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Endor study of Gd3+ in Cs2 NaYCl6
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
3 February 1984
ENDOR STUDY OF Gd 3+ IN Cs2NaYCI 6 H. BILL, G. MAGNE Department o f Physical Chemistry, Sciences 11, University o f Geneva, 30 Quai E. Ansermet, 1211 Geneva 4, Switzerland and H.U. GODEL and K. NEUENSCHWANDER Department o f Inorganic and Physical Chemistry, University o f Berne, Freiestrasse 3, 3012 Berne, Switzerland Received 3 October 1983; in final form 2 November 1983
CrystalsofCs2NaYC16 dopedwith Gda+wereinvestigated using EPR and ENDOR spectroscpy. The Gd 3+ site symmetry at 4.2 K is found to be cubic. The lattice does not undergo a structural phase change between 300 and 4.2 K. Literature ENDOR data for Cs2NaYC16 doped with Cea+ are re-interpreted.
1. Introduction The cubic elpasolite compound Cs2NaYC16 provides an ideal host lattice for the study o f trivalent transition-metal and lanthanide ions in an exactly octahedral environment. Unlike some members o f the family Cs2NaLnC16 (Ln = lanthanide), the yttrium compound does not appear to undergo a structural phase change at low temperatures. This conclusion was reached on the basis o f EPR spectroscopy on crystals doped with Ce 3+, Dy 3+ and Gd 3+ [1,2], as well as from magnetic circular dichroism measurements on the Ce3+-doped system [3]. In contrast, the authors o f a more recent ENDOR study o f Ce3+-doped Cs2NaYC16 concluded that a non-cubic distortion does occur at low temperature [4]. The deformation was reported to be so small that it could only be observed by using ENDOR, but not by using EPR. Our ENDOR investigation of Gd 3+ in Cs2NaYC16 was undertaken in order to study this phase change. In particular, we were interested in whether the reported phase change is an intrinsic property of Cs2NaYC16, or whether it depends on the nature o f the trivalent paramagnetic ion used to probe it. Our failure to fred any evidence for a non-cubic distor258
tion in Gd3+-doped Cs2NaYC16 at 4.2 K prompted us to closely examine the Ce 3+ data in ref. [4]. This led to an interpretation different from that presented earlier.
2. EPR results Cs2NaYC16 crystallizes in space group 0 5 - Fro3 m at room temperature. The cubic face-centered structure is illustrated in fig. 1. The yttrium ion is surrounded by six Ct- ions forming an exact octahedron, then by eight Cs÷ ions located on the apices o f a cube. The next shell consists o f six Na + ions forming an octahedron which is coaxial with that o f the C1- ions. Trivalent transition-metal and lanthanide ions substitute for y3+. Single crystals o f Cs2NaYC16 containing Fe 3+ (0.1%) and Gd 3÷ ( ~ 100 ppm) were grown in a Bridgman-type furnace. The samples were cut with a string saw and oriented with the aid of the Laue Xray technique. EPR data were recorded on a modified Varian Eline spectrometer with associated cryogenic equipment, at typically 9.35 GHz. A few spectra were also 0 009-2614/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS A
i O
Gd
i
i
¼
Ocl @ Cs •
Na
//
Fig. 1. Structure of the cubic unit cell of Cs2NaYC16 . recorded on our self-made 35 GHz spectrometer. The ENDOR experiments were performed on the apparatus described previously [5]. Spectra o f the Gd 3+ ion were recorded with the magnetic field rotating in (110) and (100) type planes and at 282, 77 and 4.2 K. They always consist o f one set o f seven lines. Thus, the EPR spectrum is cubic. This is further confirmed b y the successful parameterization o f its angular dependence using the conventional cubic spin hamiltonian
c~ = g~o It" S + b4(O 0 + 5 04) + b6(O 0 - 2 1 0 4 ) +~
#
(S'AU'Iu+Iu'PUlu-gu~NH'I).
