35Cl NQR study of the incommensurate phase transition in K2ZnCl4

Volume 76A, number 3,4

PHYSICS LETTERS

31 March 1980

35c1 NQR STUDY OF THE INCOMMENSURATE PHASE TRANSITION IN K 2ZnCI4 F. MILIA and M. VOUDOURIS Nuclear Research Center Demokritos, Athens, Greece Received 28 December 1979 35C1 NQR spectra can be observed only at the low Tend near T~= In the incommensurate (I)near phase 130°Cand at the high Tend T of K2ZnCI4 the 1 = 280°C,whereas they are presumably too broad to be observable in the middle of this phase. This seems to show that the “soliton lattice” model of the I phase is appropriate only close to Tc, whereas the “plane wave” modulation model is a good approximation for most of the I phase.

It is well known [1,2] that K2ZnC14 undergoes two structural phase transitions at T1 = 280°C and Tc = 130°C,which are analogous to the ones [21 observed in Rb2ZnCl4 and Rb2 ZnBr4. The high ternperature phase (T> T1) is paraelectric and belongs to the orthorhombic space group (P) Pmm(D~), the intermediate temperature phase (Tc NQR8.6 lines MHz) in K 2ZnC14 were too broad to be observable over —

,

350

most of the I phase whereas they could be continuously followed through the whole I phase in Rb2 ZnCl4 [4,6]. The temperature dependence of thelines signal-to-noise 35C1 NQR and the ratio of the high T dependence of frequency the line widths are shown in fig. 2. 35Cl NQR frequencies On heatingdecrease from —127°Cthe smoothly with increasing Tin the C phase (fig. 1). The line width and the intensity of the lines are nearly constant. Close to Tc, the lines become weak and broad (fig. 2). There is no sharp change at T~and the high frequency lines can be followed up to 146°C,i.e. they can be seen up to 16°Cabove T~ in the I phase. In the high temperature part of the I phase the lines could be seen again above 237°C, i.e. nearly 40°Cbelow T1. 35C1 NQR lines in the The disappearance of the middle of the I phase can be understood within a “plane wave” model [71of the I phase, but not within described a “solitonaslattice” model [7] where the domains I phase is consisting of commensurate separated by incommensurate domain walls (or phase solitons), where the phase changes by 2ir/3. In the “soliton limit” we would expect to see in the I phase ,

sharp commensurate lines superimposed on a broad background originating from the incommensurate domain walls. Such a behaviour is seen at the low T endInofthe the“plane I phasewave nearlimit” Tc~but elsewhere. thenot whole crystal is a unit cell and one expects [8] to see for each chemi-

PHYSICS LETTERS

Volume 76A, number 3,4

31 March 1980

104

102

100

9.8 N

I

1

a

9.6

> 9.4

9.2 9.0

8.6 ______________________________________________________________________ 140 200 300 400 500 600 T—’ K

rrc

TI

Fig. 1. Temperature dependence of the high frequency

35C1 NQR lines in K 2ZnC14.

20

~~

~Width KHz

s/n

•10

0

r.20

10

0-

——

8

_0

20

10 0

i

300

400

I

500

t~/

200

400 T

—~

K

35C1 NQR lines in K Fig. 2. Signal-to-noise ratio and line width of the high frequency

0 500

I

300

2ZnC14 versus temperature.

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Volume 76A, number 3,4

PHYSICS LETTERS

31 March 1980

cally nonequivalent paraelectric Cl site a continuous 35Cl NQR frequencies distribution of f(v) (dv/dzY’, (1)

the spectrum becomes unobservably weak. Small T gradients will — in view of the T dependence of 6 additionally smear out the spectrum. The lines can be easily observed only in the high-T part of the I

where i’ = i.’(z) measures the spatial dispersion of the NQR frequencies due to the incommensurate order parameter wave [8]. The distribution is limited by two edge singularities where dv/dz = 0. In a simple “plane wave” model of the I phase where

phase, where the order parameter and the splitting

=

~,(0)+ ~(l) cos(qz +

~),

(2)

—

with ~v =

(~,/~,(l))2] —1/2 —

References [1] K. Gesi, J. Phys. Soc. Japan 45 (1978) 1431.

[2] K. Gesi and M. lizumi, J. Phys. Soc. Japan 46 (1979) 697. [3] A.K. Moskalev, l.A. Belobrova, l.P. Aleksandrova, Sh.

one finds [8]

~ [~

between the edge singularities are still small.

(3)

Sawada and Y. Shiroishi, Phys. Stat. Sol. (a) 50 (1978) Kl57.

v

0. Here the edge singularities appear at v = ~A0)±,(1). Since ~(l) is proportional 1/2, to thethe sepaorder so edge that ~(l) ~ (T— T1) ration parameter between the singularities increases with increasing T T 1 and the intensity decreases until

[4] [5] [6] [7] [8]

F. Milia, Phys. Lett. 70A (1979) 218. I.P. Aleksandrova, Ferroelectrics, to be published. F. Milia, Ferroelectrics, to be published. AD. Bruce and R.A. Cowley, J. Phys. Cil (1978) 3609. R. Osredkar, S. Jucnic, V. Rutar, J. Seliger and R. Blinc,

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Ferroelectrics, to be published.