不同氧化度六方水钠锰矿的结构研究

通过反复试验法(trial-and-error)粉末X射线衍射(XRD)结构模拟和X射线吸收光谱(XAS)研究了不同锰平均氧化度(manganese average oxidation state,Mn AOS)六方水钠锰矿的结构变化特点.合成的系列六方水钠锰矿中(样品HB1到HB6),Mn AOS由3.92减小为3.67,晶胞参数b由2.838 (A)增加至2.848 (A).在a-b平面上的紧密堆叠尺寸(CSD)由12.0 nm减小至7.0 nm,沿c轴堆叠的锰氧八面体层数约为10.6～13.4层.随着Mn AOS逐渐降低,样品中八面体空位含量由18％减小至8％.Mn AOS与空位含量呈极显著正相关,它们与晶胞参数b呈负相关.X射线吸收近边结构光谱(XANES)线性拟合表明,样品中Mn主要以+4价存在,随着空位含量的减少,低价锰(Mn2+/3+)含量逐渐增多.扩展X射线吸收精细结构光谱(EXAFS)全多重散射模型拟合分析表明,不同氧化度六方水钠锰矿样品的晶体结构和Mn局域配位环境基本相同.由于空位含量和颗粒尺寸减小,从HB1到HB6的表观锰位点占有率(focc)由0.74减小至0.66.随着Mn AOS逐渐降低,Mn-Mn(O)配位壳平均键长增加.

Structure of hexagonal birnessites as affected by oxidation state

Powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques were used to investigate variations of the structures of hexagonal birnessites as related to average oxidation states of manganese therein (Mn AOS). As the synthesized series of hexagonal birnessites from HB1 to HB6 decrease from 3.92 to 3.67 in Mn AOS, but increase from 2.838 Å to 2.848 Å in unit cell parameter b. The series also decrease from 12.0 nm to 7.0 nm in the coherent scattering domains (CSD) in the a -b layer plane, forming a stack of 10.6~13.4 layers of manganese oxide octahedron along axis c. With the decrease in Mn AOS, the content of octahedral voids declines from 18% to 8%. Mn AOS and vacancy content have a very significant positive linear relationship, and they both show negatively linear relationships with unit cell parameter b. Linear fitting of X-ray absorption near edge structure spectroscopy (XANES) shows that Mn in these samples is dominated by Mn⁴⁺ and the proportion of Mn^2+/3+ increases with the decreasing content of octahedral voids. Fitting analysis of Mn K -edge extended X-ray absorption fine structure spectroscopy (EXAFS) with a full multiple scattering model demonstrates that the samples, though different in Mn AOS, are similar in crystal structure and Mn local coordination environment. With the decrease in content of octahedral voids and particle size, apparent Mn site occupancy ( f _occ ) decrease from 0.74 for HB1 to 0.66 for HB6, and with the decrease in Mn AOS, the average bond-length of Mn-Mn(O) shells increase.