Journal of Solid State Chemistry 203 (2013) 240–246

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Journal of Solid State Chemistry

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New high pressure rare earth tantalates RExTa2O5+1.5x (RE¼La, Eu, Yb)

Igor P. Zibrov a,n, Vladimir P. Filonenko a, Nikolai D. Zakharov b, Peter Werner b,Dmitrii V. Drobot c, Elena E. Nikishina c, Elena N. Lebedeva c

a Institute for High Pressure Physics, Russian Academy of Sciences, Kaluzhskoe Highway 14, Troitsk, Moscow 142190, Russiab Max Planck Institute for Microstructure Physics, Weinberg 2, D-06120 Halle/Saale, Germanyc Lomonosov Moscow University of Fine Chemical Technology, Prospect Vernadskogo 86, Moscow 119571, Russia

a r t i c l e i n f o

Article history:Received 11 December 2012Received in revised form6 March 2013Accepted 25 March 2013Available online 4 April 2013

Keywords:High pressureTantalum pentoxideRare-earth tantalatesX-ray diffractionElectron microscopy

96/$ - see front matter & 2013 Elsevier Inc. Ax.doi.org/10.1016/j.jssc.2013.03.057

esponding author. Fax: +7 495 8510012.ail addresses: zibrov@mail.ru, zibrov@hppi.tropi.troitsk.ru (V.P. Filonenko), zakharov@mpi-hmpi-halle.mpg.de (P. Werner), dvdrobot@ma

nick@mail.ru (E.E. Nikishina), helena_nick@m

a b s t r a c t

Rare earth tantalates La0.075Ta2O5.113, Eu0.089Ta2O5.134 and Yb0.051Ta2O5.077 have been prepared by solidstate reaction at P¼7.0 GPa and T¼1050–1100 1C and studied by X-ray diffraction, thermal analysis andelectron microscopy. Low hydrated amorphous tantalum, lanthanum, europium and ytterbium hydro-xides were used as starting materials. Aqueous as well as anhydrous compounds were obtained. Titletantalates are crystallized in the structure type of F–Ta2O5 [Zibrov et al. Russ. J. Inorg. Chem. 48 (2003)464–471] [5]. The structure was refined by the Rietveld method from X-ray powder diffractometer data:La0.075Ta2O5.113, a¼10.5099(2), b¼7.2679(1), c¼6.9765(1) Å, V¼532.90(1) Å3, Z¼6, space group Ibam;Eu0.089Ta2O5.134, a¼10.4182(3), b¼7.2685(1), c¼6.9832(1) Å, V¼528.80(2) Å3, Z¼6, space group Ibam;Yb0.051Ta2O5.077, a¼10.4557(2), b¼7.3853(1), c¼6.8923(1) Å, V¼532.21(1) Å3, Z¼6, space group Ibam.RE atoms do not replace the tantalum in its positions but the only water in the channels of the structure.Highly charged cations RE+3 compress the unit cell so that its volume becomes less than that of F–Ta2O5.Significant decrease of the unit cell volume after water removal from the structure is possible due to thepuckering of pentagonal bipyramid layers and change of the corrugation angle in the layer.

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1. Introduction

Many of the tantalates are related to materials of electronicsand optics because of their piezoelectrical, pyroelectrical, electro-optical, luminescent properties. In particular, geptatantalatesEuxLa1−xTa7O19 and others are promising in terms of their lumi-nescent properties [1–3]. So, searching for new tantalum oxides isvery important for science and new technologies. Hydrate F–Ta2O5∙2/3H2O and derived from it F–Ta2O5 oxide were obtainedby thermobaric treatment of low hydrated amorphous tantalumhydroxide [4,5]. The structure of both compounds is built up frompuckering planes of pentagonal bipyramids (PB) [TaO7], linked toeach other by the columns of bisdisphenoids (BDP) [TaO8]. Thelarge channels are separated by two planes of PB and two columnsof BDP (Fig. 1) and occupied by water molecules in the initialhydrate. After water removal at ambient pressure, the channels arereduced in size (collapse) and anhydrous oxide F–Ta2O5 is formed.The size of channels, as well as the distance between them arealmost exactly the same as the identical parameters in hexagonal

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itsk.ru (I.P. Zibrov),alle.mpg.de (N.D. Zakharov),il.ru (D.V. Drobot),ail.ru (E.N. Lebedeva).

tungsten bronzes (HTB) [6–8]. We have suggested [5] the possibi-lity of substitution of water molecules in the channels of thehydrate structure by metal atoms with the formation of oxidizedcompounds МеxTa2O5+y, or bronzes МеxTa2O5.

