Dimorphic cerium(III) oxoarsenate(III) Ce[AsO3]

Dimorphic cerium(III) oxoarsenate(III) Ce[AsO3]

Solid State Sciences 37 (2014) 164e169 Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/sssc...

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Solid State Sciences 37 (2014) 164e169

Contents lists available at ScienceDirect

Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie

Dimorphic cerium(III) oxoarsenate(III) Ce[AsO3] Florian Ledderboge a, Sebastian J. Metzger a, Gunter Heymann b, Hubert Huppertz b, Thomas Schleid a, * a b

Institute for Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innrain 80 e 82, 6020 Innsbruck, Austria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2014 Received in revised form 6 August 2014 Accepted 11 August 2014 Available online 20 August 2014

Colourless, water- and air-stable single crystals of cerium(III) oxoarsenate(III) Ce[AsO3] were prepared by the reaction of cerium metal (Ce) and arsenic sesquioxide (As2O3) in the presence of cesium chloride (CsCl) as fluxing agent at 750  C in an evacuated silica ampoule. Ce[AsO3] crystallizes monoclinically (a ¼ 902.89(8), b ¼ 782.54(7), c ¼ 829.68(7) pm, b ¼ 103.393(3) , Z ¼ 8) in the space group P21/c and is isotypic with a-Pb[SeO3]. There are two crystallographically different Ce3þ positions. (Ce1)3þ is coordinated by nine oxygen atoms (d(CeeO) ¼ 244e286 pm) and (Ce2)3þ by only eight (d(CeeO) ¼ 239 e273 pm). Both crystallographically different As3þ cations form discrete j1 tetrahedra [AsO3]3 (d(As eO) ¼ 174e179 pm), which are attached to the Ce3þ cations via edges and corners. The second monoclinic modification of Ce[AsO3] with the lattice parameters a ¼ 439.32(4), b ¼ 529.21(5), c ¼ 617.34(6) pm and b ¼ 105.369(3) with Z ¼ 2 was obtained by high-pressure synthesis (11 GPa, 1200  C) and has both a higher density (6.31 vs. 6.13 g $ cm3) and a higher calculated Madelung part of the lattice energy (15,155 vs. 15,132 kJ $ mol1). It adopts the space group P21/m, crystallizing isotypically with La[AsO3], b-Pb[SeO3], Pb[SO3] (scotlandite) or K[ClO3] and exhibits nine-fold coordinated Ce3þ cations exclusively (d(CeeO) ¼ 254e287 pm) along with tripodal [AsO3]3 anions (d(AseO) ¼ 175e176 pm). Raman spectroscopy on both phases of Ce[AsO3] shows stretching vibrations between 769 and 731 cm1 as well as asymmetric vibrations in the range of 659e617 cm1. The symmetric bending mode vibrations emerge in an interval from 340 to 410 cm1 and the asymmetric bending modes range between 230 and 290 cm1. © 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Lanthanoids Cerium Arsenic Oxoarsenates(III) Crystal structures Raman spectroscopy

1. Introduction Ternary lanthanoid(III) oxoarsenates(V) with the composition Ln[AsO4] are well known since 1934 [1] and crystallize with coordination numbers (C.N.) of eight plus one for the Ln3þ cations involved in the monoclinic monazite-type structure (space group: P21/n), which is restricted to the light lanthanoids (Ln ¼ La e Nd) [2e5] with largest cationic radii. Eightfold coordination is found in both, the tetragonal xenotime- (Ln ¼ Pm e Lu, space group: I41/amd) [1e3,6e10] and the scheelite-type crystal structures (Ln ¼ La, Ce, Pr, Nd, Sm, Tb, Dy, Er, Yb, Lu, space group: I41/a) [11,12]. On the other hand lanthanoid(III) oxoarsenates(III) were only known with the composition Ln4[As2O5]2[As4O8] (Ln ¼ Nd and Sm) [13,14] for a long time without containing discrete [AsO3]3 units. Oxoarsenates(III), where isolated [AsO3]3 units were observed, typically contain chloride anions, as it is the case for Ln5Cl3[AsO3]4 (Ln ¼ La, Pr and

* Corresponding author. E-mail address: [email protected] (T. Schleid). http://dx.doi.org/10.1016/j.solidstatesciences.2014.08.005 1293-2558/© 2014 Elsevier Masson SAS. All rights reserved.

