NaNb6Cl15 - Preparation, Structure, Ionic ... - Wiley Online Library

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M. E. Sagebarth, A. Simon*, H. Imoto, W. Weppner, and G. Kliche. Stuttgart, Germany ... clusters in the structure of Nb6F,, and bent bridges in the structures of the ...
Z. anorg. allg. Chem. 621 (1995) 1589-1596

Zeitschrift fur anorganische und allgemeine Chemie 0 Johann Ambrosius Barth 1995

NaNb,CI,,

-

Preparation, Structure, Ionic Conduction

M. E. Sagebarth, A. Simon*, H. Imoto, W. Weppner, and G. Kliche Stuttgart, Germany, Max-Planck-Institut fur Festkorperforschung Received April 4th, 1995 Abstract. NaNb,Cl,, is prepared by heating Nb,Cls, NaCl and Nb under Ar at 1 170 < T [K] < 1270 forming black regukr dodecahedra. It crystallizes in the cubic space group Ia3d (a = 2041.7(2)pm at room temperature) and transforms to a tetragonal structure below 150 K (probably 14,/acd, a = 2037.2(6), c = 2028.2(2) pm at 80 K). The Na+ ions are at

room temperature dynamically disordered in a split position. Their mobility is investigated by IR spectroscopy and electrochemical methods. Keywords: Niobium Cluster, X-Ray Structure, Phase Transition, Electrochemistry, Niobium Chloride

NaNb,Cl,, - Darstellung, Struktur, Ionenleitung Inhaltsiibersicht. NaNb6Cl,s wird durch Erhitzen von NbsC18, NaCl und Nb unter Ar auf 1170 < T [K] < 1270 in Form schwarzer regultirer Dodekaeder dargestellt. Die Vejbindung kristallisiert in der kubischen Raumgruppe Ia3d (a = 2 041,7(2) pm bei Raumtemperatur) und wandelt sich unterhalb

150 K unter Ausbildung tetragonaler Symmetrie um (wahrscheinlich I4,/acd, a = 2037,2(6), c = 2028,2(2) bei SOK). Die Na+-1onen sind bei Raumtemperatur in einer Splitposition dynamisch fehlgeordnet. Die Na+-Ionenbewegung wird IRspektroskopisch und auf elektrochemischem Weg untersucht.

Introduction

Table 1 Binary Nb and Ta halides with MsXi2 clusters

The metals niobium and tantalum form a large number of compounds with characteristic M-M bonded metal clusters known for many years [I]. In particular, M,X,,-type clusters are most frequently found with halides and oxides [2, 31. Linkage of such clusters leads to three well-investigated structure types with binary halides of these metals: Nb,CI,, [4], Ta6Br14[ 5 ] and Ta,I,, [6] are isotypic with a structure to be described as M,X,,X,,,. The clusters in these compounds have a closed-shell configuration with 16 electrons in M-M bonding states. 15-electron clusters are present in Nb,F,, [7] and Ta,X,, (X = C1, Br, I) [8, 91 which in spite of identical connectivity pattern M,X,,X,,, represent two rather different structure types. The main difference in the interconnection is due to linear M-X-M bridges between clusters in the structure of Nb6F,, and bent bridges in the structures of the tantalum halides, respectively. The remarkable differences between the two metals Nb and Ta become evident from Table 1. Here we report on the stabilization of the lh,X,,-type framework for niobium in the compound NaNb,CI,,. An isostructural representative containing a cluster which is stabilized by an interstitial atom is also known, Zr,CI,,N [lo].

c1

F Nb

Ta

Nb

Ta

Br Nb

T i

I Nb Ta

Results and Discussion 1 Preparation

In a series of experiments optimized conditions were determined for the preparation of NaNb,Cl,, from Nb,CI, [Ill, Nb and NaCl as well as from Nb6C1,, [4] and NaC1. Stoichiometric amounts of starting materials - Nb, sintered, 99,9V00, Plansee and NaCI, p. a., Merck - were sealed in niobium tubes (Plansee) which were contained in evacuated quartz glass ampoules during the reaction and afterwards quenched to room temperature. Table 2 summarizes the results of representative experiments performed with different starting materials in the appropriate stoichiometric ratio reacted at different temperatures. Obviously, NaNb,CI,, is only accessible as

Z. anorg. allg. Chem. 621 (1995)

1590 single phase material in a limited temperature region, 1170 < T[K] < 1270. At higher temperatures partial decomposition was observed, and a t lower temperatures the formation of NaNb,Cl,, was incomplete, obviously due t o the too-slow reaction of the intermediately formed compound Na,Nb,Cl,,. T h e kinetic reason for the incomplete reaction is also seen from the fact that NaNb,Cl,, did not decompose when slowly cooled from 1220 t o 870K within 150h.

