Scientific paper
Flux Syntheses and Crystal Structures of New Compounds With Decorated Kröhnkite-like Chains
Maria Wierzbicka-Wieczorek,a* Uwe Kolitschb and Ekkehart Tillmannsa
"Institute of Mineralogy and Crystallography, University of Vienna, Geocentre, Althanstr. 14, A-1090 Vienna, Austria b Department of Mineralogy and Petrography, Natural History Museum Vienna, Burging 7, A-1010 Vienna, Austria * Corresponding author: E-mail: maria.wierzbicka@univie.ac.at
Received: 15-04-2008 Dedicated to the memory of Professor Ljubo Golic
Abstract
The crystal structures of two novel compounds, KSr6Sc(SiO4)4 and Rb5ln(MoO4)4, were determined from single-crystal X-ray diffraction data collected at room temperature. Both compounds have been synthesised by the flux growth technique in the course of a research project on new micro- and nanoporous alkali-M 3+ silicates. KSr6Sc(SiO4)4 represents a new structure type and the first silicate containing decorated kröhnkite-like octahedral-tetrahedral chains. It is ortho-rhombic and crystallises in Pnma, with a = 19.137(4), b = 11.197(2), c = 7.125(1) À, V = 1526.7(5) À3, Z = 8. Rb5ln(MoO4)4 has space group P2/c with a = 11.391(2), b = 7.983(2), c = 11.100(2) À, = 113.74(3)°, V = 924.0(3) À3, Z = 2. It is isotypic with Rb5Er(MoO4)4. The topologies of both compounds are characterised by isolated infinite decorated kröhnkite-like chains that are built from either ScO6 or InO6 octahedra corner-linked and decorated by SiO4 or Mo-O4 tetrahedra. These chains are separated by alkali or alkaline earth metals. A detailed comparison to the few other compounds based on decorated kröhnkite-like chains, viz. Ba2Ca(HPO4)2(H2PO4)2, CsM3+(H1.5AsO4)2(H2AsO4) (M3+ = Ga, Cr), CsAl(H2AsO4)2(HAsO4), K(H2O)M3+(H1.5AsO4)2(H2AsO4) (Af3+ = Fe, GGa, In) and IK;5In(MIoO4)4 is given.
Keywords: Decorated kröhnkite-like chains, rubidium indium molybdate, strontium potassium scandium silicate, flux growth, single-crystal X-ray diffraction
1. Introduction
Kröhnkite-type chains (Figure 1) are infinite isolated octahedral-tetrahedral chains formed from the ladderlike corner-linkage of MO6 octahedra and XO4 tetrahedra. They are named after the mineral kröhnkite, Na3Cu(SO4)3 ■ 3H3O, in which this type of chain was first described.1 Kröhnkite-type chains are encountered in a large number of natural and synthetic compounds in different orientations and topological arrangements. Fleck, Kolitsch and coworkers provide a detailed classification and reviews of all compounds containing kröhnkite-type or kröhnkite-like chains (the latter are topologically very similar to, but not identical to kröhnkite-type chains sensu strictu), and decorated variants of the latter (the two 'free' apices of the MO6 octahedra are further decorated with one or two XO4 tetrahedra; only very few examples of such variants are known).2-6 The mentioned classification encompasses
Figure 1. Kröhnkite-type octahedral-tetrahedral chain in kröhnkite, Na3Cu(SO4)3 ■ 3H3O.
(generally hydrated) XO4 oxysalts, where X = P5+, As5+, S6+, Se6+, Cr6+, Mo6+, W6+.
During a systematic research for new micro- and na-noporous silicates in the system M1+-(M3+-)M3+-Si-O (M1+ = Na, K, Rb, Cs; M3+ = Sr, Ba; M3+ = Sc, V, Cr, Fe, In, Y, Yb, Gd) with the help of high-temperature flux growth syntheses, two novel compounds containing decorated kröhnkite-like chains, KSr6Sc(SiO4)4 and RbjIn (MoO4)4, were grown from molybdate-based flux sol-vents.7,8 During this research project, more than 30 new crystalline compounds (including isotypic compounds)
with 12 novel structure types have been studied in detail by single-crystal X-ray diffraction methods, supplemented by chemical analyses and Raman spectroscopy. The following novel silicates are described and summarised in a PhD Thesis completed at the University of Vienna7: Ba-KREESi2O7 (REE = Y,910 Yb, Sc) and isotypic SrKSc-Si2O7 (P21/n), with the three structurally related compounds BaNaScSi2O7 (P21/m), BaKYbSi2O7 (Cc) and isotypic SrKScSi2O7, BaY2Si3O10 (P21/m)1112 and isotypic BaREE2Si3O10 (REE = Gd, ^r, Yl), Sc), triclinic Sr-Y2Si3O10 (P21),12 BaRbScSi3O9 (P21/c),13,14 BaY4Si5O17 and isotypic SrREE4Si5O17 (REE = Yb, Sc) (P21/m),15 Ba2Gd2Si4O13 (C2/c),14,16 (_1VI1+)9REE7Si24Og3 (REE = Yb, Y; M1^ = K, Rb, Cs) (Ä3), Na4Sr2REE2(Si2O7)(SiO4)2 (REE ^ Y, In, Sc) (P21/c),17 Ba52RE;E13Si8O42 (IREE = Y, Ho) (/232m) and, finally, Cs3R^^Si8O19 (RE;^ = Y,9 Yb) (Pnma) and isotypic Rb3YSi8O19.
