In search of structure -High resolution X-ray powder diffraction of metal organic salts for conservation science

为了更好地了解历史上人类使用的颜料的生产和材料的腐蚀，保护科学正在寻找这些化合物的结构。像SEM-EDX和拉曼显微镜这样的方法对它们的研究是有价值的工具，但结果有限。通常，这些研究涉及碳酸盐、甲酸盐和醋酸盐等金属有机盐。其中一些是故意使用合适的化合物(如醋)制造的，另一些是由于与大气的恶化反应(湿度加上室内空气污染产生的二氧化碳或羰基化合物)而产生的。它们有可能形成金属有机骨架(MOF)。它们将有机和无机构建块结合在一起，使杂化结构具有大量有趣的特性，因此，是当前材料科学研究的重点。目前尚不清楚这些人工制品上的许多化合物的确切组成。在许多情况下，无论是从样品还是通过实验室生长，都无法获得适合于常规结构测定的单晶。现代高分辨率X射线粉末衍射(XRPD)的数据分析具有克服这一问题的潜力，正如我们在两个关于经常出现的甲酸铜钠和钙质物体上风化的镁铝榴石的晶体结构的初步研究中所表明的那样。文学作品中关于他们的公式必须加以修正。因此，从物体上取样或在实验室合成的未知结晶天然或人工腐蚀产品应由XRPD进行研究：-许多历史悠久的玻璃物体也包含金属部件。玻璃的水解可能会导致对邻近金属的腐蚀。未知的铜、锌和铅化合物与钠或钾(来自玻璃)和小的有机阴离子(来自空气)将被调查。-储存在木制橱柜中的钙质文物和自然历史物品会产生各种含有钙和醋酸盐的相，其中一些(不在ICDD中)还包括甲酸盐。-碱性醋酸铜是在2000多年前通过人工腐蚀生产出来的颜料。至少有四个相还需要确定结构。在项目期间，预计在3年内解决各种结构和公式。这将有助于更好地了解潜在的腐蚀或生产过程，并有助于保护科学的总体目标：保护文物造福人类。

To better understand the production of pigments and the corrosion of materials used by mankind during history, conservation science is in search of the structure of these compounds. Methods like SEM-EDX and Raman microscopy are valuable tools for their investigation, but are limited in their results. Often, these studies deal with metal organic salts like carbonates, formates, and acetates. Some of them have been deliberately manufactured using suitable compounds (e.g. vinegar), others derive from deterioration reactions with the atmosphere (humidity plus carbon dioxide or carbonyl compounds from indoor air pollution). They have the potential to form metal organic frameworks (MOFs). These combine organic and inorganic building blocks to give hybrid structures with plethora of interesting properties, and are, therefore, in the focus of current materials science research. The exact composition of many of these compounds occurring on artifacts is not yet known. In many cases, single crystals suitable for conventional structure determination cannot be obtained, neither from samples, nor by lab growth. The data analysis of modern high resolution X-ray powder diffraction (XRPD) has the potential to overcome this problem, as has been shown by us in two pilot studies of the crystal structure of a frequently occurring sodium copper formate and thecotrichite as efflorescence on calcareous objects. Formulas for them from literature had to be corrected. Therefore, unknown crystalline natural or artificial corrosion products either sampled from objects or synthesized in the lab shall be studied by XRPD: - Many historic glass objects also contain metal parts. Hydrolysis of the glass may cause corrosion in neighbouring metals. Unknown Cu, Zn, and Pb compounds with Na or K (from glass) and small organic anions (from air) will be investigated. - Calcareous artifacts and natural history items stored in wooden cupboards develop various calcium and acetate containing phases, some of them (not in the ICDD) also include formate. - Basic copper acetates were produced as pigment verdigris since more than 2 millennia by artificial corrosion. At least four phases still need structure determination. During the project, a variety of structures and formulas are expected to be solved within 3 years. This will help to better understand the underlying corrosion or production processes and contribute to the overall objective of conservation science: the preservation of artifacts for the benefit of mankind.