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A 19F endor study of copper-doped CdF2
Volume
70. number
1
CHEMICAL
A I9 F ENDOR STUDY OF COPPER-DOPED
PHYSICS
LETTERS
15 February
1980
CdF2
1-Y CHAN and R A. MUSHLIN Deparrmenr
of
Chenmtr~,
Waltham. d~assachusetts
Recelbed
19 November
Branders Unwemty.
0254.
USA
1979
The anIsotropy of the firstshell fluorme ENDOR IS utlhzed to elwdate the nature of the paramagnetlc speaes m copperdoped CdF2. The coordmatlon of the paramagnetlc center IS eight-fold cubic. The unusual orlentatlon of Its electronic g-tensor K e\plamed
1.
Introduction
Octahedrally coordmated cupric Ions have been exhausttvely studred by paramagnetrc resonance. In comparison, Cu?-+ situated rn an erght-fold cubic envrronment IS rare, but tt offers a convement test case for Jahn-Teller effect on a trrplet ground state. EPR of Cu7-+ m CaF? and CdF, were first reported by Zaripov et al. [I 1. Von Hoene et al. [2] suggested the existence of Jahn-Teller dtstortton for Cu2+ m CdF, based upon the stress dependence of its EPR mtenstty. These early works, however, left many rssues unanswered_ Firstly, m both CaF, and CdFZ hosts, the g-tensor is rhombrc, with prmclpal axes along the [I IO], [ liO] and [OOl ] drrectrons. As vibromc couphng of an orbital triplet with an eg mode leads to a tetragonal g-tensor along (100 1, and couplmg wrth a tlg mode results m a tngonal g along (I 1 I ), the observed-g-tensor rarses the questron as to the nature of the vtbronic couphng [3]. Furthermore, the usually promtnent four-lure copper hyperfiie structure was erther mrssmg [2], or the sphttmgs were so small that they were burred under the fluorme super hyperfine structure [I]. Instead, von Hoene et al [2] reported a stxlure hyperfine structure whtch they ascribed to a set of flue eqmvalent fluormes. Smce rt is known that CuF, IS unstable agamst reduction to Cu+ and Cue at the temperature of crystal growing [4], there are legttimate concerns as to the Identity of the paramagnetrc center m Cudoped CaF, and CdF,. In this paper, we report an ENDOR study of Cu : CdF, which sheds considerable light on these questions. 138
2. Experimental The EPR spectrometer was an X-band superheterodyne system featurmg angle-stdeband generatron of LO frequency from the srgnal klystron The experrments were conducted at 9 82 GHz and ~2 K The sample could be rotated about an axes perpendicular to the axrs of the magnettc field rotatton The ENDOR frequency source was a BCD-programmed frequency synthesizer (PRD 7828, 1 kHz-80 MHz) Above 80 MHz, a frequency doubler was also used RF current up to 1 A peak to peak was derived from two power amphfiers (EN1 310L) operated m parallel. Extensive use was made of a srgnal averager (Fabrr-tek 1062). CdF, crystals were obtamed from Optovac. All data were taken on a boule grown under HF and doped with 0 I% CuFZ. Destructrve analysrs (AA) of a typrcal fragment from the boule found a total copper concentration of 0.03 mole %.
