Estudio de las estructuras cristalinas y de las transiciones de fase en nuevos materiales de wolframio (Sr2M2+W6+O6) y de antimonio (A2M3+Sb5+O6) con estructura de perovskita doble · Perovskite Atlas

Tungstates Sr₂MWO₆ (Cd, Ca, Mn and the Cd/Ca solid solution) and antimonates A₂MSbO₆ (Al, Co, Cr, Fe, Sc, Ga, Mn, lanthanides; Ca/Sr solid solutions). Chapter map is completed in Phase 1.

status published · visibility public · id doc:thesis-02

Abdessamad Faik, 2009 · supervisor Josu M. Igartua · Universidad del País Vasco / Euskal Herriko Unibertsitatea · language es

Files: sources/theses/thesis-02.pdf, sources/theses/thesis-02.md

Figures

Figure 1: a shows the room-temperature x-ray powder diffraction patterns for the various compositions of \mathrm{Sr}_{2} \mathrm{Cd}_{1-x} \mathrm{Ca}_{x} \mathrm{WO}_{6}, in the 17-82 degree 2 \theta range. Based on a visual inspection of the roomtemperature x-ray powder diffractograms (Fig. 1b), as there is a continuous change in the position of the peaks and in their form, evolving from the Cd-containing compound to the Ca-containing one, it is clear that the substitution of the Cd cation by Ca gives rise to a continuous solid solution: all the compositions are in a double perovskite phase with good homogeneity. As mentioned, the end-members of this solid-solution crystallize, at room-temperature, in a monoclinic symmetry \left(P 2_{1} / n\right) 41, 43. Thus, we expected that the members of the solid solution also crystallize in the same space group. Indeed, for all the studied compositions, all the reflections in each diffractograms, could be indexed in the P 2_{1} / n space group. We performed two sets of refinements: in
Figure 1.13: Evolución de las amplitudes de los modos cuyas irrpes son \Gamma_{1}^{+}, \Gamma_{3}^{+}y \Gamma_{4}^{+}(modo primario) en la transición I 4 / m \rightarrow F m \overline{3} m, para tres tipos de refinamientos diferentes: puntos negros, refinamiento mediante coordenadas atómicas, con una restricción sobre una coordenada de uno de los oxígenos; estrellas en rojo, refinamiento mediante modos adaptados, también con una restricción, la amplitud del modo \Gamma_{3}^{+} se fija en cero; estrellas en azul, refinamiento por modos, con una restricción, la amplitud del modo con \Gamma_{3}^{+} fijada al valor que toma en la primera temperatura (ver texto).
Figure 1.2: Representación esquemática de la estructura de la perovskita doble \mathrm{A}_{2} \mathrm{BB}^{\prime} \mathrm{X}_{6}: (a) completamente ordenada, (b) parcialmente ordenada y (c) completamente desordenada.
Figure 1.2-2: Representación esquemática de la estructura de la perovskita doble \mathrm{A}_{2} \mathrm{BB}^{\prime} \mathrm{X}_{6}: (a) completamente ordenada, (b) parcialmente ordenada y (c) completamente desordenada.
Figure 10:
Figure 10.1: (a) and (b) Section of the diffractogram of the XRPD measurements refined in the I 2 / m and I 4 / m space groups, respectively, to show that the tetragonal symmetry does not take into account the observed splitting. (c) and (d) Another section of the diffractogram comparing the results of the refinements using the I 2 / m and the R \overline{3} space groups. The trigonal space group does not take into account the observed splitting.
Figure 11:
Figure 12: shows the evolution of diffraction intensities of this reflection with temperature. From the profile shape characteristic reflections at 2 \theta from 91 to 96 in (figure 12b), it was concluded that up to 1740 \mathrm{K} \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} does not transform to the tetragonal lattice ( I 4 / m space group) like \mathrm{Sr}_{2} \mathrm{ScSbO}_{6}. This shows that there is no change of the monoclinic distortion in all this range of temperatures; and, analyzing the peak splitting, it becomes clear that at 1740 K the unit cell is monoclinic. After examination of all diffractograms, we observed, about 1440 K (figure 12a), the continuous desapearance of diffraction peaks corresponding to primitive Bragg reflections, with indices (hkl) of the type h+k+l=2 \mathrm{n}+1. This corresponds to a continuous phase transition from monoclinic P 2_{1} / n to monoclinic I 2 / m at about 1440 K. The evolution with temperature, from 720 to 1680 K, of the refined lattice parameters for \mathrm{Ca}_{2} \mathrm{ScSbO}_{6}, are shown in figure 13. As deduced from the figure, the lattice parameters increasing countinuously with temperature, and the difference between \sqrt{2} a, \sqrt{2} b and c lattice parameters decreases. But no anomaly change is noted.
