Choudhary, R N P

Sharma, Seema

IACS Institutional Repository

Indian J. Phys. 73A (6), 741-747 (1999)U P  A- an international journalPhase transition in Na2Mo(^4 ceramicsS c e m a  S h a rm a  and  R  N  P  Choucftiary*Elepartment of Physics, A. N. College, Patna-800 013. India ^Department of Physics, IIT Kharagpur-721 302, IndiaReceived 30 November 1998, accepted 2 July J999A bstract : Polycryslallinc sample of Na2Mo04  compound was synthesized by high- temperature solid-state reaction technique. X-ray studies revealed that the compound has orthorhombic structure at room temperature. Measurements of diclectnc constant ( f ) and loss tangent (tan 6) were carried out from -190°C to 400'^C. These studies show that the compound undergoes a ferroelectric phase transition at 14®C. Studies of dielectric hysterisis of the compound showed no hysterisis loop at room temperature (i.e 29®C) suggesting nonnal (paraelectric) phase of the material.Keywords : Solid-state reaction, dielectric constant, commensurate-incommen surate phase transition.PACSNos. : 61 72.Dd, 77.84.Dy1. IntroductionPhase transitions in halogenide crystals of a general formula A2BX4, where A = monovalent alkali ions* B  =  Mn* Co* Cu, Zn* Cd* Hg, X = F* Cl, I, Br have widely been investigated in last decades [1], It has been found that, in general, they have ^-K 2S04-type orthorhmbic structure w ith m ultiple or successive phase transitions from the high-temperature normal {i.e, paraelectric  phase) to the low tem perature ferroelectric commensurate phase via a m odulated incom m ensurate phase [2-4]. Though a considerable amount of work has been done on the above crystals, there are no reports on the compound with X = Tc, W, Mo. T herefo re, to exam ine the successive, norm al-incom m ensuratc-com m cnsurate phase transition, in polycrystalline samples o f A 2BO4 (A = alkali ions, B = W, Mo and Te) family, we have  stud ied  th e ir  p relim inary  structural and detailed dielectric and electrical conductivity properties. In the present paper, we report our above work on the Na2Mo04 com pound.© 1999IA C S742 Seema Sharma andRN P Choudhary2. E xperim en ta l detailsThe polycrystalline sam ples o f  Na2Mo04 was prepared by solid-state reaction technique using the raw materials : N a2CO j (ultrapure, M /s Loba Chemical Co.) and M 0O 3 (M/s John Baker Inc. USA) in a stoichiometry. These materials were mixed thoroughly in an agate m ortar for 2 h. The mixed powder was calcined at 550°C for 5 h. The calcined powder was grounded and recalcined at SSOX for 6 h- Recalcined fine powder was then used to make pellets o f 10 mm diam eter and 1-2 mm thick under a pressure o f 5 x  10’ kg/m’ . These pellets were then sintered at 600°C for 4 h. in an alumina crucible to reduce their porosity. The formation o f the single phase compound was checked by X -ray diffraction (XRD) technique.For preliminary structural studies, X-ray diffractogram of the material was recorded at room tem perature with a m iniflex Rikagu X-ray pow der diffractom eter with CuK ^ radiation (A = 1.5418 A) in a wide range o f 2 0  (20° S 2 0  S 100°) at scanning rate o f 2°/min. The dielectric constant (£)and loss tangent (tan 8) o f N a2M o04 were obtained as a function o f  frequency (4(X) to 1(H Hz) at room temperature and o f tem perature (-1 9 0  to 400°C) at fixed frequency o f 10 kHz over a small tem perature interval (x.e. 2°C) using a CiR 1620 AP capacitance m easuring assembly in conjunction with a laboratory-made three- term inal sam ple holder which com pensated the unw anted or stray capacitance. The temperature was measured using a chromel-alumel thermocouple.The dc volume resistivity o f the pellet sample was measured as a function o f electric field ( 1 0 - 8 0  V/cm ) at room temperature, and as a function o f tem perature o f a constant electric field (10 V/cm) using a Keithley 617 program m able electrom eter. The dielectric hysterisis o f the sample was examined with the help o f modified Sawyer-Tower circuit with dual