Nonmetal-metal transition in liquid BiBiBr3 mixtures under pressure

Journal of Non-CrystallineSolids 61 & 62 (1984) 77-82 North-Holland, Amsterdam

77

NONMETAL-METAL TRANSITION IN LIQUID Bi-BiBr 3 MIXTURES UNDER PRESSURE Shinya HOSOKAWA, Hirohisa ENDO and Hideoki HOSHINO* Department of Physics, Kyoto U n i v e r s i t y , Kyoto, 606, Japan *Faculty of Education, Hirosaki U n i v e r s i t y , Hirosaki, 036, Japan. The measurements of the e l e c t r i c a l c o n d u c t i v i t y f o r the l i q u i d Bi-BiBr~ mixture were carried out in the temperature and pressure range up to 6~0°C and up to 20 kbar. In the s a l t - r i c h region the dominant conduction mechanism is a t t r i b u t e d to the electron exchange between Bi + and Bi j + . Around 50 mole% Bi the nonmetal to metal t r a n s i t i o n occurs where the sound v e l o c i t y shows abrupt change. I. INTRODUCTION There are many unsolved problems concerning the nonmetal-metal (NM-M) t r a n s i t i o n in the l i q u i d mixtures of metals with t h e i r molten salts which

-70

~

i

i

I

have an i m m i s c i b i l i t y over a wide concentration range.

In such mixtures

the microscopic inhomogeneity and

-8C

concentration f l u c t u a t i o n s are

-9c

r e f l e c t e d on the conduction mechanism. In l i q u i d Bi-BiBr 3 mixtures there appears a m i s c i b i l i t y

gap in the

concentration region between Bi and 44 mole% Bi.

The m i s c i b i l i t y

gap

~:K -100

diminishes with increasing pressure and disappears at about 16 kbar! I t

-11(

400"C

-

is i n t e r e s t i n g to study the pressure e f f e c t on the transport and thermo-

i

_1201 0

dynamic properties of l i q u i d Bi-BiBr 3 mixture over a wide concentration range.

Bi Br3

I

10

I 20

I 30

I 40

50

mole*/, B i

In t h i s paper, we report the

results of the e l e c t r i c a l c o n d u c t i v i t y and the sound v e l o c i t y f o r l i q u i d Bi-BiBr 3 mixture up to 600°C and 20 kbar.

FIGURE 1 Concentration variations of the magnetic susceptibility x for the liquid Bi-BiBr 3 mixture at atmospheric pressure and at 400°C.

0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

78

S. Hosokawa et al. / Nonmetal-metal transition in liquid Bi-BiBr 3 mixtures

2. EXPERIMENTAL PROCEDURES The electrical conductivity and sound velocity measurements under high pressure were made by using a piston-cylinder apparatus.

The details of the

experimental procedures are described elsewhere!• '2 3. RESULTS AND DISCUSSION The magnetic s u s c e p t i b i l i t y xobtained at atmospheric pressure is diamagnetic and almost independent of temperature in the low Bi concentration.

The value of

× increases gradually at elevated temperatures in the mixtures with Bi concent r a t i o n more than 30 mole%. Figure l shows the concentration variations of × for liquid Bi-BiBr 3 mixture up to 40 mole% Bi at 4OO°C. The bold line indicates the concentration variation of x calculated by assuming that the dominant cont r i b u t i o n to x is arised from two d i s t i n c t valence states Bi + and Bi 3+.

The

agreement between the experimental and calculated values is f a i r l y reasonable. Figure 2 shows the logarithm of the electrical conductivity ~ for the liquid mixture with 20 mole% Bi as a function of reciprocal absolute temperature I/T at

T ('C) 600 50O ~00

T ('C) BOOBOO i

,

i

z~O

,

i

,

300

i

I

20mok~ Bi

'

I

~

102

'

300

i

,

i

40rn°le'/" Bi

20 kb 15kb

'E u

20kb

],

"T

b

1.0

i

~

I

i

i

15 103/T

I

J

I

I 20

(K "1)

FIGURE 2 Log~ vs I/T for the liquid Bi-BiBr R mixture with 20 mole% Bi at different pressures.

