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Structure and X-ray reference diffraction patterns of (Ba6−xSrx)R2Co4O15 (x = 1, 2) (R = lanthanides)

Published online by Cambridge University Press:  25 April 2013

W. Wong-Ng ,
G. Liu ,
Y.G. Yan  and
J.A. Kaduk
W. Wong-Ng*
Affiliation:
Materials Measurement Science Division, NIST, Gaithersburg, Maryland 20899
G. Liu
Affiliation:
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China
Y.G. Yan
Affiliation:
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, People's Republic of China
J.A. Kaduk
Affiliation:
BCPS, Illinois Institute of Technology, Chicago, Illinois 60616
*
a)Author to whom correspondence should be addressed. Electronic mail: winnie.wong-ng@nist.gov
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Abstract

The structure and X-ray patterns of two series of barium lanthanide cobaltates, namely, Ba4Sr2R2Co4O15 (R = La, Nd, Sm, Eu, Gd, and Dy), and Ba5SrR2Co4O15 (R = La, Nd, Sm, Eu, and Gd) have been determined. These compounds crystallize in the space group P63mc; the unit-cell parameters of Ba4Sr2R2Co4O15 (R from La to Dy) decrease from a = 11.6128(2) Å to 11. 5266(9) Å, c = 6.869 03(11) to 6. 7630(5) Å, and V = 802.23(3) Å3 to 778.17(15) Å3, respectively. In the Ba5SrR2Co4O15 series (R = La to Gd), the unit-cell parameters decrease from a = 11.735 44(14) Å to 11.619 79(12) Å, c = 6.942 89 (14) Å to 6.836 52(8) Å, and V = 828.08(3) Å3 to 799.40(2) Å3. In the general structure of (Ba6−xSrx)R2Co4O15, there are four Co ions per formula unit occupying one CoO6 octahedral and three CoO4 tetrahedral units. Through corner-sharing of these polyhedra, a larger Co4O15 unit is formed. Sr2+ ions adopt both octahedral and 8-fold coordination environment. R3+ ions adopt 8-fold coordination (mixed site with Sr), while the larger Ba2+ ions assume both 10- and 11-fold coordination environments. The samples were found to be insulators. X-ray diffraction patterns of these samples have been determined and submitted to the Powder Diffraction File (PDF).

Keywords

X-ray reference diffraction patterns(Ba6−xSrx)R2Co4O15 (x = 1, 2) (R = La, Nd, Sm, Eu, and Gd)crystal structure
Type
Technical Articles
Information
Powder Diffraction , Volume 28 , Issue 3 , September 2013 , pp. 212 - 221
Copyright
Copyright © International Centre for Diffraction Data 2013 

I. INTRODUCTION

Continuing demands for environmentally friendly alternative-energy technologies have led to increased activities in the area of thermoelectric (TE) research. For high-temperature waste-heat conversion applications, low-dimensional layered oxides have been found to have relatively high efficiency. The efficiency and performance of TE energy conversion or cooling is related to the dimensionless figure of merit (ZT) of the TE materials, given by ZT = S 2σT/κ, where T is the absolute temperature, S is the Seebeck coefficient or TE power, σ is the electrical conductivity, and k is the thermal conductivity (Nolas et al., Reference Nolas, Sharp and Goldsmid2001). Examples of these oxides include NaCoOx (Terasaki et al., Reference Terasaki, Sasago and Uchinokura1997), Ca2Co3O6 (Mikami et al., Reference Mikami, Funashashi, Yoshimura, Mori and Sasaki2003; Mikami and Funahashi, Reference Mikami and Funahashi2005), and Ca3Co4O9 (Masset et al., Reference Masset, Michel, Maignan, Hervieu, Toulemonde, Studer and Raveau2000; Minami et al., Reference Minami, Itaka, Kawaji, Wang, Koinuma and Lippmaa2002; Grebille et al., Reference Grebille, Lambert, Bouree and Petricek2004; Hu et al., Reference Hu, Si, Sutter and Li2005). Among these materials, the most efficient material, Ca3Co4O9, is a misfit layered oxide that has two monoclinical subsystems with identical a, c, β, but different b (Masset et al., Reference Masset, Michel, Maignan, Hervieu, Toulemonde, Studer and Raveau2000). However, to have materials with high enough efficiency for large-scale industrial applications, ZT of two or higher is a requirement.