(1)
3 February 1984
Table 1 EPR parameters of Gd 3+ in Cs 3+ in Cs2NaYC16 T(K)
g
b4
b6
78 4.2
1.9932 (0.001) 1.993 (0.0015)
-17.6 (0.9) -20.8 (1.0)
+0.69 (0.3) +0.6 (0.35)
The experimental results show clearly that the electronic Zeeman term is much larger than the remaining ones. Thus, the states can approximately be labeled by [SMS) where H defines the axis o f quantification. The detailed calculations were performed using secondorder perturbation theory and then by direct computer diagonalization. The spectra along [ 100], [ 111 ] and [110] were fitted individually to eq. (1) with the aid o f a successive approximation procedure. Weighted averages o f the constants thus determined were then calculated. Finally, their validity was further checked by calculating the spectra for several directions o f H (ll (110)) and comparing them successfully with the corresponding experimental spectra. The last sum in eq. (1)represents the super-hyperfine interaction terms used only for the ENDOR part o f the study. The experimental parameters g, b 4 and b 6 are given in table 1. The signs o f the crystal-field coefficients were obtained from the 4.2 K spectrum with H parallel to [001]. The coefficients are only weakly temperature dependent, thus justifying the omission o f the 300 K results from table 1. The spin hamiltonian parameters are shown in table 2. Several other cubic Gd 3+ centers are included
Table 2 b 4 crystal-field parameters of several cubic Gd 3 ÷ centers and associated Newman-model parameters obtained using the relations [10 ] b4 = -(9/28)b4, cubic surrounding (cub.); b4 = (2/7)b4, octahedral surrounding (oct.) Host
ao a) (A)
b4 (10 -4 cm -1)
b4 (10 -4 cm -1)
Local symm.
Ref.
Cs2 NaY C16 CaO SrO CaF2 SrF2 BaF2 SrC12 ThO2
2.68 2.4 2.58 2.35 2.53 2.67 2.6
-5.9 -3£ -1.4 15.0 13.0 11.4 4.8 18.0
- 17.6 -12.4 -4.9 -46.6 -40.7 -35.0 -14.9 -56
oct. oct. oct. cub. cub. cub. cub. cub.
this work, [1 ] [61 [61 [7] [71 [71 [81 [91
a) Distance from Gd 3+ to nearest neighbor. 259
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
in table 2 for comparison. C o m m o n to all is the comparatively small b 4 (and b6) coefficient w h i c h is clearly not a simple f u n c t i o n o f the lattice constant. The same remark also applies to the N e w m a n parameters [10] w h i c h are defined and given in table 2. Ano t h e r n o t e w o r t h y fact is that b 4 has a negative sign for b o t h the cubic and the octahedral surrounding.
3. Ligand E N D O R The E P R s p e c t r u m o f Gd 3+ saturates easily at low t e m p e r a t u r e s and the best E N D O R signals were ob-
3 February 1984
rained at 1.5/~W input microwave power at 4.2 K. The crystals at our disposal all contained Fe 3+ in a d d i t i o n to the Gd 3+ centers. The EPR spectra o f these ions overlap for all directions o f H. In o r d e r to separate the c o n t r i b u t i o n s o f the two ions, E N D O R spectra were recorded on all o f the seven E P R transitions o f Gd 3+ with H oriented along each o f the three principal crystal axes ( [ 1 0 0 ] , [110], [111]). In addit i o n , the angular d e p e n d e n c e o f some o f the E N D O R lines was studied (on the ( - ~a / - ~) 1 E P R transition) w i t h H rotating b o t h in a (100) and in a (110) plane. Two sets o f lines due to Gd 3÷ were identified (see figs 2 and 3). One arises f r o m the interaction w i t h the Cs +
lOOkHz ~ ,
50 kHz
'~c~
~
1
o "1Na ~
H IIC4 i ,
H II C2
"2Na=4115MHz
r"~~1f21-3¢2)
( 1/2 I-1/2) 90" ( - 3 / 2 ) ( - 1 / 2 ) 1 1 J i l l 0° (-1/2) (-3/2)
,2Cs=1.871 MHz
13 90 35.26
I I (-1/2) (1/2) I I (1/2)(-1/2)
a
a
HIIC3
lOOkHz
(3/215/2) 19Na
SOkHz
,#Na=3.606 MHz
L_J
H II C3 (3/211/2) qCs=1.947MHz
~Cs
50 kHz ~-~ ~Na I I " |
b
1__] O" (3/2) (1/2) 70.53"
8 [
t
(1/2)(3/2)
Fig. 2. ENDOR spectra due to the neighboring Cs+ nuclei. Experiments performed at 4.2 K. The transitions are labelled by the angle between H and the principal axis (a Ca axis) of one shf tensor, and by the M S value of the dominant contribution to the actual electronic state involved. (a)//11 C2. Two pairs of lines are expected when the shf tensor is axial. The center part contains barely resolved features due to more distant Cs÷ nuclei. (b) H_IIC a • The parallel lines are rather sharp. The splitting of the +5, eta = 70.2 transition is due to a slight misalignment of the magnetic field with respect to the C3 axis. 260
b
(3/2) (5/2)
0 45
II I El I (-1/2) _ (1/2) (1/~X-1~)
HIIC2 (1/21-1~)
C
Fig. 3. ENDOR spectra due to the neighboring Na + nuclei. Experimental conditions and labeling as explained in fig. 2. (a)//II C4. This spectrum displays dearly four sets of three lines. The pair below v0 is due to the perpendicular component of the tetragonal shf tensor and the pair above is due to the parallel component. The line labeled X is a pulse. (b) H IIC3. The quadrupolar splitting is zero along this direction and one line only is observed for each M S value (thus justifying eq. (3)). (c)//IIC 2 . The spectrum compares directly with the one labeled (a) in this figure. The 45 ° components are localized at vo .
