Crystal Growth and Structural Characterization of a New Series of

Crystal Growth and Structural Characterization of a New Series of Rare Earth Palladates, LnNaPd6O8 (Ln ) Tb - Lu, Y) Samuel J. Mugavero III, Mark D. Smith, and Hans-Conrad zur Loye* Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 494–500

ReceiVed May 15, 2007; ReVised Manuscript ReceiVed September 18, 2007

ABSTRACT: Single crystals of a new series of lanthanide containing palladates, LnNaPd6O8 (Ln ) Tb - Lu, Y), were grown from sodium hydroxide fluxes in sealed silver tubes. The compounds are isostructural and crystallize in the Pm3j space group with the lattice parameter a ) 5.75650(10)-5.72500(10) Å. The crystal structure consists of slabs of LnO8 and NaO8 cubes bridged together by PdO4 square planes. The Ln and Na atoms are disordered within the slabs in all cases except in LuNaPd6O8, where a 1:1 ordered arrangement of LnO8 and NaO8 cubes is achieved. The amount of A-site disorder decreases as the ionic radius of the lanthanide cation decreases from Tb - Lu. Introduction The design of new oxide phases can be approached in a number of different ways, the most common being the incremental modification of known compositions and structures, where one element is either fully or partially substituted for by another. This very reliable approach relies on postulating the existence of specific incremental compositions, where considerations include matching the coordination preference, size, and oxidation states of the elements being substituted for one another. A more complex, but eminently more interesting, approach is based on the replacement of one element by two dissimilar elements whose average properties match the first. One example of such a substitution involves the exchange of two divalent cations by one monovalent and one trivalent cation. For example, (NaLa2)NaPtO6 was the first example of a 2Hperovskite related oxide in which the A-site substitution of a divalent alkaline earth metal cation by a lanthanide and an alkali metal cation was accomplished.1 Since then, additional A-site substituted 2H-perovskite related oxides with the general formula (A3-xNax)NaBO6 (A ) La, Pr, Nd; B ) Rh, Pt) have been reported.2 A more complex variation on this theme involves the substitution of two divalent cations by one monovalent and one trivalent cation with concomitant charge ordering of the monoand trivalent cations. For example, GdNaIrO4 is related to Ca2IrO4, via the substitution of two calcium cations for one gadolinium and one sodium cation with charge ordering of the latter two.3 On the whole, there are quite a number of such substitutions that can be achieved, including (2Ca2+ ) Ln3+ + Na+), (2Sr2+ ) Ln3+ + K+), and (2Ba2+ ) Ln3+ + K+) for differently sized lanthanides. Traditionally, to achieve such compositional changes one has relied on solid state powder synthesis to prepare materials with new compositions, where one or more of the constituent elements from a previously prepared and structurally characterized phase have been substituted. While an excellent approach for preparing new materials, it does rely on a starting point in the form of a known structure or known compositions. In this regard, crystal growth from high-temperature solutions can help, as it is an effective technique to prepare new compositions with * Author to whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Scanning electron micrographs of (a) TbNaPd6O8 and (b) LuNaPd6O8.

Figure 2. Cartoon illustrating the Ln/Na antisite disorder model utilized in the single crystal X-ray diffraction structure solutions and a table listing the amount of mixing (x) on each site. The amount of site mixing (x) was determined by averaging the single crystal X-ray diffraction data sets of three different single crystals for each composition.

new structures whose existence was not previously conceived by any deductive reasoning. This approach becomes important when one intends to prepare complex oxides, defined here as oxides having more than three constituent elements, and where it can be argued that structural complexity increases concomitantly with the incorporation of additional elements into the final product. Furthermore, this approach is also suitable for obtaining

10.1021/cg070448c CCC: $40.75  2008 American Chemical Society Published on Web 01/19/2008

Crystal Growth of New Series of Rare Earth Palladates

Crystal Growth & Design, Vol. 8, No. 2, 2008 495

Table 1. Crystallographic Data and Structural Refinement for LnNaPd6O8 (Ln ) Tb - Lu, Y) empirical formula -1

formula weight (g mol ) space group unit cell dimensions a (Å) V (Å3) Z density (calculated) (g cm-3) absorption coefficient (mm-1) F(000) crystal size (mm3) θmax (deg) index ranges reflections collected independent reflections goodness-of-fit on F2 R indices (all data) largest peak and diffraction hole (e- Å-3)

