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HRTEM, SAED and XRD investigations of RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd)
Materials Research Bulletin 53 (2014) 257–265
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HRTEM, SAED and XRD investigations of RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) Hamdi Ben Yahia a, *, Ute Ch. Rodewald a , Khalid Boulahya b , Rainer Pöttgen a a b
Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany Departamento de Química Inorgánica Facultad de Químicas Universidad Complutense, 28040 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 August 2013 Received in revised form 15 February 2014 Accepted 26 February 2014 Available online 28 February 2014
The new compounds RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) were synthesised by solid state reaction via a salt flux route and investigated by HRTEM, SAED, and single crystal X-ray diffraction. The samples crystallise with a tetragonal cell, space group P42/mnm and Z = 2. Their crystal structure consists of an alternation of [RE4O4]4+ and [ClAsO4]4– layers. The [RE4O4]4+ layer contains ORE4/4 tetrahedra which share common edges. The anions AsO43– and Cl– are located between these layers in disordered manner. SAED and HRTEM experiments confirmed this structural model and enabled us to propose an ordered model for the [ClAsO4]4– layers. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: B. Crystal growth C. X-ray diffraction C. Transmission electron microscopy (TEM) D. Crystal structure D. Layered compounds
1. Introduction The discovery of superconductivity in LaFeO1 – xFx [1] motivated broad research programs on quaternary pnictide oxides RETPnO (RE = rare earth metal, T = electron-rich transition metal; Pn = P, As, Sb) in the solid state physics and chemistry community [2–4]. Larger single crystals for property measurements of these materials can be grown either with the tin flux technique or via salt fluxes. We have repeatedly used NaCl/KCl fluxes for the growth of REZnPO, REZnAsO, and REZnSbO crystals in mm size [5–7]. In several cases we observed by-products with related structural motifs which, however, did not contain the transition metal. The quaternary compounds La3OCl[AsO3]2 [8], Nd5O4Cl[AsO3]2 [9], Ce3OCl[AsO3]2 [10], Pr5O4Cl[AsO3]2 [11], La3OBr[AsO3]2 [12], La4O4 [AsO4]Br and Pr4O4[AsO4]Br [13] all show oxygen-centred tetrahedral units which are separated and charge-balanced by the halide and arsenite, respectively arsenate anions. The structures of La4O4[AsO4]Br and Pr4O4[AsO4]Br [13] show the closest structural relationship with the pnictide oxides RETPnO. All ORE4 tetrahedra share four common edges, leading to the same layers as in the pnictide oxides. We have started a more systematic study of these complex materials with respect to substitutions of the anionic substructure. Depending on the size and combination
* Corresponding author. E-mail address: [email protected] (H. Ben Yahia). http://dx.doi.org/10.1016/j.materresbull.2014.02.025 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
of PO43–, AsO43–, Cl–, and Br– we observed different ordering patterns between the [RE4O4]4+ layers. Herein we report on the salt flux crystal growth of the series of RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) compounds and their characterisation using X-Ray diffraction, high resolution transmission microscopy (HRTEM) and selected area electron diffraction (SAED). 2. Experimental 2.1. Synthesis The title compounds were prepared by solid state reaction, from a mixture of re-sublimed arsenic (Sigma–Aldrich, 99.999%), rare earth oxides RE2O3 (RE = La, Nd, Sm–Gd) (Chempur, >99.99%) or Pr6O11 (Chempur, >96%) and a NaCl (Merck, >99.5%)/KCl (Chempur, 99.9%) salt flux (1:1 molar ratio) with a 1:2:15 molar ratio, respectively. The mixtures were put in alumina tubes, which were sealed under vacuum in silica tubes. To ensure a total reactivity of the arsenic and avoid its sublimation, the tubes were heated at 500 C for 24 h, at 600 C for 12 h, and then at 900 C for 72 h. After washing the mixtures with distilled water, we obtained mainly small RE4O4[AsO4]Cl plates and REAsO4 needles (RE = La, Nd, Sm–Gd). The former crystals were then mixed again with salt flux and heated at 900 C for 4 days. With a relatively fast decrease of the temperature at a rate of 15 C/h to r.t. we obtained larger single crystals of the title compounds. One should notice, during the second treatment, a change in the colour of Pr4O4[AsO4]Cl from
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Table 1 Cell parameters of the polycrystalline RE4O4[AsO4]Cl samples. Compounds
IRa
a (Å)
c (Å)
V (Å3)
La4O4(AsO4)Cl Pr4O4(AsO4)Cl Nd4O4(AsO4)Cl Sm4O4(AsO4)Cl Eu4O4(AsO4)Cl Gd4O4(AsO4)Cl
1.160 1.126 1.109 1.079 1.066 1.053
5.9950(3) 5.886(2) 5.8678(2) 5.8132(3) 5.8135(5) 5.772(2)
13.5587(8) 13.121(5) 13.053(1) 12.7951(8) 12.740(1) 12.690(4)
487.31(4) 455.1(3) 449.43(3) 432.39(4) 430.60(8) 422.8(2)
a
Ionic radius of RE [8].
were carried out with a Leica 420i scanning electron microscope using PrF3, SmF3, EuF3, GdF3, InAs, and KCl as standards. The experimentally observed compositions were close to the ideal ones (RE4O4[AsO4]Cl). Some impurities were identified to be the compounds REAsO4, REOCl or REAlO3 (reaction with the crucible). Fig. 1. Observed, calculated and difference plots for the XRPD profile refinement of the Sm4O4[AsO4]Cl sample obtained from flux synthesis.
2.3. X-ray diffraction
brown to green was observed, but without any significant change in the structure. Few other attempts to synthesise the RE4O4[AsO4]Cl compounds in air starting from a stoichiometric mixture of RE2O3, RECl3 and NH4H2AsO4 have been performed but led to mixtures of REOCl and REAsO4. Mixtures of re-sublimed arsenic, RE2O3, KClO4 and a NaCl/KCl salt flux (1:1 molar ratio) with a 2:4:1:15 molar ratio, respectively, have been also tested. In order to avoid the explosion of the tubes, we placed successively KClO4, NaCl/KCl, RE2O3 and As and then NaCl/KCl. The tubes were sealed under vacuum in silica tubes and heated at 500 C for 24 h, at 600 C for 12 h, and at 800 C for 48 h. This enabled us to obtain a pure sample of Sm4O4[AsO4]Cl (Fig. 1). 2.2. Electron microprobe analysis Semiquantitative EDX analyses of different single crystals including the ones investigated on the diffractometer (Fig. 2)
The polycrystalline samples were characterized by Guinier patterns (imaging plate detector, Fujifilm BAS-1800) with Cu Ka1 radiation and a-quartz (a = 4.9130, c = 5.4046 Å) as an internal standard (Table 1). Fig. 3 shows the evolution of the cell volume as a function of the ionic radius of the eight coordinated rare earth. Only Sm4O4[AsO4]Cl sample is shown to be pure (Fig. 1). For the other samples, besides the main phase we observed different impurities such as REAsO4, REAlO3 (from a side reaction with the crucible material) and /or REOCl. Crystals suitable for single crystal X-ray diffraction were selected on the basis of the size and the sharpness of the diffraction spots by Laue photographs on a Buerger camera (using white Mo radiation). The data collections were carried out on a Stoe IPDS II diffractometer using Mo Ka radiation. Data processing and first refinements were performed with the Jana2000 program package [14]. A Gaussian-type absorption correction was applied, and the shapes of the crystals were determined with the video microscope of the Stoe CCD. Details about data collections and refinements are summarized in Table 2.
[(Fig._2)TD$IG]
Fig. 2. SEM images of the RE4O4[AsO4]Cl single crystals used for the X-ray data collection.
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3. Results and discussion 3.1. Structure refinement
Fig. 3. Evolution of the cell volume as a function of the ionic radius of the eight coordinated rare earth.
2.4. SAED, HRTEM and EDX analysis Selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) were performed in a JEOL 3000 FEG electron microscope, fitted with a double tilting goniometer stage (22 , 22 ). Local composition was analysed by energydispersive X-ray spectroscopy (EDS) with an Oxford INCA analyser system attached to the above mentioned microscope. Simulated HREM images were calculated by the multislice method using the MacTempas software package [15].
