Hydrothermal Synthesis of Rare Earth Iodates from the CorrespondingPeriodates: II1). Synthesis and Structures of Ln(IO3)3 (Ln � Pr, Nd, Sm, Eu,Gd, Tb, Ho, Er) and Ln(IO3)3 · 2H2O (Ln � Eu, Gd, Dy, Er, Tm, Yb)

Paul Douglas, Andrew L. Hector*, William Levason, Mark E. Light, Melissa L. Matthews, andMichael Webster

Southampton/UK, University of Southampton, School of Chemistry

Received November 17th, 2003.

Abstract. Single crystals of lanthanide iodates have been quicklygrown by decomposition of the corresponding periodates underhydrothermal conditions. Single crystal X-ray diffraction showedthat two structure types form with the elements from Pr-Yb, ananhydrous form for Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er and a dihy-drate for Eu, Gd, Dy, Er, Tm, Yb. A detailed structure study ispresented for one representative of each of these types, along withstructure type and lattice parameters for the other materials.

Hydrothermal-Synthese von Seltenerdiodaten aus den entsprechenden Periodaten. II.Synthese und Strukturen von Ln(IO3)3 (Ln � Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er) undLn(IO3)3·2H2O (Ln � Eu, Gd, Dy, Er, Tm, Yb)

Inhaltsübersicht. Schnell wachsende Einkristalle von Lanthanidio-daten entstehen bei der Zersetzung der entsprechenden Periodateunter Hydrothermalbedingungen. Einkristall-Röntgenstrukturbe-stimmungen zeigen, daß zwei Strukturtypen gebildet werden, näm-lich die wasserfreie Form von Pr, Nd, Sm, Eu, Gd, Tb, Ho und Er,und das Dihydrat von Eu, Gd, Dy, Er, Tm und Yb. EingehendeStrukturbestimmungen werden von je einem repräsentativen Bei-spiel der beiden Reihen ausgeführt, von allen anderen Verbindun-gen werden der Strukturtyp und die Gitterparameter mitgeteilt.

Introduction

Transition metal, lanthanide, and actinide iodates continueto attract considerable attention due to their potential ap-plications in nonlinear optical and ferroelectric devices[1�4]. The water insoluble lanthanide iodates are usuallyobtained as powders from aqueous media, although crys-tals of some examples were obtained many years ago bycrystallisation from boiling concentrated mineral acids orgrowth in gels [5�10]. We recently reported that single crys-tals of Sc(IO3)3, Y(IO3)3·2H2O, La(IO3)3·1/2H2O andLu(IO3)3·2H2O could be obtained by hydrothermal meth-

1) Part 1 is reference [4]* Dr Andrew L. HectorSchool of ChemistryUniversity of SouthamptonHighfieldSouthampton SO17 1BJ/UKE-mail: A.L.Hector@soton.ac.uk

Z. Anorg. Allg. Chem. 2004, 630, 479�483 DOI: 10.1002/zaac.200300377 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 479

Tb(IO3)3: Space group P21/c, Z � 4, lattice dimensions at 120 K:a � 7.102(1), b � 8.468(1), c � 13.355(2) A, β � 99.67(1)°; R1 �

0.034.Yb(IO3)3 · 2H2O: Space group P1, Z � 2, lattice dimensions at

120 K: a � 7.013(1), b � 7.370(1), c � 10.458(2) A, � � 95.250(5),β � 105.096(5), γ � 109.910(10)°; R1 � 0.024.Keywords: Hydrothermal; Iodate; Lanthanide; Terbium; Ytter-bium; Crystal structure

Tb(IO3)3: Raumgruppe P21/c, Z � 4, Gitterkonstanten bei120 K: a � 7,102(1), b � 8,468(1), c � 13,355(2) A, β � 99,67(1)°;R1 � 0,034.

Yb(IO3)3 · 2H2O: Raumgruppe P1, Z � 2, Gitterkonstanten bei120 K: a � 7,013(1), b � 7,370(1), c � 10,458(2) A, � � 95,250(5),β � 105,096(5), γ � 109,910(10)°; R1 � 0,024.

ods from mixtures of the corresponding periodates and per-iodic acid [4]. In this second and final paper we have ex-tended this approach to the remaining lanthanide elementsand used a combination of single crystal X-ray diffraction,PXRD and IR spectroscopy to identify the productsformed. We have also attempted to relate our products topreviously reported phases.

