Stoichiometry, structure and conductivity of Nasicon

Solid State Ionics 18 & 19 (1986) 13-20 North-Holland, Amsterdam

13

STOICHIOMETRY, STRUCTUREAND CONDUCTIVITY OF NASICON A. CLEARFIELD, M.A. SUBRAMANIAN, P.R. RUDOLFand A. MOINI Department of Chemistry, Texas A&M U n i v e r s i t y , College Station, Texas, 77843, U.S.A. The crystal structures of two near stoichiometric NASICONS were determined from powder neutron d i f f r a c t i o n data. The compound prepared by Hong's s o l i d state reaction had a composition Na3.17ZrI.93Sil.9PI.IOI2 and a sol-gel preparation heated at I080°C gave Na3ZrI.93Si2P011.86. These compositions, obtained from refinement of the occupancy factors, were found to agree with elemental analysis carried out by X-ray fluorescence. The foregoing compositions are considerably d i f f e r e n t from that of a NASICON obtained by a hydrothermal route, Na3.2ZrI.68SiI.84PI.16011.54, and t h e i r unit c e l l parameters are somewhat larger. This difference results from the incorporation of approximately 0.32 moles of Na+ in the Zr 4+ sites f o r the hydrotherma] preparation. Solid state MAS NMR data are presented to support t h i s f i n d i n g , and in one sample, trapped electrons were detected. The hypothesis is proposed that the exact stoichiometry of NASICON cannot be represented by a simple s o l i d solution series, but r at her , depends on the method of preparation. 1. INTRODUCTION

of a rhombohedral NASICON by f l u x growth.

The discovery of NASICON was an important

They

determined the structure by X-ray d i f f r a c t i o n

development in the f i e l d of s o l i d e l e c t r o l y t e s

and obtained a nonstoichiometric composition,

because i t provided a superion conducting sys-

Na3.1ZrI.78Si1.24PI.76012, by refinement of occupancy factors. In a d d i t i o n , they found

tem in which conduction occurred over a three dimensional cavity network. 1'2

Thus the e l u c i -

that the end member with x = 3 contained a

dation of the mechanism of sodium ion trans-

zirconium deficiency with the charge being

port in this system became a high p r i o r i t y .

compensated by the presence of protons. I0

However, a vexing problem soon became evident.

suggested that the one dimensional NASICON

Several groups reported d i f f i c u l t y

formula proposed by Hong2

NASICON free of other phases. 3-6

in preparing The e l e c t r i c a l

They

and ceramic properties are affected by the accu-

(2) Nal+xZr2SixP3_xOl2 needs to be extended to a plane in the quaterna-

mulation of these other phases in the grain boun-

ry phase diagram.

daries of the sintered powders and reports on

should be

the i n s t a b i l i t y of NASICONS are undoubtedly i n -

(3) Nal+x+4yZr2_ySixP3_x012. 1 Meanwhile C l e a r f i e l d et a l . 1,12 prepared

fluenced by this factor. 7'8 The appearance of ZrO2 in the products made

The new general formula

monoclinic NASICON by a hydrothermal route

by high temperature s o l i d state reactions led

and found a single phase (by XRD) of t o t a l

von Alpen et a l . 6 to propose a second formula

composition given by X-ray fluorescence analysis

f o r the NASICON s o l i d s o l u t i o n , namely

as Na3.3Zrl.65Sil.9P1.I011.5. The s olid obtained from the i n i t i a l hydrothermal reaction had

(I) Nal+xZr2_x/3SixP3_xOl2_2x/3 . Although compositions of this type are known

a composition of Na4Zr2Si2PI.8015 and was stable

to e x h i b i t an X-ray pattern of a single NASICON

to about IO00°C. Above this temperature P205

phase, i t has recently been shown that these

s p l i t out to y i e l d the nonstoichiometric NASlCON.

solids also contain a glassy phase of low z i r -

Since i t is possible to have a single phase

conium content. 8 Kohler and Schulz 9 obtained single crystals 0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

appear in the X-ray pattern but to have glassy materials also present as a second phase, 8 we

14

A. ~Tearfield et al. / Stoichiometry, structure and conductivity o f NAS1CON

decided to solve the s t r u c t u r e o f our non-

prepared f o r neutron d i f f r a c t i o n

s t o i c h i o m e t r i c sample by powder neutron d i f -

sample of Alpha Inorganics NASICON was found

fraction

techniques.

