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S. A. AHMEDnd M . H . ALI:Electrical and Thermal Studies of NaNO2 517phys. stat. sol. (b) 194, 517 (1996)Subject classification: 64.60 and 77.80; S 1 l . lDepar tment of Mathematics and Science; Faculty of Petro leum and Min ing Enginee~ing ,Suez Canal Universi ty , Suez) (a) andDepar tment of Physics, Faculty of Science, Ain Shams Universi ty , Cairo ( b )

Electrical and Thermal Studies of NaN02BYS. A. AHMEDa) and M. H. ALI (b))(Receiiied May 12, 1995)

The present paper reports on experimental results on polycrystalline NaNO2 employing the dcresistivity, DTA, arid unit cell parameters (measured by X-ray diffractometry) in the tempera-ture range from 298 to 480K. All the presented results confirm, in addition to the well-knownphase transitions at 436.5 and 438 K , the existence of a second-order phase transition occurringat 448 K. We found clear evidences th at th e ferroelectric phase transition of NaNO2 is of secondorder close to the first-order one, and the rotation of the NO, radical mostly takes placearound the c-axis. The high-temperature phase transformation occurring at 448 K is thought tobe due to the completely disordered arrangement of the NO, radicals in the normal paraelectricphase.

1. IntroductionFerroelectric activity was found in sodium nitrite ( N a N 0 2 ) by Sawada et al. in 1958 [ l ] .Since then, a considerable amount of experimental and theoretical work has been carriedout on the ordered and disordered phases owing to i ts peculiarity as to the dielectric [ lto 31, spectroscopic [4 to 111, and structural [ la to 151 properties over a wide range oftemper atures.

The sodium nitrite crystal undergoes two successive phase transitions, from the disor-dered paraelectric to the ordered incommensurate phase at TNM 438 K and then to thecommensurate ferroelectric phase at TCRZ 436.5 K [16 to 181. The unit cell in the ferro-electric phase is identical with that in the paraelectric phase and contains two formulaunits in both phases. The crystal is of body-centred orthorhombic s tructure, the spacegroup is C?:-Im2m [19] in the ferroelectric phase and D ~ ~ - I m m m20 to 221 in the para-electric one. The ferroelectricity of NaNO2 is due to the permanent dipoles of the NO;radicals which in the ferroelectric phase are aligned parallel to the spontaneous polariza-tion along the &axis. In the paraelectric phase the NO ; radicals point to the fb-direc-tions, i.e. a mirror plane perpendicular to the baxis appears [17, 221. Thc narrow inter-niediate phase was first reported by Tanisaki [13, 161. Yamada et al. [17] have proposedthat this phase is a. kind of antiferroelectric with a sinusoidal modulation of the electricmoments of the NO; radicals [23]. The average order of NO; radicals in the b c plane

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I) Suez, Egypt.) Present address: Department of Physics, Tabiik Teachers College, P.O. Box 1144, Tabllk,Saudi Arabia.

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518 S. A . AHMED nd M . H. ALIvaries along the a-axis. The modulation period changes with temperature from 8.4a atTN o 1 0 . 3 ~t TC [24].

Although the phase transitions in NaNOz have been intensively studied, there aremany contrary statements in the literature. For example, some authors have found addi-tional anomalies at 373 [25] and 451 K [lS, 26 to 281. Some authors reported that thecharacter of the NaN02 phase transition a t the Curie point is close to the second-orderkind [29 to 311 and not of the first-order one a h reported by others [17, 26, 271. Also,although it is well established that the phase transitions are caused by the orientationalorder-disorder transition of NO; radicals. the rotational axis of the polarization reversalmechanism is not yet sufficiently clear. Some authors reported that the rotation of theNO; radicals takes place around the c-axis [7, 14, 15, 20, 321 while others reported itaround the a-axis [9 to 111.

To account for the above contradictions in the reported results we studied the tem-perature behaviour of dc resistivity, differential thermal analysis (DTA) , and latticeparameters of the NaNOz crystals.