Nd) [15,16] or the oxide-halide derivates Ln3OCl[AsO3]2 (Ln ¼ La, Ce, Gd and Tb) [5,17,18], as well as La3OBr[AsO3]2 [19] and Ln5O4Cl[AsO3]2 (Ln ¼ Pr and Nd) [20,21]. The simple composition Ln[AsO3] (Ln ¼ La) [22] was first realized by high-pressure (11.5 GPa) and high-temperature (1000  C) synthesis from lanthanum sesquioxide and arsenic sesquioxide in 2012. La[AsO3] adopts the monoclinic space group P21/m, crystallizes isotypically with b-Pb[SeO3] [23], Pb[SO3] (scotlandite) [24] or K[ClO3] [25,26] and exhibits nine-fold coordinated La3þ cations. 2. Experimental, material and methods For preparing a-Pb[SeO3]-type Ce[AsO3], cerium powder (Ce: 99.9%, ChemPur, Karlsruhe, Germany), arsenic sesquioxide (As2O3: 99.99% Alfa Aesar, Ward Hill, MA, USA) and cesium chloride (CsCl: suprapur, Merck, Darmstadt, Germany) were filled into silica tubes in molar ratios of 3:3:2 and sealed under dynamic vacuum. The mixture was heated up to 750  C within 2 days, kept at this temperature for 4 days and cooled down to room temperature within 7 days. After this treatment colourless crystals of Ce[AsO3] were

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Table 1 Crystallographic data for both forms of Ce[AsO3] and their isotypic relatives.

obtained, which proved to be stable against water and air. A big single crystal of elemental arsenic at the top of the ampoule could be observed as by-product according to Ce þ As2O3 / Ce[AsO3] þ As. For the synthesis of K[ClO3]-type Ce[AsO3], cerium(IV) oxide (CeO2: 99.99%, ChemPur, Karlsruhe, Germany) and arsenic sesquioxide (As2O3: 99.995%, Merck, Darmstadt, Germany) in 2:1 M ratio were filled into cylindrical boron-nitride crucibles (BN) with a fitting plate. Single crystals were obtained in a modified Walkertype module in combination with a 1000 ton press. Precast magnesium oxide octahedra (MgO) with edge lengths of 14 mm were applied as pressure medium. Eight tungsten carbide cubes (WC) with a truncation edge length of 8 mm compressed the octahedra. Further information on the construction of the assemblies is given

in references (Walker [27,28], Huppertz [29], Rubie [30], Kawai and Endo [31]). The mixture was compressed up to 11 GPa in 5.4 h, subsequently heated to 1200  C within 15 min and kept there for another 15 min. After cooling down the sample to 500  C within 150 min it was quenched to room temperature by turning off the heating. The recovered MgO octahedron was broken apart and the sample was carefully separated from the surrounding BN crucible. Besides cerium(III) oxoarsenate(V) Ce[AsO4] in the scheelite-type structure [11] K[ClO3]-type Ce[AsO3] could be found, both as colourless single crystals, formed according to 2 CeO2 þ As2O3 / Ce[AsO4] þ Ce[AsO3].

Table 2 Structure refinement for both forms of Ce[AsO3]. Structure type

a-Pb[SeO3]

Space group Collected reflections Unique reflections Rint/Rs R1/wR2 (for all reflections) Goodness of fit (GooF) Radiation Instrument Structure solution & refinement

P21/m (no. 11) P21/c (no. 14) 18,181 2573 1727 371 0.088/0.044 0.059/0.031 0.061/0.116 0.021/0.042 1.066 1.062 Mo-Ka (l ¼ 71.07 pm) k-CCD (Bruker-Nonius) Program SHELX 97 [32]

K[ClO3]

Table 3 Atomic positions and equivalent isotropic displacement parameters for both forms of Ce[AsO3]. Atom

site

x/a

y/b

z/c

Ueq/pm2

a-Pb[SeO3]-type Ce[AsO3] Ce1 Ce2 As1 As2 O1 O2 O3 O4 O5 O6

4f 4f 4f 4f 4f 4f 4f 4f 4f 4f

0.40289(8) 0.00934(8) 0.20511(15) 0.31219(15) 0.3973(11) 0.1544(11) 0.1315(11) 0.6245(11) 0.3148(11) 0.1101(11)

