Novel alkali metal amidogallates as intermediates in ammonothermal GaN crystal growth

Single crystals of lithium tetra-amidogallate, Li[Ga(NH2)4], were obtained from the reaction of Ga metal and LiNH2 in supercritical ammonia at a pressure of 250 MPa and temperature of 400 °C. Two structural modifications were characterized by single crystal X-ray diffraction: a=5.849(1) Å, b=12.640(3) Å, c=6.858(1) Å, β=92.56(3)°, Z=4, space group P21/n; a=6.005(1) Å, b=7.394(2) Å, c=6.005(1) Å, β=103.51(3)°, Z=2, space group P21. Disodium tetra-amidogallate amide, Na2[Ga(NH2)4]NH2 (a=11.748(2) Å, b=6.681(1) Å, c=9.665(2) Å, Z=4, space group Pnma), was grown ammonothermally (p=130 MPa, T=580 °C) as single crystals in the course of synthesizing wurzite GaN employing NaNH2 as an ammono-basic mineralizer. Like known Na[Ga(NH2)4], all three novel compounds contain isolated tetra-amidogallate ions [Ga(NH2)4]− as constituents and likely candidates for dominant dissolved gallium-containing species in ammonothermal GaN synthesis and crystal growth under ammono-basic conditions, accomplishing the material transport. Raman spectroscopy data for Li[Ga(NH2)4] in both modifications as well as Na2[Ga(NH2)4]NH2 are provided and discussed.