Chhonkar, N S

Mookherji, A

English

IACS Institutional Repository

LIGHT ABSORPTION IN PARAMAGNETIC IONS IN STATE OF SOLUTION. PART 111. Cr'^' IONA. MOOKHERJT and 8. CHHONKARP hysic s IjAb o i i a t o k i l s , A cjua A c; k \.{ h W r ir n J  M a r c h  27, ld(>l)ABSTRACT. J.<ighi aI>sor})lH)Ji in T r '* ' 1<>0 sails is siiiilaal liy a ‘HTlgi r’s“ UVMSP3!]K" H j)oatm j)liotom c’ tcM- in tlic' m n g r  A to  IO,0(M) A  aiui Ha* itshUk an ' (liscussc'cl in Hio light o l  crysla llina  fa'lcl H iooiy .It  is obaorved  th a t th e 'ov t 'ra ll (*ul)K* sp littin g  IS m uch largc'r in salts com paredto (hl^^ N U M im l (V)'* ion  salts. Tlu ' cn-vali*ncy fa ctor  p  as (l<*(lnccd from  tin* lowc'i - m g o f  the term  scim ration  arises from  (J-and TT-orbital overlaji in Hr'^' mn salts.The* avc'iagt* niagnc*tie inoiiK'ut and g-\allies (‘valuate'd with the h<*lj» ot the optica l data agn*<  ^ vc'ry well w ith  th ose olisc'rved (*\pf'riiii('ntall\.T h o ..(foci o f t lw  lon^t nuig.- fi-M s on tlu' xv-ator i lim lrr a'aa r.aiiul lo  on tlio valuo,,| p .  Hin.-o )Um la .lo].on(lont on  stal.ilizing     la m oasuicd t.y Ito b s a ’ vod that tins o ffo -t  «'H.s moat ,,nm oim ro(l ... t ’o -  .on ( p  U iC.) and tl.o lo..al i- .o . uoune.ed m 0r++'* ion  { f -  O .To).46I N T  7^  o  n r  <^T 1 O Nlu tbo Hccoud ]>RV( of this ]Ripor (Mookherji and (JihonkcU, l.HiO), \tlii<h \\t Bha'l n'lcr as Tart II hetoaftor. a Hvstciiiatic ovti. al iiivesti>;aliou of the coukc- quencea <.f the er.v.stalliue electric field r.i. the ..ptical and magnet i.' Itehaviottr of Ni++ ion in about twenty .saltK in atjueotis solution has been reported. A number of interesting results that have been obtained are:a) The energy of separation of the mean centrt'of the absorption bandsis almost wholly determined by the eiibie itarl. ol the crystal field and that aniKotropio part has little influence on this..,) mere is « e .l  cevelee., in NV ' i.,u fro.n tJ.e ,»r.Wevertap ...h  . .  .„d  oHHeseparation from the free ion value, /r- orbi 1salts of N i'+ ion.The „..»»»<! fluer »l,.i.*iug et the hau,.s ,lne the the fieid g.ve exeeiient agreement with the magnet.,- amm.tr.w »ai«e« obtamed from the susceptibility measurements.437438 A . Mookherji and N . S. Chhonkard) The contribution of the distant atoms to the anisotropy o f the water cluster about the Ni ^   ^ ion was found to be pronounced in certain salts; while in others they were not so pronounced.The ground state of the free Cr^++ ion is an J’-state (3d® ^F) likeNi+  ^ ion (3d®®F) but with a spin moment 3/2 instead of 1 as in Ni^ "  ^ ion. Just like oeta- hedrally coordinated Ni+‘ ions Fg orbital singlet level lies lowest in the Stark pattern for Cr+ ‘  ^ ion with similar coordination. But two facts, that the spin- orbit coupling in ‘  ^ ioji is +87 cm~  ^ (Laporte, 1928) as against —328 cm"^ in Ni++ ion and that Cr ^   ^ ion with 3 electrons as against 8 in Ni  ^ ion has Kramers spin degeneracy, make the situation for + * ion somewhat different, Moreovei*, due to larger charge on Cr+  ^ ion t he electrons of oxygons of the water cluster about CrH+ ion will have a greater tendency to move into the ceixtral ion orbitals in order to stabilize its potential eneigy and hence both cr- and 7r-orbital overlap may be present in Cr+++ ions (Owen, 1955). This stabilization may tend to reduce the secondary distortions of the octahedral cluster and hence the split­ting