Crystal Growth and Single-Crystal Structures of RERhO3 (RE )La, Pr, Nd, Sm, Eu, Tb) Orthorhodites from a K 2CO3 Flux

ReneB. Macquart, Mark D. Smith, and Hans-Conrad zur Loye*

Department of Chemistry and Biochemistry, UniVersity of South Carolina,Columbia, South Carolina 29208

ReceiVed NoVember 14, 2005

ABSTRACT: Single crystals of LaRhO3, PrRhO3, NdRhO3, SmRhO3, EuRhO3, and TbRhO3 have been grown for the firsttime using a K2CO3 flux. All the compounds were found to crystallize in the orthorhombic space groupPbnm (No. 62) andadopt the GdFeO3 distorted perovskite structure type. Transition toward a pseudo-cubic cell is noted as the rare earth cation sizeincreases.

Introduction

Rhodium-containing compounds display a number of desir-able properties that have seen them used in a wide range ofapplications, for example, as catalysts,1 in photoelectrolyticcells,2,3 and as p-type amorphous oxide semiconductors.4

Rhodium superconducting compounds are also known.5,6 Thefocus of this article is on the crystal growth and structuralcharacterization of some rare earth orthorhodites, known for theirability to act as semiconducting photocatalysts.2,3

The use of molten fluxes in the formation of single crystalsis well established.7 Hydroxide and carbonate fluxes have beenused successfully by our group in the incorporation of numerouselements (Li, Na, K, Ca, Sr, Ba, Fe, Co, Ni, Cu, Ru, Rh, Os, Ir,Pt, Pb, La, Pr, Nd, Sm, Eu, and Gd) into various metal oxidestructures.8-20 Molten K2CO3 is particularly suited for reactionswith platinum group metals and lanthanide oxides. Unlike someother flux media K2CO3 has a low toxicity and volatility andwill react readily with rare earth oxides and platinum groupmetals at around 1050°C,9,10,21and the product can easily beisolated from the flux once the crystals have formed by washingwith water. K2CO3 has the added advantage that the K+ (ionicradius 1.51 Å; eight-coordinate environment) ions are too largeto fit into the orthorhodite structure either on the eight-coordinateLa3+ (ionic radius 1.160 Å) site or the six-coordinate Rh3+ (ionicradius 0.665 Å) site.22 Use of other alkali metal fluxes such asNaOH will result in Na+ substitution onto the La3+ site as seenin the 2H-perovskite-related oxides (NaLa2)NaPtO6

11 andCa3NaRuO6.23

The first reports of rare earth orthorhodites were by Wold etal. in the late 1950s (LaRhO3)24 and early 1960s (NdRhO3).25

The structure was described in the orthorhombic space groupPbnm(No. 62) with a distorted perovskite structure similar tothat observed in gadolinium orthoferrite (GdFeO3) by Geller.26

Following this initial work, a number of attempts were madeto determine the structure of the entireRERhO3 (RE ) rareearth) series. Chazalon et al.27 reported space group andapproximate lattice parameters forRERhO3 (RE) La, Pr, Sm,Gd, Ho, or Er), while Shannon28 compiled a more accurate listfor RERhO3 (RE ) Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Lu). Data for CeRhO3 and YbRhO3 were subsequently reported

by Lazarev et al.29 and Shaplygin et al.30 in the late 1970s (noinformation has been listed for the radioactive Pm analogue).While the focus of study on these compounds has been onpowder synthesis and characterization techniques,24,25,27-32 tothe best of our knowledge there have been no reports of anysingle-crystal work except in the case of LuRhO3

3 where a PbO/PbF2/B2O3 flux was used to generate LuRhO3 crystals, thestructure of which were subsequently solved using single-crystalX-ray diffraction techniques. While no single-crystal structurewas reported, it confirmed the expected distorted perovskitestructure and regular coordination for Rh3+. Here we report forthe first time the growth and characterization of single crystalsof RERhO3 (RE) La, Pr, Nd, Sm, Eu, Tb) and highlight sometrends in the system.