3. Results The gross features of the EPR spectra of Cu-doped CdF, can be summarized by the principal g-values 2.73,2.09 and 2.13 m the [1 lo], [liO] and [OOl] dtrectrons respectively [2] _For our purpose it IS useful to consider the EPR spectrum with the magnetrc field along one of the face diagonals of the umt cell, hereafter called the [l lo] duectron. There are six magnettcally rnequtvalent sites u-r the crystal, corresponding to the SIX face diagonals of a cube Besides seeing a
CHEMICAL
Volume 70, number I
PHYSICS LETTERS
tron IS essentrally a princrpal axrs of the hyperfme tensor. It IS the short axis of the hyperfine elhpsord. These two emprrrcal facts impose a severe constramt on any model of the paramagnetic center m Cudoped CdF,. Based on the rather large g-values compared to the free-electron g, we rule out pure color centers as a possrbrhty of the paramagnetic species. Smce our EPR and ENDOR signals are present only after Cudoping, the paramagnetic center IS most likely Cu2+ in some coordination, or some Cu-associated defect centers. We have exarmned various possrbrhtres in vrew of our ENDOR results, as well as the onentation of the electronicg-tensor. The only mode1 that fits all the requuements, perhaps not so surpnsmgly, 1s a Cu2+ ion surrounded by 8 F- Ions located at the cube corners. Under this model, the ENDOR signals reported herein onginate from the four F- shown m fig. 3. It ISobvrous that the four F- remain equrvalent along any directton m the (00 1) and (110) plane. Furthermore, the [ 1 IO] direction 1s perpendrcular to the M-F duections, allowing it to be a common short prmcrpal axrs of the four hyperfine tensors Presumably, the other set of four F- ions will give rise to ENDOR signals related to the observed ones by the cubic symmetry. According to the (sunple) theory of transferred Fhyperfine mteractron [6], the hyperfme tensor of each fluorine nucleus IS characterized by two parameters A,, and A, with the cylmdrrcal axrs lying along the M-F dtrectron (a body diagonal). Thrs principal axes system 1s related to our lab frame ([l lo], [l TO], [OO11) by a sunple rotation of (Y= + 35 -26” around [l lo]. The matrix transformation is strarghtforward.
@
CU++
15 February 1980
If we neglect the very small off-diagonal elements shown in table 1, we may take A, =A ,,. = 632 Me. Ifwe further take All,-, =(cos2aA,, +sin20rA,)= 255.8 MHz, A,, = 352.09 MHz. Wrth these values of A,, and A,, we may calculateAOOt =(sin2arA,, + cos2a A,) = 159.52 MHz, which compares very favorably with the empirical value of 152 + 8 MHz. This degree of mtemal consistency is a strong indication that our model in fig. 3 is correct. It further corroborates our choice of the sign of A tto > 0. For if we take Art0 to be -63.2 MHz, and A ITo= 255.8 MHz as before, we calculate A,,, to be 96.3 MHz, which is defirte:y at odds wrth the observed value. With the essence of our model established, we may refine our data treatment in order to obtain the best values of A,, and A,. The major deficiency of eq. (1) is that it assumes the nuclei to be quantized aiong the external field, whereas in reality they are subject also to the influence of the hyperfine magnetic field. If we take the experrmental ENDOR frequency at [I 101 and [I ]OJ, and go through the treatment of Ranon and Hyde [7], we obtain A,, = 313.14 and A, = 6254 MHz. Note that thts two-stage refinement process is necessary, for the formahsm of Ranon and Hyde [-I] provrdes no umque solutron unless the sign of various quantities IS known. These tensorial values represent a very large hyperfme interactron. To our knowledge a detarled theory of transferred hyperfme interaction between a transition metal and F- ligands in cubic coordmatron has not been developed_ Last but not least we wish to comment on the PMcrpal axes orientation of the electronicg-tensor. it was this unusual rhombrc tensor along the facediagonal that brought our attention to the present problem (see section 1). Thrs unconventional g may be associated with the comphcated fine structure of our ENDOR spectra exemplified by fig. l_ We believe this fme structure isnot the “multiplet structure”of the Feuchtwang type [S]. Instead it IS a manifestation of static Jahn-Teller distortion. It is tempting to speculate that the three strong doub!ets correspond to the six possible static drstortrons of the cube of fluoride ions along the facediagonals. An orbital triplet state, such as the Cu2+ d9 system, may viironically couple to an eg and/or t2g mode. Couplmg to an e mode leads to a tetragonalg along
Fg. 3. SchematIc model of the paramagnetrccenter. The four F-k shown give rise to the ENDOR data shown III fii. 2. F-k on the other four cube-comers are not shown for clarity.
