Cu(HCO2)2(pym) (pym = pyrimidine): low-dimensional magnetic behavior and long-range ordering in a quantum-spin lattice

We synthesized and structurally and magnetically characterized the novel 3D coordination polymer Cu(HCO2)2(pym) (pym = pyrimidine). The compound crystallizes in the monoclinic space group C2/c with a = 14.4639(8) A, b = 7.7209(4) A, c = 8.5172(5) A, beta = 126.076(2) degrees, and V= 768.76(7) A3. In the structure buckled layers of Cu(HCO2)2 are interconnected by pym ligands to afford 1D Cu-pym-Cu chains. Bulk magnetic susceptibility measurements show a broad maximum at 25 K that is indicative of short-range magnetic ordering. Between 12 and 300 K a least-squares fit of the chi(T) data to a mean-field-corrected antiferromagnetic chain model yielded excellent agreement for g = 2.224(3), J/kB = -26.9(2) K, and zJ'/kB = -1.1(3) K. Below approximately 3 K a transition to long-range magnetic ordering is observed, as suggested by a sharp and sudden decrease in chi(T). This result is corroborated by muon spin relaxation measurements that show oscillations in the muon asymmetry below T(N) = 2.802(1) K and rapidly fluctuating moments above T(N).     

A2VNb6Cl18 (A=Rb, In, Tl): synthesis, crystal structure and magnetic properties of a new series of quaternary niobium chloride cluster compounds

A new series of quaternary niobium chloride cluster compounds corresponding to the general formula, A₂VNb₆Cl₁₈ (A=Rb, Tl or In), has been prepared in sealed quartz tubes from a mixture containing NbCl₅, Nb, VCl₃ and RbCl, or In or Tl metal by solid state reactions at 750°C. The structure of Tl₂VNb₆Cl₁₈ was determined using single-crystal X-ray diffraction: Crystal data: rhombohedral, (No. 148), a=9.1122(17), c=25.178(7) Å, V=1810.5(7) ų and Z=3. The full-matrix least-squares refinement against F² converged to R₁=0.0515, wR₂=0.1104 (all data). The structure consists of discrete octahedral cluster units, [Nb₆Clⁱ₁₂Cl^a₆]⁴⁻ linked by V²⁺ and A⁺ cations, located in a 6-coordinated octahedral and 12-coordinated anticubeoctahedral chloride environment, respectively. The intra-cluster bond lengths indicate 16 valence electrons per cluster. Magnetic susceptibility studies show paramagnetic behavior with a magnetic moment of 3.37 μ_B per formula unit. Electrical resistivity measurements indicate a semiconducting behavior.     

Coexistence of antiferromagnetic and paramagnetic electronic phases in quasi-one-dimensional (TMTSF)<Subscript>2</Subscript>PF<Subscript>6</Subscript>

Phase transitions occuring in a quasi-one-dimensional organic compound (TMTSF)₂PF₆ near the boundaries between the paramagnetic metallic (PM), antiferromagnetic insulator (AFI), and superconducting (SC) states were studied experimentally. A controlled transition through the phase boundary was achieved by maintaining the sample at fixed temperature T and pressure P, while the critical pressure was tuned by varying a magnetic field B. When the PM/AFI phase boundary was crossed due to the variation of a magnetic field, history effects were observed: the resistance was found to depend on the trajectory described by the system before arriving at a given point (P-B-T) of the phase space. The results of the experiment give evidence for the formation of a macroscopically inhomogeneous state characterized by the inclusions of a minor phase that is spatially separated from the major phase. Away from the phase boundary, the homogeneous state is restored. After this, upon approaching the phase boundary in the back direction, the system exhibits no features of the minor phase up to the very boundary.