Evidence for a new, stress-induced phase transition in KMnF3

Solid State Communications, Vol. 21, pp. 429-432,1977.

Pergamon Press.

Printed in Great Britain

EVIDENCE FOR A NEW, STRESS-INDUCED PHASE TRANSITION IN KMnF 3 E.

Strau~

and H.J. Riederer

Fachbereich Physik, Universitat Regensburg, Regensburg, West Germany (Received 14 July 1976 by E. Mollwo)

A new, stress-induced phase of KMnF 3 is found in emission, absorption and a.c.-susceptibility measurements. The phase transition is of first order and it occurs under uniaxial stress parallel to the [100] and to the [110] pseudocubic axes at 20.1 and 27 kp/mm:2 at 1.7 K. The experimental results are explained by a change in the amount of the tetragonal distortion in the new, stress-induced phase. THE LOW TEMPERATURE distortions from the cubic perovskite structure of KMnF3 have been investigated extensively in lattice dynamic 1 and structural 2 studies. Below 81 K KMnF 3 is supposed to have D lh -symmetry, but the deviation from the Oh-symmetry is negligible. The displacive phase transition at 186 K is caused by condensation of the R 2s -soft phonon mode. This mechanism is identical with that of the well known phase transition in SrTi0 3 at 105 K 3 and in RbCaF 3 at 200 K 4 . In SrTi0 3 a uniaxial stress of 26.5 kp/mm 2 along [111] induces a lattice instability. 5 Since a similar instability can be assumed in KMnF 3, we studied the effect ofuniaxial stress on its optical spectra in the low temperature phase. The KMnF3 crystals were thin slabs with exactly parallel end faces carefully polished to avoid inhomogeneous stress effects. The samples were cut with their faces perpendicular to the [100], the [I 10], and the [111] pseudocubic axes. They were immersed in liquid helium at 1.7 K. The apparatus used for the optical measurements has been described earlier. 6 The emission was excited with the 19,430 cm -1 line of an Ar-laser. The energy of the pure electronic transition between the ground state 6A 19 and the excited state 4T1g of the Mn2+-ion is 17,882 cm- 1 both in absorption and in emission. It is spin and parity forbidden leading to an extremely weak excitonic line. In spite of the slight tetragonal distortion of the cubic site symmetry, we do not observe a splitting of the line, unlike in the case of Cr 3+ in SrTi0 3.5 The exciton-magnon coupling and the exciton-magnon-phonon coupling generate the magnon and phonon sidebands which are observed on the low and high energy side of the exciton in emission and absorption respectively. Additional strong emission lines (X in Fig. 1) are attributed to impurity centers. 6 When uniaxial stress is applied, the excitonic absorption and emission lines and their sidebands shift by the same amount to lower energies. This shift is linear for not too high stresses and is found to be 1.5 cm -1/kp/mm 2

for all three stress directions. This can not be compared with the theoretical value because the compliance constants Cij ofKMnF3 at low temperatures are not known. We did not observe any stress splitting of the excitonic line in emission. Under [100] -stress the shift of the emission spectrum is observed to be linear only up to 20.1 kp/mm 2 • When this critical stress is exceeded the spectrum changes abruptly (Fig. 1): the exciton E' disappears and a new strong line E appears at 17,802 cm -1 separated from E' by 51 cm -1. It is accompanied by another new line M which has the characteristic shape and energy of a magnon sideband. 6 This observation was confirmed using a crystal from a different source. If one interprets the new line E as the discontinuously shifted exciton E', the same energy shift of 51 cm -1 should occur in absorption. The lines of the absorption spectrum were identified in a detailed analysis. The energy of the magnon sideband of 65 cm -1 is different from that in emission (71 cm -1) because of different exciton-magnon interaction. 6 Uniaxial stress changes this interaction in absorption while in emission it is nearly constant, as in the case of MnF 2 •9 Just below the critical stress, the energy of the magnon sideband is reduced to 61 cm-1 • The excitonic absorption line, being weak already at zero stress (a < 0.01 cm- 1), is broadened by inhomogeneous strain, but we could observe this line as a weak hump also at high stress. At the critical stress the absorption spectrum shifts discontinuously to lower energy by the same amount as the emission spectrum. In Fig. 2 we show the emission and absorption spectra at a stress of 24 kp/mm 2 • The absorption spectrum features the weak excitonic line coinciding with the emission line E, the magnon sideband M and an additional band which seems to be the associated LA phonon sideband. The discontinuous shift together with the coinCiding zero-phonon-zero-magnon lines show conclusively the intrinsic nature of the new

429

EVIDENCE FOR A NEW PHASE TRANSITION IN KMnF3

430

KMnF3 T=17K S II (100J

z

0

Vi

5 = 200 kp/mm 2

Vl ~

llJ

M

u..

