Choudhary, M K

Choudhary, R N P

Singh, N K

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Indian J. Pkys. 72A (3), 241-247 (1998)IJP A— an international journalStudies of X -rays and  electrical properties of S rM o 0 4N K Singh, M K Choudhary and R N P Choudhary* Department of Physics, H D Jain College, Ara-802 301, India ‘ Department of Physics, Indian Institute of Technology, Kharagpur-721 302, IndiaReceived 7 November 1997, accepted 20 March 1998Abstract : A polycrystalline sample of SrMoC>4 was synthesized by high-temperature solid-state reaction technique Preliminary X-ray stuoy has been carried out to check the formation of compound and to determine its preliminary crystal data. The variation of ac conductivity and dc resistivity with temperature has also been studied. Measurements of dielectric constant (e) and loss tangent (tan 8) as a function of frequency (400 Hz-10 kHz) and temperature (-180° to 300°C) show that the compound is a linear dielectricsKeywords : Solid state reaction, X-ray diffraition, dielectric constant and dc resistivityPACS Nos. : 77.22 Gm,6U0.Nz, 77 80.Bh1. IntroductionSince the discovery of ferroelectricity in BaTiO^ in 1945 f 11, a large number of oxides of different structural families have been examined [2,3] in search of new materials for device applications. It has been found that each member of an oxide family has some interesting structural and physical properties in spite of some similarities in their chemical formula or compositions [4-7]. Among all the oxides studied so far, some molybdates and tungstates, such as Gd2(Mo04i  [8], PbMo04 [9], PbWQ* [10] etc., have interesting ferroelectric and related properties. Some molybdates and tungstates of the general tormula AB04 (A = alkali ions; B = W, Mo) have very unusual successive phase (i.e. commensurate-incommensurate) transitions [11] in wide temperature range, with high electrical conductivity and dielectric loss [12] and low dielectric constant [13]. This has attracted us to synthesize and study structural, electrical and spectroscopic properties of different structural families of tungstates/molybdates, such as pervoskite, tungstan bronze (TB) [14], spinel [15} and scheelite [16]. SrMo04, suitable for laser applications [17],© 19981ACS242 NKSingh, MKChoudharyandRNPChoudharyb e lo n g s  to  th e  s c h e e li t e  s tr u c tu r a l  fa m ily  w i th  th e  s p a c e  g ro u p  I 4 | / a  [1 6 ]. D e ta i le d  l i te ra tu re  s u rv e y  o n  th is  c o m p o u n d  s u g g e s ts  th a t  e x c e p t a  fe w  s tu d ie s  [ 1 8 -2 0 ] ,  n o t m u c h  w o rk  h a v e  b e e n  d o n e  o n  it. T h e re fo re ,  w e  h a v e  c a rr ie d  o u t s y s te m a t ic  s tu d ie s  o n  s tru c tu r a l  a n d  e le c tr ic a l  p ro p e r t ie s  o f  th e  c o m p o u n d  fo r  th e  b e tte r  u n d e rs ta n d in g  o f  its  s tru c tu ra l a n d  p h y s ic a l p ro p e r t ie s  a n d  to  c h e c k  th e  e x is te n c e  o f  fe r ro e le c tr ic  p ro p e r tie s  in it.2. ExperimentalT h e  S r M o 04  s a m p le  w a s  p r e p a re d  f ro m  s tr o n t iu m  c a rb o n a te  S rC O j (9 9 %  p u re ,  M /s .  B u g o y n e  L td .)  a n d  m o ly b d e n u m  o x id e  M o 0 3 (9 9 9 .5 %  p u re , M /s .  B D H  L td .)  