The search for cobaltate compounds with improved TE properties continues worldwide. The goal of this paper is two-fold. Firstly, (Ba6−xSrx)R 2Co4O15 (x = 1, 2) compounds are investigated for their structures. The structures of Ba6La2Co4O15 and Ba5CaNd2Co4O15 have been reported by Mevs and Müller-Buschbaum (Reference Mevs and Müller-Buschbaum1990a), and Müller-Buschbaum and Martin (Reference Müller-Buschbaum and Martin1992). Since X-ray diffraction is a non-destructive technique for phase identification, X-ray diffraction patterns are especially important for phase characterization, therefore another goal of this investigation was to determine the experimental patterns for Ba4Sr2R 2Co4O15 (R = La, Nd, Sm, Gd, and Dy), and Ba5SrR 2Co4O15 (R = La, Nd, Sm, Eu, and Gd), and to make them widely available through submission to the Powder Diffraction File (PDF) (ICDD).

II. EXPERIMENTAL

A. Sample preparation

All samples were prepared by heating a stoichiometric mixture of BaCO3,R 2O3 (R = La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu), and Co3O4 in air. La2O3 and Nd2O3 were first heat treated at 550 °C overnight prior to use to ensure the absence of carbonates and hydroxides. Samples were weighed, well-mixed, and calcined at 800 °C for one day, 950 °C for one day, and subsequently at 980 °C, with intermediate grindings and pelletizations, for another 6 days. During each heat treatment, all samples were furnace cooled. The phase purity of the samples was established by powder X-ray diffraction.

B. X-ray Rietveld refinements and powder reference patterns

The Ba4Sr2R 2Co4O15 (R = La, Nd, Sm, Eu, and Gd) and Ba5SrR 2Co4O15 (R = La, Nd, Sm, Eu, Gd, and Dy) powders were mounted as ethanol slurries on zero-background cells. The X-ray powder patterns of the former samples were measured on a Bruker D2 Phaser diffractometer. The X-ray powder patterns of the latter group of samples were measured at ambient conditions on a Panalytical X'Pert Pro MPD diffractometer equipped with a PIXcel position-sensitive detector and an Anton Paar HTK1200N furnace. Patterns were measured (CuKα radiation, 45 kV, 40 mA, 0.5° divergence slit, and 0.02 rad Soller slits) from 5 to 130°2θ in 0.02° steps.

The Rietveld refinement technique (Rietveld, Reference Rietveld1969) with the software suite GSAS (Larson and von Dreele, Reference Larson and von Dreele2004) was used to determine the structure of (Ba6−xSrx)R 2Co4O15. A structural model of Ba5SrPr2Co4O15 reported previously (Müller-Buschbaum and Uensal, Reference Müller-Buschbaum and Uensal1996) was used for structural refinements. Reference patterns were obtained with a Rietveld pattern decomposition technique. Using this technique, the reported peak positions were derived from the extracted integrated intensities, and positions calculated from the unit-cell parameters. When peaks are not resolved at the resolution function, the intensities are summed, and an intensity-weighted d-spacing is reported. They are also corrected for systematic errors both in d-spacing and intensity. In summary, these patterns represent ideal specimen patterns.

C. Bond valence sum (V b) calculation

The bond valence sum values, V b, for the Ba, R, and Co sites were calculated using the Brown–Altermatt empirical expression (Brown and Altermatt, Reference Brown and Altermatt1985; Brese and O'Keeffe, Reference Brese and O'Keeffe1991). The V b of an atom i is defined as the sum of the bond valences v ij of all the bonds from atoms i to atoms j. The most commonly adopted empirical expression for bond valence v ij as a function of the interatomic distance d ij is v ij = exp[(R 0−d ij)/B]. The parameter, B, is commonly taken to be a “universal” constant equal to 0.37 Å. The values for the reference distance R 0 (Å) for Ba–O, Sr–O, Co2+–O, Co3+–O, La–O, Nd–O, Sm–O, Eu–O, Gd–O, and Dy–O are 2.29, 2.118, 1.692, 1.70, 2.172, 2.117, 2.088, 2.076, 2.065, and 2.036, respectively (Brown and Altermatt, Reference Brown and Altermatt1985; Brese and O'Keeffe, Reference Brese and O'Keeffe1991).

