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和材料的腐蚀，保护科学正在寻找这些化合物的结构。SEM-EDX和拉曼显微镜等方法是研究它们的有价值的工具，但其结果有限。通常，这些研究涉及金属有机盐，如碳酸盐，甲酸盐和乙酸盐。其中一些是故意使用合适的化合物（如醋）制造的，另一些则来自与大气的变质反应（湿度加上二氧化碳或室内空气污染的羰基化合物）。它们具有形成金属有机框架（MOFs）的潜力。这些联合收割机结合有机和无机构建块，提供具有过多有趣特性的混合结构，因此是当前材料科学研究的焦点。许多这些化合物的确切成分发生在文物尚不清楚。在许多情况下，既不能从样品中也不能通过实验室生长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）的数据分析有可能克服这个问题，正如我们在两个试点研究中所示，经常出现的甲酸铜钠和thecotrichite的晶体结构作为风化石灰质物体。必须纠正文献中的公式。因此，从物体中取样或在实验室中合成的未知晶体天然或人工腐蚀产物应通过XRPD进行研究：-许多历史悠久的玻璃物体也含有金属部件。玻璃的水解可能导致邻近金属的腐蚀。未知的铜，锌，铅化合物与钠或钾（从玻璃）和小的有机阴离子（从空气中）将进行调查。- 储存在木橱柜中的石灰质文物和自然历史物品会产生各种含钙和乙酸盐的相，其中一些（不在ICDD中）还包括甲酸盐。- 碱性醋酸铜作为颜料铜绿生产了2000多年的人工腐蚀。至少有四个阶段仍需要结构确定。在项目期间，各种结构和公式预计将在3年内解决。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：为人类的利益保护文物。

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.