Syntheses and structures of compounds with YBaCo4O7-type structure

Solid State Sciences 6 (2004) 251–266 www.elsevier.com/locate/ssscie

Syntheses and structures of compounds with YBaCo4 O7-type structure Martin Valldor Institut für Anorganische und Analytische Chemie, Universität Münster Wilhelm-Klemm-Strasse 8, 48149 Münster, Germany Received 15 October 2003; received in revised form 7 January 2004; accepted 20 January 2004

Abstract More than 20 different isostructural (YBaCo4 O7 -type, P 63 mc, a = 6.3–6.4, c = 10.1–10.3 Å) compounds in the system RBaX4 O7 (R = Ca, Y, In, Dy–Lu, X = Co, Al, Zn, Fe) have been synthesised through solid-state reaction and characterised using either X-ray single crystal or powder diffraction data. The R atom occupies an octahedral space in the close-packed oxygen lattice, the X atoms form together with oxygen a defect wurtzite like net of tetrahedra, and the Ba replaces one of the oxygen in the close packing ending up in an anticuboctahedral 12-coordination. Unit cell expansions, especially in the ab-plane, are seen with the presence of Zn2+ and larger rare-earth (RE) ions at the same time as Al3+ causes the c-axis to shrink. The distribution of Al, Co, Fe and Zn at the two crystallographic X positions is discussed. It was possible to conclude, from displacement parameters, that one certain oxygen position has significantly lower freedom to move than the others, which might affect the materials properties since the very same oxygen is also part in the metal–oxygen–metal bridge with the angle best suited for magnetic super-exchange coupling.  2004 Elsevier SAS. All rights reserved. Keywords: Cobalt oxides; Single crystal XRD; Rietveld

1. Introduction Recently a new type of magnetic compound was found, namely, the YBaCo4 O7 [1], however, its structure was previously known from LuBaZn3.09Al0.91O7 [2,3] with a statistical distribution of Zn and Al at tetrahedral positions. One chemical substitution has been reported earlier in that system, where Lu and Al were successfully replaced by In, resulting in the InBa(In,Zn)4O7 compound [4]. YBaCo4 O7 exhibited an unusual magnetic behaviour, which resembled a spin-glass (see, for example, [5]). Also, the structure itself is a geometrically frustrated system, which has a unique magnetic substructure. Chemical substitutions have been performed on the YBaCo4 O7 compound and the results thereof are presented herein.

reports [1–3]. The packing of the oxygen are stacked abac along c, i.e., the stacking is a mixture of cubic and hexagonal packing. Every eighth oxygen is substituted for Ba, leaving Ba in an anticuboctahedron coordinating 12 oxygen atoms. In turn, the Ba–O-structure can be described as a hcp of corner sharing BaO12 -polyhedra. In the tetrahedral hole in

2. Structure description A short description of RBaX4 O7 structure (Fig. 1) is here presented, but can also be found in more detail in earlier

E-mail address: [email protected] (M. Valldor). 1293-2558/$ – see front matter  2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.01.004

Fig. 1. The structure of RBaX4 O7 . The unit cell is marked with a thin line and the different atom positions are indicated with respective letters. The coordination of Ba is marked with dotted lines.

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the packing of BaO12 -polyhedra, an octahedron is formed where the R atom can be found and in the octahedral volumes in the BaO12 packing four cornersharing XO4 tetrahedra are situated.

3. Experimental The powder samples were prepared by solid state reactions in open corundum crucibles starting from stoichiometric amounts of Y2 O3 (Alfa, 99.99%), ZnO (Merck, p.a.), Al2 O3 (Baker Chem, 99%), Fe2 O3 (Aldrich, 99.99%), In2 O3 (Ventron, Ultrapure), Dy2 O3 (REacton, 99.9%), Ho2 O3 (REacton, 99.9%), Er2 O3 (Merck, p.a.), Tm2 O3 (REacton, 99.9%), Yb2 O3 (REacton, 99.9%), Lu2 O3 (ChemPur, 99.9%), CaCO3 (Merck, > 99%), BaCO3 (Merck, p.a.) and Co3 O4 (Baker Chem., p.a.). After mixing the constituents thoroughly in an agate mortar, the metal oxide mixtures were slowly heated (∼ 5 ◦ C min−1 ) up to 1100 ◦ C and reground before a final heat treatment, lasting about 20 h, at 1200 ◦ C. The samples were cooled inside the furnace at an ambient rate. In some cases the final heat treatment was not necessary to obtain pure powder. All powders containing cobalt became black and the single crystals therein were opaque, while the compounds with iron and no cobalt were red or deep red and the respective single crystals were transparent. Powder X-ray diffraction data were obtained with a Guinier–Hägg (G–H) focusing camera (radius = 40 mm) using Cu-Kα1 radiation (λ = 1.540598 Å). Pure Si or SiO2 was used as internal standard. When Si was used, the G–H films were evaluated with an LS-KEJ20 scanner and subsequently with the SCANPI [6] and the PIRUM [7]

programs. In the SiO2 cases the film was substituted for an image plate and, after exposure, scanned with a BAS-1800 II scanner from FUJIFILM. Further powder diffraction data, used in Rietveld refinements, were collected with a STOE STADI-P diffractometer in Debye–Scherrer transmission mode using Cu-Kα1 radiation (λ = 1.540598 Å). The investigated diffraction span was 15–100◦ (2θ ). The model simulations were accomplished using Fullprof2000 [8]. All Rietveld refinements were completed in the same way: the background was set manually, the pseudo-Voigt function was used to model the peak shape and the asymmetry of the peaks were accounted for up to 2θ = 50◦ . All positions were assumed to be fully occupied and the metal:metal ratios were set according to the EDX analyses. The refined parameters were scale factor, zeroposition, cell parameters, 4 variables for the peak shape, 2 peak asymmetry parameters, 9 atomic positional parameters, 5 isotropic displacement parameters (the oxygen atoms were refined as one), and if mixed occupancy was suspected, between the 2a and 6c positions, one more parameter was used for this purpose. Furthermore, in the few cases where impurity peaks were detected, EDS and rest intensity peaks were used to identify the impurity, for which, in powder refinement, only the scale factor was refined. Single crystals were investigated with a STOE imageplate detector system (IPDS) in the range ω = 0–180◦, using Mo-Kα radiation (λ = 0.7107 Å). The image-plate data was then run through the JANA2000 [9] program package after being absorption corrected empirically with X-RED [10] and X-SHAPE [11] starting from a crystal shape obtained from FACEIT, included in the STOE software. Metal:metal ratios were taken from the EDS analyses of the powders.

Table 1 Elemental analyses (atomic-%) compared with the respective starting stoichiometries for RBaX4 O7 . X is here presented with K, N and P to separate the different elements at the X position. A row indicated S.C. is the analysis of the single crystal from the sample just above this row. These single crystals were used in the structural analysis. The final composition was assumed to add up to 6 metal atoms in the formula. The STD for the last digit of the atomic percent is written within parentheses and were calculated from the 10–20 EDX results Expected stoichiometry

R (%)

Ba (%)

K (%)

N (%)

P (%)

Composition from analyses

YBaCo3 ZnO7 YBaCo2 Zn2 O7 YBaCoZn3 O7 YBaCo3 FeO7 YBaZn3 FeO7 CaBaZn2 Fe2 O7 CaBaZn2 FeAlO7 CaBaCo2 ZnAlO7 S.C. CaBaCoZn2 AlO7 CaBaCo3 AlO7 S.C. CaBaCo3 FeO7 S.C. CaBaCo2 ZnFeO7 CaBaCoZn2 FeO7 CaBaCo2 Fe2 O7 CaBaCoZnFe2 O7 CaBaCo3 ZnO7 CaBaCo2 Zn2 O7

17(2) 19(1) 17.6(4) 18(1) 17.3(5) 14.9(9) 13.8(8) 15(1) 19(2) 16.2(6) 16.0(4) 16.7(3) 16.2(2) 16.6(1) 15.2(3) 15.0(6) 16.0(5) 15.7(6) 15.7(4) 15.2(7)

