Accurate Crystal Structure Refinement of Natrolite and Localization of Free Water
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CTURE OF INORGANIC COMPOUNDS
Accurate Crystal Structure Refinement of Natrolite and Localization of Free Water A. P. Dudkaa,*, Z. V. Bedranb, M. A. Belyanchikovb, and B. P. Gorshunovb a Shubnikov
Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia b Moscow Institute of Physics and Technology, Dolgoprudnyi, Moscow oblast, 141700 Russia *e-mail: [email protected] Received April 15, 2020; revised May 22, 2020; accepted May 28, 2020
Abstract—Crystals that contain water molecules spaced by distances excluding their chemical interaction are model systems for studying the so-called “water ferroelectricity”: ordering of polar water molecules, interacting via electric dipole bonds. Natrolite crystal Na2(Al2Si3O10)(H2O)2, which contains water molecules localized in cages (isolated cavities) formed by framework ions, has been investigated by X-ray diffraction (XRD) analysis at 93 K (sp. gr. Fdd2, Z = 8, a = 18.24822(6) Å, b = 18.59561(8) Å, c = 6.57868(4) Å, R1(|F|)/wR2(|F|) = 1.149/1.294%, and Δρmin/Δρmax = –0.19/0.17 e/Å3 for 11145 independent reflections). Hydrogen atomic parameters have been refined in the anisotropic approximation of atomic displacements, which made it possible to determine the orientation of water molecules. The mineral under investigation is a model object for spectroscopic analysis of water ferroelectricity. DOI: 10.1134/S1063774520060139
INTRODUCTION Traditionally, nanoinclusions (atoms or molecules) in minerals with pores and channels have been of interest for geologists. These media can be used in practice, e.g., as matrices for storing radioactive waste and as molecular filters. Recently, researchers have been interested in these objects from the fundamental point of view, because inclusions may interact in a special way. In particular, the systems with inclusions in the form of polar water molecules have attracted attention. In the absence of chemical interaction, shortrange mutual ordering of the dipole moments of water molecules may occur because of the long-range electric dipole forces. This phenomenon may manifest itself in various (both inorganic and biological) systems. The ordered state of water molecules in a beryl crystal was revealed recently using spectroscopic analysis [1]. These systems cannot be investigated if the atomic parameters of water molecules and hydrogen atoms entering their composition are unknown. Accurate and reliable information about the characteristics of hydrogen atoms is important for studying the energy of atomic interactions in technological, biological, and pharmaceutical materials. On the whole, the role of hydrogen in chemistry, biology, and medicine can hardly be overestimated. The most important problem of materials science is the development of hydrogen-
storage devices, which may help in solving many climatic and ecological problems. Despite the importance of this subject and the interest of researchers in it [2, 3], the difficulties in determining accuratel
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