3 research outputs found
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, Ce<sub>4</sub>Li<sub>3</sub>P<sub>18</sub>N<sub>35</sub>, with a high degree of condensation >1/2. Ce<sub>4</sub>Li<sub>3</sub>P<sub>18</sub>N<sub>35</sub> consists of an
unprecedented
hexagonal framework of PN<sub>4</sub> 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>6<sub>3</sub>(Ī±, Ī²,
0)Ā0Ā(āĪ± ā Ī², Ī±, 0)Ā0. 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<sub>4ā0.5<i>x</i></sub>Li<sub>3</sub>P<sub>18</sub>N<sub>35ā1.5<i>x</i></sub>O<sub>1.5<i>x</i></sub> (<i>x</i> ā 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 Ī±-GeTe-type structure of TAGS materials
with cation vacancy concentrations <ā¼3%, quenched compounds
like Ge<sub>0.53</sub>Ag<sub>0.13</sub>ĀSb<sub>0.27</sub>ā”<sub>0.07</sub>Te<sub>1</sub> and Ge<sub>0.61</sub>Ag<sub>0.11</sub>ĀSb<sub>0.22</sub>ā”<sub>0.06</sub>Te<sub>1</sub> exhibit āparquet-likeā
multidomain
nanostructures with finite intersecting vacancy layers. These are
perpendicular to the pseudocubic āØ111ā© directions but
not equidistantly spaced, comparable to the nanostructures of compounds
(GeTe)<sub><i>n</i></sub>ĀSb<sub>2</sub>Te<sub>3</sub>. 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 Ī±-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 ā¼400 Ā°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<sub>0.53</sub>Ag<sub>0.13</sub>ĀSb<sub>0.27</sub>ā”<sub>0.07</sub>Te<sub>1</sub> and Ge<sub>0.61</sub>Ag<sub>0.11</sub>ĀSb<sub>0.22</sub>ā”<sub>0.06</sub>Te<sub>1</sub> exhibit ZT values
as high as 1.3 and 0.8, respectively, at 160 Ā°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<sub>0.53</sub>Ag<sub>0.13</sub>ĀSb<sub>0.27</sub>ā”<sub>0.07</sub>Te<sub>1</sub> and Ge<sub>0.61</sub>Ag<sub>0.11</sub>ĀSb<sub>0.22</sub>ā”<sub>0.06</sub>Te<sub>1</sub> with extended van der Waals gaps amount to 1.6 at 360 Ā°C
and 1.4 at 410 Ā°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 Ī±-GeTe-type structure of TAGS materials
with cation vacancy concentrations <ā¼3%, quenched compounds
like Ge<sub>0.53</sub>Ag<sub>0.13</sub>ĀSb<sub>0.27</sub>ā”<sub>0.07</sub>Te<sub>1</sub> and Ge<sub>0.61</sub>Ag<sub>0.11</sub>ĀSb<sub>0.22</sub>ā”<sub>0.06</sub>Te<sub>1</sub> exhibit āparquet-likeā
multidomain
nanostructures with finite intersecting vacancy layers. These are
perpendicular to the pseudocubic āØ111ā© directions but
not equidistantly spaced, comparable to the nanostructures of compounds
(GeTe)<sub><i>n</i></sub>ĀSb<sub>2</sub>Te<sub>3</sub>. 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 Ī±-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 ā¼400 Ā°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<sub>0.53</sub>Ag<sub>0.13</sub>ĀSb<sub>0.27</sub>ā”<sub>0.07</sub>Te<sub>1</sub> and Ge<sub>0.61</sub>Ag<sub>0.11</sub>ĀSb<sub>0.22</sub>ā”<sub>0.06</sub>Te<sub>1</sub> exhibit ZT values
as high as 1.3 and 0.8, respectively, at 160 Ā°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<sub>0.53</sub>Ag<sub>0.13</sub>ĀSb<sub>0.27</sub>ā”<sub>0.07</sub>Te<sub>1</sub> and Ge<sub>0.61</sub>Ag<sub>0.11</sub>ĀSb<sub>0.22</sub>ā”<sub>0.06</sub>Te<sub>1</sub> with extended van der Waals gaps amount to 1.6 at 360 Ā°C
and 1.4 at 410 Ā°C, respectively, which is at the top end of the
range of high-performance TAGS materials