13 research outputs found
Synthesis, Structure, and Electrical Property of Ce 1− x
Ce1−xSr1+xGa3O7−δ powders were synthesized by a solid state reaction method under reducing atmosphere. Single melilite phase was obtained for x≤0.2. X-ray diffraction pattern displays a preferred orientation on (002) peak of the Ce0.9Sr1.1Ga3O7−δ composition. Scanning electron microscope disclosed the nanoscale needle-like micromorphology of this material, with an average length of ~2.5 μm and a diameter of ~500 nm. Transmission electron microscopy and selected area electron diffraction reveal the single crystal nature of these individual Ce0.9Sr1.1Ga3O7−δ needles and build up a relationship between the preferred orientation observed in X-ray diffraction pattern and the micromorphology. The phase stability of Ce1−xSr1+xGa3O7−δ in air was studied and the electrical property was investigated by impedance spectroscopy
The Application of 3D-ED to Distinguish the Superstructure of Sr<sub>1.2</sub>Ca<sub>0.8</sub>Nb<sub>2</sub>O<sub>7</sub> Ignored in SC-XRD
Compared to X-rays, electrons have stronger interactions with matter. In electron diffraction, the low-order structure factors are sensitive to subtle changes in the arrangement of valence electrons around atoms when the scattering vector is smaller than the critical scattering vector. Therefore, electron diffraction is more advantageous for studying the distribution of atoms in the structure with atomic numbers smaller than that of sulfur. In this work, the crystal structure of Sr1.2Ca0.8Nb2O7 (SCNO-0.8) was analyzed using single-crystal X-ray diffraction (SC-XRD) and three-dimensional electron diffraction (3D-ED) techniques, respectively. Interestingly, the superstructure could only be identified by the 3D-ED technique, while no signal corresponding to the superstructure was detected from the SC-XRD data. The superstructure in SCNO-0.8 was disclosed to be caused by different tilting of NbO6 octahedra and the displacements of Sr/Ca atoms in the different atomic layers perpendicular to the a-axis. Therefore, the application of 3D-ED provides an effective method for studying superstructures caused by ordered arrangements of light atoms
La1+xBa1-xGa3O7+0.5x Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites
Substitution of La<sup>3+</sup> for Ba<sup>2+</sup> in LaBaGa<sub>3</sub>O<sub>7</sub> melilite
yields a new interstitial-oxide-ion conducting La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> solid solution, which only extends
up to <i>x</i> = 0.35, giving a maximum interstitial oxygen
content allowed in La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> as about half of those allowed in La<sub>1+<i>x</i></sub>(Sr/Ca)<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub>. La<sub>1.35</sub>Ba<sub>0.65</sub>Ga<sub>3</sub>O<sub>7.175</sub> ceramic displays bulk conductivity
∼1.9 × 10<sup>–3</sup> S/cm at 600 °C, which
is lower than those of La<sub>1.35</sub>(Sr/Ca)<sub>0.65</sub>Ga<sub>3</sub>O<sub>7.175</sub>, showing the reduced mobility for the oxygen
interstitials in La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> than in La<sub>1+<i>x</i></sub>(Sr/Ca)<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub>. Rietveld analysis of neutron powder diffraction data reveals
that the oxygen interstitials in La<sub>1.35</sub>Ba<sub>0.65</sub>Ga<sub>3</sub>O<sub>7.175</sub> are located within the pentagonal
tunnels at the Ga level between two La/Ba cations along the <i>c</i>-axis and stabilized via incorporating into the bonding
environment of a three-linked GaO<sub>4</sub> among the five GaO<sub>4</sub> tetrahedra forming the pentagonal tunnels, similar to the
Sr and Ca counterparts. Both static lattice atomistic simulation and
density functional theory calculation show that LaBaGa<sub>3</sub>O<sub>7</sub> has the largest formation energy for oxygen interstitial
defects among La<sub>1+<i>x</i></sub>M<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> (M = Ba, Sr, Ca), consistent with the large Ba<sup>2+</sup> cations favoring interstitial oxygen defects in melilite less than
the small cations Sr<sup>2+</sup> and Ca<sup>2+</sup>. The cationic-size
