Tae-Soo You

Pb2BO3Cl: A Tailor‐Made Polar Lead Borate Chloride with Very Strong Second Harmonic Generation

A meticulously designed, polar, non-centrosymmetric lead borate chloride, Pb2 BO3 Cl, was synthesized using KBe2 BO3 F2 (KBBF) as a model. Single-crystal X-ray diffraction revealed that the structure of Pb2 BO3 Cl consists of cationic [Pb2 (BO3 )](+) honeycomb layers and Cl(-) anions. Powder second harmonic generation (SHG) measurements on graded polycrystalline Pb2 BO3 Cl indicated that the title compound is phase-matchable (type I) and exhibits a remarkably strong SHG response, which is approximately nine times stronger than that of potassium dihydrogen phosphate, and the largest efficiency observed in materials with structures similar to KBBF. Further characterization suggested that the compound melts congruently at high temperature and has a wide transparency window from the near-UV to the mid-IR region.

ACdCO3F (A = K and Rb): new noncentrosymmetric materials with remarkably strong second-harmonic generation (SHG) responses enhanced via π-interaction

Two new noncentrosymmetric (NCS) materials, namely, ACdCO3F (A = K and Rb) containing both a d10 cation (Cd2+) and a π-conjugated parallel carbonate anion (CO32−), were synthesized through conventional solid state reactions. ACdCO3F exhibits a 3-dimensional structure that is composed of the stacked layers of [Cd(CO3)]∞. Each [Cd(CO3)]∞ layer is connected by infinite Cd–F–Cd chains and the [CO3] triangles are oriented in the same direction with a coplanar alignment. KCdCO3F and RbCdCO3F reveal remarkably strong second-harmonic generation (SHG) responses of approximately 9.0 and 7.2 times that of potassium dihydrogen phosphate (KDP), respectively, and both materials are phase-matchable. ACdCO3F exhibit wide transparent regions ranging from far UV to mid IR. Theoretical calculations confirm that the large SHG efficiencies indeed originate from enhancement via interatomic interactions between the s and p states of Cd2+ and the π-conjugated groups of the [CO3]2− unit within the [Cd(CO3)]∞ layers.

Mixed Cations and Structural Complexity in (Eu1-xCax)4In3Ge4 and (Eu1-xCax)3In2Ge3-The First Two Members of the Homologous Series A2[n+m]In2n+mGe2[n+m] (n, m = 1, 2, ...∞; A = Ca, Sr, Ba, Eu, or Yb)

Reported are the synthesis and the structural characterization of two members of a new homologous series of polar intermetallic compounds, which exist only with mixed alkaline-earth and rare-earth metal cations. Crystals of (Eu(1-x)Ca(x))(4)In(3)Ge(4) (0.35(1) <or= x <or= 0.70(1)) and (Eu(1-x)Ca(x))(3)In(2)Ge(3) (0.78(1) <or= x <or= 0.90(1)) have been grown using a molten In metal flux and structurally characterized by single-crystal X-ray diffraction. (Eu(1-x)Ca(x))(4)In(3)Ge(4) adopts the monoclinic Mg(5)Si(6)-type structure (space group C2/m, Z = 2, Pearson symbol mS22) with lattice parameters a = 16.874(1)-17.024(2) A, b = 4.496(3)-4.556(1) A, c = 7.473(4)-7.540(1) A, and beta = 107.306(10)-105.631(3) degrees . (Eu(1-x)Ca(x))(3)In(2)Ge(3) crystallizes with a novel orthorhombic structure (space group Pnma, Z = 4, Pearson symbol oP32) with lattice parameters in the ranges a = 7.382(2)-7.4010(9) A, b = 4.452(1)-4.4640(6) A, and c = 23.684(6)-23.734(3) A, depending on the Eu/Ca ratio. The polyanionic substructures in both cases are related and are based on InGe(4) edge-shared tetrahedra, Ge(2) dimers, and bridging In atoms in a nearly square-planar environment. The (Eu(1-x)Ca(x))(4)In(3)Ge(4) structure can be viewed as a 1:1 intergrowth of Mo(2)FeB(2)-like and TiNiSi-like fragments, whereas (Eu(1-x)Ca(x))(3)In(2)Ge(3) can be rationalized as a 2:1 intergrowth of the same structural motifs. Both phases exhibit fairly wide homogeneity ranges and exist only with mixed cations. The experimental results have been complemented by linear muffin-tin orbital tight-binding band structure calculations, as well as an analysis of the observed cationic site preferences.