TbNaPd6O8 948.31 Pm3j

951.89 Pm3j

5.75650(10) 190.755(6) 1 8.255 23.033 416 0.03 × 0.02 × 0.02 36.08 -9 e h e 9, -9 e k e 7, –8 e l e 5 1400 198 [R(int) ) 0.0315] 1.239 R1 ) 0.0416, wR2 ) 0.0994 5.010/-2.249

5.74890(10) 190.000(6) 1 8.319 23.650 417 0.05 × 0.04 × 0.03 40.19 -10 e h e 8, -8 e k e 10, -9 e l e 8 4001 250 [R(int) ) 0.0290] 1.301 R1 ) 0.0310, wR2 ) 0.0676 2.738/-1.899

empirical formula formula weight (g mol-1) space group unit cell dimensions a (Å) V (Å3) Z density (calculated) (g cm-3) absorption coefficient (mm-1) F(000) crystal size (mm3) θmax (deg) index ranges reflections collected independent reflections goodness-of-fit on F2 R indices (all data) largest diffraction peak and hole (e- Å-3)

HoNaPd6O8 956.65 Pm3j



TmNaPd6O8

YbNaPd6O8

958.32 Pm3j

962.43 Pm3j


5.72990(10) 188.123(6) 1 8.495 26.385 421 0.08 × 0.06 × 0.06 42.35 -10 e h e 10, -9 e k e 8 -8 e l e 10 3963 279 [R(int) ) 0.0290] 1.276 R1 ) 0.0228, wR2 ) 0.0401 2.957/-1.566


ErNaPd6O8

954.32 Pm3j


DyNaPd6O8

LuNaPd6O8

YNaPd6O8

964.36 Pm3j

878.38 Pm3j

5.72500(10) 187.640(6) 1 8.534 27.146 422 0.08 × 0.08 × 0.08 42.40 -10 e h e 9, -7 e k e 10, –9 e l e 8 2873 269 [R(int) ) 0.0286] 1.299 R1 ) 0.0249; wR2 ) 0.0501 2.192/-1.905

5.74640(10) 189.753(6) 1 7.686 21.521 390 0.04 × 0.04 × 0.04 40.21 -9 e h e 10, -8 e k e 10, –9 e l e 10 4232 250 [R(int) ) 0.0353] 1.201 R1 ) 0.0248, wR2 ) 0.0594 2.549/-0.919

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Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for LnNaPd6O8 (Ln ) Tb - Lu, Y)a TbNaPd6O8 Tb(1) Na(1a) Tb(1b) Na(1) Pd(1) O(1) DyNaPd6O8 Dy(1) Na(1a) Dy(1b) Na(1) Pd(1) O(1) HoNaPd6O8 Ho(1) Na(1a) Ho(1b) Na(1) Pd(1) O(1) ErNaPd6O8 Er(1) Na(1a) Er(1b) Na(1) Pd(1) O(1) TmNaPd6O8 Tm(1) Na(1a) Tm(1b) Na(1) Pd(1) O(1) YbNaPd6O8 Yb(1) Na(1a) Yb(1b) Na(1) Pd(1) O(1) LuNaPd6O8 Lu(1) Na(1) Pd(1) O(1) YNaPd6O8 Y(1) Na(1a) Y(1b) Na(1) Pd(1) O(1) a

occupancy

x

y

z

Ueq

0.842(8) 0.158(8) 0.158(8) 0.842(8) 1 1

0 0 ½ ½ 0.2513(3) 0.2442(10)

0 0 ½ ½ ½ 0.2442(10)

0 0 ½ ½ 0 0.2442(10)

0.006(1) 0.006(1) 0.004(1) 0.004(1) 0.009(1) 0.009(1)

0.922(5) 0.078(5) 0.078(5) 0.922(5) 1 1

0 0 ½ ½ 0.2513(3) 0.2430(6)

0 0 ½ ½ ½ 0.2430(6)

0 0 ½ ½ 0 0.2430(6)