by the tetragonal part of the field will be very small. The small positive value o f A together with the small tetragonal splitting will make the magnetic anisotropy for Cr ^   ^ < ion much smaller compared to the anisotropy of the Ni-^   ^ion and the magnetic moment wdll tend to have almost the spin-only value. Moreover, spin-orbit contribution from the upper cubic levels will be smaller compared to Ni++ ions and hence the effect of the distant atoms may be expected to be less pronounced in Cr^   ^ * ion salts. It is, however, to be remembered that owing to the lowest state in Stark pattern being an orbital singlet, Jahn-Teller distortion should be almost absent and practically the entire anisotropy should arise from the induced distortion of the octahedron by the effect of the long range atoms.It w^ ould, therefore, be interesting to study the optical absorption spectra of a number of chromic salts and view' them in the light of the findings of magne­tic susceptibility and other measurements.E X  P E H T M K N T A nThe measurements w'ere carried out by Hilger’s “ UVISPEK” spectrophoto­meter and the same procedure as in Part I of this paper (Mookherji and Chhonkar,1959) was adopted. The chemicals used were of ‘Merck’s gravimetric reagent quality Triple distilled w'ator was used for making solutions.The measurements w'ere centred round 27“C, but no observable change in the position of the absorption bands was noted for small room temperature vari­ations.K E S U L T SThe results of measurements are collected in Table I. The locations of the absorption bands for various salts of solution are given both in wavelengths andin wave numbers. In order to get prominent absorption peaks for the salts studied we had to use dilute solutions. Progressive dilution from that (con­centration at which prominent absorption peaks are obtained does not ohangc^  ^the positions of the absorption peaks.TABLE ILight Absorption m Paramagnetic lons^ etc. 439Salts Concen- trat ionn% M axim um  absorptionWave length in A Wav'c numbers (ciu'»)IOr.>(S04)30r,(SO4)3.K2SO^CrCl:jCr(N03).3Cr(C,H302)30.51 . 01 . 01 . 00.3588(»579(>5HI057755670II422540954115408541101117,01017,27017,21017,31017,64023,670 24,420 24,300 24,480 24,.330The variation of absorption in different salt solutions arc shown graphically in Figs. 1 to 2.T> I S O  V  H S T O Na) The absorption spectraIn all the five chromic salts studied by us the absorption S|ieetra (consist of two maxima, one at about 17,300 cm~  ^ and the other at about 24,300 cm~ .^ We shall designate them as T and IT respectively, J^hc maximum which lies in the ultra violet region near 38,000 cm~  ^ (Owen, 1955) will be known as ITT,The spin-orbit coupling constant A --- -f87 em“  ^ is only one fourth of that of nickel salts and hence the splitting due to the tetragonal field will be very small compared to the Ni  ^ ‘ ion. This is also v hat is observed experimentally (Krishnan, Mookherji and Bose, 1939) and hence optically we should expect only three main transitions, two being in the visible region and the other one in the ultra violet region.The maxima I and II may be identified as arising due to the transitions between the Stark levels and (r 2- P 4) respectively. In our later discussionsthese transitions will be represented as i^ Ef, and respectively (Fig. 3).b) Crystal field and energy levelsThe ground state of free Cr+++ ion is 3d* ^F and the term of the same multi- plicity i*p) lies 13,770 cm“ i above it (Moore, 1952). The tn>e of the complex ions that we shall be dealing in this paper has the 0r^-++ion at the centre of a440 A . Mookherjee and N . S. Chhonkarcompressed octahedron of water molecides i.e. four dipoles //, at ( u, 0, 0) and (0, dipolos //' at (0, 0, f h). Then the electric field potential at(.r, y, z) near the Cr'^ * ion at (0, 0, 0) is of tlie form as given by Eq. (1) of Part IJ.F ig . 