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)

We report the first four magnetic representatives of the trigonal layered A<sub>2</sub>M­(4+)­TeO<sub>6</sub> (here, M = Mn) family. Na<sub>2</sub>MnTeO<sub>6</sub> was synthesized from NaMnO<sub>2</sub>, NaNO<sub>3</sub>, and TeO<sub>2</sub> 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<sub>2</sub>MnTeO<sub>6</sub> 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<sub>2</sub>GeTeO<sub>6</sub>, trigonal (<i>P</i>3̅1<i>c</i>), 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<sup>+</sup> is so strong, due to the lone-pair effect, that its coordination is better described as trigonal pyramid. Each MnO<sub>6</sub> octahedron shares two opposite faces with AO<sub>6</sub> octahedra, whereas TeO<sub>6</sub> octahedra avoid sharing faces. Besides this double-layered structure, Na<sub>2</sub>MnTeO<sub>6</sub> 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<sub>2</sub>MnTeO<sub>6</sub> 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 <i>f</i> = Θ/<i>T</i><sub>N</sub> takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice.