Anitha, C

Mahadevan, C

Shanthini, Beaulah

IACS Institutional Repository

Indian J. Phys. 73A (6 ), 805-809 (1999)N O T EU P  A- an in tcm ationai jo u rn a lElectrical resistivity measurments on gel grown KDP crystals doped with KCS, KNO ,^ NaCl and NaNOaBeaulah Shanlhini, C Anilha, C Mahadevan* and C BalasinghD e p a r tin e n t o f  P h y s ic s , S co tt C h ris tia n  C o lle g e . N agercoil-62Q  003 ,In d iaT . H in d u  C o lleg e , N ag erco il-6 2 9  002 , Ind ia  Rrieivfd 26 April /99V. mcepted 16 June 1999A b s t r a c t  : P u re  and  im p im ty  ad d ed  (w ith  K C l. K N O ^ , NaCI and  NaNO:^) K D P s ing le  c ry s ta ls  w e re  g ro w n  by the  ge l m ethod  using  s ilica  gels. E lec in ca l resistiv ity  m easurem ents w ere e a r n e d  ou t a lo n g  b o th  the  un ique  ax is  and  p e rp e n d icu lar d irec tio n s  at room  tcm p e ia tu re  by the c o n v en tio n a l tw o -p ro b c  m e th o d  T he  p resen t s tudy  show s tha t the  im purities  w ith  no com m on ion  is ab le  to  c rea te  m o re  d efects  a long  the  pe rp en d icu lar d irection  (o -d trection ) than those w ith a c o m m o n  tonK e y w o r d s  : E lec trica l re s is tiv ity  and im purity  added K D P crystals  P A C S N o .  : 6 6 1 0  72Potassium dihydrogen orthophosphate (abbreviated as KDP), KH2PO4, belongs to the scalenohedral (twelve sided polyhedron) class of tetragonal crystal system. It has a tciramolecular unit cell having the dimensions [I] given as r/ = /? = 7.448 A and c = 6.977 A. KDP is feiTO clectric w ell b e lo w  room temperature and the curie temperature 7  ^ is 123 K [2 |.  KDP is soluble in water and the molecular weight and density rc.speclivcly are 136.09 and 2.338 g/cc [31.KDP crystals have created considerable interest among several research workers. Pure and impurity added KDP crystals were grown from aqueous solutions and also in gel media by different workers 14-6). in KDP type oi crystals the possible type of point defects which help the electrical conduction process are the ionization defects, v iz .,  (HP0 4 )2- and (H ,P0 4 ) prcxiuced as a result of proton jump from one phosphate group to another along the same bond |7]. The electrical conduction in KDP crystals has been established to be protonic [8,9]. The resistivity value of KDP crystals decreases when added© 1999 I ACS806 Beaulah Shanthini, C Anitha, C Mahadevan and C Balasinghwith oxalate (crystals grown by the gel method) [lO] and chloride (crystals grown by the slow  evaporation m ethod) [11] im purities. This was explained by considering the replacem ent o f (H 2P04)“ * ions by (C204)~2 and Cl“  ^ ions. It was also found that the activation energy values for KDP crystals do not vary much when added with oxalate impurity of various concentrations [10]. It has been observed [6] that the KDP system has becom e com plex after ion irradiation and it shows irregular behaviour with regard to conductivity property.At any particular temperature, the G ibb’s free energy o f a crystal is minimum when a certain fraction o f ions leave the normal lattice. As the temperature rises, more and more defects are produced which, in turn, decrease the resistivity [12J. In the high temperature (in trinsic) region, the effect o f im purity on electrical conduction will not change appreciably whereas in the low temperature (extrinsic) region, the presence of impurity in the crystal decreases its resistivity. Tlie electric conduction in dielectrics is mainly a defect controlled process in the low temperature region. The presence of impurities and vacancies mainly determine this region. The energy needed to form the defect is much larger than the energy needed for its drift [6, 10].W e have, in the present work, attempted to study the effect of ionic impurities (with and without the common ion added heavily (impurity added in the KDP solution used for the growth of crystals with concentration ranging from 2000 to 10000 ppm) on the