The high-temperature crystal growth of intermetallics often asks for sealing of the materials in a protective atmosphere. Here, we report on the development of a convenient sealing method for alkali-containing melts, with high vapor pressure and reactivity. Our newly designed container made of high-temperature resistant steel can be sealed manually and reliably without any air exposure of the containing material. The closed container may be heated in air up to at least 1150 °C. The containers were applied for the development and optimization of a high-temperature self-flux growth of KFe1−xAg1+yCh2 (Ch = Se, Te) single crystals. Their crystal structure and the low-temperature electrical resistance are presented. The successful growths of these air-sensitive materials out of a reactive self-flux confirm the reliability of the container.