15.3(2) 15.8(3) 16.0(5) 15.3(2) 13(1) 16(1) 14(1) 16(1) 17(3) 15.8(7) 15.1(4) 15.9(2) 15.7(1) 15.7(1) 16.5(7) 16(1) 17.7(5) 17.0(8) 17(1) 16(1)

52(1) 34.5(8) 15.4(7) 53(1) 57(2) 41(4) 39(2) 29(3) 27(7) 24(3) 51(2) 53.4(5) 49.5(6) 48.8(2) 29(1) 16(2) 34(2) 15.7(8) 48(2) 38(2)

15.5(5) 30.3(6) 51(1) 14.0(3) 12(1) 28(2) 13(1) 20(3) 17(6) 27(2) 18(2) 13.9(7) 18.5(5) 18.8(2) 24(1) 34(3) 33(1) 21(2) 19(3) 28(3)

– – – – – – 20(1) 19(2) 20(6) 18(3) – – – – 15.1(4) 18(2) – 31(1) – 2.8(7)

Y1.0(1) Ba0.92(1) Co3.12(9) Zn0.93(3) O7 Y1.16(8) Ba0.95(2) Co2.07(5) Zn1.82(3) O7 Y1.06(2) Ba0.96(3) Co0.93(4) Zn3.05(6) O7 Y1.08(9) Ba0.92(1) Co3.16(7) Fe0.84(2) O7 Y1.04(3) Ba0.82(6) Zn3.4(1) Fe0.72(7) O7 Ca0.89(5) Ba0.95(7) Zn2.4(2) Fe1.7(1) O7 Ca0.83(5) Ba0.87(6) Zn2.3(1) Fe0.79(6) Al1.18(6) O7 Ca0.91(6) Ba0.97(7) Co1.7(2) Zn1.2(2) Al1.2(1) O7 Ca1.1(1) Ba1.0(1) Co1.6(4) Zn1.0(3) Al1.2(3) O7 Ca0.97(4) Ba0.95(4) Co1.4(2) Zn1.6(1) Al1.1(2) O7 Ca0.96(2) Ba0.91(3) Co3.0(1) Al1.1(1) O7 Ca1.00(2) Ba0.95(1) Co3.21(3) Al0.84(4) O7 Ca0.97(1) Ba0.944(6) Co2.97(4) Fe1.11(3) O7 Ca0.996(8) Ba0.943(8) Co2.93(1) Fe1.13(1) O7 Ca0.91(2) Ba0.99(4) Co1.76(7) Zn1.43(8) Fe0.91(3) O7 Ca0.90(4) Ba0.99(6) Co0.97(9) Zn2.0(2) Fe1.1(1) O7 Ca0.96(3) Ba1.06(3) Co2.0(1) Fe1.96(8) O7 Ca0.94(3) Ba1.02(5) Co0.94(5) Zn1.2(1) Fe1.85(8) O7 Ca0.94(3) Ba1.05(6) Co2.9(1) Zn1.1(2) O7 Ca0.91(4) Ba0.99(6) Co2.3(1) Zn1.7(2) Al0.17(4) O7

Letter a b c d e f g h h i j j k k l m n o p q

M. Valldor / Solid State Sciences 6 (2004) 251–266

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Fig. 2. Unit cell parameters and volume for the RBaCo4 O7 samples plotted vs. ionic radii of R [11]. The error bars include 3 STDs.

Apart from a scale factor, 8 positional parameters were refined. The number of atomic displacement parameters was depending on the amount of data at hand. Generally, all atoms were refined with anisotropic displacements, but for refinements with less data the oxygen atoms were only refined isotropically. One parameter was also used to account for mixed occupancies between two sites, as in the case of the Rietveld refinements. Since the space group P 63 mc is noncentrosymmetric, an inversion twin domain was refined and, in the cases where this improved the refinement, the whole structure was inversed so that only one domain was present in the final refinement. All refinements were calculated on F and the weak reflections were weighed up until the GOF values (Sobs , Sall ) were lower than 2. All diffraction data was obtained at ambient pressure and room temperature. Scanning electron microscope (SEM) studies together with energy dispersive spectrometry (EDS) analysis were made in a SEM JEOL 820 at 20 kV with an attached LINK AN10000 or in a Leica Stereoscan 420i at 20 kV equipped with an Oxford Instrument INCA V.4.01. For each compound 10–20 crystals were checked for composition. When the single crystals were not fully covered with glue it was possible to perform elemental analyses on them, after sputtering a thin layer of gold onto the crystals using a SEM Coating Unit PS3.

Fig. 3. Cell parameters a, c, and unit cell volume plotted against changes in stoichiometry according: CaBaCo3−x Znx AlO7 (squares), CaBaCo3−x Znx FeO7 (circles), YBaCo4−x Znx O7 (triangles-down), CaBaCo2−x Znx Fe2 O7 (triangles-up). For the YBaCo4−x Znx O7 series the data x = 0 is taken from [1]. 3 STDs are included as error bars.

4. Results 4.1. Elemental analysis In the series RBaX4 O7 , where R = Y, Ca and X = Co, Zn, Fe, Al presented in Table 1, only few differences can be seen separating the starting stoichiometries with the resulting ones. For nearly all samples containing Ca, it is possible to see a small deficit of Ca accompanied by a

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Table 2 Elemental analyses compared with respective staring stoichiometries for RBaCo4 O7 . A row indicated S.C. is the analysis of the single crystal from the sample just above this row. These single crystals were used in the structural analysis. The final composition was assumed to add up to 6 metal atoms in the formula. The STD for the last digit of the atomic percent is written within parentheses and were calculated from the 10–20 EDX results Expected stoichiometry

R (%)

Ba (%)

Co (%)

Composition from analyses

InBaCo4 O7 S.C. LuBaCo4 O7 YbBaCo4 O7 TmBaCo4 O7 ErBaCo4 O7 HoBaCo4 O7 DyBaCo4 O7

15.2(7) 15.9(7) 17(1) 20(1) 15.7(3) 15.0(4) 16.0(2) 16.0(4)

18(2) 17(1) 17.6(4) 14.8(4) 15.1(3) 15.1(4) 15.5(3) 15.2(2)

67(2) 67(2) 65(1) 65(1) 69.2(3) 69.9(6) 68.5(4) 68.7(4)

In0.91(4) Ba1.1(1) Co4.0(1) O7 In0.95(4) Ba1.04(8) Co4.0(1) O7 Lu1.05(7) Ba1.06(2) Co3.89(7) O7 Yb1.23(6) Ba0.87(3) Co3.88(6) O7 Tm0.94(2) Ba0.91(2) Co4.15(2) O7 Er0.90(2) Ba0.91(2) Co4.19(3) O7 Ho0.96(1) Ba0.93(2) Co4.11(2) O7 Dy0.96(2) Ba0.91(1) Co4.12(2) O7

Letter r r s t u v w x

Table 3 Cell parameters and unit cell volume for the RBaX4 O7 (R = Y, Ca and X = Co, Fe, Al, Zn). The standard deviations are presented within parentheses Expected stoichiometry YBaCo3 ZnO7 YBaCo2 Zn2 O7 YBaCoZn3 O7 YBaCo3 FeO7 YBaZn3 FeO7 CaBaZn2 Fe2 O7 CaBaZn2 FeAlO7 CaBaCo2 ZnAlO7 CaBaCoZn2 AlO7 CaBaCo3 AlO7 CaBaCo3 FeO7 CaBaCo2 ZnFeO7 CaBaCoZn2 FeO7 CaBaCo2 Fe2 O7 CaBaCoZnFe2 O7 CaBaCo3 ZnO7 CaBaCo2 Zn2 O7

a (Å)

c (Å)

6.3053(3) 6.3137(4) 6.3205(9) 6.3145(4) 6.337(1) 6.374(2) 6.334(1) 6.312(3) 6.313(2) 6.3065(5) 6.3366(8) 6.351(2) 6.363(2) 6.350(3) 6.366(3) 6.348(3) 6.341(1)

