Rb 2 Co 1.85 Ge 1.15 O 6 : The First Quaternary, Noncentrosymmetric Rubidium Cobalt Germanate
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ORIGINAL PAPER
Rb2Co1.85Ge1.15O6: The First Quaternary, Noncentrosymmetric Rubidium Cobalt Germanate Mohammad Usman1 · Mark D. Smith1 · Hans‑Conrad zur Loye1 Received: 8 July 2020 / Accepted: 17 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Deep blue, prism-shaped, X-ray diffraction quality single crystals of a new quaternary rubidium cobalt germanate, exact composition Rb2Co1.85Ge1.15O6, were grown by soaking a pre-reacted polycrystalline powder in a molten RbCl/RbF eutectic flux (melting point = 546 °C) at 700 °C in a silver reaction vessel. The complex was characterized by single crystal X-ray diffraction and its elemental composition was semi-quantitatively confirmed by energy dispersive spectroscopy (EDS). Rb2Co1.85Ge1.15O6 crystallizes in the noncentrosymmetric orthorhombic space group C2221 with lattice parameters a = 6.5971(2) Å, b = 9.8791(3) Å and c = 10.8819(3) Å in the K 2ZnSi2O6 structure type. The crystal structure consists of a three-dimensional network, composed of Co and mixed Co/Ge tetrahedra, and features cavities occupied by Rb cations. Graphic Abstract X-ray diffraction quality single crystals of a novel rubidium cobalt germanate, R b2Co1.85Ge1.15O6, were grown by soaking a pre-reacted powder, targeted for preparing R b4.51Co2.35Ge5.10O15F1.96, in a RbCl-RbF eutectic melt at 700 °C. The complex was characterized by single crystal X-ray diffraction and found to crystallize in the orthorhombic space group C2221 in the K2ZnSi2O6 structure type.
Keywords Noncentrosymmetric · Crystal growth · Cobalt germanate Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10870-020-00868-9) contains supplementary material, which is available to authorized users. Extended author information available on the last page of the article
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Introduction Quaternary alkali cobalt germanates are exceedingly rare. A quick ICSD database search reveals that L i2CoGe3O8 and Na 2CoGeO 4 are the only two complex quaternary cobalt oxides that have been characterized as high quality single crystals [1, 2]. To the best of our knowledge, no reports exist on K or Rb containing cobalt germanates characterized via single crystal X-ray diffraction. One possible reason for the dearth of reports on the crystal structures of cobalt germanates, as well as silicates, could be the extreme complexity exhibited by their crystal structures, which is caused by Co/T (T = Si, Ge) site mixing. This was empirically observed in our recent report on complex cesium containing cobalt silicates and germanates, all of which exhibit site mixing [3]. In addition to the need for further exploration of the A/Co/Ge/O (A = alkali metal) phase space, interest in this group of materials arises from their frequent crystallization in noncentrosymmetric structure types, thereby, rendering these materials attractive candidates for applications in non-linear optics [4, 5]. Another reason for exploring the A/M/Ge/O (A = alkali metal; M
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