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The origin of the optical absorption bands of Ga+ and Ga2+ in MgO
0 0 3 8 - 1 0 9 8 , 81.0aO__ ' " " ' 1- 0 2 502 00,0
Sohd State Communicauons, Vol 38, pp 2 2 1 - 2 2 2 Pergamon Press Lid 1981 Pnnted m Great Bntam
THE ORIGIN OF THE OPTIC.XL -kBSORPTION BANDS OF Ga" .-LND Ga:" IN MgO M Moreno Departamento de Ftsica Fundamental. Facultad de Clencias Unlversldad de Santander. Santander Spare
(Recewed 4.Vovember 1980 b) S 4mehncLxl -kn interpretation of the optical absorption data on Ga" and Ga -'+ in \lgO. recently reported by Holmberg. Lee and Crawtord. IS proposed The peak at 3 3 5 e V o f G a z• M g O l s s h o w n t o b e c o n s i s t e n t w t t h a t r a n s l t l o n a r t s m g from the 4s "+ 4p transmon of free Ga :÷, whlle the peak at 4 13 eV ot Ga" MgO ts assigned to come from the 4s: ---*4s 4p (IPtl transtuon of free Ga" From these results ~t ts found that the &stance between the 4s and 4,o levels IS reduced b) a factor of about 0 4 m both systems This Iot~ reduction factor is mamly related to the strong bladelung effects appeanng In MgO
b~, recent EPR data ,,teldmg.f~ = 0 11 for TI:" KCI [5]
THE OPTICAL -~BSORPTION (OA) data concerning s and s2 ions In alkali halide cr?stals provide good lnform a u o n about covalenc} and lattice effects related to such tons [1 2] Quite recently Holmbergetal {31 have reported the first results of Ga + and Ga "e-* m MgO Their results clearly lndmate that Ga "2-" MgO gl~,es rise to an OA band peaked at 3 35 eV wtule a band appearing at 4 13 eV is due to Ga" MgO However no explanation about the ongm of both bands was raven In view ot tills we ha,,e tried to assign ~uch bands tn order to tnspe~.t the lattice effects due to MgO. a crystal formed b,, divalent ions It has recently been shown that the OA data of s-ions m tonic lattices can be well understood assummg that such ions rove rise to complexes v,nth the surroundm g a n i o n s m n e a r e s t n e l g h b o u r p o s l t l O n S [21 In the case of octahedral environment tbas model predicts three dipole allowed transmons denoted b~, tl~,lrr)-" a~g tl~(O) ~ arg and a'tg ~ tt~ where the latter arises from the s ' ~ p transition of the s-ton ~,tule the remainder are charge transter bands The positron ,,,, of the first charge transfer band r,ulTr) ~a~g is governed b~, the relation [2, 41 vrr
=
K('(A
-- X..~t)
( I )
Here K = 3 72 eV and X.a and g..u are the optical electronegattvrttes of the anion and of the metal simpurity respectively, its difference, "
a n d ] a = 0 0 8 forGa "~÷ KCl [6] where asusual, ta mean> the unpaired spin density onto a p,~ hg:md orbital Thus we can reasonabl.