%0 Journal Article %A Nalbandyan, Vladimir B. %A Shukaev, Igor L. %A Raganyan, Grigory V. %A Svyazhin, Artem %A Vasiliev, Alexander N. %A Zvereva, Elena A. %D 2019 %T Preparation, Crystal Chemistry, and Hidden Magnetic Order in the Family of Trigonal Layered Tellurates A2Mn(4+)TeO6 (A = Li, Na, Ag, or Tl) %U https://acs.figshare.com/articles/journal_contribution/Preparation_Crystal_Chemistry_and_Hidden_Magnetic_Order_in_the_Family_of_Trigonal_Layered_Tellurates_A_sub_2_sub_Mn_4_TeO_sub_6_sub_A_Li_Na_Ag_or_Tl_/8008442 %R 10.1021/acs.inorgchem.8b03445.s001 %2 https://ndownloader.figshare.com/files/14920376 %K double-layered structure %K Magnetic Order %K MnO 6 octahedron shares %K X-ray diffraction Rietveld analysis %K TeO 6 octahedra %K 2 K %K redox titration %K triple-layered rhombohedral polytype %K AO 6 octahedra %K ilmenite-like layers %K Crystal Chemistry %K edge-shared oxygen octahedra %K 2 MnTeO 6 samples %K exchange reactions %K susceptibility data exhibit %K lone-pair effect %K oxygen content %K NaMnO 2 %K antiferromagnetic order %K Na 2 MnTeO 6 %K Li %K Tl %K frustration index f %K octahedral coordination %K Trigonal Layered Tellurates %K NaNO 3 %K Ag %K Na 2 GeTeO 6 %K TeO 2 %X We report the first four magnetic representatives of the trigonal layered A2M­(4+)­TeO6 (here, M = Mn) family. Na2MnTeO6 was synthesized from NaMnO2, NaNO3, and TeO2 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 Na2MnTeO6 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 Na2GeTeO6, trigonal (P3̅1c), 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 MnO6 octahedron shares two opposite faces with AO6 octahedra, whereas TeO6 octahedra avoid sharing faces. Besides this double-layered structure, Na2MnTeO6 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 A2MnTeO6 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/TN takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice. %I ACS Publications