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 (1) crystallizes in orthorhombic space group Pbcn with lattice parameters a = 19.577(4) Å, b = 13.521(3) Å, and c = 6.885(1) Å. This compound has a three-dimensional framework structure with Sr(OH)n polyhedra, Na(OH)6 octahedra, and Pd(OH)4 square planes. Isostructural phases Li2Pt(OH)6 (2) and Na2Pt(OH)6 (3) crystallize in trigonal space group P-3 with lattice parameters of a = 5.3406(8) Å and c = 4.5684(9) Å and a = 5.7984(8) Å and c = 4.6755(9) Å, respectively. Structures of these materials consist of layers of A(OH)6 (A = Li (2), Na (3)) and Pt(OH)6 octahedra. Sr2Pt(OH)8 (4) crystallizes in monoclinic space group P2(1)/c with lattice parameters a = 5.9717(6) Å, b = 10.997(1) Å, c = 6.0158(6) Å, and β = 113.155(2)°, while Ba2Pt(OH)8 (5) crystallizes in orthorhombic space group Pbca with lattice parameters a = 8.574(2) Å, b = 8.673(2) Å, and c = 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 (3) converts cleanly via dehydration into α-Na2PtO3.