Structure and Thermodynamic Properties of (C₅H₁₂N)CuBr₃: A New Weakly Coupled Antiferromagnetic Spin-1/2 Chain Complex Lying in the 1D–3D Dimensional Cross-Over Regime

Single crystals of a metal organic complex (C5H12N)­CuBr3 (C5H12N = piperidinium, pipH for short) have been synthesized, and the structure was determined by single-crystal X-ray diffraction. (pipH)­CuBr3 crystallizes in the monoclinic group C2/c. Edging-sharing CuBr5 units link to form zigzag chains along the c axis, and the neighboring Cu­(II) ions with spin-1/2 are bridged by bibromide ions. Magnetic susceptibility data down to 1.8 K can be well fitted by the Bonner–Fisher formula for the antiferromagnetic spin-1/2 chain, giving the intrachain magnetic coupling constant J ≈ −17 K. At zero field, (pipH)­CuBr3 shows three-dimensional (3D) order below TN = 1.68 K. Calculated by the mean-field theory, the interchain coupling constant J′ = −0.91 K is obtained and the ordered magnetic moment m0 is about 0.23 μB. This value of m0 makes (pipH)­CuBr3 a rare compound suitable to study the 1D–3D dimensional cross-over problem in magnetism, since both 3D order and one-dimensional (1D) quantum fluctuations are prominent. In addition, specific heat measurements reveal two successive magnetic transitions with lowering temperature when external field μ0H ≥ 3 T is applied along the a′ axis. The μ0H–T phase diagram of (pipH)­CuBr3 is roughly constructed.