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)
Vladimir B. Nalbandyan
Igor L. Shukaev
Grigory V. Raganyan
Artem Svyazhin
Alexander N. Vasiliev
Elena A. Zvereva
10.1021/acs.inorgchem.8b03445.s001
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
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.
2019-04-17 17:40:55
double-layered structure
Magnetic Order
MnO 6 octahedron shares
X-ray diffraction Rietveld analysis
TeO 6 octahedra
2 K
redox titration
triple-layered rhombohedral polytype
AO 6 octahedra
ilmenite-like layers
Crystal Chemistry
edge-shared oxygen octahedra
2 MnTeO 6 samples
exchange reactions
susceptibility data exhibit
lone-pair effect
oxygen content
NaMnO 2
antiferromagnetic order
Na 2 MnTeO 6
Li
Tl
frustration index f
octahedral coordination
Trigonal Layered Tellurates
NaNO 3
Ag
Na 2 GeTeO 6
TeO 2