Synthesis of a New Ruthenate Ba₂₆Ru₁₂O₅₇

Abstract

Single crystals of Ba${}_{26}$Ru${}_{12}$O${}_{57}$ were grown by the floating zone method. The crystal structure is formed by an alternating stacking of pseudo-hexagonal Ru single layers and double layers. The Ru ions within the double layers are dimerized (Ru${}_2$O${}_9$) whereas the Ru ions within the single layers arrange in a distorted Kagome lattice of trigonal bipyramidally coordinated RuO${}_5$ polyhedra. Additionally, this Kagome lattice is “decorated” with RuO${}_6$ octahedra that are situated in the central free spaces within this Kagome lattice. According to the composition, the oxidation state of most of the Ru ions should be formally close to 5+.

1. Introduction

Ruthenates exhibit a broad scope of interesting physical properties ranging from itinerant ferromagnetism in SrRuO${}_3$[1] and metamagnetim in Sr${}_3$Ru${}_2$O${}_7$[2,3] to unconventional superconductivity in Sr${}_2$RuO${}_4$ [4] (see also the reviews [5,6]). Recently, it was tried to dope Ba ions into Sr${}_2$RuO${}_4$[7,8,9]. For the compound Sr${}_{1.6}$Ba${}_{0.4}$RuO${}_4$ the Ba-doping was still possible but for distinctly higher Ba-doping levels no single crystals could be grown anymore by the floating zone technique. It seems possible that a miscibility gap might appear above these Ba-doping levels within the Sr${}_{2-x}$Ba${}_x$RuO${}_4$ phase diagram (for ambient pressure, high temperature synthesis conditions). Therefore, we investigated the other side of the phase diagram around Ba${}_2$RuO${}_{4+\delta }$.

Within the ternary Ba-Ru-O system, Ba${}_1$Ru${}_1$O${}_3$ is known to exist and crystalizes in different polytypes depending on the synthesis pressure (9R, 4H, 6H and possibly 3C) [10]. The crystal structure at ambient pressure (nine-layer rhombohedral 9R) Ba${}_1$Ru${}_1$O${}_3$ consists of Ru trimers with Ru${}^{4+}$ ions sitting in face sharing Ru-oxygen octahedra [11]. For increasing Ba to Ru ratio also the compounds with the Ba to Ru ratio 4:3, 5:3 and 5:2 are known to exist. For Ba${}_4$Ru${}_3$O${}_{10}$ corner-shared Ru${}_3$O${}_{12}$ trimers are formed [12] and the material is antiferromagnetic with a magnetic ordering temperature of T${}_N$ ∼ 105 K [13]. For Ba${}_5$Ru${}_3$O${}_{12}$ Ru trimers are formed [14] whereas the compound Ba${}_5$Ru${}_2$O${}_{10}$ is crystallizing in a structure formed of Ru-Ru dimers [15]. However, for these two compounds the dimers are not interconnected with other dimers (or trimers) like in the aforementioned Ba ruthenates with lower Ba to Ru ratio. Instead, the planes with the Ru-Ru dimers are separated by Ba oxide planes. For such high levels of the Ba to Ru ratio the Ru ions (also) appear formally in a higher 5+ oxidation state. For the Ba to Ru ratio of 2:1 (which would be the end point in the Sr${}_{2-x}$Ba${}_x$RuO${}_4$ phase diagram) no compound in single crystalline form has been reported to exist so far. However, for this Ba to Ru ratio studies on thin films [16] or high pressure phases [17]—both with layered perovskite structure-exist.

2. Results and Discussion

The Ba${}_{26}$Ru${}_{12}$O${}_{57}$ single crystal was grown in a mirror furnace (HKZ,Scidre). Therefore, the starting materials were prepared by thorough grinding of BaCO${}_3$ and RuO${}_2$ with a Ba to Ru ratio of 2:1 and sintering this mixture at 1000 ${}^{°}$C for one day in air. The crystal was grown with a speed of 6 mm/h under 16 bar of an Argon/Oxygen atmosphere (with a ratio of 1:6). Thus, black, shiny single crystals of Ba${}_{26}$Ru${}_{12}$O${}_{57}$ could be finally obtained, see Figure 1. As known for other ruthenates, also during the growth of Ba${}_{26}$Ru${}_{12}$O${}_{57}$ a larger evaporation of Ru oxide could be observed during the crystal growth which explains the finally obtained Ba:Ru ratio in the grown single crystal. Also other Ba:Ru ratios of 2:1.1 and 2:1.05 have been tried for a compensation of the Ru loss, but only the ratio of 2:1 results in the growth of large impurity-free single crystals of Ba${}_{26}$Ru${}_{12}$O${}_{57}$. Its composition has been confirmed with EDX (energy dispersive X-ray spectroscopy) measurements in a scanning electron microscope yielding a molar Ba:Ru ratio of 2.07(8):1.00(5) which is within the error bars in agreement with the composition of Ba${}_{26}$Ru${}_{12}$O${}_{57}$.