Results

The method used to grow lanthanide iodate single crystalswas based on that employed previously for lanthanum andlutetium iodates [4]. Samples of good quality single crystalswere obtained in all cases tried except Ce. Cerium(IV) per-iodate did not produce cerium(III) iodate crystals underhydrothermal conditions, instead forming lumps ofamorphous gel. Hydrothermal treatment of erbium per-iodate resulted in two distinct crystal types in the samebatch and small variations in growth conditions were usedto obtain good quality samples of each. With lanthanides

P. Douglas, A. L. Hector, W. Levason, M. E. Light, M. L. Matthews, M. Webster

Fig. 1 Packing diagram showing 2 unit cells of Tb(IO3)3 (doubledin the a direction) with TbO8 polyhedra giving a partial atom num-bering.

from Pr-Yb (except Pm) we obtained two structure types,anhydrous Ln(IO3)3 (Ln � Pr, Nd, Sm, Eu, Gd, Tb, Ho,Er) and a dihydrate Ln(IO3)3·2H2O (Ln � Eu, Gd, Dy, Er,Tm, Yb). We determined the structures of most of thesamples prepared and present one example of each of thesetypes and unit cell data for the rest.

Structural studies

Tb(IO3)3

The centrosymmetric structure (Fig. 1 and Table 1) consistsof TbO8 coordination spheres linked into a 3-dimensionalnetwork structure by pyramidal [IO3]� groups (O�I�O95.9�103.2°). The I(1) and I(2) iodate groups have each oftheir three oxygen atoms coordinated to a different Tb atomwhereas I(3) has two oxygen atoms coordinated to differentTb atoms and one “dangling” oxygen. All of the O atomsexcept O(8) are coordinated to Tb (I�O�Tb127.8(3)�158.1(3)°). Two of the pyramidal [IO3]� groupiodine atoms, I(1) and I(2), also have longer range interac-tions with a fourth oxygen atom (Fig. 2), these are O(8) andO(9) which are part of the I(3) iodate group (O(9) is alsoTb-coordinated). This fourth interaction results in an over-all “saw-horse” arrangement. The Gd(IO3)3 structure hasbeen reported and is isostructural with the Tb(IO3)3 [5].

Yb(IO3)3·2H2O

The centrosymmetric structure (Fig. 3 and Table 2) consistsof the expected pyramidal [IO3]� ions (O�I�O95.1�101.3°). The eight-coordinate Yb atom comprises onewater molecule (O(10)) and seven O atoms from each of theiodate anions; I(1) and I(3) each provide two O atoms andfor I(2) all three of the O atoms are coordinated to Ybatoms (I�O�Yb 121.2(2)�142.4(2)°). In all cases iodate Oatoms are coordinated to different Yb atoms, no chelatingiodate groups are observed. The second hydrate molecule

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 479�483480

Fig. 2 Iodate group geometries in Tb(IO3)3.

Table 1 Selected bond lengths/A and angles/° for Tb(IO3)3

I(1)�O(1) 1.818(5) I(3)�O(7) 1.805(5)I(1)�O(2) 1.783(5) I(3)�O(8) 1.801(5)I(1)�O(3) 1.803(5) I(3)�O(9) 1.802(5)I(2)�O(4) 1.804(5) Tb(1)...O 2.259(5)�2.549(5)I(2)�O(5) 1.811(5)I(2)�O(6) 1.818(5)

O(1)�I(1)�O(2) 96.0(2) O(7)�I(3)�O(8) 100.5(2)O(1)�I(1)�O(3) 98.4(2) O(7)�I(3)�O(9) 97.6(2)O(2)�I(1)�O(3) 95.9(2) O(8)�I(3)�O(9) 100.9(2)O(4)�I(2)�O(5) 96.6(2) I�O�Tb(1) 127.8(3)�158.1(3)O(4)�I(2)�O(6) 103.3(2)O(5)�I(2)�O(6) 96.8(2)

(O(11)) is not involved with the Yb atom and both watermolecules are convincingly hydrogen bonded to iodate Oatoms as evidenced by the O...O distances (2.763(7) to2.942(7) A) and O�H...O angles (158(8) to 163(8)°). H...Odistances are 2.01(9)�2.17(9) A.