III. RESULTS AND DISCUSSION

Phases for Ba4Sr2R 2Co4O15, and Ba5SrR 2Co4O15 were successfully prepared only for compounds with relatively larger size of R. Based on X-ray diffraction results, in Ba4Sr2R 2Co4O15, compounds with R = Dy, Ho, Er, Yb, Tm, and Lu, and in Ba5SrR 2Co4O15, compounds with R = Ho, Er, Yb, Tm, and Lu cannot be made at all under the current synthesis conditions. X-ray diffraction data of each Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15 were indexable with a hexagonal unit cell and with a space group of P63mc (No. 186). X-ray patterns of the composition of the Ho-analog, Ba4Sr2Ho2Co4O15, indicate a different structure. The major product of this preparation has the P4mm SrHoO3 type structure, with a = 4.1301(1), c = 4.1430(2) Å, and V = 70.670(5) Å3. Refinement of this structure indicated that the A site was occupied by Sr, and that the oxygen sites were fully occupied. This sample contains 5.9(1)% mass fraction of BaSrHo4O8, 3.4(1)% mass fraction of CaCoO2+x, and 0.8(1)% mass fraction of a spinel, and traces of additional phases.

The samples are essentially isolators as they are very resistive and no reasonable Seebeck coefficient signal could be obtained.

A. Structure of Ba6−x(Sr,Ca)xR2Co4O15

Table I gives the refinement residuals for Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15. Figure 1 provides the Rietveld refinement results for Ba5SrSm2Co4O15 as an example. The observed (crosses), calculated (solid line), and difference XRD patterns (bottom) for Ba5SrSm2Co4O15, as determined by the Rietveld analysis technique, are shown. The difference pattern is plotted at the same scale as the other patterns up to 70°2θ. At higher 2θ angles, the scale has been magnified five times. The rows of tick marks refer to the calculated peak positions. The refinement residuals mainly reflect variations in the counting times, and the presence of traces of additional impurities as indicated.

Figure 1. Observed (crosses), calculated (solid line), and difference XRD pattern (bottom) for Ba5SrSm2Co4O15 by Rietveld analysis technique. The difference pattern is plotted at the same scale as the other patterns up to 70°2θ. At higher 2θ angles, the scale has been magnified five times. Phases present are indicated next to the rows of tick marks (from bottom to top: Ba5SrSm2Co4O15, (Ba, Sr)CoO3, BaCO3, BaSm2CoO5, BaCoOx, and Sm2O3).

Table I. Refinement residuals and phases present for (a) Ba4Sr2R2Co4O15 and (b) Ba5SrR2Co4O15. Values inside brackets are standard deviations.

Table II lists the unit-cell parameters for Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15. The calculated density values, D x, in both series increase as the size of R decreases. Figure 2 gives the plot of the unit-cell volumes, V, of Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15 vs. Shannon ionic radius, r(R3+). The unit-cell volume decreases across the lanthanide series from La to Dy, or with the decreasing size of the ionic radius (Shannon, Reference Shannon1976) (lanthanide contraction) of the metal ion at the octahedral site. This decreasing volume is a result of the decrease in both the a- and the c-parameters.

Figure 2. Plot of unit-cell volume of (a) Ba4Sr2R 2Co4O15 and (b) Ba5SrR 2Co4O15 vs. r(R 3+) [where “r” is the Shannon Ionic Radii (1976)].

Table II. Unit cell parameters of Ba4Sr2R2Co4O15 and Ba5SrR2Co4O15 (P63mc (No. 186), Z = 2), D x refers to calculated density. Values inside brackets are standard deviations.

The atomic coordinates, displacement parameters for the structures of Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15 are given in Tables III(a) and 3(b). Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15 are isostructural with Ba6R 2Fe4O15 (Rüter and Müller-Buschbaum, Reference Rüter and Müller-Buschbaum1990; Mevs and Müller-Buschbaum, Reference Mevs and Müller-Buschbaum1990b, Reference Mevs and Müller-Buschbaum1990c, 1992; Abe et al., Reference Abe, Doi, Hinatsu and Ohoyama2006). The structure of Ba6−xSrxR 2Co4O15 in general consists of two crystallographically independent Co sites, one is 6-fold, while the other is 4-fold coordinated. The CoO6 octahedra and CoO4 tetrahedra are linked by corner-shared oxygen ions. Specifically, a CoO6 octahedron in the center can be viewed as sharing three corners of its triangular face with three tetrahedral CoO4 units, leading to a Co4O15 cluster (Figure 3). In Ba4Sr2R 2Co4O15, all R sites are mixed with Sr (randomly occupied by 2/3R and 1/3Sr), while in Ba5SrR 2Co4O15, all R sites are mixed with Ba and Sr (randomly occupied by 2/3R, 1/6Ba, and 1/6Sr). Figure 4 gives the structure of (Ba6−xSrx)R 2Co4O15 as viewed along the c-axis. It features six units of Co4O15 and seven SrO6 octahedral units viewed along the b-axis. For clarity, the 11- and 12-fold coordinated Ba–O polyhedra and the 8-fold coordinated R/Sr–O or R/Ba/Sr–O polyhedra are not drawn. The unit-cell outline is also illustrated. There are two Co4O15 clusters per unit cell that are in turn joined by various lanthanide and alkaline-earth cations that are in 6-fold (octahedral SrO6), 8-fold (bisdisphenoid (R/Sr)O8), 10-fold (capped trigonal prism, BaO10), and 12-fold (cubic close packed, BaO12) coordination to various oxygen ions. In Ba5SrGd2Co4O15, the octahedral cation positions are randomly occupied by an equal amount of Sr and Ba, and the bisdisphenoid cation positions are randomly occupied by 2/3R, 1/6Ba, and 1/6Sr. In Ba4Sr2R 2Co4O15, the bisdisphenoid cation positions are randomly occupied by 2/3R and 1/3Sr. Figure 5 gives the coordination environment of Ba exhibiting both 10-fold and 12-fold coordination.