In the first stage we have attempted to synthesize the oxidizedphases. Rare earth elements were selected as metals which allowus to trace the evolution of structural parameters, as a function ofthe cations size. This paper presents the results obtained usinglarge lanthanum (La+3, r¼1.32 Å (C.N. 8) [9]) middle europium(Eu+3, r¼1.21 Å (C.N. 8) [9]) and small ytterbium cations (Yb+3,r¼1.12 Å (C.N. 8) [9]). Thus, the goal of this work is to replace thewater molecules in the channels of the F–Ta2O5∙2/3H2O structureby rare-earth atoms and to reveal the influence of these atoms onthe structural parameters as well.

2. Experimental

Low hydrated amorphous tantalum hydroxide was prepared byinteraction of solid tantalum pentachloride with ammonia solutionby known method [4,10]. Low hydrated amorphous lanthanum,europium and ytterbium hydroxides were prepared by a similarway—the interaction of rare earth chlorides with ammonia solu-tion [11,12]. For further synthesis of oxide phases, powder mixturescontaining the calculated amount of the starting low hydratedamorphous hydroxides in relation 1:7 which corresponds to the

Fig. 1. The structure of Yb0.051Ta2O5.077∙0.3H2O (a) and Yb0.051Ta2O5.077 (b) along axis [001]. RE atoms and water molecules are shown as red circles. Pentagonal bipyramids(PB) [TaO7] are green, bisdisphenoids (BDP) [TaO8] are brown. This figure was drawn with the help of ATOMS program [20]. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

Table 1Crystal data and structure refinement for Eu0.089Ta2O5.134∙0.28H2O andEu0.089Ta2O5.134.

Chemical formula Eu0.089Ta2O5.134∙0.28H2O Eu0.089Ta2O5.134

Chemical formula weight 465.25 460.82Space group I b a m I b a ma (Å) 10.8812(1) 10.4182(3)b (Å) 7.2681(1) 7.2685(1)c (Å) 6.8492(1) 6.9832(1)V (Å3) 541.68(1) 528.80(2)Z 6 6Dx (Mg m−3) 8.558 8.682RF 0.033 0.107RP 0.062 0.039RWP 0.088 0.041d (Durbin–Watson [18]) 0.062 0.355FWHMmin (deg) 0.17 0.22FWHMmax (deg) 0.25 0.47No. of parameters used 51 46

Table 2Crystal data and structure refinement for La0.075Ta2O5.113∙0.47H2O andLa0.075Ta2O5.113.

Chemical formula La0.075Ta2O5.113∙0.47H2O La0.075Ta2O5.113

Chemical formula weight 465.17 457.69Space group I b a m I b a ma (Å) 10.8729(1) 10.5099(2)b (Å) 7.2611(1) 7.2679(1)c (Å) 6.8401(1) 6.9765(1)V (Å3) 540.02(1) 532.90(1)Z 6 6Dx (Mg m−3) 8.582 8.557RF 0.039 0.074RP 0.062 0.065RWP 0.087 0.093d (Durbin–Watson [18]) 0.192 0.103FWHMmin (deg) 0.09 0.09FWHMmax (deg) 0.29 0.49No. of parameters used 51 52

I.P. Zibrov et al. / Journal of Solid State Chemistry 203 (2013) 240–246 241

geptatantalates RETa7O19 composition (in terms of oxide molarcomposition RE2O3:7Ta2O5) were prepared. The hydroxide mix-tures were stirred in water at 60 1C for 4 h using vibration followedby evaporation to remove water excess. The resulting powder wassubjected to microwave treatment for 1.5 h at W¼100W. Theamount of water in the final material was 10–25%.

Method of material thermobaric treatment is described in detailin [13,14]. Starting amorphous powder was pressed into pellets witha diameter of 5 mm and a height of 3–4 mm. To prevent the chemicalinteraction samples were isolated from the graphite heater by the foil(Ta, W). Synthesis was carried out at P¼7.0 GPa, T¼1050–1100 1C,t¼5 min., i.e. in the stability range of F–Ta2O5∙2/3H2O on the phasediagram, namely, P¼6.0–8.0 GPa, T¼950–1150 1C.

