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NUCLEAR QUADRUPOLE AND ELECTROACOUSTIC ECHOES IN
FERROELECTRIC SbSI
N. Krainik, S. Popov, I. Mylnikova
To cite this version:
N. Krainik, S. Popov, I. Mylnikova. NUCLEAR QUADRUPOLE AND ELECTROACOUSTIC
ECHOES IN FERROELECTRIC SbSI. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-179-
C2-182. �10.1051/jphyscol:1972261�. �jpa-00214997�
JOURNAL DE PHYSIQUE
Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page C2-179
NUCLEAR QUADRUPOLE
AND ELECTROACOU STIC ECHOES IN PERROELECTRIC SbSI
N. N. KRAINIK, S. N. POPOV and I. E. MYLNIKOVA Institute of Semiconductors of the Academy of Sciences of the U. S. S. R.,
Leningrad, D-187 nab. Kutusowa 10, U. S. S. R.
RBsumB.
- Des echos de quadrupoles nuclkaires et electroacoustiques dans les ferroklectriques ont kt6 Btudies.
Les constantes de couplage quadrupolaire et les paramhtres d'asymetrie des ions antimoine et iode sont compares aux valeurs prkalablement calcul6es d'aprh le modele des ions ponctuels polarisables.
Les temps de relaxation spin-spin et spin-reseau pour la transition
312 o112 du noyau Sb121 sont mesurbs
8,differentes tempbratures. Les maxima de frequence de relaxation sont observes a la tempkrature de Curie. Ces maxima sont probablement lies a la decroissance des frequences du mode optique mou.
En plus de l'echo
NQR,on trouve un nouveau type d'echo a deux impulsions dans les cristaux SbSI. Cet kcho se produit a toute frkquence dans l'intervalle examink (de 5 a
70Mcls). Ce n'est pas un k h o de spin. I1 disparait dans la phase parablectrique. Son amplitude dkcroit de plusieurs ordres de grandeur si I'Cchantillon est plack dans un liquide, et augmente s'il est prkalablement polarise par un,champ klectrique. L'amplitude d'echo a 6te Btudik en fonction de la longueur de l'impul- sion, de l'intervalle entre impulsions, de la temperature et de l'amplitude des impulsions. A partir des rksultats on peut conclure que I'apparition d'echo depend des vibrations piboelectriques de I'echantillon. Cet echo peut &tre dQigne comme I'kcho Blectroacoustique.
Abstract. -
Nuclear quadrupole and electroacoustic echoes in ferroelectric crystal SbSJ have been investigated. The quadrupole coupling constants and asymmetry parameters of theantimony and iodine ions are compared with previously calculated data obtained on the assumption of the point polarizable ion model. The spin-spin and spin-lattice relaxation times for the
312- 112tran- sition of the Sb121 nuclei were measured at different temperatures. The maxima of relaxation rates are observed at the Curie temperature. These maxima are probably connected with the decrease of the soft optical mode frequencies.
In addition to the NQR echo a new type of a two-pulse echo is found in SbSJ crystals. This echo occurs at any frequency in the frequency interval examined (from
5to
70Mc/s). It is not a spin echo. It disappears in a paraelectric phase. Its amplitude decreases by some order of magnitude if the sample is placed into some liquid, and increases, if the sample is previously polarized by electric field. The dependences of the echo amplitude on the pulse length, the interval between pulses, temperature, pulse amplitudes have been investigated. The results obtained give the possi- bility to conclude that the echo occurrence depends on piezoelectric vibrations of the sample. This echo may be named as the electroacoustic one.
The results of nuclear and electroacoustic echoes investigation of the interesting ferroelectric-semi- conductor SbSI are given in the present paper. Our experimental values of the antimony and iodine qua- drupole coupling constants and asymmetry para- meters q [I]-[3] are compared with these values cal- culated in paper [4]. The calculation was made on the assumption of the point polarizable ion model and the structural data of Oka et al. [5], using a computer.