~ expect that vn ~,,lll be higher for Ga > KCI than tor TI -"÷ KCI Furthermore considering the optical electronegatt,,ltles of ox,,gen (3 2) and chlorine (3 0) [4] equation (1) and the preceding a r ~ m e n t lmpb, that t.,,, in Ga-'* MgO should be higher than 4 1 e \ , a fact which discards the band at 3 35 eV as a charge transfer transition Thus ~,,e assxgn s u c h a b a n d o t Ga:* M g O a s c o m l n g t r o m t h e 4s ---,-/-p transmon of free Ga:" located at lt'o = 5 22 eV [7] This assignment imphes a reduction factor k = W,'It'o equal to 0 4 for the present ,.ale ~,.h~ch l~ ~g mficantl~ Iov,er t.han the ,,alue k = 0 6, tound for TI:* KCI and Sn'-* KCI [2 8] ¢> far as the peak a t 4 13e~ ofGa" MgOisconcerned ~e bel,e~e it come~ from the 4s:( tSop~ 454p I I p l l transition ot free Ga" In fact the s: ions In alkali hahdes give rise to the so-called 4 . B . and C band> within the V - L W ran,,e [I. 8] These bands come from the S2 ( tSo ) ~ spt 3pi 3p: 'Pi ) transltlOn~ ot tree Ions, respecu~el,,, v~bale in these ions the charge transfer band, lie at h l ~ e r energies because of the Paull pnnclple In the case o f G a ~"the small ',alue ot its s p i n - o r b i t coefficient (~eo = 0 11 eX for tree G a ' j gwe.., rt~e to ratios between the o~cIllator strengths ot bands C and 4 oCabout 500 for Ga* doped alkah hahdes [11 Considering this fact and that the oscillator strength for the 4s ---*4p transition of free Ga -'+ is 0 9 [9] ~,,e find reliable the assignment for the peak at 4 23 e\ in Ga* MgO in ~new of the oscillator strength of such a peak being comparable to that of the peak at 3 35 m Ga 2. MgO Moreover. hkel,, due to the small concentration> of Ga + and Ga"* m \lgO. both peak.,, show optical densities le~s than 0 a {3]
~"1
THE O R I G I N OF THE O P T I C A L .-kBSORPTIO\ B-X,NDS OF Ga" -k_\D Ga:" IN ' q : O '~ oI 3~ No 3 The p o s m o n of the C band at a s-" ion m an ~omc latuce h ,3e~.nbed b> the e\presston [il E.
= h,'--" + 4
+
~
"
I'~t_
In tins expression E c means the posmon ot the C band v. hde G and ~ are the exchange and s p i n - o r b i t parameters respecuvel) and i~' denote~ the dffterence
betv, een the energ'., of the ~ and p one-electron le,.els mvol,,ed in the tranattions The G..¢ and W parameters lot a s: ion m tome Iatt,ces usuali.', take smaller ,,'aluea than those corresponding to the free s z ion [1] In the present case ,.~.here ~ G h presumahl', ~mall I.~o Go = 0 08 for tree G a ' ) e q u a u o n I 2) can be ,*ell a p p r o x i m a t e d b,,
E c = kh',}+qGo
I~,)
~,,here q ,, the reduction c o e f f i o e n t tot the ex,.hange integral and [1,'o = 7 38 eV and Go = I 3') eV correspond to f r e e G a * ['7] - ~ s t h e r e d u c u o n o n t h e e x c h a n o _ e integral ts ma,nl,, produced b~, covalencx [10l ~ e can obtaan a lo,.,.er hm~t for q m the present case b,., tak,ng q = 0 3. tin~ ,,alue beLng round for Ga* KCI [8] ',~ here as alread) pointed out. more ,.o',alent eftect~ are e x p e c t e d Thus accepting the propo~edaasL__.nment for t h e 4 1 3 e \ peak a t G a " b l g O t h e p r e ~ e n t a r g u m e n t ~ and e q u a t i o n q3 ) lmpl.,, that X should ha,.e a value I'.~mg b e t ~ e e n 0 5 and 0 4 Thouda thla ~alue xs si_~ntficantlx ~maller than that found for Ga ÷ and other mono~alent s and ~" tons in alkah hahdes where k = 0 ~ [ l , 2, 8] hov,e~er tt ta clo~e to, the ',alue a r k deduced tc, r Ga -'÷ MgO It ahould be recalled m tins ~a? that comparable ~ alue., a r k are tound for TI" IO " ; and TI z" 10 61 in KCI [ 1 . 