0.006(1) 0.006(1) 0.006(1) 0.006(1) 0.007(1) 0.008(1)

0.919(5) 0.081(5) 0.081(5) 0.919(5) 1 1

0 0 ½ ½ 0.2510(1) 0.2427(6)

0 0 ½ ½ ½ 0.2427(6)

0 0 ½ ½ 0 0.2427(6)

0.007(1) 0.007(1) 0.007(1) 0.007(1) 0.007(1) 0.008(1)

0.940(5) 0.060(5) 0.060(5) 0.940(5) 1 1

0 0 ½ ½ 0.2508(1) 0.2417(5)

0 0 ½ ½ ½ 0.2417(5)

0 0 ½ ½ 0 0.2417(5)

0.007(1) 0.007(1) 0.010(2) 0.010(2) 0.007(1) 0.008(1)

0.963(4) 0.037(4) 0.037(4) 0.963(4) 1 1

0 0 ½ ½ ½ 0.2403(5)

0 0 ½ ½ 0.2507(1) 0.2403(5)

0 0 ½ ½ 0 0.2403(5)

0.007(1) 0.007(1) 0.008(2) 0.008(2) 0.006(1) 0.008(1)

0.976(4) 0.024(4) 0.024(4) 0.976(4) 1 1

0 0 ½ ½ 0.2506(1) 0.2400(4)

0 0 ½ ½ ½ 0.2400(4)

0 0 ½ ½ 0 0.2400(4)

0.008(1) 0.008(1) 0.012(2) 0.012(2) 0.007(1) 0.008(1)

1 1 1 1

0 0.5 0.2506(1) 0.2391(5)

0 0.5 0.5 0.2391(5)

0 0.5 0 0.2391(5)

0.01(10) 0.015(2) 0.008(1) 0.01(1)

0.950(8) 0.050(8) 0.050(8) 0.950(8) 1 1

0 0 ½ ½ 0.2510(1) 0.2427(6)

0 0 ½ ½ ½ 0.2427(6)

0 0 ½ ½ 0 0.2427(6)

0.007(1) 0.007(1) 0.007(1) 0.007(1) 0.007(1) 0.008(1)

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

single crystals of substitutionally modified compositions, where a complete understanding of the type of potential substitutions, such as the ones of a divalent cation by a mono- and a trivalent cation discussed above, is important. We have been interested in the discovery of new materials through crystal growth, where our efforts have been focused on studying the reactivity of lanthanide and platinum group metals in alkali metal hydroxides as a means of growing highquality single crystals of new oxide materials.3–16 Within this group of platinum group metal oxides, the chemistry of new palladates grown from alkali metal hydroxides remains relatively unexplored. Several years ago, we reported the hydroxide flux synthesis of CaPd3O4 and SrPd3O4,7 and recently, we succeeded in synthesizing LuNaPd6O8,5 a new material structurally related to the APd3O4 (A ) Ca, Sr) phases,7,17,18 from a sodium hydroxide flux. Most recently, we reported a new series of

ordered lanthanide and alkali metal containing palladates, LnKPdO3 (Ln ) La, Pr, Nd, Sm - Gd)6 from potassium hydroxide fluxes, where one lanthanide cation and one potassium cation was substituted in place of two alkaline earth cations in the A2MO3 (A ) Ca, Sr, Ba; M ) Cu, Pd) structure type.19–25 In an effort to explore the reactivity of the smaller lanthanides (Tb - Lu, Y) and palladium in hydroxide fluxes, and to take advantage of adding structural complexity via elemental substitutions, we have prepared a new series of lanthanide and alkali metal containing palladates, LnNaPd6O8 (Ln ) Tb - Lu, Y) from sodium hydroxide fluxes. This is an example of an even more complex substitution, as the parent structure, CaPd3O4, only contains a single divalent cation, such that the charge ordering of the sodium and lanthanide cations necessitates a doubling of the unit cell. Thus, in this structural series we effectively doubled the unit cell of the APd3O4 (A ) Ca, Sr) alkaline earth metal


Figure 3. CsCl-type connectivity of the LnO8 cubes (garnet) and NaO8 cubes (black) viewed along the z-direction within the unit cell of the LnNaPd6O8 series.