Figure 12-2: shows the evolution of diffraction intensities of this reflection with temperature. From the profile shape characteristic reflections at 2 \theta from 91 to 96 in (figure 12b), it was concluded that up to 1740 \mathrm{K} \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} does not transform to the tetragonal lattice ( I 4 / m space group) like \mathrm{Sr}_{2} \mathrm{ScSbO}_{6}. This shows that there is no change of the monoclinic distortion in all this range of temperatures; and, analyzing the peak splitting, it becomes clear that at 1740 K the unit cell is monoclinic. After examination of all diffractograms, we observed, about 1440 K (figure 12a), the continuous desapearance of diffraction peaks corresponding to primitive Bragg reflections, with indices (hkl) of the type h+k+l=2 \mathrm{n}+1. This corresponds to a continuous phase transition from monoclinic P 2_{1} / n to monoclinic I 2 / m at about 1440 K. The evolution with temperature, from 720 to 1680 K, of the refined lattice parameters for \mathrm{Ca}_{2} \mathrm{ScSbO}_{6}, are shown in figure 13. As deduced from the figure, the lattice parameters increasing countinuously with temperature, and the difference between \sqrt{2} a, \sqrt{2} b and c lattice parameters decreases. But no anomaly change is noted.
Figure 12.3: Evolution of the laboratory X-ray diffraction pattern at room temperature of \mathrm{Ca}_{2-x} \mathrm{Sr}_{x} \mathrm{LaSbO}_{6} (a) and \mathrm{Ca}_{2-x} \mathrm{Sr}_{x} \mathrm{SmSbO}_{6} (b) with ( x=0.5,1.0). In the insets, details from the X-ray powder diffraction patterns in 53-58^{\circ} range, to show the continuous evolution of the pattern with increasing of Sr concentration.
Figure 12.3-2: Evolution of the laboratory X-ray diffraction pattern at room temperature of \mathrm{Ca}_{2-x} \mathrm{Sr}_{x} \mathrm{LaSbO}_{6} (a) and \mathrm{Ca}_{2-x} \mathrm{Sr}_{x} \mathrm{SmSbO}_{6} (b) with ( x=0.5,1.0). In the insets, details from the X-ray powder diffraction patterns in 53-58^{\circ} range, to show the continuous evolution of the pattern with increasing of Sr concentration.
Figure 14.7: Observed X-ray patterns (symbols), calculated (line) and difference profiles for the Rietveld refinement of \mathrm{Ca}_{2} \mathrm{FeSbO}_{6} at room temperature using a structural model with P 2_{1} / n space group.
Figure 3: shows the (a) a, b and c lattice parameters, (b) monoclinic angle ( \beta) and the (c) unit-cell volume of the synthesized materials. All those parameters increase linearly with the Ca amount, a clear evidence that the Vegard’s law holds for this solid solution.
Figure 5: presents the projection of the room-temperature crystal structures of \mathrm{Sr}_{2} \mathrm{ScSbO}_{6} and \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} along the [010] double perovskite monoclinic lattice vector. The appreciable tilt of the octahedra is inferred from the deviation of \mathrm{Sc}-\mathrm{O}-\mathrm{Sb} angles from the ideal value ( 180^{\circ}), the deviation of the angle is bigger in the case of Ca compound ( 149^{\circ}) in comparison to the Sr compound \left(165^{\circ}\right), which means that the \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} is more distorted than \mathrm{Sr}_{2} \mathrm{ScSbO}_{6}.
Figure 5-2: presents the projection of the room-temperature crystal structures of \mathrm{Sr}_{2} \mathrm{ScSbO}_{6} and \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} along the [010] double perovskite monoclinic lattice vector. The appreciable tilt of the octahedra is inferred from the deviation of \mathrm{Sc}-\mathrm{O}-\mathrm{Sb} angles from the ideal value ( 180^{\circ}), the deviation of the angle is bigger in the case of Ca compound ( 149^{\circ}) in comparison to the Sr compound \left(165^{\circ}\right), which means that the \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} is more distorted than \mathrm{Sr}_{2} \mathrm{ScSbO}_{6}.