2 . S] A atm,lar situation happena tar Pb"" and Ph 3. in KC1. v.here ~ lies clo~e to 0 6 m both caaes [2. g] On the ,other hand t h e ~ , a l u e o f k l o r h o t h G a : " MgOand Ga" .",lgO ~,,~tem~ h comparable to the value k " 0 35 inferred t,_,r B~s" CaO from the data reported b,, Hu~esetal [11.12] The pre~ent dh,.u~s~on ~ugge~t sL,_.n~ficant r e d u c u o n ettecta on h' for both Ga ÷ and Ga z+ m MgO. dlu~tratmg that tin~ reducuc, n ~ not maml', due to covalenc.~ a~ ~t u~ualb, h a p p e n a l o t G a n d ~ [101 It has b e e n p o m t e d out that the reduct,on on W b related to the more draatlC Madelung repulsion on the s level than on the more e x t e r n a l p l e v e l [ 1 ] F o l l o w m g t h a s a r g u m e n t t t can reasonabl.,, be expected to find more aagntt'icant reductions on W, tar a ...m~,en s or s-" ion. ~.hen the ionic latuce ~ h e t e ,t ~s ,ntroduced ,s formed b~ divalent
matead of m,:,n,,.alent Iona Tin~ ~s Ln quahtatt',e agreement ,.~.lth the present i n t e r p r e t a u o n or the O-~ band~ at Ga" and Ga:" m MgO a.n,.~a, it ahould be re,.alle.J. that the d~tterence bet'aeen dp. atent anJ mon,v, alent ionic lattices i, not ~,.~ draatlC a~ tt could De Interred from ti~e nominal charge of the m',ol,.ed vans -~nd ,,o tollo',,.mg Pinll,p~ ~omctr': ,tale the ab,c, lute ',alue~ at the charge carried out h,, each ~,.-,n h 0 ~. and l ;'~ for NaCI and \ l g O respectv, el', [ l ; 1 Though the pre~ent asa~gnment apaears ~o be rehahle more experimental mh~rmat~,:,n t, needed for a further c o n h r m a u o n Pamcularl', the \la_'netk Ctr,.ular D~chro~m technique ma,, be ,ratable ter ,u~.h a purp,.,~e
-1ckn,~,!edge,nents - I am indebted t,:, Dr -k E H u ~ e ~ for helptt,l dhcu,st,ons and tc, r p,~lntlng out to me the results about BI 3" CaO
REFERENCES
1 2
3
4 "~
b
W B Fo',~,ier, Ph)sws olC,JlorCentres p 314 A~adem,,. Press N e w ' ~ o r k ( 1 9 o ~ j \1 \ l o r e n o d Phvs C S, qtdStatePttvs. 12 L')21 IlCj "~) S o h d S r a r e C o m m t m 29 05~ i]o'Q) (3 E Hnlmberg K H L e e a . J H Crav. torJ Pitts Re~ BI9 2 4 3 , o 1 1 ~ ' q ) ( K JOrgensen,Prog In'r'~ C;te,',' 12, l o t i icY"0) W F r e ' , . R H u ~ , H Se,~.'l& E \~,erl.mann P h v ~ S t a m s S , , h d t d b ; 6 8 25 I i o " 5 ) P G Barano,. & ~ a, Maramt~o,. Soy P h ~ S ,,',d State 20 [ 0 S 0 1 1 q 7 ~ b C E Moore, 4 t o m t c E t k c g ~ Le~cl~ NBSCIrzuIar 4o'7, 110551 S C Jaln .-k V R l&arrter & S k -~:ar,.~.al Ele,.
trontc -t h~orptton ot [mp,~nta' Ce,,ters Dnyed ,r, 4li~ahHahdeCr3'stal~ Nat Bur Sr.md q 10 [I
NSRBSNBS52(19"4) J M L ~ d a l e k & W E Ba,,h,,)" P I , ~ B -1s,v,z ~l,lec Ph~s 12 1 1 1 3 1 1 " o 1 D Bramanu. M M a n c m i & A Ran:a~,nl P h ~ Rev B3 3o~0119"711 ~ E H u ~ e ~ & ~ 3 , A R u n ~ l r n a n , J Pltl~ C S, qzd
Stare Phvs 2, 3-' q19o9D 12 13
14 15
-~ E H u ~ h e ~ & G P Pell~ Ptlv:,.Srzn~sSohd,¢a.~ 25 4 3 " l l q ~ 4 ) J C Pinlhps Covalent Bonding m C,a'stals ~L~lecule~ and Pnlymers p 146 The Llnl'.'er_.,tt, of Cincago Press, Chicago I 1'9o0) Le Sl Dang, R R o m e s t a m & D Sm~km,Ph.vs. Rev BIB o 3 t o (1978) ~ Fukuda, H J Paua & \latsu~hama Z Phys.
