Hydroflux Crystal Growth of Platinum Group Metal Hydroxides: Sr₆NaPd₂(OH)₁₇, Li₂Pt(OH)₆, Na₂Pt(OH)₆, Sr₂Pt(OH)₈, and Ba₂Pt(OH)₈

Abstract

Crystals of five complex metal hydroxides containing platinum group metals were grown by a novel low-temperature hydroflux technique, a hybrid approach between the aqueous hydrothermal and the molten hydroxide flux techniques. Sr₆NaPd₂(OH)₁₇ (<b>1</b>) crystallizes in orthorhombic space group <i>Pbcn</i> with lattice parameters <i>a</i> = 19.577(4) Å, <i>b</i> = 13.521(3) Å, and <i>c</i> = 6.885(1) Å. This compound has a three-dimensional framework structure with Sr­(OH)_<i>n</i> polyhedra, Na­(OH)₆ octahedra, and Pd­(OH)₄ square planes. Isostructural phases Li₂Pt­(OH)₆ (<b>2</b>) and Na₂Pt­(OH)₆ (<b>3</b>) crystallize in trigonal space group <i>P</i>-3 with lattice parameters of <i>a</i> = 5.3406(8) Å and <i>c</i> = 4.5684(9) Å and <i>a</i> = 5.7984(8) Å and <i>c</i> = 4.6755(9) Å, respectively. Structures of these materials consist of layers of A­(OH)₆ (A = Li (<b>2</b>), Na (<b>3</b>)) and Pt­(OH)₆ octahedra. Sr₂Pt­(OH)₈ (<b>4</b>) crystallizes in monoclinic space group <i>P</i>2₁/<i>c</i> with lattice parameters <i>a</i> = 5.9717(6) Å, <i>b</i> = 10.997(1) Å, <i>c</i> = 6.0158(6) Å, and β = 113.155(2)°, while Ba₂Pt­(OH)₈ (<b>5</b>) crystallizes in orthorhombic space group <i>Pbca</i> with lattice parameters <i>a</i> = 8.574(2) Å, <i>b</i> = 8.673(2) Å, and <i>c</i> = 10.276(2) Å. Both of these compounds have three-dimensional structures composed of Pt­(OH)₆ octahedra surrounded by either Sr­(OH)₈ or Ba­(OH)₉ polyhedra. Decomposition of these materials into condensed metal oxides, which is of importance to possible catalytic applications, was monitored via thermogravimetric analysis. For example, Na₂Pt­(OH)₆ (<b>3</b>) converts cleanly via dehydration into α-Na₂PtO₃