Afterwards, we have also synthesized Ba${}_{26}$Ru${}_{12}$O${}_{57}$ powder samples by conventional solid state reaction in order to confirm that our Ba${}_{26}$Ru${}_{12}$O${}_{57}$ single crystals crystallized in the ambient pressure phase. Therefore, stoichiometric amounts of BaCO${}_3$ and RuO${}_2$ (molar ratio of 26:12) were ground for 1/2 h and pressed into pellets which were placed in a corundum crucible and sintered at 1000 ${}^{°}$C for 12 h in air. After an intermediate re-grinding the sample was sintered again for 48 h under same conditions at ambient pressure.

For powder X-ray diffraction (XRD) measurements parts of the grown single crystals have been ground into fine powders. The XRD measurements have been performed using Cu $K_{\alpha 1}$ radiation on a Bruker D8 Discover A25 powder X-ray diffractometer. The FullProf program package [18] was used for Rietveld refinements, see Figure 2.

Using Mo $K_{\alpha }$ radiation single crystal X-ray diffraction measurements have been performed on a Bruker D8 VENTURE single crystal X-ray diffractometer equipped with a bent graphite monochromator and a Photon III detector. A crystal of roughly 10 $\mathsf{\mu }$m size has been measured, see Figure 3. Due to the small size of the sample we were able to find an almost untwined single crystal with regard to the possible orthorhombic twin domains. Nevertheless, the inversion twin could not be avoided. (For distinctly larger sample sizes we could always observe the appearance of all six possible twin domains that might appear for this structure.) The Jana2006 program suite [19] was used for the crystal structure refinement. The refinement was based on F(obs)${}^2$ with an instability factor of 0.01. For this tiny sample no extinction correction was necessary. The volume fractions of the main domain (id) and the corresponding twin domain obtained by inversion (-id) amount to 49(3)% and 40(1)% whereas the remaining four twin domains related to rotations (plus inversions) are very small (i.e., 1.6(1.6)%, 2.4(1.6)%, 2.5(1.6)% and 4.1%) with a volume fraction almost comparable to their error bars. The reduced amount (size) of twin domains in this sample with pseudo-hexagonal crystal structure underlines the reliability of the refinement results. Only for the heavier atoms Ba and Ru the anisotropic displacement parameters $U_{i,j}$ have been refined. For the oxygen atoms $U_{iso}$ has been refined. Goodness of fit, R- and weighted R-values and the obtained structural parameters and bond lengths are listed in

Table 1. Crystallographic & structural refinement data.

Empirical formula	Ba${}_{26}$Ru${}_{12}$O${}_{57}$
Formula weight (g/mol)	5696.1
Temperature	room temperature
Wavelength	Mo K${}_{\alpha }$
Crystal system	orthorhombic
Space group	Fdd2 (43)
Unit cell dimensions	a = 20.4638(12) Å
	b = 51.191(3) Å
	c = 11.7698(7) Å
Volume	12,329.5(13) Å${}^3$
Z	8
Density (g/cm${}^3$)	6.1372
Absorption coefficient $\mu$	19.234
F(000)	19,520
Crystal size	∼10 $\mathsf{\mu }$m
2${\Theta }_{max}$	64.12${}^{°}$
Index range	h: −30 → 30
	k: −76 → 76
	l: −17 → 17
Reflections in total/independant	340,765/21,135
Observed reflections/independant	256,056/19,031
Internal R-value	4.54%
Completeness up to 2${\Theta }_{max}$	99.89%
Absorption correction	multi-scan
Max./min. transmission	0.5463/0.7463
Refinement method	least squares on $F^2$
Reflections threshold	$I>3\sigma \left(I\right)$
Goodness of fit	2.19
R/R${}_w$	3.07%/7.80%
Largest minima in Fourier difference	−1.20 e${}^{-}$ Å${}^{-3}$
Largest maxima in Fourier difference	1.94 e${}^{-}$ Å${}^{-3}$

,

Table 2. Refinement results of single crystal X-ray diffraction measurements of Ba${}_{26}$Ru${}_{12}$O${}_{57}$ of the refinement with space group Fdd2 (a = 20.4638(12) Å, b = 51.191(3) Å, c = 11.7698(7) Å).

