The chemistry of ternary nitrides is reviewed with special focus on developments of the last two years (1996 and 1997). In particular, structures and properties of ternary and higher transition metal nitrides, main group nitrides, subnitrides, and nitride halides are compared, and a section on thermodynamics of ternary nitrides is included. Finally, methods for the preparation of gallium nitride single crystals are summarized.

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Ammonothermal synthesis is a method for synthesis and crystal growth suitable for a large range of chemically different materials, such as nitrides (e.g., GaN, AlN), amides (e.g., LiNH 2 , Zn(NH 2 ) 2 ), imides (e.g., Th(NH) 2 ), ammoniates (e.g., Ga(NH 3 ) 3 F 3 , [Al(NH 3 ) 6 ]I 3 · NH 3 ) and non-nitrogen compounds like hydroxides, hydrogen sulfides and polychalcogenides (e.g., NaOH, LiHS, CaS, Cs 2 Te 5 ). In particular, large scale production of high quality crystals is possible, due to comparatively simple scalability of the experimental set-up. The ammonothermal method is defined as employing a heterogeneous reaction in ammonia as one homogenous fluid close to or in supercritical state. Three types of milieus may be applied during ammonothermal synthesis: ammonobasic, ammononeutral or ammonoacidic, evoked by the used starting materials and mineralizers, strongly influencing the obtained products. There is little known about the dissolution and materials transport processes or the deposition mechanisms during ammonothermal crystal growth. However, the initial results indicate the possible nature of different intermediate species present in the respective milieus. A Alkali or alkaline-earth metal, A(1) = alkali metal, A(2) = alkaline-earth metal B Further metal, might be main group metal, transition or rare-earth metal M Transition metal, excluding Sc, Y, La R Rare-earth metal, Sc, Y, La-Lu X Halide E Other main group element

Rainer Niewa was born in Dortmund, Germany, in 1967. He studied Chemistry at the Universita ¨t-Gesamthochschule Essen and the Universita ¨t Dortmund where he received his Dr. degree in 1995. His research activities concentrate on the preparation, crystal structures, and properties of transition metal nitrides.

The high-pressure behavior of the hard material ε-Fe 3 N 1+x was studied up to 33 GPa with in situ X-ray diffraction experiments using diamond anvil cells in combination with synchrotron radiation as well as by ex situ high-temperature, high-pressure treatment at 1600(200) K in a two-stage multianvil device with a Walker-type module. Evaluation of the pressure-volume data up to 10 GPa by fitting a Murnaghan-type equation reveals a bulk modulus of B 0 ) 172( 4) GPa (B′ ) 5.7, fixed). The calculated bulk modulus (220 GPa) on the basis of density-functional theory (GGA-PAW-PBE) is in satisfying agreement with the experimental one. Single crystals of ε-Fe 3 N 1+x as obtained by ex situ high-temperature, high-pressure experiments reveal in X-ray diffraction data refinements a structural model of iron atoms in the motif of a hexagonal close packing with occupation of octahedral voids by nitrogen atoms exhibiting long-range order. The preferred structural model is described in space group P312 (a ) 4.7241(2) Å, c ) 4.3862(2) Å, V ) 84.773(6) Å 3 , Z ) 2, R(F) ) 0.0339, wR(F 2 ) ) 0.0556) and compared to a second model in P6 3 22. This choice of structural description is corroborated by the results of density-functional calculations. These yield a total energy at 0 K, which is 5 kJ/mol lower for the model in space group P312 compared to the second best alternative arrangement. Using micro-and nanoindentation techniques, a Vickers hardness of H V ) 7.4(10) GPa, a nanoindentation hardness of H ) 10.1(8) GPa, as well as a reduced elastic modulus in the amount of E r ) 178(11) GPa were measured for ε-Fe 3 N 1+x single crystals.

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The iron nitridometalates Li2[(Li(1-x)Fe(I)(x))N] display ferromagnetic ordering and spin freezing. Large magnetic moments up to 5.0mu(B)/Fe are found in the magnetization. In Mössbauer effect studies huge hyperfine magnetic fields up to 696 kOe are observed at specific Fe sites. These extraordinary fields and moments originate in an unusual ligand field splitting for those Fe species leading [within local spin density approximation (LSDA)] to a localized orbitally degenerate doublet. Including spin-orbit interaction and strong intra-atomic electron correlation (LDA+SO+U) gives rise to a large orbital momentum.

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