A New Modification of Rb[Al(NH₂)₄] and Condensation in Solid State

• Main Authors: Christian Bäucker, Rainer Niewa
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Crystals
• Subjects: ammonothermal synthesis; amide; aluminum
• Online Access: https://www.mdpi.com/2073-4352/10/11/1018

A new modification of Rb[Al(NH₂)₄] in space group <i>C</i>2/<i>c</i>, which differs from the known structural modification in the way the [Al(NH₂)₄]⁻-tetrahedra are arranged in the surrounding area of the rubidium cation, was obtained from ammonothermal synthesis at 673 K and 680 bar. The crystal structure was determined by Rietveld refinements and further investigated by infrared and Raman spectroscopy. Thermal gravimetric investigations indicate two decomposition steps up to 450 °C, which can be assigned to ammonia leaving the material while the sample liquefies. During the third and final step, volatile rubidium amide is released, leaving nano-scaled cubic AlN behind. Investigating differently aged samples implies decomposition and condensation of amidoaluminate ions already at ambient temperature, which is supported by refinements of single crystal X-ray diffraction data, revealing lower nitrogen amounts than expected. The observed single crystal also exhibits a significantly smaller volume than the reported structures, further supporting the decomposition–condensation mechanism.