B25 211 (19"b)
Sohd State Communicauons, Vol 38, pp 2 2 1 - 2 2 2 Pergamon Press Lid 1981 Pnnted m Great Bntam
THE ORIGIN OF THE OPTIC.XL -kBSORPTION BANDS OF Ga" .-LND Ga:" IN MgO M Moreno Departamento de Ftsica Fundamental. Facultad de Clencias Unlversldad de Santander. Santander Spare
(Recewed 4.Vovember 1980 b) S 4mehncLxl -kn interpretation of the optical absorption data on Ga" and Ga -'+ in \lgO. recently reported by Holmberg. Lee and Crawtord. IS proposed The peak at 3 3 5 e V o f G a z• M g O l s s h o w n t o b e c o n s i s t e n t w t t h a t r a n s l t l o n a r t s m g from the 4s "+ 4p transmon of free Ga :÷, whlle the peak at 4 13 eV ot Ga" MgO ts assigned to come from the 4s: ---*4s 4p (IPtl transtuon of free Ga" From these results ~t ts found that the &stance between the 4s and 4,o levels IS reduced b) a factor of about 0 4 m both systems This Iot~ reduction factor is mamly related to the strong bladelung effects appeanng In MgO
b~, recent EPR data ,,teldmg.f~ = 0 11 for TI:" KCI [5]
THE OPTICAL -~BSORPTION (OA) data concerning s and s2 ions In alkali halide cr?stals provide good lnform a u o n about covalenc} and lattice effects related to such tons [1 2] Quite recently Holmbergetal {31 have reported the first results of Ga + and Ga "e-* m MgO Their results clearly lndmate that Ga "2-" MgO gl~,es rise to an OA band peaked at 3 35 eV wtule a band appearing at 4 13 eV is due to Ga" MgO However no explanation about the ongm of both bands was raven In view ot tills we ha,,e tried to assign ~uch bands tn order to tnspe~.t the lattice effects due to MgO. a crystal formed b,, divalent ions It has recently been shown that the OA data of s-ions m tonic lattices can be well understood assummg that such ions rove rise to complexes v,nth the surroundm g a n i o n s m n e a r e s t n e l g h b o u r p o s l t l O n S [21 In the case of octahedral environment tbas model predicts three dipole allowed transmons denoted b~, tl~,lrr)-" a~g tl~(O) ~ arg and a'tg ~ tt~ where the latter arises from the s ' ~ p transition of the s-ton ~,tule the remainder are charge transter bands The positron ,,,, of the first charge transfer band r,ulTr) ~a~g is governed b~, the relation [2, 41 vrr
=
K('(A
-- X..~t)
( I )
Here K = 3 72 eV and X.a and g..u are the optical electronegattvrttes of the anion and of the metal simpurity respectively, its difference, "
a n d ] a = 0 0 8 forGa "~÷ KCl [6] where asusual, ta mean> the unpaired spin density onto a p,~ hg:md orbital Thus we can reasonabl.~ expect that vn ~,,lll be higher for Ga > KCI than tor TI -"÷ KCI Furthermore considering the optical electronegatt,,ltles of ox,,gen (3 2) and chlorine (3 0) [4] equation (1) and the preceding a r ~ m e n t lmpb, that t.,,, in Ga-'* MgO should be higher than 4 1 e \ , a fact which discards the band at 3 35 eV as a charge transfer transition Thus ~,,e assxgn s u c h a b a n d o t Ga:* M g O a s c o m l n g t r o m t h e 4s ---,-/-p transmon of free Ga:" located at lt'o = 5 22 eV [7] This assignment imphes a reduction factor k = W,'It'o equal to 0 4 for the present ,.ale ~,.h~ch l~ ~g mficantl~ Iov,er t.han the ,,alue k = 0 6, tound for TI:* KCI and Sn'-* KCI [2 8] ¢> far as the peak a t 4 13e~ ofGa" MgOisconcerned ~e bel,e~e it come~ from the 4s:( tSop~ 454p I I p l l transition ot free Ga" In fact the s: ions In alkali hahdes give rise to the so-called 4 . B . and C band> within the V - L W ran,,e [I. 8] These bands come from the S2 ( tSo ) ~ spt 3pi 3p: 'Pi ) transltlOn~ ot tree Ions, respecu~el,,, v~bale in these ions the charge transfer band, lie at h l ~ e r energies because of the Paull pnnclple In the case o f G a ~"the small ',alue ot its s p i n - o r b i t coefficient (~eo = 0 11 eX for tree G a ' j gwe.., rt~e to ratios between the o~cIllator strengths ot bands C and 4 oCabout 500 for Ga* doped alkah hahdes [11 Considering this fact and that the oscillator strength for the 4s ---*4p transition of free Ga -'+ is 0 9 [9] ~,,e find reliable the assignment for the peak at 4 23 e\ in Ga* MgO in ~new of the oscillator strength of such a peak being comparable to that of the peak at 3 35 m Ga 2. MgO Moreover. hkel,, due to the small concentration> of Ga + and Ga"* m \lgO. both peak.,, show optical densities le~s than 0 a {3]
~"1
THE O R I G I N OF THE O P T I C A L .-kBSORPTIO\ B-X,NDS OF Ga" -k_\D Ga:" IN ' q : O '~ oI 3~ No 3 The p o s m o n of the C band at a s-" ion m an ~omc latuce h ,3e~.nbed b> the e\presston [il E.
= h,'--" + 4
+
~
"
I'~t_
In tins expression E c means the posmon ot the C band v. hde G and ~ are the exchange and s p i n - o r b i t parameters respecuvel) and i~' denote~ the dffterence
betv, een the energ'., of the ~ and p one-electron le,.els mvol,,ed in the tranattions The G..¢ and W parameters lot a s: ion m tome Iatt,ces usuali.', take smaller ,,'aluea than those corresponding to the free s z ion [1] In the present case ,.~.here ~ G h presumahl', ~mall I.~o Go = 0 08 for tree G a ' ) e q u a u o n I 2) can be ,*ell a p p r o x i m a t e d b,,
E c = kh',}+qGo
I~,)
~,,here q ,, the reduction c o e f f i o e n t tot the ex,.hange integral and [1,'o = 7 38 eV and Go = I 3') eV correspond to f r e e G a * ['7] - ~ s t h e r e d u c u o n o n t h e e x c h a n o _ e integral ts ma,nl,, produced b~, covalencx [10l ~ e can obtaan a lo,.,.er hm~t for q m the present case b,., tak,ng q = 0 3. tin~ ,,alue beLng round for Ga* KCI [8] ',~ here as alread) pointed out. more ,.o',alent eftect~ are e x p e c t e d Thus accepting the propo~edaasL__.nment for t h e 4 1 3 e \ peak a t G a " b l g O t h e p r e ~ e n t a r g u m e n t ~ and e q u a t i o n q3 ) lmpl.,, that X should ha,.e a value I'.~mg b e t ~ e e n 0 5 and 0 4 Thouda thla ~alue xs si_~ntficantlx ~maller than that found for Ga ÷ and other mono~alent s and ~" tons in alkah hahdes where k = 0 ~ [ l , 2, 8] hov,e~er tt ta clo~e to, the ',alue a r k deduced tc, r Ga -'÷ MgO It ahould be recalled m tins ~a? that comparable ~ alue., a r k are tound for TI" IO " ; and TI z" 10 61 in KCI [ 1 . 