The flux syntheses also yielded several, partly new molybdates as reaction by-products, e.g., novel RbjEe (MoO2)2,18 K5Y(MoO2)2, RbEe(MoO2)2 and isotypic Rb-Sc(MoO2)2, and Rb2Mo2O13 (isotypic with the triclinic modification of K2Mo2O13).8
In the present paper we report the syntheses and the crystal structures (determined from single-crystal X-ray diffraction data) of KSrgSc(SiO2)2 and Rb5ln(MoO2)2, and we discuss and compare the decorated kröhnkite-like chains in their atomic arrangements with those of all other presently known compounds containing such chains.
2. Experimental
2. 1. Syntheses
The flux growth experiments were conducted in a Naber high-temperature furnace in air using platinum crucibles at Tmax = 1150 °C. Small, colourless, pseudo-tetragonal prisms of KSrgSc(SiO2)2 were grown from a MoO3 flux containing dissolved reagent-grade starting materials of K, Sr, Sc and Si (experimental parameters: 1.0012 g K2C03, 1.0001 g Sr(OH)2-8H2O, 0.1225 g Sc2O3, 0.2118 g SiO2, 1.0010 g MoO3). The Pt crucible filled with an intimate mixture of the starting materials was heated up during 12 h to 1150 °C, followed by a holding time of 3 h, and cooling during 125 h (2 K/h) to 900 °C, at which point the furnace was switched off. The reaction products were washed in distilled water, filtered and dried in air. This synthesis yielded three other reaction products, namely pseudo-octahedral crystals of SrKScSi2O7 (Cc), which is isotypic to BaKYbSi2O7, small colourless pseudohexago-nal plates of SrKScSi2O7, which is isotypic to BaKYSi2O7 and Sr3(Si3O9).
Small colourless prisms of Rb5ln(MoO2)2 were obtained as a by-product during a flux growth experiment for which the following starting mixture of reagent-grade starting material was used: 1.5002 g RbE, 0.1822 g ln2O3, 0.1019 g SiO2 and 1.7733 g MoO3. The used temperature
programme was: heating up during 2 h to 1150 °C, holding time 3 h, cooling for 166.6 h (1.2 K/h) to 900 °C, after which the furnace was switched off. The crystals were accompanied only by one additional phase, ln2O3.
2. 2. Data Collection and Structure Solution
The crystal structures of KSr6Sc(SiO2)2 and Rb5ln(MoO2)2 were determined from single-crystal X-ray diffraction data obtained from selected crystal fragments of good diffracting quality. Measurements were made with a Nonius KappaCCD four-circle X-ray diffractome-ter, equipped with a capillary optics collimator (for further details on data collection strategies and data processing see Table 1). The intensity data were processed with the Nonius program suite DENZO-SMN19 and corrected for Lorentz, polarization and background effects. Absorption was corrected according to the multi-scan method.19 The program SHELXS-97 was used for the solution of the crystal structures, employing direct methods. The structure
Table 1. Crystal data, data collection information and refinement details for KSrgScCSiO^l^ and RbsInCMoO^)^.
KSr6Sc(SiO4)4
Rb5In(MoO4)4
Space group Crystal size (mm3) a (À) b (À) c (À)
ß (°) y (À3)
Z
F (000)
Pcalc (g/cm3) ^ (mm-1) Crystal-detector distance (mm) 2emax(°), T (K) Rotation width (°) Erames
Time per frame (s)
h, k, l ranges
Total refls. measured Unique reflections
'Observed' refls.* Variables R1(F), wR2all (F2) Extinct. coefficient GooE
(A/0)max
Apmin, Apmax (e/À3) a, b**
Pnma 0.02 X 0.03 X 0.02 19.137(2) 11.197(2) 7.125(1)
90 1526.7(5) 8
1808 2.255 21.895 32
60, 293 1.5 562 200 -26/26, -10/10, -12/12, 2220 2332 (Ri„t3.12%) 1863 126 3.52, 7.95% 0.00009(2) 1.172 0.001 -1.18, 1.86 0.025, 9.6
P2/c 0.08 X 0.10 X 0.17 11.391(2) 7.983(2) 11.100(2) 113.72(3) 922.0(3) 2
1060 2.228 17.025 30
70, 293 2
519 50 -15/15, -18/18, -17/17 7876 2052 (Rint 1.56%) 3697 120 2.27, 5.72% 0.00529(16) 1.052 0.001 -1.80, 1.62 0.03, 0.9
* F„> 4ö(F„)
w = 1/[a2(Fo^) + (a X P2 + b X P]; P =[max(0, Fo2) + 2 X Fc^]/3
models were refined by standard full-matrix least-squares techniques on F2 using SHELXL-97.20
For the final refinement step of the structure model of Rb5In(MoO4)4, the atomic coordinates of isotypic Rb5Er(MoO4)4 were used as a starting model.21
The final cycles of least-squares refinement for KSr6Sc(SiO4)4 gave the residuals R1{F) = 3.55% and wR2all(F2) = 7.97% using 1863 reflections with Fo > 4g(Fo) and 146 parameters. For Rb5In(MoO4)4 the residuals are: R1(F) = 2.27%, wR2ajj(F2) = 5.72% for 3697 reflections with Fo > 4g(Fo) and 120 parameters (see Table 1). The final difference Fourier maps were fairly smooth and showed a minimum of -1.18 e/À3 (Sr position) and a maximum of 1.86 e/À3 (close to the K position) for KSr6Sc(SiO4)4. Corresponding values for Rb5In(MoO4)4 were -1.80 and 1.62 e/À3, respectively (both close to Rb positions). The final positional and displacement parameters of KSr6Sc(SiO4)4 and Rb5In(Mo-O4)4 are given in Tables 2 and 5. Anisotropic displacement parameters are listed in Tables 3 and 6. Selected interatomic distances and calculated bond-valence sums (v.u.) for the coordination polyhedra are presented in Tables 4 and
7. All figures were drawn using the program ATOMS 5.1
22
version.22
3. Results and Discussion
3. 1. Crystal Structures and Topologies
3. 1. 1. KSr6Sc(SiO4)4
KSr6Sc(SiO4)4 represents a novel structure type, the first K-Sr-Sc silicate and the first silicate with decorated kröhnkite-like octahedral-tetrahedral chains.
