Puzzling Intergrowth in Cerium Nitridophosphate Unraveled by Joint Venture of Aberration-Corrected Scanning Transmission Electron Microscopy and Synchrotron Diffraction

Thorough investigation of nitridophosphates has rapidly accelerated through development of new synthesis strategies. Here we used the recently developed high-pressure metathesis to prepare the first rare-earth metal nitridophosphate, Ce₄Li₃P₁₈N₃₅, with a high degree of condensation >1/2. Ce₄Li₃P₁₈N₃₅ consists of an unprecedented hexagonal framework of PN₄ tetrahedra and exhibits blue luminescence peaking at 455 nm. Transmission electron microscopy (TEM) revealed two intergrown domains with slight structural and compositional variations. One domain type shows extremely weak superstructure phenomena revealed by atomic-resolution scanning TEM (STEM) and single-crystal diffraction using synchrotron radiation. The corresponding superstructure involves a modulated displacement of Ce atoms in channels of tetrahedra 6-rings. The displacement model was refined in a supercell as well as in an equivalent commensurate (3 + 2)-dimensional description in superspace group <i>P</i>6₃(α, β, 0)­0­(−α – β, α, 0)­0. In the second domain type, STEM revealed disordered vacancies of the same Ce atoms that were modulated in the first domain type, leading to sum formula Ce_{4–0.5<i>x</i>}Li₃P₁₈N_{35–1.5<i>x</i>}O_1.5<i>x</i> (<i>x</i> ≈ 0.72) of the average structure. The examination of these structural intricacies may indicate the detection limit of synchrotron diffraction and TEM. We discuss the occurrence of either Ce displacements or Ce vacancies that induce the incorporation of O as necessary stabilization of the crystal structure

Nanostructures in Te/Sb/Ge/Ag (TAGS) Thermoelectric Materials Induced by Phase Transitions Associated with Vacancy Ordering

Te/Sb/Ge/Ag (TAGS) materials with rather high concentrations of cation vacancies exhibit improved thermoelectric properties as compared to corresponding conventional TAGS (with constant Ag/Sb ratio of 1) due to a significant reduction of the lattice thermal conductivity. There are different vacancy ordering possibilities depending on the vacancy concentration and the history of heat treatment of the samples. In contrast to the average α-GeTe-type structure of TAGS materials with cation vacancy concentrations <∼3%, quenched compounds like Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit “parquet-like” multidomain nanostructures with finite intersecting vacancy layers. These are perpendicular to the pseudocubic ⟨111⟩ directions but not equidistantly spaced, comparable to the nanostructures of compounds (GeTe)_<i>n</i>­Sb₂Te₃. Upon heating, the nanostructures transform into long-periodically ordered trigonal phases with parallel van der Waals gaps. These phases are slightly affected by stacking disorder but distinctly different from the α-GeTe-type structure reported for conventional TAGS materials. Deviations from this structure type are evident only from HRTEM images along certain directions or very weak intensities in diffraction patterns. At temperatures above ∼400 °C, a rock-salt-type high-temperature phase with statistically disordered cation vacancies is formed. Upon cooling, the long-periodically trigonal phases are reformed at the same temperature. Quenched nanostructured Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit ZT values as high as 1.3 and 0.8, respectively, at 160 °C, which is far below the phase transition temperatures. After heat treatment, i.e., without pronounced nanostructure and when only reversible phase transitions occur, the ZT values of Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ with extended van der Waals gaps amount to 1.6 at 360 °C and 1.4 at 410 °C, respectively, which is at the top end of the range of high-performance TAGS materials

Nanostructures in Te/Sb/Ge/Ag (TAGS) Thermoelectric Materials Induced by Phase Transitions Associated with Vacancy Ordering

Te/Sb/Ge/Ag (TAGS) materials with rather high concentrations of cation vacancies exhibit improved thermoelectric properties as compared to corresponding conventional TAGS (with constant Ag/Sb ratio of 1) due to a significant reduction of the lattice thermal conductivity. There are different vacancy ordering possibilities depending on the vacancy concentration and the history of heat treatment of the samples. In contrast to the average α-GeTe-type structure of TAGS materials with cation vacancy concentrations <∼3%, quenched compounds like Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit “parquet-like” multidomain nanostructures with finite intersecting vacancy layers. These are perpendicular to the pseudocubic ⟨111⟩ directions but not equidistantly spaced, comparable to the nanostructures of compounds (GeTe)_<i>n</i>­Sb₂Te₃. Upon heating, the nanostructures transform into long-periodically ordered trigonal phases with parallel van der Waals gaps. These phases are slightly affected by stacking disorder but distinctly different from the α-GeTe-type structure reported for conventional TAGS materials. Deviations from this structure type are evident only from HRTEM images along certain directions or very weak intensities in diffraction patterns. At temperatures above ∼400 °C, a rock-salt-type high-temperature phase with statistically disordered cation vacancies is formed. Upon cooling, the long-periodically trigonal phases are reformed at the same temperature. Quenched nanostructured Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit ZT values as high as 1.3 and 0.8, respectively, at 160 °C, which is far below the phase transition temperatures. After heat treatment, i.e., without pronounced nanostructure and when only reversible phase transitions occur, the ZT values of Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ with extended van der Waals gaps amount to 1.6 at 360 °C and 1.4 at 410 °C, respectively, which is at the top end of the range of high-performance TAGS materials