Experimental Procedures

Rhodium metal (Engelhard, powdered 99.987%) andRE2O3 (RE)La, Pr, Nd, Sm, Eu, Tb) (Alfa Aesar, 99.9%) were ground in an ace-tone slurry with an agate mortar and pestle. TheRE2O3 (RE ) La,Nd, Sm, Eu, Tb) (Alfa Aesar, 99.9%) was initially heated to 1000°Cfor 15 h to ensure dryness. Pr2O3 was obtained from Pr6O11 (AlfaAesar, 99.9%) by heating it in a tube furnace under flowing H2(5%)/N2(95%) at 1000°C for 15 h, cooling it, grinding it up, then heatingit again at 1000°C for another 15 h under flowing H2(5%)/N2(95%).The RE2O3 (RE ) La, Pr, Nd, Sm, Eu, Tb) was stored in a vacuumdesiccator when not in use. The reactants were placed in an aluminacrucible and covered with anhydrous K2CO3 (Fisher Scientific,99.8%), acting here as a flux. The crucible was covered with an alum-ina lid and heated in a tube furnace from room temperature to 1050°Cat a rate of 600°C/h, held at 1050°C for 24 h, then cooled to 800°Cat a rate of 15°C/h, held at 800°C for 1 h, and then step cooled toroom temperature by removing power to the furnace elements. Thecrucible was immersed in deionized water and sonicated to dissolvethe flux. The resulting material was filtered under suction and washedwith more deionized water. A small quantity of acetone was then usedto aid in the drying of the crystals. Initially, the reactants were com-bined in the ratio 1.5 mmol ofRE2O3 to 0.5 mmol of K2CO3 to 2 mmolof Rh and ground together in an acetone slurry with 70 mmol ofK2CO3 used as flux. Subsequent reactions were carried out using astoichiometric mixture of reactants, that is, 0.5 mmol ofRE2O3, 1 mmolof Rh, and 70 mmol of K2CO3 flux. Smaller quantities of flux (35mmol K2CO3), sufficient to cover the reactants, were also usedsuccessfully.

The crystal morphology and composition were examined usingscanning electron microscopy (SEM) and energy-dispersive X-rayanalysis (EDS). Measurements were performed with a Quanta ESEM200. A description of a typical single-crystal data collection and solutionset corresponding to LaRhO3 follows. Information for the otheranalogues is listed in Table 1. X-ray intensity data from a black crystalfragment (approximate dimensions 0.04× 0.03 × 0.02 mm3) were

* To whom correspondence should be addressed. Mailing address:University of South Carolina, Department of Chemistry and Bio-chemistry, 631 Sumter Street, Columbia, SC 29208. Phone:+1 803 777-6916. Fax: +1 803 777-8508. E-mail: zurloye@mail.chem.sc.edu. Webaddress: http://www.chem.sc.edu.

CRYSTALGROWTH& DESIGN

2006VOL.6,NO.6

1361-1365

10.1021/cg050605c CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 05/03/2006

measured at 294(1) K on a Bruker SMART APEX diffractometer (MoKR radiation,λ ) 0.710 73 Å).33 The data collection covered 99.8%of reciprocal space to 2θ ) 65.04° (average redundancy 7.8,Rint )0.048). Raw area detector data frame integration and Lp correctionswere carried out with SAINT+.33 Final unit cell parameters weredetermined by least-squares refinement of 1549 reflections withI >5σ(I) from the data set. Analysis of the data showed negligible crystaldecay during data collection. An empirical absorption correction wasapplied with SADABS.33 LaRhO3 adopts the GdFeO3 orthorhombicperovskite structure type, space groupPbnm. This structural model wasrefined by full-matrix least-squares againstF2 with SHELXTL.34 Allatoms were refined with anisotropic displacement parameters. Refine-ment of the site occupation factors for the metal atoms showed nosignificant deviation from unity occupancy. The largest residual electrondensity peaks remaining after the final refinement cycle were+2.33and-1.89 e‚Å-3, located<1 Å from La.

Results and Discussion

While reactions were performed with all the lanthanides(except Pm), only La, Pr, Nd, Sm, Eu, and Tb produced crystalsof a quality suitable for structural analysis by single-crystalX-ray diffraction. Figure 1 shows a scanning electron micro-graph of a LaRhO3 single crystal (approximately 33µm across)with truncated octahedral geometry, that is, six four-sided facesand eight six-sided faces, consistent with the orthorhombic spacegroup Pbnm. The majority of unsuccessful reactions resultedin large, flaky, black hexagonal crystals (up to 10 mm acrossand typically less than 0.5 mm thick) with an approximate ratioof elements 1K/2Rh/6O (obtained from EDS measurements) thatwere not suitable for single-crystal measurements.