Table2 Reaction of Nb3C18,Nb6Cl14,Nb, NaCl and experimental conditions Starting Materials

Temp [K] Products Time [h]

NaCI, NaC1, NaCl, NaCl, NaCl, NaCl,

1070/74 1 120/140 1170/140 12201117 1270/48 1320/1

Nb3C18, Nb Nb~C18,Nb Nb3C18, Nb Nb3C18,Nb Nb3C18,Nb NbsC11,

NbaC114, NaNb6C11g NbsC114, NaNbsCl,,, NasNbsC118 NaNb6Clls NaNbsC1,, NaNb6C1,, NaCI, Nb3CI8,NaNb6C1,5,Nb

Fourier map exhibited a well-defined peak of 3.3 e/106pm3 at Wyckoff position f, while other peaks were lower than 0.5 e/106pm3. Inclusion of Na with variable SOF led to the final parameters given in Table 4 with no residual peak in the A F map higher than 0.8 e/106 pm’. I )

Table 3 Crystal data and structure refinement of NaNb,Cl,, Formula Formula weight [a. u.] Temperature [K] Wave length [pm] Crystal system Space group Lattice constant [pm] Volume [nm3] Z Density calculated [Mg/m3] Absorption coefficient [cm-’I Diffractometer Monochromator Scan type Scan speed and range

NaNb6C1,, 1 112.22 293(2) 71.073 cubic Iajd (No 230) 2041.7(2) 8.51 1 16 3.472 48.38 Syntex P21 graphite 0

0.6 -29.3”/min; 1 background 1 off every 50 refl. 3 to 50 779 590 i,u-scans Full-Matrix least-squares ~=3.1682/(o’(F)+0.000033F’) R = 0.020, R, = 0.019 O,

These results lead t o the following optimized preparative conditions: 10.29 g (18.30mmol) Nb3C18, 0.61 g (10.44mmol) NaCl and 0.50 g (5.38 mmol) Nb are thoroughly mixed as fine powders and pressed into tablets of ca. 10 mm diameter. They are sealed under Ar in a Nb tube (approx. 70 mm length) which in turn is sealed into a quartz glass ampoule. After 90 h at 1070 K the temperature is increased by 50 K/day to 1 220 K. After another 120 h at the maximum temperature the ampoule is quenched in cold water and opened in dry Ar. Addition of LiCl as a solvent allows a much lower reaction temperature, and large crystals were easily prepared by heating NaCl and Nb3CI, (molar ratio 4:7) with addition of LiCl in a sealed Nb tube (75 h at 1080 K) followed by cooling to 780 K within 240h. The LiCl was removed by washing with ethedethanol mixture in dry Ar leaving black regular dodecahedra of approx. 2 mm diameter. These crystals break into tiny pieces when in contact with water dissolving with the characteristic olive-green colour of the Nb6Clt;’ ion. The very pronounced solubility of NaNb6C1,, compared to that of Na,Nb6C118is remarkable.

Standard refl. 28 range [“I Reflections measured Independent refl. Absorption corr. Refinement Weight Final R values”)

O

‘) R = Z(lF,I - IFcI)/ZIFoI; R, = [,Cw(IF,I - IF,I)2/Z~IF,1’]”2

Table 4 NaNb6Clls, positional and displacement parameters (standard deviations); displacement parameters [pm’] defined as exp(-2~’(UlIh2a*’+ . . . + 2Uz3klb*c*)) Atom x

Y

Z

U,,

S.O.F.

