Preparation, Crystal Chemistry, and Hidden Magnetic Order in the Family of Trigonal Layered Tellurates A₂Mn(4+)TeO₆ (A = Li, Na, Ag, or Tl)

Abstract

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/T N takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice.

Hidden magnetic order in the triangular-lattice magnet Li2MnTeO6

“…Powder samples of Li 2 MnTeO 6 were prepared by a molten-salt ion exchange reaction of the isostructural Na 2 MnTeO 6 , as reported previously [21]. Li 2 MnTeO 6 crystallizes in the space group P 3 1c.…”

“…In the present article, we examine the properties of a quasi-2D triangular lattice compound Li 2 MnTeO 6 , which belongs to the new A 2 MnTeO 6 (A = Li, Na, Ag, Tl) structural family of frustrated low-dimensional magnets [21], to find that the magnetic properties of Li 2 MnTeO 6 are similar to those magnets possessing unusual hidden magnetic order.…”

Commensurate helicoidal order in the triangular layered magnet Na2MnTeO6

“…Earlier studied Li 2 MnTeO 6 sample, whose properties will be compared with the characteristics of its Na-based counterpart, has been prepared by the exchange reactions of Na 2 MnTeO 6 and molten lithium nitrate. The analytical and x-ray diffraction data confirmed the full replacement of sodium by lithium ions in Li 2 MnTeO 6 [10,11].…”

“…Powder sample of Na 2 MnTeO 6 has been prepared by solid-state synthesis from NaMnO 2 , NaNO 3 and TeO 2 at 650-720 • C as described in Ref. [10]. Earlier studied Li 2 MnTeO 6 sample, whose properties will be compared with the characteristics of its Na-based counterpart, has been prepared by the exchange reactions of Na 2 MnTeO 6 and molten lithium nitrate.…”

“…In particular, Heisenberg-type layered systems may reach 120 °noncollinear orders of various types [6][7][8]. The family of trigonal layered quaternary tellurates [9] A 2 MnTeO 6 (A = Li, Na, Ag, Tl) provides a unique possibility to tune the interlayer exchange interaction parameters leaving virtually intact the parameters of magnetic exchange in the layers [10]. Most important in this respect is the fact that the change in the interlayer spacing influences the ratio of nearest-neighbor and next-nearest neighbor interlayer exchanges.…”