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Systematics of EPR spectra of GD3+ in some isostructural rare-earth trisulphate octahydrate hosts
SYSTEMATICS OF EPR SPECTRA OF Gd3+ IN SOME ISOSTRUCTURAL RARE-EARTH TRISULPHATE OCTAHYDRATE HOSTS
Department
of Physics,
SUSHIL K. hfIsru and P. hfIKOLAJCZAK ConcordiaUniversitySir GeorgeWilliamsCampus,Montreal, Quebec, Canada,H3GME
(Received 28 September 1978: accepted I5 November 1978) Msbrt--X-band data on Gd” ions doping some &structural sin& crystals of rare-earth trMphate octahydmte [R&SO&.8H20] single crystals (RSH) have been obtained at room temperature for the hosts characterized by R = Pr. Nd, Sm. Eu, Yb and Y, and at liquid nitrogen and liquid helium temperatures for the hosts characterized by R = Pr. Sm. Eu and Y. The data are analyzed using a rigorous least-squans fitting procedure in which all resonant line positions obtained for various orientations of the external magnetic field are fitted simultaneously to the same spin Hamiltonian parameters. The intensities of the lines obtained at liquid helium temperatures indicate the absolute S&II of the parameter b2’ to be negative for R = Pr, f31 and Y, whereas for the Sm host the sign of bzO is found to be positive. While the parameter b2’ does not exhibit a ckar cut linear dependence upon the host ion radius, the parameter b: is found to behave approximately linearly as a function of the host-ion radius. On the other hand, the zero-field splitting is foundto be a linearfunctionof the hostion radius at all three temperatures--room,liquid nibpgen and liquid helium. A comparison is presented of the systematics of the EPR spectra for the RSH hosts with those for some other isostruct&al rare-e&h hosts.
1. INTRODUClION The systematics of EPR spectra for the Gd3’ ion embedded in homologous isostructural single crystals of rareearth salts are of interest from the Point of view of understanding the interaction of the Gd” ion with its environment. It is clear that the theoretical attempts have not bee.n successful in predicting correctly the zero&Id splittings of the S-state Gd’+ ion. Wyboume( 11 computed the zero-field splitting and found it to be double in magnitude and opposite in sign to the observed value for the lanthanum ethyl sulphate host. Abraham et d[2] computed the values for the spin Hamiltonian parameter b2’ on the basis of the point charge model: these values were not in agreement with the experimental values. Newman and Urban’s[3] computation yielded zero-field splittings in the Sc, Y and Lu hosts of half the magnitude, and of the same sign, as the experimental values. The theoretical computation of the zero-field splitting of the Gd” ion thus remains an outstanding problem, and as new and more accurate experimental results become available, it is hoped that a better insight into the problem can be obtained. Host-lattice effects in the EPR spectra of the Gd3+ion have previously ken studied for the rareearth trifluotide (RTF)[4), rareearth ethyl sulfate (RES)[5-71, rare-earth trichloride hexahydrate (RCH)[8] and rareearth trinitrate hexahydrate (RNH)[9] hosts. For the RTF hosts [4], semiquantitative theoretical considerations based upon the point charge model led to the conclusion that the spin Hamiltonian Parametersdepend linearly upon the host-ion radius: this behavior was indeed found to be in accordance with the experimental results as far as the parameter b2“ was concerned, but
not for the. parameter bz2. For the. RES hosts[5-71 a linear dependence was found experimentally for all the parameters (b2’, b.‘, bh’, be”, upon the host-ion radius. (The sign of the b2’ vs host-ion radius slope was, however, found to be opposite in the RES hosts as compared with that in the RTF hosts.) Insofar as the RCH hosts are concerned, both b2’ and b2’ were found to be approximately linear functions of the host-ion radius. For the RNH hosts also, linear behaviorof both b2’, bz2 was observed as a function of the host-ion radius. The EPR of the Gd”+ ion embedded in isostructural single crystals of rare-earth trisulphate octahydrate, R#O&~8H~O (RSH), hosts is important not only from the point of view of understanding the interaction of the S-state GdN ion with its environment, but also because these host materials, when doped with Gd3‘ ions, constitute interesting materials for the purpose of constructing zero-field masers[lO]. BogJe and Symmons( I 11 studied the ground-state splitting of the Gd” ion in the RSH hosts for R = Y, Sm and Nd by directly measuring the zero-field resonance frequencies at room, liquid nitrogen and liquid helium temperatures. From their data they evaluated the absolute signs and magnitudes of the Parameters b2’, b22 and b,“. They found a dramatic change of line-shapes for the samarium salt on cooling from 80 to 4°K; as well, they found that all the lines broadened considerably a1 4°K with the two highest lines splitting into doublets. The room-temperature X-band EPR studies for the Pr host[l2] by Malhotra (at room temperature),for the Nd host[ 131by Malhotra et 01,and by Kumaraswamy et d (at 335, 300 and 80°K). and for the Sm host by Malhotra et ol.[ 141 have recently been reported; the values of all the spin Hamiltonian
