P. Shiv Halasyamani

Bulk characterization methods for non-centrosymmetric materials: second-harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity

Characterization methods for bulk non-centrosymmetric compounds are described. These methods include second-harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity. In this tutorial review with each phenomenon, details are given of the measurement techniques along with a brief history and background. Finally, data interpretation is discussed.

BiO(IO3): A New Polar Iodate that Exhibits an Aurivillius-Type (Bi2O2)2+ Layer and a Large SHG Response

A new noncentrosymmetric (NCS) and polar material containing two lone-pair cations, Bi(3+) and I(5+), and exhibiting an Aurivillius-type (Bi(2)O(2))(2+) layer has been synthesized and structurally characterized. The material, BiO(IO(3)), exhibits strong second-harmonic generation (SHG), ∼12.5 × KDP (or ∼500 × α-SiO(2)), using 1064 nm radiation, and is found in the NCS polar orthorhombic space group Pca2(1) (No. 29). The structure consists of (Bi(2)O(2))(2+) cationic layers that are connected to (IO(3))(-) anions. The macroscopic polarity, observed along the c-axis direction, may be attributed to the alignment of the IO(3) polyhedra. In addition to the crystal structure and SHG measurements, polarization and piezoelectric measurements were performed, as well as electronic structure analysis.

Polar or Nonpolar? A+ Cation Polarity Control in A2Ti(IO3)6(A = Li, Na, K, Rb, Cs, Tl)

We have synthesized a series of new alkali-metal or Tl(+) titanium iodates, A(2)Ti(IO(3))(6) (A = Li, Na, K, Rb, Cs, Tl). Interestingly the Li and Na phases are noncentrosymmetric (NCS) and polar, whereas the K, Rb, Cs, and Tl analogues are centrosymmetric (CS) and nonpolar. We are able to explain the change from NCS polar to CS nonpolar using cation-size arguments, coordination requirements, and bond valence concepts. The six materials are topologically similar, consisting of TiO(6) octahedra, each of which is bonded to six IO(3) polyhedra. These polyhedral groups are separated by the A(+) cations. Our calculations on Na(2)Ti(IO(3))(6) indicate that polarization reversal is energetically very unfavorable, rendering the material polar but not ferroelectric. For all of the materials, synthesis, structural characterization, electronic structure analysis, infrared spectra, UV-vis and thermogravimetric measurements, and ion-exchange reactions are reported. For the polar materials, second-harmonic generation, piezoelectricity, and polarization measurements were performed. Crystal data: Li(2)Ti(IO(3))(6): hexagonal, space group P6(3) (No. 173), a = b = 9.3834(11) A, c = 5.1183(6) A, Z = 1. Na(2)Ti(IO(3))(6): hexagonal, space group P6(3) (No. 173), a = b = 9.649(3) A, c = 5.198(3) A, Z = 1. K(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.2703(6) A, c = 11.3514(11) A, Z = 3. Rb(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.3757(16) A, c = 11.426(3) A, Z = 3. Cs(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.6726(5) A, c = 11.6399(10) A, Z = 3. Tl(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.4167(6) A, c = 11.3953(11) A, Z = 3.

Alignment of lone pairs in a new polar material: synthesis, characterization, and functional properties of Li2Ti(IO3)6.

A new polar noncentrosymmetric material, Li(2)Ti(IO(3))(6), has been synthesized and characterized. The material is built up from a TiO(6) octahedron that is linked to six IO(3) polyhedra. These polyhedral groups are separated by Li(+) cations. The Ti(4+) and I(5+) cations are in asymmetric polar coordination environments attributable to second-order Jahn-Teller effects. The distortion associated with the Ti(4+) cation is negligible, since the TiO(6) octahedra are completely surrounded by IO(3) polyhedra. The I(5+) cation is in a highly polar asymmetric coordination environment attributable to its stereoactive lone pair, which was qualitatively described by pseudopotential calculations of the electron localization function. The macroscopic polarity of Li(2)Ti(IO(3))(6) may be attributed to parallel alignment of the stereoactive lone pairs on the I(5+) cations. This parallel alignment profoundly influences the materials functional properties: second-harmonic generation, piezoelectricity, and pyroelectricity. The material is, however, not ferroelectric, as the polarization associated with I(5+) is not switchable.