Refinement of occupancy

studies.

to c o n t a i n l a r g e amounts of ZrO 2.

This sample

f a c t o r s should reveal the s t o i c h i o m e t r y of the

was reground and heated to IIO0°C f o r 20 hr

crystalline

and an a d d i t i o n a l 28 hr at 1150°C.

phase o n l y .

The r e s u l t 13 was a

A

This t r e a t -

f o r m u l a , Na3.2ZrL.68SiL.84P1.16011.54 , which i s very close to t h a t d e r i v e d from the a n a l y t i -

ment reduced the f r e e ZrO 2 to about 3-5 w e i g h t

cal data.

fully

The reason f o r t h i s strange s t o i c h i o -

%.

The hydrothermal p r e p a r a t i o n was described elsewhere 12 and i t s

s t r u c t u r e determina-

t i o n by neutron d i f f r a c t i o n

metry was also apparent; about 0.32 moles o f

r e p o r t e d e a r l i e r . 13

2.2 Neutron D i f f r a c t i o n

Na+ were found in the Zr 4+ s i t e s .

Neutron d i f f r a c t i o n

In c o n t r a s t to the above, Engell et a l . 14

Studies

time of f l i g h t

(TOF)

used a s o l - g e l technique to prepare s t o i c h i o m e -

data were obtained on the Special Environments

tric

phases which conformed to the Hong formu-

Powder D i f f r a c t o m e t e r (SEPD) a t the Intense

la.

This study stands in o p p o s i t i o n to the r e -

Pulsed Neutron Source (IPNS), Argonne National

s u l t s o b t a i n e d by Kuriakose e t a l . , 8 who found

L a b o r a t o r y . 17

that all

a t h i n - w a l l e d vanadium can under dry c o n d i t i o n s .

such p r e p a r a t i o n s contained ZrO 2.

We,

The samples were loaded i n t o

however, have prepared a s e r i e s of NASICONS in

Data p r e p a r a t i o n and r e d u c t i o n was conducted

which Sc3+ replaced p a r t o f the Zr 4÷.

on the 150 ° back s c a t t e r i n g banks.

The

In each

s t r u c t u r e o f two of these p r e p a r a t i o n s was de-

case the s t a r t i n g model f o r i n p u t to the R i e t -

termined by X-ray powder methods and r e f i n e d

veld program was t h a t of Hong2 f o r the composi-

by the R i e t v e l d technique. 15

t i o n Na3Zr2Si2P012.

No s i g n i f i c a n t

Refinement was c a r r i e d

change in composition over t h a t in the r e a c t a n t

out w i t h neutron TOF R i e t v e l d a n a l y s i s pro-

mix was observed.

grams 18 using a t h r e e - t e r m r e f i n a b l e background

That these p r e p a r a t i o n s are

s t o i c h i o m e t r i c lends support to the idea t h a t

f u n c t i o n and a two-term Gaussian peak shape

both s t o i c h i o m e t r i c and n o n s t o i c h i o m e t r i c forms

function.

of the compound are p o s s i b l e . 16

a s t o i c h i o m e t r i c s o l - g e l p r e p a r a t i o n (SG-AM56)

t h e s i s i s c o r r e c t , then i t the composition f i e l d exist.

I f t h i s hypo-

remains to d i s c o v e r

over which s t a b l e NASICONS

This paper i s a c o n t r i b u t i o n toward t h a t

Data were obtained f o r two samples;

heated a t 1080 ° f o r 20 hr and the reheated Alpha Inorganics N/~SICON (AI-NAS-1).

Programs

TPRMPH and TLSMPH were used to r e f i n e samples which contained ZrO 2 as they a l l o w simultaneous

end.

refinement of up to 4 phases. 19 cell profile

2. EXPERIMENTAL SECTION

Initially

and p o s i t i o n a l parameters of the

2.1 P r e p a r a t i o n o f NASICONS

NASICON p o r t i o n o f the data were r e f i n e d .