electrical resistivity at room  temperature (/.e., at a particular temperature of 32°C in the extrinsic region) o f KDP crystals grown by the gel m ethod. A report o f our work is presented and discussed.KDP crystals were grown in silica gels prepared from sodium mctasilicate (SMS) by the reduction o f solubility method. Analytical reagent (AR) grade sam ples o f KDP, potassium chloride (KCl), potassium nitrate (KNO^), sodium chloride (NaCl) and sodium nitrate (NaN03) along with double distilled water and ethyl alcohol were used.KDP was added w ith KCl, KNO 3, NaCl and NaNO^ separately (as im purities) each in six different KDP : Impurity molecular ratios, v/z. 1 : 0.000 (pure KDP), 1 : 0.002,1 : 0.004, 1 : 0.006, 1 : 0.008 and 1 : 0.010. The impurity was dissolved in the 2.5 M solution of KDP. SMS solutions with two different densities, viz. 1.06 and 1.08 g/cc were used.SMS solution o f density 1.06 or 1.08 g/cc was added to 2.5 M  KDP solution in the volume ratio 1 : 4 (SMS : KDP). Thus prepared gel solution was then transferred to test tubes (20 ml in each tube) (or diameter 2,5 cm) and was allowed to set at room temperature. Gelation occurred in about 15 to 20 hours. Then it was kept as such for one day for gel ageing. After that an equal volume o f ethyl alcohol was added slowly above the gel and the test tubes were tightly closed with rubber corks to prevent evaporation o f alcohol. Alcohol reduces the solubility o f KDP, which first precipitate at the gel alcohol interface. Diffusion o f  alcohol into the gel was follow ed by form ation o f  nuclei w hich then grew larger. Com pletion o f crystallization took about 12 to 15 days.Scalenohedral morphology was exhibited by all the crystals grown. Crystals with high transparency and large defect-free size (> 3 mm) were selected and used for the electrical resistivity m easurem ents. The extended portions of the crystals were removed off and the opposite faces were polished and coated with good quality graphite to obtain a good ohmic contact.The resistivity m easurem ents were earned out along both a- and t -dircciions for all the fourty-tw o crystals grown using the conventional two-probe technique at a particular tem perature o f 32°C. The resistance of the crystals were measuied usmg a KXX) Megohm meter. The dim ensions o f the crystals were mca.surcd using a travelling microscope (L.C. = 0.001 cm). The resistivity (p) o f the crystal was calculated using the relationp =  RA/d,where R is the m easured resistance, d  is the thickness of the sample and A  is the area of the face in contact with the lead (electrode).Results obtained in the present .study are given m Table 1. Resistivities obtained m the present study are o f the same order with those obtained by previous authors lor the KDPTable 1. R esu lts  o t e le c tr ica l re s is tiv ity  m easu rem en tsElectrical resistivity measurements on gel grown KDP crystals etc 807R esistiv ity  (x  10^ I2-m ) a longIm p u rity M o lecu la r (S M S  density 1 06  g /cc) (SM S density 1 08 g/cc)ra tiof 'd ire c tio n rt-dircction ( -d irection rt-dircctionP u re  K D P 1 . 0  0 0 0 0 6431 0 2948 0.6017 0  36(K)K C l 1 0  0 0 2 0  4109 0  2546 0  .<541 0 32271 0  004 0 .3 3 6 0 0 .2183 0  2083 0  24961 . 0 .0 0 6 0  2276 0 .1744 0  2172 0  17691 0  008 0 .1779 0  1575 0  1595 0.14.35I 0 .0 1 0 0.0891 0 .0768 0 0961 0 0960K N O 3 1 . 0 .0 0 2 0 .4269 0 1921 0  3892 0  24751 .0 .0 0 4 0 .3945 0.1435 0.2007 0  19601 ' 0  00 6 0  1144 0  0600 0  1541 0 13431 : 0 .0 0 8 0.0.S42 0  0395 0.1342 0  !095I : 0 .0 1 0 0  0438 0 0259 0  0852 0 0520NaCI 1 : 0 .0 0 2 0.3891 0.2461 0.3456 0.27331 0 .0 0 4 0.2721 0  1785 0 .23080  22131 :0 .0 0 6 0.2(X)6 0  1483 0.18970 18141 :0 .0 0 8 0 .1475 0  0653 0  13780  12321 . 0 .0 1 0 0 0 8 1 7 0 .05870 .0770 0 .0723808 Beaulah Shanthini, C Anitha, C Mahadevan and C Balasingh Tabic 1. (Cont'd.).Impurity MolecularratioResistivity (x 10^  i2-m) along(SMS density : 1.06 g/cc) (SMS density : 1.08 g/cc)(•-direction ((-direction (•-direction (/-directionNaN03 1 • 0.002 0.4348 0.0805 0.4108 0.24301 .0.004 0.4145 0.0513 0.2206 0.15971 . 