Careful examination of the Sm4O4[AsO4]Cl data set revealed a primitive tetragonal unit cell with high Laue symmetry and the extinction conditions (0 k l only for k + 1 = 2n, 0 0 l only observed for l = 2n, h 0 0 only for h = 2n) led to the space groups P42/mnm and P42nm. The structures were solved in the centro-symmetric group P42/mnm. In the case of Sm4O4[AsO4]Cl, most of the atomic sites have been located using the superflip program implemented in the Jana2000 package [14]. The use of difference-Fourier synthesis allowed us to localize the remaining oxygen atoms which led to a non-equilibrated chemical formula Sm4O4[AsO4]2. With isotropic atomic displacement parameters (ADP), the residual factors converged to R(F) = 0.0386 and wR(F2) = 0.0957 for 15 refined parameters with non-significant difference-Fourier residues. The refinement of the occupancies of the arsenic and the four oxygen atoms in its surrounding showed that they are half occupied. Then, considering the As, O3 and O4 positions half occupied and by using the difference-Fourier synthesis we were able to localize the remaining chlorine atom. The refinement of its occupancy showed that it is also half occupied. This led to the final Sm4O4[AsO4]Cl equilibrated chemical formula and the reliability factors listed in Table 2. The structures of all the other compounds (with Pr, Eu and Gd) have been solved using the structural model of Sm4O4[AsO4]Cl. Disorder of AsO43– and Cl– within the [AsO4Cl]4– layer and large ADPs for the chlorine atoms have been observed in all cases. The refined atomic positions and anisotropic displacement parameters (ADPs) are given in Tables 3 and 4, respectively. Further details on the structure refinement may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry Nos. CSD-426429; 426430
Table 2 Crystallographic data and structure refinement for RE4O4[AsO4]Cl. Formula
Pr4O4(AsO4)Cl
Pr4O4(AsO4)Cl
Sm4O4(AsO4)Cl
Eu4O4(AsO4)Cl
Gd4O4(AsO4)Cl
Crystal colour MW (g mol1) Crystal system Space group Parameters (Å)
Brown 802 Tetragonal P42/mnm a = 5.8937(7) c = 13.158(3) 457.06(12) 2 0.065 0.033 0.012 5.82 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan 7 h 8, k = 8, l = 18 3.1, 30.53 24.8 Gaussian 0.394/0.747 7579 0.064 412 293 F2 700 0.025/0.057 34 1.14 w = 1/(s 2(I) + 0.00116I2) 1.05, +1.11 1800(200)
Green 802 Tetragonal P42/mnm a = 5.8965(9) c = 13.179(3) 458.21(15) 2 0.052 0.048 0.014 5.81 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan h = 8, k = 8, l = 18 3.09, 30.48 24.8 Gaussian 0.326/0.753 4589 0.066 412 273 F2 700 0.026/0.054 34 1.03 w = 1/(s 2(I) + 0.0009I2) 1.06, +0.91 1690(180)
Colourless 839.8 Tetragonal P42/mnm a = 5.8248(9) c = 12.825(3) 435.15(13) 2 0.080 0.071 0.006 6.41 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan 7 h 8, k = 8, l = 18 3.17, 31.84 30.6 Gaussian 0.219/0.833 7709 0.038 437 367 [I 3s (I)] F2 724 0.023/0.065 34 1.20 w = 1/(s 2(I) + 0.0021I2) 1.71, +1.62 1400(200)
Colourless 846.2 Tetragonal P42/mnm a = 5.808(3) c = 12.726(9) 429.3(4) 2 0.075 0.075 0.10 6.54 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan h = 8, k = 8, l = 18 3.2, 31.83 32.9 Gaussian 0.268/0.796 11109 0.07 433 388 F2 732 0.032/0.066 34 1.20 w = 1/(s 2(I) + 0.0018I2) 2.09, +2.09 1500(200)
Colourless 867.4 Tetragonal P42/mnm a = 5.7827(4) c = 12.6623(12) 423.42(6) 2 0.064 0.040 0.014 6.80 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan h = 8, k = 8, l = 18 3.22, 31.88 35.1 Gaussian 0.030/0.354 8812 0.10 428 385 F2 740 0.024/0.056 34 1.19 w = 1/(s 2(I) + 0.0009I2) 1.22, +1.45 3490(190)
V (Å3) Z Crystal size (mm) Density calc. (g cm3) Crystal shape Temperature (K) Diffractometer Monochromator Radiation Scan-mode hkl umin, umax (deg) Linear absorption coeff. (mm1) Absorption correction Tmin/Tmax No. of reflections Rint No. of independent reflections Reflections used [I 2s (I)] Refinement F(000) R factors R(F)/wR(F2) No. of refined parameteres g.o.f. Weighting scheme Diff. Fourier residues (e/Å3) Extinction coefficient
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Table 3 Atom positions and isotropic displacement parameters (Å2) for RE4O4[AsO4]Cl.
Pr4O4[AsO4]Cl Pr1 O1 O2 As O3 O4 Cl Sm4O4[AsO4]Cl Sm O1 O2 As O3 O4 Cl Eu4O4[AsO4]Cl Eu O1 O2 As O3 O4 Cl Gd4O4[AsO4]Cl Gd O1 O2 As O3 O4 Cl
Wyckoff
Occupancy
x
y
z
Ueq
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23519(7) 1/2 1/2 0.2918(4) 0.252(4) 0.181(2) 0.186(3)
0.23519(7) 0 1/2 0.7082(4) 0.424(3) 0.819(2) 0.814(3)
0.33241(4) 1/4 0.2675(18) 1/2 1/2 0.3949(15) 1/2
0.00899(16) 0.019(5) 0.016(4) 0.0088(7) 0.017(4) 0.018(4) 0.077(6)
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23320(5) 1/2 1/2 0.2932(3) 0.2506(18) 0.1768(13) 0.1824(13)
0.23320(5) 0 1/2 0.7068(3) 0.4200(18) 0.8232(13) 0.8176(13)
0.33096(2) 1/4 0.2736(7) 1/2 1/2 0.3936(8) 1/2
0.00669(12) 0.0115(17) 0.0136(16) 0.0058(4) 0.013(3) 0.0122(18) 0.068(3)
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23279(5) 1/2 1/2 0.2942(3) 0.2480(18) 0.1755(14) 0.1830(14)
0.23279(5) 0 1/2 0.7058(3) 0.4196(18) 0.8245(14) 0.8170(14)
0.33054(3) 1/4 0.2745(8) 1/2 1/2 0.3927(8) 1/2
0.00868(13) 0.0137(18) 0.0162(18) 0.0079(4) 0.012(3) 0.0145(19) 0.070(4)
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23245(5) 1/2 1/2 0.2946(2) 0.2490(14) 0.1761(10) 0.1812(11)
0.23245(5) 0 1/2 0.7054(2) 0.4164(14) 0.8239(10) 0.8188(11)
0.330319(18) 1/4 0.2748(5) 1/2 1/2 0.3927(6) 1/2
0.00937(11) 0.0143(12) 0.0151(11) 0.0082(3) 0.0147(19) 0.0153(13) 0.079(3)
Table 4 Anisotropic displacement parameters (Å2) for RE4O4[AsO4]Cl. The anisotropic displacement factor exponent takes the form: –2p2[(ha*)2U11 + + 2hka*b*U12].
Pr4O4[AsO4]Cl Pr1 O1 O2 As O3 O4 Cl Sm4O4[AsO4]Cl Sm O1 O2 As O3 O4 Cl Eu4O4[AsO4]Cl Eu O1 O2 As O3 O4 Cl Gd4O4[AsO4]Cl Gd O1 O2 As O3 O4 Cl
U11
U22
U33
U12
U13
U23
0.0082(3) 0.007(4) 0.009(4) 0.0092(10) 0.018(8) 0.015(5) 0.094(10)
0.0082(3) 0.007(4) 0.009(4) 0.0092(10) 0.022(7) 0.015(5) 0.094(10)
0.0106(3) 0.045(14) 0.030(11) 0.0082(14) 0.012(7) 0.024(10) 0.043(10)
0.0001(2) 0 0.000(4) 0.0022(11) 0.001(8) 0.000(5) 0.043(12)
0.0007(2) 0 0 0 0 0.006(5) 0
0.0007(2) 0 0 0 0 0.006(5) 0
0.0056(2) 0.007(2) 0.0068(19) 0.0060(5) 0.021(5) 0.015(3) 0.087(7)
0.0056(2) 0.007(2) 0.0068(19) 0.0060(5) 0.014(4) 0.015(3) 0.087(7)
0.0088(2) 0.021(4) 0.027(4) 0.0053(8) 0.004(4) 0.008(4) 0.030(4)
0.00029(10) 0 0.0004(19) 0.0030(6) 0.003(4) 0.002(3) 0.062(7)
0.00038(7) 0 0 0 0 0.005(2) 0
0.00038(7) 0 0 0 0 0.005(2) 0
0.0078(2) 0.008(2) 0.010(2) 0.0082(5) 0.017(5) 0.017(3) 0.091(7)
0.0078(2) 0.008(2) 0.010(2) 0.0082(5) 0.015(5) 0.017(3) 0.091(7)
0.0104(2) 0.024(5) 0.028(4) 0.0073(8) 0.006(4) 0.009(4) 0.028(4)
0.00026(10) 0 0.001(2) 0.0018(6) 0.003(3) 0.002(4) 0.059(8)
0.00048(8) 0 0 0 0 0.004(3) 0
0.00048(8) 0 0 0 0 0.004(3) 0
0.00833(17) 0.0090(16) 0.0092(14) 0.0096(4) 0.023(4) 0.0156(18) 0.106(6)
0.00833(17) 0.0090(16) 0.0092(14) 0.0096(4) 0.011(3) 0.0156(18) 0.106(6)
0.0115(2) 0.025(3) 0.027(3) 0.0054(6) 0.010(3) 0.015(3) 0.027(3)
0.00007(8) 0 0.0008(15) 0.0011(5) 0.004(3) 0.002(3) 0.076(7)
0.00082(5) 0 0 0 0 0.0048(18) 0
0.00082(5) 0 0 0 0 0.0048(18) 0
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Fig. 4. Disordered structure of Sm 4O4[AsO4]Cl.