Extracted from the high pressure chamber samples werepurified mechanically from the protective foil and subjected toX-ray phase analysis on a diffractometer AXS with the XY position-sensitive detector (Bruker, Germany). Selected for structural ana-lysis samples were run in the Imaging Plate Guinier-camera G670(Huber, Germany) (Cu Kα1 radiation). The obtained X-ray datawere subjected to full-profile analysis procedure using the soft-ware package GSAS [15,16]. The peak shapes could be described bya symmetric, or nearly symmetric, pseudo-Voigt function.

Differential thermal analysis (DTA) combined with thermogra-vimetry (TG) were carried out in air at the ULVAC TGD 7000(Sinku-Riko, Japan).

IR absorption spectra were obtained on Fourier-spectrometerVertex-70 (Bruker, Germany) in transmission mode with a resolu-tion of 4 cm−1. Pellets for experiments were pressed from mixtureof pure KBr and 0.5% of the investigated material.

Specimens for High Resolution Transmission Electron Micro-scopy (HRTEM) investigations were prepared by crushing of smallamount of the sample in an agate mortar with following disper-sing it in butanol. Droplet of the suspension was put onto holeycarbon film supported by a Cu grid. The microscope JEM4010equipped with side entry goniometer and operated at 400 kVaccelerating voltage was used. HRTEM images were taken atScherzer underfocus Δf¼−30 nm. At this defocus cation appearas a dark spots in the image [17]. The 200 kV electron microscopePhilips CM20 equipped with high-angle side entry goniometer(maximum tilting angle 7451) and Energy Dispersive X-ray (EDX)detector was used to collect structural and compositional informa-tion as well from the same crystal fragment.

The first experiments show that after P–T treatment samplescontained 10–20% of foreign phases, which are hard to identify.Moreover, the refinement of the compounds structure, in parti-cular, occupancy of the positions in the channels show that the

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concentration of rare-earth atoms is much less than their contentin the initial mixture. Therefore, for further work starting mixtureswere diluted by half. For this purpose, to the material consisting of1 mol fraction of RE2O3 and 7 mol fractions of Ta2O5, low hydratedamorphous tantalum hydroxide powder in an amount to provide amolar ratio of 1 mol fraction of RE2O3 and 14 mol fractions ofTa2O5 was added. The powders were thoroughly mixed first withthe use of dry mix, and then under acetone. This procedure shouldgive the formula of the compound RE0.143Ta2O5.214.

Table 3Experimental details for Yb0.051Ta2O5.077∙0.3H2O and Yb0.051Ta2O5.077.

Chemical formula Yb0.051Ta2O5.077∙0.3H2O Yb0.051Ta2O5.077

Chemical formula weight 460.91 456.11Space group I b a m I b a ma (Å) 10.9056(1) 10.4557(2)b (Å) 7.2751(1) 7.3853(1)c (Å) 6.8553(1) 6.8923(1)V (Å3) 543.89(1) 532.21(1)Z 6 6Dx (Mg m-3) 8.443 8.539RF 0.042 0.073RP 0.060 0.056RWP 0.082 0.074d (Durbin–Watson [18]) 0.087 0.343FWHMmin (deg) 0.18 0.18FWHMmax (deg) 0.25 0.64No. of parameters used 51 47

Fig.2. Thermogravimetric analysis of the sample Eu0.089Ta2O5.134 � 0.28H2O. Sampleweight—45.7 mg, heating rate on air—10 ○C/min.

Fig. 3. Rietveld refinement of Eu0.089Ta2O5.134∙0.28H2O (a) and Eu0.089Ta2O5.134 (b). Obse(bottom curve) X-ray powder diffraction profiles are shown. The positions of all the allo

As it was mentioned earlier [5], the stoichiometry of the F-phaseunit cell is Me12O32. Required stoichiometry Me12O30 (Me2O5) isrealized by deficiency of two oxygen atoms per unit cell. It can bedone due to partial occupancy of the positions O(1) and O(3). Thus, theoxygen introduced into the cell by the rare earth atoms (about oneatom O) may occupy positions O(1) and O(3). During structurerefinement occupancy of the sites O(1) and O(3) was considered equalto 1.0, as slight deviations from this value had no effect on theparameters of refinement (the electron density associated with a lessthan one atom of oxygen in the cell is negligible compared to theelectron density associated with the other atoms). As the occupationfactors are highly correlated with the displacement parameters, thestrategy of refinement is based on the value of Uiso of water moleculesin the channels (OCC¼1) defined previously [5]. This value(Uiso¼0.03 Å2) was fixed during refinement. Also, like in the case ofpure F–Ta2O5, rare earths anhydrous phases are crystallized signifi-cantly worse than the aqueous ones, as evidenced by the width of thediffraction peaks (FWHM) (Tables 1–3) what led to the difficulties inrefinement. Therefore, during the refinement of the structure ofanhydrous phases the occupancy of rare earth atoms were fixed,and the value corresponded to site occupancy in the aqueous material.This step is completely justified, because removing water fromcompound cannot change rare earth content in it. Whether RE atomssubstitute Ta in BDP or PB was tested in refinement. It turned out thatthe RE atoms replace only the water, and tantalum is not (probably,because of the large difference in the size of Ta and RE).

