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EPR of orthorhombic Gd3+-M+ complexes in SrF2
Solid State Communications, Vol. 32, pp. 239—244. Pergamon Press Ltd. 1979. Printed in Great Britain. COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2 EJ. Bijvank, A.G. Zandbergen-Beishuizen and H.W. den Hartog Solid State Physics Laboratory, 1 Melkweg, Groningen, The Netherlands (Received 10 May 1979 by A.R. Miedema) In this 3~—M~ paper complexes we report (Mnew = Na, results K, Rb of and EPRAg) experiments in SrF on orthorhombic Gd 2. Special attention is payed to the second degree crystal field parameters B~and B~and a comparison with earlier results on corresponding complexes in CaF2 and BaF2 is made. We conclude that the main contributions to these crystal field parameters are of electrostatic nature. 1. INTRODUCTION IN ThE LITERATURE there have been several attempts to understand the crystal field splitting observed for the groundstate of trivalent gadolinium impurities in crystals [1—3].3~ The methods used vary systematic in series of similar hostfrom materials, whichstudies form of Gd homologous series in which various physical properties show only slight changes, to investigations which deal with one specific system [4—10].In order to contribute in a rather different we have chosen another 3~way impurity is introduced in one approach. The Gd specific host material: here, SrF2 and in the neighborhood of the impurity we introduce controlled deviations from the perfect lattice. The modifications 3~ion are of duethe to the immediate surrounding of the Gd presence of an additional M~impurity. If the M~radius is changed crystal field parameters theThe EPR signalthe of the Gd3~—M~ complexesdescribing will change. spin Hamiltonian associated with these complexes can be written as [11, 12] 0O~+ B~O~ + B° =
g~L~H . S + B2
402+ B~O~
+ B~O + B~O°6 + B~O~ + ii~o2+ ~
(1)
the mechanisms giving rise to crystal field interactions with these parameters. 2. EXPERIMENTAL PROCEDURES The crystals used for the present investigations were preparedin our crystal growing facility employing a modified Bridgman technique. Before crystal growth 10—20 ppm GdF 3, and about 2500 ppm MF were added to the starting 1 mol.% PbF material together with approximately 2 which acts as a 02- and OW scavenger. In order to reduce the concentrations of local charge compensators consisting of the trivalent Gd ion and neighboring interstitial ions we also added with approximately 500 ppmfluoride La3~impurities; in contrast the results obtained for CaF we found thatinthe intro3~ions was not2 very effective lowering duction of Ce the concentrations of Gd3~—Frdipoles (see also Bijvank, Den Hartog and Andreissen [12]). We attempted to prepare SrF 2 : Gd doped with Li~, Nat, K~,Rb~,Cs~,Tl~,Ag~and Cu~,but we have only succeeded in growing suitable materials containing Na~, K~,Rb~and Ag~.There are probably two important reasons for the failure to grow the other materials. First, the misfit of the sizes of the host ions and the impurity ions boiling temperature some of theand MFsecondly, materials.theInlow order to obtain samples of showing narrow EPR lines it was found necessary to grow at a rate of 2—3 mm hr1. The EPR experiments were carried out at room temperature under X-band (9—10 GHz) as well as under Q-band (35 GHz) conditions. The crystal field parameters were calculated by means of a least square fitting procedure which has been applied to the various rotational diagrams.
where the first term at the right hand side is the Zeeman term and7the remaining terms the interaction electron system of describe the trivalent Gd impurity of the 4f with the crystal field. The complex under consideration together with the principal axes has been shown schematically in Fig. 1. From the behavior of the crystal field parameters B~and B~as a function of the M~-radiusone can draw conclusions about the origin of the zero field splitting of Gd3~in ionic materials especially if the results are cornpared with those obtained for systems of similar nature in other crystals with the same structure, i.e. CaF 2 and BaF2. The parametersB~,B~,B~.andB~ have been determined, but we did not analyse the results because in the literature only very little has been published about 239
3. EXPERIMENTAL RESULTS 3~—M~ charge compensation Orthorhombic Gd
COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2
‘~40
Vol. 32, No.3
x~
f
I
Gd~ a
FIt)
— — —
Q~F
3~—M~ complex in the fluorite lattice structure. The Fig. 1. Three schematic representation of a Gd principal axesdimensional x,y and z have been indicated.