Na Nb Cl(1) Cl(2) Cl(3)

0.00000 0.9671 5(2) 0.02956(5) 0.87571(5) 0.57191(4)

0.25000 0.07608(2) 0.05073(4) 0.01826(4) 0.67808(4)

860(89) 147(2) 234(5) 236(5) 242(5)

0.161(4) 1.00000 1.00000 1.00000 0.50000

0.8180(6) 0.05863(1) 0.15777(4) 0.1 1lOO(5) 0.125000

2 Crystal Structure

Atom U f l

The crystal for the X-ray structure investigation had dodecahedra1shape and approx. 0.1 mm diameter. It was sealed in a glass capillary under an Ar atmosphere. Reflection data were collected with an automated four-circle diffractometer. Experimental details are summarized in Table 3. All crystallographic computations were performed by using the X-ray system (PARAM for lattice constants from powder) and SHELX 76. ORTEP was used for the graphical representation. Starting parameters were taken from the reported structure of TasCll5, space group Ia3d (No 230) [8]. Full-matrix least squares refinement with anisotropic displacement factors converged at R = 0.038 and R, = 0.054 even though Na atoms were not included in the refinement. At this stage the difference

0 1094(95) 524(74) 1013(99) 0 Na 148(2) 154(2) Nb 146(2) ll(1) -1(1) 240(5) -35(4) -31(4) Cl(1) 176(5) 287(5) Cl(2) 273(5) 230(5) 205(5) 96(4) -19(3) Cl(3) 262(6) 233(4) U22 -69(4) U33

UU

u 3 3

UIZ

u 1 3

u 2 3

-265(66) 13(1) 59(4) 6(4) -19(5)

’) Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-76344 Eggenstein-Leopoldshafen (Germany) on quoting the depository number CSD-59 101, the names of the authors, and the journal citation.

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M. E. Sagebarth et al., NaNb,ClIs - Preparation, Structure, Ionic Conduction Table 5 Comparison of interatomic distances [pm] in Ta6C1,,

and NaNb,Cl,, TasCl1s

NaNb6Clls

M-M

292.1 (2X) 292.8 (2x)

292.60(7) (2X) 293.61(6) (2X)

M-Cl'

242.7 243.6 241.3 246.2

244.7 1(9) 245.88(9) 244.5(1) 245.4(1)

M-C1"-a

256.4 (2x)

260.94(8) 261.98(8)

CP-CP

315.9 327.5 345.4 340.8 (2x) 349.5 (2x)

315.5(2) 334.9( 1) 351.1(2) 342.4(1) (2X) 343.7(2) (2x)

Cp-Cp-a

316.3 321.7 335.3 339.3

322.47(9) (2X) 320.0(1) (2x) 339.2( 1) (2 x) 344.6(1) (2x)

Na-Cl

(285.7)* (305.1)* (306.1)* (356.2)*

Na-Na

(231.1)*

*)

(2x) (2x) (2x) (2x) (2x) (2x) (2X) (2x)

293.1(9) 308.4(5) 309.5(5) 357.2(7)

(2X) (2x) (2x) (2x)

232.6(25)

refers to the coordinates of Na in NaNb,CI,,

At room temperature NaNb,Cl,, crystallizes in the cubic space group Ia3d with a = 2041.7(2)pm. The structure is projected along [001] in Figure 1, its Nb6C1,, substructure is isostructural with Ta6C1,, [8]. Interatomic distances of both are compared in Table 5. In the structure MLX,, clusters are interconnected with 6 of their 8 neighbors according to M , X ~ , X ~forming ~a a body-centered arrangement. The additional Na atoms occupy voids formed by X' and X"-" type atoms. As shown in Figure 1, each position for Na is coupled with another equivalent one at a short distance (229 pm). Therefore, each pair of positions can be regarded as a site with a double energy minimum. The center of the site is at Wyckoff position d(3/8 0 1/4) and has the symmetry of point group S, (2). If the site were completely occupied by Na, the resulting stoichiometry would be Na,,,Nb,Cl,,. In the real structure only two thirds of the available positions are occupied randomly. The site occupation factor for Na converges to 0.161(4). Within standard deviation the refined composition Na,,97(3)Nb6Cl,stherefore corresponds to a compound NaNb,CI,, indicating an ionic structure Na'Nb6Cl,,- with an anionic framework of closed shell 16-electron Nb6Cl,, clusters. The result of the structure investigation indicates interesting problems. (i) A partial occupation of equivalent Na' positions is found. There might exist an order. In the case of disorder, this could be static or dynamic in character. (ii) Ta,CI,, is only known as a binary phase. How much Na is needed to stabilize the same cluster arrangement with Nb? In other words, does a range of homogeneity, NqNb6C1,,, exist?

3 Phase Thansition X-ray powder diagrams of NaNb,Cl,, recorded with the modified Guinier technique [ 121 at room temperature exhibit very sharp lines. When linearly changing the temperature between 305 and 725 K the continuously recorded diagram [ 131 does not indicate any anomalous change but only an isotropic expansion which can be matched by the expression a = 2041.7 + 3.67 (T - 305) pm corresponding to a linear expansion coefficient a = 1.80 lo-, K-'. With decreasing temperature, however, the diagram taken in the continuous mode becomes slightly diffuse below 150 K. At that temperature an endothermic effect upon warming of a sample is detected in a DTA measurement indicating indeed a phase transition near that temperature. Cycling around the transition temperature with a continuous observation of the X-ray diagram shows that within the accuracy of the temperature reading (5 K) no hysteresis effects occur and the transition is entirely reversible, giving evidence of a second-order character. The close inspection of the diagram moreover shows that all but the reflections of hhh type, best observed with 444,

-

-

Fig. 1 Projection of the structure of NaNb,CI,, along [OOI]; Nb, clusters outlined and interconnection via CP-" bridges indicated. The ellipsoids (90% probability) for the Na atoms are

particularly large

1592

Z. anorg. allg. Chem. 621 (1995)

5

15

10

25

20

35

30

Fig. 2 (top to bottom) Guinier type diagrams of Si (room temperature) and NaNb6C1,, at 160 and 80 K compared with the calculated diagram for I4,/acd (+ reflection 4.44,* contamination Nb); representation of 0 in '

become diffuse. The first observation calls for a transition within a group-subgroup relationship and the second observation narrows down the spectrum of possible subgroups leading to I4,/acd which has the same systematic extinctions as Iajd. At 160K the cubic lattice constant a = 2036.4(2) pm is found, and at 80 K the different degrees of line broadening are met by the tetragonal lattice constants a = 2037.2(6) and c = 2028.2(2)pm. The diagram calculated on the basis of the atomic parameters at room temperature created for I4,/acd closely matches the details in the diagram of the low temperature modification, see figure 2. IR reflectivity measurements for NaNb,Cl,, and Ta,Cl,, reveal significant differences in the temperature dependences of the spectra, shown in figure 3. Whereas the reflectivity of Ta,Cl,, changes only marginally between 10 and 300K, dramatic changes are observed for NaNb,CI,, at frequencies below the range for Nb-CI 0.5 I

I

I

I

I

and C1-Nb-Cl lattice vibrations [14]. The narrow bands between 50 and 150 cm-' vanish between 100 and 200 K. Such behaviour is well known from ionic conductors. Obviously the narrow bands arrise from a local order of the Na atoms at low temperature which become mobile at the temperature of the phase transition. With respect to measurements of the Na' conduction in NaNb,CI,, it is important to know the electronic

I

0.4 "

0

0.3 22

> 2

50

100

150

200

250

300

100

150

200

250

300

I

0.2 0.1

-

-

l-

0 0.0

I

3

1

I

b

LL

2 0.3 -

7

0.2 0.1

0.0

'

I

50

I

I

I

100

150

200

I

WAVENUMBERS [ c m l ] Fig. 3 FIR spectra of (a) NaNb6C1,,and (b) Ta6C1,,at 10, 100, 200 and 298 K (top to bottom)

0

50

TEMPERATURE

[K]

Fig. 4 Dissipation curves for NaNbsCLs (a) at 400 Hz ( X decreasing, + increasing temperature) and (b) at 0.2 (+), 1 (X), 4 (*), 20 (0)and 100 kHz (a), respectively

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M. E. Sagebarth et al., NaNb,C1,5 - Preparation, Structure, Ionic Conduction

contribution to conductivity. From the reflectivity at small wave numbers a concentration of less than 4 * 1019elmole is derived. 4 Ionic Conduction 4.1 Dissipation Measurements

By transferring vibrational energy to NaNb,Cl,, and measuring the dissipation as a function of frequency and temperature information about the phase transition as well as ion motion is gained. Figure 4a shows the result of measurements at constant frequency for increasing and decreasing temperature. The pronounced chances of the dissipation factor are fully reversible, and no hysteresis effects are seen. The dissipation curves exhibit two maxima, or plateaus. The first occurs around 150K and obviously corresponds to the phase transition found in thermal and X-ray investigations. The more pronounced increase at higher temperatures is strongly dependent on the frequency of excitation (figure 4b). It starts at 170 K for the smallest frequency (0.2 kHz) and approximately 220 K for the highest (100 kHz). This feature in the dissipation curves is associated with a significant ion mobility in the crystal, obviously concerning the disordered Na+ ions observed in the structural investigations. The plot lg v vs. 1/T (50%) shows Arrhenius behavior with linearity over a large frequency range. From the slope one calculates an activation energy of 0.367 eV for the positional change of the Na' ions, This value is near the range of values found for optimized ionic conductors as e.g. Na-P-Al,O, (0.16 eV-[15]). 4.2 Electrochemical Measurements

DC measurements on a compacted disc of NaNb,Cl,, which was contacted with Pt electrodes resulted in rather the same value for the conductivity as impedance measurements, to 10-2sZ-' cm-' at 490 K. Clearly, the conductivity of the measured samples of NaNb,Cl,, is dominated by the electronic contribution.

Na, . -.-.

Fig. 5 Cells for the determination of ionic conductivity of NaNb,Cl,, via GITT measurements

To block the electronic conduction, Na-P-Al,O, was used in a setup shown in figure 5 which used liquid Na and a mixture of Na,S,/Na,S,, respectively, as ion reservoirs. The ionic conductivity of NaNb,Cl,, was determined at 470 K via GITT measurements (galvanostatic intermittant titration technique) [16]. The time dependent increase of the cell potential, after switching on the galvanostatic current, is proportional & for the first few seconds. According to

a*z

2v,

=

*

*

q * N,

i, . dE/dy

. & . dE/&

(D* chemical diffusion coefficient, v, molar volume, i, current density, E cell potential, y stoichiometry number, zcharge of moving ion, qunit charge, N, Avogadro number) the chemical diffusion coefficient can be calculated as D* = 5 10-"cm2/s. Using this value of D* the diffusivity D according to D = D*/(dlna/dlnc)

-

(

with dln a/dln c

z . q * y dE

= ___

kT

*

)

-

dy

can be calculated as D = 5 * lo-', cm2/s. From equation CJ =

cDq2/kT

(c concentration of mobile ion) the approximate value for z 3 10~9sZ-1cm-'results. the ionic conductivity These values can only be considered as estimates, since the equilibration time is quite considerable.

-

4.3

Structural Aspects

It is rather trivial to assume Na+ ion mobility within the twofold disordered position around the site d in (3/8 0 1/4) called dumbbell in the following. However, it remains unclear which pathways of the ions are available in order to lead to ionic conductivity. (i)There is one possible pathway, between dumbbells, at an angle of 108" to the dumbbell axis and involving Na' positions 547 pm apart. (ii) The other one is in the direction of the dumbbell axis. This pathway involves Na' positions which are 788pm apart. An indication as to which of these pathways might be the relevant one is provided by the experimental electron density for Na+ as well as from distance arguments. Figure 6 shows the partial Fourier synthesis for NaNb,Cl,, calculated with omission of the Na+ ions. The peak shapes indicate a pronounced anisotropy of the motion of Na' in the direction of pathway (i). The dots drawn at a separation of 41 pm are a measure for the nearest neighbor distances around the position. Large dots indicate that all atoms are more than 250 pm away, and small dots mean near-neighbor distances in the range of 230 to 250pm. These distances of neighbors are significantly larger with positions along pathway (i) compared to (ii). Pathway (ii) can therefore be excluded due to steric reasons. Even with pathway (i) a concerted chlorine atom motion is needed to open the tunnel sufficiently, as

5.0

9'0

S'O

9'0

P'O

E'O

Z'O

9'0

s.0

9'0

€.O

Z'O 1'0

Z'O €.O P'O

5.0 90'0=Z P'O

€'O

Z'O

PO'O=Z

9'0

9.0

P'O

E'O

2'0 1'0

2'0

€'O P'O

s-0

ZO'O=Z

OO'O=Z

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M. E. Sagebarth et al., NaNb6Cl15- Preparation, Structure, Ionic Conduction

The results are summarized in Table6. In all cases NaNb,Cl,, was the main product with contamination by Nb,Cl,, (x < 1) and Na,Nb,Cl,, (x > 1). The refined lattice constant for the different compositions x = 0.7, 1.0 and 1.5 was identical within three standard deviations. AS no other diagrams than those of Nb,Cl,,, NaNb,Cl,, and Na,Nb,Cl,, were detected, NaNb,CI,, is the only ternary compound on the quasi-binary line between Nb,Cl,, and Na,Nb,CI,, and it has no range of homogeneity along this line. Yet, this result only gives evidence for the stability of the Nb,Cl,, cluster with 16electrons in M-M bonding states in agreement with the expectation to find the most reduced cluster species in the presence of an excess of niobium metal. A reduction beyond the 16 electron cluster, e.g. in “Na,,,Nb,Cl,,~’, is not observed and not expected as it would lead to the occupation of a strongly M-M antibonding cluster state. On the other hand, an oxidation could be possible, however only in the absence of excess Nb metal. According to the general scheme, (p - 1) NaCl + p Nb,Cl,, + 2Nb,Cl, = (p + 1)N%Nb6C1,,, where x = (p - l)/(p + 1) experiments were performed by heating pressed pellets of appropriate amounts (total approx. 1 g) of NaC1, Nb,Cl,, and Nb,Cl, in quartz glass ampoules for 6 to 7 days at 1 170 to 1270 K. None of the reaction products was monophasic. In particular, samples with x 5 0 . 6 did not contain any Na,Nb,Cl,,. For

-

Fig. 7 Pathways for Na’ ion movement in NaNb6C1,, (path (i) bold, path (ii) thin lines, see text)

dumbbell do not lie on the surface but on either side of it. Indeed, the easy ion motion occurs across the zero potential surface. As it was discussed before, the Na’ ion conductivity a=3 Q-‘ cm-’ is significantly smaller than the electronic contribution (lo-, to lo-’ Q-‘ cm-‘) in spite of the Na’ ion concentration of 6 loz3ions/mole being at least lo4 times greater than the conduction electron concentration. This means that the measured low activation energy of 0.367 eV, compared e.g. with 1.305 eV for cm-’), must be the ion conductor Li,PO, (a = lo-“ 0-’ associated with the Na’ ion mobility within the dumbbell.

-

2.0 1.8 -

-

5

7

1.6

!z

1.4 1.2

0.4 o,2

Stoichiometry

In order to derive a possible range of homogeneity of N%Nb,Cl,, chemically, a set of preparations was performed according to the general reaction scheme 8xNaCl + (15 - x)Nb,Cl, + 3(1 + x)Nb = 8N+Nb,Cl,,.

0

~,UtttCf-X++++4++*+++++++++++++++++++++++++++++++++++ x x . x x x x x x I X x x x x x , I: x x x x x I x x x I I x x

Fig. 8 Magnetic moments of T&Cl,, (O), taken from [8], NaNb6Cl15(+) and Nb6C1,, ( x ) in Bohr magnetons

Table 6 Preparations to determine the range of homogeneity of Na,Nb6C1,, along the quasi binary line K4Nb6C1,,/Nb6C1,,;(main product bold letters); Nb ampoule, T = 1 170 K

NaCl kl (mmole)

Nb,Cl, kl (mmole)

0.031 1 (0.532) 0.0634 (1.085) 0.612 (10.47) 0.0784 (1.342) 0.0599 (1.025) 0.1160 (1.985)

0.7647 (1.360) 1.1948 (2.125) 10.293 (18.30) 1.1911 (2.118) 0.7582 (1,348) 1.2559 (2.234)

aimed composition

product

lattice constant “Na,Nb6C1,,” a [pml

“N%.7Nb6Cl15”

NbsC114, Na,Nb6CIl5 Nb&l,,, Na,Nb,Cl,, Na,Nb,Cl,, Na,Nb6Cl15,Na4Nb6Ch Na,Nb6C1,5,Na4Nb6CllX Na.Nb6CIl5,Na4Nb6C1,,

2041.45(7)

‘‘N%.sNb6Clj5’ ’ “Na,,Nb6Cl15”

‘‘Na,.,Nb6Cl,5’ ’ “Nal.3Nb6Clls” “Na,.5Nb6C1,5”

2 04 1.16(5) 2041.21(8)

1596 = 0.9,O.S and 0.71 the lattice constants a = 2041.2(2), 2041.1(3) and 2041.2(2) pm were measured. Hence, these measurements do not indicate a range of homogeneity for the compound although one has to be careful with this interpretation, as a possible variation of the Na’ content in the framework of interconnected clusters need not lead to any volume change. Strong evidence for the stoichiometric nature of NaNb6C1,, is also provided by GITT measurements discussed above. A variation Ax = 0.0003 is found by this electrochemical method. Magnetic measurements could give a conclusive answer, however, they call for pure phase samples, which could only be prepared for x = I. Susceptibility measurements on Nb6C1,, and NaNb6C1,, are used to derive the magnetic moment according to pexp= kmol - TUP)T/0.12505. Figure 8 shows a small temperature independent value for both compounds exhibiting 16 electrons in M-M bonding cluster orbitals in marked contrast to the magnetic behavior of the 15 electron cluster in Ta,Cl,,.

x

Experimental Techniques X-ray investigations: Single crystals were sealed under Ar in capillaries and investigated on a P21 diffractometer (Syntex) (table 111). Powder investigations were performed on samples in sealed capillaries using a Guinier-Simon camera [13] with CuKal radiation ( A = 154.056pm) and internal Si standard (a = 543.102pm). The heating rate for a continuous mode diagram was 12 K . h-’. The line positions were read with a coincidence device at an accuracy of 0.02 mm. Magnetic measurements: Samples were investigated in a Faraday balance (Varian magnet, 15 kG, dH/dz = 160G/cm-‘, balance Cahn R-100, Cryostat Leybold Heraeus 5 - 620 K) using selfcompensating containers of gold-coated suprasil [ 19, 201. Differential thermal analysis: The measurements between 95 and 300 K were performed in a Heraeus DTA 500 at a heating rate of 2 K min-I. Capacity measurements: Pellets of 8 mm diameter were pressed at 10 kN cm-2 and measured at 10 frequencies between 100 Hz and 100 kHz (Nz cryostat 90-300K; measurements with a HP 4274 A bridge). Impedance measurements: Pellets as before were measured between 300 and 690K using a HP4192A LF impedance analyzer, 5 Hz - 13 Mhz.

Z. anorg. allg. Chem. 621 (1995)

GITT measurements: Coulometry was performed with a precision current source (Knick, 1 PA, 3 -240 min), and the change in cell potential was measured with a Keithley 616 Electrometer. IR Spectra: Polycrystalline samples were measured in the back scattering mode at 5 - 3 0 0 K in the range 20 to 650cm-I (Bruker IFS 113 v, resolution 2 cm-l, polyethylene reference).

References H. Schafer, H. G. Schnering, Angew. Chem. 76 (1964) 833 A. Simon, Angew. Chem. 100 (1988) 164; Angew. Chem. Int. Ed. Engl. 27 (1988) 159 J Kohler, G. Svensson, A. Simon, Angew. Chem. 104 (1992) 1463; Angew. Chem. Int. Ed. Engl. 31 (1992) 1437 A. Simon, H. G. Schnering, H. Wohrle, H. Schafer, Z. anorg. allg. Chem. 339 (1965) 155 H. Schafer, D. Giegling, Z. anorg. allg. Chem. 420 (1976) I D. Bauer, H. G. v. Schnering, H. Schafer, J. Less-Common Met. 8 (1965) 388 H. Schayer, H. G. v. Schnering, K. J Niehuess, H. G. Niedervahrenholz, J. Less-Common Met. 9 (1965) 95 D. Bauer, H. G. v. Schnering, Z. anorg. allg. Chem. 361 (1968) 259 D. Bauer, H. Schafer, J. Less-Common Met. 14 (1968) 476 R. f?Ziebarth, J D. Corbett, J. Am. Chem. SOC. 107 (1985) 4571 A. Broll, A. Simon, H. G. v. Schnering, H. Schafer, Z. anorg. allg. Chem. 367 (1969) 1 A. Simon, J. Appl. Cryst. 3 (1970) 11 A. Simon, J. Appl. Cryst. 4 (1971) 138 R. Mattes, Z. anorg. allg. Chem. 364 (1969) 279 R. Collongues, . I Thery, J €? Boilot in “Solid Electrolytes, General Principles, Characterization, Materials, Applications”, Eds. €? Hagenmiiller, K von Gool, Academic Press, New York 1978, p. 253 -276 K Weppner, R. A . Huggins, J. Electrochem. SOC. 124 (1977) . , 1569 [I71 H. G. v. Schnering, R. Nesper, Angew. Chem. 99 (1987) 1097 [18] H. G. v. Schnering, R. Nesper, Angew. Chem. 98 (1986) 111 ! Zell, B. Roden, D.Wohlleben, J. Mag. Mag. Mat. 9 [I91 E (1978) 26 [20] M. Sdgebarth, Diplomarbeit, Stuttgart 1985

Anschr. d. Verf.: Prof. Dr. A. Simon Max-Planck-Institut fur Festkorperforschung Heisenbergstr. 1 D-70569 Stuttgart