control of the ability to accommodate the oxygen interstitials and
maintain high mobility for the oxygen interstitials in La<sub>1+<i>x</i></sub>M<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5x</sub> (M = Ba, Sr, Ca) gallate melilites is understood
in terms of local structural relaxation to accommodate and transport
the oxygen interstitials. The accommodation and migration of the interstitials
in the melilite structure require the tunnel-cations being able to
adapt to the synergic size expansion for the interstitial-containing
tunnel and contraction for the tunnels neighboring the interstitial-containing
tunnel and continuous tunnel-size expansion and contraction. However,
the large oxygen bonding separation requirement of the large Ba<sup>2+</sup> along the tunnel not only suppresses the ability to accommodate
the interstitials in the tunnels neighboring the Ba<sup>2+</sup>-containing
tunnel but also reduces the mobility of the oxygen interstitials among
the pentagonal tunnels
Na2CaV4O12: A low-temperature-firing dielectric with lightweight, low relative permittivity, and dielectric anomaly around 515 C
A low temperature fired Na2CaV4O12 ceramic was synthesized via a solid-state reaction route at a temperature range of 350–550 °C. The Thermal analysis confirmed the densification and melting temperature of Na2CaV4O12 to be 530 °C and 580 °C, respectively. Dielectric properties together with the electrical conductivity were characterized at a broad frequency and temperature range. A super-low relative permittivity of εr = 7.72 and loss tangent of tanδ = 0.06 were obtained at 1 MHz at room temperature. A dielectric anomaly peak took place around 515 °C, which was associated with the phase transition from P4/nbm to P 4‾ b2. Ac impedance spectrum coupled with complex modulus plots unveiled the electrical conduction mechanism, which was dominated by the short-range movement of the charge carriers at low temperatures (T ≤ 220 °C) however long-range migration of charge carriers emerged at higher temperatures
Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La 1+ x Ca 1– x Al 3 O 7+0.5 x Ceramics Synthesized by Glass Crystallization
International audienc
Acceptor Doping and Oxygen Vacancy Migration in Layered Perovskite NdBaInO<sub>4</sub>‑Based Mixed Conductors
The Ca<sup>2+</sup> and Ba<sup>2+</sup> solubility on Nd<sup>3+</sup> sites in new layered perovskite NdBaInO<sub>4</sub> mixed oxide
ionic and hole conductor and their effect on the oxide ion conductivity
of NdBaInO<sub>4</sub> were investigated. Among the alkaline earth
metal cations Ca<sup>2+</sup>, Sr<sup>2+</sup>, and Ba<sup>2+</sup>, Ca<sup>2+</sup> was shown to be the optimum acceptor–dopant
for Nd<sup>3+</sup> in NdBaInO<sub>4</sub> showing the largest substitution
for Nd<sup>3+</sup> up to 20% and leading to oxide ion conductivities
∼3 × 10<sup>–4</sup>–1.3 × 10<sup>–3</sup> s/cm within 600–800 °C on Nd<sub>0.8</sub>Ca<sub>0.2</sub>BaInO<sub>3.9</sub> composition, exceeding the most-conducting Nd<sub>0.9</sub>Sr<sub>0.1</sub>BaInO<sub>3.95</sub> in the Sr-doped NdBaInO<sub>4</sub>. Energetics of defect formation and oxygen vacancy migration
in NdBaInO<sub>4</sub> were computed through the atomistic static-lattice
simulation. The solution energies of Ca<sup>2+</sup>/Sr<sup>2+</sup>/Ba<sup>2+</sup> on the Nd<sup>3+</sup> site in NdBaInO<sub>4</sub> for creating the oxygen vacancies confirm the predominance of Ca<sup>2+</sup> on the substitution for Nd<sup>3+</sup> and enhancement
of the oxygen vacancy conductivity over the larger Sr<sup>2+</sup> and Ba<sup>2+</sup>. The electronic defect formation energies indicate
that the p-type conduction in a high partial oxygen pressure range
of the NdBaInO<sub>4</sub>-based materials is from the oxidation reaction
forming the holes centered on O atoms. Both the static lattice and
molecular dynamic simulations indicate two-dimensional oxygen vacancy
migration within the perovskite slab boundaries for the acceptor-doped
NdBaInO<sub>4</sub>. Molecular dynamic simulations on the Ca-doped
NdBaInO<sub>4</sub> specify two major vacancy migration events, respectively,
via one intraslab path along the <i>b</i> axis and one interslab
path along the <i>c</i> axis. These paths are composed by