10.276(1) 10.2822(9) 10.276(1) 10.2763(9) 10.292(1) 10.284(2) 10.163(2) 10.110(3) 10.117(3) 10.1205(8) 10.229(3) 10.238(4) 10.255(3) 10.282(4) 10.286(4) 10.229(4) 10.214(2)

surplus of the atoms, which are assumed to be situated in the tetrahedra. It is in these cases possible that Co or Fe partly occupy the octahedral positions. Another explanation for this deficit is that the Ca-standard is not perfectly in order, resulting in a small systematic error. This possible mixing at the octahedral site has, however, not been included in the refinements below. In one sample, q, there has been a significant reaction between the sample and the corundum crucible, which has been accounted for in the single crystal refinement. The elemental analyses of the single crystals reveal that the investigated crystals are good representatives of the samples in question. Only in sample j the Al content is slightly lower in the single crystal, which could be due to the difficulty in determining lighter elements if the angle of the crystal surface relative to the detector is deviating slightly from the ideal one. For the series RBaCo4 O7 (Table 2) the resulting compositions also agree well with those aimed for. Here it is also possible to see a slight deficit of the ion, which is supposed to be occupying the octahedra, except in the cases of s and t, of which the latter case probably was due to the Yb-standard, YbF3 , since both the Ba and the Co content is significantly lower than expected.

Volume (Å3 ) 353.82(7) 354.96(8) 355.50(7) 354.85(8) 357.95(8) 361.8(1) 353.13(8) 348.8(2) 349.1(1) 348.58(8) 355.7(2) 357.6(1) 359.6(1) 359.1(2) 361.0(3) 357.0(3) 355.7(1)

Letter a b c d e f g h i j k l m n o p q

4.2. X-ray powder diffraction The unit cell dimension for the series RBaCo4 O7 (R = In, Dy–Lu) is expected to have a correlation with the ionic size of the R ion. Hence, the refined unit cell volume and unit cell parameters were plotted against the Shannon–Prewitt radii of R [12]. In Fig. 2 these diagrams are shown and the unit cell volume follows the expected linear correlation with the ionic size of R. However, when a and c were compared in the same way, there is a clear difference between them. The a parameter follows a similar systematic linearity, but the c-axis is, with one exception (In), independent on the size of R. When examining Table 3 it is obvious that the Al containing samples have a significantly shorter c-axis, while the a-axis is practically the same as for the samples without Al. This is a good indication of that the Al is located at a site with great apical influence pulling the c-axis together. In contrast, Zn seems to be situated in an “in-plane” position, since the a-axis of the series presented in Fig. 3 increase, while the c-axis only increase slightly or stay constant with the increasing Zn content. For all series the Zn-content causes the unit cell volume to increase. The ionic radius of Zn2+ (rtetr = 0.60 Å) [12] often is reported as larger or

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equal to that of Co2+ (rtetr = 0.58 Å) [12], which agrees with the results presented here. The unit cell volume increase could also be affected by the fact that some Zn2+ substitute for Co3+ , which would mean a lower oxygen content in the structure and such substitutive reductions often comes with an increase of the unit cell volume. Also here the a-axis follows the unit cell volume change, while the c-axis does not. Hence, the same anisotropic unit cell expansion is seen in the Zn substituted samples as were observed for the RBaCo4 O7 series. 4.3. Rietveld refinements In Fig. 4a–4w the results from Rietveld refinements are displayed. Generally, the main phase is the dominating phase or the only phase in the powder. Only in LuBaCo4 O7 (s) and HoBaCo4 O7 (w) can the rare-earth metal oxide (RE2O3 ) be seen as a second phase. One reason for this could be the high stability of these oxides or that they were incorrectly weighed in the synthesis step. The volume fraction extracted from the refinements were 0.5(1)% (Lu2O3 ) [13] and 2.6(3)% (Ho2 O3 ) [14], respectively. Apart from the phases indicated in the diffractograms no other phase could be seen. The most obvious differences between calculated and observed pattern are at the Bragg positions and belong to the largest intensities indicating that all phases are known in the powders, but the models are not complete. The background is a deciding matter for these measurements, because Cu-Kα radiation activates fluorescence from Co and Fe within the structure, which gives angularly independent background intensity. This effect is easily spotted in the different diffractograms, since the lowest intensity/backgrounds (I /σ ) ratios are seen in the LuBaCo4 O7 and the HoBaCo4 O7 cases, where four Co are present per formula unit. The highest I /σ ratios are found with the samples containing one or two Co/Fe atoms per formula unit. The relatively low I /σ ratios worsen the standard deviations for the intensities, which in turn affect the Rietveld calculations. In all cases there is also a broad bump contributing to the background intensity in the range 15–30 degrees (2θ ). This originates from the sample holder tape, on which the sample powder is clued. In one or two cases, i.e., YBaCo3 ZnO7 (a) and maybe YBaCo2 Zn2 O7 (b), too much sample might have been used since the background intensity at higher angles increases, indicating absorption. The results from the refinements are presented in Tables 4a and 4b. 4.4. Single crystal X-ray diffraction The refinements results together with the measurement details are presented in Tables 5a, 5b and 5c. The displacement parameters all seem reasonable, however, noteworthy is that the oxygen at the second 6c site (x, −x, z; x ≈ 0.16) in several cases has large U11 , U22 and U12 values, meaning that the oxygen is modelled as having a disc shape. Either the position is split or the position is not fully occupied.

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As described above, the distribution of Al, Zn, Co and Fe at the two tetrahedral positions is difficult to predict, even though Al has fewer electrons. This was, therefore, tested in the refinements to place Al only at the in-plane tetrahedron (6c), however, the displacement parameters became negative, which means that Al does not all belong at this site but should be distributed at both sites. When the Al occupancy was refined at both sites, it turned out to be predominantly in the apical tetrahedra (2a). According to most refinements, Zn was predominantly found in the inplane tetrahedron (6c), while Fe and Co fit equally good at both sites, which probably is affected by the fact that Fe and Co can have variable oxidation states, giving flexibility, while Al and Zn cannot. Naturally, the size difference of the two sites will be closely related to the atom distribution. In Fig. 5 it is possible to see that the average metal–oxygen distance in the inplane tetrahedron (6c) is in most cases larger than that in the apical tetrahedron (2a), suggesting that the in-plane tetrahedra should contain the larger ions, such as Zn2+ and Co2+ . A comparison between bond angles, bond distances, and composition will follow in the discussion.

5. Discussion From the prepared compounds it is possible to conclude that some restriction on the stoichiometry is present, which in turn might be due to sensitivity for changes in oxygen stoichiometry. For the oxygen to be kept at 7 atoms per formula unit, the octahedral site and the four tetrahedral sites must be occupied by two 3+ ions and three 2+ ions, since Ba is unlikely to change its valence from 2+. However, since Co might be reduced at higher temperatures, the syntheses are more favourable when the starting mixture contains higher oxidation states than in the final composition. When there is not enough oxygen present from the beginning or the temperature is too high, the mixture reacts with the crucible (e.g., as in case q) or “boils” out of the crucible (e.g., as for the compound CaBaCo4 O7 ). When too much oxygen was present, e.g., as in a case like YBaCo3 AlO7 where all Co has to be reduced to 2+, the resulting mixture contained several phases. When examining which compound turned out with single crystals and which did not it is obvious that the Ca-containing compounds often had single crystals large enough for X-ray studies, while the Y-samples in most cases lacked the large single crystals. Kinetically, the Ca compounds have to be favoured in some way even though the melting point of CaO (∼ 2900 ◦ C) is somewhat higher than that of Y2 O3 (∼ 2700 ◦ C). The CO2 , released during the decomposition of CaCO3 , might assist kinetics and thus the formation of single crystals, however, there is no proof of that presented here. Increasing the reaction temperature could improve the crystal growth, but also reduce Co too much.

