Long-range magnetic order in Mn[N(CN)(2)](2)(pyz) {pyz = pyrazine} susceptibility, magnetization, specific heat, and neutron diffraction measurementsand electronic structure calculations

Using de magnetization, ac susceptibility, specific heat, and neutron diffr action, we have studied the magnetic properties of Mn[N(CN)(2)](2)(pyz) {py z pyrazine} in detail. The material crystallizes in the monoclinic space gr oup P2(1)/n with a = 7.3238(2), b = 16.7369(4), and c = 8.7905 (2) Angstrom , beta = 89.596 (2)degrees, V = 1077.65(7) Angstrom (3), and Z = 4, as dete rmined by Rietveld refinement of neutron powder diffraction data at 1.35 K. The 5 K neutron powder diffraction data reflect very little variation in t he crystal structure. Interpenetrating ReO3-like networks are formed from a xially elongated Mn2+ octahedra and edges made up of mu -bonded [N(CN)(2)]( -) anions and neutral pyz ligands. A three-dimensional antiferromagnetic or dering occurs below T-N = 2.53(2) K. The magnetic unit cell is double the n uclear one along the a- and c-axes, giving the (1/2, 0, 1/2) superstructure . The crystallographic and antiferromagnetic structures are commensurate an d consist of collinear Mn2+ moments, each with a magnitude of 4.15(6) mu (B ) aligned parallel to the a-direction (Mn-pyz-Mn chains). Electronic struct ure calculations indicate that the exchange interaction is much stronger al ong the Mn-pyz-Mn chain axis than along the Mn-NCNCN-Mn axes by a factor of approximately 40, giving rise to a predominantly one-dimensional magnetic system. Thus, the variable-temperature magnetic susceptibility data are wel l described by a Heisenberg antiferromagnetic chain model, giving g = 2.01( 1) and J/k(B) = -0.27(1) K. Owing to single-ion anisotropy of the Mn2+ ion, field-induced phenomena ascribed to spin-flop and paramagnetic transitions are observed at 0.43 and 2.83 T, respectively.