two terminal oxygen sites within the perovskite slab boundaries
Phase transformation and ionic conductivity mechanism of a low-temperature sintering semiconductor Na2CaV4O12
Alkaline earth metal vanadates have drawn attention because of their potential applications in electrochemical devices. Here, Na2CaV4O12 was prepared at extremely low temperatures (350-550 oC) and showed a semiconductor behavior with a bandgap of 2.92 eV. A phase transition from P4/nbm to P
̅
b2 occurred at 510 oC was identified by an in-situ XRD upon heating, where the 16 n site for oxygen atoms in the P4/nbm phase evolves into two distinguishable 8i sites in the P
̅
b2 phase. Ionic conduction in Na2CaV4O12 at elevated temperatures was reported for the first time in the present work. A strong correlation between ionic conductivity and phase structure of Na2CaV4O12 is observed. The charged carriers are mainly sodium ions for the low-temperature P4/nbm phase, while mixed conduction contributed by sodium ions and oxide ions happened in the transformed phase. Bond valence-based energy landscape calculations disclosed a two-dimensional interstitial diffusion mechanism for Na+ ions in the Na2Ca-layers, as well as a two-dimensional diffusion mechanism for oxide ions in the V4O12-layers. The novel semiconductor ceramic would have potential applications in all-solid sodium ions batteries or solid oxide fuel cells as electrolytes
La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites
Substitution of La<sup>3+</sup> for Ba<sup>2+</sup> in LaBaGa<sub>3</sub>O<sub>7</sub> melilite
yields a new interstitial-oxide-ion conducting La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> solid solution, which only extends
up to <i>x</i> = 0.35, giving a maximum interstitial oxygen
content allowed in La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> as about half of those allowed in La<sub>1+<i>x</i></sub>(Sr/Ca)<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub>. La<sub>1.35</sub>Ba<sub>0.65</sub>Ga<sub>3</sub>O<sub>7.175</sub> ceramic displays bulk conductivity
∼1.9 × 10<sup>–3</sup> S/cm at 600 °C, which
is lower than those of La<sub>1.35</sub>(Sr/Ca)<sub>0.65</sub>Ga<sub>3</sub>O<sub>7.175</sub>, showing the reduced mobility for the oxygen
interstitials in La<sub>1+<i>x</i></sub>Ba<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> than in La<sub>1+<i>x</i></sub>(Sr/Ca)<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub>. Rietveld analysis of neutron powder diffraction data reveals
that the oxygen interstitials in La<sub>1.35</sub>Ba<sub>0.65</sub>Ga<sub>3</sub>O<sub>7.175</sub> are located within the pentagonal
tunnels at the Ga level between two La/Ba cations along the <i>c</i>-axis and stabilized via incorporating into the bonding
environment of a three-linked GaO<sub>4</sub> among the five GaO<sub>4</sub> tetrahedra forming the pentagonal tunnels, similar to the
Sr and Ca counterparts. Both static lattice atomistic simulation and
density functional theory calculation show that LaBaGa<sub>3</sub>O<sub>7</sub> has the largest formation energy for oxygen interstitial
defects among La<sub>1+<i>x</i></sub>M<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub> (M = Ba, Sr, Ca), consistent with the large Ba<sup>2+</sup> cations favoring interstitial oxygen defects in melilite less than
the small cations Sr<sup>2+</sup> and Ca<sup>2+</sup>. The cationic-size
control of the ability to accommodate the oxygen interstitials and
maintain high mobility for the oxygen interstitials in La<sub>1+<i>x</i></sub>M<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5x</sub> (M = Ba, Sr, Ca) gallate melilites is understood
in terms of local structural relaxation to accommodate and transport
the oxygen interstitials. The accommodation and migration of the interstitials
in the melilite structure require the tunnel-cations being able to
adapt to the synergic size expansion for the interstitial-containing
tunnel and contraction for the tunnels neighboring the interstitial-containing
tunnel and continuous tunnel-size expansion and contraction. However,
the large oxygen bonding separation requirement of the large Ba<sup>2+</sup> along the tunnel not only suppresses the ability to accommodate