trace oscilloscope at a field o f 6 kV/cm.3. R esults an d  discussionThe sharp and single peaks of XRD pattern which was different from those o f ingradient com pounds, confirm ed the formation o f the single phase o f the Na2M o04 compound. All the XRD peaks (high, m edium and low intensity) were indexed in d ifferent unit cell configurations and crystal systems with the help of a standard computer program package, POW DIN. Finally, a suitable unit cell was selected on the basis o f the best agreem ent in observed and calculated d-values. The cell param eters were refined by least-squares method. They are : a  = 7.5140 A, b = 8.3080 A and c -  6.022 A. A good agreement between observed and calculated d-values (Table 1) suggests the correctness o f selected cell param eters o f the orthorhom bic crystal system at room  tem perature. Due to a lim ited num ber o f reflection data it was not possible to uniquely determine the space group o f the material.The particle size (P) o f the sample was calculated using Scherrer’s equation. P =  KA/ipifi cos 0 )  2 0  [5] (K = constant = 0.89, Pin = half peak width). The average particle size o f the sam ple was found to be 270 A.Phase transition in ceramicsTable 1. Compatison of observed and calcolaled d-values (in A) of some reflections of Na2Mo0 4  at room tem|ieiature. The enor in d is± 0.0010 A.h k i 4cal1 2 0 4.0046 4.00460 0 3 3.50t2 3.507241 1 3.25S4 3.25541 1 3 3.1646 3.15904 0 2 3.0515 3.05742 1 3 2.9619 2.96845 1 0 2.8227 2.82240 0 4 2.6301 2.63041 1 4 2.46S4 2.47364 0 4 2.1550 2.15487 0 2 1.9894 1.98772 4 2 1.8697 1.86922 ^4 1.8484 1.84870 0 6 1.7554 1.7550S 2 1 1.6894 1.68958 0 3 1.6557 1.65590 2 6 1.6156 1.6156743The variation o f  dielectric constant ( s ')  and tangent (tan 5) with frequency for the compound is shown in Figure 1. The variation o f these parameters show that both the4tan ^kHzFigure 1. Variation of dielectric confitaat (£ ') and dielectric loss (tan d) of Na2MoQ4 with firequeiicy (f) at room temperatuic.744 Seema Shama and RN  P Choudharyd ielectric  param eters decrease w ith the increase o f  frequency, w hich is an usual behaviour of a dielectric. The higher value o f e’ at low frequency is due to the presence o f all the different types o f  polarisation (viz. electronic, dipolar, interfacial, orientational, electronic etc.) in the compound at room  temperature (29°C) and at low frequency. Sim ilar dielectric behaviour with respect to frequency was found in som e o ther ferro-electric ceramics [6- 8].T he variation o f e ' and tan 5  w ith tem perature (-50®C to 4(X)®C) at a  fixed frequency o f 10 kHz is shown in Figures 2 and 3. Though the dielectric investigations weretngurc 2. Variation of dielectric constant (e') and lots tangent (tan d) of N112M0 O4  with tempeiatiire (~50”C to S0”C) at 10 kHz.carried out from  liquid nitrogen tem perature (i.e. -190°C ), it has not been show n in the F igure 2 because there  w as no sign ifican t variation o f  d ie lec tric  constan t w ithin tem perature range (-196  to -S0°C ). A  sharp dielectric anom aly was observed at 14°C w ith c L .  = 347. Figure 3 shows variation o f  the dielectric constant and dielectric loss as a function o f  tem perature (SO^C to 400°C) at a  frequency o f 10 kH z. I t  can be seen that the dielectric constant increases very sharply beyond 300**C. I t appears that this m ay be due to the presence o f space charge polarisation a t h i ^  tem perature and low frequency.The calculations o f  ac dielectric conductivity (at 10 kHz) at d iffo e n t tem peratures have been done using the relation a ^ c a e ^ e 'tm S ,  where m is the angular frequency.Phase transition in Na2Mo04 ceramics 745^  s  dielectric perm ittivity in free space. Figure 4  and Figure S show the variation of In a  with inverse o f  absolute temperature in the low (-30°C to 30°C) and high (2S0°C to 400°C)Figure 3. Variation of dielectric conitant (c') and dielectric loss (tan S) of Na2Mo04 wiUi iemperatuic (50Xto400“Q at lOkHx.Figure 4. Variation of