in d e s ir e d  s to ic h io m e try  b y  so lid  s ta te  re a c t io n  te c h n iq u e . T h e s e  c o m p o n e n t c o m p o u n d s  w e re  m ix e d  in a a g a te -m o rta r  fo r  2 h  an d  c a lc in e d  at 7 2 5 °C  in a  p la t in u m  c ru c ib le  fo r 2 0  h. T h e  c a lc in e d  p o w d e r  w a s  g ro u n d  a n d  re c a lc in e d  at 8 0 0 ° C  fo r  18 h. T h e  c a lc in e d  p o w d e r  w a s  g fo u n d  a g a in  to  m a k e  f in e  a n d  h o m o g e n e o u s  p o w d e r  w h ic h  w a s  u n ia x ia lly  c o ld  p r e s s e d  in to  p e lle ts  (d ia m e te r  =  10 m m  a n d  th ic k n e s s  =  1 -2  m m ) at a p re s s u re  o f  4 .5  x  107 k g / m 2 u s in g  a h y d ra u lic  p re ss . T h e  p e lle ts  w e re  th en  s in te re d  a t 8 2 5 °C  fo r  14 h. T h e  q u a l i ty  a n d  th e  fo rm a tio n  o f  th e  c o m p o u n d  w e re  c h e c k e d  w ith  an  X -ra y  d i f f ra c tio n  (X R D ) te c h n iq u e .F o r  p r e lim in a ry  s tru c tu ra l s tu d ie s , an  X -ra y  d i f f ra c lo g ra m  w a s  re c o rd e d  at ro o m  te m p e ra tu re  by  a  R ig a k u  X - ra y  p o w d e r  d i f f r a c to m e te r  (M in if lc x , J a p a n )  w ith  C u f r a ra d ia tio n  (A =  0 .1 5 4 1 8  n m ) fo r  a w id e  ra n g e  o f  B rag g  a n g le  20 , (1 5 °  ** 2 0  *£ 9 0 °)  a t the s c a n n in g  ra te  o f  2 ° / m i n .  T o  m e a s u re  th e  d ie le c tr ic  c o n s ta n t , th e  f la t s u rf a c e s  o f  th e  p e l l e t  S a m p le  w e re  e le c t r o d e d  w ith  h ig h  p u r i ty  a n d  u l t ra f in e  s i lv e r  p a r t ic le  p a s te . M e a s u re m e n ts  o f  d ie le c tr ic  c o n s ta n t ( e )  a n d  lo ss  ta n g e n t (tan  S) o f  th e  s a m p le  w e re  c a rr ie d  o u t b o th  a s  a  fu n c tio n  o f  fre q u e n c y  (4 0 0  H z  to  10 k H z ) and  te m p e ra tu re  ( - 1 8 0 °C  to  3 0 0 °C ) b y  G R  1 6 20  A P  c a p a c ita n c e  m e a s u r in g  a s se m b ly  in  sm a ll te m p e ra tu re  in te rv a l ( - 8 ° C ) .  M e a s u re m e n t  o f  d c  r e s is t i v i ty  w a s  d o n e  b o th  a s  a  fu n c t io n  o f  t e m p e ra tu r e  ( ro o m  t e m p e ra tu r e  to  3 2 5 ° C )  a n d  b ia s in g  e le c t r ic  f i e ld  ( 1 .5 - 8  k V / m )  b y  K e ith le y  6 1 7  p ro g ra m m a b le  e le c tro m e te r . E x is te n c e  o f  s p o n ta n e o u s  p o la r is a tio n  in th e  c o m p o u n d  w as  c h e c k e d  u s in g  la b o ra to ry  m a d e  S o w y e r -T o w e r  c irc u it.3. Results and discussionT h e  s h a rp  a n d  s in g le  d i f f r a c tio n  p e a k s  (F ig u re  1) in  th e  X - ra y  s p e c tra  (X R D ) su g g e s t  th a t  th e  c o m p o u n d  w a s  fo rm e d  in  a  s in g le  p h a se . L a tt ic e  p a ra m e te r s  a n d  d- v a lu e s  o f  th e  c o m p o u n d  w e re  c a lc u la te d  fo r  d i f f e r e n t  c ry s ta l  s y s te m s  a n d  u n it  c e ll  c o n f ig u ra t io n s  w i th  o b s e r v e d  4 - v a lu e s  o f  s t r o n g ,  m e d iu m  a n d  lo w - in te n s i ty  r e f le c t io n s  u s in g  a s ta n d a r d  c o m p u te r  p ro g ra m  " p o w d in " . F in a lly , la t t ic e  p a r a m e te r s  a n d  c ry s ta l  sy s te m  w e re  s e le c te d  o