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(a)

(b) Fig. 4. Rietveld patterns. The observed data is indicated with circles and the calculated pattern is drawn as a line. The Bragg positions are indicated with vertical lines and below them is the difference plot (Yobs − Ycalc ) presented. The stoichiometries of the samples are presented in the upper right corner of the diffractogram.

M. Valldor / Solid State Sciences 6 (2004) 251–266

(c)

(e) Fig. 4. (continued).

257

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M. Valldor / Solid State Sciences 6 (2004) 251–266

(f)

(s) Fig. 4. (continued).

M. Valldor / Solid State Sciences 6 (2004) 251–266

259

(w) Fig. 4. (continued). Table 4a Obtained data from Rietveld refinements. One STD is presented within the parentheses and these values are already multiplied by the Bérar–Lelann factor [15]. Each position is presented with: atom occ., x, y, z, Beta(iso) (Å2 ) Stoichiometry

YBaCo3 ZnO7

YBaCo2 Zn2 O7

YBaCoZn3 O7

Letter 2b

a Y 1.0 2/3, 1/3, 0.874(1) 3.9(4)

b Y 1.0 2/3, 1/3, 0.8748(8) 3.9(3)

c Y 1.0 2/3, 1/3, 0.874(1) 4.5(4)

2b

Ba 1.0 2/3, 1/3, 1/2 1.2(2)

Ba 1.0 2/3, 1/3, 1/2 1.0(1)

Ba 1.0 2/3, 1/3, 1/2 1.6(1)

2a

Co 0.7(2), Zn 0.3(2) 0, 0, 0.431(4) 2.1(5)

Co 0.7(1), Zn 0.3(1) 0, 0, 0.428(2) 1.4(3)

Co 0.2(1), Zn 0.8(1) 0, 0, 0.426(3) 2.1(4)

6c

Co 0.79(5), Zn 0.21(5) 0.172(1), 0.828(1), 0.688(2) 2.7(3)

Co 0.48(3), Zn 0.52(3) 0.1704(7), 0.8296(7), 0.687(2) 2.4(2)

Co 0.24(3), Zn 0.76(3) 0.1698(9), 0.8302(9), 0.687(2) 2.6(2)

6c

O 1.0 0.513(5), 0.487(5), 0.743(7) 4.9(7)

O 1.0 0.518(4), 0.482(4), 0.744(4) 5.2(5)

O 1.0 0.506(7), 0.494(7), 0.740(6) 6.4(8)

2a

O 1.0 0, 0, 0.243(9) 4.9(7)

O 1.0 0, 0, 0.246(7) 5.2(5)

O 1.0 0, 0, 0.246(9) 6.4(8)

6c

O 1.0 0.168(5), 0.832(5), 0.504(5) 4.9(7)

O 1.0 0.164(4), 0.836(4), 0.494(5) 5.2(5)

O 1.0 0.164(5), 0.836(5), 0.488(6) 6.4(8)

Space group a (Å) c (Å) Peaks Parameters χ2 RBragg (%) RF (%)

P 63 mc 6.3086(2) 10.2803(4) 99 25 1.42 9.35 10.8

P 63 mc 6.3173(2) 10.2850(3) 101 25 1.47 7.81 8.74

P 63 mc 6.3266(2) 10.2859(4) 102 25 2.03 9.24 10.9

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Table 4b Stoichiometry

YBaZn3 FeO7

CaBaZn2 Fe2 O7

LuBaCo4 O7

HoBaCo4 O7

Letter 2b

e Y 1.0 2/3, 1/3, 0.8741(6) 3.4(2)

f Ca 1.0 2/3, 1/3, 0.871(2) 3.0(5)

s Lu 1.0 2/3, 1/3, 0.8748(9) 6.6(4)

w Ho 1.0 2/3, 1/3, 0.871(1) 6.2(6)

2b

Ba 1.0 2/3, 1/3, 1/2 2.0(1)

Ba 1.0 2/3, 1/3, 1/2 2.7(2)

Ba 1.0 2/3, 1/3, 1/2 0.9(1)

Ba 1.0 2/3, 1/3, 1/2 1.5(2)

2a

Fe 0.05(8), Zn 0.95(8) 0, 0, 0.429(2) 2.4(2)

Fe 0.6(1), Zn 0.4(1) 0, 0, 0.436(2) 2.7(2)

Co 1.0 0, 0, 0.421(4) 2.7(5)

Co 1.0 0, 0, 0.426(7) 2.8(6)

6c

Fe 0.21(3), Zn 0.79(3) 0.1697(5), 0.8303(5), 0.688(1) 2.0(1)

Fe 0.34(4), Zn 0.66(4) 0.1708(9), 0.8292(9), 0.684(1) 2.7(2)

Co 1.0 0.172(1), 0.828(1), 0.693(3) 4.1(3)

Co 1.0 0.172(1), 0.828(1), 0.693(3) 3.2(4)

6c

O 1.0 0.478(3), 0.522(3), 0.750(3) 5.4(4)

O 1.0 0.476(4), 0.524(4), 0.743(4) 6.9(7)

O 1.0 0.511(5), 0.489(5), 0.752(6) 7.2(9)

O 1.0 0.509(6), 0.491(6), 0.743(7) 4.2(8)

2a

O 1.0 0, 0, 0.247(4) 5.5(4)

O 1.0 0, 0, 0.251(7) 6.9(7)

O 1.0 0, 0, 0.22(1) 7.2(9)

O 1.0 0, 0, 0.24(1) 4.2(8)

6c

O 1.0 0.166(3), 0.834(3), 0.495(3) 5.5(4)

O 1.0 0.161(4), 0.839(4), 0.501(5) 6.9(7)

O 1.0 0.167(5), 0.833(5), 0.504(5) 7.2(9)

O 1.0 0.161(5), 0.839(5), 0.501(6) 4.2(8)

Space group a (Å) c (Å) Peaks Parameters χ2 RBragg (%) RF (%) Impurity

P 63 mc 6.34103(9) 10.3052(2) 101 25 2.39 5.48 7.83 –

P 63 mc 6.37705(5) 10.28977(6) 104 25 1.8 8.98 11.2 –

P 63 mc 6.2619(3) 10.2235(5) 102 25 1.61 9.93 9.3 Lu2 O3 a

P 63 mc 6.3033(2) 10.2372(3) 101 25 1.13 10.3 11.9 Ho2 O3 b

a Data for secondary phases were taken from [13]. b Data for secondary phases were taken from [14].

It has proven possible only to incorporate one Al atom per formula unit. This is an additional proof of that Al when present dominates at the apical site (2a), since this position corresponds only to one forth of the tetrahedral sites. The Ba position, a 12-coordinated anticuboctahedron, seems to be too large for any of the other ions tried, La and Sr, however, if it was synthetically possible, the larger alkali metal ions may suffice. The octahedral position can hold Y, Ca, In and Dy–Lu, which means an ionic radii range of 1.00(Ca)–0.80(In) Å. When adding the smaller lanthanides (Pr–Gd) other phases appear as products, although their ionic radii are within this span. The reason for Pr, Eu and Tb to initiate formation of different compounds might be that they have mixed valences, but there is no explanation for the other lanthanides so far. The unit cell volume increased with the size of the ionic radii of the metal ion at the octahedral site. This was mainly due to the a-parameter since the c-parameter kept constant for most of the RBaCo4 O7 stoichiometries. This implies that the octahedral site has a larger influence of the inplane distances, but the octahedron is surrounded by four BaO12 -polyhedra, which should give a relatively isotropic

environment. Hence, there must be a rigid structural feature that keeps the unit cell from expanding in the unique direction for R-ions with larger radii than 1.0 Å. The only thing that has a strong connection in that direction is the net of tetrahedra, containing the magnetic ions, i.e., when the unit cell should increase isotropically the tetrahedral structure prevents the unit cell from expanding along c. When examining the M–O–M angles (Fig. 6) within the net of tetrahedra, it is obvious that the angles are in the range 100–120◦ for all samples, however, the range is narrower (107–115◦) for the samples h–q, of which many are from single crystal data. Just by looking at Fig. 6 one could estimate that the average angle is just above the ideal tetrahedral angle 109.47◦. All angles can be divided into two groups: the in-plane angles, M2–O1–M2 and M2–O2– M2, and the apical angles, M1–O2–M2 and M1–O3–M2. By comparing the different angles it might be possible to see which angles contribute with the strongest coupling between the magnetic ions, however, one must keep in mind that the measurements are done at room-temperature and that the atomic positions might be different at lower temperatures. Assuming that the super-exchange mechanism [16] can be

