)n superlattices. Appl. Phys. Lett., 75:2638-2640, 1999.

[35] A. Asthagiri, C. Niederberger, A. J. Francis, L. M. Porter, P. A. Salvador, and D. S. Sholl. SrTiO3(111) surface: An experimental and theoretical study. Surf. Sci., 537:134-152, 2003.

[36] H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-Riba, S. R. Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig, A. Roytburd, and R. Ramesh. Multiferroic BaTiO3 - CoFe2O4 nanostructures. Science, 303:661-663, 2004.

[37] N. A. Spaldin. Multiferroic materials tower up. Physics World, April, 2004.

 
   
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