n  th e  b a s is  o f  m in im u m  I M  (=  4 0ta -  4 CBj) w h ic h  w a s  c o n s is te n t  a n d  v e ry  m u c h  c o m p a r a b le  w i th  th o s e  o f  th e  c a lc u l a t e d  v a lu e s  o b ta in e d  h e re  a n dStudies of X-rays and electrical properties ofSrMo04 243values reported in JCPDS [21]. The refined lattice parameters are : a = 11.4377 (10) A and c = 12.0316 (10) A (estimated error in the parenthesis). A very good agreementhciwcen observed and calculated ^-values (Table 1) suggests the correctness of the selected cell and structure.Tabic 1. Comparison of some observed and calculated lvalues (in A) of some reflections of SrMo04 at room temperaturekhl 4** (A) c^al I/Iq,102 3 2203 3 2203 100004 3.0079 3 0071 67411 2.7023 2 7031 28132 2 4616 2 4601 6104 2.3636 2 3615 9134 2.0081 2 0076 34600 1 9013 1 9063 15306 1 7712 1.7748 29640 1.5883 1.5861 20730 1.5011 1.5018 6660 1.3474 1 3479 3823 1 3107 1.3108 10428 1.2960 1.2964 13902 1.2424 1.2434 4664 1.2296 1 2301 21419 1.2043 1.2043 15717 1 1780 1 1779 71000 1.1438 1.1438 81012 1.1179 1.1183 4though it is not possible to determine the space group from the limited powder data, the reported space group I4j/a has been confirmed with some systematically absent reflections and physical properties. The particle size of the compound calculated using Scherrer's equation [22], was found to be 363 A, which was consistent with those observed from the particle size analyser.244 N K Singh, M K Choudhary and R N  P ChoudharyThe dielectric constant (e) and loss (tan 8) decrease with increase in frequency at room temperature (Figure 2). At low frequencies, all the polarizations exist but withincrease in frequency some of the polarizations vanish. Therefore, the dielectric constant and loss decrease with increase in frequency. This suggests the normal behaviour of a dielectric. Variation of e of the compound with temperature (-180°C to 300°C) at 10 kHz shows the linear dielectric .behaviour of the compound. Below room temperature (upto -180°C), the values of e and tan 5 were found to be about 10 and 0.005 respectively,Figure 3. Variation o f dielectric constant (6 )  and loss (ton 8) o f SrMo04 with temperature at 10 kHz.(therefore, not shown in Figure 3) which are almost constant and linear. Variation o dielectric loss with temperature at a frequency 10 kHz shows a almost constant valu<Studies o f X-rays and electrical properties ofSrMo04 245(in tan 6 ) with a small anomaly at 230°C. The slow increase at low temperature is due to lattice ionic polarizability and the faster increase in high temperature region is due to space charge polarization. Similar behaviour in € and tan 6 has been observed in many molybdates and tungstates studied recently by us [23t24]. The electrical conductivity a  of the sample was calculated from the dielectric data using formula tt = co € 0e  tan 6, where e 0 = dielectric constant in vacuum and a> = angular frequency. An activation energy EA, calculated from the formula a = <70 exP (-EjK^T) (KB = Boltzmann constant) was found to be 0.03 eV (Figure 4).Figure 4. Variation o f ac conductivity (In a) o f SrMo04 with inverse o f temperature (10^/7) at 10 kHzStudy of field dependence of dc resistivity (Figure 5) shows that the resistivity decreases with increase of electric