Atom	x	y	z	U${}_{\mathbf{iso}}$ (Å${}^2$)
Ba1	0.65079(2)	0.208323(7)	0.1563	0.01467(10)
Ba2	0.999319(17)	0.085696(6)	0.20177(7)	0.00981(8)
Ba3	0.170861(19)	0.041104(7)	0.20207(6)	0.01739(10)
Ba4	0.82290(2)	0.219793(7)	0.24056(5)	0.01327(10)
Ba5	0.354973(17)	0.032130(7)	0.21110(6)	0.01728(9)
Ba6	0.32153(2)	0.216648(7)	0.17447(5)	0.01444(11)
Ba7	0.167785(18)	0.124818(6)	0.20302(8)	0.01096(8)
Ba8	0.70055(2)	0.041117(7)	0.21087(6)	0.02095(11)
Ba9	0.89407(3)	0.040235(9)	0.00885(5)	0.02325(13)
Ba10	0.500566(18)	0.080283(7)	0.20496(7)	0.01392(9)
Ba11	0.000746(16)	0.164118(6)	0.20603(7)	0.00952(8)
Ba12	0.501251(17)	0.162389(6)	0.20568(7)	0.00925(8)
Ba13	0.666246(17)	0.124834(6)	0.20821(8)	0.01113(8)
Ru1	0.83331(2)	0.103263(7)	0.20564(8)	0.00665(10)
Ru2	0.33370(2)	0.151882(7)	0.20507(8)	0.00661(10)
Ru3	0.33503(2)	0.097675(7)	0.20691(8)	0.00672(10)
Ru4	0.83275(2)	0.156070(8)	0.20657(8)	0.00678(10)
Ru5	0.5	0	0.21206(10)	0.00769(15)
Ru6	0.50515(3)	0.249290(8)	0.21046(10)	0.01119(11)
Ru7	0	0	0.19733(10)	0.01396(19)
O1	0.1229(2)	0.57513(9)	0.0740(4)	0.0096(9)
O2	0.2552(2)	0.58445(8)	0.2062(5)	0.0141(8)
O3	0.1258(3)	0.32816(11)	0.0876(5)	0.0163(11)
O4	0.25532(19)	0.17114(8)	0.2035(5)	0.0108(7)
O5	0.2429(2)	0.42234(8)	0.2063(5)	0.0147(8)
O6	0.1296(3)	0.82271(11)	0.0898(5)	0.0194(12)
O7	0.2545(2)	0.67576(8)	0.2060(5)	0.0153(8)
O8	0.2028(3)	0.37512(8)	0.0939(4)	0.0090(10)
O9	0.1223(3)	0.07778(10)	0.0714(4)	0.0102(9)
O10	0.1262(3)	0.42173(10)	0.0887(4)	0.0155(10)
O11	0.1281(3)	0.91493(9)	0.0864(4)	0.0119(9)
O12	0.2042(2)	0.87099(9)	0.0928(4)	0.0074(9)
O13	0.09182(19)	0.87039(7)	0.2025(5)	0.0098(7)
O14	0.09121(19)	0.37525(7)	0.2059(5)	0.0091(7)
O15	0.1229(2)	0.17197(10)	0.0751(4)	0.0107(9)
O16	0.1229(3)	0.66528(9)	0.0729(4)	0.0142(10)
O17	0.0472(3)	0.12503(9)	0.0677(4)	0.0112(10)
O18	0.0421(2)	0.47838(10)	0.0913(4)	0.0166(9)
O19	0.0279(3)	0.96359(10)	0.1966(6)	0.0312(12)
O20	0.0786(2)	0.52270(8)	0.2076(5)	0.0168(8)
O21	0.0150(4)	0.71228(13)	0.1873(7)	0.0492(19)
O22	0.0078(3)	0.21343(12)	0.2369(6)	0.0392(16)
O23	0.0756(3)	0.00744(11)	0.1199(5)	0.0265(12)
O24	0.0452(3)	0.62015(10)	0.0702(4)	0.0106(9)
O25	0.25	0.75	0.0983(6)	0.0206(14)
O26	0.0736(3)	0.25183(10)	0.1366(5)	0.0223(11)
O27	0.0785(3)	0.75492(12)	0.1292(5)	0.0287(12)
O28	0.2101(3)	0.22822(10)	0.0825(4)	0.0173(10)
O29	0.2418(4)	0.49459(14)	0.1135(6)	0.0428(16)

,

Table 3. Refinement results of single crystal X-ray diffraction measurements of Ba${}_{26}$Ru${}_{12}$O${}_{57}$.