Synthesis, structure, chemical bonding, and magnetism of the series RELiGe2 (RE = La-Nd, Sm, Eu).

This article focuses on the synthesis and the crystal chemistry of six members of a series of rare-earth metal based germanides with general formula RELiGe(2) (RE = La-Nd, Sm, and Eu). The structures of these compounds have been established by single-crystal X-ray diffraction (CaLiSi(2) structure type, space group Pnma, Z = 4, Pearson symbol oP16). The chemical bonding within this atomic arrangement can be rationalized in terms of anionic germanium zigzag chains, conjoined via chains of edge-shared LiGe(4) tetrahedra and separated by rare-earth metal cations. The structure can also be viewed as an intergrowth of AlB(2)-like and TiNiSi-like fragments, or as the result of the replacement of 50% of the rare-earth metal atoms by lithium in the parent structure of the REGe monogermanides. Except for LaLiGe(2) and SmLiGe(2), the remaining four RELiGe(2) phases exhibit Curie-Weiss paramagnetism above about 50 K. In the low temperature regime, the localized 4f electrons in CeLiGe(2), PrLiGe(2), and SmLiGe(2) order ferromagnetically, while antiferromagnetic ordering is observed for NdLiGe(2) and EuLiGe(2). The calculated effective magnetic moments confirm RE(3+) ground states in all cases excluding EuLiGe(2), in which the magnetic response is consistent with Eu(2+) configuration (J = S = 7/2). The experimental results have been complemented by tight-binding linear muffin-tin orbital (TB-LMTO) band structure calculations.

Quantitative Advances in the Zintl–Klemm Formalism

The Zintl–Klemm formalism has enjoyed tremendous success for rationalizing numerous network- and cluster-based structures involving main group elements. As research continues to explore the applicability of this potentially predictive concept, developments in theoretical and computational chemistry allow the study of larger and heavier molecular and solid-state building blocks to test this powerful formalism semiquantitatively, as well as improved handling of interatomic interactions involving widely disparate elements. Inherent in the Zintl–Klemm formalism is a coexisting tension between anisotropic, covalent bonding interactions, and isotropic, ionic, or metallic bonding forces collected in a system whose equilibrium volume is governed by atomic sizes via core repulsions. This chapter summarizes recent applications and quantitative developments of the Zintl–Klemm formalism, emphasizing results of first-principles calculations on molecules and extended solids, as well as selected experimental results that address the general validity of using this simple concept.

Closely related rare-earth metal germanides RE2Li2Ge3 and RE3Li4Ge4 (RE = La-Nd, Sm): synthesis, crystal chemistry, and magnetic properties.