field. This may be due to ionisation of gases andFigure S. Variation of dc resistivity of SrMoC>4 with Figure 6. Variation of dc resistivity (In a ) ofapplied electric field at room temperature. SrMo0 4  with inverse of absolute temperature (I /T ).246 NKSingh, MKChoudhary a ndR NP  Choudharymoisture present in the pores/cracks of the compounds, thus increasing the conductivity of the ceramic samples in general [25]. Variation of In a  with inverse of absolute temperature at constant electric field (7.87 kv/m-1) is shown in Figure 6. The decrease of resistivity with increasing temperature can be explained on the basis that the insulators have no free carriers, but due to thermal energy, electrons can be set free from oxygen ions. Hence, conductivity of SrMo04 increases due to generation of electrons [25]. However, the compound shows a negative temperature coefficient of resistance (NTCR) similar to an extrinsic semiconductors above 100°C. As no D-E hystersis loop in wide temperature range (liquid nitrogen temperature to 300°C) was observed, we concluded that the compound is non ferroelectric, which is very much consistent to our other studies. It is finally concluded that unlike many molybdates, this compound does not show any ferroelectric behaviour.AcknowledgmentThe authors wish to thank Sri S Bera and Ms. T Kar for their kind help in some experimental work.References[IJ  B Wul and L M  Goldman C. R Acad. Sci. USSR46 123 ( 1945)[2] E C Subbarao Ferroelectncs S 267 (1973) #13] K K Deb Ferroelectncs 82 45 ( 1988)[4] K S Aleksandary, A T  Anistrator, S V Metrikova, P V Klevsov and V N Voronov Phys. Stal. Sol J H  377 (1981)[5] K S Aleksandary, D H Blat, V I Zinenki, 1 M Iskomev and A I K ruglink Ferroelectncs 54 233 (1984)[6J S Bera and R N P  Choudhary Indian J. Pure Appl. Phys. 33 306 (1995)[7] S Bera and R N P  Choudhary Mater Lett. 22 197 (1995)[8] E T Keve, S C Abrahams, K Nassau and A M Glass Solid State Commun. 8 1517 (1970)[9] W Bucs and H Gehrke Z  Anorg. Allgem. Chem. 288 307 (1956)[10] G M  Clark and W P Doyle Spectrochim. Acta 22 1441 ( 1966)[11] T Janssen and A Janner Adv. Phys. 36 519 (1987)[12] A T  Moulson and J M Herbert Electroceramics (Materials Properties and Application) (London : Chapman and Hall) (1985)[131 S Bern and R N P Choudhary Mater Sci. Lett. 15 251 (1996)[14] M  H Fremcombe Acta Cryst. 13 313 (1960)[15] K S Singh, Sati and R N P Choudhary Parmana 48 161 (1992)[16] R W G W yekoff Crystal Structure 3(2) 21 (1964)[17] L  F Johson J. Appl. Phys, 34 897 (1963)[181 R Loudon Adv. Phys. 13 423 (1964)119 1 J P Russell and R Loudon Proc. Phys. Soc. 85 1029 (1965)Studies o f X-rays and electrical properties o f SrM o04 247[20] J P Russell J. Phys. (Pans) 26 620 (1965)[21 ] Powder Diffraction File Set Voi 6-/0 fRevised) Inorganic Vol. No. PDIS15 p 404 |22] P Scherrer Cottin Nachricht 2 98 ( 19 18)[23] T Kar and R N P Choudhary Mater. Lett. 32 109 (1997)[24] R N P Choudhary and N K Mishra Indian J. Pure Appi Phys. 31945 (1993)125] R C Buchanan Ceramic Materials for Electronics (New Y ork : Marcel D e k k e r) ( 1986)

Studies of X-Rays and Electrical Properties of SrMoO4

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Studies of X-Rays and Electrical Properties of SrMoO4

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