(Pr4O4[AsO4]Cl), CSD-426431 (Sm4O4[AsO4]Cl), CSD-426427 (Eu4O4[AsO4]Cl), and CSD-426428 (Gd4O4[AsO4]Cl). Different attempts to lower the symmetry (according to the subgroups listed in the International Tables, Suppl. A1) have been performed and it was possible to obtain a perfectly ordered structure using the space group Pmn21, however very large ADPs have been observed for the chlorine atoms and some atoms (O1, O2 and Sm2) exhibited non-positive definite ADP matrix. When only isotropic displacement parameters are refined for these three atoms, the final residual factors converged to the values reported in Table S1. Although, the TEM experiments showed the presence of n-glide planes perpendicular to a and b, which is in agreement with the space group P42/mnm and excludes the space group Pmn21 (see
TEM section), we will describe the ordered Pmn21-model in the next sections. The HRTEM images, which have been taken along the layers, indicate that this ordered Pmn21-model describes very well a major part of the Sm4O4[AsO4]Cl structure. The refined atomic positions and anisotropic ADPs in the ordered Pmn21model are given in Tables S2 and S3, respectively. 3.2. Crystal structure 3.2.1. Disordered RE4O4[AsO4]Cl structure (P42/mnm -model) The crystal structure of the title compounds RE4O4[AsO4]Cl consists of alternating [RE4O4]4+ and [ClAsO4]4– layers (Fig. 4). The [RE4O4]4+ layer contains ORE4/4 tetrahedra which share common
[(Fig._5)TD$IG]
Fig. 5. Different layered structures containing LaO, La2O2 or La3O3 layers as in the title compounds.
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Table 5 Interatomic distances (Å) and bond valence (*: BVS) for RE4O4[AsO4]Cl. Average distances are given in brackets. Distances Pr Pr–O1 (2) Pr–O2 Pr–O2 Pr–O3 Pr–O4 Pr–Cl
As–O4 (2) As–O3 (2) As–Cl
Eu Eu–O1 (2) Eu–O2 Eu–O2 Eu–O3 Eu–O4
B.V.
2.3539(6) 2.362(13) 2.368(9) 2.474(7) 2.606(13) <2.419> 3.336(12)
0.557 0.546 0.537 0.403 0.282 *2.886 0.107 *2.993
1.667(18) 1.695(15) <1.681> 0.883(16) 3.984(16)
1.310 1.215 *5.05
B.V.
2.3105(5) 2.318(3) 2.342(5) 2.428(5) 2.541(8) <2.375> 3.263(6)
0.551 0.540 0.506 0.401 0.296 *2.845 0.115 *2.960
1.667(9) 1.689(11) <1.678> 0.912(8) 3.918(2)
1.310 1.235 *5.09
2.2880(3) 2.2980(18) 2.321(3) 2.399(4) 2.512(6) <2.351> 3.229(5)
0.557 0.542 0.509 0.413 0.304 *2.882 0.120 *3.002
As–O4 (2) As–O3 (2)
1.669(7) 1.692(8) <1.680>
1.303 1.225 *5.06
As–Cl
0.927(6) 3.891(6)
Sm–Cl
As–O4 (2) As–O3 (2) As–Cl
2.2994(14) 2.308(3) 2.333(6) 2.416(5) 2.522(9) <2.362> 3.251(7)
0.560 0.531 0.497 0.397 0.298 *2.843 0.116 *2.959
As–O4 (2) As–O3 (2)
1.678(10) 1.684(11) <1.681>
1.272 1.251 *5.05
As–Cl
0.914(9) 3.920(2)
Eu–Cl
Distances Sm Sm–O1 (2) Sm–O2 Sm–O2 Sm–O3 Sm–O4
Gd Gd–O1 (2) Gd–O2 Gd–O2 Gd–O3 Gd–O4 Gd–Cl
BV = e(r0 – r)/b with the following parameters: b = 0.37, r0 (PrIII–O) = 2.138, r0 (SmIII–O) = 2.090, r0 (EuIII–O) = 2.2.074, r0 (GdIII–O) = 2.071, r0 (PrIII–Cl) = 2.509, r0 (SmIII–Cl) = 2.461, r0 (EuIII–Cl) = 2.454, r0 (GdIII–Cl) = 2.443 and r0 (AsV–O) = 1.767 [16,17].
edges. The chlorine Cl– and the tetrahedrally coordinated As5+ atoms are located between these layers in disordered manner. Although this is a new type of structure, similar [RE4O4]4+ layers have been observed in many other compounds, e. g. in LaOCl, La3O3PO4, LaOFeP, LaOS, La2O2SO4 or La2O2Te (Fig. 5). The interatomic distances and bond valence sums are listed in Table 5. In the RE4O4[AsO4]Cl compounds (RE = Pr, Sm, Eu and Gd), the rare earth cations are coordinated to six oxygen and one chlorine atoms forming a distorted, mono-capped trigonal prism (Fig. 6). The RE–O2 distances within the [RE4O4]4+ layers are very similar to those observed in the layered compounds of Fig. 5. The decrease of the average RE–O distances from 2.419 to 2.351 Å in the RE4O4
Fig. 6. Coordination of RE in the RE4O4[AsO4]Cl compounds (RE = Pr, Sm, Eu and Gd).
[AsO4]Cl compounds (RE = Pr, Sm, Eu and Gd) is in good agreement with the decrease of the rare earth size and with the sum of the ionic radius of O2– and RE3+ [18]. This behaviour is at the origin of the decrease of the cell parameters and consequently of the cell volume (Fig. 3). This induces a decrease of the Cl–As and Cl–O3 distances within the [AsO4Cl]4 layer from 3.984 to 3.891 Å and from 2.936 to 2.835 Å for Pr4O4[AsO4]Cl and Gd4O4[AsO4]Cl, respectively. This may explain why compositions with RE smaller than Gd did not form. The AsO4 tetrahedra are quite regular in shape with average distances ranging from 1.678 to 1.681 Å. These values are slightly lower than the value of 1.717 Å, expected from the sum of the effective ionic radii of the four-coordinated As5+ and O2– [18]. The calculations of the bond valence sums (BVS) for the RE and As atoms are listed in Table 5. They are in very good agreement with the expected 3+ and 5+ values, respectively. 3.2.2. Ordered Sm4O4[AsO4]Cl structure (Pmn21-model) In the Pmn21-model of Sm4O4[AsO4]Cl, a perfect ordering of the AsO43– and Cl– anions between the [RE4O4]4+ layers is observed (Fig. 7a). In the [ClAsO4]4– layer, Cl– is surrounded by four AsO43– tetrahedra with an average As–Cl distance of 4.122 Å (Fig. 7b). Such ordering is different from that in Pr4O4[AsO4]Br (Fig. 7c). The interatomic distances and bond valence sums are listed in Table S4. 3.3. SAED and HRTEM analyses The cationic composition, determined on several small crystallites by energy dispersive X-ray analysis in the electron microscope, is in agreement with the nominal one. Selected area electron diffraction (SAED) was used to fully reconstruct the reciprocal space. Fig. 8 shows the [0 0 1], ½1 1 0 and [0 1 0] zone
[(Fig._7)TD$IG]
Fig. 7. The ordered structure of Sm4O4[AsO4]Cl obtained by lowering the symmetry from P42/mnm (a1 = b1 6¼ c1) to Pmn21 (a2 = c1, b2 = c2 = a1) (a), a view of the [ClAsO4]4– layer (b), and a view of the [BrAsO4]4– layer in Pr4O4[AsO4]Br .
[(Fig._8)TD$IG]
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Fig. 9. HREM image of Sm4O4[AsO4]Cl along [0 0 1] (a) and the corresponding FT (b). Calculated image is depicted at the inset.