M. Valldor / Solid State Sciences 6 (2004) 251–266

261

Table 5a Results from single crystal refinements. One STD is presented within parentheses. Every position is presented as follows: Atom occ., x, y, z, Uiso , U11 , U22 , U33 , U12 , U13 , U23 . For the positions 2a and 2b the U13 and U23 parameters have been left out, since they cannot be refined Stoichiometry

YBaCo3 FeO7

CaBaZn2 FeAlO7

CaBaCo2 ZnAlO7

CaBaCoZn2 AlO7

Letter 2b

d Y 1.0 2/3, 1/3, 0.8731(2) 0.0153(6) 0.016(1), 0.016(1), 0.014(1), 0.0080(5)

g Ca 1.0 2/3, 1/3, 0.8721(6) 0.019(1) 0.019(2), 0.019(2) 0.019(2), 0.0097(8)

h Ca 1.0 2/3, 1/3, 0.8723(4) 0.0137(7) 0.0148(8), 0.0148(8) 0.011(2), 0.0074(4)

i Ca 1.0 2/3, 1/3, 0.8730(2) 0.0097(5) 0.0095(6), 0.0095(6) 0.010(1), 0.0047(3)

2b

Ba 1.0 2/3, 1/3, 1/2 0.0189(6) 0.0197(7), 0.0197(7) 0.0173(9), 0.0099(4)

Ba 1.0 2/3, 1/3, 1/2 0.0232(5) 0.0230(6), 0.0230(6) 0.0235(9), 0.0115(3)

Ba 1.0 2/3, 1/3, 1/2 0.0180(3) 0.0183(3), 0.0183(3) 0.0173(6), 0.0091(2)

Ba 1.0 2/3, 1/3, 1/2 0.0148(2) 0.0150(3), 0.0150(3) 0.0145(4), 0.0075(1)

2a

Co 0.8(2), Fe 0.2(2) 0, 0, 0.4402(7) 0.0120(6) 0.0114(7), 0.0114(7) 0.013(1), 0.0057(4)

Fe 0.57(4), Al 0.43(4) 0, 0, 0.4390(6) 0.015(1) 0.016(2), 0.016(2) 0.013(2), 0.0081(8)

Co 0.44(2), Al 0.56(2) 0, 0, 0.4400(5) 0.0101(8) 0.0101(9), 0.0101(9) 0.010(2), 0.0051(5)

Co 0.49(1), Al 0.51(1) 0, 0, 0.4402(4) 0.0098(6) 0.0094(7), 0.0094(7) 0.010(1), 0.0047(4)

6c

Co 0.79(6), Fe 0.21(6) 0.1715(1), 0.8285(1), 0.6861(4) 0.0134(4) 0.0134(5), 0.0134(5) 0.0136(6), 0.0069(5) −0.001(1), 0.001(1)

Zn 0.7182, Fe 0.06(1), Al 0.23(1) 0.1714(2), 0.8286(2), 0.6840(4) 0.0170(7) 0.0162(8), 0.0162(8) 0.019(1), 0.0082(8) −0.0006(7), 0.0006(7)

Co 0.417(7), Al 0.187(7), Zn 0.395 0.1713(1), 0.8286(1), 0.6847(3) 0.0130(4) 0.0132(5), 0.0132(5) 0.0128(8), 0.0069(4) 0.0002(4), −0.0002(4)

Co 0.391(5), Al 0.087(5), Zn 0.5217 0.17120(9), 0.82880(9), 0.6842(2) 0.0113(3) 0.0111(3), 0.0111(3) 0.0124(5), 0.0061(3) 0.0004(3), −0.0004(3)

6c

O 1.0 0.497(2), 0.503(2), 0.752(2) 0.044(4) 0.022(4), 0.022(4) 0.078(7), 0.004(4) −0.018(3), 0.018(3)

O 1.0 0.490(2), 0.510(2), 0.739(3) 0.054(9) 0.027(7), 0.027(7) 0.11(2), 0.011(7) −0.020(5), 0.020(5)

O 1.0 0.495(2), 0.505(2), 0.738(2) 0.052(5) 0.028(4), 0.028(4) 0.09(1), 0.007(4) −0.028(4), 0.028(4)

O 1.0 0.495(1), 0.505(1), 0.738(1) 0.049(4) 0.028(3), 0.028(3) 0.085(8), 0.011(3) −0.024(2), 0.024(2)

2a

O 1.0 0, 0, 0.251(2) 0.016(3) 0.015(4), 0.015(4) 0.018(5), 0.007(2)

O 1.0 0, 0, 0.257(3) 0.028(6) 0.029(7), 0.029(7) 0.03(1), 0.014(4)

O 1.0 0, 0, 0.255(2) 0.018(3) 0.020(3), 0.020(3) 0.014(6), 0.010(2)

O 1.0 0, 0, 0.258(1) 0.014(2) 0.016(2), 0.016(2) 0.010(4), 0.008(1)

6c

O 1.0 0.163(1), 0.837(1), 0.500(1) 0.039(6) 0.070(7), 0.070(7) 0.015(4), 0.065(7) 0.001(2), −0.001(2)

O 1.0 0.154(1), 0.846(1), 0.501(2) 0.06(1) 0.10(2), 0.10(2) 0.023(7), 0.09(2) −0.005(3), 0.005(3)

O 1.0 0.1558(8), 0.8442(8), 0.499(2) 0.046(8) 0.09(1), 0.09(1) 0.012(5), 0.09(1) 0.001(2), −0.001(2)

O 1.0 0.1539(6), 0.8461(6), 0.4978(9) 0.049(6) 0.099(7), 0.099(7) 0.012(3), 0.096(7) −0.001(2), 0.001(2)

Space group a (Å) c (Å) Parameters S (obs.) S (all) R (obs.) Rw (obs.) R (all) Rw (all) Z Calc. ρ (g cm−3 ) Transm. Abs. coeff. (mm−1 ) F (000) θ range

P 63 mc 6.314(1) 10.296(2) 31 1.32 1.07 0.0337 0.0336 0.085 0.0374 2 5.337 0.819/0.4991 22.385 516 3.7◦ –34.9◦

P 63 mc 6.329(3) 10.150(6) 31 0.96 0.89 0.0651 0.0839 0.1144 0.1038 2 4.727 0.6246/0.5295 15.117 460 3.7◦ –35◦

P 63 mc 6.3033(6) 10.101(1) 31 1.97 1.78 0.0453 0.0626 0.0579 0.0636 2 4.747 0.1337/0.0419 14.478 456 3.7◦ –35◦

P 63 mc 6.3065(8) 10.093(2) 31 1.19 1.05 0.0299 0.0301 0.0485 0.0314 2 4.876 0.8373/0.681 15.640 468 3.7◦ –35◦ (continued on next page)

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Table 5a (continued) Stoichiometry

YBaCo3 FeO7

CaBaZn2 FeAlO7

CaBaCo2 ZnAlO7

CaBaCoZn2 AlO7

hkl range Ref. total Indep. ref. Ref. (I > 3σ ) Peak/hole (e Å−3 ) CSD Nr.a

±10, ±10, ±16 4961 635 354 11.17/−13.93 413463

−10–9, −10–9, ±16 4586 631 365 5.46/−8.98 413466

±10, −10–9, −16–15 4978 618 496 3.42/−4.10 413474

−9–10, −10–8, ±16 4906 620 451 3.61/−3.31 413469

a Data obtainable from FIZ, Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany.

