Chemical Bonding Center | Research Theme I: Superferroelectrics & non-linear optical (NLO) materials

The focus of this theme is to create a new generation of ``super-ferroelectrics'' and ``super-nonlinear optical materials''. These phenomena require non-centrosymmetricity, and so only exist in solids that have polar space groups In most cases, polar behavior is achieved by off-centering of ions in polyhedra, which in turn is driven by local chemical bonding. Three distinctly different mechanisms (described below) have been identified for off-centering. The goal in this work is to combine two or all three of them in the same material so as to enhance the polar response.

Physical property inter-relationships between
non-centrosymmetric crystal classes

The first off-centering mechanism, known as a second order Jahn-Teller (SOJT) effect[1,1,2,3], is the ligand field stabilization of a d⁰ transition metal cation by its surrounding anions. This is the origin of the off-center displacement of the small cation in well-known perovskite ferroelectrics such as BaTiO₃[4]. The off-centering can occur toward the edge, face, or corner of the MO₆ octahedron[5], and often changes its orientation as a function of temperature. The second mechanism, also classified as a SOJT effect, occurs around cations that have an ns² valence electron configuration. These ns² ions tend to lose inversion symmetry by the formation of a localized lobe of electron density, often referred to as a ``lone pair''. Such stereochemical activity is the driving force for off-center distortion in the group IV chalcogenides like GeTe[6] and in Bi-based perovskites, such as BiMnO₃[7]. Finally a third mechanism, based on electrostatics, coordination geometry and size effects, rather than chemical bonding, has recently been identified by Spaldin in the hexagonal manganite, YMnO₃[8]. Sometimes two contributing mechanisms occur naturally in the same material, such as in PbTiO₃, which has a stereochemically active lone pair on the Pb ion, as well as SOJT off-centering of the Ti[9]. However no material has yet been identified in which all three of mechanisms combine constructively.

We will pursue two parallel research tracks within this theme. We will immediately initiate an empirical exploration of new materials that combine two or all of the off-centering mechanisms described above. Here we will draw on Halasyamani's recent successes in synthesizing several new second-order non-linear optical materials with strong doubling efficiencies by combining d⁰ transition metal and lone-pair cations[10,11,12]. A substantial challenge will be to control the cooperativity of the displacements. In (Na,Bi)TiO₃ for example, both Bi and Ti are active ions but they act in opposite directions resulting in a small overall polarization[13]. Halasyamani's early results show that the lone-pair cation often supports and reinforces the direction of the out-of-center distortion of the d⁰ transition metal[14], however nothing is yet known about the interaction of mechanism (3) with the SOJT distortions. Synthesis of new materials will first be pursued using traditional chemical approaches in the groups of Halasyamani, Seshadri and Woodward. However promising materials that remain elusive using the traditional approaches will be candidates for thin film growth by Salvador and Stemmer.

In parallel with the empirical effort, a major research component of this theme will be a fundamental study on the details of off-centering in solids. First we will conduct a comprehensive literature search of non-centric materials, and tabulate the results according to the mechanism type and the amount of distortion. Continuous symmetry measures that provide a statistical description of how far coordination polyhedra deviate from regularity[15] have been applied to organic SHG materials[15] and distortions of tetrahedral and planar complexes of d⁹ Cu^II[16]. We will extend the use of these statistical tools to characterize distorted coordination polyhedra in polar materials. Our particular focus will be to understand the factors that control the magnitude of off-center distortions of potential SOJT ions (d⁰ and ns² cations). Our research team is well-positioned to make progress on this problem. For example, Woodward's recent results suggest that the magnitude of the out-of-center distortion for the d⁰ transition metal scales with the electronegativity of the cation[17]. Spaldin and Seshadri have recently shown that the computed crystal orbital hamiltonian population (COHP) between cation s and ligand p states can be used as an indicator of trends in lone pair stereochemical activity[6]. In fact our computational methods are ideally suited to this component of the research, since the effects of size, alloying, chemistry and electronics in promoting or inhibiting off-centering can be isolated independently[18].

This theme is central to the CDM. Not only will the synergy between theory, synthesis and characterization provide a foundation towards the rational design of superferroelectric and NLO oxides, but also the fundamental understanding of local off-centering and long-range cooperative behavior that we develop will be directly applicable to the other themes.