Figure 4. Unit cell of LnNaPd6O8 viewed along the z-direction. The Pd2+ cations (light grey) square planes connect the slabs of LnO8 (garnet) and NaO8 (black) cubes. The dark grey spheres represent oxygen atoms.

palladates to yield the general formula AA′Pd6O8, while taking advantage of the (2Ca2+ ) Ln3+ + Na+) type substitution. The crystal growth and structural characterization of the new palladates LnNaPd6O8 (Ln ) Tb - Lu, Y) are reported herein. Experimental Procedures Crystal Growth. Black, cube-shaped single crystals of LnNaPd6O8 (Ln ) Tb - Lu, Y) were grown from molten sodium hydroxide fluxes. Tb2O3 (prepared by heating Tb4O7 (Alfa Aesar, 99.9%, 0.75 mmol) in a reducing atmosphere at 1000 °C for several days), Dy2O3 (Alfa Aesar, 99.9%, 0.75 mmol), Ho2O3 (Alfa Aesar, 99.9%, 0.75 mmol), Er2O3 (Alfa Aesar, 99.9%, 0.75 mmol), Tm2O3 (Alfa Aesar, 99.9%, 0.75 mmol), Yb2O3 (Alfa Aesar, 99.9%, 0.75 mmol), Lu2O3 (Alfa Aesar, 99.9%, 0.75 mmol), or Y2O3 (Alfa Aesar, 99.9%, 0.75 mmol), Pd metal (Engelhard, 99.9%, 1 mmol) or Pd(NH3)2Cl2 (prepared according to the literature),26 and NaOH (Fisher, ACS reagent, 4.0 g) were loaded into silver tubes that had been previously flame sealed at one end. The

Crystal Growth & Design, Vol. 8, No. 2, 2008 497 other ends of the tubes were crimped and folded three times before placing the tubes upright into a programmable box furnace. The tubes were heated to 700 °C in 1 h, held at that temperature for 24 h, and then cooled to room temperature by shutting off the furnace. The crystals were removed from the flux matrix by dissolving the flux in water and isolating the crystals by vacuum filtration. Scanning Electron Microscopy. Single crystals were analyzed by scanning electron microscopy using an FEI Quanta environmental scanning electron microscope (ESEM) instrument utilized in the low vacuum mode. Scanning electron micrographs of single crystals of TbNaPd6O8 and LuNaPd6O8 are shown in Figure 1, panels a and b, respectively and are representative of the crystal morphology observed for the entire LnNaPd6O8 (Ln ) Tb - Lu, Y) series. Energy dispersive spectroscopy verified the presence of Ln, Na, Pd, and O in all of the phases and within the detection limits of the instrument, confirmed the absence of extraneous elements, such as silver. Structure Determination. Crystals of LnNaPd6O8 (Ln ) Tb - Lu, Y) formed as black reflective cubical blocks. X-ray diffraction intensity data measurements were performed at 294(2) K using a Bruker SMART APEX diffractometer (Mo KR radiation, λ ) 0.71073 Å.27 Highly redundant data sets were collected to 2θmax of typically 80° to 85°. The raw area detector data frames were integrated and corrected for Lorentz-polarization effects with SAINT+.27 The final unit cell parameters for each data set were determined by least-squares refinement of all reflections with I > 5σ(I) from each data set. The data were corrected for absorption effects with SADABS.27 Full-matrix leastsquares refinement against F2 and difference Fourier calculations were performed with SHELXTL.28 The compounds crystallize in the space group Pm3j (No. 200). Systematic absences in each intensity data set clearly excluded screw axis and glide plane symmetry elements. The compounds are isostructural with the previously described Ln ) Lu compound. Difficulties during refinement were encountered for all compounds except Ln ) Lu. The problem took the form of a zero or vanishingly small displacement parameter for the 1b site when assigned as 100% sodium, as in LuNaPd6O8.5 Refinement of the Na1 (site 1b) occupation factor (hereafter sof) always showed an increase in sodium occupancy equivalent to ca. 120% of a sodium atom for Ln ) Yb to ca. 255% of a sodium atom for Ln ) Tb. The increase in scattering power from this site increased smoothly with Ln radius. Problematically, in these cases the sofs for the other atomic sites showed no significant deviation from unity occupancy. To exclude the possibility of an anomalous crystal, data sets from a minimum of three different crystals for each Ln were collected. All showed similar results. Additionally, qualitative SEM elemental analysis for each compound detected the presence of only Ln, Na, Pd for each, along with the absence of other elements (e.g., Ag+ from the reaction tube) within the detection limits of the SEM instrument. The best model to describe the atomic distribution in these compounds is an antisite disorder model where Ln3+ substitutes for Na+ on the 1b site, compensated by an equal substitution of Na+ onto the Ln3+ 1a site. It was necessary to introduce Na+ mixing onto the Ln3+ (1a) site to maintain charge balance in the systems, even though the deviation from unity occupancy of the Ln3+ 1a site sofs was small or zero. This model resulted in the most chemically and crystallographically reasonable model that preserved charge neutrality. The model also agrees with the SEM data, which gave an equal Ln/Na atomic percentage in each compound. The lack of an apparent decrease in the Ln (1a) sof may be due to the small percentage of admixture of a light atom (Na) onto a heavy atom (Ln) site. For the final refinements, one occupancy parameter was refined to describe both sites, with the total occupancy of each site constrained to sum to unity, that is, occupancy (Ln1, site 1a) ) x and occupancy (Na1A, site 1a) ) 1 - x, while, simultaneously, occupancy (Na1, site 1b) ) x and occupancy (Ln1B, site 1b) ) 1 - x. The Ln/Na site mixing and a table of x values are illustrated in Figure 2. In the case of the three data sets collected for Ln ) Lu, only one crystal showed more than a 2% extent of Lu/ Na mixing. Therefore, all Ln ) Lu crystals were refined as fully ordered. It must be noted that a proportion of Lu/Na mixing just below the limit of reliability of the X-ray data mixing may exist. For all compounds, Pd and O atoms were refined with anisotropic displacement parameters. Ln and Na atoms have crystallographically imposed m-3 site symmetry (Th point symmetry) and are therefore inherently isotropic. All crystals were refined as merohedral twins emulating the higher Laue class m-m, with the matrix (010 100 001j). The twin fractions in each case refined to near 50%. A summary of

