Low-temperature structure and dynamics of brucite
Low-temperature structure and dynamics of brucite
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DOI：
10.1021/jp972225a
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发表时间：
1997-11-13
影响因子：
3.3
通讯作者：
Schultz, AJ
中科院分区：
文献类型：
--
作者：
Chakoumakos, BC;Loong, CK;Schultz, AJ
Mg（OD）（2）的飞行时间中子粉末衍射数据和Mg（OH）2的非弹性中子散射测量的结构改进表明，热收缩的机制主要是八面体厚度的减小，其次是层间厚度的减小。在300-15 K的温度区间内，八面体层厚度的收缩是中间层厚度收缩的4倍。水镁石在15 K时的体积相当于在0.2 GPa时的体积，这不足以观察高压下八面体层之间的氢键;然而，三位分裂原子模型，其中O-D方向与3倍c轴成角a，比单位模型更适合每个温度的数据。D原子的大原子位移参数使单原子点模型的O-D距离明显缩短，但如果对O-D距离进行“骑"运动校正，它会延长到与三原子点分裂模型确定的值大致相同的值。晶格参数的温度依赖关系为a = 3.1435 + 2.911 × 10（-5）T + 9.944 × 10（-8）T（2）-1.965 × 10（-11）T（3）-4.789 × 10（-13）T（4），c = 4.7478 + 1.793 x 10（-4）T + 4.496 x 10（-7）T（2）+ 3.910 x 10（-10）T（3），V = 40.635 + 2.330 x 10（-3）T + 5.592 x 10（-6）T（2）+ 1.748 x 10（-9）T（3），T单位为摄氏度。由非弹性散射测量得到的Mg（OH）（2）中氢在15 K时的振动谱显示出在461.8 meV处的尖锐OH伸缩带，在47.5 meV处的E-u OH振动，以及由于晶格振动和组合激发而产生的其它宽的特征。在低温下没有发现增强氢键的证据。
Structural refinements of time-of-flight neutron powder diffraction data for Mg(OD)(2) and inelastic neutron scattering measurements for Mg(OH)2 demonstrate that the mechanism of thermal contraction is primarily the reduction of the octahedral thickness and secondarily the reduction of the interlayer thickness. The contraction of the octahedral layer thickness is 4 times as great as the contraction of the interlayer thickness over the temperature interval 300-15 K. The volume of brucite at 15 K is equivalent to that at 0.2 GPa which should not be sufficient to observe the H-bonding between the octahedral layer that occurs at high pressure; however, the three-site split-atom model, in which the O-D direction makes an angle a with the 3-fold c axis, provides a better fit to the data for each temperature than the single-site model. The O-D distance from the single-site model is markedly shortened by the large atomic displacement parameter of the D atom, but if the O-D distance is corrected for ''riding'' motion, it lengthens roughly to the same values determined from the three-site split-atom model. The temperature dependence of the lattice parameters are given by a = 3.1435 + 2.911 x 10(-5)T + 9.944 x 10(-8)T(2) -1.965 x 10(-11)T(3) -4.789 x 10(-13)T(4), c = 4.7478 + 1.793 x 10(-4)T + 4.496 x 10(-7)T(2) + 3.910 x 10(-10)T(3), V = 40.635 + 2.330 x 10(-3)T + 5.592 x 10(-6)T(2) + 1.748 x 10(-9)T(3), for T in degrees C. The vibrational spectrum of hydrogen in Mg(OH)(2) at 15 K obtained from inelastic scattering measurements shows a sharp OH stretch band at 461.8 meV, an E-u OH libration at 47.5 meV, and other broad features due to lattice vibrations and combination excitations. No evidence of enhanced hydrogen bonding was found at low temperatures.