Other reported scandium silicates containing alkali and/or alkaline earth cations comprise K2Sc(Si2O6),23 K2ScF(Si4O10	2 2 6
j,24 K3Sc(Si2O7),25 Ba9Sc2(SiO4)6,26 as well
Figure 2. View of KSr6Sc(SiO4)4 along [001], and perpendicular to the decorated kröhnkite-like octahedral-tetrahedral chains (in the central part of the figure, Sr atoms have been omitted in order to show the SiO4 tetrahedra more clearly).
Figure 3. The structure of KSr6Sc(SiO4)4 projected along [010], i.e., parallel to the isolated decorated kröhnkite-like octahedral-te-trahedral chains.
as the above mentioned six compounds BaKScSi2O7, BaNaScSi2O7, BaSc2Si3O10, BaRbScSi3O9, SrSc4Si5O17, Na4Sr2Sc2(Si2O7)(SiO4)2. None of these compounds shows a close structural relation to KSr6Sc(SiO4)4.
The new structure type of KSr6Sc(SiO4)4 (Figures 2, 3) contains one octahedrally coordinated Sc site, three crystallographically non-equivalent SiO4 tetrahe-dra, and five Sr sites, three of which are occupied by a mixture of Sr and K atoms (one with K > Sr) (Table 2). The majority of these sites are located on mirror planes in y = Va. The ScO6 octahedra share all of their oxygen corners with SiO4 tetrahedra, forming isolated decorated octahedral-tetrahedral chains running parallel to [010], i.e., decorated kröhnkite-like chains (see Figure 2). The Sr/K atoms are located between these chains. The [8]- or [9]-coordinated Sr atoms have average Sr-O bond lengths of 2.61, 2.73 and 2.74 À (Table 4). The average K-O bond length of the [8+2]-coordinated K atom is 2.87 À. The average Si-O bond lengths lie within a small range from 1.62 to 1.64 À. The isolated ScO6 octahedron is only slightly distorted, with O-Sc-O bond angles ranging between 83.14° and 94.48 and between 170.9 and 177.58°. The average Sc-O bond length (2.10 À) corresponds very well with literature values for oxidic Sc compounds (2.10 À).27 The bond-valence sums (BVSs) of Si atoms range between 3.86 and 4.02 v.u. (valence units). The BVSs for the Sr1 (1.84 v.u.), Sr2 (2.14 v.u.) and Sr3 (1.77 v.u.) atoms are close to the theoretical value.
In contrast, BVSs for the Sr4 (1.61 v.u.) and K5 (1.51 v.u.) atoms reflect the mixed (Sr/K) occupancies of these two sites (Table 4). We would like to point out that unrestrained occupancy refinements of the mixed Sr/K sites resulted in a nearly charge-balanced empirical formula («KSrgSc(SiO4)4), with 63.9 positive charges v^. 64 negative charges. For the final refinement the occupancies were slightly modified and then fixed to achieve a completely charge-balanced formula.
Table 2. Fractional atomic coordinates and equivalent isotropic displacement parameters (À2) for KSr6Sc(SiO4)4. yequjv according to Fischer & Tillmanns.28
Atom	x	.y	z	U equiv	Occupancy*
Srl	0.41000(2)	0.00028(5)	0.33110(7)	0.01021(12)	0.950
K1	0.41000(2)	0.00028(5)	0.33110(7)	0.01021(12)	0.050
Sr2	0.22493(2)	0.02370(5)	0.48859(7)	0.01168(12)	
Sr3	0.48151(4)		-0.00159(10)	0.01361(16)	
Sr4	0.33940(5)		0.72011(13)	0.0154(2)	0.70
K4	0.33940(5)		0.72011(13)	0.0154(2)	0.30
K5	0.32136(6)		0.65769(15)	0.0125(2)	0.60
Sr5	0.32136(6)		0.65769(15)	0.0125(2)	0.40
Sc	0.49987(7)		0.48125(19)	0.0070(3)	
Sil	0.32788(10)		0.2314(3)	0.0088(4)	
Si2	0.34565(10)		0.2244(3)	0.0087(4)	
Si3	0.42300(7)	-0.00434(13)	0.76985(19)	0.0078(3)	
Ol	0.3229(3)		0.0064(8)	0.0169(12)	
O2	0.2905(2)	-0.3687(4)	0.3133(6)	0.0176(8)	
O3	0.4082(3)		0.3152(8)	0.0130(11)	
O4	0.3902(3)		0.0323(8)	0.0220(13)	
O5	0.29935(19)	0.3700(3)	0.2466(6)	0.0147(8)	
O6	0.4036(3)		0.3985(9)	0.0205(13)	
O7	0.41657(19)	-0.0331(3)	0.9907(5)	0.0137(8)	
O8	0.34586(19)	0.0088(4)	0.6753(6)	0.0170(8)	
O9	0.46801(18)	0.1152(3)	0.7105(5)	0.0119(8)	
O10	0.46390(19)	-0.1175(4)	0.6657(5)	0.0151(8)	
■ Occupancy fixed to achieve completely charge-balanced formula (see text).