3. Results

3.1. DTA-TG investigations and Rietveld refinement.

Thermogravimetric analysis of the samples after P–T treatmentshow (Fig. 2, Supplementary Figs. S1, S2) that the water is removedfrom the material in the range T≈400–650 1C (sample with Eu),T≈280–650 1C (sample with La) and T¼550–650 1C (sample withYb). The total amount is 1.1% in case of Eu, 1.9%—La and 1.2%—Yb.The breaking on the TG curves is associated with a strongexothermic effect at T¼450–650 1C.

Since at fully occupied positions in the channels water amount is2.65% (which follows from the formula Ta2O5∙2/3H2O), then the siteoccupancy of water in the channels is 1.1/2.65¼0.415 for Eu, 1.9/2.65≈0.7 for La and 1.2/2.65¼0.45 for Yb. When conducting full-profile analysis this values were fixed and not refined. The structureof the phases is preserved upon heating in air up to T¼650 1C. Thus,the anhydrous materials were obtained whose structural parameters

rved (+), calculated (solid line) and the difference between observed and calculatedwed Bragg reflections are indicated by the row of vertical tick marks.

Fig+1.5

Fig. 5. [001] HRTEM image of Eu0.089Ta2O5.134. The projection of unit cell isinserted: red balls are Eu, blue balls are Ta. A-OCCEu is empty, B-OCCEu is fullyoccupied. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

I.P. Zibrov et al. / Journal of Solid State Chemistry 203 (2013) 240–246 243

are also refined. The results of full-profile analysis for compoundsEu0.089Ta2O5.134∙0.28H2O and Eu0.089Ta2O5.134, La0.075Ta2O5.113∙0.47H2O and La0.075Ta2O5.113, Yb0.051Ta2O5.077∙0.3H2O and Yb0.051Ta2O5.077

are shown in Fig. 3, Supplementary Figs. S3, S4 and presented inTables 1–3. Atomic coordinates, site occupancy and isotropic thermalparameters for these compounds are presented in SupplementaryTables S4–S6 and selected interatomic distances with bond valences—in Tables S7–S9.

3.2. HRTEM investigation.

The goal of the TEM investigation is to analyze the occupation ofatomic positions by Eu in Eu0.089Ta2O5.134 crystal structure. Theschematic model of the structure and its [001] projection are shownin Fig. 4a, b. The oxygen atoms are not shown for simplicity. They arevery weak scatters for high energy electrons in comparison with Euand Ta and as a result do not change the contrast in the imagenoticeably. The HRTEM image of the EuxTa2O5+1.5x structure is shownin Fig. 5. It was taken at 400 kV accelerating voltage in [001] crystal-lographic direction under Scherzer phase contrast conditions (defocusΔ¼−35 nm, thickness less than at least 2 nm). At this imagingconditions the Eu and Ta form the columns along electron beamdirection running in [001] direction (Fig. 4) which are imaged as darkspots. The darkness depends very much on scattering amplitude ofelement as well as on occupation factor of this position OCC. One canfind two systems of dark spots in the image Fig. 5. The Ta positions(blue balls) are of the same darkness (OCCTa¼1), while the darkness ofEu positions (red balls) very much fluctuates from one atomic columnto another due to variation of occupancy factor 0 oOCCEuo1 (see A, Bin Fig. 5). For example, position A is empty OCCEu¼0, whereas positionB is almost fully occupied OCCEu¼1.The computer simulation supportsthis interpretation of contrast variations (see Fig. 6). Comparison ofexperimental images with simulated ones gives the average OCCEu�0.3 in this particular crystal fragment. It means that Eu positions areoccupied randomly with average OCCEu¼0.3, giving the averagecomposition x¼OCC � NEu=NTa≈0:1 (Eu0.1TaO2.65), where NEu¼4,NTa¼12—are the numbers of Eu and Ta atomic positions in unit cellrespectively. This value is in a good agreement with the result of EDXanalysis. However, in most cases we do not observed any contrast atEu positions at all. It means that the Eu occupation of these positions isbelow our detection limit OCCEuo0.15.