ac conductivity (In a ) of Na2Mo04 vrith inverse of absolute temperature from (-30“C to 30"C) at 10 kHz.tem perature regions, respectively. The activation energy, calculated from Figure 4 (between —30®C to 10“C) is 0.7 eV is observed. In the high-temperature region, theFigure 5. Variatioii of ac con<|oetivity (In o) of Na2Mo04 with invene of absolute temperature horn (2S0*C to 400*0 at 10 kHz.Figure 6. Variation of dc leastivity (In pac> Na2Mo04 with biasing electric field at room temperature.activation energy increases from  1.08 to 1.72 eV as temperature increases, rom tem perature (29"C) to 2 0 0 ^ ,  tint value o f In <r ranges from -1 4 3  to -1 . . w c that over a  vride range o f  tem perature the material shows an ac conducuvity wvariation wifli tem perature is not very significant 73A(6)-4746 Seema Sharma and RN P ChoudharyFigure 6 shows the variation o f dc resistivity 0® Pdc) with the biasing electric field at room  tem perature. It is observed that the dc resistivity o f this com pound decreases with increase in biasing electric field. This effect was observed by others for a num ber o f compounds {9-11].T he varia tion  o f dc volum e resistiv ity  (In w ith the inverse o f  absolute tem perature is show n in Figure 7. D ecrease o f the dc fesistivity of the com pound w ith increase in tem perature is due to the progressive increase o f therm al energy inside the material which gives rise to more and more free electrons, and hence p^c decreases. Below 68°C, Pic is observed to decrease with the decrease in temperature. Similar anomaly hasFigure 7. Variation of dc resistivity (In Na2Mo04 with inverse ofabsolute temperatuie.been observed in the case o f Rb2Te04 , T l2Te04 and Na2W 04 compounds [12-14]. At low temperatures, decreasing (rend o f with decrease in temperature has been found in many superconductor ceramics.No hysterisis loop (E vs P) for N a2M o04 com pound a t room  tem perature wasfound.4. ConclusioiisIt is concluded that N a2M o04 has an orthorhom bic structure at room  tem perature and exhibits a phase transition at 14“C. Although it was observed that in many com pounds o f  the A iBO a-type, there exists successive phase transition [ferroelectric commensurate fdiase- incom m ensurate phase-paraelectric  norm al phase] w ithin the tem perature range o f  investigation in N a2M o04 . Unlike m any other com pounds o f  the fam ily, only one phase transition is observed. Sim ilar behaviour in N a2W 04  and TI2M 0O4 o f  the A2BO4 famil) [14,15] was also found.AcknowledgeniciitThe authors are thankful to Mr. R P  M ukheqee for his kind help in X -ray experiments.Phase transition in Na2Mo04 ceramics 747Rdercficcsf 1 ] K Gcm ./ Phy Sol J a p a n  53 3850 ( 1948)[2] J D Axe and G Shiiane Im o n w ien su rate  P h a se s  m D ie l a t n c s , ed R Blinc and A P Levanyuk (North- Holland, Amsterdam) Vol 2 Chap 10 p 1(1986)[3J F Shimjzii. M Takashige, S vSawada and T Yainaguchi J .  Phy, S a c  Ja p a n  62 2964 (1993)|4) K Gesi J  Phy S m . J a p a n  54 3694 (1985)f5| P Scherrer G ott N ai hr 2 98 ( 1918) '[6| N K Misra, R Sail and R N P Choudhaur)' Mmer Lett. 24 313 (1995)|7 | S Bcia and R N P Choudharv J  M ater  S a ,  Lett. 14 J68 (1995)[8J S Bera and R N P Choudhary Ind J  P ure a n d A p p l f h y s  33 306 (1995)|9 | R C' Buchanan C en im u  M ateria ls Jar E lev tw n ics (f^ few York, Marcel Dekker) p 1 (1986)110| S N Choudhary and R N P Choudhary M ateria ls  Len s 34 411 (1998)[11] R N f* Choudhary, S Sharma and M L NandaGoswami J .  M ater. S i r  Letts. 16 908 (1997)[12] S Sharma and R N P Choudhary F e n o e le i  tries 227 41 (1999)[13] S Sharma and R N P Choudhary J  E leetroceian tic s 3 :4  443 (1999)114] S Shanna, R N P Choudhary and S R Shannigrahi M ater Letts (in press)f 15| S Sharma and R N P Choudhary J  Phy s' Chem So lid s 60 743 (1999)

Phase Transition in Na2MoO4Ceramics

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Phase Transition in Na2MoO4Ceramics

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