I

IO

I

I

I

i

I

I

1.5 103/T (K-l)

I

I

2.0

FIGURE 3 Log~ vs I/T for the liquid Bi-BiBr R mixture with 40 mole% Bi at different pressures.

S, Hosokawa et al, / Nonmetal-metal transition in liquid Bi-BiBr 3 mixtures

10, 15 and 20 kbar.

The arrows denote the melting point of the mixture.

79 The

loga vs I/T curve shows linear behavior at high temperatures and high pressures, where the value of a lies between I and lO ohm-lcm- l .

The slope of the loga vs

I/T curves becomes large with increasing pressure. In Fig.3 the logarithm of a for the liquid mixture with 40 mole% Bi is plotted as a function of I/T at 10, 15 and 20 kbar.

In the intermediate temperature

range the loga vs I/T curve shows linear behavior, where the value of a lies between l and 10 ohm-lcm- l .

At high temperatures the curve bends. With in-

creasing pressure a increases and the slope of the loga vs I/T curves becomes large. The contribution of electrons to a was confirmed by using the residual potent i a l method. Figure 4 shows the results for liquid Bi-BiBr 3 mixtures at 500°C and at 3 kbar.

In the mixture with 20 mole% Bi the residual potential drops

sharply just after switching off the current, and then damps gradually with time which indicates the dominant conduction due to electrons and a slight ionic conduction.

At 40 mole% Bi

there is no indication of the ionic conduction.

liq, Bi-BiBr 3 20mote'l, Bi --

5

3 kbor

"P

'sk°Z

7

5OO'C O.5mA O

I

o

40

t(sec) I

51

1 0

~

I

80

I

120

i

I

tiq.Bi- BiBr3 40 mole'l.Bi--

~

3 kbar 5OO'(:

Bi Br3

mole'kBi

Bi

5 mA

o

I

I

I

20

t(sec)

40

I

J

60

FIGURE 4 Residual potential vs time for the liquid Bi-BiBr 3 mixtures with 20 and 40 mole% Bi at 500 C and at 3 kbar.

FIGURE 5 Concentration variations of loga fsr the liquid Bi-BiBr R mixture at 400 C and at 15 kbar, together with the pressure variations of the phase boundary of the m i s c i b i l i t y gap from reference ( l ) .

80

S. Hosokawa et al. / Nonmetal-metal transition in liquid Bi-BiBr 3 mixtures

In Fig.5 the logarithm of ~ for the liquid Bi-BiBr 3 mixture is plotted as a function of Bi concentration at 15 kbar and at 400°C. The inset shows the pressure variations of the phase boundary of the m i s c i b i l i t y gap in the mixture! One notes that ~ shows exponential dependenceon the concentration in the saltrich region.

I t is interesting that the value of ~ increases sharply and

approaches to 200 ohm-lcm-l (minimum metallic conductivity) around 50 mole% Bi, which indicates the occurrence of the NM-Mt r a n s i t i o n . Raleigh 3 proposed that the predominant conduction process in the s a l t - r i c h region of liquid Bi-Bil 3 mixtures is described by a simultaneous two electron transfer between Bi + and Bi 3+.

As mentioned above, ~ shows exponential depend-

ence on the concentration and the loge vs I/T curve is almost linear in the s a l t - r i c h region.

The magnetic s u s c e p t i b i l i t y data suggest that the s a l t - r i c h

region contains at least two d i s t i n c t valence states Bi + and Bi 3+.

These facts

encourage us to believe that the conduction in the s a l t - r i c h region of liquid Bi-BiBr~ mixture is caused by the simultaneous two electron transfer between Bi .3and BI .

+

This mixed valence mixture is one of simple examples of so-called

"negative U" effect 4 in which the Coulomb repulsion U of two electrons on one s i t e (M+) is more than compensated by the gain in i n t e r i o n i c Coulomb energy and structural relaxation with formation of the complex anion (NX4)°. The information on the ionic arrangement is indispensable to understand the conduction mechanism in the mixed valence mixture. see the c r y s t a l l i n e structure 5 of BiBr 3 or BiBr. of BiBr 3, Bi 3+ is surrounded octahedrally by Br-. sufficient number of empty holes. by four Br-.

