87Rb and 85Rb NQR study of phase transitions in RbH 3(SeO3)2

HAL Id: jpa-00209626

https://hal.archives-ouvertes.fr/jpa-00209626

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

87Rb and 85Rb NQR study of phase transitions in RbH 3(SeO3)2

J. Seliger, V. Žagar, R. Blinc, L.A. Shuvalov

To cite this version:

J. Seliger, V. Žagar, R. Blinc, L.A. Shuvalov. 87Rb and 85Rb NQR study of phase transitions in RbH 3(SeO3)2. Journal de Physique, 1983, 44 (4), pp.521-524. �10.1051/jphys:01983004404052100�.

�jpa-00209626�

87Rb and 85Rb NQR study ^of phase transitions in RbH3(SeO3)2

J. Seliger, ^V. 017Dagar, ^{R. Blinc}

J. Stefan Institute, Department ^of Physics, ^E. Kardelj University ^of Ljubljana, Ljubljana, Yugoslavia

and L. A. Shuvalov

Institute of Crystallography, Academy ^{of Sciences} USSR, Moscow, ^USSR

(Reçu ^le 7 juillet 1982, révisé le I S novembre, accepté le 7 décembre 1982)

Résumé.

Nous avons mesuré le spectre de résonance quadrupolaire ^{de 85Rb et 87Rb dans le} composé RbH3(SeO3)2 par ^une technique de double résonance proton-Rb. L’accroissement de un à quatre du nombre de sites non équivalents ^et la variation avec la température ^des constantes de couplages quadrupolaires ^et des para- mètres d’asymétrie ^{dans la} phase ferroélectrique ^{sont en accord avec un} modèle de mode mou où l’on prend ^en compte la rotation d’un type de groupe SeO3. ^{Nous avons} confirmé l’existence d’une phase incommensurable intermédiaire entre Tc et Tc ^{+ 4 K.}

Abstract.

The temperature dependence of the nuclear quadrupole ^resonance spectra ^of 85Rb and 87Rb in

RbH3(SeO3)2 ^{has been measured} by ^a proton-Rb ^{double resonance} technique ^{in the} laboratory frame. The increase in the number of chemically non-equivalent ^{sites from one to} four and the observed temperature dependence ^{of the} quadrupole coupling ^{constants and the} asymmetry parameters ^on going ^into the ferroelectric phase agrees with the

proposed soft mode motion involving rotations of one type ^of SeO3 groups. The existence of ^an intermediate incommensurate phase ^between Tc ^and Tc ^{+ 4 K} has been confirmed.

Classification Physics ^Abstracts

76.70F - 77.80B

1. Introduction.

RbH3(Se03)2 (henceforth ^desi- gnated RHS) ^{is the} only example ^{of an 0-H--0}

bonded ferroelectric of the KH2po4 ^or NaH3(Se03)2 family where the ferroelectric phase transition [1, 2]

around T,,, = - ^{120 OC is} preceded by ^{an incommen-}

surate phase.

The paraelectric phase of RHS is orthorhombic

(space group P212121, ^{z =} 4) and consists of two

types of chains formed by ^{0-H--0 bonded} Seo3

groups [3, 4]. The ferroelectric phase ^transition

around T,,, = - ^{120°C is induced} by ^a doubly degenerate ^mode [5, 6] (involving twist motions of the

Se03 groups of one chain) which becomes soft in the

vicinity ^{of the} Z-point (c*/2) of the Brillouin zone.

The low temperature phase ^is monoclinic (space

group P21, ^{z =} 8) ^{and has a} very small spontaneous

polarization along the b-axis. In contrast to other 0-H--0 bonded ferroelectrics there is a small

negative isotope ^shift in ll§ ^on deuteration [7, 8]

and instead of increasing ^the spontaneous polarization

decreases with increasing ^{deuterium content} [8].

The hydrogens ⁱⁿ the 0-H--0 bonds seem to be ordered even in the paraelectric ^phase.