Table 3. Anisotropic displacement parameters for (À2) KSr6Sc(SiO4)4.
Atom	U11	U22	U33	U23	U13	U12
Sr1	0.0099(2)	0.0114(2)	0.0093(2)	-0.00067(19)	-0.00048(17)	0.00082(19)
K1	0.0099(2)	0.0114(2)	0.0093(2)	-0.00067(19)	-0.00048(17)	0.00082(19)
Sr2	0.0114(2)	0.0148(3)	0.0088(2)	-0.00096(19)	0.00127(17)	0.00062(18)
Sr3	0.0176(4)	0.0145(4)	0.0088(3)	0.0	0.0004(3)	0.0
Sr4	0.0147(4)	0.0132(5)	0.0182(4)	0.0	0.0037(3)	0.0
K4	0.0147(4)	0.0132(5)	0.0182(4)	0.0	0.0037(3)	0.0
K5	0.0141(5)	0.0146(6)	0.0088(5)	0.0	0.0008(4)	0.0
Sr5	0.0141(5)	0.0146(6)	0.0088(5)	0.0	0.0008(4)	0.0
Sc	0.0086(6)	0.0084(6)	0.0041(6)	0.0	-0.0002(5)	0.0
Si1	0.0090(9)	0.0074(10)	0.0100(9)	0.0	-0.0005(7)	0.0
Si2	0.0083(9)	0.0078(10)	0.0100(9)	0.0	-0.0008(7)	0.0
Si3	0.0086(6)	0.0079(7)	0.0071(6)	-0.0005(5)	0.0008(5)	-0.0009(5)
O1	0.029(3)	0.009(3)	0.012(3)	0.0	-0.007(2)	0.0
O2	0.020(2)	0.009(2)	0.024(2)	0.0023(16)	0.0016(16)	-0.0036(16)
O3	0.012(2)	0.013(3)	0.014(3)	0.0	-0.001(2)	0.0
O4	0.024(3)	0.024(3)	0.018(3)	0.0	0.012(2)	0.0
O5	0.0132(18)	0.0095(19)	0.021(2)	0.0008(16)	0.0013(15)	0.0017(15)
O6	0.016(3)	0.023(3)	0.022(3)	0.0	-0.011(2)	0.0
O7	0.0152(18)	0.0171(19)	0.0089(17)	0.0017(16)	0.0014(15)	0.0006(15)
O8	0.0095(16)	0.026(2)	0.0150(18)	0.0000(17)	-0.0031(15)	0.0006(17)
O9	0.0136(18)	0.010(2)	0.0117(18)	0.0027(15)	0.0033(14)	-0.0030(15)
O10	0.0176(19)	0.014(2)	0.0140(19)	-0.0060(16)	0.0039(15)	0.0018(15)
3.1.2. Rb5ln(MoO4)4
Rb5ln(MoO4)4 belongs to the large group of isoelec-tronic compounds with the general formula A5M 3+(ZO4)4 (A = Rb, K, Tl; M = REE, Bi, Fe, In; X = Mo, W). Most of them crystallise in a variety of layered structure types,
related to the mineral palmierite, K2Pb(SO4)2.31,32 In contrast, Rb5Er(MoO4)4 and isotypic Rb5ln(MoO4)4 represent a chain-based structure type, which is built from decorated kröhnkite-like chains. The crystal structure of Rb5Er(MoO4)4 was determined by Klevtsova and Glinska-
Table 4. Selected interatomic distances (À) and calculated bond-valence sums (v.u.) for the coordination polyhedra in KSr6Sc(SiO4)4'.
KSr6Sc(SiO4)4
Sr1-O7	2.458(4)	0.399
Sr1-O5	2.637(4)	0.246
Sr1-O9	2.685(4)	0.216
Sr1-O2	2.724(4)	0.194
Sr1-O8	2.745(4)	0.184
Sr1-O10	2.747(4)	0.182
Sr1-O3	2.8050(8)	0.156
Sr1-O6	2.8396(13)	0.142
Sr1-O10	2.913(4)	0.117
Sr1-O9**	3.193(4)	0.055
Mean	2.73	1.84
Sr2-O2	2.480(4)	0.376
Sr2-O5	2.533(4)	0.326
Sr2-O5	2.560(4)	0.303
Sr2-O2	2.625(4)	0.254
Sr2-O8	2.636(4)	0.246
Sr2-O8	2.675(4)	0.222
Sr2-O1	2.697(2)	0.209
Sr2-O7	2.710(4)	0.202
Mean	2.61	2.14
Sr3-O4	2.466(6)	0.390
Sr3-O3	2.658(5)	0.232
Sr3-O7 (2x)	2.729(4)	0.192
Sr3-O (2x)	2.741(4)	0.187
Sr3-O10 (2x)	2.817(4)	0.152
Sr3-O1	3.036(6)	0.087
Mean	2.74	1.77
Sr4-O4	2.426(6)	0.435
Sr4-O6	2.600(6)	0.272
Sr4-O8 (2x)	2.723(4)	0.195
Sr4-O9 (2x)	2.888(4)	0.125
Sr4-O2 (2x)	2.896(4)	0.122
Mean	2.76	1.61
K5-O1	2.485(6)	0.383
K5-O5 (2x)	2.747(4)	0.189
K5-O2 (2x)	2.852(4)	0.142
K5-O8 (2x)	2.938(4)	0.113
K5-O3	2.952(5)	0.108
K5-O10 (2x)	3.106(4)	0.071
Mean	2.87	1.51
Sc-O6	2.036(5)	0.624
Sc-O10 (2x)	2.098(4)	0.528
Sc-O3	2.117(5)	0.501
Sc-O9 (2x)	2.127(4)	0.488
Mean	2.10	3.16
Si1-O1	1.606(6)	1.050
Si1-O2 (2x)	1.618(4)	1.016
Si1-O3	1.648(5)	0.937
Mean	1.62	4.02
Si2-O4	1.613(6)	1.030
Si2-O5 (2x)	1.617(4)	1.019
Si2-O6	1.664(6)	0.898
Mean	1.64	3.86
Si3-O7	1.611(4)	1.036
Si3-O8	1.629(4)	0.987
Si3-O9	1.647(4)	0.940
SÌ3-O10	1.664(4)	0.898
Mean	1.63	3.97