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
3 February 1984
Table 3 ENDOR results for the M3÷-Na + and the M3+-Cs + (M 3+ = Gd 3+, Ce 3+) nearest-neighbor interactions in Cs2NaYC16: spin hamiltonian parameters (in kHz) a) M3÷
Neighbour nucleus
Gd 3+
Na ÷
Gd 3÷
Cs ÷
Ce 3 +
Na ÷
I 3 5 7 5 3 ~
A~I
A~
AsUb)
A~ c)
p~
+218 (12)
- 1 8 0 (15)
-47
1 34
+25.8
+190 (l 5)
- 1 3 0 (18)
-24.6
106.6
+228 (10)
-28
+80.8
a) Principal axis along [ 111 ] for Cs ÷ and along [ 100 ] for Na ÷. 1
b) As = -~(All + 2A±). c) A p = A s - A±.
neighbors. The set shows an axial angular dependence with the principal axis of the super-hyperfme tensor parallel to a trigonal axis of the crystal. An accurate parameterization of the angular dependence was obtained with the equation
v~ = [v~ - Ms(A~cos20.+ A~sin2O u)l= [vr~[ (/1 = 1, ..., 8 ) ,
(2)
where v~ is the free precession frequency of the nucleus involved (the Cs nucleus in this case) and A~I and A~ are the components of the axial shf tensor. Finally, the angle 0g is measured between H and the principal axis of the tensor. The experimental parameters are given in table 3. The fact that (2) provides a good description of the ENDOR line positions can be traced back to the experimental result that the ISM S) quantization of the electronic spin states is reasonably good. The only major deviation from this model is the observation in several spectra of ENDOR lines which are not due to the two M S states directly involved in the monitored EPR transition (see, for example, fig. 2). No quadrupolar splittings were observed in these spectra. However, this fact is not a strong argument in favor of almost unperturbed local cubic symmetry at the Cs sites adjacent to the Gd 3+ ions because the quadrupole coupling constant is rather small ( ~ 47 times smaller than that of sodium). The other set of lines observed in the ENDOR spectra is due to the neighboring Na + nuclei. The angular dependence of these lines is very well explained under the assumption that super-hyperfine and quadrupole interactions are present. Two triplets are observed when H is parallel to a fourfold crystal axis
(see fig. 3). We verified that the triplet splittings are not due to any misalignment of the crystal. Indeed, splittings of the ENDOR lines due to this latter effect are readily produced (see fig. 2), but are also readily recognized. The very narrow lines (a few kHz wide) split when the dewar is tilted only 0.5 ° away from a [100] axis and/or out of a symmetry plane. The formal parameterization of the spectra was performed with the aid of v({ 1½) = [Vr~+ ~P~I(3 cos20 u - 1)i,
1
2
- i P i l ( 3 c o s 0 ~ - 1)i, (/2 = 1, ... 6),
(3)