2 . S] A atm,lar situation happena tar Pb"" and Ph 3. in KC1. v.here ~ lies clo~e to 0 6 m both caaes [2. g] On the ,other hand t h e ~ , a l u e o f k l o r h o t h G a : " MgOand Ga" .",lgO ~,,~tem~ h comparable to the value k " 0 35 inferred t,_,r B~s" CaO from the data reported b,, Hu~esetal [11.12] The pre~ent dh,.u~s~on ~ugge~t sL,_.n~ficant r e d u c u o n ettecta on h' for both Ga ÷ and Ga z+ m MgO. dlu~tratmg that tin~ reducuc, n ~ not maml', due to covalenc.~ a~ ~t u~ualb, h a p p e n a l o t G a n d ~ [101 It has b e e n p o m t e d out that the reduct,on on W b related to the more draatlC Madelung repulsion on the s level than on the more e x t e r n a l p l e v e l [ 1 ] F o l l o w m g t h a s a r g u m e n t t t can reasonabl.,, be expected to find more aagntt'icant reductions on W, tar a ...m~,en s or s-" ion. ~.hen the ionic latuce ~ h e t e ,t ~s ,ntroduced ,s formed b~ divalent
matead of m,:,n,,.alent Iona Tin~ ~s Ln quahtatt',e agreement ,.~.lth the present i n t e r p r e t a u o n or the O-~ band~ at Ga" and Ga:" m MgO a.n,.~a, it ahould be re,.alle.J. that the d~tterence bet'aeen dp. atent anJ mon,v, alent ionic lattices i, not ~,.~ draatlC a~ tt could De Interred from ti~e nominal charge of the m',ol,.ed vans -~nd ,,o tollo',,.mg Pinll,p~ ~omctr': ,tale the ab,c, lute ',alue~ at the charge carried out h,, each ~,.-,n h 0 ~. and l ;'~ for NaCI and \ l g O respectv, el', [ l ; 1 Though the pre~ent asa~gnment apaears ~o be rehahle more experimental mh~rmat~,:,n t, needed for a further c o n h r m a u o n Pamcularl', the \la_'netk Ctr,.ular D~chro~m technique ma,, be ,ratable ter ,u~.h a purp,.,~e
-1ckn,~,!edge,nents - I am indebted t,:, Dr -k E H u ~ e ~ for helptt,l dhcu,st,ons and tc, r p,~lntlng out to me the results about BI 3" CaO
REFERENCES
1 2
3
4 "~
b
W B Fo',~,ier, Ph)sws olC,JlorCentres p 314 A~adem,,. Press N e w ' ~ o r k ( 1 9 o ~ j \1 \ l o r e n o d Phvs C S, qtdStatePttvs. 12 L')21 IlCj "~) S o h d S r a r e C o m m t m 29 05~ i]o'Q) (3 E Hnlmberg K H L e e a . J H Crav. torJ Pitts Re~ BI9 2 4 3 , o 1 1 ~ ' q ) ( K JOrgensen,Prog In'r'~ C;te,',' 12, l o t i icY"0) W F r e ' , . R H u ~ , H Se,~.'l& E \~,erl.mann P h v ~ S t a m s S , , h d t d b ; 6 8 25 I i o " 5 ) P G Barano,. & ~ a, Maramt~o,. Soy P h ~ S ,,',d State 20 [ 0 S 0 1 1 q 7 ~ b C E Moore, 4 t o m t c E t k c g ~ Le~cl~ NBSCIrzuIar 4o'7, 110551 S C Jaln .-k V R l&arrter & S k -~:ar,.~.al Ele,.
trontc -t h~orptton ot [mp,~nta' Ce,,ters Dnyed ,r, 4li~ahHahdeCr3'stal~ Nat Bur Sr.md q 10 [I
NSRBSNBS52(19"4) J M L ~ d a l e k & W E Ba,,h,,)" P I , ~ B -1s,v,z ~l,lec Ph~s 12 1 1 1 3 1 1 " o 1 D Bramanu. M M a n c m i & A Ran:a~,nl P h ~ Rev B3 3o~0119"711 ~ E H u ~ e ~ & ~ 3 , A R u n ~ l r n a n , J Pltl~ C S, qzd
Stare Phvs 2, 3-' q19o9D 12 13
14 15
-~ E H u ~ h e ~ & G P Pell~ Ptlv:,.Srzn~sSohd,¢a.~ 25 4 3 " l l q ~ 4 ) J C Pinlhps Covalent Bonding m C,a'stals ~L~lecule~ and Pnlymers p 146 The Llnl'.'er_.,tt, of Cincago Press, Chicago I 1'9o0) Le Sl Dang, R R o m e s t a m & D Sm~km,Ph.vs. Rev BIB o 3 t o (1978) ~ Fukuda, H J Paua & \latsu~hama Z Phys.
B25 211 (19"b)