Halasyamani's group has synthesized and structurally characterized three new polar materials, Li₂Ti(IO₃)₆, Na₂Ti(IO₃)₆, and BaNbO(IO₃)₅. All of the compounds exhibit second-harmonic generation as well as ferroelec- tricity. Calculations to understand differences in behavior between the compounds, and variable temperature powder X-ray diffraction measurements are in progess. Seshadri is performing calorimetry studies of these compounds in order to locate possible phase transitions from non-centrosymmetric room temperature phase to a centrosymmetric high temperature phase.

Woodward has studied the relationships between electronic structure, crystal structure and bonding in ternary oxides containing lone-pair cations and d0 cations. The compounds investigated include polymorphs of SnWO₄, BiVO₄, BiNbO₄ as well as PbWO₄. The study involves synthesis, spectroscopic characterization and electronic band structure calculations. In collaboration with Seshadri, Woodward's graduate student Matt Stoltzfus has been calculating Electron Localization Functions for these lone-pair compounds; locating the lone pair and understanding its character is an important collective goal of the proposal.
Woodward's work on synthesizing new lone-pair active perovskites and pyrochlores is described in Theme VI.

Salvador and Spaldin are investigating the stability of the hexagonal YMnO₃ structure (which we proposed as a starting material for \superferroelectrics") using pulsed laser deposition to deposit thin films, and density functional calculations to understand stabilities. More details are given in the discussion for Theme V.

The stability region of hexagonal ferroelectric ReMnO₃ materials can be extended using thin film growth techniques. For example, metastable hexagonal GdMnO₃ and DyMnO₃ can be grown by depositing either on epitaxial buffer layers or on polycrystals of YMnO₃ . The enhanced stability is the result of the use of appropriate substrates, which lead to low energy interfaces, and stabilize the metastable polymorph over the bulk form.

Computational studies show that metal 5s - O 2p antibonding interactions are responsible for the strong stereoactive electron lone pair distortions associated with Sn²⁺ , Sb³⁺ and Te⁴⁺ ions in oxides. Among their 6th period analogues, particularly Pb²⁺ and Bi³⁺ , this antibonding interaction is much weaker due to relativistic effects. Consequently, there is a much greater variety in the symmetry of the local coordination environment for these ions. Symmetric environments for the 6th period ions are favored by strong and symmetry bonding between oxygen and the transition metal ion.