1. A b sorp tion  ciirvoa (aquooun so lu tion ) o f (1) O.50/0 Cr,(S04),(T I) l.O o/o € r2 (S 0 4 )3 .K a S 0 4  (iri) 1.0o/„ CvC].The crystal field conforming to a potential as given by Eq.(l) of Part II splits the ground state into a number of levels whose approximate energies, accord­ing to Owen (1956) are shown in Fig, 3 and also given by Eq. (2) of Part II.The measured magnetic anisotropy for Cr+-*'  ^ ion in crystals is very sihall (Krishnan et aL, 1939), and tetragonal separation in state of solution will beLight Absorption in Paramagnetic Ions, etc. 441ao70II904 0i2010hOK ig. 2. A b sorp tion  ourvos (ncpioous so lu tion ) o f ( I )  1 .0 %  C .(N O ;,).,(II) 0 3% ().);,corrcsponrliiigly less and hence neglecting 1\ and 1\ ternis in Ecp (2) of Part TI we havehandK^ X  -- AAVK =  hE,We have observed both experimentally (Table I) and henceK  and X  ean be approximately evaluated. These are given in Table II.442 A . MooTcherjee and N . S. Chhonkar< e'-x - 1X7 H ' X  ♦ 3^ -Tj -loy31^  loy^ ^F ig . 3 S tark  s jilittin g  o f  th(‘ grouiKl s in to  o f  O r^ '' inn.TABLE IISiilt.s Kc i n »Xcm -1 cm  1f2<!r.,(S( ) , ) 3 3r>,720 6050 0870 0.717:n;,2ri7 6606 10950 0.795Crdl, 36,u o 66S0 J0650 0.773.36,350 668(» 100.30 0.730( ) . « ! , IT:,0 ,0 ;, 37,045 7120 0810 0.712I'hosc* A"-values ijtdicate tliai- in stale of solution all the six nienibors of the eluster afiout the (V++ < ion may lx* the same in all the salts exeept the last iji Table IT studied by us.It would be interesting to eom])are tlie magnitudes of iT-values of and Ni*  ^ ions with those of ' ion. We find that in ordinary ionic salts of Ou' ' ion (I^art f, 11)50) and of Ni  ^+ ion (Part II) the magnitudes of K  are about 2 bOOO em“ * and 17,700 em “' respectively, while in ordinary ehromie salts its value is apjwoximately 30,300 (*m-b Henee the overall separations (10/21 7T) in Ou ' ' ion and (0/7/v) in Ni  ^ * ion and Cr  ^ ion are I2,400enr^, 15,200 em~  ^and 24,400 em  ^ respectively and henee it is highest in Cr^  ' ‘ ion salts, e) Emluaiioti of the term, separation [E')For Cr^++ ion where‘*F-term lies lowest, the term separation E' in crystals which is aji important spectroseopie constant., can be evaluated from the expres- sion (1) as given below (Owen, 1955):X ) +  [ (where symbols have their usual meanings.E'A- I  A'+ A )T (1)Light Ahsovption in PoTuningn^ ttc I  on ft, ftc, 44.*}Now utilising our oalculatf^d K -  and X-values (Tal.lc II) u,- liavo ovaluate<l ^'-values from Eq. (1). These are given in Table II. It is seen that there is a lowering of the term separation for Or+ ' ' ions in crystals from the five ion value E  =  13,770 cm >. Following Owen (1!).').7) this lowering may he attrihuted to tin- eovaleney fa ctor /^  w h o r e a r i s i n g  from partial overlap of the 3d- orbitals with a-  and Tr-orbitals of tho sumimidiiis^ atoms. Af-rordiuglv,E 'K rFrom Kq. (2) one can (jaloiiiate/ -  which are given in Tabic II.Primarily this rovalciicy fWlor