Figure 3. The structure motif of Co4O15 which consists of three corner-shared [CoO4] tetrahedral units with one CoO6 octahedral unit at the center.

Figure 4. Crystal structure of Ba4Sr2R 2Co4O15 at room temperature showing the unit cell outline, and different coordination environment of tetrahedral [CoO4] and octahedral [CoO6] units. The 8-fold coordinated (R, Sr)O8 bisdisphenoids and BaO10 and BaO12 polyhedra were not shown for clarity.

Figure 5. Crystal structure of Ba4Sr2Gd2Co4O15 at room temperature showing the capped trigonal prism BaO10 and cubic close packed BaO12 coordination environment.

Table III(a). Atomic coordinates and isotropic displacement factors for Ba4Sr2R 2Co4O15; values inside brackets are standard deviations.

Table III(b). Atomic coordinates and isotropic displacement factors for Ba5SrR 2Co4O15. Values inside brackets are standard deviations.

Tables IV(a) and IV(b) give the bond distances of Ba/Sr–O, R–O, and Co–O, and bond valence sum values (V b) for Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15, respectively. The results of bond valence calculations show that Co atoms in both octahedral and tetrahedral sites are all of a 3+ valence instead of a 2+ valence. From R = La to R = Gd, all Co3+ sites experience tensile stress, or underbonding (in an over-sized cage environment) as V b values are all smaller than the ideal valence of 3+. However, the V b values for the octahedral Co site in the Dy-analog are substantially greater than 3.0 (compressive strain). The V b values for Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15 suggest that all Ba2+ and Sr2+ sites (except for Ba3) are under tensile stress. The V b of Ba3 changes from 1.886 to 3.642 in Ba4Sr2R 2Co4O15 and from 1.883 to 3.031 in Ba5SrR 2Co4O15 as the ionic radius decreases from La3+ to Dy3+ and from La3+ to Gd3+, respectively, the Ba3 cage changes from under tensile stress to compressive stress. In the R = Dy analog, the large compressive stress at Ba3 (V b = 3.6) and at Co6 (V b = 3.3) imply maximum strain or the last member (with the smallest lanthanide ion) that Ba4Sr2R 2Co4O15 can form. Note that most of the V b values for R–O in Ba4Sr2R 2Co4O15 and Ba5SrR 2Co4O15 are significantly greater than the ideal value for that site. Since all R sites are mixed with Sr or with Sr and Ba, the ideal V b value is 2.666 (2/3 × 3 + 1/3 × 2 = 2.666), the values here, ranging from 2.842 to 3.144 are all greater than 2.666, representing a large compressive stress or overbonding for the R sites except for the phase with R = Dy. All V b values for the cobalt sites are mostly less than 3.0, suggesting compressive stress.

Table IV(a). Bond distances and bond valence sum values (V b) for Ba4Sr2R 2Co4O15. Values inside brackets are standard deviations.

Table IV(b). Bond distances and bond valence sum values (V b) for Ba5SrR 2Co4O15. Values inside brackets are standard deviations.

B. Reference X-ray diffraction patterns

An example of the reference patterns of Ba4Sr2Gd2Co4O15 is given in Table V. In this pattern, the symbols “M” and “ + ” refer to peaks containing contributions from two and more than two reflections, respectively. The symbol * indicates that the particular peak has the strongest intensity of the entire pattern and has been designated a value of “999.” The intensity values reported are integrated intensities rather than peak heights. All patterns have been submitted for inclusion in the Powder Diffraction File (PDF) (ICDD).