Longer I-O interactions feature strongly inYb(IO3)3·2H2O (Fig. 4). I(3) exhibits the overall saw-horseshape observed in two of the Tb(IO3)3 iodate groupswhereas I(1) and I(2) each have three longer interactionsresulting in an overall approximately octahedral shape, re-sulting from their proximity to each other, they effectivelyform chains of alternating I(1) and I(2) along the b-axis.Yb(IO3)3·2H2O is isostructural with one of the polymorphsof Y(IO3)3·2H2O [4].

Other measurements

IR spectra were obtained from a small number of crystalstaken from the batches used for X-ray studies. The presenceof bands due to water at �3300 and 1620 cm�1 identifiedthe Tm, Yb and one form of the Eu, Gd and Er iodatecrystals as hydrates. One form of Eu, Gd and Er, and thesamples of Pr, Nd, Sm, Tb, Dy, Ho were anhydrous. Pre-

Hydrothermal Synthesis of Rare Earth Iodates

Fig. 3 Unit cell of Yb(IO3)3·2H2O with YbO8 polyhedra giving a partial atom numbering.

Table 2 Selected bond lengths/A and angles/° for Yb(IO3)3·2H2O

I(1)-O(1) 1.807(4) I(3)-O(7) 1.794(5)I(1)-O(2) 1.814(4) I(3)-O(8) 1.814(4)I(1)-O(3) 1.805(4) I(3)-O(9) 1.821(4)I(2)-O(4) 1.809(4) Yb(1)...O(I) 2.304(5)-2.390(4)I(2)-O(5) 1.813(4) Yb(1)...O(10) 2.330(5)I(2)-O(6) 1.824(5)

O(1)-I(1)-O(2) 95.8(2) O(7)-I(3)-O(8) 98.6(2)O(1)-I(1)-O(3) 96.5(2) O(7)-I(3)-O(9) 101.3(2)O(2)-I(1)-O(3) 96.9(2) O(8)-I(3)-O(9) 96.1(2)O(4)-I(2)-O(5) 99.6(2) I-O-Yb(1) 121.2(2)-142.4(2)O(4)-I(2)-O(6) 95.1(2)O(5)-I(2)-O(6) 97.7(2)

vious studies of powdered iodates [6, 7] reported IR spectraand TGA data and attempted to distinguish different struc-tural forms, although it appears that some products mayhave contained HIO3, and in other cases mixtures may haveformed complicating identification [7, 8].

Our anhydrous lanthanide iodate phases for Sm-Tb andHo have IR spectra which correspond well with the “anhy-dride type 1” [7], whilst the Ln(IO3)3·2H2O (Ln � Eu, Gd,Er, Tm, Yb) have spectra in moderate agreement with “di-hydrate type II” [7]. For Dy alone the IR spectra showedthe presence of the “anhydride type I”, this sample musthave been a mixture since the crystal chosen for the X-raystudy was of the dihydrate Dy(IO3)3·2H2O.

In an attempt to determine whether the bulk samplesmatched the picked single crystals some were dried in vacuoat room temperature and ground for study by powder X-

Z. Anorg. Allg. Chem. 2004, 630, 479�483 zaac.wiley-vch.de 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 481

Fig. 4 Iodate group geometries in Yb(IO3)3·2H2O.

ray diffraction. Gd(IO3)3 yielded a pattern which closelymatched one generated from the single crystal determinedstructure. However, dried and ground Yb(IO3)3·2H2O was

P. Douglas, A. L. Hector, W. Levason, M. E. Light, M. L. Matthews, M. Webster

Table 3 Cell dimensions for hydrothermally synthesized lanthanide iodates at 120 Ka)

Ln Colour a b c � β γ Notes

La White 7.219(2) 11.139(4) 10.708(3) � 91.86(1) � b), ref. [4]Pr Green 7.260(1) 8.528(1) 13.551(2) � 100.39(5) � c)

Nd Off-white 7.238(1) 8.520(1) 13.515(2) � 100.26(1) � c)

Sm Off-white 7.174(1) 8.489(1) 13.436(2) � 99.93(1) � c)

Eu Off-white 7.130 8.467 13.386 � 99.91 � c)

Eu Off-white 7.131(2) 7.368(2) 10.659(4) 94.84(1) 105.39(2) 109.84(1) d)

Gd Off-white 7.085(1) 7.349(2) 10.576(3) 94.947(5) 105.32(2) 109.82(1) d),e)