Figure 6.3: Evolution, as a function of increasing calcium content, ( x=0,0.2,0.4,0.6,0.8,1.0), at room temperature, of the (a) unit-cell volume, (b) monoclinic angle ( \beta) and the (c) unit-cell parameters of the synthesized materials. Dotted line is a guide for the eyes.
Figure 7.1: Observed ( \cdot), calculated ( -) and difference profiles for the Rietveld refinement of \mathrm{Sr}_{2} \mathrm{Mn}^{2+} \mathrm{WO}_{6} at room temperature using a structural model with the P 2_{1} / n space group. In (a), the impurity \mathrm{SrWO}_{4} ( <3.35 \%) is included in the refinement as a known additional phase. In (b), the impurity \mathrm{MnO}(<1 \%) is included in the refinement as a known additional phase. The top barred pattern corresponds to \mathrm{Sr}_{2} \mathrm{Mn}^{2+} \mathrm{WO}_{6} and the bottom barred pattern to the impurity.
Figure 7.1-2: Observed ( \cdot), calculated ( -) and difference profiles for the Rietveld refinement of \mathrm{Sr}_{2} \mathrm{Mn}^{2+} \mathrm{WO}_{6} at room temperature using a structural model with the P 2_{1} / n space group. In (a), the impurity \mathrm{SrWO}_{4} ( <3.35 \%) is included in the refinement as a known additional phase. In (b), the impurity \mathrm{MnO}(<1 \%) is included in the refinement as a known additional phase. The top barred pattern corresponds to \mathrm{Sr}_{2} \mathrm{Mn}^{2+} \mathrm{WO}_{6} and the bottom barred pattern to the impurity.
Figure 8: shows the evolution with temperature of the lattice parameters of \mathrm{Sr}_{2} \mathrm{CoSbO}_{6}. In this figure we have represented the unit cell in the rhombohedral description ( a=b=c, \alpha=\beta=\gamma), instead of the hexagonal one ( a=b, \gamma=120^{\circ}). This representation is more convenient for observing the disappearance of the trigonal distortion. As it can be observed in Fig. 8, the rhombohedral angle \alpha decreases continuously with the temperature, and at \approx 470 \mathrm{K} it becomes equal to 60 deg, the value at which the cubic symmetry is attained.
Figure 9: shows the phase transition temperatures as a function of the tolerance factor t. From this figure, we conclude that the phase transition temperatures increase with decreasing of the value of
Figure 9.3: Raman spectra of \mathrm{Sr}_{2} \mathrm{AlSbO}_{6} (sample 1) at different temperatures (excitation 514.5 nm). No significant changes are observed at lower temperatures, indicating that the cubic structure observed at 300 K remains stable down to, at least, 79K.
Figure 9.3-2: Raman spectra of \mathrm{Sr}_{2} \mathrm{AlSbO}_{6} (sample 1) at different temperatures (excitation 514.5 nm). No significant changes are observed at lower temperatures, indicating that the cubic structure observed at 300 K remains stable down to, at least, 79K.
Figure 9.3-3: Raman spectra of \mathrm{Sr}_{2} \mathrm{AlSbO}_{6} (sample 1) at different temperatures (excitation 514.5 nm). No significant changes are observed at lower temperatures, indicating that the cubic structure observed at 300 K remains stable down to, at least, 79K.
Figure 9.6: Rietveld refinement of \mathrm{Sr}_{2} \mathrm{CoSbO}_{6}, at room temperature, using the R \overline{3} and R \overline{3} m space groups. Experimental (symbols) and calculated (line) and difference powder diffraction profiles. The bars represent the Bragg peak positions. The structural model in R \overline{3} fits better the intensity of the peak.
Figure 9.6-2: Rietveld refinement of \mathrm{Sr}_{2} \mathrm{CoSbO}_{6}, at room temperature, using the R \overline{3} and R \overline{3} m space groups. Experimental (symbols) and calculated (line) and difference powder diffraction profiles. The bars represent the Bragg peak positions. The structural model in R \overline{3} fits better the intensity of the peak.
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Tables

