Polar Noncentrosymmetric ZnMoSb₂O₇ and Nonpolar Centrosymmetric CdMoSb₄O₁₀: d¹⁰ Transition Metal Size Effect Influencing the Stoichiometry and the Centricity

Two new quaternary molybdenum­(VI) antimony­(III) oxides, ZnMoSb₂O₇ and CdMoSb₄O₁₀, have been synthesized in phase-pure form. The title compounds consist of highly polarizable cations, i.e., d⁰ (Mo⁶⁺) and d¹⁰ (Zn²⁺ or Cd²⁺), and lone-pair cations (Sb³⁺). ZnMoSb₂O₇ exhibits a three-dimensional framework with ZnO₄, MoO₄, and SbO₄ polyhedra in the polar space group <i>P</i>2₁, whereas CdMoSb₄O₁₀ exhibits one-dimensional tubule structures with CdO₆, MoO₄, and SbO₃ polyhedra in the space group <i>P</i>2₁/<i>m</i>. Several synthetic efforts suggest that the the dissimilar radii of Zn²⁺ and Cd²⁺ that can accommodate polyhedra of Sb³⁺ cations influence the stoichiometry as well as the centricity for the reported materials. Spectroscopic, thermal, and elemental analyses are reported along with dipole moment calculations. Nonlinear optical properties and their structural origin are examined for polar ZnMoSb₂O₇ as well

A Polar Titanium–Organic Chain with a Very Large Second-Harmonic-Generation Response

A noncentrosymmetric (NCS) titanium–organic compound, [H₂N­(CH₃)₂]­TiO­{[NC₅H₃(CO₂)₂]­[NC₅H₄(CO₂)]} (CAUMOF-18), has been synthesized by a solvothermal reaction. The aligned unidimensional polar chain structure of CAUMOF-18 consisting of corner-shared distorted TiO₅N₂ pentagonal bipyramids is attributed to strong hydrogen-bonding and π–π interactions. CAUMOF-18 reveals a very strong second-harmonic-generation efficiency of 400 times that of α-SiO₂ and is phase-matchable (type I). Water-molecule-driven reversible centricity conversion and topotactic transformation to TiO₂ microrods for CAUMOF-18 are also presented

Variable Asymmetric Chains in Transition Metal Oxyfluorides: Structure–Second-Harmonic-Generation Property Relationships

Four novel transition metal oxyfluorides, [Zn­(pz)₃]­[MoO₂F₄]·0.1H₂O (<b>1</b>), [Zn­(pz)₂F₂]­[Zn­(pz)₃]₂[WO₂F₄]₂ (<b>2</b>), [Cd­(pz)₄]­[Cd­(pz)₄(H₂O)]­[MoO₂F₄]₂·0.625H₂O (<b>3</b>), and [Zn­(mpz)₃]₂[MoO₂F₄]₂ (<b>4</b>) (pz = pyrazole; mpz = 3-methyl pyrazole) have been synthesized. Compounds <b>1</b> and <b>4</b> contain helical chains. Compound <b>2</b> accommodates zigzag chains, and compound <b>3</b> has quasi-one-dimensional linear chains. The variable chain structures are found to be attributable to the different structure-directing anionic groups and hydrogen bonding interactions. Compound <b>4</b> crystallized in the noncentrosymmetric (NCS) polar space group, <i>Pna</i>2₁, is nonphase-matchable (Type I), and reveals a moderate second-harmonic-generation (SHG) efficiency (10 × α-SiO₂). The observed SHG efficiency of compound <b>4</b> is due to the small net polarization occurring from the arrangement of ZnN₃F₂ trigonal bipyramids. Spectroscopic and thermal characterizations along with calculations for the title materials are reported

Li₆M(SeO₃)₄ (M = Co, Ni, and Cd) and Li₂Zn(SeO₃)₂: Selenites with Late Transition-Metal Cations