12 5G SrF 2 :Gd,Ag
H~II<1OO>
I5~
15~ 3/2
5/2
-3/2 •I/2
-5/2
7/2 C
C
C
C C
C
3~in SrF Fig. 2. EPR spectrum 3~have of Gd been indicated 2 :Gd,(C); Ag the obtained remaining for the lines situation are dueH0 to IIorthorhombic [100] at 300 Gd3~—Ag~ K. The EPR centers lines associand in ated somewith casescubic the type Gd of transition has been shown by referring to the S~ I *~S~transition as S~. —
centers have been studied systematically with EPR. Just as for CaF2, BaF2 and SrC12 we found significant vanations theKcrystal parameters the2 different M~ (= of Na~, ~,Rb~field and Ag~) ions. Infor Fig. we show the EPR spectrum observed for a sample SrF 2 : Gd, Ag
with H 0 [100]. The EPR lines due to cubic Gd ions have been indicated; the extra linesA are associated with 3~—Ag~ centers. diagram obtained orthorhombic from a rotationGd of the magnetic field in the (100) plane has been given in Fig. 3. The broken lines are associated
EPR OF ORThORHOMBIC Gd~—M~ COMPLEXES IN SrF2
Vol. 32, No.3 14-
241
3+
7/2
SrF2:Gd +Ag
/
7/2----~ -1/2
~\
5/2
13
/
-5/2
~
\~______:,
~‘
‘~
~E~12
11
<110>
<100>
<100>
-
0
30
‘p
60
90
Fig. 3. Rotational diagram of the EPR 3~centers; lines observed the drawn forlines SrF2correspond : Gd, Ag. The withaxis theof various rotation transitions is [100].of The orthorhombic broken lines are associated The Gd3~—Ag4centers. withtransitions cubic Gd S~ 1 S~have been indicated by S~. —
~
with the cubic EPR spectrum and the drawn lines are due to orthorhombic centers. It should be noted that the EPR lines from orthorhombic centers can be devided in various different groups because of the fact that the principal z-axis can be along all the (110> direction of the SrF2 crystal. In some cases it is easy to distinguish between the possible directions, sometimes it is impossible to do so. It is for example impossible from a simple EPR experiment to decide that the outer EPR lines observed for H0 II [110] (see Fig. 3) are due to parallel or perpendicular complexes. This leads to two different results in terms of the solution of the crystal field parameters which both give an equal description of the experimental observations, Electric field effect experiments on each of the centers in SrF2 will eventually us the opportunity distinguish between parallel andgive perpendicular centers. to These experiments bic centers in CaF have been carried out for orthorhom2 by Lefferts etal. [13] and Bijvank et a!. [14] and have lead to a confirmation of the choice made by Bijvank and Den Hartog [11] and Bijvank,
Den Hartog and Andriessen [12] on physical grounds. In Table 1 we have compiled the values of the various crystal field parameters for the crystals SrF 2 : Gd, M; here we have given the most acceptable results on physical grounds, but also the alternative interpretations have been given (compare Bijvank, Den Hartog and Andniessen [12]; in this reference we have also given more details about the computer program employed to interpret the rotational diagrams). In Fig. 4 we have plotted the values for B~found for orthorhombic complexes in SrF2 together with the corresponding results for CaF2 and BaF2 and it can be seen that this crystal field parameter behaves quite similar for all fluorite type materials investigated. The values for SrF2 are3~—Na~ between those for CaF2 BaF2 complex. Also theand values for except for well the Gd B~ fit very in between those for CaF 2 and BaF2 (see 3~—Ag Fig. 5). From the values of B~and B~for3~—Na~, the Gd complex asand compared to the for Gd Gd3~—K~ Gd3~—Rb~ we ones conclude that the silver impurity within the complex is monovalent in contrast
COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2
242
200
Vol. 32, No.3
Ca
150_
I