High temperature s o l i d s t a t e r e a c t i o n s were

was f o l l o w e d by o p t i m i z a t i o n o f the minor phase

c a r r i e d out in sealed p l a t i n u m tubes to prevent

parameters and in the f i n a l

the escape of v o l a t i l e

parameters were s i m u l t a n e o u s l y r e f i n e d .

c o n s t i t u e n t s . 11

The r e -

a c t a n t s c o n s i s t e d o f c a r e f u l l y weighed out quantities

o f Na3PO4, ZrP207, ZrO 2 and SiO 2 to give

In the case o f sample SG-AM56, the d i f f r a c contributing reflections

bed f u l l y

I.OA down.

paper. 11

were 20-50 hr. a t 1150°C.

Heating times

Sol-gel preparations

refinements a l l

t i o n peaks were extremely broad w i t h over 150

the d e s i r e d composition and t r e a t e d as d e s c r i in an e a r l i e r

per data p o i n t from

No ZrO 2 was detected in the p a t t e r n

However, the broadness o f the peaks was respon-

f o l l o w e d the procedure given by Engel114 w i t h

s i b l e f o r a slow approach to convergence.

minor v a r i a t i o n s .

final

Two s p e c i a l samples were

This

The

d i f f e r e n c e p l o t showed t h a t the problem

A. Clearfield et al. / Stoichiornetry, structure and conductivity o f NASICON

15

with peak asymmetry could not be eliminated.

tained.

These were phase I , Na5Zr(P04) 3 and

Therefore, the refinement was not as complete

y-Na3PO4.

This l a t t e r phase is probably s t a b i -

as f o r other samples and consequently the esd's

l i z e d by the presence of Zr 23 and has the compo-

are s l i g h t l y l a r g e r .

As part of each refinement

s i t i o n Na ~ Z r . . . . Z.JJ

(PO,)~ previously represent-

2~

U.tO/

the occupancy factors were released and only

ed as Na7Zro.5(P04) 3. ~

s i g n i f i c a n t changes retained.

monoclinic NASICON obtained.

Elemental analysis was carried out by X-ray

In no case was a single Therefore, in

order to obtain some idea of the stoichiometry,

fluorescence (Micron, In c ., A n a l y t i c a l Services

we prepared a sample (AI-NAS-1) whose s t a r t i n g

Laboratory).

composition was close to Na3Zr2Si2POt2.

Because

a large amount of sample was required f o r the 3. RESULTS

neutron study we reheated a commercial product

In the high temperature platinum tube reac-

(Alpha Inorganics NASICON) u n t i l no f u r t h e r

t i o n s , we explored a range of compositions giv-

reduction in the ZrO2 content was observed.

en by eq.(3) in which x = 0-3 and y = 0-0.3.

Full d e t a i l s of the structure determination

The results are shown in Figure 1 as a p l o t of

and results w i l l be published elsewhere. 24

x versus y.

the portions pertinent to this discussion are

A completely open c i r c l e indicates

Only

that only a single phase (rhombohedral NASICON)

given in Table 2.

was observed in the X-ray pattern and a t r i a n -

unit c e l l dimensions are almost i d e n t i c a l to

gle indicates the monoclinic phase.

I t is to be noted that the

Partial

blackening on the l e f t hand side of the t r i a n -

No1+4y+xZr2-yS1xP3-x012

gle or c i r c l e indicates the presence of ZrO2 and

ZrO~

on the r i g h t hand side of the f i g u r e , a phase

~)

other than ZrO2 or NASICON. This was usually

L~ Monoclinic Rhombohedral

phase I , a sodium zirconium phosphate with a composition range Na4.8_4.52ZrI.12_1.05(P04)3 .20'21 I t is seen that only a small number of the compositions consisted of a single phase and none of these were compositions in the high conduction region.

~.o~'~

xf

1.5(

In some cases the samples were reground

and reheated f o r up to 100 hr with only a small decrease in the amount of minor phases.

Howev-

er, i t is i n s t r u c t i v e to examine the u n i t c e l l

,o( )

O

@

@

@

@

@

072

o13

parameters of the NASICON phases which are col-

o.C)

lected in Table i . Along the j o i n represented by y = O, the unit c e l l dimensions increase in much the same way as given by Hong2 except that in the high Si region our a axis is considerably larger.