A series of lithium metal selenites, Li₆M­(SeO₃)₄ (M = Co, Ni, and Cd) and Li₂Zn­(SeO₃)₂, were synthesized by hydrothermal and solid-state reactions. Li₆M­(SeO₃)₄ is composed of Li⁺ cations, MO₆ octahedra, and SeO₃ polyhedra, while Li₂Zn­(SeO₃)₂ consists of Li⁺, Zn­(Li)­O₄ tetrahedra, and SeO₃ polyhedra. Isostructural Li₆Co­(SeO₃)₄ and Li₆Ni­(SeO₃)₄ crystallize in the rhombohedral space group <i>R</i>3̅, forming a three-dimensional distorted cubic lattice. Li₂Zn­(SeO₃)₂ crystallizes in the orthorhombic space group <i>Pbam</i> and reveals a layered structure in the <i>bc</i> plane. Li₆Cd­(SeO₃)₄ revealing a unidimensional structure crystallizes in the polar non-centrosymmetric space group <i>C</i>2, attributed to the parallel alignment of distorted CdO₆ octahedra. The direct-current magnetic susceptibility measurements unveil that Li₆Co­(SeO₃)₄ is a canted antiferromagnet with <i>T</i>_N = 25 K, while Li₆Ni­(SeO₃)₄ undergoes an antiferromagnetic transition at <i>T</i>_N = 54 K, having a negligible canted moment. The weak ferromagnetism observed in Li₆Co­(SeO₃)₄ indicates the significance of spin–orbit coupling, bringing about anisotropic exchange interactions. Li₆Cd­(SeO₃)₄ reveals a second harmonic generation (SHG) efficiency of 10 × α-SiO₂. Dipole moment calculations on Li₆Cd­(SeO₃)₄ indicate that the cooperative interaction of CdO₆ and SeO₃ is responsible for the observed SHG properties. Band gaps of the compounds are enlarged as atomic number increases. The effect of late transition-metal cations with different coordination numbers on the framework structures and the subsequent physical properties will be also discussed

Influence of Thermally Activated Solid-State Crystal-to-Crystal Structural Transformation on the Thermoelectric Properties of the Ca_5–<i>x</i>Yb_<i>x</i>Al₂Sb₆ (1.0 ≤ <i>x</i> ≤ 5.0) System

The solid-solution Zintl compounds with the mixed cations of Ca²⁺and Yb²⁺ in the Ca_5–<i>x</i>Yb_<i>x</i>Al₂Sb₆ (1.0 ≤ <i>x</i> ≤ 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba₅Al₂Bi₆-type and Ca₅Ga₂Sb₆-type phases have been characterized for seven compounds with 2.5 ≤ <i>x</i> ≤ 5.0 and three compounds with 1.0 ≤ <i>x</i> ≤ 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group <i>Pbam</i> (<i>Z</i> = 2, <i>oP</i>26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al₂Sb₈] double chains and isolated space-filling Ca²⁺/Yb²⁺ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca_1.5Yb_3.5Al₂Sb₆ from the Ba₅Al₂Bi₆-type to the Ca₅Ga₂Sb₆-type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable structure to the thermodynamically more stable crystal structure on the basis of theoretical calculations. Discrepancies of the local coordination geometries of the anionic [Al₂Sb₈] units and the geometrical arrangements of structural building moieties in the two distinct phases provoke the different electrical properties of metallic and semiconducting conduction, respectively, for the Ba₅Al₂Bi₆-type and Ca₅Ga₂Sb₆-type phases. Density of states and crystal orbital Hamilton population analyses based on tight-binding linear muffin-tin orbital calculations prove that the band-gap opening in the Ca₅Ga₂Sb₆-type phase should mainly be attributed to an extended bond distance of the bridging Sb–Sb in the [Al₂Sb₈] unit. A series of thermoelectric (TE) property measurements indicates that the phase transition via the postannealing process eventually results in an enhancement of the TE performance of Yb-rich Ca_1.5Yb_3.5Al₂Sb₆

Photoconversion Mechanisms and the Origin of Second-Harmonic Generation in Metal Iodates with Wide Transparency, NaLn(IO₃)₄ (Ln = La, Ce, Sm, and Eu) and NaLa(IO₃)₄:Ln³⁺ (Ln = Sm and Eu)

