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 Ni^IITe^VIO<inf>4</inf> and Cu^IITe^IV<inf>2</inf>O<inf>5</inf> 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 Ni^IITe^VIO<inf>4</inf> reported so far (from now on named α-) was corrected. Both α-(space group P2<inf>1</inf>/c, Z = 2) and the new β-polymorph of Ni^IITe^VIO<inf>4</inf> (space group I4<inf>1</inf>/a, Z = 8) can be considered derivatives (hettotypes) of the rutile structure (aristotype), as shown by detailed symmetry relationships. For CuTe<inf>2</inf>O<inf>5</inf> also, only one crystalline phase has been described so far (from now on named α-) that corresponds to the mineral rajite (space group P2<inf>1</inf>/c, Z = 2). Its anion comprises two different trigonal-pyramidal TeO<inf>3</inf> groups linked through corner-sharing into a ditellurite unit. The anion part of the new β-CuTe<inf>2</inf>O<inf>5</inf> modification (space group P2<inf>1</inf>/c, Z = 2), likewise, comprises two Te^IV atoms but is more complex. Here, one Te^IV atom exhibits a coordination number of 4 and is part of a (Formula presented.) TeO<inf>2/2</inf>O<inf>2/1</inf>] chain, and the other has a coordination number of 5 and is part of a (Formula presented.) TeO<inf>2/2</inf>O<inf>3/1</inf>]<inf>2</inf> dimer. The two types of anions are linked into a tri-periodic framework where both Te^IV atoms are stereochemically active. The α- and β-CuTe<inf>2</inf>O<inf>5</inf> 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.