which are first-order expressions of the shf and quadrupole interactions within the model given above. This approach is well justified because the nuclear Zeeman energy term is much larger than the others. The transitions are labeled by the two nuclear states involved, Vr~ is the term of eq. (2) (for Na), and PII is the (only independent) matrix element (/°33) of the axial quadrupole tensor (eq. (1)). The experimental parameters are given in table 3. Both the shf and the quadrupole tensor are axial and have their principal axis oriented parallel to a fourfold crystal direction. These results provide compelling evidence that the local site symmetry of the Gd 3+ ion is cubic. On the basis of these results, we also expect a quadrupolar splitting of the Na + lines in the ENDOR data of Ce3+-doped Cs2NaYC16 in ref. [4]. This leads to a new, and in our view more plausible, interpreta261
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
tion o f the spectra and their angular dependence (figs. 4, 5 and 6 in ref. [4]). Indeed,we find that their spectra are reproduced very well by our eqs. (3) and the parameters given in table 3 of this letter under the heading "Ce 3+-. The principal axes of both the shf tensor and the quadrupole tensor are oriented parallel to a fourfold crystal axis. A closer look at figs. 4 and 6 o f ref. [4] convincingly shows two pairs o f three lines when HII C4 . The partners o feach pair correspond 1 1 to the M S = ~ and - ~ electromc Zeeman states, respectively, of the Ce 3+ ion. Further, when HIIC 3 only two lines remain, because the quadrupolar splitting is zero for this direction. If this interpretation is accepted, it is clear that the Ce 3+ ion is also on a cubic site at 4.2 K in Cs2NaYC16 . Thus, there appears to be no evidence of a phase transition of this host between room temperature and 4.2 K.
Acknowledgement Some o f the samples were oriented by Mr. Navarro.
262
3 February 1984
The drawing (fig. 1) is due to Mr. Chambaz. This work was supported by the Swiss National Science Foundation.
References [1 ] R.W. Schwartz and N.J. Hill, J. Chem. Soc. Faraday Trans. II 70 (1974) 124. [2] R.W. Schwartz, S.W. Watkins, C.J. O'Connor and R.L. Carlin, J. Chem. Soc. Faraday Trans. II 72 (1976) 565. [3] R.W. Schwartz and P.N. Schatz, Phys. Rev. B8 (1973) 3229. [4] G~E. Fish and H.J. Stapleton, J. Chem. Phys. 69 (1978) 4068. [5 ] C. Balestra, H. Bill and J .L. Clignez, J. Phys. E12 (1979) 824. [6] M.M. Abraham, L.A. Boatner, Y. Chen, J .L. Kolopus and R.W. Reynolds, Phys. Rev. B4 (1971) 2853. [7] N. Miyata and B.E. Argyle, Phys. Rev. 157 (1967) 448. [8 ] R.W. Reynolds, L.A. Boatner and M.M. Abraham, J. Chem. Phys. 52 (1970) 3851. [9] M.M. Abraham and L.A. Boatner, J. Chem. Phys. 51 (1969) 3134. [10] DJ. Newman, J. Phys. C8 (1975) 1862.
CHEMICAL PHYSICS LETTERS
3 February 1984
ENDOR STUDY OF Gd 3+ IN Cs2NaYCI 6 H. BILL, G. MAGNE Department o f Physical Chemistry, Sciences 11, University o f Geneva, 30 Quai E. Ansermet, 1211 Geneva 4, Switzerland and H.U. GODEL and K. NEUENSCHWANDER Department o f Inorganic and Physical Chemistry, University o f Berne, Freiestrasse 3, 3012 Berne, Switzerland Received 3 October 1983; in final form 2 November 1983
CrystalsofCs2NaYC16 dopedwith Gda+wereinvestigated using EPR and ENDOR spectroscpy. The Gd 3+ site symmetry at 4.2 K is found to be cubic. The lattice does not undergo a structural phase change between 300 and 4.2 K. Literature ENDOR data for Cs2NaYC16 doped with Cea+ are re-interpreted.