K[ClO3]-type Ce[AsO3] Ce 2e 0.33667(8) As 2e 0.07900(14) O1 2e 0.6715(11) O2 4f 0.1913(7)

0.16544(9) 0.31088(9) 0.51457(16) 0.59181(16) 0.4595(12) 0.0335(12) 0.3239(12) 0.3646(12) 0.3647(12) 0.6093(11)

0.27132(9) 0.34377(9) 0.04892(16) 0.43871(16) 0.0941(12) 0.3340(12) 0.1167(12) 0.3509(12) 0.4553(12) 0.4097(12)

95(2) 87(2) 89(3) 92(3) 130(19) 136(19) 111(18) 127(19) 119(18) 119(18)

1

0.70369(6) 0.14520(11) 0.1189(8) 0.3404(5)

121(2) 120(2) 197(12) 134(8)

/4 /4 1 /4 0.0023(7) 1

Fig. 1. Oxygen coordination polyhedra around (Ce1)3þ (left) and (Ce2)3þ (right) in a-Pb[SeO3]-type Ce[AsO3].

Table 4 Selected interatomic distances (d/pm) and angles (£/ ) in a-Pb[SeO3]-type Ce[AsO3]. [(Ce1)O9]15 polyhedron Ce1 Ce1 Ce1 Ce1 Ce1 Ce1 Ce1 Ce1 Ce1

e e e e e e e e e

O5 O1 O4 O4 O5 O2 O1 O3 O1

[(Ce2)O8]13 polyhedron 244.1 248.3 250.1 255.3 256.8 262.9 272.5 278.1 286.2

[(As1)O3]3e j1 tetrahedron As1 e O1 As1 e O2 As1 e O3 O1 e As1 e O2 O1 e As1 e O3 O2 e As1 e O3

Ce2 Ce2 Ce2 Ce2 Ce2 Ce2 Ce2 Ce2

e e e e e e e e

O3 O3 O6 O2 O2 O6 O6 O5

239.6 251.0 252.0 252.7 254.6 260.0 265.0 273.1

[(As2)O3]3e j1 tetrahedron 174.2 177.6 177.7 100.2 98.7 96.4

As2 e O4 As2 e O5 As2 e O6 O4 e As2 e O5 O4 e As2 e O6 O5 e As2 e O6

174.0 178.2 179.0 97.1 102.0 94.7

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Fig. 4. View along [100] at the double layer of (Ce1)3þ cations (left) and the layer of (Ce2)3þ cations (right) in a-Pb[SeO3]-type Ce[AsO3]. Fig. 2. View at the crystal structure of a-Pb[SeO3]-type Ce[AsO3] along [001].

3. Results and discussion Crystallographic data and structure refinement parameters for the two dimorphic Ce[AsO3] compounds are given in Tables 1 and 2. The atomic positions and equivalent isotropic displacement parameters are shown in Table 3. Isotypic to the a-Pb[SeO3] crystal structure [33] Ce[AsO3] crystallizes monoclinically in the space group P21/c with the lattice parameters a ¼ 902.89(8), b ¼ 782.54(7), c ¼ 829.68(7) pm and b ¼ 103.392(3) for Z ¼ 8. There are two crystallographically different Ce3þ positions. (Ce1)3þ is coordinated by nine and (Ce2)3þ only by eight oxygen atoms (Fig. 1). Each [(Ce1)O9]15 polyhedron is connected to further five [(Ce1)O9]15 polyhedra via edges and to three [(Ce2)O8]13 polyhedra, one each via a corner, an edge and a face. In the case of the [(Ce2)O8]13 polyhedra there is the same coordinative situation with switched positions. The distances given in Table 4 for both polyhedra (d(CeeO) ¼ 239e286 pm) are in the known range for cerium(III)- oxygen bonds and compare well with Ce2O3 (A-type structure: d(CeeO) ¼ 234e269 pm) [34]. The distance for the hypothetical ninth oxygen ligand (O4)2 for (Ce2)3þ is

Fig. 5. View at the network of [CeO9]15 polyhedra with integrated As3þ cations in the structure of K[ClO3]-type Ce[AsO3] along [010].