2. ExperimentsThe powder sample of sodium nitrite (NaN02) in the present s tudy was obtained fromBritish Drug Houses, Ltd. (London) and had a specified purity of 99.99%. It was puri-fied by recrystallization, dried, crushed, and finally filtered through a 44 ym sieve togive fairly uniform particle size.

The samples used for the study of dc resistivity (ed,)were prepared by compressingthe test powder sample in the form of pellets of diameter 13 mm and thickness 10 mmby using hydraulic pressure. A silver paste was painted on the opposite faces of thepellet as electrodes. The resistivity data were obtained on raising the temperature fromroom temperature to 480 K and also on cooling. The sample was heated firstly to about380 K for 2 h before taking observations to avoid surface conduction. The temperatureof the sample was measured by a digital thermometer (type Crisori 637) provided with aPt thermocouple of accuracy 10.1 K. The rate of temperature change was about 0.2 K /min in the vicinity of the transition points and 2 K/min in the other regions. The dcresistance was measured by two methods: directly with a digital multimeter (type Keith-ley 177) ranged from 20Q to 20MR and using a conventional circuit. Values of edcobtained from the two methods are in good agreement. The experimental runs werecarried out on a group of samples and repeated several times for each sample to ensurereproducibility.

The measurements of a high-sensitive differential thermal analysis (DTA) were carriedout at atniospheric pressure using a DTA30 system ( type Shimadzu, Japan). A series ofDTA runs have been made under different conditions and the best DTA thermogram ispresented on heating and cooling.

A high-precision X-ray diffractometer (type Philips P W 1050) attached to a smallheating device was used in our investigation for the precise determination of lattice param-eters (with a relative accuracy of rim). All measurements were performed on heat-ing over a temperature range of 298 to 480 K . X-ray CuKa radiation (2 = 0.1542 nm)monochromatized by a graphite crystal was used. A range of scattering angles15" < 20 < 80" was covered in 0.02" steps (A20) with a pre-set time of 20 s per step inthe step mode operation.

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Electrical and Thermal Studies of NaNO:! 5193. Results and Discussion

The temperature variation of the dc resistivity in the vicinity of the transition tempera-tures, as given in Fig. 1, shows slight anomalies at 438 and 448 K in addition to the ex-pected one corresponding to the well-known ferroelectric-incommensurate phase transitiontemperature a t 436.5 K . The data presently obtained for the natural logarithm of dc resis-tivity (In Qdr) as a function of 1/T are shown in Fig. 2 . Except in the vicinity of the transi-tion points (from 432 to 438 K ) , the exponential law e = P o exp (U I l c T ) is obeyed in threedistinct regions labelled I, 11, and I11 in Fig. 2 star ting from the low-temperature end. Ac-cordingly, anomalous behaviour of Qdr appeared as a change in the slopes of the differentregions (Fig. 2 ) . Anyway, it is impossible to decide whether the relation 4 = eo exp ( U / k T )holds or not in the sinusoidal phase because of the extremely narrow temperature range(about 1 K ) of this phase. On the other hand, there is no temperature hysteresis at Tc sincethe heating and cooling curves coincide. The activation energies for conduction obtainedfrom the slopes of the three linear regions I, 11, and I11 are 1.22, 2.07, and 1.64 eV, respec-tively. Data from literature and the present ones are compared in Table 1. It can be notedthat our values are in relatively good agreement with the values of Takagi and Gesi [30].

Fig. 3 shows the DTA thermogram obtained for NaN02. The thermogram indicatesthat the endothermic phase transition of NaN02 takes place at 437 K while heating butin cooling the reversibility of the exothermic transformation appeared a t 436 K . A step-like anomaly is clearly shown at 448 K in the DTA thermogram. On the other hand, wedid not find any appreciable change corresponding to the incommensurate-paraelectrictransformation at 438 K by the DTA technique.

