New crystalline phases of an equiatomic K-Cs alloy at low temperature

J. Phys Chtm. Solid1 Vol. 42. pp. 19-22 Pcrgdmon Press Ltd.. 1981. Printed hn Greal Bnufn

NEWCRYSTALLINEPHASESOFANEQUIATOMICK-CsALLOY ATLOWTEMPERATURE U. SHMUELI Department

of Chemistry,

Tel Aviv

UniversiIy.

Ramat

Aviv.

Israel

and V. STElNgERG,t Department

of Physics

(Received

and A. VORONEL

T. SVERBILOVA

and Astronomy, 25 April

Tel Aviv

University.

1979; accepted

2 May

Ramaf

Aviv,

Israel

1980)

Abstract-Solid equiatomic K-Cs alloys have been investigated by X-ray diffraction Ihroughout the temperature range 3WIOOK. The results indicate that a phase separation occurs below 18SK accompanied b the appearance of x and c = I l.W2) A an ordered phase in this range. This phase has a hexagonal lattice with parameters: (I =9.32(l) (at 170Kl. Evidence from our other studies[‘l] indicates that its composilion is K$Zs. Another phase transformation in this ordered crvstal is observed below I2OK. There is no change of lattice symmetry but the unit cell constants (at IGK). The Iransformaiion can be ascribed IO a shrink to the values: a =9.llII)A and c = 10.8&2)1/ rearrangement of the electronic structure of Cs.

INTROWCTION

The present communication deals with the X-ray patterns of equiatomic K-Cs alloys at low temperatures. The simplicity and ease of preparation of these alloys make them attractive objects of study for trying to understand the structural properties of alloys, especially the formation of disordered solid solutions[ I]. The crystal structures of these disordered solid solutions were investigated below 234K and it was concluded]?, 31 that their structure is b.c.c. Simon et al. [4] found a phase separation K-Cs below 18OK and have reported the existence of two intermetallic compounds, KzCs and K,Ck, under the separation curve. On the other hand, no phase separation above 78K was found in earlier thermographic experiments by Tracy Hall el al.[S], presumably because of the high rate of cooling employed in their measurements. Our own resistivity, slow thermographic and heat capacity measurements for various K-Cs alloy compositions[6,7] confirm the occurrence of phase separation in K-Cs below 185K. However, when the rate of cooling was increased above lOV/hr, no phase separation was found in agreement with[5]. We found no evidence for the existence of a K,Cs, phase, but our data definitely confirms the persistence of a K,Cs phase[7]. Since an assessment of the results obtained in[7] suggested to us that the phenomenon of atomic ordering may also be of importance, we decided to carry out an X-ray diffraction study of K-Cs below 18SK. X-Ray diffraction patterns from equimolar K-Cs solutions were obtained throughout the temperature range: 295-IODK. The composition of this alloy corresponds to an azeotropic point, which ensures no segregation as a

result of crossing the liquidus-solidus line under equilibrium conditions (see Fig. I). AlLdjfFraction patterns of K-Cs taken in the range 233-385K contain no powder lines, but they show a diffuse background which increases with decreasing temperature and they display a small number of single-crystal spots. This is consistent with previous[2] evidence on the formation of a chemically disordered (e.g. atoms K and Cs randomly distributed) solid solution in this temperature range. A sharply detailed powder pattern appears and persists throughout the l85-120K range and further cooling below 120K resulted in the appearance of another new powder pattern. The single-crystal spots, characteristic of the “warmest” solid phase persist throughout the temperature range investigated. So we have reason to believe that below 185K we obtain a two-phase system consisting of the atomically ordered KzCs crystalline phase and disordered K,Cs,_, mixed crystals. This could be in agreement with the coexistence curve reporTZ 70 I------

0K

tFrom July 1980. the authors address is-Department of Physics. University of California. Santa Barbara, CA 93106 USA

Fig.

19

1

---&----

L --40 m

I. Liquidus-solidus

Atomic

Eio -.-.

per ten?

relationship the K-Cs

in the diagram

system.

j

00

CS

of state for

20

U. SHMUELI

ted in141 if we remove the powder

patterns

our samples

the K,Cs,,-compound.

we obtain

are completely

However,

for the ordered

portion

wherein of

the

cylindrical

different.

formation

of

transfer The samples

of K-Cs

cesium

(Merck,

99.95%)

in a glove

amounts

of Hz0

After

the alloys

carefully

All

prepared and

box

under

from

commercial

potassium

(Ventron,

argon

and O2 impurities were prepared,

weighed mm) which

the samples

by thoroughly they

(diameter were

sealed

atmosphere,

not exceeding

components.

into glass capillaries -0.01

were

99.98%)

had equimolar

side

the

the glove

compositions

inside

box.

of K and

The capillary single-crystal axis

employed distance filtered

with the K-Cs goniometer

of a precession

camera.

as a flat plate

camera,

of 60 mm, using unfiltered Molybdenum

Temperature cooling

sample

head which

system,

radiation

control

consists

nitrogen

evaporator

of

The with

Exposures,

an with

Fig. 2. Single-crystal

instrument

on a to the was

specimen-to-film

the

(AtKa)