This

has also been found to be the case by others. t4'22

As y increases, in the region of i n -

terest f o r high conduction (x = 1.5-2) ZrO2 as well as sodium zirconium phosphates were ob-

oC Y

FIGURE 1 Phases formed by high temperature solid state reactions

16

A. Clearfield et al.

/ Stoichiometry, structure and conductivity o f NASICON

TABLE 1 Unit Cell Parameters and Phases Obtained f o r D i f f e r e n t NASICON Preparations Exp. No.

Composition

Other* Phases Present

MAS-13

Nal.5Zr2Sio.5P2.5012

WW-122

NaI.9ZrI.9Sio.5P2.5012

L a t t i c e Parameters a(A)

b(A)

c(A)

~(deg)

None

8.8792(7)

-

22.768(1)

-

ZrO 2 + I

8.8804(6)

-

22.771(1)

-

ZrO 2 + I

8,8863(6)

-

22,775(1)

-

ZrO 2

8.9426(7)

-

22.838(2)

-

-

22.849(2)

-

WW-120

Na2.3Zr1.8Sio.5P2.5012

MAS-12

Na2Zr2SiP2012

MAS-18

Na2.2Zr1.95SiP2012

MAS-15

Na2.4ZrI.9SiP2012

ZrO 2 + I + U

8.9485(5)

N~AS-17

Na2.8ZrI.8SiP2012

ZrO 2 + U

8.9625(5)

-

22.866(2)

-

ZrO 2

8.9904(5)

-

22.901(3)

-

MAS-23

ZrO 2 + U

MAS-24

Na2.5Zr2Sil.5Pl.5012 Na2.7Zrl.95Sil.5PI.5012 Na2.9ZrI.90Sil.5PI.5012 Na3.3Zrl.8oSil.5PI.5012

MAS-26

Na2.8Zr2Sil.8Pi.2012

MAS-21 MAS-19

MAS-25 MAS-11

Na3.4ZrI.85SiI.8PI.2012

15.5807(6)

8.9978(5)

9.219(4)

123.86(2)

15.591(1)

9.003(I)

9.208(1)

123.76(2)

15.6065(9)

9.0216(12)

9.2219

123.61(2)

ZrO 2 + I

15.605(2)

9.020(1)

9.212(1)

123.61(1)

ZrO 2 + I + U

15.646(2)

9.048(1)

9.214(1)

123.60(2)

ZrO2

15.654(2)

9.050(1)

9.212(1)

123.58(2)

ZrO 2 + U ZrO 2 + U + y-Na3PO 4

Na3Zr2Si2POI2

* I = S o l i d S o l u t i o n (see t e x t ) U = Na5Zr(P04) 3 those of MAS-11 in Table I which was obtained

t h i s formula is t h a t c a l c u l a t e d to j u s t s a t i s f y

from the platinum tube r e a c t i o n of the required

a l l the cation charge.

initial

elemental analysis in t h i s case is not as good

stoichiometry.

The formula f o r t h i s

Agreement with the

monoclinic NASICON phase, derived from the occu-

as f o r the previous sample but t h i s is to be

pancy f a c t o r s of the r e f i n e d neutron d i f f r a c t i o n

expected as the neutron peaks were broadened

data, is N a 3 . 1 7 Z r l . 9 3 S i l . 9 P i . 1 0 1 2 . This formula f i t s the scheme presented in e q . ( 3 ) , t h a t i s ,

ever when t h i s sample was heated to 1150°C,

a zirconium d e f i c i e n c y compensated by 4Na+.

a small amount of ZrO 2 formed accompanied by

El-

so t h a t the refinement was less accurate.

How-

emental a n a l y s i s f o r t h i s phase (XRF) is given

a considerable sharpening of the X-ray powder

in Table 3 along w i t h that c a l c u l a t e d from the

reflections.

neutron d i f f r a c t i o n

data.

We note t h a t except

Another i n t e r e s t i n g f e a t u r e of t h i s prepara-

f o r the low c a l c u l a t e d Zr value the agreement

t i o n is the almost complete segregation of

is e x c e l l e n t .

the s i l i c o n in T2 s i t e s and the phosphorus

However, we estimated from the

refinement t h a t about 4 weight % ZrO 2 was also

in T1 s i t e s .

present in the s o l i d .

segregation is real is found in a c o n s i d e r a t i o n

When allowance is made

f o r the ZrO 2, the best f i t

to the elemental ana-

Corroborating evidence that t h i s

of the bond distances.