U - L - L - .'428 432 436 440 444 448 452 1

T I K )Fig. 1. Temperature dependence of dc resistivity for N a N 0 2 in the vicinity of the transition points

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520 S. A . A H M E Dnd M . H. ALI

1 I 1 I I2.10 2.18 2.26 2.34 iI00 T (K- I

The lattice parameters and subsequently the unit cell volume of NaN02 were ob-tained from the accurate measurements of the Bragg angles of at least six well-resolvedspecial ( h O O ) , (OkO),and (001) reflections at room temperature and as functions o f tem-perature. The obtained lattice parameters of the NaNOa crystal at room temperature(298 K) are: a = 0.3569 nm, b = 0.5569 nm, and c = 0.5384 nm in good agreement witht.hose previously reported [19] a = 0.3560 nm, b = 0.5563 nm, and c = 0.5384 nm. Varia-tions of the lattice parameters a , b , c arid unit cell volume V are shown in Fig. 4 and 5,respectively. It can be seen that a , b, arid V increase with increasing temperature up to423 K , then decrease with increasing temperature in the temperature range of 9 K justbelow Tc. On the other hand, as is shown in Fig. 4, the length of the c-axis decreasesmoderately with increasing temperature up to 432 K , followed by a continuous decreaseup to 436 K . Also similar small anomalies in the NaN02 unit cell parameters a , b, c , andV are clearly shown at 448 K .

Plots of ed, versus T and 1nedcversus 1/T, given in Fig. 1 and 2, show three anoma-lies at 436.5, 438, and 448 K . The temperature indicated by the first anomaly corre-sponds to the well-known transition point TCwhere the spontaneous polarization disap-pears [33], the second one at 438 K corresponds to the incommensurate-paraelectric phasepoint TN , nd the last one at 448 K corresponds to a highest-temperature phase transi-