[ltXl],andcouphgtoa t 2g mode results m a trigonal g along [l Ill. If both eg and tk modes are coupled, in genera1 either the tetragonal or trigonal distortion
CHEMICAL PHYSICS LETTERS
Volume 70, number 1
has the rninunum energy, whereas the other dlstortlon is a maximum. In either case, the [I lo] direction 1s a saddle pomt on the potential energy surface. O’Brien [9] has pomted out, however, that under some circumstances, the tetragonal and trigonal &stortlon, as well as the saddle pomt at [I IO] ail have the
Acknowledgement
same energy. The condltlon duced Jahn-Teller energy,
References
v;/?/l,w~
1s an equality of the reor, more specifically,
= 2 v; /3p,o;,
where V ISthe potential energy, p the effective mass, o the vibrational frequency, and the subscripts e and t denotes the eg and t2g modes respectlkely. Under this condition, there exists a twodlmenslonal region of Q-space connectmg the above three points of mterest m which the energy remains constant. It IS there-
fore plausible that, under the considerable strain [4] of doping copper into CdF, [4], the [ 1 lo] distortion actually assumes the lowest energy. While it is dlscomfottmg mg
to count
on a numerical
comcldence
tn explam-
the gross-feature of an EPR spectrum, our clearcut ENDOR results suggest that the overall picture IS correc :t.
142
15 February 1980
Thus research was supported m part by the Natlonai Scrence Foundation (DMR 18095).
111 hI hf Zarlpov, V S Kropotov,
L D. Lwanova and V G. Stepanov, Soviet Phys Sohd State 9 (1968) 2347; 10 (1969) 2122. PI D C von Hoene and R.C Fedder, Phys Letters 30A (1969) 1. 131 F S Ham- m Electron paramagnetlc resonance, ed S Geschwmd (Plenum Press, New York, 1969) p. 1. [41 H. von Waterberg, Z Anorg Alig Chem. 241 (1939) 393 [51 CA Hutchson Jr and G.A. Pearson, J Chem Phys 47 (1967) 520. [61 J. Owen and J H hl. Thornley, Rept. Progr Phys 29 (1966) 675, and references therem 171 U Ranon and J S Hyde, Phys Rev. 141 (1966) 259 Phys Rev 126 (1962) 1628 181 TE Feuchtwang, 191 M.C.M. O’Brien, Phys. Rev. 187 (1969) 407
70. number
1
CHEMICAL
A I9 F ENDOR STUDY OF COPPER-DOPED
PHYSICS
LETTERS
15 February
1980
CdF2
1-Y CHAN and R A. MUSHLIN Deparrmenr
of
Chenmtr~,
Waltham. d~assachusetts
Recelbed
19 November
Branders Unwemty.
0254.
USA
1979
The anIsotropy of the firstshell fluorme ENDOR IS utlhzed to elwdate the nature of the paramagnetlc speaes m copperdoped CdF2. The coordmatlon of the paramagnetlc center IS eight-fold cubic. The unusual orlentatlon of Its electronic g-tensor K e\plamed
1.
Introduction
Octahedrally coordmated cupric Ions have been exhausttvely studred by paramagnetrc resonance. In comparison, Cu?-+ situated rn an erght-fold cubic envrronment IS rare, but tt offers a convement test case for Jahn-Teller effect on a trrplet ground state. EPR of Cu7-+ m CaF? and CdF, were first reported by Zaripov et al. [I 1. Von Hoene et al. [2] suggested the existence of Jahn-Teller dtstortton for Cu2+ m CdF, based upon the stress dependence of its EPR mtenstty. These early works, however, left many rssues unanswered_ Firstly, m both CaF, and CdFZ hosts, the g-tensor is rhombrc, with prmclpal axes along the [I IO], [ liO] and [OOl ] drrectrons. As vibromc couphng of an orbital triplet with an eg mode leads to a tetragonal g-tensor along (100 1, and couplmg wrth a tlg mode results m a tngonal g along (I 1 I ), the observed-g-tensor rarses the questron as to the nature of the vtbronic couphng [3]. Furthermore, the usually promtnent four-lure copper hyperfiie structure was erther mrssmg [2], or the sphttmgs were so small that they were burred under the fluorme super hyperfine structure [I]. Instead, von Hoene et al [2] reported a stxlure hyperfine structure whtch they ascribed to a set of flue eqmvalent fluormes. Smce rt is known that CuF, IS unstable agamst reduction to Cu+ and Cue at the temperature of crystal growing [4], there are legttimate concerns as to the Identity of the paramagnetrc center m Cudoped CaF, and CdF,. In this paper, we report an ENDOR study of Cu : CdF, which sheds considerable light on these questions. 138
2. Experimental The EPR spectrometer was an X-band superheterodyne system featurmg angle-stdeband generatron of LO frequency from the srgnal klystron The experrments were conducted at 9 82 GHz and ~2 K The sample could be rotated about an axes perpendicular to the axrs of the magnettc field rotatton The ENDOR frequency source was a BCD-programmed frequency synthesizer (PRD 7828, 1 kHz-80 MHz) Above 80 MHz, a frequency doubler was also used RF current up to 1 A peak to peak was derived from two power amphfiers (EN1 310L) operated m parallel. Extensive use was made of a srgnal averager (Fabrr-tek 1062). CdF, crystals were obtamed from Optovac. All data were taken on a boule grown under HF and doped with 0 I% CuFZ. Destructrve analysrs (AA) of a typrcal fragment from the boule found a total copper concentration of 0.03 mole %.