Table V. X-ray powder pattern for Ba4Sr2Gd2Co4O15 (P6 3mc (No. 186), a = 11.5872(4) Å, c = 6.8169(2) Å, V = 792.63(6) Å3, Z = 2, and D x = 6.35 g cm−3). The symbols “M” and “ + ” refer to peaks containing contributions from two and more than two reflections, respectively. The symbol * indicates that the particular peak has the strongest intensity of the entire pattern and is designated a value of “999.”

IV. SUMMARY

Crystal structure, reference patterns, and TE properties of Ba4Sr2R 2Co4O15 (R = La, Nd, Sm, Eu, Gd, and Dy), and Ba4Sr2R 2Co4O15 (R = La, Nd, Sm, Eu, and Gd) series of compounds have been determined. The small size of Sr (as compared to Ba) apparently gives rise to the stability of Ba4Sr2Dy2Co4O15, whereas the corresponding Ba5SrDy2Co4O15 phase is not stable. Bond valence sum calculations indicated that all Co's adopt 3+ valence states in these compounds. In the Ba4Sr2Dy2Co4O15 analog the large compressive stress at Ba3 (V b = 3.6) and at Co6 (V b = 3.3) imply maximum strain or the last member (with the smallest lanthanide ion) that Ba4Sr2R 2Co4O15 can form.

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Figure 0

Figure 1. Observed (crosses), calculated (solid line), and difference XRD pattern (bottom) for Ba5SrSm2Co4O15 by Rietveld analysis technique. The difference pattern is plotted at the same scale as the other patterns up to 70°2θ. At higher 2θ angles, the scale has been magnified five times. Phases present are indicated next to the rows of tick marks (from bottom to top: Ba5SrSm2Co4O15, (Ba, Sr)CoO3, BaCO3, BaSm2CoO5, BaCoOx, and Sm2O3).

Figure 1

Table I. Refinement residuals and phases present for (a) Ba4Sr2R2Co4O15 and (b) Ba5SrR2Co4O15. Values inside brackets are standard deviations.

Figure 2

Figure 2. Plot of unit-cell volume of (a) Ba4Sr2R2Co4O15 and (b) Ba5SrR2Co4O15 vs. r(R3+) [where “r” is the Shannon Ionic Radii (1976)].

Figure 3

Table II. Unit cell parameters of Ba4Sr2R2Co4O15 and Ba5SrR2Co4O15 (P63mc (No. 186), Z = 2), Dx refers to calculated density. Values inside brackets are standard deviations.

Figure 4

Figure 3. The structure motif of Co4O15 which consists of three corner-shared [CoO4] tetrahedral units with one CoO6 octahedral unit at the center.

Figure 5

Figure 4. Crystal structure of Ba4Sr2R2Co4O15 at room temperature showing the unit cell outline, and different coordination environment of tetrahedral [CoO4] and octahedral [CoO6] units. The 8-fold coordinated (R, Sr)O8 bisdisphenoids and BaO10 and BaO12 polyhedra were not shown for clarity.

Figure 6

Figure 5. Crystal structure of Ba4Sr2Gd2Co4O15 at room temperature showing the capped trigonal prism BaO10 and cubic close packed BaO12 coordination environment.

Figure 7

Table III(a). Atomic coordinates and isotropic displacement factors for Ba4Sr2R2Co4O15; values inside brackets are standard deviations.

Figure 8

Table III(b). Atomic coordinates and isotropic displacement factors for Ba5SrR2Co4O15. Values inside brackets are standard deviations.

Figure 9

Table IV(a). Bond distances and bond valence sum values (Vb) for Ba4Sr2R2Co4O15. Values inside brackets are standard deviations.

Figure 10

Table IV(b). Bond distances and bond valence sum values (Vb) for Ba5SrR2Co4O15. Values inside brackets are standard deviations.

Figure 11

Table V. X-ray powder pattern for Ba4Sr2Gd2Co4O15 (P63mc (No. 186), a = 11.5872(4) Å, c = 6.8169(2) Å, V = 792.63(6) Å3, Z = 2, and Dx = 6.35 g cm−3). The symbols “M” and “ + ” refer to peaks containing contributions from two and more than two reflections, respectively. The symbol * indicates that the particular peak has the strongest intensity of the entire pattern and is designated a value of “999.”