0,(K)6 0.3360 0.0450 0.2193 0.11181 . 0.(K)8 0.0771 0.0315 0.2073 0.09501 * 0.010 0.0332 0.0160 0.0968 0.0385crystals (x  10* ohm -m etre) [6 ,10-11,13]. It can be seen that the electrical resistivity decreases (consequently, electrical conductivity increases) with the increase in impurity concentration (in the solution used for the growth o f crystals).The resistivities are more along the c-direction than those along the a-dircction (i.e., perpendicular to the c- or unique axis direction). This is in correspondence with the results obtained by the previous authors [10,13]. Density o f the SMS solution u.sed for the growth o f crystals has some effect. For all the dopants considered, the resistivity along a-direction increases with the increase in SMS density. The resistivity along the c-direction decreases with the increase in SMS density for the chloride impurities while no systematic variation observed for the nitrate impurities. This gives a rough idea that the KDP crystals grown with SMS density o f 1.08 g/cc are less defective than those grown with SMS density of1.06 g/cc.R esistivities for the NaCl added KDP crystals are considerably less than those observed for the KCl added KDP crystals along both the a- and c-directions. Resistivities for the N aN O i added KDP crystals are considerably more and less respectively along c- and a-directions than tho.se observed for the KNO^ added KDP crystals. This .shows that the impurities with no common ion are able to create more defects along a-direction than those w ith a com m on ion. H ow ever, along c-d ireclion , the n itrates and ch lorides behave oppositely. Reason for this is not understood.Pure and im purity added KDP single crystals were grown and resistivities were m easured along both the c- and a-directions at a tem perature o f  32“C. The present study indicates that the resistivity decreases with the increase in im purity concentration. The resistivities are more along the unique axis direction than those along the perpendicular (a-) direction. The im purities with no com m on ion are able to create m ore defects along a- direction than those with a common ion.E le c tr ic a l  resistivity m easurem ents on g e l  grow n K D P  crystals etc 809References[IJ  R  W  G  W y c k o f f  Crystal Structures (N ew  Y ork  ; In te rsc iencc) V ol. 3 11 E dn p 160 (1960) f2J E  C  S u b b a ra o  Ferroelectries 5 267 (1973)[31 Jo h n  A  D e an  (E d n ) Lange's Handbook of Chemistry (N ew  Y ork  M cG raw  Hill Book C om pany) X II Edn p  4 ^ 9 4  (1 9 7 9 )[4] H  K  H c n isc h  Crystals in Gels and Uesegang Rinfgs (C am bridge  . C am b n d g e  U niversity  Press) (1988)[5] L  N  R ash k o v ic h  K D P-Family Single Crystals (N ew  Y ork  : A dam  H ilger) (1991)[6 ] K  S o m a s e k h a ra  U d u p a , P  M o h an  R ao , S n ram a n a  A itha l, A P B hat and O K A vasthi Bull Mmo Sn2 0  1 0 6 9 (1 9 9 7 )[7 ] A  W  G e o rg e  Nuci Instrum & Meth. Phys. Res. B 29  708 (1989)[8 ] M  O ’ K eeffe  and  C  T  P e m n o  J Phys & Chem Solid 2 H 2 1 1 (1967)[9 J M  S h an m u g h a rn , F D  G n an an i and P R am asam y  Indian J Pure & Appl Phys 20  579 (1982)110] M  S h a n in u g h a m , F  D  G n an am  and  P R am asam y  biduin J Pme A Appl Pti\s 23 82 ( 198S)[11] P  S e k a r  R am asu b ra iiian ian  and  C  M ahadevan  Indian J Pure & Appl Phy\ 29 28S (1991)[12] S C  Ja in  and  S L  D ah ak e  Indian J. Pure A Appl Phys 2  7 1 (1 9 6 4 )[ 1 3 1  L B  H a m s  and  G  J V e lla  J Appl Chem 17 151 (1966)73A(6)-12

Electrical Resistivity Measurements on Gel Grown KDP Crystals Doped with KCl, KNO3, NaCl and NaNO3

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Electrical Resistivity Measurements on Gel Grown KDP Crystals Doped with KCl, KNO3, NaCl and NaNO3

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