Polymorphism of the Transition Metal Oxidotellurates NiTeO₄ and CuTe₂O₅

Abstract

As part of crystal growth experiments on transition metal oxidotellurates using chemical vapor transport reactions or hydrothermal conditions, single crystals of NiˡˡTeⱽˡO₄ andⱽˡ₂O₅ were obtained for the first time in the form of new modifications, as revealed by crystal structure determinations from X-ray data. In the course of these investigations, the crystal structure model of the only phase of NiˡˡTeⱽˡO₄ reported so far (from now on named α-) was corrected. Both α-(space group P21/c, Z = 2) and the new β-polymorph of NiˡˡTeⱽˡO₄ (space group I41/a, Z = 8) can be considered derivatives (hettotypes) of the rutile structure (aristotype), as shown by detailed symmetry relationships. For CuTe₂O₅ also, only one crystalline phase has been described so far (from now on named α-) that corresponds to the mineral rajite (space group P21/c, Z = 2). Its anion comprises two different trigonal-pyramidal TeO3 groups linked through corner-sharing into a ditellurite unit. The anion part of the new β-CuTe2O5 modification (space group P21/c, Z = 2), likewise, comprises two TeIV atoms but is more complex. Here, one TeIV atom exhibits a coordination number of 4 and is part of a [∝1 TeO2/2O2/1] chain, and the other has a coordination number of 5 and is part of a [∝1 TeO2/2O3/1]2 dimer. The two types of anions are linked into a tri-periodic framework where both TeIV atoms are stereochemically active. The α- and β-CuTe₂O₅ modifications show no closer structural relationship, which is also reflected in their clearly different Raman spectra. Data mining for knowledge discovery in a structure database reveals that polymorphism is a rather common phenomenon for the family of inorganic oxidotellurates.