Atom	U${}_{11}$ (Å${}^2$)	U${}_{22}$ (Å${}^2$)	U${}_{33}$ (Å${}^2$)
Ba1	0.01598(19)	0.01027(16)	0.01775(19)
Ba2	0.01068(15)	0.00795(12)	0.01081(16)
Ba3	0.01431(16)	0.01217(14)	0.02569(19)
Ba4	0.01802(19)	0.00930(15)	0.01250(17)
Ba5	0.01249(15)	0.01051(14)	0.02885(19)
Ba6	0.0205(2)	0.00795(15)	0.0148(2)
Ba7	0.00906(14)	0.01393(13)	0.00988(16)
Ba8	0.02028(18)	0.01712(17)	0.0254(2)
Ba9	0.0248(2)	0.0231(2)	0.0218(2)
Ba10	0.01128(16)	0.01918(15)	0.01129(16)
Ba11	0.00932(14)	0.00804(12)	0.01119(15)
Ba12	0.00949(14)	0.00785(12)	0.01041(15)
Ba13	0.00979(15)	0.01347(13)	0.01013(16)
Ru1	0.00610(17)	0.00638(16)	0.00747(17)
Ru2	0.00665(18)	0.00582(16)	0.00736(18)
Ru3	0.00650(17)	0.00651(16)	0.00716(17)
Ru4	0.00673(18)	0.00627(15)	0.00733(18)
Ru5	0.0082(3)	0.0076(2)	0.0073(3)
Ru6	0.01110(19)	0.00691(16)	0.0156(2)
Ru7	0.0232(3)	0.0082(2)	0.0105(4)
Ba1	−0.00120(14)	−0.00363(15)	−0.00249(13)
Ba2	−0.00129(11)	−0.00031(16)	0.00017(15)
Ba3	−0.00332(13)	0.00150(19)	−0.00277(16)
Ba4	0.00047(13)	0.00177(13)	0.00054(12)
Ba5	0.00103(12)	0.00066(19)	0.00177(16)
Ba6	0.00056(13)	−0.00395(14)	−0.00033(12)
Ba7	0.00103(12)	−0.0013(2)	−0.00129(15)
Ba8	−0.00772(14)	0.0032(2)	0.00001(18)
Ba9	0.01344(18)	−0.00860(18)	−0.01232(16)
Ba10	−0.00026(13)	0.00042(17)	0.0016(2)
Ba11	−0.00102(11)	0.00087(18)	−0.00106(16)
Ba12	−0.00057(11)	0.00028(18)	0.00026(17)
Ba13	0.00243(11)	−0.0001(2)	0.00155(15)
Ru1	0.00016(14)	−0.00077(19)	−0.0001(2)
Ru2	−0.00025(14)	0.0000(2)	0.0002(2)
Ru3	−0.00054(14)	−0.00053(19)	−0.0008(2)
Ru4	0.00022(14)	0.0000(2)	−0.0001(2)
Ru5	0.0001(2)	0	0
Ru6	−0.00109(14)	−0.0059(2)	0.00244(17)
Ru7	−0.0059(3)	0	0

and

Table 4. Ru-oxygen distances in Ba${}_{26}$Ru${}_{12}$O${}_{57}$.

