Mössbauer study of the hydration phenomena in some alkaline ferricyanides

Votume 13, number.4 ,,

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CHE,MICAL‘PjWSICS LETTERS ‘_:

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.m SOME AI_*~U~E FERR@YANIDEG :

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,. : Changes of h)dration in lithium and so&m fekcyanides abtair&d by e&& to humidity.and &y tieezing kqucous ~It3t~ons of.the complex are ~ccomp~~ie~ by marked alter&m oi,their. hiijssba~e~ pec’tra. : ‘.‘. ,:

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Lithium ferricyatiide, which’is reported’to &ystallize with four wa& moleculesas Li3Fe(CN)6-4H20, is a very hygroscopic substance. Because of its hygro-

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known that tee va{ue df the q~adrup~Ie spIitt& in .’ 7 ‘. ferricyanides is stroiigly dependent on’th~.c~tions surl : rounding the compteti’ion [ 1,3,].. :‘: .scopic character it tends to absorb humidity from.the. ’ Sodium ferri~y~nide* ~a~Fe(C~)~ :H& is a!so .attiosphere and this.hydration process is accompanied ’ hygroscopic bl;t this hydrate iS more stable than by a re~~arkable aiteration of the Nilssbauer spectrum .’ Ii,Fe(CN),.&$O. The Mssbauer spectrum of a. of the compound. frozen acjueous sohi tion qf sodium’ ferr~cyanide is The hlb‘ssbaixr spectrum of LijFe(CN)6*4H30 at markedly different from thgt of the pure solid safkq r&m. temperature is r&produced in fig. 1a, with-a is illustrated ii-~fig. 2. The ~uadr~pole spIit~i~~, which. value of 0.80 mm/see for the quadrupole splitting and .’ has a yafue of 02X58 m&se? at liquid nitrogen tern,.-0.30 mmfsec for the isomer shift, relative to a 57co ‘. perature in the solid phase’, cojlapses in the froieri .source in a Pd matrix. When we aIlow,the absorption ,. ‘solution a~ the satie temperature and dne observesa of humidity so that all moiecules in the timple have single line soinewhat brcradened.:By evapqrating the .’ absorbed water (which happens in a f9w seconds in a water one finds again the, value korresponditig t,o’that .” humidity saturated a&n&sphere at rodm temperature) ‘, of the ptire salt.‘The ab&nce of an eIectric fieid: the value of the quadrupole splitting becomes gradient (EFG) in a’ferricyanide is’unexpecteh since r’: ‘0.25 mmfsec, while the isomer shift remains the same, to the 3ds jaw-spiri c~n~guratio~ of’iion in. these cornai is illustrated in fi,g. If. This.phenomenon, which, plexes, a tiontribution @the. EFG ‘From the vtilence ” .. prob;ibly tiorresponds tq. the formation of a.difTerent electrons 131 ‘.must ctirrespotid. The absence &qua- :: ‘.:‘: hydrate Li3FefC~)6~nl$$, is ieversible, since by , : d&pole splitting in-the f&en &h&ion of sodium ” :’ :: gra+~ally with~r3w~~ the water mdtecules through fe&yani+ can be explairied by asSu@@i& a dif-: :” ~~u~~ the intensity Of s~e~frum f that at first’was .’ : ferent hydrate, is Segregated-oti &eiing the acjue&s..’ .’ IdO&, becomes &T&T while the in&& ofspec~. salu tion. The.g&omeYric$ Configuration df the;.Na’ .“, :. trum a that at fiist *as zero, increas&, So that at t&j ‘. ions in.this hid&& rilu$t ije.such.that‘they.induce: &i ,.l k-r&; wh+ the .ex&ess of humidit? has been ;em&ed EFG at the irori site. oE titi’ ~~~~~~~nide ion~w~~h, 1.:’i : f orie g~ts~lOW%~ofa’~nd no.contrib&&of f &‘can be ,hacp&s to-be”~~~h~-~~i?.ot~d-~f &%gr&dti &id.&-,‘l’ ‘&enfrom’fig. I,., .’ .: ..,., ‘. ..:. ;_ ;.,. _,/ poslte..in.sign’to.that &f,the &leixc&‘ql~c.trd&, && ,&$t: ::-_:, ;:YZ, ” The ba&tion’of the ‘quadrupole splitting of.littQtim’ .iest$i dppearing &h cubic ~yti~~tj~‘at the. iron-+ ferriqyanide is, probably due. to 5 xedistkibutiori .qf the j of the tiitister. -!n‘fkt ‘a s&kuti. f@$yanidg hydrate,,. :.:.j ‘tiC’&tions around ihk.fe&&nide ion-with the’in” ‘::,,...I_‘ ~a~.~e(C~j~‘- i Oi@ ha~~~~e~~~-~~~~~ted~ [4] ; with: :-;I::,,:‘i:~ creas&in knuqber b~.tiat#:mgjec@s sincei~is,~: .. ..,,.’. $+drupole ‘s&i&g ob’~‘.~;q5’mm%sec.~nd is&e6 .?:: _..,__ ., : :‘.

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VOfUm6.13; number, 4

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C~HEMICAL h-IYSI& L&& ._ '. : _'

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,, “’ ‘. ” _’ ’ The ~~~te~ce’~~s~veral hydr$es,of the aEk&ne shift -0.240 mm/s&, which corresponds well to.the values obtained by us for the sodium fenicyanide in :,. .-. ~ferri&an@e~ with markedty different values for the frozen solution: : quadruptile splitting- may ~kxplaifi~ the discrepancy of :: some.va1ue.s reported in+he literatq&.[7] . The formation. of a hydrate of sodium ferricya~i~~

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