with 351.4 pm much longer than the distances for the bonding oxygen atoms (Fig. 1, right). This is a small difference between a-Pb[SeO3] and the isotypic structure of Ce[AsO3], but in the a-Pb[SeO3] structure a loss of connectivity for this O2 anion can also be observed [23]. Layers of (Ce2)3þ and double layers of (Ce1)3þ cations are stacked in an alternating sequence along [100] (Fig. 2). The holes created by the structure of the double layers are filled with two crystallographically different As3þ cations, which both form

Table 5 Selected interatomic distances (d/pm) and angles (£/ ) in K[ClO3]-type Ce[AsO3]. [CeO9]15 polyhedron Ce e O2 Ce e O2 Ce e O1

252.8 (2x) 254.2 (2x) 259.5 (1x)

Ce e O2 Ce e O1

262.8 (2x) 286.8 (2x)

O1 e As e O2 O2 e As e O2

99.1 (2x) 96.2 (1x)

[AsO3]3e j1 tetrahedron Fig. 3. Double layer of (Ce1)3þ cations (top) and layer of (Ce2)3þ cations (bottom) in aPb[SeO3]-type Ce[AsO3].

As e O1 As e O2

175.4 (1x) 175.9 (2x)

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Fig. 6. Coupling of the [CeO9]15 polyhedra in K[ClO3]-type Ce[AsO3] via (O1)2 (bottom left), via (O2)2 (bottom right) and in total (top).

isolated [AsO3]3 oxoanions (d(AseO) ¼ 175e176 pm, £(OeAseO) ¼ 95e102 , see Table 4) with all available oxygen atoms. Each layer comprises two different types of rhomboid (Ce3þ)4 squares. The first type of rhomboid is created by two [OCe3As]10þ tetrahedra, with each one pointing upward and one downward from the plane (Fig. 3). As shown in Fig. 4, the second type of rhomboid is surrounded by four rhomboids of the first type and vice versa like in a distorted chessboard. It is built up by four Ce3þ, four O2 and two As3þ ions, the latter with its lone pair electrons aiming towards the Ce3þ cations at the opposite. Such as La[AsO3] [22] before, Ce[AsO3] can also adopt the crystal structure of K[ClO3] with the lattice constants a ¼ 439.32(4), b ¼ 529.21(5), c ¼ 617.34(6) pm and b ¼ 105.369(3) for Z ¼ 2. In this monoclinic structure there is only one crystallographically unique position for each Ce3þ and each As3þ cation (Fig. 5). Ce3þ is coordinated by nine oxygen atoms (Table 5), which can be split into seven with short bond lengths (d(CeeO) ¼ 254e263 pm) and two with a more than ten percent