The space group and cell dimensions of the rare earthorthorhodites have been determined previously using powdertechniques.24,25,27-30 Table 1 shows the lattice parameters andrefinement statistics forRERhO3 (RE ) La, Pr, Nd, Sm, Eu,

Table 1. Structural Data and Refinement Statistics forRERhO3 (RE ) La, Pr, Nd, Sm, Eu, or Tb)

empirical formula LaRhO3 PrRhO3 NdRhO3 SmRhO3 EuRhO3 TbRhO3

formula weight(g mol-1)

289.82 291.82 295.15 301.26 302.87 309.83

temp (K) 294(1) 294(1) 294(1) 294(1) 294(1) 294(1)space group Pbnm Pbnm Pbnm Pbnm Pbnm Pbnmunit cell dimensionsa (Å) 5.5242(12) 5.4167(2) 5.3758(3) 5.3231(3) 5.2978(3) 5.2538(3)b (Å) 5.7005(12) 5.7405(2) 5.7524(3) 5.7566(3) 5.7574(3) 5.7454(3)c (Å) 7.8968(17) 7.8032(3) 7.7703(4) 7.7084(4) 7.6786(4) 7.6254(5)V (Å3) 248.68(9) 242.637(15) 240.29(2) 236.21(2) 234.21(2) 230.17(2)Z 4 4 4 4 4 4density (calcd)

(g cm-3)7.741 7.989 8.159 8.471 8.589 8.941

absorption coefficient(mm-1)

23.303 26.356 27.946 31.307 33.282 37.341

F(000) 504 512 516 524 528 536crystal size (mm3) 0.04× 0.03× 0.02 0.04× 0.04× 0.03 0.03× 0.02× 0.02 0.04× 0.02× 0.02 0.04× 0.02× 0.02 0.03× 0.02× 0.02θ range for data

collection (deg)4.50-32.52 4.58-36.34 4.61-32.61 4.65-33.12 4.67-33.15 4.71-35.22

reflns collected 4000 5218 4138 4334 4307 4383independent reflns 481 618 463 475 469 551

(Rint ) 0.0478) (Rint ) 0.0341) (Rint ) 0.0371) (Rint ) 0.0353) (Rint ) 0.0299) (Rint ) 0.0483)goodness-of-fit

onF21.141 1.161 1.105 1.115 1.141 1.067

final R indices[I > 2σ(I)]

R1 ) 0.0290,wR2 ) 0.0567

R1 ) 0.0221,wR2 ) 0.0440

R1 ) 0.0282,wR2 ) 0.0623

R1 ) 0.0295,wR2 ) 0.0553

R1 ) 0.0246,wR2 ) 0.0569

R1 ) 0.0295,wR2 ) 0.0500

R indices (all data) R1 ) 0.0356,wR2 ) 0.0588

R1 ) 0.0252,wR2 ) 0.0447

R1 ) 0.0309,wR2 ) 0.0632

R1 ) 0.0330,wR2 ) 0.0564

R1 ) 0.0255,wR2 ) 0.0574

R1 ) 0.0355,wR2 ) 0.0512

largest diffractionpeak and hole(e‚Å-3)

2.327 and-1.890

1.497 and-2.088

1.894 and-2.755

3.595 and-3.786

2.023 and-2.296

2.442 and-2.692

Figure 1. Scanning electron micrograph of a LaRhO3 single crystal(truncated octahedron 4668, Pbnm).

Figure 2. Lattice parameters and volume as a function of ionicradius (IR) of the rare earth ions forRERhO3 (RE ) La, Pr, Nd, Sm,Eu, Tb).