Table 10.1: 2 \theta ranges and the (hkl) indices of the reflections for the I 2 / m, I 4 / m, R \overline{3} and F m \overline{3} m (the ideal cubic symmetry) space groups.
Table 10.4: Main bond distances ( \AA) and selected angles ( { }^{\circ}) for \mathrm{Sr}_{2} \mathrm{CrSbO}_{6} from XRPD at room temperature. \mathrm{rCr}{ }^{3+}(\mathrm{VI})=0.615 \AA \mathrm{rSb} b^{5+}(\mathrm{VI})=0.6 \AA.
Table 11.3: Main bond distances ( \AA) and selected angles ( { }^{\circ}) for \mathrm{Sr}_{2} \mathrm{SmSbO}_{6} and \mathrm{Sr}_{2} \mathrm{LaSbO}_{6} from XRPD at room temperature.
Table 12.2: Crystal structure data and refinement results for \mathrm{Ca}_{2} \mathrm{SmSbO}_{6} at room temperature. The atomic positions (in fractional coordinates) and isotropic atomic displacement parameters were refined in the space group P 2_{1} / n. (Note: a=5.6062(1) \AA; b=5.8553(1) \AA; c=8.0977(1) \AA; \beta=90.23(1)^{\circ}; R_{\mathrm{wp}}=11.6 \%; R_{\mathrm{B}}=5.13 \%; \chi^{2}=1.72.)
Table 13.2: Crystal structure and R. factors for \mathrm{Ca}_{2} \mathrm{ScSbO}_{6}, the atomic positions were refined in space group P 2_{1} / n at room temperature from x-ray and E9 neutron data. The cell parameters are: a=5.5088(1) \AA; \mathrm{b}=5.6226(1) \AA; c=7.8601(1); \beta=90.02(1) \circ; \mathrm{V}=243.46(1) \AA^{3}
Table 13.3: Main bond distances ( \AA) and selected angles ( { }^{\circ}) for \mathrm{Sr}_{2} \mathrm{ScSbO}_{6} and \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} from XRPD at room temperature. \mathrm{rS} c^{3+}(\mathrm{VI})=0.745 \AA \mathrm{rS} b^{5+}(\mathrm{VI})=0.6 \AA.
Table 15.1: Structural details and preparation conditions of \mathrm{Sr}_{2} \mathrm{MSbO}_{6} compounds. The data shown are as follows: Highest temperature of treatment; ionic radii in [3]; \Delta \mathrm{r}=\left|r_{M^{+3}}-r_{S b^{5+}}\right|; experimentally observed degree of cation ordering; tolerance factor; symmetry of the crystal structure at room temperature and literature reference. PW stands for present work.
Table 3: lists the main interatomic distances and angles for \mathrm{Sr}_{2} \mathrm{ScSbO}_{6} and \mathrm{Ca}_{2} \mathrm{ScSbO}_{6} phases at room temperature. The structural analysis of both compounds indicates that the \mathrm{Sc}^{3+} and \mathrm{Sb}^{5+} are octahedrally coordinated with the oxygen atoms. The typical \mathrm{Sc}-\mathrm{O} bond lengths of \mathrm{ScO}_{6} octahedra range between 2.04 and 2.10 \AA. Similarly, the \mathrm{Sb}-\mathrm{O} bond lengths of \mathrm{SbO}_{6} octahedra range between 1.96 and 2.01 \AA. The average \langle\mathrm{Sc}-\mathrm{O}\rangle and \langle\mathrm{Sb}-\mathrm{O}\rangle are also close to the calculated
Table 7.2: Crystal structure data and refinement results for \mathrm{Sr}_{2} \mathrm{Mn}^{3+} \mathrm{WO}_{6+\delta} at room temperature. The atomic positions (in fractional coordinates) and temperature factors were refined in the space group I 4 / m. (Note: a=5.6353(1) \AA; c=8.0149(1) \AA and V=254.53(1)^{3}; R_{\mathrm{p}}=10.6 \%; R_{\mathrm{wp}}=15.1 \%; R_{\text {exp }}=5.42 \%; \chi^{2}=7.73).
Table 7.3: Main bond distances ( \AA) for \mathrm{Sr}_{2} \mathrm{Mn}^{2+} \mathrm{WO}_{6} and \mathrm{Sr}_{2} \mathrm{Mn}^{3+} \mathrm{WO}_{6+\delta} from XRPD at room temperature.
Table 9.3: Crystal structure for \mathrm{Sr}_{2} \mathrm{CoSbO}_{6}, at room temperature, in the R \overline{3} m and R \overline{3} space groups. The atomic positions in the R \overline{3} m space group are: \mathrm{Sb} 1 / \mathrm{Co} 2(3 \mathrm{ b}: 0,0,1 / 2), \mathrm{Co} 1 / \mathrm{Sb} 2(3 \mathrm{a}: 0,0,0), \mathrm{Sr}(6 \mathrm{c}: 0,0, z), \mathrm{O} 1(18 \mathrm{ h}: x,-x, z). The atomic positions in the R \overline{3} space group are: \mathrm{Sb} 1 / \mathrm{Co} 2(3 \mathrm{ b}: 0,0,1 / 2), \mathrm{Co} 1 / \mathrm{Sb} 2(3 \mathrm{a}: 0,0,0), Sr (6c: 0,0, z), O1(18f: x, y, z).