Fig. 4. Behavior of the 3~—M~ crystal centers field parameter in the three B~as fluorite a function type alkaline of the ionic earth radius fluoride of the crystals. additional The host monovalent materials metalbeen have impurity indicated for Gd by the corresponding alkaline earth symbol. with our findings for the Gd3~—Agcomplex in SrC1 2, where the silver impurity is assumed to be neutral,
second degree crystal field parameters: (i) the Coulomb interaction due to the effective negative charge associated with the M~impurity, (II) the electrostatic inter-
4 DISCUSSION
action with the neighboring ions, displacement (iii) the electrostatic dipoles created interactions at the with
From our earlier investigations it was found that the behavior of the crystal field parameters B~and B~for orthorhombic complexes in CaF2 and BaF2 as a function of the M’~-radiusis similar, although the parameter B~ for BaF2 is rather irregular. We have also observed that there are various contributions to the above mentioned
the induced dipoles at the neighboring ions and (iv) the second order contribution from odd crystal field terms (especially for B~).It is clear that with these contributions, which are of approximately the same size and sometimes have opposite signs, it is difficult to interpret the experimental results. In this respect the new results
Vol. 32, No.3
COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2
243
0-
/
Ca
2~0
R(A) Fig. 5. Behavior of the 3~—M~ crystalcenters field parameter in the three B~fluorite as a function type alkaline of the ionic earthradius fluoride of the crystals. additional The host monovalent materials metalbeen have impurity indicated for Gd by the corresponding alkaline earth symbol. obtained for orthorhombic complexes in SrF 2 will provide additional information for a better understanding of the crystal field splitting of trivalent Gd in ionic materials. From Fig. 4 it can be seen that the B~value for the various materials behaves quite similarly. The onset of the steep rise, which occurs for CaF2 at an ionic radius for the M~impurity of about 1.2 A is for BaF2 situated at approximately 1.4 A. For SrF2 the result is between those of forthe CaF2 and BaF2. Itstraight can alsoline be for seenlarge that the slope approximately M’~radii decreases on going from CaF 2 to BaF2. This suggests that the increase of B~for large M~ions is due to the 34’ outward shift of the F-ions both and the M1’-impurity (see neighboring Fig. 4). In addition the Gd the decrease of the slope of the linesB~VSRM+ shows that the changes of the crystal field interactions due to the displacements of the F -ions mentioned are decreasing on going from CaF 2 to BaF2 because the relative variations of the distance between these F-ions and the central Gd~-iondecrease! 4’-radius The behaviorofB~as a function of the M
is rather irregular for CaF2, SrF2 and BaF2. B~increases with increasing M~-radiusfor CaF2, it is approximately constant for SrF2 and it shows a maximum for RM~= 1.3 A for BaF2. These results suggest that there are at least two contributions to B~of approximately the same magnitude, which behave quite differently upon vanation of RM+. This is also suggested by the results of theoretical calculations ofB~from an electrostatic model, in 3~—M~ which the ionic positions in the by neighborhood complex were evaluated means of a of the Gd potential energy minimization procedure [15].
4.
REFERENCES C.A. Hutchison, B.R. Judd & D.F.D. Pope, Proc. Phys. Soc. B70, 514 (1957). ~ D.J. Newman & W. Urban,J. Phys. CS, 3101
5. 6.
(1972). A. Vishwamittar & S.P. Puri, J. Ozein. Phys. 61, 3720 (1974). Phys. ClO, 3977 (1977). H. Grundel,J.
1.
~:
244 7.
EPR OF ORTHORHOMBIC Gd34’ —M4’ COMPLEXES IN SrF2
R.W. Reynolds & L.A. Boatner, J. Chem. Phys. 54, 1827 (1971). 8. D. Zevenhuijzen, JA. van Winsum & H.W. den Hartog,/. Phys. C9, 3113(1976). 9. D. van Ormondt, M.G. van den Oord & W. Biesiot, Phys. Lett. ASS, 54(1975). 10. H. Wever & H.W. den Hartog,Phys. Status Solidi (b) 70,253 (1975).