Reported are the syntheses, crystal structures, and magnetic susceptibilities of two series of closely related rare-earth metal-lithium germanides RE(2)Li(2)Ge(3) and RE(3)Li(4)Ge(4) (RE = La-Nd, Sm). All title compounds have been synthesized by reactions of the corresponding elements at high temperatures, and their structures have been established by single-crystal X-ray diffraction. RE(2)Li(2)Ge(3) phases crystallize in the orthorhombic space group Cmcm (No. 63) with the Ce(2)Li(2)Ge(3) structure type, while the RE(3)Li(4)Ge(4) phases crystallize in the orthorhombic space group Immm (No. 71) with the Zr(3)Cu(4)Si(4) structure type, respectively. Both of their structures can be recognized as the intergrowths of MgAl(2)Cu- and AlB(2)-like slabs, and these traits of the crystal chemistry are discussed. Temperature-dependent direct-current magnetization measurements indicate Curie-Weiss paramagnetism in the high-temperature regime for RE(2)Li(2)Ge(3) and RE(3)Li(4)Ge(4) (RE = Ce, Pr, Nd), while Sm(2)Li(2)Ge(3) and Sm(3)Li(4)Ge(4) exhibit Van Vleck-type paramagnetism. The data are consistent with the local-moment magnetism expected for RE(3+) ground states. At temperatures below ca. 20 K, magnetic ordering transitions have been observed. The experimental results have been complemented by tight-binding linear muffin-tin orbital electronic-band-structure calculations.

Synthesis, crystal chemistry, and magnetic properties of RE7Li8Ge10 and RE11Li12Ge16 (RE = La-Nd, Sm): new members of the [REGe2](n)[RELi2Ge](m) homologous series.

Eight new rare-earth metal-lithium-germanides belonging to the [REGe(2)](n)[RELi(2)Ge](m) homologous series have been synthesized and structurally characterized by single-crystal X-ray diffraction. The structures of the title compounds can be rationalized as linear intergrowths of imaginary RELi(2)Ge (MgAl(2)Cu structure type) and REGe(2) (AlB(2) structure type) slabs. The compounds with general formula RE(7)Li(8)Ge(10) (RE = La-Nd, Sm), i.e., [REGe(2)](3)[RELi(2)Ge](4), crystallize in the orthorhombic space group Cmmm (No. 65) with a new structure type. Similarly, the compounds with general formula RE(11)Li(12)Ge(16) (RE = Ce-Nd), i.e., [REGe(2)](5)[RELi(2)Ge](6), crystallize in the orthorhombic space group Immm (No. 71) also with its own structure type. Temperature-dependent DC magnetization measurements indicate Curie-Weiss paramagnetism in the high-temperature regime and hint at complex magnetic ordering at low temperatures. The measured effective moments are consistent with RE(3+) ground states in all cases. The experimental results have been complemented by tight-binding linear muffin-tin orbital (TB-LMTO) electronic structure calculations.

To What Extent Does the Zintl-Klemm Formalism Work? The Eu(Zn1-xGex)2 Series

The series of ternary polar intermetallics Eu(Zn(1-x)Ge(x))(2) (0 < or = x < or = 1) has been investigated and characterized by powder and single-crystal X-ray diffraction as well as physical property measurements. For 0.50(2) < or = x < 0.75(2), this series shows a homogeneity width of hexagonal AlB(2)-type phases (space group P6/mmm, Pearson symbol hP3) with Zn and Ge atoms statistically distributed in the planar polyanionic 6(3) nets. As the Ge content increases in this range, a decreases from 4.3631(6) A to 4.2358(6) A, while c increases from 4.3014(9) A to 4.5759(9) A, resulting in an increasing c/a ratio. Furthermore, the Zn-Ge bond distance in the hexagonal net drops from 2.5190(3) A to 2.4455(3) A, while the anisotropy of the displacement ellipsoids significantly increases along the c direction. For x < 0.50 and x > 0.75, respectively, orthorhombic KHg(2)-type and trigonal EuGe(2)-type phases occur as a second phase in mixtures with an AlB(2)-type phase. Diffraction of the x = 0.75(2) sample shows incommensurate modulation along the c direction; a structural model in super space group P3m1(00gamma)00s reveals puckered 6(3) nets. Temperature-dependent magnetic susceptibility measurements for two AlB(2)-type compounds show Curie-Weiss behavior above 40.0(2) K and 45.5(2) K with magnetic moments of 7.98(1) mu(B) for Eu(Zn(0.48)Ge(0.52(2)))(2) and 7.96(1) mu(B) for Eu(Zn(0.30)Ge(0.70(2)))(2), respectively, indicating a (4f)(7) electronic configuration for Eu atoms (Eu(2+)). The Zintl-Klemm formalism accounts for the lower limit of Ge content in the AlB(2)-type phases but does not identify the observed upper limit. In a companion paper, the intrinsic relationships among chemical structures, compositions, and electronic structures are analyzed by electronic structure calculations.

