Khastgir, S R

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A NOTE ON VOLUME RECTIFICATION OF CRYSTALSS. R. KHASIGIK( K c i C i v C i i  l o i  I f i i b l U a h o n ^  / g / ^ lRecently a raper has been 1‘ublishal on v.il.nnc rectilicalion by H. K . Sen (ip4j). riie paper appears to be an claboralioii of what was reported by tlie same autlior (1937) about five years ago. Deaglio (103^1) at that time, m a note published in Nature, made a clear statement about volume rectification which is only a natural consequence of the lack of symmetry in polar crystals. He also icfened to the experimental evidence of such volume rectification in his expel imeiits with carborundum and made the present ]iosition with regard to the lectifying properties of crystals abundantly clear. \'et it has been concluded by Sen in his recent paper that “ the volume rectification effect is not confined to crystals like carborundum, zincite and silicon alone, which have no centres of .symmetry but that it is also exhibited by crystals like galena, iroii-pyrites and pyrolusilc, i<ossessing centres of .symmetry.”  In view of .such conclusion, a brief review of the subject is given here to leave no room for any misunder­standing about the existence of volume rectification in polar crystals.My experiments with l)as-(lupta (Khastgir, 1935) .showed that when carborundum, zincite or silicon crystal was placed between two mercury elec­trodes giving large contact areas, there was con.siderable rectification. Similar experiments with symmetrical crystals like iron-iiyrites, galena, etc., showed no rectification. The carborundum crystal under examination exhibited one jilane face and other facets and it consi.sted of at least tw'O crystals twinned together. Of the symmetrical crystals, the iron-pyrites and magnetite ciystals used in these exiieriinents could be regarded as single crystals. The results of these experiments strongly .siii'gei/ed asymmetric conductance in crystals having 110 ceutres of .symmetry, 'riie fact that the rectification obseived in carborun­dum, zincite and silicon crystals did not change, when one of the cry.stal surfaces was heated, seemed also to support this suggestion. As there was no experimental test that the .surface effect was entirely eliminated in our experi­ments, the evidence of volume rectification in polar crystals was not regarded as conclusive. Crucial tests were therefore considered necessary. Such a crucial test was made a few months later by Deaglio (1935) who performed two sets of experiments. In the first set of experiments, the plane natural face of the carborundum crystal which was perpendicular to the axis of six-fold .symmetry and the face opposite to it, scraped and made plane, were coated v\'ith gold orplatinum by cathodic sputtering. The I-V  characteiistic curve obtained with13112 S, l i .  Khasim rsuL’h crystal Ixlueci) luo declrodcs inakiuL' large crnilact areas showed consi- derat>ie rcctiflcalion. No cleclrolytic iHdarihalicm wa.s nl)Served l)ut there was l^holo-elcctric effect on the ciyslal snrfatv. 'flic observed reclificaljun W'as considered as nitunlx due lo l ec'lifkalioii of tlie ban ier layer '’-type. In the second set ol ex]»eriineiils, bulli Ihe faces of a cai Ikji undiiin single crystal woie s(aai)c:d and made sinooLlc 'Hie eleclKJcks weie gold or ])ia1iniiin films deposited (jii (lie scjaj*cd siirfat'es, Tbeie ^\as iu> tiace of any electrolytic polarisatic^n and llieve was also no evidence of any Ueml)er effect or any internal pljoto- eiettiic effect. Tlie characieristic' curves weie tlien drawn for two different tliieknesses of tile same cr\slal keeping the contact-areas the same. 'I'lie recti- fic'ation obscived was ojjposite in sign to that r)I)served in the first set of experiments. When enn ves ueie drawn sinewing f U}uni  against vuiiaj^c pci ( lu, for Ihe Iw o dillcrejil tliickiiesses of tJie t i N slal, iJiey were found to be almost identical. I'his showed llial llie ol)Servcd recliJicalicm ^\as not due to any surface eflect. Tlieie \^as thus no room for any doubt as to tlic existence of volume reclilic'ation^ at least in carboruiidiiiii crystals.'this asymmetric conduclivity was stndicd later in the experiments of Khastgir and Chakravarty (013}) when the iKvleiitial fall in the body of the I'arbomndum er>stal \vas observed for an ai plied voltage sending current in the two optK)sile directions. Witli cailioi uncliim, the ])oteiitial distribution was found to be different for the two opposite directions. In the single ciyxstals of iroii-jiyntcs and magnetite, liowever, tlie iioteiitial distribution remained exactly the same for llie two o])posite directiems of the current. ,It is likely llait 'I'issot (nutd and l^ierce (icjj e) also delected vedume recti­fication velars agc> in some ci yxslals. d'liey showed that witli a certain groii]) of crystals, even relatively large polished plates betv\cc‘ii metallic electrodes made very sensitive  ^ detectors. It Is exidenl that tlie reelificalion ol)served liy lliem in 1 he symmelvical Cl \ stals w as a MM/m I ellccl ; whereas, in the ci ystals with no c'entres of .synmieliy, the cfiecl must have been a combination of surjucc and viW///nr leclilicatioii.1 hi the Iheoietic'al side, 1\. de Ivionpj (ii|j(|) showed in a geiicial wa}^  that the ciyslal 1 ecliJicalion cniiUl l»e cliK' lo asyiiiiiielric liindiiig of the ions into positions <d' eiiuilibi iiiiii l)y resloi ing fcaces, not syniniclrical for efjiial ami opposite disidaceinents. Considering the isolIiLi iiial variation of the electrical resistance witli ihe ajiplied voltage (wliich was called “  elcctro-rc^sistive "  effect), t)steibcig alst^  ga '^c a theoietical lieatmcnt of rci’tificatioJi in carborniidum(’lystals. The resistance was show'ii tlieorelicaliy to cliange, on reversing the sign of tlie a])|>lied voltage hy an amount dei)eiiding upon tlie elcctro-resistivc coustaiils of the ciystal. Carbbimtdnm cry.stals were actually found to rectify current in the maiiiiei incdietcd along the direction of tlie axis of six-fold symmetry.In conclusion, it can be said that vSen's observations of the rectification effect in syinniclrical aystals like galena, iron-pyrilcs, pyrolnsile, etc., placedA Note on Volume Rectification of Crystals 1 1 3between mercury electrodes of larf>e contact areas, merely showed that the surface effects were not eliminated in his exi)eriineiits.Pnvsics l)r;pAk'i‘Mi'NT,J)Aa \ Univkksitvli P' !•: R 1{ N C 15 SlK'ii«Jio, uj3,s.C<mii)le.sKemlus, 200, 1 J 0 3 ; .see al.M, A l i i  l< A u u i .  I h  Uc .Sanir .c  (ehLssl*li\'sK\s) IvXX (ig36).Dea^ l^io, 1938, Saiitrc,  141, jo iiKliiislKir, 1^35, NiiUirc, 135, 13S; sec also Kha.st.i-ir and Das Cii],(a, 1Q3.S, Imi Jo in .  I ’liys. 9, 35SKhas(j>ir und Chakravart_v, 1937, / c i lsc ln  ili  hh J 'hysik.  10S, S8.Kroiiij^, jyjg, 123, 31^ ),Oskrbci^, ig^6, PJ iys  R e v  5 0 , 1187.I’iem*, 19J2, hUctiyiLian 69, 66. v^ cii, 1937, Rialtnc, 140, 110:2. vSc*n, H>l?, JtuL J o in . Phys.,  16,Ti.sstjl, jgK', L li lccU iciaii, 39, 331,

A Note on Volume Rectification of Crystals

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A Note on Volume Rectification of Crystals

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