Magnetische struktur von η-Mn 3N 2

Abstract

Crystals of ϑ-Mn 6N 5 and η-Mn 3N 2 have been prepared in high pressure autoclaves by reaction of MnI 2 with NaNH 2 (I:2) in ammonia at 6 kbar and temperatures from 400 to 450 °C and 450 to 600 °C respectively. Single-crystal X-ray investigations and Guinier powder patterns give no indication of an ordered arrangement of nitrogen in ϑ-Mn 6N 5. For neutron diffraction experiments microcrystalline η-Mn 3N 2 was prepared by controlled thermal decomposition of the ternary amide Na 2[Mn(NH 2) 4] at 500 °C. The precursor was synthesized by reacting the elements manganese and sodium with liquid ammonia at 100 °C and 100 bar in an autoclave. Investigations into η-Mn 3N 2 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; λ=2.016 A ̊ ; f( Mn 2+); ( a= b; μ∥[100]). At 291 K: a=4.200(1) A ̊ ; c=12.129(4) A ̊ . Mn(1) at 4a: B eq =3.4(7) A ̊ 2 ; μ=4.4(5) μ B . Mn(2) at 8i: z=0.333(1); B eq =1.2(4) A ̊ 2 ; μ=3.4(2) μ B . N at 8i: z=0.1588(6); B eq =1.9(3) A ̊ 2 . R wp/ R tot/ R mag=15%/4.1%/3.6%. At 11 K: a=4.189(1) A ̊ ; c=12.104(4) A ̊ . Mn(1) at 4a: B eq =2.6(6) A ̊ 2 ; μ=4.3(5) μ B. Mn(2) at 8i: z=0.330(1); B eq =0.7(3) A ̊ 2 ; μ=3.4(2) μ B. N at 8i: z=0.1594(5); B eq =2.0(4) A ̊ 2 . R wp/ R tot/ R mag=12%/3.0%/2.4%. The magnetic ordering in η-Mn 3N 2 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.