Abstract:
Crystal growth of new chemical compounds as well as functional materials often proceeds from solvents and fluxes. This presentation discusses products and chemistry of crystal growth of main group element nitrides from ammonia and molten alkali metal as two prominent approaches.
The ammonothermal synthesis typically uses ammonobasic or ammonoacidic mineralizers in supercritical ammonia at elevated temperatures and pressures, and has been shown to be able to grow a wide range of interesting materials such as AlN crystals and free-standing GaN wafers. The latter have proven to be ideal substrates for manufacturing blue and white LEDs with superior performance. However, the crystal growth mechanism in the ammonothermal synthesis in terms of the involved solute species was poorly understood, although the chemical processes during dissolution of the starting material, mass transport, and crystallization of product crystals are of paramount importance. Intermediates and their influence on crystal growth in supercritical ammonia depending on the chemical nature of the mineralizer and the pressure and temperature conditions are discussed with a focus on GaN and InN.
Sodium and other alkali metal melts except for lithium on the contrary are unable to dissolve elemental nitrogen, however, with addition of alkaline earth metals synthesis of a wide range of novel compounds with an extremely broad range of combinations of oxidation states for example in tetrelides and nitridotetrelates is possible. Formation and structures of such rather exotic compounds is discussed.