Crystal Growth and Single-Crystal Structures of RERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb) Orthorhodites from a K2CO3 Flux Rene´ B. Macquart, Mark D. Smith, and Hans-Conrad zur Loye* Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1361-1365
ReceiVed NoVember 14, 2005
ABSTRACT: Single crystals of LaRhO3, PrRhO3, NdRhO3, SmRhO3, EuRhO3, and TbRhO3 have been grown for the first time using a K2CO3 flux. All the compounds were found to crystallize in the orthorhombic space group Pbnm (No. 62) and adopt the GdFeO3 distorted perovskite structure type. Transition toward a pseudo-cubic cell is noted as the rare earth cation size increases. Introduction Rhodium-containing compounds display a number of desirable properties that have seen them used in a wide range of applications, for example, as catalysts,1 in photoelectrolytic cells,2,3 and as p-type amorphous oxide semiconductors.4 Rhodium superconducting compounds are also known.5,6 The focus of this article is on the crystal growth and structural characterization of some rare earth orthorhodites, known for their ability to act as semiconducting photocatalysts.2,3 The use of molten fluxes in the formation of single crystals is well established.7 Hydroxide and carbonate fluxes have been used successfully by our group in the incorporation of numerous elements (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 oxide structures.8-20 Molten K2CO3 is particularly suited for reactions with platinum group metals and lanthanide oxides. Unlike some other flux media K2CO3 has a low toxicity and volatility and will react readily with rare earth oxides and platinum group metals at around 1050 °C,9,10,21 and the product can easily be isolated from the flux once the crystals have formed by washing with water. K2CO3 has the added advantage that the K+ (ionic radius 1.51 Å; eight-coordinate environment) ions are too large to fit into the orthorhodite structure either on the eight-coordinate La3+ (ionic radius 1.160 Å) site or the six-coordinate Rh3+ (ionic radius 0.665 Å) site.22 Use of other alkali metal fluxes such as NaOH will result in Na+ substitution onto the La3+ site as seen in the 2H-perovskite-related oxides (NaLa2)NaPtO611 and Ca3NaRuO6.23 The first reports of rare earth orthorhodites were by Wold et al. in the late 1950s (LaRhO3)24 and early 1960s (NdRhO3).25 The structure was described in the orthorhombic space group Pbnm (No. 62) with a distorted perovskite structure similar to that observed in gadolinium orthoferrite (GdFeO3) by Geller.26 Following this initial work, a number of attempts were made to determine the structure of the entire RERhO3 (RE ) rare earth) series. Chazalon et al.27 reported space group and approximate lattice parameters for RERhO3 (RE ) La, Pr, Sm, Gd, Ho, or Er), while Shannon28 compiled a more accurate list for RERhO3 (RE ) Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu). Data for CeRhO3 and YbRhO3 were subsequently reported * To whom correspondence should be addressed. Mailing address: University of South Carolina, Department of Chemistry and Biochemistry, 631 Sumter Street, Columbia, SC 29208. Phone: +1 803 7776916. Fax: +1 803 777-8508. E-mail:
[email protected]. Web address: http://www.chem.sc.edu.
by Lazarev et al.29 and Shaplygin et al.30 in the late 1970s (no information has been listed for the radioactive Pm analogue). While the focus of study on these compounds has been on powder synthesis and characterization techniques,24,25,27-32 to the best of our knowledge there have been no reports of any single-crystal work except in the case of LuRhO33 where a PbO/ PbF2/B2O3 flux was used to generate LuRhO3 crystals, the structure of which were subsequently solved using single-crystal X-ray diffraction techniques. While no single-crystal structure was reported, it confirmed the expected distorted perovskite structure and regular coordination for Rh3+. Here we report for the first time the growth and characterization of single crystals of RERhO3 (RE ) La, Pr, Nd, Sm, Eu, Tb) and highlight some trends in the system. Experimental Procedures Rhodium metal (Engelhard, powdered 99.987%) and RE2O3 (RE ) La, Pr, Nd, Sm, Eu, Tb) (Alfa Aesar, 99.9%) were ground in an acetone slurry with an agate mortar and pestle. The RE2O3 (RE ) La, Nd, Sm, Eu, Tb) (Alfa Aesar, 99.9%) was initially heated to 1000 °C for 15 h to ensure dryness. Pr2O3 was obtained from Pr6O11 (Alfa Aesar, 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 heating it 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 vacuum desiccator when not in use. The reactants were placed in an alumina crucible and covered with anhydrous K2CO3 (Fisher Scientific, 99.8%), acting here as a flux. The crucible was covered with an alumina lid and heated in a tube furnace from room temperature to 1050 °C at a rate of 600 °C/h, held at 1050 °C for 24 h, then cooled to 800 °C at a rate of 15 °C/h, held at 800 °C for 1 h, and then step cooled to room temperature by removing power to the furnace elements. The crucible was immersed in deionized water and sonicated to dissolve the flux. The resulting material was filtered under suction and washed with more deionized water. A small quantity of acetone was then used to aid in the drying of the crystals. Initially, the reactants were combined in the ratio 1.5 mmol of RE2O3 to 0.5 mmol of K2CO3 to 2 mmol of Rh and ground together in an acetone slurry with 70 mmol of K2CO3 used as flux. Subsequent reactions were carried out using a stoichiometric mixture of reactants, that is, 0.5 mmol of RE2O3, 1 mmol of Rh, and 70 mmol of K2CO3 flux. Smaller quantities of flux (35 mmol K2CO3), sufficient to cover the reactants, were also used successfully. The crystal morphology and composition were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDS). Measurements were performed with a Quanta ESEM 200. A description of a typical single-crystal data collection and solution set corresponding to LaRhO3 follows. Information for the other analogues is listed in Table 1. X-ray intensity data from a black crystal fragment (approximate dimensions 0.04 × 0.03 × 0.02 mm3) were
10.1021/cg050605c CCC: $33.50 © 2006 American Chemical Society Published on Web 05/03/2006
1362 Crystal Growth & Design, Vol. 6, No. 6, 2006
Macquart et al.