Tb Off-white 7.102(1) 8.468(1) 13.355(2) � 99.67(1) � c)

Dy Off-white 7.053(2) 7.362(2) 10.524(4) 95.14(2) 105.18(2) 109.90(1) d)

Ho Pink 7.054(1) 8.449(1) 13.305(2) � 99.668(6) � c)

Er Orange 7.036(1) 8.443(2) 13.289(2) � 99.67(1) � c)

Er Pink 7.037(2) 7.369(2) 10.509(3) 95.20(1) 105.10(2) 109.87(2) d)

Tm Off-white 7.026(2) 7.374(2) 10.469(4) 95.23(2) 104.93(2) 109.80(2) d)

Yb Off-white 7.013(2) 7.370(1) 10.458(2) 95.25(2) 105.10(2) 109.91(5) d)

Lu White 7.2652(9) 7.4458(2) 9.3030(3) 79.504(1) 84.755(1) 71.676(2) f), ref. [4]

a) Single crystal data recorded at 120 K using a Bruker Nonius CCD diffractometer. Cell dimensions in A and degrees (°). The su (esd)values taken from the diffractometer software are unreasonably small [11] and the values here are � 5.b) La(IO3)3·1/2H2O. Monoclinic, Z � 2. Space group Pn (no. 7).c) Ln(IO3)3. Monoclinic, Z � 4. Space group P21/c (no. 14).d) Ln(IO3)3·2H2O. Triclinic, Z � 2. Space group P1 (no. 2).e) Crystals of type (c) were also obtained with similar cell dimensions to those previously published [5].f) Different Ln(IO3)3·2H2O polymorph. Triclinic, Z � 2. Space group P1 (no. 2).

found to be amorphous by PXD. Similarly a mixed phaseEr sample gave a powder pattern corresponding to onlyEr(IO3)3 even though the sample used clearly containedpink and orange crystals, demonstrated by the single crystalexperiments to be the dihydrate (major product) and theanhydrous material (minor product) respectively. Thus itseemed we could only use PXD to demonstrate the presenceof the anhydrous iodate, the dihydrate lost crystallinity ondrying and grinding.

Discussion

The lanthanide iodate materials which we have been able toobtain as single crystals by decomposition of the periodatesunder hydrothermal conditions are listed in Table 3. This isa quick and reproducible method to produce large singlecrystals of these materials. Only La(IO3)3·1/2H2O is non-centrosymmetric and thus of any potential use as a secondharmonic generator. Cerium is oxidised to Ce4� in the pres-ence of periodate and hydrothermal treatment of ce-rium(IV) periodate under the conditions used herein led tolumps of an amorphous gel-like material. A cerium(III) iod-ate has been reported previously [12] but we could not pro-duce one by these methods.

According to the classification employed previously byNassau and co-workers [7], the anhydrous lanthanide iodatecompounds produced herein all correspond to “anhydridetype I”. This is consistent with the range of phases observedcrystallographically and with the IR spectra. Two dihydratestructure types were originally assigned from IR data [7],type I which they obtained for Tm-Lu and type II for Nd-Er. We have previously determined structures for type I di-hydrate phases with hydrothermally grown Lu(IO3)3·2H2O[4] and Y(IO3)3·2H2O [13] grown by evaporation. The dihy-drates herein are type II, we also found this structure type

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 479�483482

for a second polymorph of Y(IO3)3·2H2O [4]. Hence wefind significant overlap in where these two structures formin the lanthanide series.

The iodate groups observed in all the rare earth iodatematerials we have examined are the expected pyramidal[IO3]� structure. Frequently a further 1 or 3 longer I...Ointeractions around 2.7 A and with O-I...O around 170° arefound with approximate “saw-horse” or a highly distortedoctahedral geometry. We have not found any cases in therare earth iodates of the symmetric [IO4]3� groups recentlyreported in Ag4(UO2)4(IO3)2(IO4)2O2 [14].

Experimental Section

50 mg amorphous lanthanide periodate, 2 g H5IO6 (Avocado) and15 cm3 distilled water were placed in a 23 cm3 Parr 4749 Teflonlined hydrothermal bomb. This was heated at 160 °C for 48h andwhen allowed to cool, crystals were collected from the resultantsolution. The exception was Ho, where this method led to a powderbut increasing the temperature to 170 °C yielded single crystals.