ya21; it is monoclinic, with space group P3/c and the unit-cell parameters a = 11.44, b = 7.99, c =11.19 À, ß = 113.3°. We point out that the crystal structure data of Rb5Er(MoO4)4 are missing in the latest edition of the Inorganic Crystal Structure Database (ICSD). The ICDD-PDF contains a measured X-ray powder diffraction pattern of Rb5In(MoO4)4 (PDF-entry no. 36-1367).33 The pattern is indexed on the basis of a monoclinic (space group not given) unit cell with a = 18.78, b = 7.984, c = 12.30 À, ß = 91.58°, V = 1843.56 À3 which corresponds numerically to a B-centered cell with double volume as compared to the unit-cell parameters reported here (a = 11.391, b = 7.983, c = 11.100 À, ß = 113.74°, V = 924.0 À3, space group P3/c; transformation matrix P- to B-centred cell [10-1 010 101]). A comparison of the powder diffraction data reported in PDF-entry no. 36-1367 33 with those calculated for the compound described in this work shows that all
Figure 4. View of Rb5In(MoO4)4 along [001], parallel to the decorated kröhnkite-like octahedral-tetrahedral chains.
Bond-valence sums (v.u.) for the oxygen atoms O1 to O10 are as follows: O1 = 1.94, O2 = 2.10, O3 = 2.09, O4 = 1.86, O5 = 1.89, O6 = 2.01, O7 = 1.83, O8 = 1.90, O9 = 2.01, O10 = 1.95.
* Bond-valence calculations are based on parameters of Brese & O'Keeffe29 and for Sc atoms, on Brown30 (updated values from webmirrors/i_d_brown). Bond-valence sums for the mixed Sr/K sites Sr4 and K5 were calculated taking into account the respective occupancies (see Table 2).
** Not used for calculation of mean distances and BVSs.
strong and medium strong reflections of Rb5In(MoO4)4 appear in the pattern. However, there are a number of additional strong reflections reported that clearly do not belong to the powder diffraction pattern of Rb5In(MoO4)4.
The asymmetric unit of Rb5In(MoO4)4 contains one In site, three Rb, two Mo and eight O sites. Two atoms, Rb1 and In, lie on special positions, whereas all remaining atoms are in general positions. The main building unit of its crystal structure (see Figures 4, 5) is a decorated kröhnkite-like [100] chain built from a distorted InOg octahedron corner-linked by MoO4 tetrahedra and decorated by additional MoO4 tetrahedra. These chains are separated in different directions by three non-equivalent Rb atoms. A somewhat layered character of the atomic arrangement parallel to (100) is evident from the view in Figure 5.
Figure 5. View of Rb5In(MoO4)4 along [010], perpendicular to the decorated kröhnkite-like octahedral-tetrahedral chains.
The InO6 octahedra are only slightly distorted with an average In-O bond length of 2.136 À and O-In-O bond angles between 81.01 and 99.33 and between 156.08 and 178.74°. The Rb atoms are coordinated by eight or ten O atoms with average Rb-O bond lengths of 3.026 À (for [8]-coordination), and 3.093 and 3.120 À (for [10]-coor-dination).
According to the classification of molybdates with composition A5M 3+(MoO4)4 (A = K, Rb; M 3+ = REE, Y, In, Sc, Fe, Al, Bi) by Lazoryak and Efremov31, Rb5Er (MoO4)4 and isotypic Rb5In(MoO4)4 adopt structure type VII. This type is structurally similar to type V adopted by K5M3+(MoO4)4 (M 3+ = Tm-Lu, In, Sc), and represented by K5In(MoO4)4 discussed below.
3. 2. Comparison With Other Compounds Based on Decorated Kröhnkite-like Octahedral-Tetrahedral Chains
As already mentioned in the introduction, the kröhnkite-type chain sensu strictu is encountered in a fairly large number of compounds. In contrast, there exist only few compounds containing decorated kröhnkite-like chains. Figure 6 compares the chain units of nine known and two new compounds with decorated kröhnkite-like chains. All these compounds contain more or less distorted MO6 octahedra linked and decorated by the XO4 tetra-hedra. However, there are distinct differences concerning both linkage and orientation of XO4 tetrahedra with respect to the octahedra.