The possible existence of an incommensurate phase

in RHS intermediate between the paraelectric ^{and the}

ferroelectric phases ^{has been} predicted ^{on the basis}

of _group theoretical arguments [9]. ^{In the} study ^of

the temperature dependence ^{of the elastic} compliance [10] in addition to the anomaly ^at

-

120°C another anomaly has been observed at

-

118 °C thus providing the first evidence for the existence of an intermediate phase. ^{Neutron scatte-} ring [11], ^{Cr3 + EPR} [12], ultrasonic [13] ^{and macro-} scopic quadrupole ^moment [14] measurements have confirmed the existence of an incommensurate phase

in RHS. CW 8’Rb NMR [15] and deuteron NMR [16]

measurements have however failed to provide any evidence for the existence of an intermediate incom- mensurate phase ^{in RHS. In a recent} study [17] ^{of the}

dielectric and elastic properties ^{of RHS it has been}

furthermore suggested ^that only ^one sharp ^ferro-

electric transition occurs at Tc in pure samples ^and

that the appearance of more than one transition is related to the _presence of impurities ^{and inhomo-} geneities ^{of the} samples. ^{To make} things ^{even more} complicated ^{there is some} evidence for the existence

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01983004404052100

522

of another transition [14, 18] within the intermediate

phase.

In order to settle the problem ^{of the existence of the}

incommensurate phase ^{in RHS we decided to} study

the _pure nuclear quadrupole ^resonance (NQR) spec- tra of 8’Rb and 85Rb ^{in a} RbH3(Se03)2 single crystal

as a function of temperature. ^The shape ^{of an incom-}

mensurate spectrum ^{can be} easily distinguished ^from

the shape ^{of a line broadened} by impurities. ^We

also hoped ^{to throw some new} light ^{on the nature of}

the soft mode displacements ^at the ferroelectric phase

transition.

2. Experimental.

^{The Rb} NQR spectra ^{of RHS}

were obtained with the help ^{of a} Rb-proton ^double

resonance technique involving ^level crossing ^{in the} laboratory ^frame [19]. ^The measurements were per- formed on a home built double resonance spectro-

meter. The temperature ^{of the} sample ^{was controlled}

and measured with an accuracy of ± 0.3 OC. The

single crystal ^{used was} grown in a refrigerator by lowering ^the temperature ^{of a} saturated solution from 8.5 OC.

8’ Rb has a spin ^{I =} 3/2, ^{whereas 85Rb has a} spin

I ⁼ 5/2. ^The natural abundance of these two isotopes

is 27.85 % ^{for 87 Rb and} 72.15 % ^{for 85Rb. The ratio}

of their quadrupole ^{moments is :}

The eigenstates ^{E of the} quadrupolar Hamiltonian

are for I ⁼ 3/2 ^obtained [20] from the secular equation

and for I ⁼ 5/2 ^from [20]

with x = E/A, A = e2q Q/4 /(2 ^{1 -} 1). ^{For 87Rb}

we thus get ^{for each} chemically non-equivalent ^site

a single NQR frequency corresponding ^{to Am = ± 1 as}

whereas we get ^for 85Rb for each chemically ^non- equivalent ^{site two low} frequency ^{Am = ± 1} NQR

transitions. For small values of the asymmetry para-

meter ?I they ^{can be} approximated by

and

For larger ^values of 17 ^{we have to solve} equation (3) numerically. The asymmetry of the EFG tensor is

immediately apparent ^from the ratio of VQ/VQ.

For q ^{# 0 a third} high frequency NQR ^transition

becomes allowed for I ⁼ 5/2. This transition was not

investigated ^{in the} present study.

3. Results and discussion.

The temperature dependences of the observed 87Rb and 85Rb NQR frequencies ^are presented ⁱⁿ figure ^{1. In} agreement with the structural data [3-6] ^{we find one 8’Rb and}

two 85Rb NQR frequencies (corresponding ^{to one} chemically non-equivalent ^Rb site) ^{in the} high ^tem- perature paraelectric phase.

Fig. ^1.