the interstitials in the tunnels neighboring the Ba<sup>2+</sup>-containing
tunnel but also reduces the mobility of the oxygen interstitials among
the pentagonal tunnels
Localization of Oxygen Interstitials in CeSrGa<sub>3</sub>O<sub>7+δ</sub> Melilite
The
solubility of Ce in the La<sub>1–<i>x</i></sub>Ce<sub><i>x</i></sub>SrGa<sub>3</sub>O<sub>7+δ</sub> and
La<sub>1.54–<i>x</i></sub>Ce<sub><i>x</i></sub>Sr<sub>0.46</sub>Ga<sub>3</sub>O<sub>7.27+δ</sub> melilites
was investigated, along with the thermal redox stability in air of
these melilites and the conductivity variation associated with oxidization
of Ce<sup>3+</sup> into Ce<sup>4+</sup>. Under CO reducing atmosphere,
the La in LaSrGa<sub>3</sub>O<sub>7</sub> may be completely substituted
by Ce to form the La<sub>1–<i>x</i></sub>Ce<sub><i>x</i></sub>SrGa<sub>3</sub>O<sub>7+δ</sub> solid solution,
which is stable in air to ∼600 °C when <i>x</i> ≥ 0.6. On the other side, the La<sub>1.54–<i>x</i></sub>Ce<sub><i>x</i></sub>Sr<sub>0.46</sub>Ga<sub>3</sub>O<sub>7.27+δ</sub> compositions displayed much lower
Ce solubility (<i>x</i> ≤ 0.1), irrespective of the
synthesis atmosphere. In the as-made La<sub>1–<i>x</i></sub>Ce<sub><i>x</i></sub>SrGa<sub>3</sub>O<sub>7+δ</sub>, the conductivity increased with the cerium content, due to the
enhanced electronic conduction arising from the 4f electrons in Ce<sup>3+</sup> cations. At 600 °C, CeSrGa<sub>3</sub>O<sub>7+δ</sub> showed a conductivity of ∼10<sup>–4</sup> S/cm in
air, nearly 4 orders of magnitude higher than that of LaSrGa<sub>3</sub>O<sub>7</sub>. The oxidation of Ce<sup>3+</sup> into Ce<sup>4+</sup> in CeSrGa<sub>3</sub>O<sub>7+δ</sub> slightly reduced the
conductivity, and the oxygen excess did not result in apparent increase
of oxide ion conduction in CeSrGa<sub>3</sub>O<sub>7+δ</sub>. The Ce doping in air also reduced the interstitial oxide ion conductivity
of La<sub>1.54</sub>Sr<sub>0.46</sub>Ga<sub>3</sub>O<sub>7.27</sub>. Neutron powder diffraction study on CeSrGa<sub>3</sub>O<sub>7.39</sub> composition revealed that the extra oxygen is incorporated in the
four-linked GaO<sub>4</sub> polyhedral environment, leading to distorted
GaO<sub>5</sub> trigonal bipyramid. The stabilization and low mobility
of interstitial oxygen atoms in CeSrGa<sub>3</sub>O<sub>7+δ</sub>, in contrast with those in La<sub>1+<i>x</i></sub>Sr<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub>, may be correlated with the cationic size contraction
from the oxidation of Ce<sup>3+</sup> to Ce<sup>4+</sup>. These results
provide a new comprehensive understanding of the accommodation and
conduction mechanism of the oxygen interstitials in the melilite structure
Solid-State <sup>29</sup>Si NMR and Neutron-Diffraction Studies of Sr<sub>0.7</sub>K<sub>0.3</sub>SiO<sub>2.85</sub> Oxide Ion Conductors
K/Na-doped
SrSiO<sub>3</sub>-based oxide
ion conductors were recently reported as promising candidates for
low-temperature solid-oxide fuel cells. Sr<sub>0.7</sub>K<sub>0.3</sub>SiO<sub>2.85</sub>, close to the solid-solution limit of Sr<sub>1–<i>x</i></sub>K<sub><i>x</i></sub>SiO<sub>3–0.5<i>x</i></sub>, was characterized by solid-state <sup>29</sup>Si
NMR spectroscopy and neutron powder diffraction (NPD). Differing with
the average structure containing the vacancies stabilized within the
isolated Si<sub>3</sub>O<sub>9</sub> tetrahedral rings derived from
the NPD study, the <sup>29</sup>Si NMR data provides new insight into
the local defect structure in Sr<sub>0.7</sub>K<sub>0.3</sub>SiO<sub>2.85</sub>. The Q<sup>1</sup>-linked tetrahedral Si signal in the <sup>29</sup>Si NMR data suggests that the Si<sub>3</sub>O<sub>9</sub> tetrahedral rings in the K-doped SrSiO<sub>3</sub> materials were
broken, forming Si<sub>3</sub>O<sub>8</sub> chains. The Si<sub>3</sub>O<sub>8</sub> chains can be stabilized by either bonding with the
oxygen atoms of the absorbed lattice water molecules, leading to the
Q<sup>1</sup>-linked tetrahedral Si, or sharing oxygen atoms with
neighboring Si<sub>3</sub>O<sub>9</sub> units, which is consistent
with the Q<sup>3</sup>-linked tetrahedral Si signal detected in the <sup>29</sup>Si NMR spectra