1362 Crystal Growth & Design, Vol. 6, No. 6, 2006 Macquart et al.

Tb). TheR indices show a good solution in each case. The unitcell dimensions are comparable to those reported by Shannon28

in Pbnm (cell settingcab, space group no. 62). If the cubicaristotype35 parent structure (Pm3hm) has unit cell dimensionsac × ac × ac then the orthorhombicPbnm hettotype hasdimensionsa ≈ x2ac < b ≈ x2ac, andc ) 2ac. Figure 2 showsthe lattice parameters (a < c/x2 < b) of the orthorhombic unitcell as a function of rare earth cation size. Thec lattice parameteris displayed asc/x2 for ease of comparison. As the cation sizeincreases, the lattice parameters tend to converge witha andc/x2 increasing andb decreasing. Above∼1.11 Å (cationslarger than Nd3+), there is an increase in the rate of convergencesuggesting the beginnings of a transition to a pseudo-cubic phasewith a ≈ c/x2 ≈ b. This trend has been noted by others in

LaTiO3, LaVO3, and LaFeO3.36 The unit cell volume increasesconsistently with increase in rare earth cation size irrespectiveof lattice parameter convergence, Figure 2.

Table 2 shows the atomic coordinates and equivalent isotropicatomic displacement parameters obtained from the single-crystalstructure solutions ofRERhO3 (RE) La, Pr, Nd, Sm, Eu, Tb),while Table 3 shows selected interatomic bond lengths, dis-tances, and angles. Figure 3 shows the structure of LaRhO3,which is representative of all the orthorhodites studied here.La3+ cations are situated in the cavity formed by tilted corner-sharing [RhO6] octahedra. The coordination environment ofthe rare earth was based on the number of oxygens at a dis-tance closer than the nearest rhodium. By this method, theenvironment was found to be eight-coordinate for all the rareearth compounds; however, there was an oxygen located justbeyond the nearest rhodium suggesting the possibility of anine-coordinate environment with a reduction in tilting, Table3. The [RhO6] octahedra deviate slightly from the regularoctahedral geometry expected in perovskites with two differentdiagonally opposite sets of Rh-O(2) bonds of equal length inthe equatorial plane (2.051(4) and 2.067(4) Å) and shorter axialRh-O(1) bonds (2.042(2) Å), Figure 3 and Table 3. Figure 4shows the degree of Rh-O bond variation (∆d) plotted as afunction of the eight-coordinate ionic radius of the rare earth

Table 2. Atomic Coordinatesa and Equivalent Isotropic AtomicDisplacement Parameters forRERhO3 (RE ) La, Pr, Nd, Sm,

Eu, Tb)

x y z Ueq (Å2)b

LaRhO3

La 0.5140(1) 0.4430(1) 1/4 0.009(1)Rh 1/2 0 0 0.007(1)O(1) 0.4082(11) 0.0204(10) 1/4 0.009(1)O(2) 0.8030(7) 0.2022(7) 0.0475(6) 0.010(1)

PrRhO3

Pr 0.5187(1) 0.4330(1) 1/4 0.007(1)Rh 1/2 0 0 0.005(1)O(1) 0.3977(6) 0.0315(7) 1/4 0.008(1)O(2) 0.8103(4) 0.1973(5) 0.0516(3) 0.008(1)

NdRhO3

Nd 0.5209(1) 0.4295(1) 1/4 0.008(1)Rh 1/2 0 0 0.006(1)O(1) 0.3939(11) 0.0325(11) 1/4 0.008(1)O(2) 0.8131(8) 0.1972(7) 0.0529(6) 0.009(1)

SmRhO3

Sm 0.5238(1) 0.4252(1) 1/4 0.008(1)Rh 1/2 0 0 0.008(1)O(1) 0.3856(12) 0.0372(12) 1/4 0.009(1)O(2) 0.8168(8) 0.1945(8) 0.0553(6) 0.009(1)

EuRhO3

Eu 0.5248(1) 0.4230(1) 1/4 0.007(1)Rh 1/2 0 0 0.006(1)O(1) 0.3827(10) 0.0395(9) 1/4 0.009(1)O(2) 0.8176(6) 0.1939(6) 0.0560(5) 0.008(1)

TbRhO3

Tb 0.5266(1) 0.4208(1) 1/4 0.006(1)Rh 1/2 0 0 0.005(1)O(1) 0.3765(11) 0.0474(10) 1/4 0.007(1)O(2) 0.8194(8) 0.1922(7) 0.0593(6) 0.008(1)

a RE at 4c; Rh at 4b; O(1) at 4c; O(2) at 8d site. b Ueq is defined asone-third of the trace of the orthogonalizedUij tensor.