167

longer bond length (d(CeeO) ¼ 287 pm). On the one hand Ce3þ cations and (O1)2 anions form chains in a rope-ladder structure (Fig. 6, bottom left), which connects the [CeO9]15 polyhedra via edges, and on the other hand these polyhedra are linked by a plane of [(O2)6]2 prisms (Fig. 6, bottom right). All these prisms share one edge with six other prisms each, where two prisms show the same orientation and four point into the opposite direction. In the case of the As3þ cations both structures of dimorphic Ce [AsO3] are more similar. Every As3þ cation forms [AsO3]3 j1 tetrahedra with all available oxygen atoms, each one with a stereochemically active lone pair on top (Fig. 7), which are connected to the Ce3þ cations via edges and corners. The structure of K[ClO3]type Ce[AsO3] (Table 5) is more symmetric and the distribution of the bond lengths is a little bit closer (d(AseO) ¼ 175e176 pm) as compared to a-Pb[SeO3]-type Ce[AsO3] (d(AseO) ¼ 174e179 pm, Table 4). However, the bond lengths are generally somewhat shorter than in As2O3 (arsenolite: d(AseO) ¼ 179 pm) since the [AsO3]3 units are isolated and not oxygen-linked in the binary oxide. There are short As3þ$$$Ce3þ contacts in the pointing direction of lone pairs at the As3þ cation to the Ce3þ cations in K[ClO3]type Ce[AsO3] (d(As$$$Ce) ¼ 322 pm) and (As1)3þ to Ce3þ in a-Pb[SeO3]-type Ce[AsO3] (d(As1$$$Ce2) ¼ 327 pm) as shortest ones. In the case of the (As2)3þ cation in a-Pb[SeO3]-type Ce[AsO3] the nearest Ce3þ cation is farther away (d(As2$$$Ce1) ¼ 336 pm) and not in the pointing direction of the lone pair. The next Ce3þ cation in this direction is only the third nearest and has a much longer distance (d(As2$$$Ce1) ¼ 347 pm). Most of the oxygen atoms show also the same coordination modes with two cerium atoms bridging to the other oxygen atoms and a third one completing the [OAsCe3]10þ tetrahedron. As an exception from this, O4 and Ce2 belonging to As2 in a-Pb[SeO3]-type Ce[AsO3] are not connected to each other, as it should be supposed. This may also effects the displacement of the As3þ cations from their O,O,O-planes within the isolated [AsO3]3 pyramids. In a-Pb[SeO3]-type Ce[AsO3] the deviation of the (As1)3þ cation form its O,O,O-plane is 86 pm, which is exactly the same value as found for As3þ in K[ClO3]-type Ce[AsO3] and marginally less than the deviation of the (As2)3þ cation from its O,O,O-plane (87 pm). The orientation of the [AsO3]3 anions in the structure of K[ClO3]-type Ce[AsO3] shows a much higher symmetry, as it should be for a high-pressure structure. So the triangular areas as the base of the j1 tetrahedra lie all within the [101] plane in K[ClO3]-type Ce [AsO3], but are tilted in its a-Pb[SeO3]-type polymorph (Fig. 8). Calculations of the MAdelung Part of the Lattice Energy (MAPLE) [35,36] reveal A- or K[ClO3]-type Ce[AsO3] with MAPLE ¼ 15,155 kJ $ mol1 as slightly more stable than B- or a-Pb[SeO3]-type Ce[AsO3] (MAPLE ¼ 15,132 kJ $ mol1). An even higher MAPLE difference shows the pair a-Pb[SeO3] (MAPLE ¼ 16,823 kJ $ mol1) and b-Pb[SeO3] (MAPLE ¼ 16,628 kJ $ mol1), where the mineral molybdomenite (b-Pb[SeO3]) has surprisingly a lower density than its low-temperature polymorph (a-Pb[SeO3]).

Fig. 7. Isolated [AsO3]3 j1 tetrahedra with the coordinating Ce3þ cations in K[ClO3]-type Ce[AsO3] (left) and a-Pb[SeO3]-type Ce[AsO3] (right).

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Fig. 8. View along [001] at the j1-tetrahedral [AsO3]3 anions in the monoclinic structures of a-Pb[SeO3]-type Ce[AsO3] (left) and in K[ClO3]-type Ce[AsO3] (right).

More recent investigations of the mineralogical Pb[SeO3] system [37e39] seem to contradict these findings, however. 4. Spectroscopy The Raman experiments were done with a XploRA Raman microscope (HORIBA Jobin Yvon GmbH, Bensheim, Germany). The single crystal Raman spectrum of a-Pb[SeO3]-type Ce[AsO3] (Fig. 9) shows four bands between 600 and 900 cm1. Assigned to the stretching vibrations are the two strong bands (v1: 769 and 731 cm1), which represent the symmetric and two weaker bands (n3: 659 and 617 cm1) for the asymmetric vibrations. The symmetric bending mode vibrations emerge in the range of 340e410 cm1 and the asymmetric bending modes range between 431 and 467 cm1. Compared to the Raman spectrum of K[ClO3]type Ce[AsO3] (Fig. 9) most bands seems to be split into two or more bands. This could be explained with the two crystallographically different As3þ cations, especially because they do not share the

same coordination of Ce3þ cations as it is shown in Fig. 7. However the Raman spectrum of K[ClO3]-type Ce[AsO3] considers nine signals. The two strong bands are also the stretching vibrations with 742 cm1 for the symmetric one and 641 cm1 for the asymmetric vibration. Also for the bending mode vibrations the bands correspond to the bands of a-Pb[SeO3]-type Ce[AsO3]. For the symmetric bending mode vibrations we obtain two signals (n2: 403 and 375 cm1) and one for the asymmetric bending mode (n4: 474 cm1). The weak bands in the range from 430 to 510 cm1 may also include CeeO vibrations, as these bands are normally not assigned, but present in different Raman spectra of compounds containing trivalent cerium and oxygen. For the assignment of the vibration bands a comparison with different oxoarsenates(III), oxoarsenates(V) and cerium(III) minerals was carried out. So Raman bands around 850 cm1 are assigned to n1 vibration modes and bands around 800 cm1 to n3 vibrations in the vivianite-type oxoarsenate(V) group (M3[AsO4]2$8H2O) with M ¼ Mg, Fe, Co, Ni and Zn. The assignment for the bending modes is 225 cm1 for n2 as