Fig. 8. SAED patterns corresponding to Sm4O4[AsO4]Cl, [0 0 1] (a), ½1 1 0 (b) and [0 1 0] (c) zone axes.
axis, all maxima can be indexed on the basis of a tetragonal cell, with parameters a = 5.8 Å and c = 12.8 Å for Sm4O4[AsO4]Cl. Similar results were found for Gd4O4[AsO4]Cl (a = 5.7 Å and c = 12.6 Å). Besides, the reflection conditions are compatible with space group P42/mnm (136) in both cases. Additional strong streaking can be observed along the c-axis suggesting short range ordering. Such disorder has been observed by X-ray diffraction and associated to the AsO43– and Cl– disorder within the [AsO4Cl]4– layer. To elucidate the origin of the ordering pattern, HRTEM was performed along [0 0 1] and ½1 1 0 , shown in Figs. 9a and 10a, respectively. The corresponding HREM micrograph (Fig. 9a) shows an apparently well-ordered material with d-spacings of 5.8 Å, corresponding to d100 and d010. Fourier transformation was performed on the HREM micrograph, looking for the existence of different domains that could evidence the presence of additional ordering of the structure. However, the whole crystal results apparently homogeneous and only the maxima corresponding to the basal simple plane (ab) were observed (Fig. 9a). Simulated image using tetragonal primitive unit cell fits nicely with the experimental image for Dt = 8 nm and Df = 25 nm (shown at the inset of Fig. 9). However, the HREM image along ½110 (Fig. 10a) clearly reveals the presence of different domains in the sample Sm4O4[AsO4]Cl. Actually, the careful analysis of this micrograph shows the presence of structural domains (outlined by white circles in Fig. 9a) corresponding to a new ordering; doubling the [11 0] direction with intergrowth in a simple basic tetragonal matrix. The optical Fourier transform corresponding to both types of structural domains are depicted in Fig. 10b and c. These patterns correspond to the ½1 1 0 zone axis of a basic tetragonal unit cell, respectively. The contrast variation observed in the image corresponds to zig–zag of dark dots alternating with less dark dots along the c-axis such contrast corresponds to layers of Sm4O4 alternating with [AsO4]Cl layers as depicted in Fig. 10d. A calculated image using the tetragonal primitive unit cell fits nicely with the experimental image for Dt = 3 nm and Df = 20 nm (shown at the inset in zone A). Moreover, by simulating the images along the thick part of the image; Dt = 7 nm and Df = 35 nm (shown at the inset in zone B), the new ordering seems to be related only to the [AsO4]Cl layers as it can be observed in Fig. 10e. Such an ordering can be explained as an alternation of AsO4 groups with Cl atoms
[(Fig._10)TD$IG]
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Fig. 10. HREM image of Sm4O4[AsO4]Cl along ½1 1 0 (a), the corresponding FT in outlined domains (b), the corresponding FT of the non-outlined domains (c), and enlarged images showing [RE4O4]4+ and [ClAsO4]4– layers (d and e). Calculated images are depicted at the inset.
along the [11 0] direction (marked by arrows in Fig. 10a). Along the c-axis, layers of Sm4O4 alternate with [AsO4]Cl layers in an ordered way, but the disorder observed along the [AsO4]Cl layers leads to no new ordering along c-axis, this fact is consistent with the lines of diffuse scattering observed in the diffraction pattern of Fig. 10b.
Based on the HRTEM analysis, the ordering of AsO43– and Cl– in Sm4O4[AsO4]Cl corresponds perfectly to the ordered model obtained by decreasing the symmetry from P42/mnm via Pnnm to Pmn21 (Fig. 7b), although the SAED experiment excludes Pmn21 as possible space group. The average As–Cl distance of 4.122 Å,
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265
Fig. 11. Transformation of the ordered Pmn21- to the disordered P42/mnm-model.
obtained by single crystal diffraction (Fig.7b and Table S4), is in very good agreement with the distance of 4.1 Å reported in Fig. 10a of the HRTEM analysis of Sm4O4[AsO4]Cl. For crystallographers and others, the presence of large ADPs, half occupied atomic positions (Cl, As, O3, and O4) and short As–Cl distances of 0.9 Å in the P42/mnm model might be inacceptable. However, these anomalies are very simple to explain. If one starts from the ordered Pmn21 structure (Fig. 11a) and adds 42 screw axes as depicted in Fig. 11b, that does not affect the [RE4O4]4+ layers but generates new atomic positions in the [ClAsO4]4– layers (Fig. 11c), consequently all the atoms in these layers have to be half occupied in order to achieve an equilibrated chemical formula RE4O4[AsO4] Cl. This is in perfect agreement with the disordered P42/mnm model (Fig. 11d). It is therefore concluded that the occurrence of such RE4O4[AsO4]Cl disordered structures is mainly due to the intergrowth of locally ordered Pmn21-[ClAsO4]4– layers with different orientations in the (1 0 0) planes. 4. Conclusion The arsenate(V) chlorides RE4O4[AsO4]Cl show structural features similar to the large family of equiatomic pnictide oxides. Polycationic [RE4O4]4+ layers of edge-sharing ORE4/4 tetrahedra are charge-balanced and separated by the AsO43– and Cl– anions. Based on SAED and X-ray diffraction analyses, while we observed ordering for the bromides La4O4[AsO4]Br and Pr4O4[AsO4]Br [13], in the case of the chlorides we can present only an average structure (S. G.: P42/mnm) with disorder of the anionic groups. However, based on HRTEM analyses, an ordering of AsO43– and Cl– in RE4O4[AsO4]Cl could be proposed. This ordering is in agreement with the model obtained by decreasing the symmetry from P42/mnm via Pnnm to Pmn21. The corresponding phosphates RE4O4[PO4]Cl (RE = La–Nd, Gd) have been recently published [19], and the bromides RE4O4 [PO4]Br (RE = La, Pr, Nd, Sm) will be reported in forthcoming contributions. Acknowledgments This work was financially supported by the Deutsche Forschungsgemeinschaft through SPP 1458 Hochtemperatursupraleitung in
Eisenpnictiden. H.B.Y. is indebted to the Alexander von Humboldt Foundation for a research stipend. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull. 2014.02.025. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, Journal of the American Chemical Society 130 (2008) 3296–3297. [2] R. Pöttgen, D. Johrendt, Zeitschrift für Naturforschung 63b (2008) 1135– 1148. [3] D.C. Johnston, Advances in Physics 59 (2010) 803–1061. [4] S. Muir, M.A. Subramanian, Progress in Solid State Chemistry 40 (2012) 41–56. [5] H. Lincke, T. Nilges, R. Pöttgen, Zeitschrift fur Anorganische und Allgemeine Chemie 632 (2006) 1804–1808. [6] H. Lincke, R. Glaum, V. Dittrich, M.H. Möller, R. Pöttgen, Zeitschrift fur Anorganische und Allgemeine Chemie 635 (2009) 936–941. [7] I. Schellenberg, H. Lincke, W. Hermes, V. Dittrich, R. Glaum, M.H. Möller, R. Pöttgen, Zeitschrift für Naturforschung 65b (2010) 1191–1198. [8] D.-H. Kang, T. Schleid, Zeitschrift fur Anorganische und Allgemeine Chemie 633 (2007) 1205–1210. [9] D.-H. Kang, J. Wontcheu, T. Schleid, Solid State Sciences 11 (2009) 299–304. [10] H. Ben Yahia, U.C. Rodewald, R. Pöttgen, Zeitschrift für Naturforschung 64b (2009) 896–900. [11] H. Ben Yahia, A. Villesuzanne, U.C. Rodewald, T. Schleid, R. Pöttgen, Zeitschrift für Naturforschung 65b (2010) 549–555. [12] H. Ben Yahia, U.C. Rodewald, R. Pöttgen, Zeitschrift für Naturforschung 65b (2010) 1289–1292. [13] H. Ben Yahia, U.C. Rodewald, K. Boulahya, J.M. Gonzalez-Calbet, R. Pöttgen, Solid State Sciences 13 (2011) 239–243. [14] V. Petricek, M. Dusek, L. Palatinus, Structure Determination Software Programs, Institute of Physics, University of Prague, Prague (Czech Republic), 2006. [15] http://www.totalresolution.com/. [16] I.D. Brown, D. Altermatt, Acta Crystallographica. Section B: Structural Crystallography and Crystal Chemistry 41 (1985) 244–247. [17] N.E. Brese, M. O’Keefe, Acta Crystallographica. Section B: Structural Crystallography and Crystal Chemistry 47 (1991) 192–197. [18] R.D. Shannon, Acta Crystallographica. Section A: Crystal Physics, Diffraction, Theoretical and General Crystallography 32 (1976) 751– 767. [19] H. Ben Yahia, U. Ch. Rodewald, C. Feldmann, M. Roming, F. Weill, R. Pöttgen, Journal of Materials Chemistry C2 (2014) 1131–1140.