Fig. 5. Average metal–oxygen distances for the two tetrahedra in the compounds indicated on the x-axis. The distances for the 2a position are indicated with closed rings, while the distances for the 6c position are marked with closed squares. The YBaCo4 O7 (∗) [1] compound is included for comparison. One STD is presented for every data point as an error bar.

Fig. 6. Metal–oxygen–metal bond angles between the tetrahedally-coordinated positions for the different samples indicated on the x-axis. The YBaCo4 O7 (∗) [1] compound is included as a comparison. The angles are indicated as follows: M2–O1–M2 (open circles), M1–O3–M2 (thick line), M1–O2–M2 (filled squares-line), M2–O2–M2 (thin line). One STD is included for each data point as an error bar.

applied in these cases, the angles closest to 90◦ (or 180◦ ) are stronger than the rest. Thus, it is possible to say that the in-plane angles are in average closer to 90◦ and, in particular, the M2–O2–M2 angle is, in many of the samples, the smallest angle in the tetrahedral structure. When discussing the distribution of oxidation states in the structure it is important always to have in mind that the

CaBaX4 O7 should end up with two 3+ and two 2+ ions at the X-site, while in the cases of YBaX4 O7 the presence of Y3+ changes this ratio into one 3+ and three 2+ ions. Note, this discussion will only make sense if there are 7 oxygen per every formula unit as it was shown for YBaCo4 O7 [1]. Hence, it is assumed that this is also valid for the compounds presented here. Adding Zn2+ in different amount to the X position might give a clue about the bond nature in the tetrahedra, since the addition of the stable 2+ ions forces the 3+ ions to choose a site. The comparisons presented in Fig. 7 show that the Zn-doping affects the presented stoichiometry series in different ways. This is understandable since the compounds, where x = 0, have already different amounts of rigid 3+ ions. The 3+ ion stability is assumed to be the following: Al3+ > Fe3+ > Co3+ . Hence, the series CaBaCo3−x Znx AlO7 and CaBaCo3−x Znx FeO7 could show similarities, since the stable 3+ ions will probably occupy the smaller tetrahedron (2a) and the added Zn2+ will almost only enter the larger tetrahedron (6c). First, what can be found in all four series is a relative long M2–O2 distance in comparison with the other M2–O distances. This indicates that the M2 (6c) tetrahedra are far from symmetric and stay equally distorted with increasing Zn content. This fact could be important for the materials properties and this M2–O2 distance stays constant even though the unit cell increases with the addition of Zn2+ (Fig. 3). Thus, the oxygen in the middle (O2) is strongly held in place, which can also be observed in the structural refinements. In Fig. 8 it is possible to see the displacements for all oxygen in the single crystal refinements. Obvious is that the O2 has a relatively smaller Uiso in comparison with the other two oxygen positions, which could be interpreted in different ways: (i) the thermal movement of O2 is restricted due to its environment or (ii) O1 and O3 are extremely mobile. Situation (ii) is of course possible, but the oxygen mobility has not been measured yet and is less likely than (i) since the system is more ionic than covalent. If the option (i) is valid, it is possible to explain the displacement of M2 from the centre of the 6c tetrahedron as being a result of lack of space. O2 is positioned inside a tetrahedron of three M2 and one M1. As a guideline the Co–Co, Co–Zn and Zn– Zn distances in the series YBaCo4−x Znx O7 (x = 0–3) were compared with those in the wurtzite structures of CoO [17] and ZnO [18]. The previously reported ZnO contains Zn– Zn distances in the range 3.20–3.22 Å, while the Co–Co distances in CoO were all close to 3.21 Å. The compounds in

M. Valldor / Solid State Sciences 6 (2004) 251–266

263

Table 5b Stoichiometry

CaBaCo3 AlO7

CaBaCo3 FeO7

CaBaCo2 ZnFeO7

CaBaCoZn2 FeO7

CaBaCo2 Fe2 O7

Letter 2b

j Ca 1.0 2/3, 1/3, 0.8728(7) 0.007(2) 0.008(2), 0.008(2) 0.006(3), 0.004(1)

k Ca 1.0 2/3, 1/3, 0.8724(9) 0.006(2) 0.007(2), 0.007(2) 0.004(3), 0.004(1)

l Ca 1.0 2/3, 1/3, 0.8721(5) 0.0137(7) 0.0127(8), 0.0127(8) 0.016(1), 0.0063(4)

m Ca 1.0 2/3, 1/3, 0.8728(4) 0.0148(9) 0.014(1), 0.014(1) 0.016(2), 0.0070(5)

n Ca 1.0 2/3, 1/3, 0.8720(3) 0.0149(5) 0.0152(6), 0.0152(6) 0.0143(9), 0.0076(3)

2b

Ba 1.0 2/3, 1/3, 1/2 0.0172(7) 0.0166(9), 0.0166(9) 0.018(1), 0.0083(4)

Ba 1.0 2/3, 1/3, 1/2 0.0169(8) 0.017(1), 0.017(1) 0.016(1), 0.0086(5)

Ba 1.0 2/3, 1/3, 1/2 0.0213(3) 0.0220(4), 0.0220(4) 0.0201(6), 0.0110(2)

Ba 1.0 2/3, 13, 1/2 0.0212(4) 0.0223(4), 0.0223(4) 0.0189(7), 0.0112(2)

Ba 1.0 2/3, 1/3, 1/2 0.0229(2) 0.0232(3), 0.0232(3) 0.0221(4), 0.0116(2)

2a

Co 0.46(3), Al 0.54(3) 0, 0, 0.4413(7) 0.008(2) 0.008(2), 0.008(2) 0.006(3), 0.004(1)

Co 0.0(3), Fe 1.0(3) 0, 0, 0.4395(6) 0.010(1) 0.008(2), 0.008(2) 0.003(3), 0.0041(8)

Co 0.11, Fe 0.89 0, 0, 0.4404(3) 0.0113(5) 0.0105(5), 0.0105(5) 0.013(1), 0.0053(3)

Co 0.75(8), Zn 0.25(8) 0, 0, 0.4399(4) 0.0131(6) 0.0124(7), 0.0124(7) 0.014(1), 0.0062(4)

Fe 1.0 0, 0, 0.4388(2) 0.0155(4) 0.0156(4), 0.0156(4) 0.0154(8), 0.0078(2)

6c

Co 0.81(1), Al 0.19(1) 0.1717(2), 0.8283(2), 0.6832(4) 0.008(1) 0.011(1), 0.011(1) 0.006(2), 0.009(1) −0.0029(6), 0.0029(6)

Co 0.96(9), Fe 0.04(9) 0.1722(2), 0.8378(2), 0.6836(4) 0.010(1) 0.014(2), 0.014(2) 0.009(2), 0.013(1) −0.0018(7), 0.0018(7)

Co 0.535, Zn 0.465 0.1714(1), 0.8286(1), 0.6840(3) 0.0149(4) 0.0133(5), 0.0133(5) 0.0184(7), 0.0070(4) −0.0001(4), 0.0001(4)

Co 0.05(3), Zn 0.59(3), Fe 0.3574 0.1715(1), 0.8285(1), 0.6842(3) 0.0142(4) 0.0149(5), 0.0149(5) 0.0135(6), 0.0080(5) 0.0002(5), −0.0002(5)

Co 0.6667, Fe 0.3333 0.17165(8), 0.82835(8), 0.6844(2) 0.0155(3) 0.0152(4), 0.0152(4) 0.0167(5), 0.0080(4) −0.0004(3), 0.0004(3)

6c

O 1.0 0.495(1), 0.505(1), 0.735(2) 0.036(6)

O 1.0 0.493(1), 0.507(1), 0.738(2) 0.034(6)

O 1.0 0.496(1), 0.504(1), 0.741(2) 0.074(2) 0.014(3), 0.014(3) 0.12(2), −0.005(2) −0.028(4), 0.028(4)