The range of values

l y s i s o v e r a l l is found f o r 3 weight % ZrO 2 (Na,

f o r the P-O bonds (in T1) is 1.518(11)-1.537(9)A

12.4%; S i , 9.8%; P, 6.3%; Zr, 35.5%).

and f o r the Si-O bonds (in T2) 1.616(14) to

S i m i l a r l y refinement of the neutron data f o r

1.683(17)A.

The accepted average values f o r

the s o l - g e l sample (SG-AM56) y i e l d e d the formula

Si-O t e t r a h e d r a l bond distances are 1.620(2)A

Na3ZrI.93Si2P011.86.

and f o r P-O 1.520(3)A. 25

The amount of oxygen in

By way of contrast

A. Clearfield et al. / Stoichiornetry, structure and conductivity of NASICON

17

TABLE 2 Crystal Structure Data f o r Several NASICONS Prepared by D i f f e r e n t Synthetic Methods AI-NAS-I Space Group

SG-AM56

C2/c

C2/c

Hydrothermal C2/c

SCH(-x=2) C2/c

o

a(a)

15.6451(4)

15.643(4)

15.6209(8)

15.6407(6)

b

9.0491(2)

9.0484(7)

9.0326(5)

9.0498(3)

c

9.2151(2)

9.2214(22)

9.2172(5)

9.2102(4)

B(deg)

123.74(1)

No. of c o n t r i b u t i n g reflections

123.87(1)

123.67(1)

123.71(3)

977

975

997

R wp Rp

0.0334

0.0443

0.0285

-

0.0539

0.0765

0.0664

*

Re

0.0183

0.0200

0.0124

0.0528

0.0622

Occupancy f a c t o r Zr

1.93(1)

1.93(1)

1.67

2

P1

0.48

0.95

0.30

0.75

Sil

0.51

0.05

0.70

0.25

P2

0.61

0.05

0.86

0.26

Si2

0.39

1.95

1.13

1.74

Nal

0.27(1)

0.22(2)

0.265

1.00

Na2

1.00(1)

1.00(3)

1.00

0.732

Na3

1.90(6)

1.77(9)

1.608

1.268

Na(Zr)

-

0.326

• Not given the comparable Si-O and P-O distances

n AI-

treatment the s o l i d had a blue c o l o r a t i o n .

NAS-1, where s i l i c o n and phosphorus atoms are

Therefore we obtained i t s esr spectrum and

more randomly dispersed, are T I , 1.586-I.606A

t h i s revealed the presence of a radical with

o

and T2, 1.569-I.602A, respectively.

Data f o r

the NASICON prepared by a hydrothermal route are also given in Tables 2 and 3.

For t h i s sam-

ple we note that the sodium content is higher

g

= 2.047.

Reheating the sample in vacuum

removed some of t h i s species leaving free electrons in the l a t t i c e defects as shown by a signal at g = 2,0023.

and the zirconium content s i g n i f i c a n t l y lower

Preliminary NMR data also indicates that

than f o r the other samples as required by the

the hydrothermal preparation d i f f e r s from the

formula Na3.2ZrI.68SiI.84PI.16011.54. Furthermore the a and b axes are s i g n i f i c a n t l y smaller

near stoichiometric ones. In NaZr2(P04) 3 the 31p chemical s h i f t is -24.2ppm r e l a t i v e to

than f o r the other samples.

H3PO4 and the peak is narrow.

To confirm the s t o i -

chiometry we prepared a sample of the above comp o s i t i o n by the sol-gel method and unlike the

S i m i l a r l y in the stoichiometric compound Na3ScZrSiP201215 a much broadened 31p peak is observed at -12.4ppm,

contain ZrO2 even on s i n t e r i n g at 1200°C f o r 20

However in the hydrothermal preparation a simil a r 31p peak is observed at -11.7ppm but a

hr.

second much weaker (and broader) one is also

previous sol-gel sample (SG-AM56) i t did not However, we noticed that a f t e r t h i s heat

18

A. Clearfield et al. / Stoichiometry, structure and conductivity o f N A S I C O N

TABLE 3 Comparison of Observed Elemental Analysis (XRF) and Calculated Values from Neutron Refined Data Element

Obs.(XRF) %

Na

AI-NAS-1 Calc.(Neutron Data)

SG-AM56 Obs. % Calc.