All samples appeared to be monophasic when examined under amicroscope except those with Er, which always yielded a mixtureof pink and pale orange crystals. Using the conditions above weobtained good quality crystals of pink Er(IO3)3·2H2O but the or-ange material was polycrystalline. Using 50 mg erbium periodatewith 1 g H5IO6 and 8 cm3 water and the same heating cycle weobtained a good quality orange Er(IO3)3 crystal, in this case thepink material was polycrystalline. In all cases the pink material wasabout 80�90 % of the sample volume.

Powder X-ray diffraction data were collected in Bragg-Brentanogeometry using a Siemens D5000 diffractometer with Cu-Kα1 radi-ation over a 2θ range 10�70°. IR spectra were obtained using CsIplates on a Perkin Elmer 983G spectrometer over the range4000�200 cm�1, samples were powdered and mulled with Nujol.

Hydrothermal Synthesis of Rare Earth Iodates

Table 4 Crystal data and structure refinement dataa)

Tb(IO3)3 Yb(IO3)3·2H2O

Formula I3O9Tb H4I3O11YbFormula weight 683.62 733.77Crystal system monoclinic triclinicSpace group P21/c (no. 14) P1 (no. 2)a/A 7.102(1) 7.013(1)b/A 8.468(1) 7.370(1)c/A 13.355(2) 10.458(2)α/° 90 95.250(5)β/° 99.67(1) 105.096(5)γ/° 90 109.91(1)V/A3 791.8(2) 480.9(1)Crystal size/mm 0.06 � 0.08 � 0.10 0.06 � 0.06 � 0.04Z, Dcalcd (g cm�3) 4, 5.735 2, 5.067F(000)/e 1184 642Total no. of observns 8622 6411No. of unique observns (Rint) 1812 (0.083) 1682 (0.037)No. of parameters 119 145µ(Mo-Kα)/mm�1 20.65 19.40S 1.17 1.08Residual electron density/e A�3 �3.12 to �2.22 �1.19 to �1.07Rb (Fo > 4 (Fo)) 0.033 0.021R (all data) 0.034 0.024wR2b (all data) 0.081 0.054

a) Radiation Mo-Kα (λ� 0.71073 A); T � 120 K.b) R � Σ�Fo� � �Fc�/Σ�Fo�. wR2 � [Σw(Fo2 � Fc2)2/ΣwFo4]1/2.

X-ray Crystallography

Cell dimensions and intensity measurements were made using aBruker Nonius Kappa CCD diffractometer fitted with graphitemonochromated Mo-Kα radiation with the crystals maintained at120 K in a nitrogen gas stream. A multi-scan absorption correction(SORTAV [15] or SADABS (Yb) [16]) was routinely employed butbecause of the large linear absorption coefficient for these materi-als, efforts were made to use analytical corrections from indexedfaces. All the crystals examined were of fragments cleaved fromlarger crystals and several were found to be split. The materialswere hard and our efforts to grind spheres using a local version ofthe Bond [17] apparatus were not successful unlike the Gd(IO3)3

structure [5]; the art of grinding spheres is declining. We report ourbest anhydrous (Tb) and dihydrate (Yb) iodate structures (Table 4),and all the crystals examined fall into one of these two structuraltypes with the cell dimensions reported in Table 3. For theYb(IO3)3·2H2O convincing positions for all the H atoms wereidentified and supported by the O-H...O hydrogen-bonded dis-tances and angles. The structure solution [18] and refinement wereroutine with full-matrix least-squares refinement on F 2 [19]. Thelargest positive and negative peaks in the final difference electron-density map were close to the heavy atoms.

Z. Anorg. Allg. Chem. 2004, 630, 479�483 zaac.wiley-vch.de 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 483

Further details of the crystal structures are available from the Fach-informationszentrum Karlsruhe, D-76344 Eggenstein-Leopolds-hafen (Germany), on quoting the depository numbers CSD-413351(Tb), -413352 (Yb), the names of the authors, and the citation ofthe paper.

Acknowledgements. We thank the Royal Society for a UniversityResearch Fellowship (ALH), the EPSRC for financial support(MLM) and Professor M. B. Hursthouse for diffractometer access.We wish to acknowledge use of the EPSRC’s chemical databaseservice at Daresbury [20].

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