11. 12. 13. 14. 15.
Vol. 32, No.3
E.J. Bijvank & H.W. den Hartog, Phys. Rev. B12, 4646 (1975). E.J. Bijvank, H.W. den Hartog & J. Andriessen, Phys. Rev. B16, 1008(1977). A.N. Lefferts, E.J. Bijvank & H.W. den Hartog, Phys. Rev. B17, 4214 (1978). E.J. Bijvank, P.W. van Hasselt & H.W. den Hartog (unpublished). E.J. Bijvank & H.W. den Hartog (unpublished).
g~L~H . S + B2
402+ B~O~
+ B~O + B~O°6 + B~O~ + ii~o2+ ~
(1)
the mechanisms giving rise to crystal field interactions with these parameters. 2. EXPERIMENTAL PROCEDURES The crystals used for the present investigations were preparedin our crystal growing facility employing a modified Bridgman technique. Before crystal growth 10—20 ppm GdF 3, and about 2500 ppm MF were added to the starting 1 mol.% PbF material together with approximately 2 which acts as a 02- and OW scavenger. In order to reduce the concentrations of local charge compensators consisting of the trivalent Gd ion and neighboring interstitial ions we also added with approximately 500 ppmfluoride La3~impurities; in contrast the results obtained for CaF we found thatinthe intro3~ions was not2 very effective lowering duction of Ce the concentrations of Gd3~—Frdipoles (see also Bijvank, Den Hartog and Andreissen [12]). We attempted to prepare SrF 2 : Gd doped with Li~, Nat, K~,Rb~,Cs~,Tl~,Ag~and Cu~,but we have only succeeded in growing suitable materials containing Na~, K~,Rb~and Ag~.There are probably two important reasons for the failure to grow the other materials. First, the misfit of the sizes of the host ions and the impurity ions boiling temperature some of theand MFsecondly, materials.theInlow order to obtain samples of showing narrow EPR lines it was found necessary to grow at a rate of 2—3 mm hr1. The EPR experiments were carried out at room temperature under X-band (9—10 GHz) as well as under Q-band (35 GHz) conditions. The crystal field parameters were calculated by means of a least square fitting procedure which has been applied to the various rotational diagrams.
where the first term at the right hand side is the Zeeman term and7the remaining terms the interaction electron system of describe the trivalent Gd impurity of the 4f with the crystal field. The complex under consideration together with the principal axes has been shown schematically in Fig. 1. From the behavior of the crystal field parameters B~and B~as a function of the M~-radiusone can draw conclusions about the origin of the zero field splitting of Gd3~in ionic materials especially if the results are cornpared with those obtained for systems of similar nature in other crystals with the same structure, i.e. CaF 2 and BaF2. The parametersB~,B~,B~.andB~ have been determined, but we did not analyse the results because in the literature only very little has been published about 239
3. EXPERIMENTAL RESULTS 3~—M~ charge compensation Orthorhombic Gd
COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2
‘~40
Vol. 32, No.3
x~
f
I
Gd~ a
FIt)
— — —
Q~F
3~—M~ complex in the fluorite lattice structure. The Fig. 1. Three schematic representation of a Gd principal axesdimensional x,y and z have been indicated.