RbMgCO3F: A New Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material

A new deep-ultraviolet nonlinear optical material, RbMgCO3F, has been synthesized and characterized. The achiral nonpolar acentric material is second harmonic generation (SHG) active at both 1064 and 532 nm, with efficiencies of 160 × α-SiO2 and 0.6 × β-BaB2O4, respectively, and exhibits a short UV cutoff, below 190 nm. RbMgCO3F possesses a three-dimensional structure of corner-shared Mg(CO3)2F2 polyhedra. Unlike other acentric carbonate fluorides, in this example, the inclusion of Mg(2+) creates pentagonal channels where the Rb(+) resides. Our electronic structure calculations reveal that the denticity of the carbonate linkage, monodentate or bidendate, to the divalent cation is a useful parameter for tuning the transparency window and achieving the sizable SHG response.

Design and Synthesis of the Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material Ba3(ZnB5O10)PO4

Access to the elusive deep-ultraviolet by direct second harmonic generation (SHG) enabled by a new beryllium-free zincoborate-phosphate crystal is reported. Ba3(ZnB5O10)PO4, exhibits large SHG responses at 1064 and 532 nm and a short 180 nm absorption edge. Centimeter-size crystals are grown, and quantum mechanical calculations demonstrate the key role played by ZnO4 tetrahedra in the enhanced optical responses.

The role of polar, lamdba (Λ)-shaped building units in noncentrosymmetric inorganic structures.

A methodology for the design of polar, inorganic structures is demonstrated here with the packing of lambda (Λ)-shaped basic building units (BBUs). Noncentrosymmetric (NCS) solids with interesting physical properties can be created with BBUs that lack an inversion center and are likely to pack into a polar configuration; previous methods to construct these solids have used NCS octahedra as BBUs. Using this methodology to synthesize NCS solids, one must increase the coordination of the NCS octahedra with maintenance of the noncentrosymmetry of the bulk. The first step in this progression from an NCS octahedron to an inorganic NCS solid is the formation of a bimetallic BBU. This step is exemplified with the compound CuVOF(4)(H(2)O)(7): this compound, presented here, crystallizes in an NCS structure with ordered, isolated [Cu(H(2)O)(5)](2+) cations and [VOF(4)(H(2)O)](2-) anions into Λ-shaped, bimetallic BBUs to form CuVOF(4)(H(2)O)(6)·H(2)O, owing to the Jahn-Teller distortion of Cu(2+). Conversely, the centrosymmetric heterotypes with the same formula MVOF(4)(H(2)O)(7) (M(II) = Co, Ni, and Zn) exhibit ordered, isolated [VOF(4)(H(2)O)](2-) and [M(H(2)O)(6)](2+) ionic species in a hydrogen bond network. CuVOF(4)(H(2)O)(7) exhibits a net polar moment while the heterotypes do not; this demonstrates that Λ-shaped BBUs give a greater probability for and, in this case, lead to NCS structures.

Role of acentric displacements on the crystal structure and second-harmonic generating properties of RbPbCO3F and CsPbCO3F.