Temperature dependence ^{of the 85Rb and 87Rb} NQR frequencies ^{in the} paraelectric (P), incommensurate (I)

and ferroelectric (F) phases ^{of RHS.}

In the low temperature ferroelectric phase ^{we find}

four chemically non-equivalent sites which we

designate A 1, A2, B 1 ^and B2. ^This agrees with the 87Rb NMR data of reference [15]. The increase in the number of non-equivalent ^{Rb sites} corresponds ^{to the} doubling of the unit cell and the disappearance ^of

the two-fold screw axes along ^{the a and c} directions in the ferroelectric phase [5, 6]. ^{It is not due to} the pre-

sence of domains. NQR ^is (in contrast to NMR)

insensitive to the _presence of ferroelectric or ferro- elastic domains.

’

The temperature dependences ^{of the 85 Rb} quadru- pole coupling ^constants e 2q Q/h and of the asymmetry parameters q ^{in the} paraelectric ^and ferroelectric

phases ^are presented ⁱⁿ figures ^{2 and 3. On} going ^into

the ferroelectric phase e 2q Q/h increases for sites A,

and B2 whereas it decreases for sites A2 ^and B1. ^The

value of il increases for sites A 1 and B1 1 whereas it decreases for sites A2 ^and B2.

The above changes in the Rb EFG tensor reflect

Fig. ^2.

Temperature dependence ^of the 8 5 Rb quadrupole coupling ^{constants in RHS.}

Fig. ^3.

Temperature dependence ^{of the} asymmetry para- meter q ^{in RHS.}

the displacements of the ions due to the condensation of the soft mode. The Rb ion in RHS is located in the centre of a distorted octahedron of _oxygen ions

belonging ^{to the two different} ((HSeOg) ^and (H2Se03))

chains. Only ^one of these chains is involved in the

Fig. ^4.

^{Observed 85Rb} NQR lineshape ^{in the} P, ^{I and} F phases.

soft mode motion [5, 6] whereas the other is essentially

unaffected. Within the chain involved in the phase

transition mechanism adjacent Seo3 groups ^rotate in opposite directions. The observed EFG tensor

changes qualitatively agree with the nature of the

proposed ^{soft mode} displacements [5, 6].

Between T c ^and T c ^{+ 7 K an} incommensurate

broadening [21] of the low frequency ^{85Rb line has}

been observed (Fig. 4). This confirms the existence of an intermediate incommensurate phase ⁱⁿ undoped

RHS. The observed line-shape ^{is not as} expected ^for

a one-dimensional modulation but more resembles that expected ^{for a} two-dimensional modulation [22].

The relatively ^low resolution of the present experiment however prevented ^a quantitative fit between the observed and theoretical lineshape. ^{It should be}

stressed that critical fluctuations cannot be _respon- sible for the observed broadening ^{of the} NQR ^lines.

The observation of the splitting of the low frequency

" Rb line which starts at Ti ^and continuously ^increases through T c (Fig. 1) ^is characteristic for ^an incom-

mensurate transition and rules out critical fluctua- tions as a source of the observed line broadening.

The incommensurate modulation in RTS seems to have a relatively pronounced soliton character between Tc ^and Tc ^{+ 3 K as} demonstrated by ^the

continuation of the ferroelectric lines into the I phase.

Between T c ^{+ 3 K and} T c ^{+ 7 K} the soliton lines

disappear ^{and the} line-shape changes. ^This may be

connected with the apparent existence of another

transition within the I phase [18].

524

References

[1] SHUVALOV, ^L. A., IVANOV, ^N. R., GORDEYEVA, ^N. V., KIRPICHNIKOVA, ^L. F., Kristallografiya ¹⁴ (1969) 658; Engl. ^{trans. Sov.} Phys. Crystallogr. ¹⁴ (1970) 554.

[2] SHUVALOV, ^{L. A. et} al., Kristallografiya ²⁰ (1975) 336; Engl. ^{trans. Sov.} Phys. Crystallogr. ²⁰ (1975) 206.

[3] TOVBIS, ^A. B., DAVYDOVA, ^T. S., SIMONOV, ^V. I., Kristallografiya ¹⁷ (1972) 103 ; Engl. ^{trans. Sov.}

Phys. Crystallogr. ¹⁷ (1972) ^81.