12 5G SrF 2 :Gd,Ag
H~II<1OO>
I5~
15~ 3/2
5/2
-3/2 •I/2
-5/2
7/2 C
C
C
C C
C
3~in SrF Fig. 2. EPR spectrum 3~have of Gd been indicated 2 :Gd,(C); Ag the obtained remaining for the lines situation are dueH0 to IIorthorhombic [100] at 300 Gd3~—Ag~ K. The EPR centers lines associand in ated somewith casescubic the type Gd of transition has been shown by referring to the S~ I *~S~transition as S~. —
centers have been studied systematically with EPR. Just as for CaF2, BaF2 and SrC12 we found significant vanations theKcrystal parameters the2 different M~ (= of Na~, ~,Rb~field and Ag~) ions. Infor Fig. we show the EPR spectrum observed for a sample SrF 2 : Gd, Ag
with H 0 [100]. The EPR lines due to cubic Gd ions have been indicated; the extra linesA are associated with 3~—Ag~ centers. diagram obtained orthorhombic from a rotationGd of the magnetic field in the (100) plane has been given in Fig. 3. The broken lines are associated
EPR OF ORThORHOMBIC Gd~—M~ COMPLEXES IN SrF2
Vol. 32, No.3 14-
241
3+
7/2
SrF2:Gd +Ag
/
7/2----~ -1/2
~\
5/2
13
/
-5/2
~
\~______:,
~‘
‘~
~E~12
11
<110>
<100>
<100>
-
0
30
‘p
60
90
Fig. 3. Rotational diagram of the EPR 3~centers; lines observed the drawn forlines SrF2correspond : Gd, Ag. The withaxis theof various rotation transitions is [100].of The orthorhombic broken lines are associated The Gd3~—Ag4centers. withtransitions cubic Gd S~ 1 S~have been indicated by S~. —
~
with the cubic EPR spectrum and the drawn lines are due to orthorhombic centers. It should be noted that the EPR lines from orthorhombic centers can be devided in various different groups because of the fact that the principal z-axis can be along all the (110> direction of the SrF2 crystal. In some cases it is easy to distinguish between the possible directions, sometimes it is impossible to do so. It is for example impossible from a simple EPR experiment to decide that the outer EPR lines observed for H0 II [110] (see Fig. 3) are due to parallel or perpendicular complexes. This leads to two different results in terms of the solution of the crystal field parameters which both give an equal description of the experimental observations, Electric field effect experiments on each of the centers in SrF2 will eventually us the opportunity distinguish between parallel andgive perpendicular centers. to These experiments bic centers in CaF have been carried out for orthorhom2 by Lefferts etal. [13] and Bijvank et a!. [14] and have lead to a confirmation of the choice made by Bijvank and Den Hartog [11] and Bijvank,
Den Hartog and Andriessen [12] on physical grounds. In Table 1 we have compiled the values of the various crystal field parameters for the crystals SrF 2 : Gd, M; here we have given the most acceptable results on physical grounds, but also the alternative interpretations have been given (compare Bijvank, Den Hartog and Andniessen [12]; in this reference we have also given more details about the computer program employed to interpret the rotational diagrams). In Fig. 4 we have plotted the values for B~found for orthorhombic complexes in SrF2 together with the corresponding results for CaF2 and BaF2 and it can be seen that this crystal field parameter behaves quite similar for all fluorite type materials investigated. The values for SrF2 are3~—Na~ between those for CaF2 BaF2 complex. Also theand values for except for well the Gd B~ fit very in between those for CaF 2 and BaF2 (see 3~—Ag Fig. 5). From the values of B~and B~for3~—Na~, the Gd complex asand compared to the for Gd Gd3~—K~ Gd3~—Rb~ we ones conclude that the silver impurity within the complex is monovalent in contrast
COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2
242
200
Vol. 32, No.3
Ca
150_
I
Fig. 4. Behavior of the 3~—M~ crystal centers field parameter in the three B~as fluorite a function type alkaline of the ionic earth radius fluoride of the crystals. additional The host monovalent materials metalbeen have impurity indicated for Gd by the corresponding alkaline earth symbol. with our findings for the Gd3~—Agcomplex in SrC1 2, where the silver impurity is assumed to be neutral,