= 0.7107 with

by Enruf-Nonius. automatically a double

diffraction

A).

tition

the aid of a Delft. refilled

stream

patlern

reached

con-

tube.

was removed,

of flow

Dewar

temperature.

were

were

usu-

In several as follows:

a

the outlet

of

between

sample.

the required resulting

tubes

that the specimen

quenched

was inserted the

was tem-

system.

the required

and

The

precise below,

measurements

background

typical

temperature are based

and

after

the

temperature,

in a rapid of

the

on specific

on equimolar

for a disordered

a

on the warmer rate

or Debye-Scherer,

was

of

the cold N? stream

Glass

the cooling laue

aid

? mm away

cold

the par-

freezing

of the

alloy. quoted

wi:h diffuse

tube

had reached

The

a

outlet

and the required

by heating

the alloy

partition

liquid

arrangement.

each exposure

either

transfer

stream

A constant

the

about

of the sample,

transfer

however,

cardboard

The

the capillary with

after it had been ascertained

had slowly cases,

as well as zirconium-

was achieved

produced

system

was mounted was attached

part

throughout

ally taken

by

thus preventing

sample.

with

measured

obtained

the outlet

employed

cs.

dial

during were

the

was positioned

the gas stream.

peratures

thickness

on

co-axial

was

which

the irradiated of

is surrounded

N2 stream,

sample.

temperature

maintained

introduced

wall

off inside

from

mixing

were

-02mm.

the

stream

crystals

the K-Cs

The

gas

was kept

thermocouple,

I ppm.

N2

ice

tube

taining

EXPERMENTAL

cold

shell of a warm

K-Cs

phase

transitions.

heat and resistivity samples[7].

cubic Aid

solution.

New

crystalline

phases of an equiatomic

RESULTS

A qualitative

description

observed

in the various

explored.

is presented

The only feature the intensity of

temperature in Table

ranges

distribution

of the powder

which

were

the

patterns

is that

concentration

from

equiatomic

The

lower

from

equilibrium

rather

patterns

by cooling several regularly

random

taken or by

single

tensity

intensity

dis-

representative

observed

in ranges

(III)

reproduced

increases

distortions

up the Since

as the lower with

(II).

spots.

the intensity limit

deviations

scattering

temperature.

temperature pattern.

goes taken

with

is brought seen in (II).

0

0

I I

0

I 0

I 0 0

I I

2 0

3

1 0

I 0

3 I

0 I

I 4

2 0 4 3 I 0 2 I 4 4 0 0 3 0 4 2 I 5 3 I 5 4 I 4 3 I 6 2 2 7 4 3 I 5 0 6 7 0 0 (1 =9.32(1)A c= ll.8Of2)A

8.25 A

us s (‘s v(‘s

L’s

ms w

m m L‘s

s

m m ms m w

m w I4

w w H Iv

6.68 4.58 4.01 3.67 3.05 2.91 2.69 2.62 2.49

8.07 A 6.60 4.66 4.03 366 3.05 2.95 2.69 2.62 2.49

superimposed The

patterns patterns

2.38 2.24 2.10 2.02 I .98 I .88 I .62 I.51 I .48 1.37 1.32 1.24 I.15

2.38 2.24 2.12 2.02 I.99 I .87 I.62 I.51 1.48 1.37 I .32 I .25 I.15

0 0

I 2 2 2 I 3 I 4

I 0 0 I 0 0 0 0

2 4 4 3 4 5 6

0 I 2 I I I

I

c=

into

2. Qualitative

description

range

(‘s

I 0 0 I 0 4 2 5 0

5 cs N’S (‘5 s rs s

5 I 0 6 5 4 3

at

l70-IIOK.

However.

in the coldest

range (IV)

observed

H H !A

spots

a =9.11(1) 10.86f2) A

of the diffraction

in ranges

7.89 6.38 4.55 3.94 3.71 2.98 2.57 2.37 2.09 I .97

I.91 1.70 I .49 1.39 I .35 I.25 I.14

1.90 1.70 I .49 1.39 I.35 I.26 I.14

A r,,,, = 780 A’

patterns

II

paltern

(2)

+ Single crystal pattern (no diffuse background)

IRSK Powder

pattern

( I)

+ Single crystal pattern (no diffuse background)

I 233K

Single crystal pattern

Broad liquid rings al:

+ Diffuse

background

(see Fig. 2)

0= 2.47: 4.91: 8.47.

persist

in the

differ

in range (III).

1.16 8, 6.41 4.54 3.97 3.70 2.95 2.53 2.31 2.10 I .98

: H u

the

single-crystal

those observed

1

(III).

crystal

taken

patterns.