In KSrgSc(S1O4)4 (Figure 6a) and the protonated phosphate compound Ba2Ca(HPO4)2(H2PO4)2,33 the two decorating tetrahedra are attached to the trans apices of the MOg octahedra. Specifically, in KSrgSc(S1O4)4 two Si3-centred tetrahedra corner-link neighbouring octahedra, whereas the two remaing S11- and Si2-centred tetra-hedra only decorate the chain. Although the overall topo-
Table 5. Fractional atomic coordinates and equivalent isotropic displacement parameters (Ä2) for Rb5In(MoO4)4. yequ,v according to Fischer & Tillmanns.28
Atom	x	.y	z	U equiv
Rb1	Vi	0.38964(4)	V	0.02177(7)
Rb2	0.10260(2)	0.34909(3)	0.65569(2)	0.02542(6)
Rb3	0.40615(2)	0.86800(3)	0.35455(2)	0.01999(5)
In	0.0	0.08625(3)	V	0.01074(5)
Mo1	0.195171(15)	0.85392(2)	0.555916(16)	0.01067(4)
Mo2	0.283066(18)	0.33993(2)	0.442361(17)	0.01253(5)
O1	0.11442(15)	0.8913(2)	0.38167(14)	0.0185(3)
O2	0.35569(15)	0.9074(2)	0.60628(16)	0.0211(3)
O3	0.12492(15)	0.9706(2)	0.64853(15)	0.0182(3)
O4	0.17831(19)	0.6432(2)	0.58073(19)	0.0279(4)
O5	0.11423(15)	0.2778(2)	0.37620(16)	0.0229(3)
O6	0.34680(16)	0.3243(2)	0.61421(15)	0.0211(3)
O7	0.30474(18)	0.5421(2)	0.39453(18)	0.0254(4)
O8	0.37157(17)	0.2057(2)	0.38808(17)	0.0239(3)
Table 6. Anisotropic displacement parameters (Ä2) for Rb5In(MoO4)4.
Atom	U11	U22	U33	U23	U13	U12
Rb1	0.02211(15)	0.01526(13)	0.02713(15)	0.0	0.00905(12)	0.0
Rb2	0.02270(12)	0.02601(13)	0.02473(12)	0.00352(9)	0.00662(9)	-0.00855(9)
Rb3	0.02290(11)	0.01753(10)	0.01843(10)	-0.00186(8)	0.00716(8)	0.00038(8)
In	0.00952(8)	0.01248(9)	0.00981(8)	0.0	0.00345(6)	0.0
Mo1	0.00984(8)	0.01184(8)	0.01064(7)	0.00178(5)	0.00443(6)	0.00112(5)
Mo2	0.01297(8)	0.01153(8)	0.01255(8)	-0.00024(5)	0.00456(6)	-0.00114(5)
O1	0.0187(7)	0.0220(8)	0.0124(6)	0.0036(6)	0.0040(6)	0.0069(6)
O2	0.0122(7)	0.0289(9)	0.0221(7)	0.0005(7)	0.0069(6)	0.0001(6)
O3	0.0189(7)	0.0214(8)	0.0179(7)	0.0011(6)	0.0112(6)	0.0036(6)
O4	0.0382(11)	0.0141(8)	0.0363(10)	0.0053(7)	0.0203(9)	0.0008(7)
O5	0.0168(7)	0.0244(8)	0.0254(8)	-0.0106(7)	0.0063(6)	-0.0065(6)
O6	0.0238(8)	0.0221(8)	0.0149(7)	0.0015(6)	0.0052(6)	0.0007(6)
O7	0.0353(10)	0.0151(7)	0.0263(8)	0.0040(6)	0.0131(7)	-0.0036(7)
O8	0.0268(8)	0.0216(8)	0.0278(8)	-0.0024(7)	0.0157(7)	0.0022(7)
Table 7. Selected interatomic distances (À) and calculated bond-valence sums (v.u.) for the coordination polyhedra in Rb5ln(MoO4)4*.
Rb5In(MoO4)4
Rb1-O6 (2x)	2.9007(19)	0.177	Rb3-O6	3.1003(18)	0.103
Rb1-O8 (2x)	2.9069(18)	0.174	Rb3-O2	3.1328(17)	0.094
Rb1-O2 (2x)	2.9594(19)	0.151	Rb3-O2	3.1407(18)	0.092
Rb1-O4 (2x)**	3.389(2)	0.047	Rb3-O3**	3.355(2)	0.052
Rb1-O7 (2x)**	3.446(2)	0.040	Rb3-O1**	3.4602(17)	0.039
Mean	2.92	1.00	Mean	3.01	1.10
Rb2-O4	2.7436(19)	0.270	ln-O5 (2x)	2.1274(17)	0.544
Rb2-O7	2.858(2)	0.199	ln-O1 (2x)	2.1730(16)	0.481
Rb2-O6	3.0033(18)	0.134	ln-O3 (2x)	2.1886(15)	0.461
Rb2-O1	3.0236(18)	0.127	Mean	2.16	2.97
Rb2-O3	3.0362(19)	0.123	Mo1-O4	1.7275(18)	1.624
Rb2-O1	3.1181(17)	0.098	Mo1-O2	1.7364(16)	1.586
Rb2-O5	3.2089(18)	0.077	Mo1-O3	1.7963(15)	1.349
Rb2-O4	3.220(2)	0.075	Mo1-O1	1.8011(16)	1.331
Mean	3.03	1.10	Mean	1.77	5.90
Rb3-O8	2.7715(19)	0.251	Mo2-O8	1.7366(17)	1.587
Rb3-O6	2.9119(17)	0.172	Mo2-O7	1.7474(17)	1.541
Rb3-O7	2.9517(18)	0.154	Mo2-O6	1.7511(17)	1.524
Rb3-O8	3.016(2)	0.130	Mo2-O5	1.8295(17)	1.235
Rb3-O2	3.0871(17)	0.107	Mean	1.77	5.89
Bond-valence sums (v.u.) for the oxygen atoms O1 to O8 are as follows: O1 = 2.07, O2 = 2.03, O3 = 1.98, O4 = 2.02, O5 = 1.86, O6 = 2.11, O7 = 1.94, O8 = 2.14
* Bond-valence calculations are based on parameters of Brese & O'Keeffe.29 ** Not used for calculation of mean distances and BVSs.
logy of the decorated chains in both compounds is identical, the decorating P1-centred tetrahedra in Ba2Ca (HPO4)2(H2PO4)2 are considerably tilted with respect to the chain direction, and the chain is somewhat corrugated (see Figure 6b).
ln Rb5ln(MoO4)4 (Figure 6c) the chain is formed by a different octahedral-tetrahedral linkage scheme: Mo1-centred tetrahedra link the lnO6 octahedra alternatively in a horizontal and vertical plane, whereas the two Mo2-centred decorating tetrahedra are attached to the cis apices of the lnO6 octahedra. This scheme results in a slightly wavy character of the chain.