1. Introduction The cubic elpasolite compound Cs2NaYC16 provides an ideal host lattice for the study o f trivalent transition-metal and lanthanide ions in an exactly octahedral environment. Unlike some members o f the family Cs2NaLnC16 (Ln = lanthanide), the yttrium compound does not appear to undergo a structural phase change at low temperatures. This conclusion was reached on the basis o f EPR spectroscopy on crystals doped with Ce 3+, Dy 3+ and Gd 3+ [1,2], as well as from magnetic circular dichroism measurements on the Ce3+-doped system [3]. In contrast, the authors o f a more recent ENDOR study o f Ce3+-doped Cs2NaYC16 concluded that a non-cubic distortion does occur at low temperature [4]. The deformation was reported to be so small that it could only be observed by using ENDOR, but not by using EPR. Our ENDOR investigation of Gd 3+ in Cs2NaYC16 was undertaken in order to study this phase change. In particular, we were interested in whether the reported phase change is an intrinsic property of Cs2NaYC16, or whether it depends on the nature o f the trivalent paramagnetic ion used to probe it. Our failure to fred any evidence for a non-cubic distor258
tion in Gd3+-doped Cs2NaYC16 at 4.2 K prompted us to closely examine the Ce 3+ data in ref. [4]. This led to an interpretation different from that presented earlier.
2. EPR results Cs2NaYC16 crystallizes in space group 0 5 - Fro3 m at room temperature. The cubic face-centered structure is illustrated in fig. 1. The yttrium ion is surrounded by six Ct- ions forming an exact octahedron, then by eight Cs÷ ions located on the apices o f a cube. The next shell consists o f six Na + ions forming an octahedron which is coaxial with that o f the C1- ions. Trivalent transition-metal and lanthanide ions substitute for y3+. Single crystals o f Cs2NaYC16 containing Fe 3+ (0.1%) and Gd 3÷ ( ~ 100 ppm) were grown in a Bridgman-type furnace. The samples were cut with a string saw and oriented with the aid of the Laue Xray technique. EPR data were recorded on a modified Varian Eline spectrometer with associated cryogenic equipment, at typically 9.35 GHz. A few spectra were also 0 009-2614/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS A
i O
Gd
i
i
¼
Ocl @ Cs •
Na
//
Fig. 1. Structure of the cubic unit cell of Cs2NaYC16 . recorded on our self-made 35 GHz spectrometer. The ENDOR experiments were performed on the apparatus described previously [5]. Spectra o f the Gd 3+ ion were recorded with the magnetic field rotating in (110) and (100) type planes and at 282, 77 and 4.2 K. They always consist o f one set o f seven lines. Thus, the EPR spectrum is cubic. This is further confirmed b y the successful parameterization o f its angular dependence using the conventional cubic spin hamiltonian
c~ = g~o It" S + b4(O 0 + 5 04) + b6(O 0 - 2 1 0 4 ) +~
#
(S'AU'Iu+Iu'PUlu-gu~NH'I).
(1)
3 February 1984
Table 1 EPR parameters of Gd 3+ in Cs 3+ in Cs2NaYC16 T(K)
g
b4
b6
78 4.2
1.9932 (0.001) 1.993 (0.0015)
-17.6 (0.9) -20.8 (1.0)
+0.69 (0.3) +0.6 (0.35)
The experimental results show clearly that the electronic Zeeman term is much larger than the remaining ones. Thus, the states can approximately be labeled by [SMS) where H defines the axis o f quantification. The detailed calculations were performed using secondorder perturbation theory and then by direct computer diagonalization. The spectra along [ 100], [ 111 ] and [110] were fitted individually to eq. (1) with the aid o f a successive approximation procedure. Weighted averages o f the constants thus determined were then calculated. Finally, their validity was further checked by calculating the spectra for several directions o f H (ll (110)) and comparing them successfully with the corresponding experimental spectra. The last sum in eq. (1)represents the super-hyperfine interaction terms used only for the ENDOR part o f the study. The experimental parameters g, b 4 and b 6 are given in table 1. The signs o f the crystal-field coefficients were obtained from the 4.2 K spectrum with H parallel to [001]. The coefficients are only weakly temperature dependent, thus justifying the omission o f the 300 K results from table 1. The spin hamiltonian parameters are shown in table 2. Several other cubic Gd 3+ centers are included
Table 2 b 4 crystal-field parameters of several cubic Gd 3 ÷ centers and associated Newman-model parameters obtained using the relations [10 ] b4 = -(9/28)b4, cubic surrounding (cub.); b4 = (2/7)b4, octahedral surrounding (oct.) Host
ao a) (A)
b4 (10 -4 cm -1)
b4 (10 -4 cm -1)
Local symm.
Ref.