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Theme I: Superferroelectrics & non-linear optical (NLO) materials. Theme Leader: Halasyamani The focus of this theme is to create a new generation of ``super-ferroelectrics'' and ``super-nonlinear optical materials''. These phenomena require non-centrosymmetricity, and so only exist in solids that have polar space groups In most cases, polar behavior is achieved by off-centering of ions in polyhedra, which in turn is driven by local chemical bonding. Three distinctly different mechanisms (described below) have been identified for off-centering. The goal in this work is to combine two or all three of them in the same material so as to enhance the polar response. Physical property inter-relationships between non-centrosymmetric crystal classes The first off-centering mechanism, known as a second order Jahn-Teller (SOJT) effect[1,1,2,3], is the ligand field stabilization of a d⁰ transition metal cation by its surrounding anions. This is the origin of the off-center displacement of the small cation in well-known perovskite ferroelectrics such as BaTiO₃[4]. The off-centering can occur toward the edge, face, or corner of the MO₆ octahedron[5], and often changes its orientation as a function of temperature. The second mechanism, also classified as a SOJT effect, occurs around cations that have an ns² valence electron configuration. These ns² ions tend to lose inversion symmetry by the formation of a localized lobe of electron density, often referred to as a ``lone pair''. Such stereochemical activity is the driving force for off-center distortion in the group IV chalcogenides like GeTe[6] and in Bi-based perovskites, such as BiMnO₃[7]. Finally a third mechanism, based on electrostatics, coordination geometry and size effects, rather than chemical bonding, has recently been identified by Spaldin in the hexagonal manganite, YMnO₃[8]. Sometimes two contributing mechanisms occur naturally in the same material, such as in PbTiO₃, which has a stereochemically active lone pair on the Pb ion, as well as SOJT off-centering of the Ti[9]. However no material has yet been identified in which all three of mechanisms combine constructively. We will pursue two parallel research tracks within this theme. We will immediately initiate an empirical exploration of new materials that combine two or all of the off-centering mechanisms described above. Here we will draw on Halasyamani's recent successes in synthesizing several new second-order non-linear optical materials with strong doubling efficiencies by combining d⁰ transition metal and lone-pair cations[10,11,12]. A substantial challenge will be to control the cooperativity of the displacements. In (Na,Bi)TiO₃ for example, both Bi and Ti are active ions but they act in opposite directions resulting in a small overall polarization[13]. Halasyamani's early results show that the lone-pair cation often supports and reinforces the direction of the out-of-center distortion of the d⁰ transition metal[14], however nothing is yet known about the interaction of mechanism (3) with the SOJT distortions. Synthesis of new materials will first be pursued using traditional chemical approaches in the groups of Halasyamani, Seshadri and Woodward. However promising materials that remain elusive using the traditional approaches will be candidates for thin film growth by Salvador and Stemmer. In parallel with the empirical effort, a major research component of this theme will be a fundamental study on the details of off-centering in solids. First we will conduct a comprehensive literature search of non-centric materials, and tabulate the results according to the mechanism type and the amount of distortion. Continuous symmetry measures that provide a statistical description of how far coordination polyhedra deviate from regularity[15] have been applied to organic SHG materials[15] and distortions of tetrahedral and planar complexes of d⁹ Cu^II[16]. We will extend the use of these statistical tools to characterize distorted coordination polyhedra in polar materials. Our particular focus will be to understand the factors that control the magnitude of off-center distortions of potential SOJT ions (d⁰ and ns² cations). Our research team is well-positioned to make progress on this problem. For example, Woodward's recent results suggest that the magnitude of the out-of-center distortion for the d⁰ transition metal scales with the electronegativity of the cation[17]. Spaldin and Seshadri have recently shown that the computed crystal orbital hamiltonian population (COHP) between cation s and ligand p states can be used as an indicator of trends in lone pair stereochemical activity[6]. In fact our computational methods are ideally suited to this component of the research, since the effects of size, alloying, chemistry and electronics in promoting or inhibiting off-centering can be isolated independently[18]. This theme is central to the CDM. Not only will the synergy between theory, synthesis and characterization provide a foundation towards the rational design of superferroelectric and NLO oxides, but also the fundamental understanding of local off-centering and long-range cooperative behavior that we develop will be directly applicable to the other themes. Current research activities: Halasyamani's group has synthesized and structurally characterized three new polar materials, Li₂Ti(IO₃)₆, Na₂Ti(IO₃)₆, and BaNbO(IO₃)₅. All of the compounds exhibit second-harmonic generation as well as ferroelec- tricity. Calculations to understand differences in