3.3. IR absorption spectra study.

After heating of aqueous compounds till 650 1C the anhydrousmaterials were obtained. IR spectroscopy was used to prove thatanhydrous phases had no water molecules in the structure. IRabsorption spectra of La0.075Ta2O5.113∙0.47H2O and La0.075Ta2O5.113,so as spectra of F–Ta2O5∙2/3H2O and F–Ta2O5 are presented in

. 4. (a) Unit cell of EuxTa2O5+1.5x oxide. Small balls are Eu, big balls are Ta. Oxygen atomsx. Columns of Eu and Ta atoms are separated in this projection.

Fig. 7. The water in the channels of the structure gives peaks at∼850 cm−1, ∼1100 cm−1, ∼1400 cm−1, ∼3100 cm−1. The IR spectra oflanthanum tantalates almost exactly coincide with the spectra ofF–Ta2O5∙2/3H2O and F–Ta2O5. The peaks associated with lantha-num are not visible on the absorption spectrum because of the lowconcentration of rare earth.

4. Discussion

The experiments show that the RE atoms substitute for watermolecules in the studied structure. The site occupancy is low andcorresponds to the formula RE0.05-0.1Ta2O5+y. There is a strongexothermal peak at T¼550–600 1C in all three DTA curves (Fig. 2,

are skipped. a¼1.042, b¼0.7268, c¼0.698 nm. (b) [001] Projection of EuxTa2O5

Fig. 6. Simulated TEMHR images of EuxTa2O5+1.5x. (a) OCCEu¼0.0, (b) OCCEu¼0.5, (c) OCCEu¼1.0. Big and small circles are Ta and Eu respectively.

Fig. 7. IR absorption spectra of La0.075Ta2O5.113∙0.47H2O and F–Ta2O5∙2/3H2O (a, b); La0.075Ta2O5.113 and F–Ta2O5 (c, d). Solid line—F–Ta2O5∙2/3H2O and F–Ta2O5; dashed line—La0.075Ta2O5.113∙0.47H2O and La0.075Ta2O5.113.

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Supplementary Figs. S1, S2) which is associated with rearrange-ment of the structure after the removal of water: reduction of theunit cell volume due to the collapse of channels. The breakings onthe TG curves at the same time are associated with the “kick” effectwhen the heated by thermal effect air rises, pushing down thecrucible. For fully occupied positions in the channels by RE atomsthe formula RE0.667Ta2O5 corresponds exactly to the formula of the

HTB Me0.33WO3. The size of channels in the aqueous tantalates(r∼4.1 Å along the a axis (Table 4) and r∼4.5 Å along the b axis) andin anhydrous compounds after the collapse of the channels (r∼3.0-3.7 Å along the a axis (Table 4) and r∼4.7 Å along the b axis) isclose to the size of channels in the HTB (r∼4.7 Å). The distancebetween the channels is close too: r∼7.4 Å in the HTB and r∼7.3 Åin the F-phase.

Fig

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The oxotantalates MxTa3O6 (M¼Na (x¼0.74), Li, Mn (x¼0.58), Yb(x¼0.44)) [21] have most similar structure to the investigatedcompounds: similar unit cell parameters and channels in which Matoms are placed. However, the coordination of Ta atoms is differentand x value is much more in comparison with title tantalates.

The coordination polyhedron of rare-earth atoms in the chan-nels of F-phase is distorted square prism (C.N. 8) (Tables S7–S9,Fig. 8) which is normal for the RE atoms. For instance, suchcoordination number RE atoms have in the fergusonite structuretype REMeO4 (Me¼Nb, Ta). These square prisms have commonedges along c axis. It is seen from the Table 4 that highly chargedcations RE+3 compress the unit cell so that its volume is signifi-cantly reduced compared to pure Ta2O5, most of anhydrousphases. The calculated valence of RE atoms (Tables S7–S9) isdirectly correlated with the concentration of RE+3 in anhydroustantalates and close to 3 for Eu, i.e. Eu–O bond lengths are close tonormal (rav.exp¼2.507 Å (Table S7); average bond valence per 1Eu–O bond is 3/8 and calculated rav.calc¼2.437 Å, r0¼2.074 Å [19]). Inthe case of La, and especially Yb, [REO8] square prisms are stronglystretched (calculated valence of Yb is 1.494 (Table S9)). In aqueousphases the size of the channel and hence RE–O bond lengths aredetermined by a large molecule of water, so the calculated valenceof rare earths is considerably less than 3.