Two lead fluorocarbonates, RbPbCO3F and CsPbCO3F, were synthesized and characterized. The materials were synthesized through solvothermal and conventional solid-state techniques. RbPbCO3F and CsPbCO3F were structurally characterized by single-crystal X-ray diffraction and exhibit three-dimensional (3D) crystal structures consisting of corner-shared PbO6F2 polyhedra. For RbPbCO3F, infrared and ultraviolet–visible spectroscopy and thermogravimetric and differential thermal analysis measurements were performed. RbPbCO3F is a new noncentrosymmetric material and crystallizes in the achiral and nonpolar space group P6̅m2 (crystal class 6̅m2). Powder second-harmonic generation (SHG) measurements on RbPbCO3F and CsPbCO3F using 1064 nm radiation revealed an SHG efficiency of approximately 250 and 300 × α-SiO2, respectively. Charge constants d33 of approximately 72 and 94 pm/V were obtained for RbPbCO3F and CsPbCO3F, respectively, through converse piezoelectric measurements. Electronic structure calculations indicate that the nonlinear optical response originates from the distorted PbO6F2 polyhedra, because of the even–odd parity mixing of the O 2p states with the nearly spherically symmetric 6s electrons of Pb2+. The degree of inversion symmetry breaking is quantified using a mode-polarization vector analysis and is correlated with cation size mismatch, from which it is possible to deduce the acentric properties of 3D alkali-metal fluorocarbonates.

A3V5O14 (A = K+, Rb+, or Tl+), new polar oxides with a tetragonal tungsten bronze related structural topology: synthesis, structure, and functional properties.

Three polar noncentrosymmetric (NCS) oxide materials, A(3)V(5)O(14) (A = K(+), Rb(+), or Tl(+)), have been synthesized by hydrothermal and conventional solid state techniques. Their crystal structures and functional properties (second-harmonic generation, piezoelectricity, and polarization) have been determined. The iso-structural materials exhibit a layered structural topology that consists of corner-sharing VO(4) tetrahedra and VO(5) square pyramids. The layers stack parallel to the c-axis direction and are separated by the K(+), Rb(+), or Tl(+) cations. Powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicate the materials exhibit moderate SHG efficiencies of approximately 100 x alpha-SiO(2). Additional SHG measurements, that is, particle size versus SHG efficiency, indicate the materials are type-I phase-matchable. Converse piezoelectric measurements for K(3)V(5)O(14), Rb(3)V(5)O(14), and Tl(3)V(5)O(14) revealed d(33) values of 28, 22, and 26 pm/V, respectively. Pyroelectric measurements, that is, temperature-dependent polarization measurements, resulted in pyroelectric coefficients of -2.2, -2.9, and -2.8 microC/m(2) x K at 65 degrees C, for K(3)V(5)O(14), Rb(3)V(5)O(14), and Tl(3)V(5)O(14) respectively. Frequency-dependent polarization measurements confirmed that all of the materials are nonferroelectric, consistent with our first principle density functional theory (DFT) electronic structure calculations. Infrared, UV-vis, thermogravimetric, and differential scanning calorimetry measurements were also performed. Crystal data: K(3)V(5)O(14), trigonal, space group P31m (No. 157), a = 8.6970(16) A, c = 4.9434(19) A, V = 323.81(15), and Z = 1; Rb(3)V(5)O(14), trigonal, space group P31m (No. 157), a = 8.7092(5) A, c = 5.2772(7) A, V = 346.65(5), and Z = 1; Tl(3)V(5)O(14), trigonal, space group P31m (No. 157), a = 8.7397(8) A, c = 5.0846(10) A, V = 336.34(8), and Z = 1.

The First Organically Templated Layered Uranium(IV) Fluorides: (H3N(CH2)3NH3)U2F10⋅2 H2O, (H3N(CH2)4NH3)U2F10⋅3 H2O, and (H3N(CH2)6NH3)U2F10⋅2 H2O

A new series of layered organically templated uranium(IV) fluorides has been exclusively synthesized under hydrothermal conditions. The unprecedented materials contain [U2 F10 ]2- anionic layers that are separated by charge balancing cationic templates and occluded water molecules (see structure depicted). The templates can be fully ion-exchanged for a variety of metals (Na+ , K+ , and Co2+ ) at room temperature.