should not be dilTonMit tor all the salts in which + ion is similarly coordinated with six \ui\w  molccuh^s, }>nt then' might bo an appreciable change iji the ovcilap betvvccJL ioji and oxygenand hence in the eovalency factor from salt to salt arising from the (‘ftcct of distant atoms. In state of solution distant atom eth'ct will lx* ju'gligibh' and covaleney factor should jiot vary a])])rcriably IVom salt to salt with similar coordijuition. Bose and Milra (11102) in tlu'ir mastia ly sur\<‘y of th(‘ com])ai*at ive influence of short and long range fields on the magJK'tir behaximir ol salts of iron group of elements came to the conclusion that long raugi* fic'ld (*ff(‘ct in t-r' ‘ ' ion salts will be less pronounced thaii N i' * ion salts.ft  is seen that these /^-values are considerably smaller than thos(' of Pu ' ' (0.85) and (d*fl) ions. This demaiids a strojigcr bondijig in Pr' < ' ion, wiiicliis to be attributed to the larger charge on it. As a result of this larger charge the electrons on the oxygt'n have got a tendemy to movi* into th(‘ c(‘)itral chromic i()]i in order to (wen out the charge* distribution. Lhidcr sindi state* of afiairs we are no longer justified to take* ;r-bondiug as weak and Ih'Jici*/ -  in them will consist of both / , -  and Moreover, if er-boJiding is strong then* may be some amoimtof 77’-l>e)ndiiig whiedi will di^crease tin* liypenlhie stTucturi* hi hydratt'd com])l(‘X(\s of Cr' *’ ions as actually observtxl by Bowers (11152) ami Bake*r and Bleaney (1952).From the above} discussion it s e e m s  that in | fV(H.^O) .^]' '  ^ complexes/^*'* will tend to a value ^  0.9 and to a v a lu e r  0.S5, making - 0.7fi asactually observed (Table II).d) C a lcu la tio n  o f  m a^jnetic m om en t valuers :The exprossiein for the?. me*au magimtic monunit for C-r^ ”' ' ion will \)V alrneist similar to that for Ni"*""*^  ion excepting that the* spin-only value will be 15 instead of 8, and will be given by the Eef. (9) of Part II and henex* the mean magnetic moment is given by^  15[l+(8A--3A;T).a'l •••where444 A . Moohherji and N . S. OhhonkarP - 2.1Kh is interestiT)j{ to note that the last two terms in the expression (3) which re})iesent the (hwiiuion of the effective magnetic moment from its spin-only value are of opposite sign. The comparative strengtiis of the two terms at will l)cSA 090 cm"^and ^ k T  030 cm ^The difference in them is only | 00 cni“ i which when multiplied hy a' having a negative value will bring down the spin-only value by about 0.004. Hence the relative contribution from these two terms is very very small as is shown below ://2 .. 15(1 - 0.04H-0.037)It is therefore clear that in chromium salts there will be very small dtwiatioJi (::::; 0.4%) from the spin-only value.Now utilising our calculated values of K , p  and the Eq. (3) we have eva­luated //-values for ion in state of solution for different salts. These aregiven in Table FIT. For comparison we have included solutif)n values in th(‘ Table. It is seen that the two values (optical and magnetic) agrees well within the experimental errors.These values of the mean magnetic moment for 0r^ ~^^ ion salts have almost the spin-only value (3.87) as has already been shown above.TABLE III/ -^Values g-ValuesSaltsOptical(soln.)Magnetic(ftoln.)Ojitical ( r^ysf al0r,(SO4);, 3 .8t>7 3.850*2 1.970 1.980*Cra(S0 4 )3 .K,S (>4 3.86(> 3.778*3 1.908 1.980*C!rCl,, 3.867 3.850*2 1.989 1.990**Cr(NOa),, 3.8673.8683.745**3.756*J1.968 J .9722.260****Ting and WillianiR, 1051.