O 1.0 0.495(2), 0.505(2), 0.737(2) 0.057(6) 0.026(4), 0.026(4) 0.11(2), 0.006(5) −0.032(4), 0.032(4)

O 1.0 0.4952(9), 0.5048(9), 0.740(2) 0.058(4) 0.020(2), 0.020(2) 0.120(11), 0.000(2) −0.026(3), 0.026(3)

2a

O 1.0 0, 0, 0.257(3) 0.017(4)

O 1.0 0, 0, 0.251(4) 0.025(5)

O 1.0 0, 0, 0.253(2) 0.021(3) 0.014(3), 0.014(3) 0.034(8), 0.007(2)

O 1.0 0, 0, 0.250(2) 0.017(3) 0.019(4), 0.019(4) 0.015(5), 0.009(2)

O 1.0 0, 0, 0.252(2) 0.021(2) 0.022(3), 0.022(3) 0.019(4), 0.011(1)

6c

O 1.0 0.153(1), 0.847(1), 0.491(2) 0.042(4)

O 1.0 0.155(1), 0.845(1), 0.494(3) 0.042(5)

O 1.0 0.1567(9), 0.8433(9), 0.496(2) 0.08(2) 0.15(2), 0.15(2) 0.031(7), 0.15(2) 0.001(2), −0.001(2)

O 1.0 0.1574(9), 0.8429(9), 0.499(2) 0.06(1) 0.12(2), 0.12(2) 0.026(6), 0.12(2) −0.001(2), 0.001(2)

O 1.0 0.1566(6), 0.8434(6), 0.500(1) 0.064(9) 0.13(1), 0.13(1) 0.015(3), 0.12(1) 0.002(1), −0.002(1)

Space group a (Å) c (Å) Parameters S (obs.) S (all) R (obs.) Rw (obs.) R (all) Rw (all) Z Calc. ρ (g cm−3 ) Transm. Abs. coeff. (mm−1 ) F (000) θ range

P 63 mc 6.321(1) 10.147(3) 24 1.91 1.89 0.0458 0.0748 0.0474 0.0754 2 4.633 0.2812/0.1331 13.127 449 3.7◦ –23.9◦

P 63 mc 6.341(1) 10.213(3) 24 1.96 1.96 0.0515 0.0809 0.0517 0.081 2 4.870 0.2091/0.1301 14.973 478 3.7◦ –23.8◦

P 63 mc 6.3058(9) 10.183(2) 30 1.89 1.95 0.0623 0.1034 0.0657 0.1043 2 5.031 0.514/0.3216 16.726 487 3.7◦ –35.1◦

P 63 mc 6.361(1) 10.240(2) 31 1.98 1.74 0.048 0.0576 0.0656 0.0588 2 4.949 0.692/0.3803 16.937 490 3.7◦ –35.1◦

P 63 mc 6.3382(7) 10.247(1) 30 1.99 1.93 0.0504 0.0765 0.0531 0.0771 2 4.834 0.5454/0.3364 14.68 476 3.7◦ –35◦ (continued on next page)

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Table 5b (continued) Stoichiometry

CaBaCo3 AlO7

CaBaCo3 FeO7

CaBaCo2 ZnFeO7

CaBaCoZn2 FeO7

CaBaCo2 Fe2 O7

hkl range Ref. total Indep. ref. Ref. (I > 3σ ) Peak/hole (e Å−3 ) CSD Nr.a

±7, ±7, ±11 2135 234 227 3.16/−2.54 413473

±7, ±7, ±11 2044 225 224 3.30/−3.02 413471

−9–10, ±10, ±16 4884 634 587 4.30/−8.76 413472

±10, −9–10, ±16 5170 650 492 4.02/−8.25 413468

−10–9, ±10, ±16 5055 639 592 3.27/−4.54 413476

a Data obtainable from FIZ, Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany.

Fig. 7. Metal–oxygen distances plotted as functions of Zn content (x) for the compositions written in the graphs. The distances are presented as follows: M1–O2 (circles), M1–O3 (squares), M2–O1 (triangles down), M2–O2 (triangles up), M2–O3 (no markers). The tetrahedral structure shows the corresponding distances in the structure. One STD is added to each data point as an error bar. The data for YBaCo4 O7 originate from [1].

the compared series have corresponding distances between 3.08 and 3.26 Å. This indicates that, even though the MO4 tetrahedra are distorted, the O2M4 formation should not shrink, since the same M–O2 distances are present as in the wurtzite structure. The shortest distance 3.08 Å is caused by the Co3+ ions that are somewhat closer to the central O2. These relatively short Co–Co distances might play a role in the magnetic properties and will be discussed further in a following paper. The M2–O2, discussed above, does not change with larger x, however, the M2–O3 distance decreases at the same time as the M2–O1 increases with higher Zn content. In fact, the two distances become more equal as Zn substitutes for Co, which is understandable because Zn2+ is a d10 and more symmetrical than the d7 ion Co2+ , even though the d7 high spin tetrahedral ligand-field splitting is highly symmetrical. The M1 site, which should be dominantly occupied by 3+ ions, contains the shortest M–O distance, the M1–O3 distance, that seems to increase marginally with more Zn while the M1–O2 stays the same or shrinks. This results in a more symmetric M1 site. A further series of compounds can be compared, CaBaCo3 XO7 (X = Al3+ , Fe3+ or Zn2+ ), to see possible changes in the two tetrahedra. These three ions can be discussed as having preferable sites: Al3+ is almost only at the 2a site, Fe3+ is preferably at the 2a site, Zn2+ ends mostly up at the 6c site. Co possesses 3/4 of the tetrahedral positions and can be classified as having no preferable site. In the graph

Fig. 8. Uiso plotted for the O1 (circles), O2 (squares) and O3 (triangles) atoms in the examined single crystals. The samples are marked on the x-axis with respective letters. One STD is added as an error bar to every data point.

(Fig. 9) there seems to be correlations between the atomic type and metal oxygen distances. The M1 site increases in size through the series Al–Fe–Zn. M1–O3 is again the shortest interatomic distance in the structure and increases in an equal relative amount compared with the M1–O2. The increase is then due to the ionic change and at the M1 site according: Al3+ –Fe3+ –Co3+ , since Zn2+ is assumed to enter the M2 site. A fraction of Zn2+ could enter the M1 site, however, the average metal oxygen distances for this site are significantly shorter than those in ZnO (1.97 Å)

M. Valldor / Solid State Sciences 6 (2004) 251–266

265

Table 5c Stoichiometry

CaBaCoZnFe2 O7

CaBaCo3 ZnO7

CaBaCo2 Zn2 O7

InBaCo4 O7

DyBaCo4 O7

Letter 2b

o Ca 1.0 2/3, 1/3, 0.8732(6) 0.018(1) 0.017(1), 0.017(1) 0.019(2), 0.0086(6)

p Ca 1.0 2/3, 1/3, 0.8723(3) 0.0135(5) 0.0130(5), 0.0130(5) 0.0146(9), 0.0065(3)

q Ca 1.0 2/3, 1/3, 0.8724(2) 0.0110(3) 0.0109(4), 0.0109(4) 0.0110(7), 0.0055(2)

r In 1.0 2/3, 1/3, 0.8744(2) 0.0032(8) 0.0012(7), 0.0012(7) 0.007(2), 0.0006(4)

x Dy 1.0 2/3, 1/3, 0.8719(3) 0.0085(6) 0.0093(7), 0.0093(7) 0.007(1), 0.0047(3)

2b

Ba 1.0 2/3, 1/3, 1/2 0.0245(4) 0.0255(5), 0.0255(5) 0.0226(7), 0.0127(3)

Ba 1.0 2/3, 1/3, 1/2 0.0209(2) 0.0220(3), 0.0220(3) 0.0188(4), 0.0110(1)

Ba 1.0 2/3, 1/3, 1/2 0.0195(2) 0.0204(2), 0.0204(2) 0.0176(3), 0.0102(1)

Ba 1.0 2/3, 1/3, 1/2 0.0135(8) 0.0148(9), 0.0148(9) 0.011(2), 0.0074(5)

