Synthesis and Characterization of Niobium Substituted Hexagonal TungstenBronzes

A. Hussaina, A. Ul-Monira, M. M. Mursheda, and C. H. Rüscherb,*a Dhaka/Bangladesh, Department of Chemistry, University of Dhakab Hannover, Institut für Mineralogie und ZFM der Universität

Received July 20th, 2000; revised July 20th, 2001.

Abstract. Alkali metal tungsten bronzes, MxWO3, and its niobiumsubstituted forms, MxNbyW1-yO3, have been prepared withM 5 K and Rb and nominal compositions of x 5 0.20, 0.25, 0.30and 0.0 # y # 0.20 at temperatures between 600 and 900 °C. TheX-ray powder patterns reveal that single phases of niobium substi-tuted hexagonal tungsten bronze (HTB) can be prepared for

Synthese und Charakterisierung Niob substituierter hexagonalerWolframbronzen

Inhaltsübersicht. Alkalimetall Wolframbronzen MxWO3 mitM 5 K, Rb und die mit Niob substituierten MischkristalleMxNbyW1-yO3 wurden mit den nominellen Zusammensetzungenx 5 0.20, 0.25, 0.30 und 0.0 # y # 0.20 bei Temperaturen zwischen600 und 900 °C hergestellt. Die Röntgenbeugungsdiagramme zei-gen, dass einphasige Produkte vom Typ hexagonaler Wolframbron-

Introduction

The ternary metal oxides, MxWO3, are known as tungstenbronzes where M is generally an electropositive metal atomand x can vary between 0 and 1. For this family of com-pounds four different structure types have been reported sofar, namely Perovskite Tungsten Bronzes, PTB [1, 2], Tet-ragonal Tungsten Bronzes, TTB [3], Hexagonal TungstenBronzes, HTB [4], and Intergrowth Tungsten Bronzes, ITB[5].

The crystal structure of the HTB can be described as athree dimensional network of corner sharing WO6 oc-tahedra containing hexagonal and trigonal tunnels alongthe c direction (Fig. 1). The metal atoms, M, occupy specificsites only within the hexagonal tunnels. The homogeneityrange of the HTB phase MxWO3 with M 5 K, Rb, Cs isfound to be mostly restricted to x between 0.19 and 0.33 [6,7], but the lower stability limit is greatly depending on thepreparation conditions and can be extended to about x 5

* Dr. C. H. RüscherInstitut für Mineralogie der UniversitätWelfengarten 1D-30167 HannoverTel.: 0049-(0)511-7624888e-mail: C.Ruescher@mineralogie.uni-hannover.de

416 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 004422313/02/628/4162420 $ 17.501.50/0 Z. Anorg. Allg. Chem. 2002, 628, 4162420

x 5 0.2, y # 0.05 ; x 5 0.25, y # 0.125 and x 5 0.3, y # 0.15.Investigations of the optical reflectivity and the infrared absorptionof Rb0.3NbyW1-yO3 indicate a decreasing concentration of free car-rier with increasing niobium content.

Keywords: Niobium; Tungsten bronzes

zen (HTB) für x 5 0.2 bis zu Niobanteilen von y 5 0.05, für x 5

0.25 bis y 5 0.125 und für x 5 0.3 bis y 5 0.15 existieren. Untersu-chungen der optischen Reflexion und der IR-Absorption für dieMischkristalle Rb0.3NbyW1-yO3 zeigen eine Abnahme der freienLadungsträgerkonzentration mit zunehmendem Niobgehalt.

Fig. 1 The structure of Hexagonal Tungsten Bronze projectedalong c axis.

0.15 [8]. The upper stability limit of the HTB phases (x 50.33) corresponds to the maximum content of sites for Mwithin the hexagonal tunnels. The HTB systems MxWO3

with M 5 K, Rb, Cs show interesting electrical propertieslike the appearance of superconductivity. The supercon-ducting transition temperatures (Tc) are between 1 and 9 K,increasing gradually with decreasing x [9212]. In this con-text it is interesting to note that the specific conductivities(above Tc) of these Drude type metals [13] systematicallydecrease with decreasing x which is related with a decrease

Synthesis and Characterization of Niobium Substituted Hexagonal Tungsten Bronzes

of their effective carrier concentration. Thus a transitionfrom metallic to semiconducting properties could be ex-pected similar to the metal insulator (M/I) transition ob-served for the NaxWO3 system (see [14] and references citedtherein), following the x dependence to lowest values as faras possible.

