Two Spin-Peierls-like Compounds Exhibiting Divergent Structural Features, Lattice Compression, and Expansion in the Low- Temperature Phase

详细信息    查看全文

• 作者：Zhengfang Tian ; Haibao Duan ; Xiaoming Ren ; Changsheng Lu ; Yizhi Li ; You Song ; Huizhen Zhu ; Qingjin Meng
• 刊名：Journal of Physical Chemistry B
• 出版年：2009
• 出版时间：June 18, 2009
• 年：2009
• 卷：113
• 期：24
• 页码：8278-8283
• 全文大小：246K
• 年卷期：v.113,no.24(June 18, 2009)
• ISSN：1520-5207

文摘

Two quasi-one-dimensional (quasi-1D) compounds, [4′-CH₃Bz-4-RPy][Ni(mnt)₂] (mnt²⁻ = maleonitriledithiolate), where 4′-CH₃Bz-4-RPy⁺ = 1-(4′-methylbenzyl)pyridinium (denoted as compound 1) and 1-(4′-methylbenzyl)-4-aminopyridinium (denoted as compound 2), show a spin-Peierls-like transition with T_C ≈ 182 K for 1 and T_C ≈ 155 K for 2. The enthalpy changes for the transition are estimated to be ΔH = 316.6 J·mol⁻¹ for 1 and 1082.1 J·mol⁻¹ for 2. From fits to the magnetic susceptibility, the magnetic exchange constants in the gapless state are calculated to be J = 166(2) K with g = 2.020(23) for 1 versus J = 42(0) K with g = 2.056(5) for 2. In the high-temperature (HT) phase, 1 and 2 are isostructural and crystallize in the monoclinic space group P2₁/c. The nonmagnetic cations and paramagnetic anions form segregated columns with regular anionic and cationic stacks. In the low-temperature (LT) phase, the crystals of the two compounds undergo a transformation to the triclinic space group P-1, and both anionic and cationic stacks dimerize. In the transformation from the HT to LT phases, the two compounds exhibit divergent structural features, with lattice compression for 1 but lattice expansion for 2, due to intermolecular slippage. Combined with our previous studies, it is also noted that the transition temperature, T_C, is qualitatively related to the cell volume in the HT phase for the series of compounds [1-(4′-R-benzylpyridinium][Ni(mnt)₂] (where R represents the substituent). When there is a single substituent in the para position of benzene, giving a larger cell volume, the transition temperature increases.