Phase Transformation of a K <subscript>2</subscript> GeF <subscript>6</subscript> Polymorph for Phosphors Driven by a Simple Precipitation-Dissolution Equilibrium and Ion Exchange.

Tuning crystal phase transformations is very important for obtaining polymorphs for phosphors with the ideal optical properties and stability. Mn 4+ -doped K 2 GeF 6 (KGF) is a typical polymorphic phosphor, but the thermodynamic and kinetic mechanism of its phase transformation is still unclear. Herein, the phase transformation of polymorphs varying from P 6 3 mc KGF and trigonal KGF to P 6 3 mc Si 4+ -doped KGF is realized by introducing the synergistic action of an HF solution and Si 4+ ions. The full structural refinements of KGF polymorphs at room temperature and the electronic band structure calculations were performed. The results show that the Si 4+ -doped hexagonal KGF polymorph with good photoluminescence properties is the most stable phase according to the calculated total energy landscape and relative formation energy. The morphologic changes were monitored in situ to clearly understand the rapid phase transformation mechanism, which proves that the phase transformation is driven by a simple precipitation-dissolution equilibrium and ionic exchange.