[4] TELLGREN, R., AHMAD, D., LIMINGA, R., ^{J. Solid} State Chem. 6 (1973) ^250.

[5] GRIMM, H., FITZGERALD, ^W. J., ^Acta Cryst. ^{A 34} (1978) 268.

[6] GRIMM, H., FITZGERALD, W. J., J. Phys. ^{C 11} (1978)

829.

[7] SHUVALOV, ^L. A., IVANOV, ^N. R., KIRPICHNIKOVA, L. F., GORDEYEVA, ^N. V., Phys. ^{Lett. A 33} (1970)

490.

[8] GLADKII, ^V. V., MAGATAEV, ^V. K., SHUVALOV, ^L. A., FEDOSYNK, ^R. M., Phys. ^{Lett. A59} (1976) ^391.

[9] SANNIKOV, ^D. G., LEVANYUK, ^A. P., ^{Fiz. Tverd.}

Tela 19 (1977) 118 ; Engl. ^{trans. Sov.} Phys. ^Solid

State 19 (1977) ^67.

[10] GLADKII, ^V. V., KIRIKOV, ^V. A., MAGATAEV, ^V. K., SHUVALOV, ^L. A., ^Fiz. Tverd. Tela 19 (1977) 291 ; Engl. ^{trans. Sov.} Phys. Solid State 19 (1977)

167 ;

GLADKII, ^V. V., KIRIKOV, V. A., MAGATAEV, ^V. K., SHUVALOV, ^L. A., Ferroelectrics 21 (1978) ^511.

[11] GESI, K., IIZUMI, M., ^J. Phys. ^Soc. Jpn. ⁴⁸ (1980)

697.

[12] WAPLAK, S., JERZAK, S., STANKOWSKI, J., SHUVALOV, L. A., Physica ^{B 106} (1981) ^251.

[13] ESAYAN, S. M., LEMANOV, ^V. V., MAMATKULOV, M., SHUVALOV, ^L. A., Kristallografiya ²⁶ (1981) ^1086.

[14] GLADKII, ^V. V., KALLAEV, ^S. N., KIRIKOV, ^V. A., SHUVALOV, L. A., Fiz. Tverd. Tela 21 (1979) 3732; Engl. ^{trans. Sov.} Phys. Solid State 21

(1979) 2155 ;

GLADKII, ^V. V., KALLAEV, ^S. N., KIRIKOV, ^V. A., LEVANYUK, ^A. P., SHUVALOV, ^L. A., ^J. Phys.

Soc. Jpn. 49; Suppl. ^B (1980) ^83.

[15] TAKOSHIMA, T., KASAHARA, M., TATSUZAKI, I., ^J.

Phys. ^Soc. Jpn. ⁴⁶ (1979) ^1804.

[16] SAKAI, A., KASAHARA, M., TATSUZAKI, I., ^J. Phys.

Soc. Jpn. ⁴⁷ (1979) ^161.

[17] GOTO, ^Y., SAWAGUCHI, E., ^J. Phys. ^Soc. Jpn. ⁴⁹ (1980) ^2255.

[18] GLADKII, ^V. V., KALLAEV, S. N., KIRIKOV, ^V. A., SHUVALOV, ^L. A., ^Fiz. Tverd. Tela 23 (1981)

1335.

[19] SLUSHER, ^{R. E. and} HAHN, ^E. L., Phys. ^{Rev. 166} (1968) ^332.

See also SELIGER, J., BLINC, R., MALI, M., OSREDKAR, ^R.

Iand PRELESNIK, A., Phys. ^{Rev. 11} (1975) ^27.

[20] ABRAGAM, A., ^The Principles of ^Nuclear Magnetism (Oxford University ^Press, London) ^1961.

[21] ^{For a review see} BLINC, R., Physics Reports ⁷⁹ (1981) 331, and references therein.

[22] ZUMER, ^{S. and} SELIGER, J., Ferroelectrics 36 (1981)

301.