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HRTEM, SAED and XRD investigations of RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) Hamdi Ben Yahia a, *, Ute Ch. Rodewald a , Khalid Boulahya b , Rainer Pöttgen a a b
Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany Departamento de Química Inorgánica Facultad de Químicas Universidad Complutense, 28040 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 August 2013 Received in revised form 15 February 2014 Accepted 26 February 2014 Available online 28 February 2014
The new compounds RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) were synthesised by solid state reaction via a salt flux route and investigated by HRTEM, SAED, and single crystal X-ray diffraction. The samples crystallise with a tetragonal cell, space group P42/mnm and Z = 2. Their crystal structure consists of an alternation of [RE4O4]4+ and [ClAsO4]4– layers. The [RE4O4]4+ layer contains ORE4/4 tetrahedra which share common edges. The anions AsO43– and Cl– are located between these layers in disordered manner. SAED and HRTEM experiments confirmed this structural model and enabled us to propose an ordered model for the [ClAsO4]4– layers. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: B. Crystal growth C. X-ray diffraction C. Transmission electron microscopy (TEM) D. Crystal structure D. Layered compounds
1. Introduction The discovery of superconductivity in LaFeO1 – xFx [1] motivated broad research programs on quaternary pnictide oxides RETPnO (RE = rare earth metal, T = electron-rich transition metal; Pn = P, As, Sb) in the solid state physics and chemistry community [2–4]. Larger single crystals for property measurements of these materials can be grown either with the tin flux technique or via salt fluxes. We have repeatedly used NaCl/KCl fluxes for the growth of REZnPO, REZnAsO, and REZnSbO crystals in mm size [5–7]. In several cases we observed by-products with related structural motifs which, however, did not contain the transition metal. The quaternary compounds La3OCl[AsO3]2 [8], Nd5O4Cl[AsO3]2 [9], Ce3OCl[AsO3]2 [10], Pr5O4Cl[AsO3]2 [11], La3OBr[AsO3]2 [12], La4O4 [AsO4]Br and Pr4O4[AsO4]Br [13] all show oxygen-centred tetrahedral units which are separated and charge-balanced by the halide and arsenite, respectively arsenate anions. The structures of La4O4[AsO4]Br and Pr4O4[AsO4]Br [13] show the closest structural relationship with the pnictide oxides RETPnO. All ORE4 tetrahedra share four common edges, leading to the same layers as in the pnictide oxides. We have started a more systematic study of these complex materials with respect to substitutions of the anionic substructure. Depending on the size and combination
* Corresponding author. E-mail address: [email protected] (H. Ben Yahia). http://dx.doi.org/10.1016/j.materresbull.2014.02.025 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
of PO43–, AsO43–, Cl–, and Br– we observed different ordering patterns between the [RE4O4]4+ layers. Herein we report on the salt flux crystal growth of the series of RE4O4[AsO4]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) compounds and their characterisation using X-Ray diffraction, high resolution transmission microscopy (HRTEM) and selected area electron diffraction (SAED). 2. Experimental 2.1. Synthesis The title compounds were prepared by solid state reaction, from a mixture of re-sublimed arsenic (Sigma–Aldrich, 99.999%), rare earth oxides RE2O3 (RE = La, Nd, Sm–Gd) (Chempur, >99.99%) or Pr6O11 (Chempur, >96%) and a NaCl (Merck, >99.5%)/KCl (Chempur, 99.9%) salt flux (1:1 molar ratio) with a 1:2:15 molar ratio, respectively. The mixtures were put in alumina tubes, which were sealed under vacuum in silica tubes. To ensure a total reactivity of the arsenic and avoid its sublimation, the tubes were heated at 500 C for 24 h, at 600 C for 12 h, and then at 900 C for 72 h. After washing the mixtures with distilled water, we obtained mainly small RE4O4[AsO4]Cl plates and REAsO4 needles (RE = La, Nd, Sm–Gd). The former crystals were then mixed again with salt flux and heated at 900 C for 4 days. With a relatively fast decrease of the temperature at a rate of 15 C/h to r.t. we obtained larger single crystals of the title compounds. One should notice, during the second treatment, a change in the colour of Pr4O4[AsO4]Cl from
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Table 1 Cell parameters of the polycrystalline RE4O4[AsO4]Cl samples. Compounds
IRa
a (Å)
c (Å)
V (Å3)
La4O4(AsO4)Cl Pr4O4(AsO4)Cl Nd4O4(AsO4)Cl Sm4O4(AsO4)Cl Eu4O4(AsO4)Cl Gd4O4(AsO4)Cl
1.160 1.126 1.109 1.079 1.066 1.053
5.9950(3) 5.886(2) 5.8678(2) 5.8132(3) 5.8135(5) 5.772(2)
13.5587(8) 13.121(5) 13.053(1) 12.7951(8) 12.740(1) 12.690(4)
487.31(4) 455.1(3) 449.43(3) 432.39(4) 430.60(8) 422.8(2)
a
Ionic radius of RE [8].
were carried out with a Leica 420i scanning electron microscope using PrF3, SmF3, EuF3, GdF3, InAs, and KCl as standards. The experimentally observed compositions were close to the ideal ones (RE4O4[AsO4]Cl). Some impurities were identified to be the compounds REAsO4, REOCl or REAlO3 (reaction with the crucible). Fig. 1. Observed, calculated and difference plots for the XRPD profile refinement of the Sm4O4[AsO4]Cl sample obtained from flux synthesis.
2.3. X-ray diffraction
brown to green was observed, but without any significant change in the structure. Few other attempts to synthesise the RE4O4[AsO4]Cl compounds in air starting from a stoichiometric mixture of RE2O3, RECl3 and NH4H2AsO4 have been performed but led to mixtures of REOCl and REAsO4. Mixtures of re-sublimed arsenic, RE2O3, KClO4 and a NaCl/KCl salt flux (1:1 molar ratio) with a 2:4:1:15 molar ratio, respectively, have been also tested. In order to avoid the explosion of the tubes, we placed successively KClO4, NaCl/KCl, RE2O3 and As and then NaCl/KCl. The tubes were sealed under vacuum in silica tubes and heated at 500 C for 24 h, at 600 C for 12 h, and at 800 C for 48 h. This enabled us to obtain a pure sample of Sm4O4[AsO4]Cl (Fig. 1). 2.2. Electron microprobe analysis Semiquantitative EDX analyses of different single crystals including the ones investigated on the diffractometer (Fig. 2)
The polycrystalline samples were characterized by Guinier patterns (imaging plate detector, Fujifilm BAS-1800) with Cu Ka1 radiation and a-quartz (a = 4.9130, c = 5.4046 Å) as an internal standard (Table 1). Fig. 3 shows the evolution of the cell volume as a function of the ionic radius of the eight coordinated rare earth. Only Sm4O4[AsO4]Cl sample is shown to be pure (Fig. 1). For the other samples, besides the main phase we observed different impurities such as REAsO4, REAlO3 (from a side reaction with the crucible material) and /or REOCl. Crystals suitable for single crystal X-ray diffraction were selected on the basis of the size and the sharpness of the diffraction spots by Laue photographs on a Buerger camera (using white Mo radiation). The data collections were carried out on a Stoe IPDS II diffractometer using Mo Ka radiation. Data processing and first refinements were performed with the Jana2000 program package [14]. A Gaussian-type absorption correction was applied, and the shapes of the crystals were determined with the video microscope of the Stoe CCD. Details about data collections and refinements are summarized in Table 2.
[(Fig._2)TD$IG]
Fig. 2. SEM images of the RE4O4[AsO4]Cl single crystals used for the X-ray data collection.
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[(Fig._3)TD$IG]
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3. Results and discussion 3.1. Structure refinement
Fig. 3. Evolution of the cell volume as a function of the ionic radius of the eight coordinated rare earth.
2.4. SAED, HRTEM and EDX analysis Selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) were performed in a JEOL 3000 FEG electron microscope, fitted with a double tilting goniometer stage (22 , 22 ). Local composition was analysed by energydispersive X-ray spectroscopy (EDS) with an Oxford INCA analyser system attached to the above mentioned microscope. Simulated HREM images were calculated by the multislice method using the MacTempas software package [15].