second degree crystal field parameters: (i) the Coulomb interaction due to the effective negative charge associated with the M~impurity, (II) the electrostatic inter-
4 DISCUSSION
action with the neighboring ions, displacement (iii) the electrostatic dipoles created interactions at the with
From our earlier investigations it was found that the behavior of the crystal field parameters B~and B~for orthorhombic complexes in CaF2 and BaF2 as a function of the M’~-radiusis similar, although the parameter B~ for BaF2 is rather irregular. We have also observed that there are various contributions to the above mentioned
the induced dipoles at the neighboring ions and (iv) the second order contribution from odd crystal field terms (especially for B~).It is clear that with these contributions, which are of approximately the same size and sometimes have opposite signs, it is difficult to interpret the experimental results. In this respect the new results
Vol. 32, No.3
COMPLEXES IN SrF EPR OF ORTHORHOMBIC Gd3~—M~ 2
243
0-
/
Ca
2~0
R(A) Fig. 5. Behavior of the 3~—M~ crystalcenters field parameter in the three B~fluorite as a function type alkaline of the ionic earthradius fluoride of the crystals. additional The host monovalent materials metalbeen have impurity indicated for Gd by the corresponding alkaline earth symbol. obtained for orthorhombic complexes in SrF 2 will provide additional information for a better understanding of the crystal field splitting of trivalent Gd in ionic materials. From Fig. 4 it can be seen that the B~value for the various materials behaves quite similarly. The onset of the steep rise, which occurs for CaF2 at an ionic radius for the M~impurity of about 1.2 A is for BaF2 situated at approximately 1.4 A. For SrF2 the result is between those of forthe CaF2 and BaF2. Itstraight can alsoline be for seenlarge that the slope approximately M’~radii decreases on going from CaF 2 to BaF2. This suggests that the increase of B~for large M~ions is due to the 34’ outward shift of the F-ions both and the M1’-impurity (see neighboring Fig. 4). In addition the Gd the decrease of the slope of the linesB~VSRM+ shows that the changes of the crystal field interactions due to the displacements of the F -ions mentioned are decreasing on going from CaF 2 to BaF2 because the relative variations of the distance between these F-ions and the central Gd~-iondecrease! 4’-radius The behaviorofB~as a function of the M
is rather irregular for CaF2, SrF2 and BaF2. B~increases with increasing M~-radiusfor CaF2, it is approximately constant for SrF2 and it shows a maximum for RM~= 1.3 A for BaF2. These results suggest that there are at least two contributions to B~of approximately the same magnitude, which behave quite differently upon vanation of RM+. This is also suggested by the results of theoretical calculations ofB~from an electrostatic model, in 3~—M~ which the ionic positions in the by neighborhood complex were evaluated means of a of the Gd potential energy minimization procedure [15].
4.
REFERENCES C.A. Hutchison, B.R. Judd & D.F.D. Pope, Proc. Phys. Soc. B70, 514 (1957). ~ D.J. Newman & W. Urban,J. Phys. CS, 3101
5. 6.
(1972). A. Vishwamittar & S.P. Puri, J. Ozein. Phys. 61, 3720 (1974). Phys. ClO, 3977 (1977). H. Grundel,J.
1.
~:
244 7.
EPR OF ORTHORHOMBIC Gd34’ —M4’ COMPLEXES IN SrF2
R.W. Reynolds & L.A. Boatner, J. Chem. Phys. 54, 1827 (1971). 8. D. Zevenhuijzen, JA. van Winsum & H.W. den Hartog,/. Phys. C9, 3113(1976). 9. D. van Ormondt, M.G. van den Oord & W. Biesiot, Phys. Lett. ASS, 54(1975). 10. H. Wever & H.W. den Hartog,Phys. Status Solidi (b) 70,253 (1975).
11. 12. 13. 14. 15.
Vol. 32, No.3
E.J. Bijvank & H.W. den Hartog, Phys. Rev. B12, 4646 (1975). E.J. Bijvank, H.W. den Hartog & J. Andriessen, Phys. Rev. B16, 1008(1977). A.N. Lefferts, E.J. Bijvank & H.W. den Hartog, Phys. Rev. B17, 4214 (1978). E.J. Bijvank, P.W. van Hasselt & H.W. den Hartog (unpublished). E.J. Bijvank & H.W. den Hartog (unpublished).