0

E'

>!::::

Vl

S = 201 kp/mm 2

Z

llJ ~

~ S1cm-1------1

~

E

--HS =202 kp/mm 2

17700

17750

17800

WAVENUMBER

Vol. 21, No.5

resulting in an increase of the crystal field at the site of the Mn 2+-ion. The first-order character of the transition is manifested not only by the discontinuous shifts but also by hysteresis effects. The low-stress phase is observed to be stable up to 24 kp/mm 2 • Above the critical value of 20.1 kp/mm 2 , however, an increase of temperature always initiates the transition. All described effects are independent on the rate of cooling, nor was there any supercooling in linear birefringence measurements of all other known phase transitions. 7 Therefore we believe that we have found the assumed stress-induced lattice-instability in KMnF3' In order to investigate the symmetry of the new phase we applied stress also in other directions. For the analysis of the results we decompose a given applied stress S into its various symmetry components a(ri). They are different for different stress directions:

a(100)

= -} a(rn - 3~2 al(r;)

0(110)

1 +) = "3o(r1 -

17850

[em-I]

Fig. 1. Emission spectra ofKMnF below and above the critical stress in the region ofthe 4T 1g -exciton E'. The spectrum is shifted discontinuously by 51 cm -1. The bandwidth of the exciton is larger than the spectral resolution and is mainly due to inhomogeneous strain. line E. The exciton emission is predominantly a-polarized (perpendicular to the stress) while the magnon sideband is 1T-polarized (parallel to the stress). At the critical stress the degree of polarization increases discontinuously. The observed discontinuity can be explained with a first order stress-induced phase transition at 20.1 kp/mm 2

1 (+) 1 (+) 3../2 al r3 + ../2 al rs

0(111) = -} a(rn + 3~2

{al(rn + a2(rn + a3(rm·

Using these relations we can decide on the symmetry of the distortion at the phase transition if we apply the stress also parallel to the [110] and [111] pseudo cubic axes. The dependence of the energies of the emission lines under [110] -stress is shown in Fig. 3. We observe the phase transition at a critical stress of 27 kp/mm 2 • It is also of first order and the jump of the excitonic line

KMnF 3 2'0 kp/mm2 II [100J 17K

LA

E

e

M

iJi

III

1-----'1-

~

70cm-1 - - - l - - - - 61cm-1

I

eOcm-1 --f----j

UJ

LL

o

--Uo-

>-

I-

iJi

z

UJ

EMISSION

~

17700

Fig.

ABSORPTION

17750

17800 17850 WAVENUMBER [cm-'l

17900

17950

2. Emission and absorption spectra in the high-stress phase. The zero-phonon-zero-magnon lines E coincide in emission and absorption.

Vol.

~I,

EVIDENCE FOR A NEW PHASE TRANSITION IN KMnF3

No.5

431

17890 r - - - - - - - - - - - - - - - - - - - - - - - - - - ,

Eu 17850 a::

III ~

~

o

:It:~----------~:::-~-:."- -----jb_~:::.:. -::::::= ---"-- x-x- x-x

w

17800

----------o-----------O-_____._____ -o.. ____~:-x_ x-- x_ ><- -x'_x -x_ x_

W

~

""'O--.o-.-o..-O...-o.....,Q

~

~---1fn:

Exciton o Magnon sIdeband x Impuroty lines

D.

17750

T= 1,7 K 511[1101

II

I

~-.o<- - I

I

It 6--0-___ _ 1

17710 . L - - - - + - - - - - f - - - - - - - 1 - - - - - + - - - - + - - - _ _ + ' 25 10 15 20 30 5

STRESS [kp/mm 2 j

Fig. 3. Shift of exciton, magnon sideband and impurity-lines under [11 0] -stress. The first order character of the phase transition is evident. The polarization of the lines is also indicated. The impurity lines are not discussed in this paper.