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Table 3. Selected Interatomic Distances (Å) and Bond Angles (°) for LnNaPd6O8 (Ln ) Tb - Lu, Y) TbNaPd6O8 Tb(1)/Na(1a)-O(1) × 8 Na(1)/Tb(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

DyNaPd6O8 2.435(10) 2.550(10) 2.036(6) 87.3(5) 177.7(4)

Dy(1)/Na(1a)-O(1) × 8 Na(1)/Dy(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

2.415(5) 2.559(5) 2.032(2) 86.8(3) 177.3(2)

Er(1)/(1a)-O(1) × 8 Na(1)/Er(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

HoNaPd6O8 Ho(1)/Na(1a)-O(1) × 8 Na(1)/Ho(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

ErNaPd6O8

TmNaPd6O8 Tm(1)/Na(1a)-O(1) × 8 Na(1)/Tm(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

2.402(5) 2.568(5) 2.031(2) 86.1(2) 177.0(2)

YbNaPd6O8 2.386(5) 2.580(5) 2.030(3) 85.5(2) 176.61(19)

Yb(1)/Na(1a)-O(1) × 8 Na(1)/Yb(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

2.371(5) 2.587(5) 2.027(3) 85.0(2) 176.29(18)

Y(1)/Na(1a)-O(1) × 8 Na(1)/Y(1b)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

LuNaPd6O8 Lu(1)-O(1) × 8 Na(1)-O(1) × 8 Pd(1)-O(1) × 4 cis-O(1)-Pd(1)-O(1) trans-O(1)-Pd(1)-O(1)

2.420(6) 2.559(6) 2.034(3) 86.8(3) 177.3(2)

2.382(4) 2.580(4) 2.028(2) 85.4(2) 176.58(16)

YNaPd6O8

the crystallographic data and refinement statistics is listed in Table 1, and the atomic coordinates with equivalent isotropic displacement parameters are listed in Table 2. Further details of the crystal structure investigations can be obtained from the Fachinformationszentrum Karlsruhe, 76344 EggensteinLeopoldshafen, Germany (fax: (49) 7247-808-666; e-mail: crystdata@ fiz-karlsruhe.de) on quoting the depository number CSD - 416619, 418038 - 418044.