When calculating the field gradient a t the ion under consideration the influence of the field gradient a t the surrounding ions was not taken into account.
The number of ions in the summation volume was equal t o 2 200-2 400. 420 versions of VE tensor were calculated for various ion polarizabilities ai and the electric charge ei. Some results are given in Table.
The agreement between the experimental and calcu- lated results is better for the ferroelectric phase than for the paraelectric one. A better agreement between the calculated and the experimental results was obtained when using the ion polarizabilities different for para- and ferroelectric phases, especially for the
antimony. Obviously, this suggests that the character of the antimony chemical bonds is essentially covalent.
The maximum of spin-lattice relaxation rate a t the Curie point was obtained in some order-disorder type ferroelectrics in recent works of Blinc et al. and Bonera et al. [6]-[8]. The investigation of NQ Rrela- xation in SbSJ-type ferroelectrics is of a great interest because of their phase transitions being of the displa- cive type. According to the theoretical treatment the soft mode frequency decrease as well as the critical polarization fluctuations in the vicinity of the ferroelec- tric Curie point lead to the maximum of the spin- tlatice relaxation rate (E. g. see the papers by Bonera et al., Tokunaga et al. [S], [9]). The optical soft mode frequencies in SbSJ decrease a t the Curie temperature according to the optical investigations of Chisler et al., Petzelt, Balkansky et al. and Others [lo]-[12].
We investigated the NQR relaxation times T I and
T; for the 312 tt112 transition of the Sb12' nuclei at different temperatures. In order t o weaken the influence of the piezoelectric effect the sample was immersed into a damping liquid. The temperature dependences
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972261
C2-180 N. N. KRAINIK, S. N. POPOV AND I. E. MYLNIKOVA
TABLE
Calculated and experimental values of nuclear quadrupole coupling constants /?; and asymmetry parameter f/j for SbSJ
O D
Paraelectri c phas e
es
"S U
03
d
13 U 1,35 3,15 3,15 3,15
Exp 0,15 0,15 0,15 0,15 0,15
Exi
a
s5,5 5,5 5,5 5,5 leriment;
5,5 5,5 8,0 8,0 8,0 Derimem
5,8 5,0 5,0 5,8 al data
8,2 8,2 6,6 6,6 7,4 tal data
es b / e es / e
+ 3 - 2 + 3 - 2
2,25 - 1,5 2,25 - 1,5 T = - 196 °C
2 — 2 2,25 - 1,5 3 - 2 2,25 - 1,5 2,25 - 1,5 i T = + 30 °C
eI / e
- 1 - 1 - 0 , 7 5 - 0,75 - 1 - 0,75 - 1 - 0,75 - 0,75
T> 1 2 1
Bs b
202,54 273,85 205,39 203,89 318,36 196,81 147,61 210,85 158,15 203,33 295,35
R 123
Us b
259,68 351,11 263,33 261,42 398,11 252,33 189,25 270,33 202,76 260,69 376,4
h
sb1,00 0,412 0,412 0,396 0,311 0,02 0,02 0,07 0,07 0,09 0,09
B /
2 7860,78 1 341,75
944,8 975,2 669,95 1 832,5 1 298,1 1 832,0 1 295,6 1 527,2 518,08
0,607 0,509 0,509 0,588 0,645 0,399 0,399 0,454 0,454 0,435 0,581 of l/T
7! and \\T\ for different temperatures are shown
in figure 1. The maxima of relaxation rate are observed at the Curie temperature region. This result is in accordance with the theoretical predictions.
In addition to the NQR echo a new type of two- pulse echo was observed in the ferroelectric phase of the SbSJ crystal [13]. The new echo may be named the electroacoustic one. This echo has the following features : it occurs at any frequency throughout the range from 5 to 70 Mc/s. The new echo exceeds the NQR echo for 10 to 100 times in amplitude. The echo amplitude decreases with the increase of the pulse interval and increases with the lowering of the tempe- rature. In a polarized sample the echo is several times as large as that in an unpolarized one. When a sample is supplied with electrodes and especially when there is a short circuit between them the echo amplitude considerably diminishes. The echo amplitude increses near the piezoresonance frequencies of the sample.