Atoms	Distance	Coordination
Ru1-O2	1.867(4)	octahedral
Ru1-O11	1.861(5)
Ru1-O12	2.022(5)
Ru1-O13	2.042(4)
Ru1-O16	1.861(5)
Ru1-O24	2.068(5)
Ru2-O3	1.909(6)	octahedral
Ru2-O4	1.883(4)
Ru2-O8	2.044(5)
Ru2-O9	1.893(5)
Ru2-O14	2.071(4)
Ru2-O17	2.053(5)
Ru3-O5	1.895(4)	octahedral
Ru3-O8	2.075(5)
Ru3-O10	1.884(5)
Ru3-O14	2.049(4)
Ru3-O15	1.884(5)
Ru3-O17	2.062(5)
Ru4-O1	1.874(5)	octahedral
Ru4-O6	1.914(6)
Ru4-O7	1.892(4)
Ru4-O12	2.069(5)
Ru4-O13	2.054(4)
Ru4-O24	2.044(5)
Ru5-O18	1.997(5)	octahedral
Ru5-O18	1.997(5)
Ru5-O20	1.986(4)
Ru5-O20	1.986(4)
Ru5-O28	1.980(5)
Ru5-O28	1.980(5)
Ru6-O21	1.924(7)	trigonal bipyr.
Ru6-O22	1.952(6)
Ru6-O26	1.832(5)
Ru6-O27	1.803(6)
Ru6-O29	1.818(7)
Ru7-O19	1.949(5)	trigonal bipyr.
Ru7-O19	1.949(5)
Ru7-O23	1.835(6)
Ru7-O23	1.835(6)
Ru7-O25	1.777(7)

.

Structure

The crystal structure of Ba${}_{26}$Ru${}_{12}$O${}_{57}$ alternatingly consists of layers of single Ru ions and of layers of dimerized Ru-ions located in face sharing octahedra which are denoted in Figure 4 as layers ’A#’ (Ru single layers) and ’B#’ (Ru double layers) respectively. Within these Ru-layers the Ru ions arrange in a pseudohexagonal manner, see Figure 5. Two third of the Ru ions within the ’A#’ layers have a trigonal bipyramidal oxygen coordination—see Figure 5a—with an enhanced oxidation state compared to the other Ru ions, see

Table 5. Bond valence sums (BVS) for Ba and Ru ions together with their oxygen coordination. Parameters were taken from Ref. [24]. The values in the brackets indicate the number of the of oxygen ions that are (i) distinctly below, (ii) roughly within and (iii) clearly above the plane of the Ru ions (that is perpendicular to the b-axis) for a certain layer.

Atom	BVS	av. O-Distance	O-Coordination
Ba1	2.25(1)	2.79(16)	8-fold (2-5-1)
Ba2	2.25(1)	2.88(17)	10-fold (3-6-1)
Ba3	2.23(1)	2.67(12)	octahedral (3-0-3)
Ba4	2.41(1)	2.76(14)	8-fold (3-4-1)
Ba5	2.25(1)	2.80(20)	8-fold (3-4-1)
Ba6	2.40(1)	2.77(19)	8-fold (3-4-1)
Ba7	1.95(1)	2.98(11)	12-fold (3-6-3)
Ba8	2.17(1)	2.81(16)	8-fold (3-3-2)
Ba9	2.16(1)	2.82(18)	8-fold (3-3-2)
Ba10	1.77(1)	3.05(22)	12-fold (3-6-3)
Ba11	2.36(1)	2.87(18)	10-fold (3-6-1)
Ba12	2.30(1)	2.87(16)	10-fold (3-6-1)
Ba13	2.06(1)	2.96(12)	12-fold (3-6-3)
average	2.2(2)
Ru1	4.48(2)	1.95(10)	octahedral (3-0-3)
Ru2	4.19(2)	1.98(9)	octahedral (3-0-3)
Ru3	4.21(2)	1.97(10)	octahedral (3-0-3)
Ru4	4.21(2)	1.97(9)	octahedral (3-0-3)
Ru5	3.96(2)	1.99(1)	octahedral (3-0-3)
Ru6	4.65(4)	1.87(7)	trigonal bipyr. (1-3-1)
Ru7	4.63(4)	1.87(8)	trigonal bipyr. (1-3-1)
average	4.3(3)

. These Ru ions form a distorted Kagome lattice. The other Ru ions are octahedrally coordinated by the oxygen ions, see Figure 5a and are situated in the free space of the Kagome lattice. This difference in the Ru sites becomes also apparent in their Ba coordination which can be seen in Figure 5b. The RuO${}_6$ octahedra are located in quite compact RuBa${}_6$ octahedra (red) wheras the RuO${}_5$ bipyramids are situated in much larger RuBa${}_{12}$ icosahedra (yellow). The dimerized Ru ions within the ’B#’ layers all form Ru${}_2$O${}_9$ polyhedra consisting of two face sharing RuO${}_6$ octahedra. The corresponding Ru-Ba polyhedra (orange) exhibit more uniform sizes compared to the ones within the ‘A#’ (single-)layers, see Figure 5d. It is the different stacking (‘A1-B1-A2-B2-A3-B1${}^{\prime }$-A4-B2${}^{\prime }$-A1’) of these layers that is responsible for the large value of the lattice constant in b-direction, compare Figure 4. Note, that also the heavy Ba-ions follow this stacking.