Results and Discussion Single crystals of LnNaPd6O8 (Ln ) Tb - Lu, Y) were grown from molten sodium hydroxide flux reactions in sealed silver

2.416(5) 2.561(5) 2.033(3) 86.6(2) 177.39(18)

tubes. As we previously reported, LuNaPd6O85 is structurally related to APd3O4 (A ) Ca, Sr)7,17,18 and NaPt3O4,29 which crystallize in the cubic space group Pm3j. The crystal structures of LnNaPd6O8 (Tb - Yb, Y) are isostructural with LuNaPd6O8 and can be described as being of the cesium chloride-type, where the lanthanide and sodium atoms are both arranged in a cubic coordination environment. Eight lanthanide atoms are located at the corners of the cubic unit cell and each corner-share one sodium atom located in the center of the unit cell as shown in Figure 3. As shown in Figure 4, the palladium atoms are situated in a square planar coordination environment, corner-shared

Figure 5. Cartoon displaying the change in cell volume for both the AA′PdO3 and AA′Pd6O8 structures as Ln3+ and M+ (alkali metal) cations are substituted in place of two M2+ (alkaline earth metal) cations. On the basis of cell volume, phases that have been synthesized are indicated by a star (g), and phases that have not been synthesized and should not form are indicated by a “no symbol” (L).


perpendicularly with each other and simultaneously edge-shared to the NaO8 and LnO8 cubes. The Na-O and Ln-O bond distances of the NaO8 and LnO8 cubes located on the disordered crystallographic 1a and 1b sites range from 2.371(5) to 2.587(5) Å with the shortest Ln-O and Na-O bond distances existing in the lutetium analogue. The Pd-O bond lengths in the PdO4 square planes range from 2.027(3) Å in the lutetium analogue (the smallest lanthanide in this series) to 2.036(6) Å in the terbium analogue (the largest lanthanide in this series). The PdO4 square planes each deviate slightly from the ideal square planar geometry in that the two cis-O-Pd-O bond angles range from 85.0(2)° to 94.9(2)° and the trans-O-Pd-O bond angle ranges from 176.29(18)° to 177.4(4)°. The deviation from ideal geometry can be attributed to the fact that the corner-shared NaO8 and LnO8 cubes are of different dimensions, which causes the square planes to be slightly puckered. The Ln-O, Na-O, and Pd-O interatomic distances and the O-Pd-O bond angles are compiled in Table 3. The LnNaPd6O8 (Ln ) Tb - Lu, Y) phases differ from the APd3O4 (A ) Ca, Sr) phases,7,17,18 in that, in place of two Ca2+ or Sr2+ cations, a 1:1 substitution of Ln3+ and Na+ cations is observed. The overall charge on the A-site remains +4 but is maintained by one Ln3+ and one Na+ atom, and we observe an effective doubling of the unit cell. The general formula AA′Pd6O8, where A and A′ are two divalent cations or the combination of one trivalent and one monovalent cation, can be utilized to describe the stoichiometry. In the Ln1-xNa1+xIrO4 (Ln ) Gd - Er, Y; x ) 0.04 – 0.25) series of lanthanide containing iridates,3 the substitution of Ln3+ and Na+ cation in place of two Ca2+ cations resulted in adjustments in the overall composition to maintain structural stability and thus an increase in the oxidation state of iridium. In contrast, the LnNaPd6O8 (Ln ) Tb - Lu, Y) phases maintain structural stability through an antisite mixing (disorder) on the 1a and 1b sites, where Ln3+ substitutes for Na+ on the 1b site, compensated by an equal substitution of Na+ onto the Ln3+ 1a site. The overall 1:1 stoichiometry of Ln/Na atoms is maintained, and the oxidation state of palladium remains unchanged at +2. The antisite mixing is present in all phases of this series, except in the lutetium analogue LuNaPd6O8, where a 1:1 ordered arrangement of LuO8 and NaO8 cubes is observed. The difference in size of the Na+ (1.18 Å) and Lu3+ (0.977 Å) cations30 is large and most likely the dominant contributor for ordering. When analyzing the structural relationship that exists within the Ln3+ - M+ - Pd - O (Ln3+ ) lanthanide; M+ ) Na, K) phase space, where A-site substitutions into the A2PdO3 (A ) Sr, Ba)17,20 and APd3O4 (A ) Ca, Sr)7,17,18 structure types have resulted in the formation of the LnKPdO3 (Ln ) La, Pr, Nd, Sm - Gd)6 and LnNaPd6O8 (Ln ) Tb Lu, Y) compositions, we can consider the existence of a structure–stability limit based on the size of the A-cations. The cartoon in Figure 5 offers a visual representation of the change in cell volume as a result of varying the size of the A- and A′-cations. For the larger cations, where the AA′PdO3 structure type is stable, the Ba2PdO3 phase roughly represents the upper limit of stability because barium is the largest A-site cation to be incorporated into this structure type and has the largest cell volume. The ionic radii of Ba2+ and Sr2+ cations in a seven coordinate environment are 1.38 and 1.21 Å, respectively; therefore, the range of the average ionic radii on the A-site of the LnKPdO3 (1.26–1.23 Å) phases ideally falls between 1.38 and 1.21 Å. For the smaller cations, where the AA′Pd6O8 structure type is stable, LuNaPd6O8 roughly represents the lower limit of stability because it has the

