Na₂TeS₃, Na₂TeSe₃-<i>mP</i>24, and Na₂TeSe₃-<i>mC</i>48: Crystal Structures and Optical and Electrical Properties of Sodium Chalcogenidotellurates(IV)

Pure samples of Na₂TeS₃ and Na₂TeSe₃ were synthesized by the reactions of stoichiometric amounts of the elements Na, Te, and Q (Q = S, Se) in the ratio 2:1:3. Both compounds are highly air- and moisture-sensitive. The crystal structures were determined by single-crystal X-ray diffraction. Yellow Na₂TeS₃ crystallizes in the space group <i>P</i>2₁/<i>c</i>. Na₂TeSe₃ exists in a low-temperature modification (Na₂TeSe₃-<i>mP</i>24, space group <i>P</i>2₁/<i>c</i>) and a high-temperature modification (Na₂TeSe₃-<i>mC</i>48, space group <i>C</i>2/<i>c</i>); both modifications are red. Density functional theory calculations confirmed the coexistence of both modifications of Na₂TeSe₃ because they are very close in energy (Δ<i>E</i> = 0.18 kJ mol^–1). To the contrary, hypothetic Na₂TeS₃-<i>mC</i>48 is significantly less favored (Δ<i>E</i> = 1.8 kJ mol^–1) than the primitive modification. Na₂TeS₃ and Na₂TeSe₃-<i>mP</i>24 are isotypic to Li₂TeS₃, whereas Na₂TeSe₃-<i>mC</i>48 crystallizes in its own structure type, which was first described by Eisenmann and Zagler. The title compounds have two common structure motifs. Trigonal TeQ₃ pyramids form layers, and the Na atoms are surrounded by a distorted octahedral environment of chalcogen atoms. Raman spectra are dominated by the vibration modes of the TeQ₃ units. The activation energies of the total conductivity of the title compounds range between 0.68 eV (Na₂TeS₃) and 1.1 eV (Na₂TeSe₃). Direct principal band gaps of 1.20 and 1.72 eV were calculated for Na₂TeSe₃ and Na₂TeS₃, respectively. The optical band gaps are in the range from 1.38 eV for Li₂TeSe₃ to 2.35 eV for Na₂TeS₃

Influence of Lattice Dynamics on Na⁺ Transport in the Solid Electrolyte Na₃PS_4–<i>x</i>Se_<i>x</i>

Li⁺- and Na⁺-conducting thiophosphates have attracted much interest because of their intrinsically high ionic conductivities and the possibility to be employed in solid-state batteries. Inspired by the recent finding of the influence of changing lattice vibrations and induced lattice softening on the ionic transport of Li⁺-conducting electrolytes, here we explore this effect in the Na⁺ conductor Na₃PS_4–<i>x</i>Se_<i>x</i>. Ultrasonic speed of sound measurements are used to monitor a changing lattice stiffness and Debye frequencies. The changes in the lattice dynamics are complemented by X-ray diffraction and electrochemical impedance spectroscopy. With systematic alteration of the polarizability of the anion framework, a softening of the lattice can be observed that leads to a reduction of the activation barrier for migration as well as a decreased Arrhenius prefactor. This work shows that, similar to Li⁺ transport, the softening of the average vibrational frequencies of the lattice has a tremendous effect on Na⁺-ionic transport and that ion–phonon interactions need to be considered in solid electrolytes