When the sample is immersed into a viscid liquid, the echo is damped by several orders of magnitude.
The echo vanishes in the paraelectric phase in accor- dance with the disappearance of the piezoeffect.
Therefore, one can suppose that there is some connec- tion between the echo origin and the piezovibrations of ferroelectric domains and perhaps, of the domain boundaries in SbSJ. An analogous echo was observed in powdered piezoelectric crystals by Kessel et al. [14].
The dependence of the echo amplitude on RF voltage is shown in figure 2. The echo amplitude versus the first pulse length t
1at fixed lengths of the second pulse f
2is shown in figure 3. It is clear from figure 3 that the relation between the pulse lengths for an electroacoustic echo differs from that of a spin echo.
It is possible to suppose that in our case the echo origin is connected with some nonlinearity of the piezoeffect. The piezoeffect nonlinearity in SbSJ is very high. E. g. at 10 °C and 40 kc/s
d3 3E = 3 5o v /d3 3E = o « 0>8 5
(E is RF field amplitude) [15].
Supposing the nonlinear connection between the
FIG. 1. — The S b "1 spin-lattice relaxation rate \)T\ (curve 1) and relaxation rate XjTi (curve 2) as functions of the temperature
for the 3/2 <-> 1/2 transition.
NUCLEAR QUADRUPOLE AND ELECTROACOUSTIC ECHOES
FIG. 2. - Electroacoustic echo amplitude as a function of RF voltage at frequency of 25 Mc/s and the temperature of - 120 OC.
FIG. 3.
-
Electroacoustic echo amplitudes as a function of the first pulse length t a for four fixed lengths t z of the second pulse (1-1 ps, 2-2 ys, 3-3 ps, 4-4 ps) at 25 Mc/s and- 120 OC.N . N. KRAINIK, S. N . POPOV A N D I. E. MYLNIKOVA
0
-200 -150 -100 -50 0 50
z c4.s c)
FIG.
4. - Relaxation time Tz of the electroacoustic echo as a function of temperature at 25 Mc/s.dipole moment p and the displacement c of a damping oscillator p(t) - c3(t) Kessel et al. [14] obtained the approximate expression of the echo signal. This expression agrees with some of our experimental data.
The temperature dependence of the relaxation time
T,in SbSJ is shown in figure 4.
T,values were deter- mined from the echo envelope. It would be possible to suppose that
T,decreases with the increase of the ultrasonic wave attenuation, but this supposition is not consistent with the experimental data of the ultrasonic wave attenuation in SbSJ obtained by Samulionis et al. [16].
The increase of
T,at low temperature may be
connected with the increase of the coercive force which prevents RF domain switching. The weakening of the domain orientation processes and consequently the weakening of the domain dimensions changes must bring to the decrease of the piezovibration frequency changes and of the vibration phase changes.
T,
has to increase due to these processes.
It seems that the connection between the echo and the piezoelectric vibrations is proved. What kind of nonlinear interactions are responsible for the electro- acoustic echo is not quite clear up to now.
The authors would like to thank Prof. G. A. Smo- lensky and Dr. A. R. Kessel for helpful discussions.
References
KRAINIK (N. N.), POPOV (S. N.), MYLNIKOVA
(I.E.),
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Popov (S. N.), KRAINIK (N. N.), MYLNIKOVA (I. E.),
Izv. Akad. Nauk S S S R , Ser. fiz., 1969, 33, 271.P o ~ o v (S. N.), KRAINIK
(N.N.), MYLNIKOVA (I. E.),
J. Phys. Soc. Japan, 1970, 28,suppl.,
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