Cs2 NaY C16 CaO SrO CaF2 SrF2 BaF2 SrC12 ThO2
2.68 2.4 2.58 2.35 2.53 2.67 2.6
-5.9 -3£ -1.4 15.0 13.0 11.4 4.8 18.0
- 17.6 -12.4 -4.9 -46.6 -40.7 -35.0 -14.9 -56
oct. oct. oct. cub. cub. cub. cub. cub.
this work, [1 ] [61 [61 [7] [71 [71 [81 [91
a) Distance from Gd 3+ to nearest neighbor. 259
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
in table 2 for comparison. C o m m o n to all is the comparatively small b 4 (and b6) coefficient w h i c h is clearly not a simple f u n c t i o n o f the lattice constant. The same remark also applies to the N e w m a n parameters [10] w h i c h are defined and given in table 2. Ano t h e r n o t e w o r t h y fact is that b 4 has a negative sign for b o t h the cubic and the octahedral surrounding.
3. Ligand E N D O R The E P R s p e c t r u m o f Gd 3+ saturates easily at low t e m p e r a t u r e s and the best E N D O R signals were ob-
3 February 1984
rained at 1.5/~W input microwave power at 4.2 K. The crystals at our disposal all contained Fe 3+ in a d d i t i o n to the Gd 3+ centers. The EPR spectra o f these ions overlap for all directions o f H. In o r d e r to separate the c o n t r i b u t i o n s o f the two ions, E N D O R spectra were recorded on all o f the seven E P R transitions o f Gd 3+ with H oriented along each o f the three principal crystal axes ( [ 1 0 0 ] , [110], [111]). In addit i o n , the angular d e p e n d e n c e o f some o f the E N D O R lines was studied (on the ( - ~a / - ~) 1 E P R transition) w i t h H rotating b o t h in a (100) and in a (110) plane. Two sets o f lines due to Gd 3÷ were identified (see figs 2 and 3). One arises f r o m the interaction w i t h the Cs +
lOOkHz ~ ,
50 kHz
'~c~
~
1
o "1Na ~
H IIC4 i ,
H II C2
"2Na=4115MHz
r"~~1f21-3¢2)
( 1/2 I-1/2) 90" ( - 3 / 2 ) ( - 1 / 2 ) 1 1 J i l l 0° (-1/2) (-3/2)
,2Cs=1.871 MHz
13 90 35.26
I I (-1/2) (1/2) I I (1/2)(-1/2)
a
a
HIIC3
lOOkHz
(3/215/2) 19Na
SOkHz
,#Na=3.606 MHz
L_J
H II C3 (3/211/2) qCs=1.947MHz
~Cs
50 kHz ~-~ ~Na I I " |
b
1__] O" (3/2) (1/2) 70.53"
8 [
t
(1/2)(3/2)
Fig. 2. ENDOR spectra due to the neighboring Cs+ nuclei. Experiments performed at 4.2 K. The transitions are labelled by the angle between H and the principal axis (a Ca axis) of one shf tensor, and by the M S value of the dominant contribution to the actual electronic state involved. (a)//11 C2. Two pairs of lines are expected when the shf tensor is axial. The center part contains barely resolved features due to more distant Cs÷ nuclei. (b) H_IIC a • The parallel lines are rather sharp. The splitting of the +5, eta = 70.2 transition is due to a slight misalignment of the magnetic field with respect to the C3 axis. 260
b
(3/2) (5/2)
0 45
II I El I (-1/2) _ (1/2) (1/~X-1~)
HIIC2 (1/21-1~)
C
Fig. 3. ENDOR spectra due to the neighboring Na + nuclei. Experimental conditions and labeling as explained in fig. 2. (a)//II C4. This spectrum displays dearly four sets of three lines. The pair below v0 is due to the perpendicular component of the tetragonal shf tensor and the pair above is due to the parallel component. The line labeled X is a pulse. (b) H IIC3. The quadrupolar splitting is zero along this direction and one line only is observed for each M S value (thus justifying eq. (3)). (c)//IIC 2 . The spectrum compares directly with the one labeled (a) in this figure. The 45 ° components are localized at vo .