Careful examination of the Sm4O4[AsO4]Cl data set revealed a primitive tetragonal unit cell with high Laue symmetry and the extinction conditions (0 k l only for k + 1 = 2n, 0 0 l only observed for l = 2n, h 0 0 only for h = 2n) led to the space groups P42/mnm and P42nm. The structures were solved in the centro-symmetric group P42/mnm. In the case of Sm4O4[AsO4]Cl, most of the atomic sites have been located using the superflip program implemented in the Jana2000 package [14]. The use of difference-Fourier synthesis allowed us to localize the remaining oxygen atoms which led to a non-equilibrated chemical formula Sm4O4[AsO4]2. With isotropic atomic displacement parameters (ADP), the residual factors converged to R(F) = 0.0386 and wR(F2) = 0.0957 for 15 refined parameters with non-significant difference-Fourier residues. The refinement of the occupancies of the arsenic and the four oxygen atoms in its surrounding showed that they are half occupied. Then, considering the As, O3 and O4 positions half occupied and by using the difference-Fourier synthesis we were able to localize the remaining chlorine atom. The refinement of its occupancy showed that it is also half occupied. This led to the final Sm4O4[AsO4]Cl equilibrated chemical formula and the reliability factors listed in Table 2. The structures of all the other compounds (with Pr, Eu and Gd) have been solved using the structural model of Sm4O4[AsO4]Cl. Disorder of AsO43– and Cl– within the [AsO4Cl]4– layer and large ADPs for the chlorine atoms have been observed in all cases. The refined atomic positions and anisotropic displacement parameters (ADPs) are given in Tables 3 and 4, respectively. Further details on the structure refinement may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry Nos. CSD-426429; 426430
Table 2 Crystallographic data and structure refinement for RE4O4[AsO4]Cl. Formula
Pr4O4(AsO4)Cl
Pr4O4(AsO4)Cl
Sm4O4(AsO4)Cl
Eu4O4(AsO4)Cl
Gd4O4(AsO4)Cl
Crystal colour MW (g mol1) Crystal system Space group Parameters (Å)
Brown 802 Tetragonal P42/mnm a = 5.8937(7) c = 13.158(3) 457.06(12) 2 0.065 0.033 0.012 5.82 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan 7 h 8, k = 8, l = 18 3.1, 30.53 24.8 Gaussian 0.394/0.747 7579 0.064 412 293 F2 700 0.025/0.057 34 1.14 w = 1/(s 2(I) + 0.00116I2) 1.05, +1.11 1800(200)
Green 802 Tetragonal P42/mnm a = 5.8965(9) c = 13.179(3) 458.21(15) 2 0.052 0.048 0.014 5.81 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan h = 8, k = 8, l = 18 3.09, 30.48 24.8 Gaussian 0.326/0.753 4589 0.066 412 273 F2 700 0.026/0.054 34 1.03 w = 1/(s 2(I) + 0.0009I2) 1.06, +0.91 1690(180)
Colourless 839.8 Tetragonal P42/mnm a = 5.8248(9) c = 12.825(3) 435.15(13) 2 0.080 0.071 0.006 6.41 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan 7 h 8, k = 8, l = 18 3.17, 31.84 30.6 Gaussian 0.219/0.833 7709 0.038 437 367 [I 3s (I)] F2 724 0.023/0.065 34 1.20 w = 1/(s 2(I) + 0.0021I2) 1.71, +1.62 1400(200)
Colourless 846.2 Tetragonal P42/mnm a = 5.808(3) c = 12.726(9) 429.3(4) 2 0.075 0.075 0.10 6.54 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan h = 8, k = 8, l = 18 3.2, 31.83 32.9 Gaussian 0.268/0.796 11109 0.07 433 388 F2 732 0.032/0.066 34 1.20 w = 1/(s 2(I) + 0.0018I2) 2.09, +2.09 1500(200)
Colourless 867.4 Tetragonal P42/mnm a = 5.7827(4) c = 12.6623(12) 423.42(6) 2 0.064 0.040 0.014 6.80 Plate 293(1) Stoe IPDS II Oriented graphite Mo Ka (l = 0.71073 Å) Multi-scan h = 8, k = 8, l = 18 3.22, 31.88 35.1 Gaussian 0.030/0.354 8812 0.10 428 385 F2 740 0.024/0.056 34 1.19 w = 1/(s 2(I) + 0.0009I2) 1.22, +1.45 3490(190)
V (Å3) Z Crystal size (mm) Density calc. (g cm3) Crystal shape Temperature (K) Diffractometer Monochromator Radiation Scan-mode hkl umin, umax (deg) Linear absorption coeff. (mm1) Absorption correction Tmin/Tmax No. of reflections Rint No. of independent reflections Reflections used [I 2s (I)] Refinement F(000) R factors R(F)/wR(F2) No. of refined parameteres g.o.f. Weighting scheme Diff. Fourier residues (e/Å3) Extinction coefficient
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Table 3 Atom positions and isotropic displacement parameters (Å2) for RE4O4[AsO4]Cl.
Pr4O4[AsO4]Cl Pr1 O1 O2 As O3 O4 Cl Sm4O4[AsO4]Cl Sm O1 O2 As O3 O4 Cl Eu4O4[AsO4]Cl Eu O1 O2 As O3 O4 Cl Gd4O4[AsO4]Cl Gd O1 O2 As O3 O4 Cl
Wyckoff
Occupancy
x
y
z
Ueq
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23519(7) 1/2 1/2 0.2918(4) 0.252(4) 0.181(2) 0.186(3)
0.23519(7) 0 1/2 0.7082(4) 0.424(3) 0.819(2) 0.814(3)
0.33241(4) 1/4 0.2675(18) 1/2 1/2 0.3949(15) 1/2
0.00899(16) 0.019(5) 0.016(4) 0.0088(7) 0.017(4) 0.018(4) 0.077(6)
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23320(5) 1/2 1/2 0.2932(3) 0.2506(18) 0.1768(13) 0.1824(13)
0.23320(5) 0 1/2 0.7068(3) 0.4200(18) 0.8232(13) 0.8176(13)
0.33096(2) 1/4 0.2736(7) 1/2 1/2 0.3936(8) 1/2
0.00669(12) 0.0115(17) 0.0136(16) 0.0058(4) 0.013(3) 0.0122(18) 0.068(3)
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23279(5) 1/2 1/2 0.2942(3) 0.2480(18) 0.1755(14) 0.1830(14)
0.23279(5) 0 1/2 0.7058(3) 0.4196(18) 0.8245(14) 0.8170(14)
0.33054(3) 1/4 0.2745(8) 1/2 1/2 0.3927(8) 1/2
0.00868(13) 0.0137(18) 0.0162(18) 0.0079(4) 0.012(3) 0.0145(19) 0.070(4)
8j 4d 4e 4f 8i 8j 4f
1 1 1 0.5 0.5 0.5 0.5
0.23245(5) 1/2 1/2 0.2946(2) 0.2490(14) 0.1761(10) 0.1812(11)
0.23245(5) 0 1/2 0.7054(2) 0.4164(14) 0.8239(10) 0.8188(11)
0.330319(18) 1/4 0.2748(5) 1/2 1/2 0.3927(6) 1/2
0.00937(11) 0.0143(12) 0.0151(11) 0.0082(3) 0.0147(19) 0.0153(13) 0.079(3)
Table 4 Anisotropic displacement parameters (Å2) for RE4O4[AsO4]Cl. The anisotropic displacement factor exponent takes the form: –2p2[(ha*)2U11 + + 2hka*b*U12].
Pr4O4[AsO4]Cl Pr1 O1 O2 As O3 O4 Cl Sm4O4[AsO4]Cl Sm O1 O2 As O3 O4 Cl Eu4O4[AsO4]Cl Eu O1 O2 As O3 O4 Cl Gd4O4[AsO4]Cl Gd O1 O2 As O3 O4 Cl
U11
U22
U33
U12
U13
U23
0.0082(3) 0.007(4) 0.009(4) 0.0092(10) 0.018(8) 0.015(5) 0.094(10)
0.0082(3) 0.007(4) 0.009(4) 0.0092(10) 0.022(7) 0.015(5) 0.094(10)
0.0106(3) 0.045(14) 0.030(11) 0.0082(14) 0.012(7) 0.024(10) 0.043(10)
0.0001(2) 0 0.000(4) 0.0022(11) 0.001(8) 0.000(5) 0.043(12)
0.0007(2) 0 0 0 0 0.006(5) 0
0.0007(2) 0 0 0 0 0.006(5) 0
0.0056(2) 0.007(2) 0.0068(19) 0.0060(5) 0.021(5) 0.015(3) 0.087(7)
0.0056(2) 0.007(2) 0.0068(19) 0.0060(5) 0.014(4) 0.015(3) 0.087(7)
0.0088(2) 0.021(4) 0.027(4) 0.0053(8) 0.004(4) 0.008(4) 0.030(4)
0.00029(10) 0 0.0004(19) 0.0030(6) 0.003(4) 0.002(3) 0.062(7)
0.00038(7) 0 0 0 0 0.005(2) 0
0.00038(7) 0 0 0 0 0.005(2) 0
0.0078(2) 0.008(2) 0.010(2) 0.0082(5) 0.017(5) 0.017(3) 0.091(7)
0.0078(2) 0.008(2) 0.010(2) 0.0082(5) 0.015(5) 0.017(3) 0.091(7)
0.0104(2) 0.024(5) 0.028(4) 0.0073(8) 0.006(4) 0.009(4) 0.028(4)
0.00026(10) 0 0.001(2) 0.0018(6) 0.003(3) 0.002(4) 0.059(8)
0.00048(8) 0 0 0 0 0.004(3) 0
0.00048(8) 0 0 0 0 0.004(3) 0
0.00833(17) 0.0090(16) 0.0092(14) 0.0096(4) 0.023(4) 0.0156(18) 0.106(6)
0.00833(17) 0.0090(16) 0.0092(14) 0.0096(4) 0.011(3) 0.0156(18) 0.106(6)
0.0115(2) 0.025(3) 0.027(3) 0.0054(6) 0.010(3) 0.015(3) 0.027(3)
0.00007(8) 0 0.0008(15) 0.0011(5) 0.004(3) 0.002(3) 0.076(7)
0.00082(5) 0 0 0 0 0.0048(18) 0
0.00082(5) 0 0 0 0 0.0048(18) 0
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261
Fig. 4. Disordered structure of Sm 4O4[AsO4]Cl.