I t would be instructive to In the high temperature form This structure leaves a

In c r y s t a l l i n e BiBr, Bi cation is surrounded

I t is a reasonable assumption that the structure of the liquid Bi-BiBr 3 mixture in the mixed valence state may contain a unit Bi+(Bi3+Br4 )-.

A possible model

for the configuration of the mixture is shown in Fig.6.

Small closed

circles denote Bi 3+, large closed circles Bi + and open circles Br-. In this structure Bi 3+ bonds with four Br . Bi ÷ site

When the two electrons on are transferred to Bi 3+

site there appears an alternation of FIGURE 6 A model for the configuration of the liquid Bi-BiBr~ mixture in the mixed valence state, j

the surrounding configurations of Bi ions.

S. Hosokawa et al. / Nonmetal-metal transition in liquid Bi-BiBr 3 mixtures

81

A p p l i c a t i o n of pressure a f f e c t s s t r o n g l y the jumping frequency of electrons

,(R).

The decrease in

the average distance R between Bi ÷ and Bi 3+ would increase

v(R),

'

which leads to the increase in e as seen in Figs. 2 and 3.

,

'

'

I

.

i

i

.

.

.

500"C 12kb

The slope of

the log~ vs I / T curve increases with r a i s i n g pressure as shown in Figs. 2 and 3, which suggests t h a t the reduction of volume by compression tends to keep cations separated in local i n t e r s t i c e s .

The f a c t t h a t

the log~ vs I / T curves bend at high i

temperatures f o r the mixture with 40 mole% Bi is associated with

L

~

i

50 rnole*/.Bi

BiBr3

lO0 Bi

increasing concentration of Bi ° because the f o l l o w i n g r e a c t i o n is expected to be s h i f t e d to the l e f t with i n c r e a s i n g temperature. Bier 3 + 2Bi

~

FIGURE 7 Concentration v a r i a t i o n s of the sound v e l o c i t y V f o r l i q u i d Bi-BiBr^ mixtures aS 500°C and at 12 kb~r.

3Bier.

Above 50 mole% Bi one might consider t h a t the Bi+-Bi 3+ exchange process would be p r o g r e s s i v e l y replaced by a more rapid Bi°-Bi + one-electron process.

It is

expected t h a t Bi ° acts as an e a s i l y i o n i z a b l e e l e c t r o n source and consequently there occurs the d e l o c a l i z a t i o n of electrons from t h i s source. Figure 7 shows the sound v e l o c i t y Vs f o r the l i q u i d Bi-BiBr 3 mixture as a f u n c t i o n of Bi concentration at 500°C and at 12 kbar~

In the s a l t - r i c h

region

Vs is independent of Bi concentration and s t a r t s to increase r a p i d l y around 50 mole% Bi at which the nonmetal to metal t r a n s i t i o n

occurs.

The i n f l e c t i o n

Vs against concentration curve i n d i c a t e s the change in the bonding nature. The abrupt change in Vs around 50 mole% Bi under pressure implies t h a t the rapture of Bi+-Bi 3+ pairs takes place on the t r a n s i t i o n . ACKNOWLEDGEMENT The authors are pleased to acknowledge s t i m u l a t i n g discussions with Dr.K. Tamura and Mr.M.Mushiage. REFERENCES I) K.Tamura, H.Hoshino and H.Endo, Ber. Bunsenges. Phys. Chem. 84 (1980) 235.

in

82

~Hosokawa etal./Nonme~l-memltmnsition m liquid Bi-BiBr3 m ~ t u r ~

2) M.Mushiage, M.Misonou, H.Endo and E.Rapoport, Solid State Comm. 41 (1982) 181 3) D.O.Raleigh, J. Chem. Phys. 38 (1963) 1677. 4) W.W.Warren, Jr., G.Sch~nherr and F.Hensel, Chem. Phys. Lett. 96 (~983) 505. 5) H. von Benda, Thesis (U. Stuttgart, 1977). 6) H.Hoshino, K.Tamura, S.Hosokawa, K.Suzuki, M.Misonou and H.Endo, J. Phys. (France) 41 (1980) C8-52.