Four new metal iodates, namely, NaLn­(IO₃)₄ (Ln = La, Ce, Sm, and Eu), and a series of NaLa­(IO₃)₄:Ln³⁺ (Ln = Sm and Eu) solid solutions were synthesized through hydrothermal reactions. The structures of the title compounds are similar to that of NaY­(IO₃)₄ crystallizing in the acentric monoclinic space group <i>Cc</i>. The iodate materials reveal layered structures composed of LnO₈ square antiprisms and IO₃ polyhedra, in which each layer is connected by the I···O interactions. NaLa­(IO₃)₄ suggests a great potential as a matrix for optical source attributed to its acentricity and broad transparency from visible to mid-IR region. The photoluminescence properties depending on the concentration of Sm³⁺ reveal that NaLa­(IO₃)₄:Sm³⁺ undergoes a self-quenching relaxation over 7 mol % of Sm³⁺ by dipole–quadrupole interactions. Attributable to the asymmetric coordination environment of Ln³⁺, stronger electric dipole transitions compared to magnetic dipole transitions were observed for both compounds. In addition, the materials exhibit strong second-harmonic generation (SHG) responses and are type I phase-matchable. The structural origin of the SHG properties for the reported iodates is elucidated

Syntheses, Structures, and Characterization of Quaternary Tellurites, Li₃MTe₄O₁₁ (M = Al, Ga, and Fe)

Three new quaternary lithium metal tellurites, Li₃MTe₄O₁₁ (M = Al, Ga, and Fe), have been synthesized through hydrothermal and solid-state reactions by heating a mixture of LiOH·H₂O, TeO₂, and M₂O₃. The structures of the title compounds have been determined by single-crystal and powder X-ray diffraction. Li₃MTe₄O₁₁ reveal three-dimensional (3D) frameworks that consist of MO₆ octahedra, TeO₃ trigonal pyramids, and TeO₄ polyhedra. The variable coordination mode of Te⁴⁺ within the framework leads to the formation of 1D channels that host Li⁺ cations on both tetrahedral and octahedral sites. The bulk and grain boundary Li⁺ ion conductivities for a Li₃FeTe₄O₁₁ pellet in open air are estimated to be 1.0 × 10^–4 and 2.7 × 10^–6 S cm^–1, respectively, at room temperature from the impedance profile analysis. A lower activation energy of 19.9 kJ mol^–1 is obtained for the system, which is similar to that of Li₁₀GeP₂S₁₂ (24 kJ mol^–1). Detailed characterizations such as thermal, spectroscopic, and magnetic properties for the reported materials are also reported

Li₁₃Mn(SeO₃)₈: Lithium-Rich Transition Metal Selenite Containing Jahn–Teller Distortive Cations

A novel lithium-rich transition metal selenite, Li₁₃Mn­(SeO₃)₈, that is composed of a Jahn–Teller distortive cation, Mn³⁺, in the high spin d⁴ state, and a second-order Jahn–Teller (SOJT) distortive lone pair cation, Se⁴⁺, has been synthesized via hydrothermal and high temperature solid state reactions. The selenite is classified as a molecular compound consisting of MnO₆ octahedra, SeO₃ trigonal pyramids, and Li⁺ cations. Considering the Li–O interactions, the structure of Li₁₃Mn­(SeO₃)₈ may be described as a pseudo-three-dimensional framework as well. The title compound is thermally stable up to 500 °C and starts decomposing above the temperature attributable to the volatilization of SeO₂. While the MnO₆ octahedra in Li₁₃Mn­(SeO₃)₈ exhibit six identical Mn–O bond distances at room temperature due to the dynamic Jahn–Teller effect, a clear elongation of two Mn–O bonds along a specific direction is observed at 100 K. A series of isostructural selenites with different transition metals, i.e., Li₁₃M­(SeO₃)₈ (M = Sc, Cr, and Fe), have been also successfully obtained in phase pure forms using similar synthetic methods. Magnetic properties, spectroscopic characterizations, and local dipole moments calculations for all the synthesized selenites are presented