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Structure and thermodynamic properties of a weakly-coupled antiferromagnetic spin-1/2 chain compound (C5H12N)CuBr3

Single crystals of a metal organic complex \ce{(C5H12N)CuBr3} (\ce{C5H12N} = piperidinium, pipH for short) have been synthesized and the structure was determined by single-crystal X-ray diffraction. \ce{(pipH)CuBr3} crystallizes in the monoclinic group $C$2/$c$. Edging-sharing \ce{CuBr5} units link to form zigzag chains along the $c$ axis and the neighboring Cu(II) ions with spin-1/2 are bridged by bi-bromide ions. Magnetic susceptibility data down to 1.8 K can be well fitted by the Bonner-Fisher formula for antiferromagnetic spin-1/2 chain, giving the intrachain magnetic coupling constant $J$ $\sim$ 17 K. At zero field, \ce{(pipH)CuBr3} shows three-dimensional (3D) order below $T_N$ = 1.68 K. Calculated by the mean-field theory, the interchain coupling constant $J'$ = 0.65 K is obtained and the ordered magnetic moment $m_0$ is about 0.20 $μ_B$. This value of $m_0$ makes \ce{(pipH)CuBr3} a rare compound suitable to study the dimensional crossover 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 $H \geq$ 3 T is applied along the $a'$ axis. The $H$ - $T$ phase diagram of \ce{(pipH)CuBr3} is roughly constructed. The interplay between exchange interactions, dimensionality, Zeeman energy and possible Dzyaloshinkii-Moriya interaction should be the driving force for the multiple phase transitions.