♦♦Kozyrev, Salikhov and Shanionin, 1952. ♦♦♦LancaHter o.nd Gordy, 1951.♦I Mookhorji, T. (Unpublished)♦2 Cabrora, Harquina, 1922♦3 Welo,*4 Fahlenbrach.1929,1932.:: 1 For complete reforenco consult Solwood’s, Mag- netoohemistry.Light Absorption in Paramagnetic Ions, etc. 445e) Calculation, of g-valwji :The Hpoctroseopic splitting factor </ for 0)-+-t ' ion according to Owen (Iflfir) is given by25.2Ar/ -- \r (^)Substituting the values ot‘ /\ and/- from Table II, </-values could l)e calculated vhich are given in Table 111. In the absence of any (‘XjKU’imeiital (/-values in state of solution for Cr^  ^* ion we have included crystal f/-values in tlic table for comparison. It is seeji that fy-valucs in state of solution and hi crystal do not vary appreciably.From our ojitical measurements we observe that /--valiu‘s in r^^  ^ ( h i * ' ,  Ni' * and Oo++ ions are ~0.75, ~0.S5, and ~t).9r> respectively (Mookherjiand Chhonkar, 1959, 19()0). /--Values give the covalent chara(;ter of the ions; so the covalent character will go on decreasing as we go from Cr^  * ' to (h)' ' ions. Further, more covalent the ion is, the mort' stable will be the water cluster sur­rounding it. As a result, the stabilizing energy in case of (V^*" ion will be larger comjiared to others and that this will tend to minimise tin* s(‘condary distor­tions of the octahedral cluster. W<‘ hav(^  already seen that these seeimdary distortions Avill be produced by the ellect o( the distant atoms alojic and that loo will be minimised by the high stability of the water cluster. That this effeid is very small is also supported by magnetic anisotropy measurements (Krishnan (/ (//, 1939). It is therefore, clear from the above discussion that the distant atom effect in Cr^++ ion will not be pronounced; whereas this should be most pronounced in (Jo*^  ion, less in Ni* ' and almost negligible in (hi* ' ions.A K N I) w  L >: D (; M t: iV T SThe work was carried out at the Physics Laboratories ol Agra (^ollege, Agra,. We wish to (‘xpress our sinc.ere thanks to Vnivcrsity Grants (hunmission whose generous grants enabled us to (;arry out this ])iecti ol work.Oar sincere thanks are also dia^  to JVofossor A. Bose, D.Sc., F.N.I. for Ins helpful criticisms and concrete suggestions.K .E V  E J< E N SBowm-fi, K . D . H»r>2, P ro c , J^htps. Nor., 65, SliO.Uak<«r .1. M. .uid iJIcanoy. U. l!»r.2. P ror. Ph/l^. Soc.. 65,A . ttud M ilm . S. K . /n ,f. . / .  VViyv.. 26. .-.4S.Ko/.yrcv, B. M. Sulikliov, S. (i. and Shmnoniii Yu. Ya. 1!*62, J . E j j,. Thcor. Phyx. 22, .M'.Kimhnan. K. S- Mookh.'. ji, A. and Jiow. J’liil. T r.w x.. Koy Son. 289A, 12.'5.Baucastm-. K. W. an.l Gordy. W., 1951 , J . ('he,,,. Ph./x., 19, IIHJ.Eajiorto, O. 192S, !fC. Phiff*. 47, /<)i.Mookhorji, A. and OhKonkar, N. S. 19.)!), J„(l. ./. Phi/x. 33, 74^Mookliarji. A. and ( ’Wionkm-. N.S., J9ti0. I,id. -J. Ph,/x. 34. 147, 336. 363.M oore , O .E ., 1952. A to m ic  K norj?y I.ovcIb -A'xt. p » r .  S ta „d . C irc . 4«)7, \ o l. 11.Owen. J. 195.5, P r o c .  H o y ,  S o , - . , 227, 1S3.Ting, Y. and Williams. D., 1951. /Vo's. H cc. 82, 597.2

Light Absorption in Paramagnetic Ions in State of Solution. Part III. Cr+++  Ion

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Light Absorption in Paramagnetic Ions in State of Solution. Part III. Cr+++ Ion

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