Two new ternary nitrides, Sr,CrN, and Ba,CrN,, were synthesised from the appropriate alkaline-earth-metal nitride and chromium metal or the chromium nitride, Cr,N, at high temperatures in sealed stainless-steel capsules. Structure determination from single-crystal X-ray data showed them to be isostructural in the hexagonal space group P6,/m, Z = 2, with lattice constants a = 7.724(2), c = 5.249(1) A for Sr,CrN, and a = 8.201(2), c = 5.497(1) A for Ba,CrN,. The structures contain trigonal-planar [CrN,I6-anions and either Sr2 + or Ba2 + cations.Whilst a very large number of ternary oxides have been prepared containing alkaline-earth and transition elements it is only in the past few years that the corresponding ternary nitride systems have begun to be investigated in detail. Even now a wide range of often novel structure types have been shown to exist in ternary nitrides. The valences and co-ordinations exhibited by transition metals in these materials can be unusual often leading to interesting physical properties. '7' Examples of such nitrides are the A,MN, (313) (A = Ca, Sr or Ba; M = V, Cr, Mn or Fe)3-7 and A6MN, (615) (A = Ca; M = Ga, Fe or Mn)8*9 series of materials. A particular feature of these compounds is the presence of a discrete [MN,I6-anion possessing trigonal-planar co-ordination of M to nitrogen. In the strontium and barium compounds of manganese and iron, namely Sr,MnN,, Ba,MnN,7 and Ba,FeN,,6 the [MN,I6anion has D,, symmetry, whilst in the corresponding calcium compounds of vanadium, chromium and manganese, Ca3VN3,4 Ca,CrN,3 and Ca3MnN3,' these anions have Czo symmetry. In the case of the chromium nitride this change in symmetry was attributed to a Jahn-Teller instability for the low-spin Cr3+ ion., Interestingly, the Ca,MN, compounds appear to contain triangular units of D,, symmetry * q 9 although magnetic measurements suggest there may be a local distortion of the anions.'In this paper we report the first preparation and characterisation of two ternary nitrides, Sr,CrN, and Ba,CrN,, which both contain the [CrN,I6-anion having D,, symmetry as opposed to the C,, symmetry preyiously seen in Ca,CrN,.3 We also outline the effects likely to govern the crystal structure and transition-metal co-ordination in A,MN, materials in general and how these factors are likely to relate to their magnetic properties. With the crystal data now emerging, we can begin the task of establishing coherent structural correlations for these new materials.

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The new ternary nitrides SrZrN2 and SrHfN2 have been synthesized by the solid state reactions of Sr2N with ZrN and HfN and characterized by powder X-ray diffraction. Both nitrides form structures in the hexagonal space group R3̄m (SrZrN2, a = 3.37302(5) Å, c = 17.6756(3) Å, Z = 3; SrHfN2, a = 3.34481(3) Å, c = 17.6779(2) Å, Z = 3) and are isostructural with α-NaFeO2. The structures are constructed of alternating layers of strontium−nitrogen edge-sharing octahedra and either zirconium−nitrogen or hafnium−nitrogen edge-sharing octahedra stacked along the c axis. Alternatively, Sr2+ can be regarded as occupying octahedral holes between Zr(Hf)N2 octahedral layers. The title nitrides are essentially diamagnetic at room temperature, consistent with the chemical composition. These compounds are the first alkaline earth ternary nitrides observed to have the α-NaFeO2 layered structure.

A new ternary nitride, SrTiN(2), has been synthesized by the solid-state reaction of Sr(2)N with TiN and characterized by powder X-ray diffraction. SrTiN(2) crystallizes in the tetragonal space group P4/nmm (a = 3.8799(2) Å, c = 7.6985(4) Å, Z = 2) and is isostructural with KCoO(2). Titanium is coordinated to five nitrogens in a distorted square-based pyramidal geometry, forming layers of edge-sharing pyramids which stack along the (001) direction. Strontium is situated between the Ti-N layers and is coordinated to five nitrogen atoms. The title compound is only the third example of a ternary titanium nitride.