Table 3. Selected Interatomic Distances, Bond Lengths (Å), and Angles (deg) forRERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb)

LaRhO3 PrRhO3 NdRhO3 SmRhO3 EuRhO3 TbRhO3

RE-O(1) 2.374(6) 2.325(4) 2.308(6) 2.273(6) 2.261(5) 2.239(5)RE-O(2)× 2 2.400(4) 2.357(3) 2.348(4) 2.319(5) 2.313(3) 2.280(4)RE-O(1) 2.479(6) 2.396(4) 2.383(6) 2.351(7) 2.332(5) 2.286(5)RE-O(2)× 2 2.644(4) 2.593(3) 2.569(4) 2.539(5) 2.523(4) 2.491(4)RE-O(2)× 2 2.750(4) 2.715(3) 2.705(4) 2.688(5) 2.679(4) 2.678(4)RE-Rh 3.2062(6) 3.1614(2) 3.1447(4) 3.1176(4) 3.1036(3) 3.0821(3)RE-O(1) 3.2224(6) 3.2114(4) 3.2012(6) 3.2093(6) 3.2098(5) 3.2192(5)Rh-O(1)× 2 2.042(2) 2.036(1) 2.033(2) 2.032(2) 2.031(2) 2.032(2)Rh-O(2)× 2 2.051(4) 2.059(2) 2.052(4) 2.056(4) 2.055(3) 2.057(4)Rh-O(2)× 2 2.067(4) 2.066(2) 2.071(4) 2.069(4) 2.065(3) 2.059(4)Rh-O(1)-Rh 150.5(3) 146.7(2) 145.7(3) 143.0(3) 142.0(3) 139.5(3)Rh-O(2)-Rh 149.1(2) 146.2(1) 145.4(2) 143.8(2) 143.5(2) 142.1(2)O(2)-Rh-O(2)angle,R

90.08(6) 91.06(4) 91.56(7) 91.93(8) 92.12(6) 92.11(7)

∆d (×10-5) 2.5(5) 3.9(3) 5.7(7) 5.6(7) 4.8(5) 3.6(5)tilt angleφ 19.1 21.2 21.7 22.9 23.2 24.0tilt angleψ 1.52 1.87 1.96 2.18 2.23 2.41

Figure 3. Structure of LaRhO3 looking along the [110] axis (top left)and down the [001] axis (bottom) displaying thea-a-b+ tilting. Theunit cell is indicated in yellow. Rh-O(2) bond lengths are given in Å.The tilting about the [010] axis is shown on the right.

Growth and Single-Crystal Structures ofRERhO3 Crystal Growth & Design, Vol. 6, No. 6, 20061363

cations inRERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb).∆d ≡1/6∑n)1-6[(dn - ⟨d⟩)/⟨d⟩]2 and shows the variation of theoctahedraldσ ) (Rh-O)σ bond lengths.36 ∆d increases with theionic radius of the rare earth cation from Tb to Nd and thendecreases sharply from Nd to La.

Figure 5 shows the variation of O(2)-Rh-O(2) angle (R)along the equatorial plane of the [RhO6] octahedra with ionicradius of the eight-coordinate rare earths and provides anothermeans of examining octahedral distortion. The greater thedeviation of the O(2)-Rh-O(2) angle away from 90°, thegreater the octahedral distortion is. As the size of the cation isincreased from Tb to Nd, there is a gradual decrease in angulardistortion, and then from Nd to La, the distortion decreasesmarkedly so that the [RhO6] octahedra in LaRhO6 are almostregular in shape coincident with the drop in∆d observed earlier.Figure 5 also shows the change in the octahedral tilt angleφ asa function of cation radius. As the cations get larger theoctahedral tilting decreases. Values ofφ were obtained usingSPuDS37,38 and are based on the equations given by O’Keefeand Hyde39 for calculating the octahedral tilt about the primaryaxis in space group 62. In the current setting, the primary axisof tilt is the b axis, which corresponds to a [110]c axis of thecubic aristotype. A secondary tilt angle (ψ) is given byψ )tan-1[x2(1 - cosφ)/(2 + cosφ)]. This tilt is coupled toφ andshows the same behavior asφ with cation size increase but liesalong thec axis in Pbnm([001]c axis in cubic parent) and issignificantly smaller, Table 3. Two schemes are used to describethe tilting in the system. In Figure 6, the tilts are described about