Ba 1.0 2/3, 1/3, 1/2 0.020(1) 0.022(1), 0.022(1) 0.014(2), 0.0111(5)

2a

Co 0.6(2), Zn 0.4(2) 0, 0, 0.4397(5) 0.0145(7) 0.0134(9), 0.0134(9) 0.017(1), 0.0067(4)

Co 0.89(5), Zn 0.11(5) 0, 0, 0.4419(2) 0.0127(4) 0.0127(5), 0.0127(5) 0.0129(7), 0.0063(2)

Co 0.24(3), Zn 0.60(3), Al 0.16 0, 0, 0.4416(2) 0.0106(3) 0.0107(3), 0.0107(3) 0.0105(6), 0.0053(2)

Co 1.0 0, 0, 0.4331(9) 0.0080(9) 0.009(1), 0.009(1) 0.006(2), 0.0045(5)

Co 1.0 0, 0, 0.4400(8) 0.008(1) 0.008(1), 0.008(1) 0.006(2), 0.0042(7)

6c

Co 0.11(8), Fe 0.49(8), Zn 0.401 0.1716(1), 0.8284(1), 0.6837(3) 0.0159(5) 0.0157(6), 0.0157(6) 0.0163(8), 0.0079(6) −0.0001(5), 0.0001(5)

Co 0.67(2), Zn 0.33(2) 0.17127(7), 0.82873(7), 0.6851(2) 0.0141(3) 0.0136(3), 0.0136(3) 0.0157(5), 0.0073(3) −0.0005(3), 0.0005(3)

Co 0.66(1), Zn 0.34(1) 0.17110(4), 0.82890(4), 0.6849(1) 0.0109(2) 0.0107(2), 0.0107(2) 0.0123(4), 0.0061(2) −0.0001(2), 0.0001(2)

Co 1.0 0.1725(1), 0.8275(1), 0.6864(7) 0.0076(7) 0.0066(8), 0.0066(8) 0.009(1), 0.0030(6) −0.0053(6), 0.0053(6)

Co 1.0 0.1706(2), 0.8294(2), 0.6864(5) 0.010(1) 0.009(1), 0.009(1) 0.012(2), 0.005(1) 0.0052(9), −0.0052(9)

6c

O 1.0 0.496(2), 0.504(2), 0.735(2) 0.045(6) 0.026(5), 0.026(5) 0.07(1), 0.004(5) −0.028(4), 0.028(4)

O 1.0 0.4966(9), 0.5034(9), 0.737(1) 0.058(4) 0.022(2), 0.022(2) 0.11(1), 0.001(2) −0.030(3), 0.030(3)

O 1.0 0.4963(5), 0.5037(5), 0.740(1) 0.060(3) 0.017(1), 0.017(1) 0.13(1), −0.001(1) −0.028(2), 0.028(2)

O 1.0 0.5025(6), 0.4975(6), 0.756(2) 0.018(3)

O 1.0 0.504(2), 0.496(2), 0.750(2) 0.042(5)

2a

O 1.0 0, 0, 0.251(3) 0.020(4) 0.017(5), 0.017(5) 0.024(8), 0.009(2)

O 1.0 0, 0, 0.250(1) 0.018(2) 0.019(2), 0.019(2) 0.017(4), 0.009(1)

O 1.0 0, 0, 0.2527(9) 0.013(1) 0.014(1), 0.014(1) 0.013(3), 0.0068(7)

O 1.0 0, 0, 0.248(2) 0.016(5)

O 1.0 0, 0, 0.256(3) 0.020(8)

6c

O 1.0 0.157(1), 0.843(1), 0.495(2) 0.08(2) 0.14(2), 0.14(2) 0.04(1), 0.14(2) −0.004(3), 0.004(3)

O 1.0 0.1571(6), 0.8429(6), 0.497(1) 0.07(1) 0.15(1), 0.15(1) 0.015(3), 0.148(14) −0.0003(13), 0.0003(13)

O 1.0 0.1571(4), 0.8429(4), 0.4987(8) 0.072(8) 0.15(1), 0.15(1) 0.010(2), 0.15(1) 0.0002(8), −0.0002(8)

O 1.0 0.1661(7), 0.8339(7), 0.498(2) 0.023(4)

O 1.0 0.158(2), 0.842(2), 0.501(2) 0.042(6)

Space group a (Å) c (Å) Parameters S (obs.) S (all) R (obs.) Rw (obs.) R (all) Rw (all) Z Calc. ρ (g cm−3 ) Transm. Abs. coeff. (mm−1 ) F (000)

P 63 mc 6.364(1) 10.273(2) 31 1.97 1.8 0.0753 0.104 0.0867 0.106 2 4.856 0.4968/0.2335 15.793 484

P 63 mc 6.3308(6) 10.202(1) 31 1.97 1.90 0.0384 0.0611 0.0416 0.0615 2 4.990 0.2479/0.1911 16.510 487

P 63 mc 6.3331(6) 10.196(1) 31 1.93 1.89 0.0386 0.0625 0.0434 0.0638 2 4.973 0.3248/0.0519 16.727 485

P 63 mc 6.206(2) 10.087(3) 23 1.92 1.90 0.0270 0.0473 0.0306 0.0482 2 5.92 0.6641/0.4407 18.753 538

P 63 mc 6.322(1) 10.245(3) 23 1.97 1.93 0.0403 0.0591 0.0433 0.0598 2 6.062 0.1088/0.0589 24.994 572 (continued on next page)

266

M. Valldor / Solid State Sciences 6 (2004) 251–266

Table 5c (continued) Stoichiometry

CaBaCoZnFe2 O7

CaBaCo3 ZnO7

CaBaCo2 Zn2 O7

InBaCo4 O7

DyBaCo4 O7

θ range hkl range Ref. total Indep. ref. Ref. (I > 3σ ) Peak/hole (e Å−3 ) CSD Nr.a

3.7◦ –35.1◦ ±9, −10–9, ±16 5132 650 526 4.38/−9.63 413467

3.7◦ –35◦ ±10, −8–10, ±16 5032 637 589 3.07/−5.92 413470

3.7◦ –34.9◦ ±10, −9–10, −14–16 5126 991 915 4.90/−3.06 413475

3.8◦ –23.8◦ ±7, ±7, −10–11 1723 203 193 2.03/−1.47 413464

3.7◦ –24◦ ±7, ±6, ±11 1988 246 233 2.90/−1.98 413465

a Data obtainable from FIZ, Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany.

Acknowledgements I thank the Foundation Blanceflor Boncompagni-Ludovisi, née Bildt and the Alexander von Humboldt Foundation for endorsing this project through research stipends. Thanks are extended to Pedro Berastegui at Stockholm University for supplying an oxide. It was nice to do some of the syntheses work with Hong Peng and Malin Hedman at Stockholm University. Rainer Pöttgen at Münster University is also acknowledged for his hospitality, giving me the chance to finish this work. Fig. 9. Metal–oxygen distances plotted for different X (Al, Fe, Zn) in the general formula CaBaCo3 XO7−δ . The distances are presented as follows: M1–O2 (circles), M1–O3 (squares), M2–O1 (triangles down), M2–O2 (triangles up), M2–O3 (no markers). One STD is added to each data point as an error bar.

[18] and therefore unlikely. The M2–O2 distance is again the longest of the distances in the tetrahedra, but decreases somewhat with larger atoms at the M1 site. Note that the M2–O1 distance does not decrease, probably because it is short already to begin with. The other two M2–O distances decrease to result in a more symmetrical tetrahedron, which could be due to the presence of Zn2+ ions. Magnetic properties were expected for these samples, due to the interesting new substructure with magnetic ions with interatomic angles in the range for super-exchange mechanism. The previously reported YBaCo4 O7 contained a magnetic feature which could not fully be explained. I hope that the nature of this magnetic structure type will be better understood when studying the substituted samples. Property measurements have already been done for most of the samples and will be presented in a following paper.

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