The structure can greatly change its volume without destructiondue to the presence of puckered planes of PB [TaO7] (Fig. 1). Thefigure shows that the angle of the corrugation is the angle betweenthe axes of neighboring PB, i.e. angle Ta1–O2–Ta1. For greater clarity,a single channel is shown in Fig. 8. Corrugation increases inanhydrous tantalate (Fig. 8), i.e. angle Ta1–O2–Ta1 decreases(Table 4). The size of the channel along the b axis (the distanceO1–O1) increases after water removal (Fig. 8) but the size along thea axis (the distance O2–O2) decreases significantly (Table 4).

Table 4Unit cell parameters a, b, c (Å), volume V (Å3), angle Та1–О2–Та1 α (deg) and O2–O2dista

Compound a b

F-Ta2O5∙2/3H2O 10.9039(2) 7.2774(1)La0.075Ta2O5.113∙0.47H2O 10.8729(1) 7.2611(1)Eu0.089Ta2O5.134∙0.28H2O 10.8812(1) 7.2681(1)Yb0.051Ta2O5.077∙0.3H2O 10.9056(1) 7.2751(1)F–Ta2O5 10.4546(3) 7.3485(2)La0.075Ta2O5.113 10.5099(2) 7.2679(1)Eu0.089Ta2O5.134 10.4182(3) 7.2685(1)Yb0.051Ta2O5.077 10.4557(2) 7.3853(1)

. 8. Channel in the structure of Yb0.051Ta2O5.077∙0.3H2O (a) and Yb0.051Ta2O5.077 (b). O2 are

A similar decrease in unit cell volume is observed in some ofthe HTB, particularly for Ba [22] and to a lesser extent, for K [23], Inand Tl [24]. For many other elements the unit cell volume eitherclose or slightly greater than that of the WO3-hex. [25].

Heating in vacuum up to T¼450–500 1C leads to the waterremoval without destroying of the material although the structureof the anhydrous compounds is strongly distorted.

It should be noted that the structure of both aqueous andanhydrous phases contains a very short (2.0–2.2 Å) O–O distances(Tables S7–S9) which is not quite normal. Such a short distances weobserved in high-pressure modification Z–Me2O5 (Me¼Nb, Ta[13,26]). Perhaps, this is due to the fact that the material is subjectedto extreme pressure and temperature effects.

Distribution of RE cations through the channels is irregular: itcan be seen both empty and partially or completely filled channels(Fig. 5). There is no order in the alternation of empty and filledchannels. Previously, we observed a similar distribution of La andNd in the HTB RExWO3 [8,27]. X-ray experiment allows us to refinethe integrated concentration of rare earth on the entire sampleand electron microscopy shows the local distribution.

5. Conclusions

Experiments have shown that RE atoms replace the watermolecules in the structure of F–Ta2O5∙2/3H2O. The water alsoremains in the channels and occupies approximately 40% (Eu, Yb)and ∼70% (La) of positions. Lanthanum, europium and ytterbiumdo not replace the tantalum in the PB and BDP but the only waterin the channels. Highly charged cations RE+3 compress the unit cellso that its volume becomes less than that of F–Ta2O5 and RE–Obond lengths in coordination polyhedron [REO8] become normal.

nce r (Å) of investigated compounds.

c V α r

6.8529(1) 543.79(1) 138.1 4.0656.8401(1) 540.02(1) 140.9 4.1436.8492(1) 541.68(1) 140.0 4.1506.8553(1) 543.89(1) 137.6 4.0446.9576(2) 534.52(2) 112.3 2.8326.9765(1) 532.90(1) 131.7 3.6806.9832(1) 528.80(2) 113.6 2.8866.8923(1) 532.21(1) 126.2 3.401

the vertices of PB. This figure was drawn with the help of ATOMS program [20].

I.P. Zibrov et al. / Journal of Solid State Chemistry 203 (2013) 240–246246

It happens when the concentration of RE is close to RE0.1Ta2O5.15

and unit cell volume is reduced to less than 528 Å3. Significantchange in the unit cell volume without its destroying is possibledue to the puckering of PB layers and change of the corrugationangle in the layer.

Acknowledgments

This work was partially supported by RFBR, Grant 11-03-00308.The authors thank Yu.A. Velikodnii for the help in X-ray datacollection and Lyapin S.G. for assistance in taking of the IR spectra.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jssc.2013.03.057.

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