0

III IIOK

Powder

is

range-(111

Range and temperature IV

A

192K.

and a powder

I&.,, = 887 A’

Table

in-

down.

single

observed

from

The powder

I I

at

has

Its

disappears

I. Powder patterns of ordered K-G in ranges III and IV. The unit cell dimensions are given along with their estimated standard deviations fin parentheses. in units of Ihe last decimal place)

I

further crystal.

in Fig. 2.

significantly

of

for this

the

distorted

diffuse

on

diffraction

is obtained.

powder

that

real

with termal

temperature

diffraction

of the

temperature

is the

when

background.

pattern

on a

to conclude

local

diffuse

specimen,

superimposed

it is reasonable

connected

Table

2 2

spots

background.

is approached,

2 I

warming

range

are responsible

the

dependence

decreases

overall

throughout

crystal

peaked

of this background range (II)

connected

opposite

As the

Diffraction whether

thermal

the

rings in this range

21

phenomenon.

and (IV).

display

alloy at low !rmpcraturc

Background mentioning,

of the broad

resembles

patterns,

2.

of range (I 1, worth

temperatures

tribution

of the diffraction

K-C<

A

(Ill)

and (IV)

22

u. SHMUELI

were indexed

by the procedure

possible

to account

for

assuming

hexagonal

lattices

and c axes, in the above procedure,

indices,

well

as calculated

cell

dimensions

and

led

to

unit

cell

these

thermal results,

between

different

a axes

angle

values.

intensities, are given

in Table

point

III.

IV

cell volume shrinkage

due

without that

Similar contraction

A,

sures exceeding

c =

precise

However.

of pure

40 Kbar

of

this

arrange-

it seems

plausible

change

of volume

is

structure

of

of the electronic

observations

to

with the

nature

of the atomic

the above-mentioned

volume

be ascribed

above. the

a knowledge

to a rearrangement

could

I) in comparison

mentioned

to explain

in this structure.

assume

than

(see Table

at l20K

is difficult

ment

is far more

Cs]lO].

were crystalline

reported

for

Cs under

to

the pres-

[ I I ].

aniso-

According

to region

which contraction

I. It

a =9.33(l)

in region

thermal

taken at

V = 879 A’, thus indicating

expansion

as

and unit

photograph

dimensions

The

observed

shrinkage

It

by their

of the reflections,

the transition

by

the usual indexing

Bragg

volumes,

It was

lines

were refined

out that a powder

I I .66(2) 8, and volume tropic

After

relative

spacings

should be pointed

et 01[8].

observed

with slightly

ranges.

fit to the indexed

including

120K

the

the unit cell dimensions

least-squares results,

of Henry

all of

to

must

lie

Acknowledgemenls-The United States-Israel

work is partially supported Binational Science Foundation.

by

the

I10 and 120 K. REFERENCS CONCLUSIOY

The

existence

agreement K-Cs,

of

with

described

However, Simon

a new

the

phase

by Steinberg

the

powder

el a/.[41

differs

In fact,

it was

from

new feature

the

volume

existing

ordered phases. as well,

III

can KzCs

to index

unit

cell

higher

that

the

our

work

by

I (111).

pattern

(III)

author. is our evidence

phase below

120K.

being a sharp decrease compared

diffraction

with

patterns

to a phase

disordered

The same separation but the ordered

of

the

in

phase

temperature.

be ascribed and

observed

in Table

by the above

difference

of the

at slightly

It appears range

that shown

of the present

is in

ordering

and by Simon[9].

pattern

of a new crystalline

its most conspicuous

l85K

and

et o/.(7]

not possible

for the existence

below

diffraction

using the unit cell published The

phase separation

K,Cs,,_,,

seems to persist phase

observed

separation

undergoes

in into

crystalline in range IV a significant

I. Hansen M. and Anderko K.. Consfifufion of Binary A//OK -.,-. 2nd Edn. McGraw-Hill, New York (1958). 2. Rinck E.. Comof. Rend. 197. 1404 (1933). 3. Bohm B.‘and Klemm W.. Z. Anorg. Chem. 243, 69 (1939) 4. Simon A., Bramer W., Hillenkotter B. and Kullmann H. J.. Z. Anonz Alla. Chem. 419. 253. (1976). 5. Goat&J. R.. Ott J. B. and Tracy Hall H. Jr., 1. Chem. Engng. Data 16. 83 (1971). 6. Steinberg V.. Sverbilova T. and Voronel A.. Proc 7th @mu on Thetkophysical Properties, May 1977, NBS, Gaithersburn Marvland. U.S.A. (1977): and Proc. l5fh Inf. Conf ~WI Temderafure Physics. (LT-IS) Grenoble. France (l97j78;: 7. Steinberg V.. Sverbilova T. and Voronel T.. 1. Phys. Chem. Solids. to be published (1980). 8. Henrv N. F. hf.. Lipson H., Wooster W. A.. The Inferprefofion of X-ray Diflrucfion Phofographs. Macmillan. London (l%lJ. 9. Bauhofer W.. Simon A.. Z. Nafurforsch 32a. 1275 (1977). IO. Sternheimer R.. Phys. Rev. 78. 235 (1950). II. Jayaraman A.. Newton R. C. and McDonough J. M.. Phys. Rev. IS9. 527 (1967).

Low