单核Gd化合物的场依赖磁弛豫和磁热效应研究

Field-Dependent Magnetic Relaxation and Magnetocaloric Effect in Mononuclear Gd Complexes

合成了两例具有局域D4d对称性的单核钆配合物:Gd(THD)3L(THD=2,2,6,6-四甲基庚烷-3,5-二酮,L=Phen(1,10-菲罗琳,1),Bpy(2,2'-联吡啶,2).磁性研究表明其在低温下具有较大的磁热效应,在2.5 K时,0到5 T外磁场下的磁熵变(-ΔSm)分别高达17.0和18.4 J?kg-1?K-1.研究发现,化合物1,2还同时具有明显的场依赖磁弛豫性质.通过分析弛豫时间随外加磁场和温度变化的关系,发现其低场下的弛豫行为可以用共振声子俘获机制来解释,高场下的弛豫行为则可能来源于直接过程.

Two new mononuclear Gd complexes Gd（THD）3L （THD=2,2,6,6-tetramethylheptane-3,5-dione, L-Phen （I）, Bpy （2）） based on b-diketone type ligand with Phen and Bpy as capping ligands were synthesized, respectively. To obtain the complexes, the THD liangd （3 mmol） and capping ligand （1 mmol） were mixed and dissolved in ethanol （10 mL）, tbllowed by adding of 5 mL aqueous solution of KOH （3 mmol） and slowly heating to remove the a-H ofTHD ligand. And then 5 mL aqueous solution of Gd（NO3）3 was added dropwise. The white precipitate was collected and recrystallized in the mixture of ethanol and acetone to get colorless transparent crystals. Their structures were determined by the single-crystal X-ray diffraction method, tbund crystallized in P-1 space group, and gave an eight-coordinated coordination environment ofGd^3＋ with D4d local symmetry. Magnetic susceptibility measurements of powder samples were carried out by SQUID magnetometer at 1000 Oe dc field from 2 to 300 K, and the ХmT value at room temperatures were 7.89 and 8.25 cm^3·K·mol^-1, respectively, in good agreement with the expected value. The magnetization measurements reveal that the magnetic entropy changes （-△Sm） at 2.5 K from 0 to 5 Tesla magnetic field are as large as 17.0 and 18.4 J·kg^-1·K^-1 for 1 and 2, respectively. The ac susceptibility shows an obvious slow magnetic relaxation phenomenon. By analyzing the temperature and magnetic field dependences of the relaxation time, we lbund that the resonant phonon trapping mechanism is predominant when the field is lower than I T and the direct process plays a role when the field is larger than 2 T.