(Pr4O4[AsO4]Cl), CSD-426431 (Sm4O4[AsO4]Cl), CSD-426427 (Eu4O4[AsO4]Cl), and CSD-426428 (Gd4O4[AsO4]Cl). Different attempts to lower the symmetry (according to the subgroups listed in the International Tables, Suppl. A1) have been performed and it was possible to obtain a perfectly ordered structure using the space group Pmn21, however very large ADPs have been observed for the chlorine atoms and some atoms (O1, O2 and Sm2) exhibited non-positive definite ADP matrix. When only isotropic displacement parameters are refined for these three atoms, the final residual factors converged to the values reported in Table S1. Although, the TEM experiments showed the presence of n-glide planes perpendicular to a and b, which is in agreement with the space group P42/mnm and excludes the space group Pmn21 (see
TEM section), we will describe the ordered Pmn21-model in the next sections. The HRTEM images, which have been taken along the layers, indicate that this ordered Pmn21-model describes very well a major part of the Sm4O4[AsO4]Cl structure. The refined atomic positions and anisotropic ADPs in the ordered Pmn21model are given in Tables S2 and S3, respectively. 3.2. Crystal structure 3.2.1. Disordered RE4O4[AsO4]Cl structure (P42/mnm -model) The crystal structure of the title compounds RE4O4[AsO4]Cl consists of alternating [RE4O4]4+ and [ClAsO4]4– layers (Fig. 4). The [RE4O4]4+ layer contains ORE4/4 tetrahedra which share common
[(Fig._5)TD$IG]
Fig. 5. Different layered structures containing LaO, La2O2 or La3O3 layers as in the title compounds.
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Table 5 Interatomic distances (Å) and bond valence (*: BVS) for RE4O4[AsO4]Cl. Average distances are given in brackets. Distances Pr Pr–O1 (2) Pr–O2 Pr–O2 Pr–O3 Pr–O4 Pr–Cl
As–O4 (2) As–O3 (2) As–Cl
Eu Eu–O1 (2) Eu–O2 Eu–O2 Eu–O3 Eu–O4
B.V.
2.3539(6) 2.362(13) 2.368(9) 2.474(7) 2.606(13) <2.419> 3.336(12)
0.557 0.546 0.537 0.403 0.282 *2.886 0.107 *2.993
1.667(18) 1.695(15) <1.681> 0.883(16) 3.984(16)
1.310 1.215 *5.05
B.V.
2.3105(5) 2.318(3) 2.342(5) 2.428(5) 2.541(8) <2.375> 3.263(6)
0.551 0.540 0.506 0.401 0.296 *2.845 0.115 *2.960
1.667(9) 1.689(11) <1.678> 0.912(8) 3.918(2)
1.310 1.235 *5.09
2.2880(3) 2.2980(18) 2.321(3) 2.399(4) 2.512(6) <2.351> 3.229(5)
0.557 0.542 0.509 0.413 0.304 *2.882 0.120 *3.002
As–O4 (2) As–O3 (2)
1.669(7) 1.692(8) <1.680>
1.303 1.225 *5.06
As–Cl
0.927(6) 3.891(6)
Sm–Cl
As–O4 (2) As–O3 (2) As–Cl
2.2994(14) 2.308(3) 2.333(6) 2.416(5) 2.522(9) <2.362> 3.251(7)
0.560 0.531 0.497 0.397 0.298 *2.843 0.116 *2.959
As–O4 (2) As–O3 (2)
1.678(10) 1.684(11) <1.681>
1.272 1.251 *5.05
As–Cl
0.914(9) 3.920(2)
Eu–Cl
Distances Sm Sm–O1 (2) Sm–O2 Sm–O2 Sm–O3 Sm–O4
Gd Gd–O1 (2) Gd–O2 Gd–O2 Gd–O3 Gd–O4 Gd–Cl
BV = e(r0 – r)/b with the following parameters: b = 0.37, r0 (PrIII–O) = 2.138, r0 (SmIII–O) = 2.090, r0 (EuIII–O) = 2.2.074, r0 (GdIII–O) = 2.071, r0 (PrIII–Cl) = 2.509, r0 (SmIII–Cl) = 2.461, r0 (EuIII–Cl) = 2.454, r0 (GdIII–Cl) = 2.443 and r0 (AsV–O) = 1.767 [16,17].
edges. The chlorine Cl– and the tetrahedrally coordinated As5+ atoms are located between these layers in disordered manner. Although this is a new type of structure, similar [RE4O4]4+ layers have been observed in many other compounds, e. g. in LaOCl, La3O3PO4, LaOFeP, LaOS, La2O2SO4 or La2O2Te (Fig. 5). The interatomic distances and bond valence sums are listed in Table 5. In the RE4O4[AsO4]Cl compounds (RE = Pr, Sm, Eu and Gd), the rare earth cations are coordinated to six oxygen and one chlorine atoms forming a distorted, mono-capped trigonal prism (Fig. 6). The RE–O2 distances within the [RE4O4]4+ layers are very similar to those observed in the layered compounds of Fig. 5. The decrease of the average RE–O distances from 2.419 to 2.351 Å in the RE4O4
Fig. 6. Coordination of RE in the RE4O4[AsO4]Cl compounds (RE = Pr, Sm, Eu and Gd).
[AsO4]Cl compounds (RE = Pr, Sm, Eu and Gd) is in good agreement with the decrease of the rare earth size and with the sum of the ionic radius of O2– and RE3+ [18]. This behaviour is at the origin of the decrease of the cell parameters and consequently of the cell volume (Fig. 3). This induces a decrease of the Cl–As and Cl–O3 distances within the [AsO4Cl]4 layer from 3.984 to 3.891 Å and from 2.936 to 2.835 Å for Pr4O4[AsO4]Cl and Gd4O4[AsO4]Cl, respectively. This may explain why compositions with RE smaller than Gd did not form. The AsO4 tetrahedra are quite regular in shape with average distances ranging from 1.678 to 1.681 Å. These values are slightly lower than the value of 1.717 Å, expected from the sum of the effective ionic radii of the four-coordinated As5+ and O2– [18]. The calculations of the bond valence sums (BVS) for the RE and As atoms are listed in Table 5. They are in very good agreement with the expected 3+ and 5+ values, respectively. 3.2.2. Ordered Sm4O4[AsO4]Cl structure (Pmn21-model) In the Pmn21-model of Sm4O4[AsO4]Cl, a perfect ordering of the AsO43– and Cl– anions between the [RE4O4]4+ layers is observed (Fig. 7a). In the [ClAsO4]4– layer, Cl– is surrounded by four AsO43– tetrahedra with an average As–Cl distance of 4.122 Å (Fig. 7b). Such ordering is different from that in Pr4O4[AsO4]Br (Fig. 7c). The interatomic distances and bond valence sums are listed in Table S4. 3.3. SAED and HRTEM analyses The cationic composition, determined on several small crystallites by energy dispersive X-ray analysis in the electron microscope, is in agreement with the nominal one. Selected area electron diffraction (SAED) was used to fully reconstruct the reciprocal space. Fig. 8 shows the [0 0 1], ½1 1 0 and [0 1 0] zone
[(Fig._7)TD$IG]
Fig. 7. The ordered structure of Sm4O4[AsO4]Cl obtained by lowering the symmetry from P42/mnm (a1 = b1 6¼ c1) to Pmn21 (a2 = c1, b2 = c2 = a1) (a), a view of the [ClAsO4]4– layer (b), and a view of the [BrAsO4]4– layer in Pr4O4[AsO4]Br .
[(Fig._8)TD$IG]
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263
Fig. 9. HREM image of Sm4O4[AsO4]Cl along [0 0 1] (a) and the corresponding FT (b). Calculated image is depicted at the inset.