The chain in K5ln(MoO4)435 (Figure 6d) is related to that in Rb5ln(MoO4)4, but considerably more distorted. The linkage of the lnO6 octahedra by Mo2- and Mo4-centred tetrahedra in a twisted sequence, and the cis decorating Mo1- and Mo3-centred tetrahedra result in a distinct zigzag-like character of the chain.
In the protonated arsenate compounds CsM3+-(H15AsO4)2(H2AsO4) (M = Ga, Cr), CsAl(H2As-O4)2(HAsO4) and K(H2O)M (H1.5AsO4)2(H2AsO4) (M = Fe, Ga, In) described by Schwendtner and Kolitsch,36-38 all AsO4 tetrahedra are involved in the linkage system forming the chain, i.e., each AsO4 tetrahedron corner-links
a)
b)
d)
c)
Figure 6. Decorated kröhnkite-like octahedral-tetrahedral chains in a) KSr6Sc(SiO4)4, b) Ba2Ca(HPO4)2(H2PO4)2, c) Rb5ln(MoO4)4, d) K5ln(MoO4)4, e) CsM 3+(Hj5AsO4)2(H2AsO4) (M = Ga, Cr), f) Cs-Al(H2AsO4)2(HAsO4), g) K(^2O)MW3+(Hj5AsO4)2(H2AsO4) (M = Ee, Ga, In).	.
two MO6 octahedra (see Eigure 6 e-g). The overall topology of the chains in these arsenates is identical. lt is probable that this chain type will also be found in protonated phosphates.
4. Conclusions
The present work demonstrates that decorated kröhnkite-like chains are not only encountered among ar-senates, phosphates and molybdates, but also in silicates, with KSr6Sc(SiO2)2 being the first such example. A comparison of all presently known compounds containing decorated kröhnkite-like chains reveals, firstly, different systems of corner-linkage forming the chains, and, secondly, different roles of additional tetrahedra (some only play a decorating role, while others provide an additional corner-linkage along the chain direction).
5. Acknowledgement
We thank two anonymous referees for critical and constructive comments on the manuscript. This research was financially supported by the Austrian Science Eoun-dation (EWE) (Grant P17623-N10).
6. References
1.	B. Dahlman, Ark. Mineral. Geol. 1952, 1, 339-366.
2.	M. Eleck, U. Kolitsch, B. Hertweck, Z. Kristallogr. 2002, 217, 2353443.
3.	M. Eleck, U. Kolitsch, Z. Kristallogr. 2003, 218, 553-567. 2. U. Kolitsch, M. Eleck, Z. Kristallgr. 2005, 220, 31-41.
5.	U. Kolitsch, M. Eleck, Eur. J. Mineral. 2006, 18, 271-482.
6.	U. Kolitsch, Acta Crystallogr. 2006, C62, i353i37.
7.	M. Wierzbicka-Wieczorek, Syntheses, crystal structures and crystal chemistry of new mixed-framework silicates and a
new molybdate structure type. Ph.D. Thesis, Institute of Mineralogy and Crystallography, University of Vienna, 2007, 186 pp.
8.	M. Wierzbicka-Wieczorek, U. Kolitsch, E. Tillmanns, Z. Kristallogr. 2008 submitted.
9.	U. Kolitsch, M. Wierzbicka-Wieczorek, E. Tillmanns Can. Mineral. 2008 (accepted).
10.	U. Kolitsch, M. Wierzbicka, E. Tillmanns, An unusually narrow Si-O-Si angle in the new disilicate BaKYSi2O7, in: DGK (Ed.), Annual Conference of the German Society for Crystallography, Freiburg, Germany, Z. Kristallogr. 2006, Suppl. 24, 137.
11.	U. Kolitsch, M. Wierzbicka, E. Tillmanns, Acta Crystallogr. 2006, C62, i97-i99.
12.	U. Kolitsch, M. Wierzbicka, E. Tillmanns, Monoclinic Ba-Y2Si3O10 and triclinio SrY2Si3O10: flux growth and atomic arrangements of two new silicates containing finite Si3O10 units, in: SCS and CCA (Eds.), 15th Annual Conference of Slovenian Crystallographic Society and Croatian Crystallo-graphic Association, Jezersko, Slovenia, Book of Abstracts, 2006, 22.
13.	M. Wierzbicka-Wieczorek, U. Kolitsch, E. Tillmanns, Can. Min. 2008, in prep.
14.	M. Wierzbicka, U. Kolitsch, E. Tillmanns, Flux growth and crystal structure determination of new M1+-M2+-M3+ di-, tri- and tetrasilicates (M1* = Na, K; M2+ = Sr, Ba; M3+ = Sc, Y, Gd, Er, Yb), in: ICSSC-5 and PCCG-8 (Eds.), 5th International Conference on Solid State Crystals and 8th Polish Conference on Crystal Growth, Zakopane, Poland, Book of Abstracts, 2007, 70-71.