behavior between the compounds, and variable temperature powder X-ray diffraction measurements are in progess. Seshadri is performing calorimetry studies of these compounds in order to locate possible phase transitions from non-centrosymmetric room temperature phase to a centrosymmetric high temperature phase. Woodward has studied the relationships between electronic structure, crystal structure and bonding in ternary oxides containing lone-pair cations and d0 cations. The compounds investigated include polymorphs of SnWO₄, BiVO₄, BiNbO₄ as well as PbWO₄. The study involves synthesis, spectroscopic characterization and electronic band structure calculations. In collaboration with Seshadri, Woodward's graduate student Matt Stoltzfus has been calculating Electron Localization Functions for these lone-pair compounds; locating the lone pair and understanding its character is an important collective goal of the proposal. Woodward's work on synthesizing new lone-pair active perovskites and pyrochlores is described in Theme VI. Salvador and Spaldin are investigating the stability of the hexagonal YMnO₃ structure (which we proposed as a starting material for \superferroelectrics") using pulsed laser deposition to deposit thin films, and density functional calculations to understand stabilities. More details are given in the discussion for Theme V. Research findings: New polar compounds that exhibit second-harmonic generation can be systematically synthesized using standard solid state chemistry approaches. Examples include Li₂Ti(IO₃)₆ , Na₂Ti(IO₃)₆ , and BaNbO(IO₃)₅ , and the tungsten bronzes ABW₂O₉ (A = K, Rb or Cs; B = Nb or Ta). Strong second-harmonic generating efficiency has been observed ( ~ 500 times that of α-SiO₂ ). The stability region of hexagonal ferroelectric ReMnO₃ materials can be extended using thin film growth techniques. For example, metastable hexagonal GdMnO₃ and DyMnO₃ can be grown by depositing either on epitaxial buffer layers or on polycrystals of YMnO₃ . The enhanced stability is the result of the use of appropriate substrates, which lead to low energy interfaces, and stabilize the metastable polymorph over the bulk form. Computational studies show that metal 5s - O 2p antibonding interactions are responsible for the strong stereoactive electron lone pair distortions associated with Sn²⁺ , Sb³⁺ and Te⁴⁺ ions in oxides. Among their 6th period analogues, particularly Pb²⁺ and Bi³⁺ , this antibonding interaction is much weaker due to relativistic effects. Consequently, there is a much greater variety in the symmetry of the local coordination environment for these ions. Symmetric environments for the 6th period ions are favored by strong and symmetry bonding between oxygen and the transition metal ion. References [1] P. S. Halasyamani and K. R. Poeppelmeier. Non-centrosymmetric oxides. Chem. Mater., 10:2753-2769, 1998. [2] R. G. Pearson. A symmetry rule for predicting molecular structures. J. Am. Chem. Soc., 91:4947-4955, 1969. [3] R. G. Pearson. The second-order Jahn-Teller effect. J. Molec. Struc. (THEOCHEM), 103:25-34, 1983. [4] N. A. Hill. Why are there so few magnetic ferroelectrics? J. Phys. Chem. B, 104(29):6694-6709, 2000. [5] J. B. Goodenough. Jahn-Teller phenomena in solids. Annu. Rev. Mater. Sci., 28:1-27, 1998. [6] U. V. Waghmare, N. A. Spaldin, H. C. Kandpal, and R. Seshadri. First principles indicators of metallicity and cation off-centricity in the IV-VI rock-salt chalcogenides of divalent Ge, Sn and Pb. Phys. Rev. B, 67:125111, 2003. [7] R. Seshadri and N. A. Hill. Visualizing the role of Bi 6s ``lone pairs'' in the off-center distortion in ferromagnetic BiMnO₃. Chem. Mater., 13:2892-2899, 2001. [8] B. B. van Aken, T. T. M. Palstra, A. Filippetti, and N. A. Spaldin. Origin of ferroelectricity in magnetoelectric YMnO₃. Nature Mater., 3:164-170, 2004. [9] R. E. Cohen and H. Krakauer. Electronic-structure studies of the differences in ferroelectric behavior of BaTiO₃ and PbTiO₃. Ferroelectrics, 136(1-4):65-83, 1992. [10] 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. [11] 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. [12] 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. [13] G. O. Jones and P. A. Thomas. Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na_0.5Bi_0.5TiO₃. Acta Crystallogr. B, 58:168-78, 2002. [14] P. S. Halasyamani. Primary and secondary distortion effects in d⁰ transition metal cations: Oxides containing both octahedrally coordinated d⁰ transition metals and lone-pair cations. Chem. Mater., 2003. submitted. [15] D. R. Kanis, J. S. Wong, T. J. Marks, M. A. Ratner, H. Zabrodsky, S. Keinan, and D. Avnir. Continuous symmetry analysis of hyperpolarizabilities. Characterization of the second-order nonlinear optical response of distorted benzene. J. Phys. Chem., 99:11061-11066, 1995. [16] S. Keinan and D. Avnir. Continuous symmetry analysis of tetrahedral/planar distortions. Copper chlorides and other AB₄ species. Inorg. Chem., 40:318-323, 2001. [17] H. W. Eng, P. W. Barnes, B. M. Auer, and P. M. Woodward. Investigations of the electronic structure of d⁰ transition metal oxides belonging to the perovskite family. J. Solid State Chem., 175:94-109, 2003. [18] A. D. Walkingshaw, N. A. Spaldin, and E. Artacho. A density-functional study of charge doping in WO₃. Phys. Rev. B, 2004. submitted.
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Theme I: Superferroelectrics & non-linear optical (NLO) materials. Theme Leader: Halasyamani

References

Theme I: Superferroelectrics & non-linear optical (NLO) materials.
Theme Leader: Halasyamani