Fig. 8. SAED patterns corresponding to Sm4O4[AsO4]Cl, [0 0 1] (a), ½1 1 0 (b) and [0 1 0] (c) zone axes.
axis, all maxima can be indexed on the basis of a tetragonal cell, with parameters a = 5.8 Å and c = 12.8 Å for Sm4O4[AsO4]Cl. Similar results were found for Gd4O4[AsO4]Cl (a = 5.7 Å and c = 12.6 Å). Besides, the reflection conditions are compatible with space group P42/mnm (136) in both cases. Additional strong streaking can be observed along the c-axis suggesting short range ordering. Such disorder has been observed by X-ray diffraction and associated to the AsO43– and Cl– disorder within the [AsO4Cl]4– layer. To elucidate the origin of the ordering pattern, HRTEM was performed along [0 0 1] and ½1 1 0 , shown in Figs. 9a and 10a, respectively. The corresponding HREM micrograph (Fig. 9a) shows an apparently well-ordered material with d-spacings of 5.8 Å, corresponding to d100 and d010. Fourier transformation was performed on the HREM micrograph, looking for the existence of different domains that could evidence the presence of additional ordering of the structure. However, the whole crystal results apparently homogeneous and only the maxima corresponding to the basal simple plane (ab) were observed (Fig. 9a). Simulated image using tetragonal primitive unit cell fits nicely with the experimental image for Dt = 8 nm and Df = 25 nm (shown at the inset of Fig. 9). However, the HREM image along ½110 (Fig. 10a) clearly reveals the presence of different domains in the sample Sm4O4[AsO4]Cl. Actually, the careful analysis of this micrograph shows the presence of structural domains (outlined by white circles in Fig. 9a) corresponding to a new ordering; doubling the [11 0] direction with intergrowth in a simple basic tetragonal matrix. The optical Fourier transform corresponding to both types of structural domains are depicted in Fig. 10b and c. These patterns correspond to the ½1 1 0 zone axis of a basic tetragonal unit cell, respectively. The contrast variation observed in the image corresponds to zig–zag of dark dots alternating with less dark dots along the c-axis such contrast corresponds to layers of Sm4O4 alternating with [AsO4]Cl layers as depicted in Fig. 10d. A calculated image using the tetragonal primitive unit cell fits nicely with the experimental image for Dt = 3 nm and Df = 20 nm (shown at the inset in zone A). Moreover, by simulating the images along the thick part of the image; Dt = 7 nm and Df = 35 nm (shown at the inset in zone B), the new ordering seems to be related only to the [AsO4]Cl layers as it can be observed in Fig. 10e. Such an ordering can be explained as an alternation of AsO4 groups with Cl atoms
[(Fig._10)TD$IG]
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Fig. 10. HREM image of Sm4O4[AsO4]Cl along ½1 1 0 (a), the corresponding FT in outlined domains (b), the corresponding FT of the non-outlined domains (c), and enlarged images showing [RE4O4]4+ and [ClAsO4]4– layers (d and e). Calculated images are depicted at the inset.
along the [11 0] direction (marked by arrows in Fig. 10a). Along the c-axis, layers of Sm4O4 alternate with [AsO4]Cl layers in an ordered way, but the disorder observed along the [AsO4]Cl layers leads to no new ordering along c-axis, this fact is consistent with the lines of diffuse scattering observed in the diffraction pattern of Fig. 10b.
Based on the HRTEM analysis, the ordering of AsO43– and Cl– in Sm4O4[AsO4]Cl corresponds perfectly to the ordered model obtained by decreasing the symmetry from P42/mnm via Pnnm to Pmn21 (Fig. 7b), although the SAED experiment excludes Pmn21 as possible space group. The average As–Cl distance of 4.122 Å,
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265
Fig. 11. Transformation of the ordered Pmn21- to the disordered P42/mnm-model.
obtained by single crystal diffraction (Fig.7b and Table S4), is in very good agreement with the distance of 4.1 Å reported in Fig. 10a of the HRTEM analysis of Sm4O4[AsO4]Cl. For crystallographers and others, the presence of large ADPs, half occupied atomic positions (Cl, As, O3, and O4) and short As–Cl distances of 0.9 Å in the P42/mnm model might be inacceptable. However, these anomalies are very simple to explain. If one starts from the ordered Pmn21 structure (Fig. 11a) and adds 42 screw axes as depicted in Fig. 11b, that does not affect the [RE4O4]4+ layers but generates new atomic positions in the [ClAsO4]4– layers (Fig. 11c), consequently all the atoms in these layers have to be half occupied in order to achieve an equilibrated chemical formula RE4O4[AsO4] Cl. This is in perfect agreement with the disordered P42/mnm model (Fig. 11d). It is therefore concluded that the occurrence of such RE4O4[AsO4]Cl disordered structures is mainly due to the intergrowth of locally ordered Pmn21-[ClAsO4]4– layers with different orientations in the (1 0 0) planes. 4. Conclusion The arsenate(V) chlorides RE4O4[AsO4]Cl show structural features similar to the large family of equiatomic pnictide oxides. Polycationic [RE4O4]4+ layers of edge-sharing ORE4/4 tetrahedra are charge-balanced and separated by the AsO43– and Cl– anions. Based on SAED and X-ray diffraction analyses, while we observed ordering for the bromides La4O4[AsO4]Br and Pr4O4[AsO4]Br [13], in the case of the chlorides we can present only an average structure (S. G.: P42/mnm) with disorder of the anionic groups. However, based on HRTEM analyses, an ordering of AsO43– and Cl– in RE4O4[AsO4]Cl could be proposed. This ordering is in agreement with the model obtained by decreasing the symmetry from P42/mnm via Pnnm to Pmn21. The corresponding phosphates RE4O4[PO4]Cl (RE = La–Nd, Gd) have been recently published [19], and the bromides RE4O4 [PO4]Br (RE = La, Pr, Nd, Sm) will be reported in forthcoming contributions. Acknowledgments This work was financially supported by the Deutsche Forschungsgemeinschaft through SPP 1458 Hochtemperatursupraleitung in
Eisenpnictiden. H.B.Y. is indebted to the Alexander von Humboldt Foundation for a research stipend. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull. 2014.02.025. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, Journal of the American Chemical Society 130 (2008) 3296–3297. [2] R. Pöttgen, D. Johrendt, Zeitschrift für Naturforschung 63b (2008) 1135– 1148. [3] D.C. Johnston, Advances in Physics 59 (2010) 803–1061. [4] S. Muir, M.A. Subramanian, Progress in Solid State Chemistry 40 (2012) 41–56. [5] H. Lincke, T. Nilges, R. Pöttgen, Zeitschrift fur Anorganische und Allgemeine Chemie 632 (2006) 1804–1808. [6] H. Lincke, R. Glaum, V. Dittrich, M.H. Möller, R. Pöttgen, Zeitschrift fur Anorganische und Allgemeine Chemie 635 (2009) 936–941. [7] I. Schellenberg, H. Lincke, W. Hermes, V. Dittrich, R. Glaum, M.H. Möller, R. Pöttgen, Zeitschrift für Naturforschung 65b (2010) 1191–1198. [8] D.-H. Kang, T. Schleid, Zeitschrift fur Anorganische und Allgemeine Chemie 633 (2007) 1205–1210. [9] D.-H. Kang, J. Wontcheu, T. Schleid, Solid State Sciences 11 (2009) 299–304. [10] H. Ben Yahia, U.C. Rodewald, R. Pöttgen, Zeitschrift für Naturforschung 64b (2009) 896–900. [11] H. Ben Yahia, A. Villesuzanne, U.C. Rodewald, T. Schleid, R. Pöttgen, Zeitschrift für Naturforschung 65b (2010) 549–555. [12] H. Ben Yahia, U.C. Rodewald, R. Pöttgen, Zeitschrift für Naturforschung 65b (2010) 1289–1292. [13] H. Ben Yahia, U.C. Rodewald, K. Boulahya, J.M. Gonzalez-Calbet, R. Pöttgen, Solid State Sciences 13 (2011) 239–243. [14] V. Petricek, M. Dusek, L. Palatinus, Structure Determination Software Programs, Institute of Physics, University of Prague, Prague (Czech Republic), 2006. [15] http://www.totalresolution.com/. [16] I.D. Brown, D. Altermatt, Acta Crystallographica. Section B: Structural Crystallography and Crystal Chemistry 41 (1985) 244–247. [17] N.E. Brese, M. O’Keefe, Acta Crystallographica. Section B: Structural Crystallography and Crystal Chemistry 47 (1991) 192–197. [18] R.D. Shannon, Acta Crystallographica. Section A: Crystal Physics, Diffraction, Theoretical and General Crystallography 32 (1976) 751– 767. [19] H. Ben Yahia, U. Ch. Rodewald, C. Feldmann, M. Roming, F. Weill, R. Pöttgen, Journal of Materials Chemistry C2 (2014) 1131–1140.