Fig. 9. Single crystal Raman spectra of K[ClO3]-type Ce[AsO3] (red, bottom) and a-Pb[SeO3]-type Ce[AsO3] (blue, mid) recorded at l ¼ 638.0 nm and power infrared spectrum of (impure) a-Pb[SeO3]-type Ce[AsO3] (green, top). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 6 Raman shifts (~ n/cm1) of K[ClO3]-type Ce[AsO3] and a-Pb[SeO3]-type Ce[AsO3] recorded at l ¼ 638.0 nm compared with ludlockite (PbFe4As10O22) [42] (l ¼ 633.0 nm) and monazite (Ce[PO4]) [41] (l ¼ 514.5 nm). K[ClO3]-type Ce[AsO3]

a-Pb[SeO3]-type Ce[AsO3]

Assignment

Ludlockite PbFe4As10O22

Assignment

Monazite Ce[PO4]

742

769 731 659 617

n1 n1 n3 n3

798 774 756 743 674 666 549 536 501 486 470 421 381 368 348 332 287 266

n1 n1 n3 n3

1071 1054 1025 990 970 618 588 568 535 467 414 396 270 254 227 219 183 172 158 152 100 88

641

505 467 431

n4 n4

403 375

388 375 357

n2 n2 n2

250

276 230 189 170 149 126

474

192 133 126

well as 460 and 444 cm1 for n4 [40]. Compared to the cerium(III) oxophosphate(V) monazite [41] (Table 6) the vibration bands are found at about 200 cm1 lower wavenumbers and the order of the n1 and n3 stretching modes is interchanged. This change can also €hnite (Fe4As5O13) and ludlockite be found for schneiderho (PbFe4As10O22) [42] (Table 6) with even lower wavenumbers for the stretching vibrations and matches well with the cerium(III) oxoarsenates(III) reported here. In the case of the bending modes both cerium(III) oxoarsenates(III) show a shift of the v4 bands to higher wavenumbers comparable to these in the vivianite cases. A Nicolte iS5 FT-IR spectrometer (Thermo Fisher Scientific, Inc., Waltham, MA, USA) with ZnSe ATR was used for the powder infrared experiment with (impure) a-Pb[SeO3]-type Ce[AsO3]. IR bands were observed at 985, 845, 794, 748, 693 and 606 cm1, but detailed assignments are hampered by the included impurities. 5. Conclusion Both polymorphs of Ce[AsO3] show many similarities in the structure, but in detail the a-Pb[SeO3]-type Ce[AsO3] is deformed somewhat due the loss of the connection of the ninth oxygen ligand at (Ce2)3þ. In further publications we want to name the K[ClO3]type Ce[AsO3] as the A-type Ce[AsO3], because it is the denser (Dx ¼ 6.31 g $ cm3), more stable (MAPLE ¼ 15,155 kJ $ mol1) and higher ordered structure. The less dense (Dx ¼ 6.13 g $ cm3), less stable (MAPLE ¼ 15,132 kJ mol1) and more disordered polymorph with the a-Pb[SeO3]-type structure we want to name B-type Ce[AsO3]. This is quite the other way around as in the Pb[SeO3] case (see Table 1), in which a-Pb[SeO3] has a higher density (Dx ¼ 6.99 g $ cm3) than b-Pb[SeO3] (Dx ¼ 6.93 g $ cm3). Acknowledgments We thank Dr. Falk Lissner for the X-ray single-crystal diffraction measurements. References [1] M. Strada, G. Schwendimann, Gazz. Chim. Ital. 64 (1934) 662e674. [2] A. Brahimi, M.M. Ftini, H. Amor, Acta Crystallogr. 58 (2002) 98e99. [3] D.-H. Kang, Th. Schleid, Z. Anorg. Allg. Chem. 631 (2005) 1799e1802.

n4 n4 n2

Assignment

n3 n1 n4

n2

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