Fig. 2. Temperature dependence ofthe logarithmic resistivity for NaNOz

F 2

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Electrical and Thermal Studies of NaNOz 521T a b l e 1Activation energies (in eV)region I region I1 region I11 ref.

~~~

1.05 1.39 1.23 13013.10 r331.43 -1.22 2.07 1.64 present study

tion. The transition point at 448 K , observed in the present study, is somewhat lowerthan reported in previous works [26, 271. The high-temperature anomaly a t 448 K , sup-ported by similar anomalies in DTA, lattice parameters, and unit cell volume, confirmsthe presence of a second-order phase transition occurring at this temperature. Yamadaet al. [17]have measured the temperature dependence of the X-ray diffraction intensity.They reported that the satellite peaks completely disappear from the diffraction patternsat a critical temperature (Tcr= 453 K) and concluded that the NO, radicals can beregarded as completely disordered and NaNOz being paraelectric above 453 K. Theanomaly observed here at 448 K confirmed the above conclusion about the existence ofsuch a high critical point at which the disorder of the NO; radicals is completed.

The activation energy of the charge transport obtained from the lned, versus l /Tcurve amounts to 1.22, 2.07, and 1.64 eV in regions I, 11, and 111, respectively. The rate

T,=4 36 KI

TT,=437 K

of variation of conductivity CI is higher inregion I1 (from T = 438 to 448 K ) than inthe other two regions. This might be relatedto the fact that the concentration of anionsin region I1 is less than in the other t w o re-gions. The conductivity (5 depends on thecharge transfer and the mean distancemoved by the cations before recombiningwith a nearest nitrite ion. Hence, the recorn-bination probability in region I1 should behigher. On the other hand, the complete dis-ordering of the NO, radicals, occurring atthe critical temperature (T = 448 K ) causesa loosening of the ionic bonds [34]. This ef-fect tends to reduce the height of the energybarrier. The reduction of the height of theenergy barriers leads to an increasing mobil-ity of the cations, giving rise to the ob-served increase in conductivity at 448 K .

Fig. 3. Differential thermal analysis (DTA) ther-mogram for the NaNOz crystal during a) heatingand b) cooling

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522 S. A. AHUED nd M. H. ALI

E.565hE 0.561cvFig. 4. Ternperature dependence of thelattice constants a , b , and c for theN a N O l crystal

The linear thermal expansions along the a-, h-, and c-axes show anomalies in the vicin-ity of Tc with continuous hehaviour in the temperature range of about 9 K below Tc.This fact suggests t,hat the phase transition occurring at Tc is of second order near tothe first-order one. The ferroelectric-incommensurate phase transition at Tc is accompa-nied by contractlions along the a-, b-, and c-axes and of the unit cell volume. The con-traction starts at 423 K and reaches its rriaxirriurri at 436 K. The temperature indicatedat the beginning of the contractioris is thought to be related to the onset, of a processinvolving orieritational disorder of the NO , radicals, while the other one at, 36 K corre-

mEcrnIz>

1 1 0 1

lTc=436.5 KI I , I I I280 320 360 400 440 480

T ( K )Fig. 5 . Temperature dcpendence of the unit cell volume for the N a N 0 2 crystal

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Elwtrical and Thermal Stiidies of NaNOz 523sporids to the Ciirie temperature (Tc).An explanation of the volume contraction ofthe NaNOz crystal in the vicinity of the Curie point may be obtained qualitativelyby consideration of the change in covalent bond distance when the NO, radical inthe fully polarized lattice is misoriented. On the other hand, the enhanced rotationof electric dipoles associated with NO; radicals is expected to increase the expansioncoefficients of the lattice parameters. This assumption was confirmed from the sud-den expansion in the lattice parameters a , b arid unit cell volume V of NaN02 aboveTc. The anomalies observed here for the first time in the unit cell volume at 448 Kcorifirnied the conclusion about the existence of a critical temperature (Tc,.) t thispoint. On the other hand, the expansion was not observed in the c-direction aboveTc. This might confirm the conclusion that the rotation of NO, radicals takes placearound the c-axis.

There is no t,emperature hysteresis at Tc on heating arid cooling in the resistivitymeasurements. On the other hand, in DTA, the ferroelectric-incommensurate transitionpoints (Tc) are 437 and 436 K during heating and cooling, respectively, indicating atemperature hysteresis of about 1 K around Tc.

The small temperature hysteresis observed only in DTA on heating and cooling isthought to be related to t,he poor accuracy of the digital temperature display providedwith the DTA3O (51K ) . Accordingly, there is no remarkable temperature hysteresisas observed in the order-disorder phase transition of first order [18] in this material.Again this result clearly shows that, the phase transition at Tc is of second order nearto the first-order one. The mean value of the ferroelectric-incommensurate point (Tc)judged from Fig. 1 to 5 is 436.5 K and is in good agreement with the previouslyreported one.

4. ConclusionsOn the basis of the present results of electrical and thermal studies on N a N 0 2 onemight conclude the following points:

1. The unit cell parameters at room temperature (ferroe1ect)ric phase) are :n = 0.3569 nm: b = 0.5569 nm, and c = 0.5384 nni.

2. The ferroelectric-incommensurate phase transition at the Ciirie point was fourid tobe of second order close to the first-order one. The Curie temperature Tc was deter-niined as 436.5 K . The incommensurate-paraelectric transition point was also deter-mined as 438 K .

3. The results of the temperature dependence of dc resistivity and lattice parametersconfirmed the existence of the highest-temperature phase transition at 448 K . The high-est phase transition point was also checked by DTA. The phase transformation at 448 Kin NaNOz is thought to be of second order.

4. Above 448 K , the NO, radicals probably take the completely disordered arrange-ment in the paraelectric phase.

5. R.otation of NO; radicals niostly takes place around the c-axis.Acknowledgements The X-ray diffraction experiments were carried out at the Pliys-ics Department of Sheffield University (England). The authors wish to thank Dr. N .Cowlarn for his generous help throughout the course of this study.

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524 S. A. AHMEDnd M. H. AM:Electrical and Thermal Studies of NaN02References

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