3. Results The gross features of the EPR spectra of Cu-doped CdF, can be summarized by the principal g-values 2.73,2.09 and 2.13 m the [1 lo], [liO] and [OOl] dtrectrons respectively [2] _For our purpose it IS useful to consider the EPR spectrum with the magnetrc field along one of the face diagonals of the umt cell, hereafter called the [l lo] duectron. There are six magnettcally rnequtvalent sites u-r the crystal, corresponding to the SIX face diagonals of a cube Besides seeing a
CHEMICAL
Volume 70, number I
PHYSICS LETTERS
tron IS essentrally a princrpal axrs of the hyperfme tensor. It IS the short axis of the hyperfine elhpsord. These two emprrrcal facts impose a severe constramt on any model of the paramagnetic center m Cudoped CdF,. Based on the rather large g-values compared to the free-electron g, we rule out pure color centers as a possrbrhty of the paramagnetic species. Smce our EPR and ENDOR signals are present only after Cudoping, the paramagnetic center IS most likely Cu2+ in some coordination, or some Cu-associated defect centers. We have exarmned various possrbrhtres in vrew of our ENDOR results, as well as the onentation of the electronicg-tensor. The only mode1 that fits all the requuements, perhaps not so surpnsmgly, 1s a Cu2+ ion surrounded by 8 F- Ions located at the cube corners. Under this model, the ENDOR signals reported herein onginate from the four F- shown m fig. 3. It ISobvrous that the four F- remain equrvalent along any directton m the (00 1) and (110) plane. Furthermore, the [ 1 IO] direction 1s perpendrcular to the M-F duections, allowing it to be a common short prmcrpal axrs of the four hyperfine tensors Presumably, the other set of four F- ions will give rise to ENDOR signals related to the observed ones by the cubic symmetry. According to the (sunple) theory of transferred Fhyperfine mteractron [6], the hyperfme tensor of each fluorine nucleus IS characterized by two parameters A,, and A, with the cylmdrrcal axrs lying along the M-F dtrectron (a body diagonal). Thrs principal axes system 1s related to our lab frame ([l lo], [l TO], [OO11) by a sunple rotation of (Y= + 35 -26” around [l lo]. The matrix transformation is strarghtforward.