Hydrothermal Obs. % Calc.

13.8

13.8

14.2

13.2

14.8

14.7

Si

9.9

10.1

10.2

10.8

10.8

10.3

P

6.3

6,4

6.4

5.9

7.2

7.2

Zr

34.8

33.3

34.5

33.7

29.8

30.9

obtained at +6.07ppm.

At this stage of our stu-

dy we tend to a t t r i b u t e this resonance to the

4. DISCUSSION There would appear to be no doubt, on the

influence of Na+ in the Zr 4+ sites acting on

basis of this study, that a nonstoichiometric

the phosphorus nuclei.

form of NASICON exists.

In fact the scandium

The stoichiometry

compound also gave a resonance at +5.8ppm, but

based on the refinement of the neutron d i f f r a c -

i t was so weak as to be barely perceptible. An 23 Na

t i o n data and elemental analysis carried out

NMR spectrum f o r Na5Zr(P04) 3 showed that

caused large s h i f t s in the 31p resonance.

There

by an independent laboratory agree w i t h i n the experimental error of the procedures.

Further-

were c l e a r l y v i s i b l e resonances of -4.36,

more, t h i s agreement rules out the presence

-0.97, +1.67 and +4.94ppm.*

of an amorphous or glassy phase in the hydro-

This compound has

a NASICON-Iike structure26 in which a l l of the

thermal preparation.

cavities are f i l l e d as well as half the Zr4÷

from the synthesis of the nonstoichiometric

sites.

phase by a sol-gel rather than a hydrothermal

The varied interactions of 23Na coupled

to 31p cause the m u l t i p l i c i t y of resonances. A similar anomaly was observed in the 29Si MAS NMR spectra.

The stoichiometric scandium

The f i n a l proof stems

technique. The sol-gel product was stable even to s i n t e r i n g at 1200°C without the appearance of a second phase. The key feature ex h ib it e d by the nonstoichio-

compound gave a single resonance peak at -90.3ppm while the hydrothermal preparation not

metric NASICON is the presence of 0.32 Na+ in

only gave this resonance (-90.1ppm) but another

the Zr 4+ sites.

very broad one of about I/3 intensity at -101.9.

ten (from refinement of the neutron data)

This l a t t e r resonance is indicative of a conden-

Na2.88(Zrl.68Nao.32)Sil.84Pl.16011.54 . This formula requires a deficiency of 0.23 moles

sed s i l i c a t e group such as pyrosilicate.

Thus, the formula can be w r i t -

3.1 Conductivity Data

of ZrO2 and 0.09 moles of Zr 4+ compensated

Conductivity data for the hydrothermally pre-

by 0.36 moles of Na÷.

In contrast the high

pared NASICON12 and the near stoichiometric sam-

temperature s o l i d state reaction and the sol-

ple, AI-NAS-1, are not too d i f f e r e n t even though

gel technique y i e l d products with more Zr,

the l a t t e r sample contained some ZrO2.

However,

i.e.,

1.93 moles and l a r g e r a and b unit c e l l

t h e i r corrosion behavior in contact with molten

parameters.

Their stoichiometries can be ration-

sodium may prove to be somewhat d i f f e r e n t and

a l i z e d as follows.

this aspect needs to be explored.

formula Na3Zr1.93Si2POiL.86, is d e f i c i e n t in

The sol-gel preparation,

0.07 moles of Zr02; f o r the s o l i d - s t a t e reaction NMR data collected by A. K. Cheetham and N. Clayden, University of Oxford.

the formula Na3.17Zr1.93SiI.9P1.I012 requires that the deficiency of 0.07 moles of Zr be

A. Clearfield et al. / Stoichiometry, structure and conductivity of NASICON

19

i n d i v i d u a l Si-O and P-O distance.

compensated by by 0.28 moles of Na+. One may question the stoichiometries just

There is a l -

most no difference in bond lengths between atoms

presented, but there are several factors which

in the T1 and T2 sites.

indicate they are accurate w i t h i n about 3 times

SG-AM56, where the Si and P atoms are segregated

the given esds.

i n t o sites T2 and TI , r e s p e c t i v e l y , the bond

F i r s t the agreement between

However, f or sample

the composition as determined from neutron d i f -

lengths are those of nearly pure Si-O and P-O

f r a c t i o n and XRF (obtained from an independent

distances, respectively.

laboratory) is e x c e l l e n t .