The formula of tungsten bronzes can be written as[Mx(Wx

51W1-x61)O3] indicating the presence of formally

pentavalent tungsten ions. Pentavalent tungsten ions can bereplaced by other pentavalent ions of suitable sizes such asV, Nb, and Ta. Galasso et al. [15] have reported a phase ofTTB type in the system KTaO3-WO3. Deschanvres et al. [16]have substituted W atoms in K-HTB and K-TTB accordingto Kx Mex W1-xO3 with Me 5 Nb, Ta and with 0.2 < x <0.33 and 0.45 < x < 0.65, respectively. These are fully oxid-ized forms of tungsten bronzes, i.e. the number of intro-duced electrons by the potassium atoms are totally compen-sated by the amount of pentavalent ions. In the presentpaper, we report the results of the synthesis and characteris-ation of niobium substituted HTB phases, MxNbyW1-yO3

with nominal compositions M 5 K, Rb; x 5 0.2, 0.25, 0.3and y # 0.2 where the number of electrons introduced byalkali metal atoms are only partly compensated by penta-valent niobium.

Experimental

The starting materials were high quality reagent grade K2WO4,Rb2WO4 (BDH Laboratory Supplies), WO3 (Ventron GmbH),WO2 (Alfa Aesar), and Nb2O5 (BDH Laboratory Supplies). Nom-inal compositions of MxWO3 and Mx NbyW1-yO3, with M 5 Kand Rb, x 5 0.20, 0.25, 0.3 and 0.0 # y # 0.20 were preparedaccording to the following chemical reaction:

1/2 x M2WO4 1 (1-x-1/2 y)WO3 1 1/2(x-y)WO2 1 1/2 yNb2O5

R Mx NbyW1-yO3.

The reactants were mixed intimately in an agate motor, transferredinto silica tubes, evacuated (1.3 Pa) at room temperature for 223hours, sealed, and then heated in a Muffle Furnace at temperaturesbetween 600 and 900 °C for periods between 2 to 7 days. Thesamples were quenched to room temperature. All samples werecharacterised by X-ray powder diffraction using a Guinier-Häggfocusing camera, CuKα1 radiation and Si as an internal standard.The reflection positions in the x-ray films having coincident scalein it were read manually with help of a film projector. The d-valuesand the lattice parameters were then calculated by using standardcomputer programs [17]. Some of the x-ray films were also evalu-ated with a computerised LS 18 line film scanner [18 ] to crosscheck the data obtained from x-ray films with coincident scale. Thed-values and lattice parameters obtained with these two methodsshow excellent agreement. The preparation conditions of thebronzes and the results of lattice constants determined by using thestandard computer programs [17] are given in Table 1.

The optical reflectivity was measured directly from the undilutedpowders against KBr powder as a reference. For the detection anarrangement with a broad angle input and output beam (Spectra-Tec.) was used. For the investigation of the infrared absorptionpellets were pressed from 1.5 mg of finely powdered samples dis-persed into 200 mg KBr. The spectra shown are given in absorption

Z. Anorg. Allg. Chem. 2002, 628, 4162420 417

Table 1 Preparation conditions and results of unit cell parameterrefinement. (h.t. 5 heating temperature; h.p. 5 heating period; a 5

cell parameters of HTB phase only, 1 5 trace of WO3, 11 5

trace of WO3 plus extra reflections, nr 5 not refined). Number inparentheses are mean standard deviation in the last digits.