the orthorhombicb axis (φ) labeled [010]o and the orthorhombicc axis (ψ) labeled [001]o. In Figure 3, LaRhO3 adopts ana-a-b+

tilt system using the Glazer notation,40 which is based on theoctahedral tilting relative to the axes of the cubic aristotype([100]c denoting, here, the cubica axis). The first scheme,illustrated in Figure 6, displays the tilt angles in the specificorthorhombic system, while the Glazer notation, Figure 3, isconceptually useful when comparing various hettotypes basedon how they distort away from the cubic aristotype. Lookingdown thec axis in Figure 3, the [RhO6] octahedra in adjacentlayers are all tilted in the same sense (to giveb+), while lookingalong the [110]o axis ([100]c axis of the cubic aristotype), theoctahedra are tilted in the opposite sense but by an equal amount(to givea-a-), Table 3. Note that while the tilting about thecaxis (ψ) seems larger in Figure 3 than that listed in Table 3,the majority of the tilt is due to the primary tiltφ about the[010]o axis. The octahedral rotation is the driving force behindthe convergence of the lattice parameters toward a pseudo-cubicphase; however, it is the reduction in octahedral distortion,principally the decrease in∆d, that causes the increase in therate of convergence for cations larger than Nd3+, Figure 2.

Conclusions

We have used a K2CO3 flux to grow single crystals ofRERhO3 (RE) La, Pr, Nd, Sm, Eu, Tb) for the first time. Thecrystals were found to adopt a GdFeO3 type distorted perovskitestructure in orthorhombic space groupPbnmin good agreementwith previous polycrystalline studies. A description of the tiltingbased on two different schemes has been given. As the rareearth cation size increases, the orthorhodites tend toward apseudo-cubic geometry due to a decrease in octahedral tiltingand a lessening of octahedral distortion in the equatorial planegiven by angleR. For cations with ionic radius greater than∼1.11 Å (Nd3+), the octahedral bond mismatch,∆d, lessensnoticeably thereby speeding the transition toward a pseudo-cubiccell, a ≈ c/x2 ≈ b.

Figure 4. ∆d variation with ionic radius (IR) of the rare earth ions forRERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb).

Figure 5. Octahedral tilt (φ) and octahedral distortion O(2)-Rh-O(2)angle (R) as a function of the ionic radius (IR) of the rare earth ionsfor RERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb).

Figure 6. Comparison of the unit cell of the cubic aristotype (bold,dashed) and the undistorted orthorhombic perovskite hettotype (bold,solid). In the ideal perovskite,a ) x2ac. Distortion of the perovskitestructure fromPm3hm to Pbnmresults in a new unit cella ≈ x2ac <b ≈ x2ac andc ) 2ac due to octahedral tilting about the [010]o and[001]o axes (φ and ψ, respectively) and distortion of the regularoctahedra so that the octahedral metal-oxygen bond lengths (d) varyand the oxygen-metal-oxygen bond angles (R) in the equatorial planediverge from 90°. The rare earth cation is displaced away from thecenter of the cubooctahedral cavity formed by the corner-linkedoctahedra (12-fold oxygen coordination) resulting in an 8-fold coordina-tion environment. The distorted perovskite structure (Pbnm) is shownon the right. Note the rotation in the opposite direction of corner-linkedoctahedra.

1364 Crystal Growth & Design, Vol. 6, No. 6, 2006 Macquart et al.

Acknowledgment. Financial support from the Departmentof Energy through Grant DE-FG02-04ER46122 and the NationalScience Foundation through Grant DMR:0450103 is gratefullyacknowledged. R.M. extends thanks to M. W. Lufaso for hisinsightful comments.

Supporting Information Available: Twelve X-ray crystallographicinformation files forRERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb), six inPbnm (reference no. 415915, 415913, 415911, 415909, 415907,415905) and six in the standard settingPnma(reference no. 415916,415914, 415912, 415910, 415908, 415906), have been deposited atthe ICSD. This material is available free of charge via the Internet athttp://pubs.acs.org.

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