@
CU++
15 February 1980
If we neglect the very small off-diagonal elements shown in table 1, we may take A, =A ,,. = 632 Me. Ifwe further take All,-, =(cos2aA,, +sin20rA,)= 255.8 MHz, A,, = 352.09 MHz. Wrth these values of A,, and A,, we may calculateAOOt =(sin2arA,, + cos2a A,) = 159.52 MHz, which compares very favorably with the empirical value of 152 + 8 MHz. This degree of mtemal consistency is a strong indication that our model in fig. 3 is correct. It further corroborates our choice of the sign of A tto > 0. For if we take Art0 to be -63.2 MHz, and A ITo= 255.8 MHz as before, we calculate A,,, to be 96.3 MHz, which is defirte:y at odds wrth the observed value. With the essence of our model established, we may refine our data treatment in order to obtain the best values of A,, and A,. The major deficiency of eq. (1) is that it assumes the nuclei to be quantized aiong the external field, whereas in reality they are subject also to the influence of the hyperfine magnetic field. If we take the experrmental ENDOR frequency at [I 101 and [I ]OJ, and go through the treatment of Ranon and Hyde [7], we obtain A,, = 313.14 and A, = 6254 MHz. Note that thts two-stage refinement process is necessary, for the formahsm of Ranon and Hyde [-I] provrdes no umque solutron unless the sign of various quantities IS known. These tensorial values represent a very large hyperfme interactron. To our knowledge a detarled theory of transferred hyperfme interaction between a transition metal and F- ligands in cubic coordmatron has not been developed_ Last but not least we wish to comment on the PMcrpal axes orientation of the electronicg-tensor. it was this unusual rhombrc tensor along the facediagonal that brought our attention to the present problem (see section 1). Thrs unconventional g may be associated with the comphcated fine structure of our ENDOR spectra exemplified by fig. l_ We believe this fme structure isnot the “multiplet structure”of the Feuchtwang type [S]. Instead it IS a manifestation of static Jahn-Teller distortion. It is tempting to speculate that the three strong doub!ets correspond to the six possible static drstortrons of the cube of fluoride ions along the facediagonals. An orbital triplet state, such as the Cu2+ d9 system, may viironically couple to an eg and/or t2g mode. Couplmg to an e mode leads to a tetragonalg along
Fg. 3. SchematIc model of the paramagnetrccenter. The four F-k shown give rise to the ENDOR data shown III fii. 2. F-k on the other four cube-comers are not shown for clarity.
[ltXl],andcouphgtoa t 2g mode results m a trigonal g along [l Ill. If both eg and tk modes are coupled, in genera1 either the tetragonal or trigonal distortion
CHEMICAL PHYSICS LETTERS
Volume 70, number 1
has the rninunum energy, whereas the other dlstortlon is a maximum. In either case, the [I lo] direction 1s a saddle pomt on the potential energy surface. O’Brien [9] has pomted out, however, that under some circumstances, the tetragonal and trigonal &stortlon, as well as the saddle pomt at [I IO] ail have the
Acknowledgement
same energy. The condltlon duced Jahn-Teller energy,
References
v;/?/l,w~
1s an equality of the reor, more specifically,
= 2 v; /3p,o;,
where V ISthe potential energy, p the effective mass, o the vibrational frequency, and the subscripts e and t denotes the eg and t2g modes respectlkely. Under this condition, there exists a twodlmenslonal region of Q-space connectmg the above three points of mterest m which the energy remains constant. It IS there-
fore plausible that, under the considerable strain [4] of doping copper into CdF, [4], the [ 1 lo] distortion actually assumes the lowest energy. While it is dlscomfottmg mg
to count
on a numerical
comcldence
tn explam-
the gross-feature of an EPR spectrum, our clearcut ENDOR results suggest that the overall picture IS correc :t.
142
15 February 1980
Thus research was supported m part by the Natlonai Scrence Foundation (DMR 18095).
111 hI hf Zarlpov, V S Kropotov,
L D. Lwanova and V G. Stepanov, Soviet Phys Sohd State 9 (1968) 2347; 10 (1969) 2122. PI D C von Hoene and R.C Fedder, Phys Letters 30A (1969) 1. 131 F S Ham- m Electron paramagnetlc resonance, ed S Geschwmd (Plenum Press, New York, 1969) p. 1. [41 H. von Waterberg, Z Anorg Alig Chem. 241 (1939) 393 [51 CA Hutchson Jr and G.A. Pearson, J Chem Phys 47 (1967) 520. [61 J. Owen and J H hl. Thornley, Rept. Progr Phys 29 (1966) 675, and references therem 171 U Ranon and J S Hyde, Phys Rev. 141 (1966) 259 Phys Rev 126 (1962) 1628 181 TE Feuchtwang, 191 M.C.M. O’Brien, Phys. Rev. 187 (1969) 407