Preparation, Crystal Chemistry, and Hidden Magnetic Order in the Family of Trigonal Layered Tellurates A 2 Mn(4+)TeO 6 (A = Li, Na, Ag, or Tl)

• Published in: Inorganic chemistry 2019-05, Vol.58 (9), p.5524-5532
• Main Authors: Nalbandyan, Vladimir B, Shukaev, Igor L, Raganyan, Grigory V, Svyazhin, Artem, Vasiliev, Alexander N, Zvereva, Elena A
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.8b03445

We report the first four magnetic representatives of the trigonal layered A M(4+)TeO (here, M = Mn) family. Na MnTeO was synthesized from NaMnO , NaNO , and TeO at 650-720 °C, but analogues for which A = Li and K could not be obtained by direct synthesis. However, those for which A = Li, Ag, and Tl (but not K) were prepared by exchange reactions between Na MnTeO and the corresponding molten nitrates. The oxygen content was verified by redox titration. According to the X-ray diffraction Rietveld analysis, the four new compounds are isostructural with Na GeTeO , trigonal ( P3̅1 c), based on ilmenite-like layers of edge-shared oxygen octahedra occupied by Mn(4+) and Te(6+) in an ordered manner. These layers are separated by cations A, also in a distorted octahedral coordination. However, off-center displacement of Tl is so strong, due to the lone-pair effect, that its coordination is better described as trigonal pyramid. Each MnO octahedron shares two opposite faces with AO octahedra, whereas TeO octahedra avoid sharing faces. Besides this double-layered structure, Na MnTeO was often accompanied by a transient triple-layered rhombohedral polytype. However, it could not be prepared as a single phase and disappeared on annealing at 700-720 °C. All A MnTeO samples (A = Ag, Li, Na, or Tl) revealed the unusual phenomenon of hidden magnetic order. Low-field magnetic susceptibility data exhibit a Curie-Weiss type behavior for all samples under study and do not show any sign of the establishment of long-range magnetic order down to 2 K. In contrast, both the magnetic susceptibility in sufficiently high external magnetic fields and the zero-field specific heat unambiguously revealed an onset of antiferromagnetic order at low temperatures. The frustration index f = Θ/ T takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice.