Solid State Ionics 38 (1990) 275-281 North-Holland

IONIC MOTION IN A2In2MosO~6 [A=Na, K, (Na, Li)] PHASES

J.P. MORALES 1, L. HERNAN 1, E. WANG, M.T. TSAI, J.-G. LEE and M. GREENBLATT 2 Department of Chemistry Rutgers, the State University of New Jersey, New Brunswick, New Jersey 08903, USA

Received 30 May 1989; accepted for publication t 8 March 1990

lonic conductivity of the three-dimensional network solids A21n2Mo5O16 with A = Na, K was measured by ac impedance. The ionic conductivity of the K analogue,~-(548K) = 7.48 x i0 "7 (-~ cm -I) with E a = 0.63eV is slightly higher than that of the Na isomorph, O~(548K) - 2.38 x 10 .7 (~cm -I) with E a = 0.79eV. Lithium insertion reactions via n-butyllithium and electrochemically show that -one Li/unit formula may be inserted topotactically in Na21n2Mo5O16.

i. INTRODUCTION

Recently, Na21n2MosO16 has been synthesized and characterized (i). The crystal structure was shown to consist of layers of edge-sharing triangular cluster units of Mo3013, InO 6 octahedra and MoO 4 tetahedra. The sodium ions are located in three different environments created by the In-Mo-O network (Fig. i). One sodium ion Na(1) has trigonal antiprismatic geometry and the site is fully occupied. The other two Na ions Na(2) and Na(3) occupy ten - coordinate sites with occupancies of 62 and 42%, respectively.

The large cavities in which the Na + ions are located and the presence of empty sites in the structure suggest possible mobility of the monovalent cations. Moreover, the empty positions might accommodate additional cations via insertion reactions with concomitant changes in the number of electrons available for Me-Me bonding in the trigonal cluster (2).

In this paper the electrical conductivity and results of lithium insertion investigation are reported for this compound. Synthesis and ionic conductivity of the isostructural K

analogue, K21n2MosO16 are also reported.

2. EXPERIMENTAL

Na21n2Mo5O16 in powder form was prepared according to reference (i). Polycrystalline

Present address: Department of Chemistry, University of Cordova, Cordova, Spain.

2 To whom correspondence should be addressed.

0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )

K21n2MosO16 was prepared by reacting stoichiometric quantities of K2MoO 4 (prepared from K2CO 3 and MOO3), In203, MOO3, and Me metal in an evacuated sealed quartz tube at 600°C for 62h. Trace amounts of MoO 2 impurity were removed by washing the product with 3N HNO 3 at room temperature. X-ray diffraction patterns were obtained by a Scintag PAD V diffractometer using monochromatized Cul~ radiation. Mica (fluorophlogopite) was used as an internal standard for the determination of the lattice parameters. Differential Thermal Analysis (DTA) and thermogravimetrio analysis (TGA) were carried out on a DuPont 9900 Thermoanalyzer. Chemical lithiation was carried out by treating the samples with a solution of 2N n- butyllithium in hexane (n-BuLi). A 0.1N solution of iodine in acetonitrile (12/CH3CN) was used as the deintercalating reagent. Lithium content was determined by plasma emission spectroscopic analysis.

For the electrochemical study, the cell was constructed using Li metal foil as anode, the ternary oxide as cathode and LiCI04 in anhydrous propylene carbonate (PC) as the liquid electrolyte.

The electrical conductivities were measured by ac impedance measurements using a Solartron 1250 frequency response analyzer over a frequency range of i0 Hz to 65 KHz. Details of the impedance equipment and sample probe are described in ref. (3).

3. RESULTS AND DISCUSSION

The powder X-ray diffraction data of K21n2Mo5O16 indexed on the bases of space group P3ml, and refined unit cell parameters a =

276 J.P. Morales et al. /Ionic motion in Aeln2Mos016

1 C

MO3 1 I 1 I I

I 1 1

Hal ( ~ ~t I.J~7 c~ )a 7.1., ¢_)

In1

Fig. i. View of the atomic arrangement in Na2In2Mo5Ol6 in the ab plane, showing the stacking perpendicular to c (numbered atoms are oxygens). Na-O bonds are omitted for clarity.

5.808(2), c = II.997(I)A are listed in Table I. K2In2Mo6016 is isostructural with Na2In2Mo5016 (a = 5.7856(8)A, c = II.266(I)A) (i). The expansion of the unit cell of the K compound along the c axis by -0.7A is consistent with the layered structure, where the layers of indium and molybdenum oxygen polyhedra are stacked along the c axis and the alkali metal ions are located in the interlayer cavities (see Fig. i). Thus the increase in c dimension for the potassium containing material is consistent with the larger ionic radius of K +.

The impedance measurements were carried out in the temperature range 298-973 K under dynamic N 2 atmosphere and in air. Fig. 2 shows typical examples of impedance spectra for

K2In2Mo5016 (similar impedance plots were obtained for Na2In2Mo5Ol6 ) that are characterized by well defined semicircles in the low temperature region. The semicircles correspond to bulk relaxation; straight lines in the region of low frequencies that correspond to interracial processes associated with the blocking nature of the electrodes were not observed (4). As the temperature increases, the points on the semicircles move to lower values of Z', the real part of the impedance. At temperatures above 673 K the experimental points are virtually located on the x-axis of the plot, with no additional semicircles in the region of low frequency.

This seems to indicate that we are observing the bulk conduction process throughout the temperature range and not bulk and grain or electrode processes separately.

The temperature variation of electrical conductivity of Na21n2Mo5O16 is shown in Fig. 3. The Arrhenius plot shows three changes in the slope in the low temperature region: The conductivity ranges from a(498 K)-I0 -8 to a(591 K)-IO "6 (~cm) "I with E~ -0.79 eV; a(591K) -10 -6 to a(629 K) -3.3 x I0 "b (~cm) -I with E a -0.93eV; a(629 K) -3.3 x 10 .6 to a(731 K) -1.6 x I0 -5 (~cm) -I with E a -0.72 eV. Above 773 K, the conductivity increases sharply up to 843 K, where another change in the slope is observed with E a -0.39eV.

Similar conductivity behavior is seen in K21n2Mo5016 (Fig. 4). In the low temperature regime: from a(425 K) -1.4 x 10 .8 to o(606 K) -2.5 x 10 .6 (~cm) -I and lower Ea-O.63eV. The anomaly at 606 K in the Arrhenius plot of K2In2Mo5O16 (vs 623 K in the Na analogue) leads to a lowering of E a -0.25eV and a slower increase of conductivity (a -lO-6(~cm) -I) with increasing temperature up to 712 K, where the conductivity rises sharply with increasing temperature. The activation energies and pre- exponential factors for Na2In2Mo5Ol6 and K2In2Mo5O16 at various temperature regimes are listed in Table II.

DTA and TGA results of K2In2Mo5016 measured

J.P. Morales et al. / Ionic motion in AelneMos016 277

Table I. X-ray Powder Diffraction Data for

K21n2MosO16 dobs.(A) deale.(A) h k I I/I o

12.001 11.997 0 0 1 i00

5.998 5.999 0 0 2 4

4.638 4.638 i 0 1 2

3.999 3.999 0 0 3 30

3.855 3.854 1 0 2 i0

3.130 3.130 i 0 3 20

2.998 2.999 0 0 4 6

2.903 2.904 i i 0 i

2.822 2.822 i i i 3

2.614 2.614 i 1 2 2

2.576 2.576 i 0 4 1

2.515 2.515 2 0 0 i

2.461 2.461 2 0 1 3

2.399 2.399 0 0 5 3

2.349 2.350 i 1 3 1

2.156 2.166 1 0 5 2

2.129 2.129 2 0 3 1

1.999 2.000 0 0 6 17

1.927 1.927 2 0 4 2

1.812 1.812 2 1 2 i

1.736 1.736 2 0 5 4

1.718 1.717 2 1 3 2

1.623 1.622 I 0 7 I

1.565 1.565 2 0 6 I

1.477 1.476 i 1 7 I

in air and in N 2 atmosphere are shown in Fig. 5. A sharp exothermic peak in the DTA at -823 K is indicative of a large transformation. The TGA indicates oxygen uptake from -723 K and completion of reaction by 823 K. X-ray powder diffraction of the product obtained in the DTA and TGA above 823 K shows the formation of NalnMo208; the weight gain observed in the TGA is consistent with the reaction: Na21n2Mo5016 + 1.502 ~ 2NalnMo208 + MoO 3. The DTA/TGA results of K21n2Mo5016 are similar to that of Na21n2Mo5016 except for the lower oxidation reaction temperature (798 K) of the Na compound.

Examination of the samples used in the impedance measurement (in air) above each of the transition temperatures observed,

respectively, (e.g., 606, 712, and 823 K for

K21n2Mo5016 in Fig. 4) by powder X-ray diffraction shows no observable change up to 700 K; above this temperature a mixed phase

region (i.e., corresponding to K21n2Mo5016, KlnMo208 and MOO3) is observed; above 823 K, KlnMo208 and MoO 3 are present. Qualitative measurement of dc conductivity at room temperature by two probe measurement on a sintered pellet of Na21n2Mo5O16 indicates that RDC >> RAC ; this suggest that the conductivity due to electrons is negligible. We can now attempt to interpret the conductivity results: above 843 and 823 K the high conductivity (a-10 -2 (~cm) -I) is attributed to the AlnMo208 (A = Na, K) phases, respectively. The conductivity of the Na compound (a(843 K) -1.8 x 10 .2 (~cm) -I) increases with temperature and E a -0.39eV is low. This was further confirmed by the synthesis of a single phase NalnMo208 pellet, which showed similar AC conductivity and E a in this temperature region. The conductivity of KlnMo208 does not appear to change with increasing temperature above 823K (Fig. 4). The region of rapidly changing conductivity from 731-843 K (Fig. 3) and 712-823K (Fig. 4) for the Na and K samples, respectively, correspond to multiphase regions as indicated by the powder X-ray results. Thus intrinsic/extrinsic ionic bulk conductivity in the A21n2Mo5O16 (A = Na, K) phases due to motion of the Na + and K + ions may be attributed to the low temperature regions of Fig. 3 and 4, below 731 and 712 K, respectively.

The breaks in the slope of the plot of Figs. 3 and 4 suggest different types of conduction mechanism via Na+/K + motion and may be interpreted by taking into account the different crystallographic positions that the Na+/K + ions occupy in the crystal lattice (i). As commented above, these ions are located in three different cavities created by the In-Mo-O network. Na(1) is in a twelve-coordinate oxygen "cage," which is fully occupied with six Na-O distances at -2.50 ~ and six at -3.35 ~. Na(2) and Na(3) are located in ten coordinated cavities with similar environments and are 62 and 42% partially occupied, respectively. A relatively short distance Na-O (2.30 ~) is observed for one of the oxygens while the nine remaining oxygens are located at longer average distances (six at 2.97 ~ and three at -2.49 ~). It is likely that the mobility of Na(3) and Na(2), with high concentrations of unoccupied sites of similar energy, is higher than that of the Na(1) ions. The low temperature region of conductivity (Fig. 3 and 4) may be ascribed to the motion of Na(2) and Na(3).

The nature of the change in the ionic motion indicated by the change in the slope at 629 and 591 K in the Arrhenius plots of Na21n2Mo5016 and K21n2Mo5016 , respectively (Figs. 3, 4), can only be speculated upon. The motion of both

278 J.P. Morales et al. / Ionic motion in A flneMos016

V

3OOO

4701(

z' ( 1 0 0 0 n )

Fig . 2. Complex impedance d i a g r a m s f o r K21n2Mo5O16 recorded at different temperatures.

5OOO

Fig. 3.

115 1:9 1000/T(K)

1000 - 1 , , , , , ,

-2 +

+

-3

+

-4- +

+

-5

-6

-7

-8 .7 111

~C 100

O

20oo

i

2.3 2.7

Temperature dependence of the specific conductivity o of Na21n2Mo5016.

S ?

-I I0~, .....

+ + +

-4 4"

4.

4.

-6

-7

Fig. 4.

-8 .7

~C I00

4.

i

1'.1 1.N 1.'9 2.3 2.7 1000/T(K)

Temperature dependence of the specific conductivity 0 of K21n2Mo5O16.

Table II.

ZP. Moralesetal./lonic motioninAzln2MosO~6

Activation Energies and Pro-Exponential Factors for Various Temperature Regimes

Na2In2Mo5Ol6 Ea(eV) ao(~-cm)-I

490 - 591 K 0.79 1.48

591 - 623 K 0.93 4.60

623 - 731 K 0.72 0.66

279

K21n2Mo5016 Ea(eV) ao(~-cm)-i

425 - 606 K 0.63 0.54

606 - 712 K 0.25 3.46 x 10 .4

Na(2)/K(2) and Na(3)/K(3) is possible only in two dimension; i.e., Na(2)/K(2) can jump into a Na(2)/K(2) vacancy and Na(3)/K(3) can jump into Na(3)/K(3) vacancy, but there is no chance of motion along the c direction, since the No(1) sites are fully occupied (Fig. i). The distortion of the semicircle in Fig. 2 at 470 K may be due to two possible conduction mechanism for (Na(2)/K(2) and Na(3)/K(3) and may not be associated with a depression of the are below the real axis. However, if at higher temperature, (i.e., above 590 K) the Na(1) ions also begin to move, the possiblity of three- dimenstional ionic motion becomes possible with

concomitant increase of the activation energy. The higher activation energy is required for the formation of Na(1) vacancies (i.e., jumping of No(1) into empty No(2) or Na(3) sites). This phenomenon was not observed in

K2In2Mo5O16 , possibly because the larger radius of K(1) ion does not allow K + to pass through the "bottle-neck" in the structure. The slower increase in the conductivity in the region 630- 730K for Na and 590-720 K for K respectively suggests, however, that the number of mobile cations is smaller in this activated regime; this may be duo to cluster formation of ion/vacancy defects for example. The lower

6"

3-

2 -

1

o

-1

-2

-3

. 5 -

- 6 -

-7

t DTA

in air

TGA in air J

1~o ~o ~ ~o ~o ~o T e m p e r a t u r e ( ' C )

105.0

104.5

104.0

103.5

103.0

102.5

102.0

101.5

101.0

100.5

100.0

99.5 700

Fig. 5. DTA and TGA of K21n2Mo5O16 in air and N2.

280 J.P. Morales et al. /Ionic motion in A fin2Mos016

temperature of anomaly for K (603 K) compared to Na (623 K) may indicate that the defect clusters are easier to form for the K than the Na compound. (The entropy change decreased significantly for the K compound.)

The substantially lower activation energy and higher conductivity of the K compound compared to the Na analogue in the low temperature regime (below 590 K) may be ascribed to differences in polarization by the cations; similar behavior has been seen in other Na/K analogues (6). Potassium is more mobile, because it is less polarizing than sodium. The "bottle-necks" for motion of (Na(2)/K(2) and Na(3)/K(3) are sufficiently large for both. The very different values of the pre- exponantial factors (Table II) in the various linear regimes of the Arrhenius plot support

the model involving different kinds of ions and mechanisms of motion.

The presence of empty tunnels in the structure of the Na containing compound was further confirmed by the insertion of lithium, carried out by chemical and electrochemical methods. Chemical lithiation with n-BuLi yielded a product of nominal composition Li3.7Na21n2Mo5016 (as determined by chemical analysis). The powder X-ray diffraction data of the lithiated product, together with the electrochemically lithiated phase

LiNa21n2Mo5Ol6 and the parent compound is given in Table III. The X-ray lines of the lithiated phases are broader and significantly less intense than those corresponding to the parent compound.

The data in Table III indicate that the

Table III. X-ray Powder Diffraction Data of Na21n2Mo5016 and Its Lithiated Analogs

"L___i3.7N__~a21__n2M__o5~16"~ LiNa21n2Mo5~16~ Na21__n2Mo5~16~

dob I/Io dob I/Io dob I/Io hkl

11+26 i00 11.44 20 11.25 96 001 10.87 65 11.13 32 5.62 15 5.63 21 002 5.47 i0

4.58 25 4.58 14 4.58 14 i01 3.75 80 3.743 i00 3.749 I00 003,102 3.712 65 3.652 15 3.010 50 3.002 58 3.004 82 103

2.915 27 2.907 20 2.898 27 2.893 17 Ii0 2.817 I0 2.806 ii 2.815 7 004

2.802 7 iii 2.576 20 2.575 16 2.573 9 112 2.526 i0 2.513 I0 2.509 I0 2.463 20 2.453 25 2.449 31 2.293 i0 2.291 I0 2.254 20 2.249 ii 2.193 I0 2.187 I0

1.874 25 1.871 29 1.857 20 1.803 I0 1.797 15 1.796 15

1.694 I0 1.691 13 1.678 I0 1.673 16

2.505 4 200

2.446 21 201 2.293 8 113 2.253 41 005

2.085 5 203 2.055 8 105 1.878 22 006

1.796 13 212 1.778 6 115 1.758 13 106 1.692 13 213 1.676 14 205

a.

b. c.

Product of excess n-BuLi reaction at RT Product of electrochemical reaction ref. I.

ZP. Moralesetal./lonic motioninA21n2M050t6

Table IV. Unit Cell Parameters of Na21n2Mo5O16 and Its Lithiated Analogs

Compound a(~) c(~) V(~ 3 ) Ref.

281

Na2In2Mo5Ol6 5.7856(8) 11.266(1) 326.6(2) 1

LiNa2In2Mo5Ol6 a 5.796(2) 11.247(6) 327.1(3) this work

LixNa2In2Mo5016 b 5.800(3) 11.28(1) 328.5(5) this work

a. electrochemically lithiated b. lithiated with excess n-BuLi

compound with nominal composition, "Li3.7Na21n2Mo5016" is not a single phase. The apparent splitting of the X-ray diffraction lines (particularly dominant in the 001 reflections) suggest the presence of at least two phases. Thus, re~tction with excess n-BuLi leads to a two (or more) phase lithiated product. Based on the values of dob s in Table II, one of the lithiated phases, likely with the smaller Li content, leads to a decrease in the unit cell volume: the other phase, probably with the larger Li content results in a larger unit cell. Fitting the observed d values of the lithiated phase with the larger unit cell yields the cell parameters assigned to LixNa21n2Mo5Ol6 in Table IV.

Lithium deintercalation of "Li3.7Na21n2O16" with 12/CH3CN solution for seven days yielded "LiNa21n2Mo5Ol6" as determined by Li analysis of the product. However, the powder X-ray

A

3 "

0 o.0

X X X

X

• • i . . , • • i • • , • • , - •

0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 .2

Fig. 6.

X

Open-circuit voltage vs. x for Li[IM LiCIO 4, PC[LixNa2In2Mo5016, graphite.

diffraction of the delithiated phase indicates more than one phase.

Electrochemical lithiation carried out on a polycrystalline pellet of Na2In2Mo5016 (Fig. 6) shows that one mole of Li/formula unit may be inserted topotactically and reversibly. Although the overall cell volume of the LiNa2In2Mo5016 phase is -2% larger than that of the host, the c dimension actually decreases in the lithiated phase indicating that the highly polarizing Li + ions occupying sites in positions stacking along the c axis induce contraction of the lattice along c for this composition. The increasing trend of the lattice parameters and unit cell volumes in Table IV suggest that in the phase labeled LixNa2In2Mo5Ol6 (product of n-guLi reaction) x > i.

ACKNOWLEDGEMENT

The work was partly supported by the Office of Naval Research. L.H. and J.M. thank the Junta de Andalucia (Spain) for a Research Fellowship.

REFERENCES

(I) B. T. Collins, S. M. Fine, J. A. Potenza, P. P. Tsai and M. Greenblatt, Inorg. Chem., in press.

(2) C. C. Torardi and R. E. McCarley, Inorg. Chem. 24, 476 (1985).

(3) E. Nomura and M. Greenblatt, Solid State Ionics 13, 249 (1984).

(4) W. I. Archer and R. D. Armstrong, Specialist Periodical Reports. Electrochemistry Vol. 7 Chemical Society (1980).

(5) M. A. Subramanian and C. C. Torardi, Mat. Res. Bull. 22, 957 (1987).

(6) S. Chandra, Superionic Solids: Principles and Applications (North-Holland, Amsterdam, 1981) p. 28.