For sample AI-NAS-I

the difference in Zr values in fact predicts the presence of 4% ZrO2.

We also note that

I t is impossible at this time to t e l l whether the near stoichiometric NASICONS have any Na+ in the Zr 4+ sites.

We believe t h i s information

Schioler 22 carried out a neutron d i f f r a c t i o n

w i l l eventually arise from a d e t a i l e d NMR study

study of a stoichiometric NASICON sample of ex-

now underway.

pected composition Na3Zr2Si2P012 and we have

presence of free electrons in the nonstoichiome-

included her results in Table 2.

t r i c NASICON prepared by the sol-gel method.

The cell d i -

mensions of S c h i o l e r ' s preparation are almost

A surprising discovery is the

This could arise from an oxygen deficiency com-

i d e n t i c a l to those of AI-NAS-1 and SG-AM56 and

pensated by electrons in the oxygen sites.

the s o l i d also contained a small amount of ZrO2.

aspect also needs f u r t h e r i n v e s t i g a t i o n particu-

This

Therefore the composition of her NASICON should

l a r l y as to the nature of the u n i d e n t i f i e d ra-

be very close to that of AI-NAS-I.

dical and the e f f e c t on the c onduc t iv it y .

However,

Schioler constrained her t o t a l Na occupancy to equal 3 and that of Zr to be 2.

The refinement

I t is i n t e r e s t i n g that the conductivity of the near stoichiometric and nonstoichiometric

on t h i s basis produced several non-positive tem-

NASICONS are also i d e n t i c a l .

perature factors and poorer agreement factors.

not too surprising as the number of conducting

In our refinements, a l l the temperature factors

ions d i f f e r by only about 10% and t h e i r s i t e

were p h y s i c a l l y reasonable.

occupancy is nearly the same. Apparently the

Unfortunately

However, t h i s is

S c h i o l e r ' s data suffered from the presence of

bottleneck sizes also do not d i f f e r by much as

an impurity, probably Ti metal, and i t is not

the a c t i v a t i o n energies f o r conduction are also

known how much t h i s affected her results.

almost the same. F i n a l l y , we have proposed 13'15 that NASICON

We also note that our d i s t r i b u t i o n of sodium ions in the three c a v i t y sites d i f f e r s consider-

does not precisely conform to the s o l i d solu-

ably from that presented by Schioler.

t i o n formulas such as those represented by eqs.

She in-

dicates complete occupancy of the Nal s i t e

(1)-(3).

Rather the f i n a l composition is to

whereas we f i n d t h i s p o s i t i o n only 1/4 f i l l e d .

some extent path dependent; that i s , depends up-

In contrast in our study the Na2 s i t e was a l -

on the experimental conditions.

ways f i l l e d ,

the Na+ ions v ib r a t e so e n e r g e t i c a l l y at the

but Schioler found that s i t e to

We believe that

temperature required f o r NASICON formation that

be approximately 3/4 f i l l e d . Another feature i n d i c a t i n g the r e l i a b i l i t y

they tend to disrupt the l a t t i c e framework.

By

of our refinement is the way the Si-O and P-O

capturing some of these ions in t i g h t l y held oc-

bond distances r e f l e c t the d i s t r i b u t i o n of the

tahedral sites ( i . e . Zr-O distances are smaller

s i l i c o n and phosphorus atoms.

than Na-O bond distances) part of the stress is

When the d i s t r i -

bution is nearly random, the average bond dis-

alleviated.

tance l i e s close to the weighted mean of the

w i l l develop with f u r t h e r study by the methods

No doubt other r a t i o n a l i z a t i o n s

A. CTearfield et al. / Stoichiometry, stn~cture and conductivity o / N A S I C O N

20

indicated here including studies at elevated tem-

12.