System nominal composition h.t. h.p. unit cell parameterin °C in days a/A c/A

K2HTB K0.20WO3 800 2 7.3876 (6) 7.5135 (10)K0.20Nb0.05W0.95O3 800 3 7.3864 (6) 7.5107 (10)K0.20Nb0.10W0.90O3 800 2 7.3850 (8) 7.5098 (12)a 11K0.25WO3 800 3 7.3875 (6) 7.5137 (10)K0.25’Nb0.02W0.98O3 800 3 7.3864 (5) 7.5085 (10)K0.25Nb0.05W0.95O3 800 4 7.4864 (5) 7.5102 (7)K0.25Nb0.08W0.92O3 800 3 7.3852 (6) 7.5017 (8)K0.25Nb0.10W0.90O3 800 4 7.3841 (6) 7.5055 (11)K0.25Nb0.12W0.88O3 800 3 7.3855 (8) 7.5036 (11)a 1K0.25Nb0.15W0.85O3 800 4 nr nr 11K0.30WO3 800 4 7.3839 (6) 7.5266 (8)K0.30Nb0.02W0.98O3 800 3 7.3837 (8) 7.5260 (12)K0.30Nb0.05W0.95O3 800 4 7.3829 (4) 7.5253 (7)K0.30Nb0.08W0.92O3 800 3 7.3827 (9) 7.5114 (10)K0.30Nb0.10W0.90O3 800 4 7.3810 (6) 7.5103 (10)K0.30Nb0.12W0.88O3 800 3 7.3766 (5) 7.5197 (7)K0.30Nb0.15W0.85O3 800 4 7.3753 (9) 7.5135 (10)K0.30Nb0.17W0.83O3 800 3 7.3732 (13) 7.5241 (15)a 1K0.30Nb0.20W0.80O3 800 3 nr nr 11K0.30Nb0.05W0.95O3 700 3 7.3844 (4) 7.5102 (6)K0.30Nb0.10W0.90O3 700 3 7.3818 (7) 7.5133 (10)K0.30Nb0.15W0.85O3 700 4 7.3816 (6) 7.5122 (9)K0.30Nb0.05W0.95O3 600 7 7.3836 (6) 7.5284 (12)K0.30Nb0.10W0.90O3 600 7 7.3819 (6) 7.5250 (9)K0.30Nb0.05W0.95O3 900 3 7.3808 (6) 7.5076 (8)K0.30Nb0.10W0.90O3 900 3 7.3818 (9) 7.5084 (11)K0.30Nb0.15W0.85O3 900 3 7.3768 (7) 7.5276 (12)a 1

Rb-HTB Rb0.30WO3 800 5 7.3905 (4) 7.5623 (6)Rb0.30Nb0.025W0.975O3 800 5 7.3913 (5) 7.5539 (3)Rb0.30Nb0.05W0.95O3 800 5 7.3890 (4) 7.5539 (6)Rb0.30Nb0.075W0.925O3 800 5 7.3895 (4) 7.5538 (7)Rb0.30Nb0.10W0.90O3 800 5 7.3878 (6) 7.5515 (4)Rb0.30Nb0.125W0.875O3 800 5 7.3853 (10) 7.5425 (6)Rb0.30Nb0.15W0.85O3 800 5 7.3879 (5) 7.5495 (5)Rb0.30Nb0.175W0.825O3 800 5 7.3874 (7) 7.5450 (11)a 1Rb0.30Nb0.20W0.80O3 800 5 7.3873 (8) 7.5518 (10)a 11Rb0.25WO3 600 7 7.3889 (6) 7.5619 (10)Rb0.25Nb0.025W0.975O3 600 7 7.3890 (7) 7.5567 (12)Rb0.25Nb0.05W0.95O3 600 7 7.3881 (7) 7.5599 (15)Rb0.25Nb0.075W0.925O3 600 7 7.3889 (5) 7.5666 (8)Rb0.25Nb0.10W0.90O3 600 7 7.3860 (5) 7.5668 (8)Rb0.25Nb0.125W0.875O3 600 7 7.3872 (7) 7.5602 (8)Rb0.25Nb0.15W0.85O3 600 7 7.3855 (8) 7.5625 (11)a 1Rb0.25Nb0.175W0.825O3 600 7 7.3876 (8) 7.5649 (12)a 1Rb0.25Nb0.20W0.80O3 600 7 7.3905 (6) 7.5617 (10)a 11

units with α 5 log (I0/I). I0 and I are transmitted intensities throughthe reference pellet (KBr) and the KBr diluted sample pellet,respectively. The spectroscopic measurements were carried outusing FTIR spectrometer (Bruker IFS88 for the range 3000224000cm21; IFS66v for the range 20024000 cm21).

