Crystals of theta-Mn6N5 and eta-Mn3N2 have been prepared in high pressure autoclaves by reaction of MnI2 with NaNH2 (1:2) in ammonia at 6 kbar and temperatures from 400 to 450-degrees-C and 450 to 600-degrees-C respectively. Single-crystal X-ray investigations and Guinier powder patterns give no indication of an ordered arrangement of nitrogen in theta-Mn6N5. For neutron diffraction experiments microcrystalline eta-Mn3N2 was prepared by controlled thermal decomposition of the ternary amide Na2[Mn(NH2)4] at 500-degrees-C. The precursor was synthesized by reacting the elements manganese and sodium with liquid ammonia at 100-degrees-C and 100 bar in an autoclave. Investigations into eta-Mn3N2 with unpolarized neutrons at 11 K and 291 K and measurements of the magnetic susceptibility between 3.5 K and room temperature indicated an antiferromagnetic structure: F(C)mm'm' (C(A)mca); Z = 4; lambda = 2.016 angstrom; f(Mn2+); (a = b; mu-parallel-to[100]). At 291 K: a = 4.200(1) angstrom; c = 12.129(4) angstrom. Mn(1) at 4a: B(eq) = 3.4(7) angstrom2; mu = 4.4 (5)mu(B). Mn(2) at 8i: z = 0.333(1); B(eq) = 1.2(4) angstrom2; mu = 3.4(2)mu(B). N at 8i: z = 0.1588(6); B(eq) = 1.9(3) angstrom2. R(wp)/R(tot)/R(mag) = 15%/4.1%/3.6%. At 11 K: a = 4.189(1) angstrom; c = 12.104(4) angstrom. Mn(1) at 4a: B(eq) = 2.6(6) angstrom2; mu = 4.3(5)mu(B). Mn(2) at 8i: z = 0.330(1); B(eq) = 0.7(3) angstrom2; mu = 3.4(2)mu(B). N at 8i: z = 0.1594(5); B(eq) = 2.0(4) angstrom2. R(wp)/F(tot)/R(mag) = 12%/3.0%/2.4%. The magnetic ordering in eta-Mn3N2 corresponds to a collinear antiferromagnetic arrangement of spins. The exchange interaction is that of ferromagnetic ordering in planes parallel to (001) and aligned + - + - + - along [001]. The magnetic moments in both partial manganese structures are parallel to [100]. The use of the Landau theory shows that the magnetic structure of the F(c)mm'm' symmetry is related to the two-dimensional irreducible corepresentation, which can be induced by the small E(g) at k=(0.5, 0.5, -0.5) in the first Brillouin zone of I4/mmm.
