2 research outputs found
The Fluorite-Like Phase Nd<sub>5</sub>Mo<sub>3</sub>O<sub>16±δ</sub> in the MoO<sub>3</sub>–Nd<sub>2</sub>O<sub>3</sub> System: Synthesis, Crystal Structure, and Conducting Properties
This paper describes a study of the
system MoO<sub>3</sub>–Nd<sub>2</sub>O<sub>3</sub> using a
combination of X-ray powder diffraction (XRD), neutron powder diffraction
(NPD), thermogravimetric analysis (TGA), and ac impedance spectroscopy
(IS). A phase-pure material is observed at a composition of 45.5 mol
% Nd<sub>2</sub>O<sub>3</sub>, which corresponds to an ideal stoichiometry
of Nd<sub>5</sub>Mo<sub>3</sub>O<sub>16.5</sub>. XRD and NPD show
that the crystal structure is a superstructure of the fluorite arrangement,
with long-range ordering of the two cation species leading to a doubled
unit cell parameter. The sample is found to be significantly oxygen
deficient, i.e. Nd<sub>5</sub>Mo<sub>3</sub>O<sub>15.63(4)</sub>,
when it is prepared by a solid-state reaction at 1473 K in air. TGA
measurements indicate that the sample loses only minimal mass on heating
to 1273 K in O<sub>2</sub>. IS studies of the mean conductivity under
different atmospheres show that the sample is a mixed conductor between
ambient temperature and 873 K, with a dominant electronic component
at higher temperatures, as demonstrated by measurements under inert
atmosphere. NPD measurements indicate that the anion vacancies are
preferentially located on the O2 sites, while studies of the temperature
dependence performed under an O<sub>2</sub> atmosphere to 1273 K show
significantly anisotropic thermal parameters of the anions. Together
with analysis of the total neutron scattering data, this supports
a model of oxygen ions hopping between O2 positions, with a vacancy,
rather than interstitial, mechanism for the anion diffusion
Sr<sub>2</sub>GaScO<sub>5</sub>, Sr<sub>10</sub>Ga<sub>6</sub>Sc<sub>4</sub>O<sub>25</sub>, and SrGa<sub>0.75</sub>Sc<sub>0.25</sub>O<sub>2.5</sub>: a Play in the Octahedra to Tetrahedra Ratio in Oxygen-Deficient Perovskites
Three different perovskite-related phases were isolated
in the
SrGa<sub>1–<i>x</i></sub>Sc<sub><i>x</i></sub>O<sub>2.5</sub> system: Sr<sub>2</sub>GaScO<sub>5</sub>, Sr<sub>10</sub>Ga<sub>6</sub>Sc<sub>4</sub>O<sub>25</sub>, and SrGa<sub>0.75</sub>Sc<sub>0.25</sub>O<sub>2.5</sub>. Sr<sub>2</sub>GaScO<sub>5</sub> (<i>x</i> = 0.5) crystallizes
in a brownmillerite-type structure [space group (S.G.) <i>Icmm</i>, <i>a</i> = 5.91048(5) Ã…, <i>b</i> = 15.1594(1)
Ã…,
and <i>c</i> = 5.70926(4) Ã…] with complete ordering
of Sc<sup>3+</sup> and Ga<sup>3+</sup> over
octahedral and tetrahedral positions, respectively. The crystal structure
of Sr<sub>10</sub>Ga<sub>6</sub>Sc<sub>4</sub>O<sub>25</sub> (<i>x</i> = 0.4) was determined by the Monte Carlo method and refined
using a combination of X-ray, neutron, and electron diffraction data
[S.G. <i>I</i>4<sub>1</sub>/<i>a</i>, <i>a</i> = 17.517(1) Ã…, <i>c</i> = 32.830(3) Ã…].
It represents a novel type of ordering of the B cations and oxygen
vacancies in perovskites. The crystal structure of Sr<sub>10</sub>Ga<sub>6</sub>Sc<sub>4</sub>O<sub>25</sub> can be described as a
stacking of eight perovskite layers along the <i>c</i> axis
...[−(Sc/Ga)ÂO<sub>1.6</sub>–SrO<sub>0.8</sub>–(Sc/Ga)ÂO<sub>1.8</sub>–SrO<sub>0.8</sub>−]<sub>2</sub>.... Similar
to Sr<sub>2</sub>GaScO<sub>5</sub>, this structure features a complete
ordering of the Sc<sup>3+</sup> and Ga<sup>3+</sup> cations over octahedral
and tetrahedral positions, respectively, within each layer. A specific
feature of the crystal structure of Sr<sub>10</sub>Ga<sub>6</sub>Sc<sub>4</sub>O<sub>25</sub> is that one-third of the tetrahedra have one
vertex not connected with other Sc/Ga cations. Further partial replacement
of Sc<sup>3+</sup> by Ga<sup>3+</sup> leads to the formation of the
cubic perovskite phase SrGa<sub>0.75</sub>Sc<sub>0.25</sub>O<sub>2.5</sub> (<i>x</i> = 0.25) with <i>a</i> = 3.9817(4)
Ã….
This compound incorporates water molecules in the structure forming
SrGa<sub>0.75</sub>Sc<sub>0.25</sub>O<sub>2.5</sub>·<i>x</i>H<sub>2</sub>O hydrate, which exhibits a proton conductivity of ∼2.0
× 10<sup>–6</sup> S/cm at 673 K