A. C l e a r f i e l d , M. A. Subramanian, W. Wang and P. Jerus, Solid State lonics 9/10 (1983) 895.

13.

P. R. Rudolf, M. A. Subramanian, A. C l e a r f i e l d and J. D. Jorgensen, Mat. Res. B u l l . 20 (1985) 643.

14.

J. Engell, S. Mortenson and L. Moller , Sol i d State lonics 9/10 (1983) 877.

15.

M. A. Subramanian, P. R. Rudolf and A.

peratures. 5. ACKNOWLEDGEMENT Part of t h i s work was supported by the National Science Foundation under grant no. DMR 8025184 and by the Center f o r Energy and Mineral Resources of Texas A&M University f o r which grateful acknowledgement is made. We also thank Drs. Anthony K. Cheetham and Nigel Clayden, Universi-

C l e a r f i e l d , J. Solid State Chem., in press 16.

A. C l e a r f i e l d , Solid State lonics 9/10 (1983) 823.

17.

J. D. Jorgensen and J. Faber, J r . , in Proc. 6th Mtg. Int. Collab. of Advan. Neutron Sources. Ed. J. M. Carpenter. Pp 105-114. Argonne National Laboratory, June 28 - July 2, 1982. (ANL publication ANL-82-80).

f o r the use of t h e i r f a c i l i t i e s .

18.

R. B. Von Dreele, J. D. Jorgensen and C. G. Windsor, J. Appl. Cryst. 15 (1982) 581.

6. REFERENCES

19.

Programs from Materials Science and Technology D i v i s i o n , Argonne National Laborat o r y , Argonne, l l l i n o i s .

Hong, Mat. Res. B u l l . , 11 (1976)

20.

A. C l e a r f i e l d , R. Guerra, A. Oskarsson, M. A. Subramanian and W. Wang, Mat. Res. Bull. 18 (1983) 1561.

3.

M. L. Bayard and G. G. Barna, J. Electroanal. Chem. 91 (1978) 201.

21.

S. J. Milne and A. R. West, Mat. Res. Bull. 19 (1984) 705.

4.

J. P. B o i l o t , J. P. Salanie, G. Desplances and D. Le P o t i e r , Mat. Res. Bull. 14 (1979) 1469.

22.

L. J. Schioler, Ph.D. d i s s e r t a t i o n , Mass. Inst. Tech., Feb. 1983.

5.

R. S. Gordon, G. R. M i l l e r , B. J. McEntire, E. D. Beck and J. R. Rasmussen, Solid State lonics 3/4 (1981) 243.

23.

S. J. Milne and A. R. West, J. Solid State Chem. 57 (1985) 166.

24.

6.

U. von Alpen, M. F. Bell and H. H. Hoefer, Solid State lonics 3/4 (1981) 215.

P. R. Rudolf, A. C l e a r f i e l d and J. D. Jorgensen, J. Solid State Chem., in preparation.

7.

H. Schmid, L. C. De Jonghe and C. Cameron, Solid State lonics 6 (1982) 57.

25.

C. T. Prewitt and R. D. Shannon, Trans. Amer. Cryst. Assoc. 5 (1969) 51.

8.

A. K. Kuriakose, T. A. Wheat, A. Ahmad and J. Dirocco, J. Am. Ceram. Soc. 67 (1984) 179.

26.

J. P. B o i l o t , G. C o l l i n and R. Comes, Sol i d State lonics 9/10 (1983) 829.

9.

H. Kohler and H. Schulz, Solid State lonics 9/10 (1983) 795.

ty of Oxford, f o r taking the NMR data and Dr. James D. Jorgensen, Argonne National Laboratory f o r assistance with the neutron d i f f r a c t i o n data. The Intense Pulsed Neutron Source (IPNS) is operated under the auspices of the U. S. Department of Energy to whom thanks are extended

i.

J. B. Goodenough, H. Y.-P. Hong and J. A. Kafalas, Mat. Res. B u l l . I I (1976) 204.

2.

H, Y . - P .

173.

i0.

H. Kohler, H. Schulz and O. Melnikov, Mat. Res. B u l l . 18 (1983) 589.

Ii.

A. C l e a r f i e l d , P. Jerus and R. N. Cotman, Solid State l o n i c s , 5 (1981) 301.