Crystal Growth & Design, Vol. 8, No. 2, 2008 499 Table 4. Cell Volumes of the A2Pd6O8-type Oxides composition

cell volume (Å3)

Sr2Pd6O8 (SrPd3O4) TbNaPd6O8 DyNaPd6O8 YNaPd6O8 HoNaPd6O8 ErNaPd6O8 Ca2Pd6O8 (CaPd3O4) TmNaPd6O8 YbNaPd6O8 LuNaPd6O8

196.33(4) 190.755(6) 190.000(6) 189.753(6) 189.475(6) 188.991(6) 188.68(2) 188.576(6) 188.123(6) 187.640(6)

smallest average ionic radius on the A-site and thus the smallest cell volume of this structure type. The average ionic radii of the A and A′-site cations in an eight coordinate environment in the AA′Pd6O8 compounds ranges from 1.07 Å in LuNaPd6O8 to 1.26 Å in Sr2Pd6O8, and the cell volumes range from 187.640(6) Å3 in LuNaPd6O8 to 196.44(3) Å3 in Sr2Pd6O8 (Table 4). As a consequence of these structural limitations, the analogous LnNaPd6O8 (Ln ) La - Gd) and LnKPd6O8 (Ln ) La - Lu) should not be stable because the average ionic radii on the A-site and the cell volumes in these compounds fall above that of Sr2Pd6O8, which appears to be the upper limit of this structure type. Similarly, the magnesium analogue Mg2Pd6O8 (MgPd3O4) has not been reported and should not be stable because the ionic radius of Mg2+ (0.89 Å) appears to be too small to maintain stability within this structure type. Interestingly, the strontium analogues, Sr2PdO3 and SrPd3O4, exist for both structure types and thus represent a halfway point from both a cation size and a cell volume perspective, and thus, the structural stability limit for the two structure types can be placed in between Sr2PdO3 and SrPd3O4. Conclusion A new series of complex palladium oxides, LnNaPd6O8 (Ln ) Tb - Lu, Y), have been prepared from sodium hydroxide fluxes. The materials consist of slabs of LnO8 and NaO8 cubes, where in all phases except for LuNaPd6O8 disorder exists on the Ln and Na sites within the slabs. Interestingly, LuNaPd6O8 represents the first example of an ordered substitution of a lanthanide metal and an alkali metal for an alkaline earth metal on the A-site of a platinum group metal oxide. Acknowledgment. Financial support from the Department of Energy through Grant DE-FG02-04ER46122 and the National Science Foundation through Grant DMR:0450103 is gratefully acknowledged.

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CG070448C

Crystal Growth and Structural Characterization of a New Series of

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