N

~ 5

KMnF3

>u

T=15K 511 [100]

Z l!J

::::> l.

a a:

l!J LL I

g;

3

4 ...J

St;:4

...J

i3

~ 2 LL 0

f o ,11MHZ

l!J (!J

Z <{

I

u

12

15

rr 25

20

STRESS [kp/mm 2 ]

Fig. 4. Dependence of oscillator frequency on applied [IOO]-stress. An increase of frequency corresponds to a decrease of a.c.-susceptibility X" which has its minimum at the critical stress. is 54 cm -1 to lower energies. As in the case of [100]stress, the a-polarization of the exciton and the 1T-polarization of the magnon sideband increase abruptly at the transition point. In the high stress phase the line shift is linear again and no splitting of the exciton is observed. In contrast to this, [Ill] -stress does not induce any discontinuous line shift up to 40 kp/mm 2 , not even at temperatures up to 80 K. The crystals could not withstand higher stress. Only the constant shift due to the of the uniaxial stress was totally symmetric part observed.

acrn

The following analysis of the results for the three different stress directions leads to the conclusion that the phase transition has to be explained as an additional tetragonal distortion. According to the decomposition given above, none of these stress directions contains orthorhombic components a2Cn). The total symmetric part cannot cause the phase transition since it is not observed under [Ill] -stress. Furthermore, a trigonal deformation is ruled out because the transition is observed under [lOO]-stress which has no trigonal component The tetragonal component is

a(rn

alcr;).

alcrn

432

EVIDENCE FOR A NEW PHASE TRANSITION IN KMnF3

Vol. 21, No .. 5

•

not present under [111] -stress but it exists under [100]- first order stress-induced phase transition with a change in the amount of the tetragonal distortion is different and [110] -stress. As the phase transition is observed from the results for SrTi0 3 where the stress-induced only for these two directions a tetragonal distortion is phase transition changes the symmetry from tetragonal the only possible explanation. to trigonal. 5 Further support for the above interpretation Using a method described by Maartense 8 we could of our KMnF3 emission and absorption measurements show that the stress-induced phase transition is manican be drawn from preliminary Raman measurements fested also in the magnetic behaviour. The sample was placed in the coil of an 11 MHz oscillator and the change which give no evidence for a change of symmetry at the critical stress. of frequency was recorded when [100] -stress was applied. At the critical stress a maximum of the frequency was observed (Fig. 4). This maximum corresponds to a minimum of the a.c.-susceptibility parallel to the stress. The microscopic interpretation of the effect Acknowledgements - We wish to thank Dr. V. Gerhardt, Prof. M. Creuzburg, Prof. W. Gebhardt, and Dr. as well as of the hysteresis shown in Fig. 4 are as yet J.P. Srivastava for helpful discussions, Dr. G. Nitschmann unclear. for providing crystal material, and F. Koch and H. Kett The interpretation of the experimental results as a for their technical assistance. REFERENCES 1.

LOCKWOOD D.I. & TORRIE RH.,J. Phys. C7, 2729 (1974).

2.

HIDAKA H., J. Phys. Soc. Japan 39, 180 (1975).

3.

MINKIEWICZ V.I., FUJII Y. & YAMADA Y.,J. Phys. Soc Japan 28,443 (1970).

4.

RUSHWORTH A.I. & RYAN J.F., Solid State Commun. 18, 1239 (1976).

5.

BURKE W.J. & PRESSLEY R.J., Solid State Commun. 7, 1187 (1969).

6.

STRAUB E., GERHARDT V. & RIEDERER H.J.,J. Lumine.,sc. 12/13,239 (1976).

7. 8.

KELLNER U.C. & GERMANN K.H., Unpublished, University Regensburg (1975).

9.

DIETZ R.E., MEIXNER A.E., GUGGENHEIM H.J. & MISETICH A., J. Luminesc. 1/2,279 (I 970).

MAARTENSE I., Solid State Commun. 12, 1133 (1973).