15.	M. Wierzbicka-Wieczorek, U. Kolitsch, E. Tillmanns, Flux growth and topology of novel mixed sorosilicates: Ba-Y4(Si2O7)(Si^O1g) and isotypic SrREE4(Si2O7)(Si3O10) (REE = Yb, Sc), in: DGK (Ed.), 16th Annual Conference of the German Society for Crystallography, Abstract CD, DGK, Erlangen, Germany, 2008, 2E-01.
16.	M. Wierzbicka-Wieczorek, U. Kolitsch, E. Tillmanns, Ba2Gd2Si4O13: a rare example of a silicate with finite Si4O13 chains. Acta Crystallogr. C 2008, in prep.
17.	M. Wierzbicka, U. Kolitsch, E. Tillmanns, Flux synthesis and crystal structure of the new mixed silicates Na4Sr2M 3+2(Si2O7)-(SiO4)2 (M3+ = Y, In, Sc), in: SCS and CCA (Eds.), 16th Annual Conference of Slovenian Crystallograp-hic Society and Croatian Crystallographic Association, Petr-cane, Croatia, Book of Abstracts, 2007, 37.
18.	M. Wierzbicka, U. Kolitsch, E. Tillmanns, Rb5Fe(MoO4)4 - a novel type with isolated Fe2Mo8O32 clusters, in: DGK (Ed.), Annual Conference of the German Society for Crystallography, Freiburg, Germany, Z. Kristallogr. 2006, Suppl. 24, 176.
19.	Z. Otwinowski, D. Borek, W. Majewski, W. Minor, Acta Crystallogr. 2003, A59, 228-234.
20.	G. M. Sheldrick, SHELXS97 and SHELXL97. University of Göttingen, Germany, 1997.
21.	R. F. Klevtsova, L. A. Glinskaya, Dokl. Akad. Nauk SSSR 1976, 230, 1337-1340 (in Russian).
22.	Shape Software Atoms for Windows and Macintosh, Version 5.1. Shape Software, Kingsport, TN 37663, USA, 1999.
23.	Yu. A. Pyatenko, T. A. Zhdanova, A. A. Voronkov, Dokl. Akad. Nauk SSSR 1979, 248, 868-871 (in Russian).
24.	U. Kolitsch, E. Tillmanns, Eur. J. Min. 2004, 16, 143-149.
25.	J. D. Napper, R. C. Layland, M. D. Smith, H.-C. zur Loye, J. Chem. Crystallogr. 2004, 34, 347-351.
26.	L.-H. Wang, L. F. Schneemeyer, R. J. Cava, T. Siegrist, J. Solid State Chem. 1994, 113, 211-214.
27.	W. H. Baur, in: M. O'Keeffe, A. Navrotsky, (Eds.), Structure and bonding in crystals, Vol. II, Academic Press, New York, 1981, 31-52.
28.	R. X. Fischer, E. Tillmanns, Acta Crystallogr. 1988, C44, 775-776.
29.	N. E. Brese, M. O'Keeffe, Acta Crystallogr. 1991, B47, 192-197.
30.	I. D. Brown, J. Appl. Crystallogr. 1996, 29, 479-480.
31.	B. I. Lazoryak, V. A. Efremov, Sov. Phys. Crystallogr. 1987, 32, 220-224.
32.	V. A. Morozov, B. I. Lazoryak, O. I. Lebedev, S. Amelinckx, G. Van Tendeloo, J. Solid State Chem. 2003, 176, 76-87.
33.	V. A. Efremov, V. K. Trunov, Yu. A. Velikodnyi, Zh. Neorg. Khim. 1971, 16, 1052-1055 (in Russian).
34.	M. Toumi, S. Chabchoub, L. Smiri-Dogguy, Y. Laligant, Eur. J. Solid State Inorg. Chem. 1997, 34, 1249-1257.
35.	O. V. Kudin, V. A. Efremov, V. K. Trunov, Yu. A Velikodnyi, Zh. Neorg. Khim. 1981, 26, 2734-2739.
36.	K. Schwendtner, U. Kolitsch, Acta Crystallogr. 2005, C61, i90-i93.
37.	K. Schwendtner, U. Kolitsch, Acta Crystallogr. 2007, C63, i17-i20.
38.	K. Schwendtner, U. Kolitsch, Eur. J. Min. 2007, 19, 399409.
Povzetek
Z metodo rentgenske difrakcije sta bili pri sobni temperaturi določeni kristalni strukturi dveh novih spojin KSr6Sc (SiO4)4 in Rb5In(MoO4)4. Obe spojini sta bili sintetizirani iz fluksa pri študiju novih mikro in nanoporoznih alkalijskih-M 3+ silikaktov. KSr6Sc(SiO4)4 predstavlja nov strukturni tip in je to prvi silikat, ki vsebuje dekorativne kröhnkitu podobne oktaedrsko-tetraedrske verige. Spojina je ortorombska v Pnma, with a = 19.137(4), b = 11.197(2), c = 7.125(1) À, V = 1526.7(5) À3, Z = 8. Rb5In(MoO4)4 kristalizira v P2/c with a = 11.391(2), b = 7.983(2), c = 11.100(2) À, ß = 113.74(3)°, V = 924.0(3) À3, Z = 2. Spojina je izotipična z Rb5Er(MoO4)4. Narejena je bila tudi primerjava z ostalimi spojinami, ki vsebujejo kröhnkitu podobne verige npr. Ba3Ca(HPO4)3(H3PO4)3, CsM3+(H1.5AsO4)3(H3AsO4) (M3+ = Ga, Cr), CsAl(H3AsO4)3(HAsO4), K(H3O)M3+(H1.5AsO4)3(H3AsO4) (M3"2 = Fe, Ga, In) in K5Ii:i(Mo04)4.