Støckler Kristoffer Andreas Holm, Roth Nikolaj, Grønbech Thomas Bjørn Egede, Iversen Bo Brummerstedt
Center for Integrated Materials Research, Deparetment of Chemistry and iNANOAarhus University, Langelandsgade 140, Aarhus 8000, Denmark.
IUCrJ. 2022 Jun 29;9(Pt 4):523-532. doi: 10.1107/S2052252522006315. eCollection 2022 Jul 1.
The scattering pattern of a crystal obeys the symmetry of the crystal structure through the corresponding Laue group. This is usually also true for the diffuse scattering, containing information about disorder, but here a case is reported where the diffuse scattering is of lower symmetry than the parent crystal structure. The mineral bixbyite has been studied by X-ray and neutron scattering techniques since 1928 with some of the most recent studies characterizing the low-temperature transition to a magnetically disordered spin-glass state. However, bixbyite also exhibits structural disorder, and here single-crystal X-ray and neutron scattering is used to characterize the different modes of disorder present. One-dimensional rods of diffuse scattering are observed in the cubic mineral bixbyite, which break the expected symmetry of the scattering pattern. It is shown that this scattering arises from epitaxial intergrowths of the related mineral, braunite. The presence of this disorder mode is found to be directly observable as well-defined residuals in the average structure refined against the Bragg diffraction. An additional three-dimensional diffuse scattering component is observed in neutron scattering data, which is shown to originate from the substitutional disorder on the Fe/Mn sites. This occupational disorder gives rise to local relaxations of the oxide sublattice, and the pattern of oxide displacements can be rationalized based on crystal-field theory. The combined use of neutron and X-ray single-crystal scattering techniques highlights their great complementarity. In particular, the large sample requirements for neutron scattering experiments prove to be an obstacle in solving the intergrowth disorder due to several growth orientations, whereas for X-ray scattering the one-dimensional nature of the intergrowth disorder renders solving this a more tractable task. On the other hand, the oxide relaxations cannot be resolved using X-rays due to the low Mn/Fe contrast. By combining the two approaches both types of disorder have been characterized.
晶体的散射图案通过相应的劳厄群服从晶体结构的对称性。对于包含无序信息的漫散射来说通常也是如此，但本文报道了一个案例，其中漫散射的对称性低于母晶体结构。自1928年以来，矿物方铁锰矿一直通过X射线和中子散射技术进行研究，一些最新研究对其低温转变为磁无序自旋玻璃态进行了表征。然而，方铁锰矿也表现出结构无序，本文使用单晶X射线和中子散射来表征存在的不同无序模式。在立方矿物方铁锰矿中观察到一维漫散射棒，这打破了散射图案预期的对称性。结果表明，这种散射源于相关矿物布劳奈矿的外延共生。发现这种无序模式的存在可以直接观察到，作为在根据布拉格衍射精修的平均结构中明确的残余。在中子散射数据中观察到另一个三维漫散射分量，结果表明它源自Fe/Mn位点上的替代无序。这种占位无序导致氧化物亚晶格的局部弛豫，并且可以基于晶体场理论对方格位移模式进行合理化解释。中子和X射线单晶散射技术的联合使用突出了它们的巨大互补性。特别是，中子散射实验对大样品的要求被证明是解决由于几种生长取向导致的共生无序的一个障碍，而对于X射线散射，共生无序的一维性质使得解决这个问题成为一项更易于处理的任务。另一方面，由于Mn/Fe对比度低，使用X射线无法分辨氧化物弛豫。通过结合这两种方法，两种类型的无序都得到了表征。