Table 1. Structural Data and Refinement Statistics for RERhO3 (RE ) La, Pr, Nd, Sm, Eu, or Tb) empirical formula formula weight (g mol-1) temp (K) space group unit cell dimensions a (Å) b (Å) c (Å) V (Å3) Z density (calcd) (g cm-3) absorption coefficient (mm-1) F(000) crystal size (mm3) θ range for data collection (deg) reflns collected independent reflns goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diffraction peak and hole (e‚Å-3)
LaRhO3 289.82
PrRhO3 291.82
NdRhO3 295.15
SmRhO3 301.26
EuRhO3 302.87
TbRhO3 309.83
294(1) Pbnm
294(1) Pbnm
294(1) Pbnm
294(1) Pbnm
294(1) Pbnm
294(1) Pbnm
5.5242(12) 5.7005(12) 7.8968(17) 248.68(9) 4 7.741
5.4167(2) 5.7405(2) 7.8032(3) 242.637(15) 4 7.989
5.3758(3) 5.7524(3) 7.7703(4) 240.29(2) 4 8.159
5.3231(3) 5.7566(3) 7.7084(4) 236.21(2) 4 8.471
5.2978(3) 5.7574(3) 7.6786(4) 234.21(2) 4 8.589
5.2538(3) 5.7454(3) 7.6254(5) 230.17(2) 4 8.941
23.303
26.356
27.946
31.307
33.282
37.341
504 0.04 × 0.03 × 0.02 4.50-32.52
512 0.04 × 0.04 × 0.03 4.58-36.34
516 0.03 × 0.02 × 0.02 4.61-32.61
524 0.04 × 0.02 × 0.02 4.65-33.12
528 0.04 × 0.02 × 0.02 4.67-33.15
536 0.03 × 0.02 × 0.02 4.71-35.22
4000 481 (Rint ) 0.0478) 1.141
5218 618 (Rint ) 0.0341) 1.161
4138 463 (Rint ) 0.0371) 1.105
4334 475 (Rint ) 0.0353) 1.115
4307 469 (Rint ) 0.0299) 1.141
4383 551 (Rint ) 0.0483) 1.067
R1 ) 0.0290, wR2 ) 0.0567 R1 ) 0.0356, wR2 ) 0.0588 2.327 and -1.890
R1 ) 0.0221, wR2 ) 0.0440 R1 ) 0.0252, wR2 ) 0.0447 1.497 and -2.088
R1 ) 0.0282, wR2 ) 0.0623 R1 ) 0.0309, wR2 ) 0.0632 1.894 and -2.755
R1 ) 0.0295, wR2 ) 0.0553 R1 ) 0.0330, wR2 ) 0.0564 3.595 and -3.786
R1 ) 0.0246, wR2 ) 0.0569 R1 ) 0.0255, wR2 ) 0.0574 2.023 and -2.296
R1 ) 0.0295, wR2 ) 0.0500 R1 ) 0.0355, wR2 ) 0.0512 2.442 and -2.692
measured at 294(1) K on a Bruker SMART APEX diffractometer (Mo KR 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 corrections were carried out with SAINT+.33 Final unit cell parameters were determined by least-squares refinement of 1549 reflections with I > 5σ(I) from the data set. Analysis of the data showed negligible crystal decay during data collection. An empirical absorption correction was applied with SADABS.33 LaRhO3 adopts the GdFeO3 orthorhombic perovskite structure type, space group Pbnm. This structural model was refined by full-matrix least-squares against F2 with SHELXTL.34 All atoms were refined with anisotropic displacement parameters. Refinement of the site occupation factors for the metal atoms showed no significant deviation from unity occupancy. The largest residual electron density peaks remaining after the final refinement cycle were +2.33 and -1.89 e‚Å-3, located