Theoretical Interpretation of the Structural Variations along the Eu(Zn1−xGex)2 (0 ≤ x ≤ 1) Series

The electronic structures of EuZn(2), Eu(Zn(0.75)Ge(0.25))(2), Eu(Zn(0.5)Ge(0.5))(2), Eu(Zn(0.25)Ge(0.75))(2), and EuGe(2) have been investigated using tight-binding, linear muffin-tin orbital (TB-LMTO) and pseudopotential methods to understand the structural preferences influenced by valence electron counts and to explain the observed homogeneity range of the AlB(2)-type phases as reported in the companion article. A crystal orbital Hamilton population (COHP) analysis for Zn-Zn contacts in EuZn(2) suggests a possible homogeneity width for the KHg(2)-type phase, which is indicated from analysis of X-ray powder diffraction patterns. Total electronic energy comparisons, as well as density of states (DOS) and COHP analysis for a hypothetical Zn-rich compound, Eu(Zn(0.75)Ge(0.25))(2), indicate that two distinct phases, KHg(2)-type EuZn(2) and AlB(2)-type Eu(Zn(1-x)Ge(x))(2) (0.5 < or = x < or = 0.70), are more favorable than a single Zn-rich composition adopting the AlB(2)-type phase. Among 10 structural models of Eu(Zn(0.5)Ge(0.5))(2), the one with heteroatomic Zn-Ge interactions both within and perpendicular to the 6(3) nets is energetically the most favorable structure. The experimentally observed Zn-Ge bond distance is attributed to the contribution of both sigma- and pi-bond interactions. Zn-Ge, Eu-Zn, and Eu-Ge COHP curves of the minimum energy form of Eu(Zn(0.5)Ge(0.5))(2) show bonding character above the Fermi level and explain the observed wide homogeneity width of the AlB(2)-type phase. In the Ge-rich case, Eu(Zn(0.25)Ge(0.75))(2), the planar hexagonal nets are not energetically favorable due to the significant antibonding character of Ge-Ge bonding at the Fermi level. Structural relaxation using pseudopotentials also revealed that the hexagonal nets tend to pucker rather than being planar, in agreement with the observed incommensurately modulated superstructure. An electron localization function analysis for Eu(Zn(0.5)Ge(0.5))(2) reveals that there exists no two-center, two-electron bond or multicentered interactions between interlayer Zn...Ge contacts.

Rb3VO(O2)2CO3: A Four‐in‐One Carbonatoperoxovanadate Exhibiting an Extremely Strong Second‐Harmonic Generation Response

A nonlinear optical (NLO) carbonatoperoxovanadate, Rb3 VO(O2 )2 CO3 , was synthesized through a simple solution-evaporation method in phase-pure form. Single-crystal X-ray diffraction revealed that the structure of Rb3 VO(O2 )2 CO3 consists of important noncentrosymmetric (NCS) chromophores, that is, π-delocalized (CO3 )2- groups, a second-order Jahn-Teller (SOJT) distortive V5+ cation, and π-localized distorted O22- groups, as well as charge-balancing polarizable Rb+ ions. The powder second-harmonic generation (SHG) measurements indicated that Rb3 VO(O2 )2 CO3 is phase-matchable (Type I) and exhibits a remarkably strong SHG response circa 21.0 times that of potassium dihydrogen phosphate (KDP), which is the largest efficiency observed among carbonate NLO materials. First-principles calculation analysis suggests that the extremely large SHG response of Rb3 VO(O2 )2 CO3 is attributed to the synergistic effect of the cooperation of all the constituting NCS chromophores.