Abstract
A new modification of Rb[Al(NH2)4] in space group C2/c, 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.
Highlights
Within the last decade, the continuing rise in the importance of semiconductors—for example, for LED technology—has driven the need for new and improved semiconducting materials such as AlN [1], GaN [2], InN [3], InAs [4] or ZnO [5]
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
When heating aluminum metal and RbNH2 in an autoclave in supercritical ammonia to a furnace temperature of 673 K with a maximum pressure of 680 bar, a colorless substance is obtained from the hot area at the bottom of the autoclave
Summary
The continuing rise in the importance of semiconductors—for example, for LED technology—has driven the need for new and improved semiconducting materials such as AlN [1], GaN [2], InN [3], InAs [4] or ZnO [5]. The synthesis route for these materials often involves chemical vapor deposition (CVD) [6,7]. With CVD being a highly energy-consuming and elaborate method, ammonothermal synthesis—using supercritical ammonia as reaction medium (critical data Tc = 134.5 ◦ C and pc = 119 bar)—combines the scalability and production of freestanding superior high-quality nitride single crystals. The ammonothermal method derived from hydrothermal synthesis, the more commonly known solvothermal technique, was first utilized by Jacobs and Juza in 1966 [8]. It gained popularity as a superior method to obtain nitride crystals with high purity and low defect concentration. Mineralizers are typically ammonoacids or -bases and serve to increase solubility
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