Results and Discussion

The X-ray powder patterns of all samples of K0.2Nb1-

yWyO3 prepared at 800 °C show single phase of HTB onlywhen y # 0.05 (see Tab. 1). Otherwise traces of WO3 andsome weak extra reflections are observed, which were, how-ever, not used for cell constant refinement.

K0.25Nb1-yWyO3 prepared at 800 °C (324 days) andRb0.25NbyW1-yO3 prepared at 600 °C (7 days) show tracesof WO3 when y > 0.1 and 0.125 respectively. Single phasesof HTB can be prepared for compositions K0.3NbyW1-yO3

and Rb0.3NbyW1-yO3 with y # 0.15. Optical microscopic

A. Hussain, A. Ul-Monir, M. M. Murshed, C. H. Rüscher

studies and X-ray powder patterns of the samples revealthat the crystallinity and homogeneity of the samples be-come better at higher preparation temperatures. Attemptsto prepare pure phases of fully oxidised analogues of HTBwith niobium up to 900 °C were not successful. The X-raypattern of one fully oxidised sample1), K0.25Nb0.25W0.75O3,prepared at 950 °C also shows traces of WO3 and extra re-flections other than HTB.

Recently, structure refinements by Rietveld method forsamples of nominal compositions Rb0.3Nb0.1W0.9O3 andRb0.3Nb0.2W0.8O3 [19] support the incorporation of ni-obium on tungsten sites which agree well with nominalcomposition of the starting materials. The obtained latticeparameters are in good agreement with data given in Tab.1. It may be noted that the data for structure refinement inRef. [19] were collected in a STOE STADI P diffractometerin a 0.02 mm capillary tube and that the very weak extrareflections other than HTB in the sample Rb0.3Nb0.2W0.8

O3 were not detected. The cell parameters of pure MxWO3

phases of potassium and rubidium, prepared in this studyat different temperatures, agree well with the previously re-ported values [6].

The unit cell constants for the HTB single phases ofseries M0.30NbyW1-yO3, prepared at 800 °C, andM0.25NbyW1-yO3 prepared at 800 °C (M 5 K) and 600 °C(M 5 Rb) are shown in Fig. 2 and 3, respectively. The solidlines result from linear regression analysis. There is a sys-tematic decrease of lattice constants with increasing Nbcontent except for Rb0.25Nb1-yWyO3 for the c values (Fig.3). The decrease in lattice constants for K0.30NbyW1-yO3

and Rb0.30NbyW1-yO3 indicates a decrease in unit cell vol-ume of 0.3% for y 5 0.1 with respect to y 5 0.0. For com-parison, the unit cell volume of RbxWO3 decreases by about0.15% (from 357.53 A3 to 357.02 A3) for x 5 0.30 to x 50.20 [6]. This indicates a similar increase of strain intro-duced by Nb substitution in alkali metal HTB compared tothe decrease of alkali metal content. A similar change involume can also be deduced from the variation in latticeconstants for K0.25NbyW1-yO3 but negligible change inRb0.25NbyW1-yO3 (Fig. 3). This could be due to the factthat Rb0.25NbyW1-yO3 system was prepared at 600 °C(Table 1).

Since single crystals of suitable size could not be preparedyet for conductivity measurement, however, some import-ant hints of the effect of the substitution of tungsten byniobium could be obtained for the system Rb0.30Nby-W1-yO3 using the optical and infrared properties of theirpowders. The reflectivities of the powder samples show acharacteristic minimum at about 16000 cm21 (Fig. 4). Thereflectivity increases strongly with decreasing wavenumbershowing another minimum like feature at about 10000cm21. For a better understanding these data may be com-

1) The sample was kindly supplied by Dr. Margareta Sundberg ofStockholm University, Sweden.

Z. Anorg. Allg. Chem. 2002, 628, 4162420418

Fig. 2 Variation of lattice constants as a function of nominal ni-obium content in Rb0.3NbyW1-yO3 and K0.3NbyW1-yO3. Solid linesare results of linear regression analysis.