For many Ba-containing compounds the Ba ions are twelve-fold coordinated by anions as is observed for Ba7, Ba10 and Ba13 in Ba${}_{26}$Ru${}_{12}$O${}_{57}$. An example in literature is BaNiO${}_3$[20]. But also a plethora of other oxygen coordinations have been reported for Ba in literature. The Ba1 ions in our new crystal structure exhibit an 8-fold oxygen coordination that can be also found e.g., in Ba${}_3$OSiO${}_4$[21]. A different 8-fold oxygen coordination in our new crystal structure can be observed for the Ba4, Ba5 and Ba6 ions. This latter oxygen coordination was reported for Ba${}_3$(BO${}_3$)${}_2$[22]. Also an octahedral anionic coordination is known for Ba (although an octahedral oxygen coordination is much more typical for somewhat smaller but still very large Sr${}^{2+}$ ions) and has been reported for Ba${}_2$CuO${}_2$Cl [23]. The oxygen coordination of the Ba ions is also summarized in Table 5. The Ba-O polyhedra are characterized by three values given in brackets. The central values denotes the amount of oxygen ions that are situated very roughly in the plane of the Ba-ion (perpendicular to the pseudohexagonal axis). The first and the last values denote the number of oxygen ions far above and below this plane. [e.g., an octahedral coordination would be abbreviated by (3-0-3).] A cif-file is provided as a Supplementary Materials File for a detailed visualization.

The nominal Ru oxidation state should be slightly higher than 5+ according to the composition of Ba${}_{26}$Ru${}_{12}$O${}_{57}$. Note, that the absolute values of the bond valence sums (listed in Table 5) are not necessarily equal (or even close) to the real oxidation states, especially also because of the huge complexity of the crystal structure of Ba${}_{26}$Ru${}_{12}$O${}_{57}$. Note that there is a scattering in the BVS values of the Ba${}^{2+}$ ions (oxygen ions) of $\pm 0.315$ ($\pm 0.246$) around 2.085+ (1.888−) which provides an indication for the reliability of the BVS in this very complex structure. A similar scattering range of $\pm 0.345$ of the BVS values around 4.305+ can be observed for the Ru ions. If one would take the values serious, the bond valence sum (BVS) formalism would indicate that the oxidation state of the Ru ions with trigonal bipyramidal oxygen coordination is higher than that of the other all octahedrally oxygen coordinated Ru ions. Such a higher oxidation state of the Ru ions with trigonal bipyramidal oxygen coordination would be in agreement with the total oxygen composition. Note, that a trigonal bipyramidal oxygen coordination is known to exist for Ru ions e.g., in K${}_2$Ru(OH)${}_2$O${}_3$ [25]. Nevertheless, the scattering of all the BVS values is somewhat enhanced (see above) and future X-ray absorption spectroscopy measurements are required to study the real Ru valencies in Ba${}_{26}$Ru${}_{12}$O${}_{57}$.

Besides floating zone grown single crystals also conventional solid state reaction under ambient pressure conditions (air) was successful. We conclude that Ba${}_{26}$Ru${}_{12}$O${}_{57}$ is the ambient pressure phase within the phase diagram. Note, that high pressures of several GPa are needed to stabilize Ba ruthenates with layered perovskite (K${}_2$NiF${}_4$) structure for Ba to Ru ratios of 2:1 [17]. For smaller Ba:Ru ratios (like 2:1.1), we observed the formation of a Ba${}_4$Ru${}_3$O${}_{10}$ impurity phase that is growing under the same growth conditions during our floating zone growth.

3. Conclusions

In summary, we synthesized the new Ba ruthenate compound Ba${}_{26}$Ru${}_{12}$O${}_{57}$. Within the ternary system Ba-Ru-O this compound is stable at ambient pressure for Ba to Ru ratios close to 2:1. Its pseudo-hexagonal crystal structure has been determined by means of single crystal X-ray diffraction and consists of alternating double and single layers of Ru. In the double layers the Ru ions are dimerized (Ru${}_2$O${}_9$) whereas the Ru ions in the single layers with trigonal bipyramidal coordination (RuO${}_5$) are forming a distorted Kagome lattice which is “decorated” by octahedrally oxygen coordinated Ru ions (RuO${}_6$) in the central free space within the Kagome lattice.