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
3 February 1984
Table 3 ENDOR results for the M3÷-Na + and the M3+-Cs + (M 3+ = Gd 3+, Ce 3+) nearest-neighbor interactions in Cs2NaYC16: spin hamiltonian parameters (in kHz) a) M3÷
Neighbour nucleus
Gd 3+
Na ÷
Gd 3÷
Cs ÷
Ce 3 +
Na ÷
I 3 5 7 5 3 ~
A~I
A~
AsUb)
A~ c)
p~
+218 (12)
- 1 8 0 (15)
-47
1 34
+25.8
+190 (l 5)
- 1 3 0 (18)
-24.6
106.6
+228 (10)
-28
+80.8
a) Principal axis along [ 111 ] for Cs ÷ and along [ 100 ] for Na ÷. 1
b) As = -~(All + 2A±). c) A p = A s - A±.
neighbors. The set shows an axial angular dependence with the principal axis of the super-hyperfme tensor parallel to a trigonal axis of the crystal. An accurate parameterization of the angular dependence was obtained with the equation
v~ = [v~ - Ms(A~cos20.+ A~sin2O u)l= [vr~[ (/1 = 1, ..., 8 ) ,
(2)
where v~ is the free precession frequency of the nucleus involved (the Cs nucleus in this case) and A~I and A~ are the components of the axial shf tensor. Finally, the angle 0g is measured between H and the principal axis of the tensor. The experimental parameters are given in table 3. The fact that (2) provides a good description of the ENDOR line positions can be traced back to the experimental result that the ISM S) quantization of the electronic spin states is reasonably good. The only major deviation from this model is the observation in several spectra of ENDOR lines which are not due to the two M S states directly involved in the monitored EPR transition (see, for example, fig. 2). No quadrupolar splittings were observed in these spectra. However, this fact is not a strong argument in favor of almost unperturbed local cubic symmetry at the Cs sites adjacent to the Gd 3+ ions because the quadrupole coupling constant is rather small ( ~ 47 times smaller than that of sodium). The other set of lines observed in the ENDOR spectra is due to the neighboring Na + nuclei. The angular dependence of these lines is very well explained under the assumption that super-hyperfine and quadrupole interactions are present. Two triplets are observed when H is parallel to a fourfold crystal axis
(see fig. 3). We verified that the triplet splittings are not due to any misalignment of the crystal. Indeed, splittings of the ENDOR lines due to this latter effect are readily produced (see fig. 2), but are also readily recognized. The very narrow lines (a few kHz wide) split when the dewar is tilted only 0.5 ° away from a [100] axis and/or out of a symmetry plane. The formal parameterization of the spectra was performed with the aid of v({ 1½) = [Vr~+ ~P~I(3 cos20 u - 1)i,
1
2
- i P i l ( 3 c o s 0 ~ - 1)i, (/2 = 1, ... 6),
(3)
which are first-order expressions of the shf and quadrupole interactions within the model given above. This approach is well justified because the nuclear Zeeman energy term is much larger than the others. The transitions are labeled by the two nuclear states involved, Vr~ is the term of eq. (2) (for Na), and PII is the (only independent) matrix element (/°33) of the axial quadrupole tensor (eq. (1)). The experimental parameters are given in table 3. Both the shf and the quadrupole tensor are axial and have their principal axis oriented parallel to a fourfold crystal direction. These results provide compelling evidence that the local site symmetry of the Gd 3+ ion is cubic. On the basis of these results, we also expect a quadrupolar splitting of the Na + lines in the ENDOR data of Ce3+-doped Cs2NaYC16 in ref. [4]. This leads to a new, and in our view more plausible, interpreta261
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
tion o f the spectra and their angular dependence (figs. 4, 5 and 6 in ref. [4]). Indeed,we find that their spectra are reproduced very well by our eqs. (3) and the parameters given in table 3 of this letter under the heading "Ce 3+-. The principal axes of both the shf tensor and the quadrupole tensor are oriented parallel to a fourfold crystal axis. A closer look at figs. 4 and 6 o f ref. [4] convincingly shows two pairs o f three lines when HII C4 . The partners o feach pair correspond 1 1 to the M S = ~ and - ~ electromc Zeeman states, respectively, of the Ce 3+ ion. Further, when HIIC 3 only two lines remain, because the quadrupolar splitting is zero for this direction. If this interpretation is accepted, it is clear that the Ce 3+ ion is also on a cubic site at 4.2 K in Cs2NaYC16 . Thus, there appears to be no evidence of a phase transition of this host between room temperature and 4.2 K.
Acknowledgement Some o f the samples were oriented by Mr. Navarro.
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3 February 1984
The drawing (fig. 1) is due to Mr. Chambaz. This work was supported by the Swiss National Science Foundation.
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