Fig. 3 Variation of lattice constants as a function of nominal ni-obium content in Rb0.25NbyW1-yO3 and K0.25NbyW1-yO3. Solidlines are results of linear regression analysis.

pared with the data of single crystal work for Rb0.3WO3

and Rb0.2WO3 [13]. The spectra given in the insert of Fig.4 were calculated averaging according to:

<R> 5 1/3 Rc 1 2/3 Ra (1)

Synthesis and Characterization of Niobium Substituted Hexagonal Tungsten Bronzes

Fig. 4 Reflection spectra of powder samples (Rp) of selectedRb0.3NbyW1-yO3 in relative units. Spectra are shifted verticallyagainst each other for better comparison. Insert: Average reflectiv-ity (R in %, according to equation (1) in text) of single crystals,Rb0.2WO3 and Rb0.3WO3 calculated using data given in Ref. [13].

where Rc and Ra denote the reflectivity polarized paralleland perpendicular to the hexagonal c axis orientation,respectively. It can be seen that the superposition of themain component Ra and Rc leads to a typical line shape ofthe reflectivity also observed from the powder samples. Thesingle crystal spectral properties of RbxWO3-HTB has beenexplained in terms of a Drude-free carrier description withtwo plasmafrequencies to be separated for the componentsof the electical field (E) parallel (Rc) and perpendicular (Ra)to the hexagonal c axis [13]. Therefore, it is concluded thatthe spectral behaviour of Rb0.3Nb1-yWyO3 with W replacedby Nb can be explained by a Drude-free carrier model withtwo different plasmafrequencies related to the hexagonalcrystal system. The second observation is the shift of thereflection minima with incrasing Nb content towards lowerwavenumbers. This effect indicates a decreasing free carrierconcentration which is formally given via the decreasingconcentration of W51 states according to:

RbxNby51W1-y

61Wx-y51O3 .

Such an effect is observed also for RbxWO3 for decreasingx from 0.3 to 0.2 (insert of Fig. 4) [13]. It may be noted thatfor Rb0.30NbyW1-yO3 with y > 0.1 the minimum features inthe reflectivity becomes weaker. This could be due to anincrease in the free carrier damping or could even indicatea localisation of free carriers. The existence of such localis-ation of free carriers are well known in binary WO3-x,NbO2.5-x and reduced ternary Nb/W/O systems [20223].Here the number of localised charge carriers are correlated

Z. Anorg. Allg. Chem. 2002, 628, 4162420 419

Fig. 5 IR absorption spectra of few selected samples ofRb0.3NbyW1-yO3 in the range between 300 and 1400 cm21. Spectraare shifted vertically against each other for better comparison. Verti-cal line marks wavenumber position at which the intensities (Int*)relative to an extrapolated background line as indicated by solidlines were measured. Insert: The dependence of (Int*) on niobiumcontent y.

with the formal number of W51 states up to a certain satu-ration point of about 5 to 10 percent per formula unit.Above this saturation concentration theses oxides becomemetallic possessing a typical Drude free carrier behaviourtogether with the appearence of the related free carrier plas-mafrequency in the near infrared spectral range.

The IR absorption of Rb0.3NbyW1-yO3 with y between0.0 and 0.2 reveal a concave like shape of increasing inten-sity towards higher wavenumbers. The superimposed fea-ture seen below about 1000 cm21 (Fig. 5) is due to phononabsorption. The effect of the IR-active phonon absorptionbecomes usually rarther small in metallic conducting sys-tems with effective free carrier plasma frequencies above thephonon absorption frequency. A tentative evaluation of thisabsorption feature as a function of y shows a significantincrease of its intensity for y > 0.1 (see insert of Fig. 5). Asimilar effect is expected by a break down of the Drude-freecarrier concept (Fig. 4). Therefore, the increased phononabsorption intensity indicates a significant decrease in thefree carrier contribution which is consistent with the changein spectral feature in the near infrared reflectivity forsamples with Nb content y > 0.1.Acknowledgements. We are grateful to Professor Lars Kihlborg ofStockholm University, Sweden, for useful discussions. Thanks aredue to anonymous referees for their comments to improve the pre-sent study. This work is part of a research project financially sup-ported by Deutsche Gesellschaft für Technische Zusammenarbeitand Deutsche Forschungsgemeinschaft.

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