Hydrothermal Synthesis and Crystal Structure of Polar and Nonpolar Compounds in Indium Iodate Family

Abstract

Using hydrothermal synthesis, a series of indium iodates α-K3In(IO3)6 (1), β-K3In(IO3)6(2), In(OH)(IO3)2·H2O (3), and In(IO3)3 (4) from zero-dimensional, one-dimensional, to two-dimensional have been prepared. Compound 1 crystallizes in an orthorhombic system with space group Fdd2, a = 39.596(7) Å, b = 8.2984(14) Å, c = 11.258(2) Å, V = 3699.1(11) Å3. Compounds 2, 3, and 4 are obtained in the same reaction reagents with different pH values. At a pH value of 0.5, compound 2 is formed, which crystallizes in a triclinic system with space group P1̅, a = 6.9441(14) Å, b = 7.0685(14) Å, c = 10.639(12) Å, α = 98.14(3)°, β = 108.77(3)°, γ = 102.72(3)°, V = 469.4(20) Å3. The increase of pH value to 1.5 gives rise to compound 3, which crystallizes in a triclinic system with space group P1̅, a = 6.7321(13) Å, b = 7.5255(15) Å, c = 7.6798(15) Å, α = 100.43(3)°, β = 91.96(3)°, γ = 106.64(3)°, V = 365.09(12) Å3. When the pH value is further raised to 3, compound 4 is obtained in high yield (about 90% based on indium). The structure of compound 1 is made up of zero-dimensional units consisting of [In(IO3)6]3− anions separated by K+ cations. The indium centers are ligated by six monodentate iodate anions, of which the terminal oxygen atoms form long ionic contacts with K+ cations. Distinctively, compound 1 crystallizes in the polar space group Fdd2. The polarity in the structure is imparted by the alignment of the stereochemically active lone pairs of electrons of the iodate anions along the c-axis. Second-harmonic generation of 532 nm light from a 1064 nm laser source yields a response approximately similar to that of KDP. The same reaction stoichiometric undergoes pH-mediated structural transformation leading to the formation of zero-dimensional compound 2, one-dimensional compound 3, and two-dimensional compound 4. Compound 2 is also composed of [In(IO3)6]3− anions separated by K+ cations but is nonpolar. Compound 3 displays a one-dimensional ribbon structure constructed from [InO6] octahedra and [IO3] pyramids and separated by water molecules. [InO6] octahedra share corners to form a chain, of which two sides are decorated by [IO3] pyramids by bridging the adjacent octahedra. Compound 4 is known to contain Te4O9-type structure, in which iodate anions bridge In3+ cations to form two-dimensional layers. The reaction conditions and thermal stabilities of these compounds are investigated.