477
418
S. K. h4rsru and P.
parameters were evaluated from the data obtained only for the external magnetic field orientation along the 2 and X axes. Since no data were obtained at liquid helium temperatures the absolute signs of the parameters could not be determined. It is the purpose of this paper to report the X-band EPR measurements on the Gd3+ ion in the RSH hosts, for R = Pr, Nd, Sm, Eu, Yb and Y at room temperature, and for R = Pr, Sm. Eu and Y at both liquid nitrogen and liquid helium temperatures. It is possible to obtain EPR spectra in the. parpmrypleticlattices here as the host spin-lattice relaxation times are short enough to enable one to observe the Gd3’ resonances. The data are analyzed using a rigorous least-squares fitting procedure[IS] in which all the resonant* line positions obtained for several orientations of the magnetic field in the 2X plane are fitted simultaneously. This decreases the probable error, for it is well knowa[161 that the mean square error is inversely proportional to the diflerence between the number of data points fitted and the number of parameters. The details of the crystal structure, sample preparation and the spin Hamiltonian for the various hosts are provided in Section 2. The room-temperature data, thei analysis and the resulting systematics are presented in Section 3. Section 4 deals with the details of the IOWtemperature.data. A general discussion of the linewidth is give.1 in Section 5. Some concluding remarks are made in Section 6.
MIKOLAJCZAK
the Am3’ ion is coordinated to eight oxygens: four belonging to (SG#- and the other four to the water molecules. Further, according to them the symmetry of the polyhedron formed by the eight oxygen ions is an intermediate case between’s distorted antiprism (&) and a distorted dodecahedron (023) implying that the site symmetry of the Am’+ ion will be one of the subgroups Ci, C2, or Ci of the spacegroup CL.
Samplepnparation None of the RSH salts is very soluble in water. Moreover, the water solubilities decrease with increasing tempetature[l9]. The samples were prepared by crystallion by slow evaporation of aqueous solutions containing small amounts of Gd#0,)3~8H&J at room temperature. The RIGd ratio was chosen to be 500 for all samples except for Eu, for which it was 100. It took from about two to eight ‘weeks for crystals of reasonable size to form. The crystals are not deliquescent.
Spin Hamiltonian The site symmetry seen by a Gd3+ ion in the RSH hosts is most probably monoclinic, which is the same as that seen in the RCH and RNH hosts. This is further supported by the symmetry of the angular variation of the data. It is ‘found that the spectra obtained for the magnetic field orientation at angle 8 = l&UPto 90’ from theZaxisintheZXplanearethemirrorimaees(about the X-axis) of those obtained for B = do to 90” in the samepIane.[TbeXYZaxesarechoaentobethose directions’of the external magnetic field for which the BMQLTONUN overall splittings of the EFR lines are maximum: of these crystal stractllre the overall splitting is maximum along the 2 axis and All the host crystals studied in this paper are monosmallest along the Y axis.] The data are thus fitted to the clinic and their unikell parameters are listed in Tabk 1. following spin Hamiltonian (for further justification see There are eight metafhc ions per unit ceU and the space Ref. [20]), expressed in the notation of Abragam and group is A2/a(Ch). Thus tbere are only two magneticBleaney[21]. ally inequivalent sites for the RM ion: these are related by the twofold symmetry axis of the crystal[ll]. Ac- a! = jig *R *s + &QO + &Q2 + BP040 + B:o,2 cording to the partial determination of Fttzwater and t B,'O,' t Be”O,”t &,'O,' t &,*a4 t Bc.“O,“. (1) RundIe[l’l] nine oxygen atoms are coordmated to the rare-earth ion for the Nd host. However, Burns and Baybarz[lS] found the crystal structure for the Nd host In the following, the parameters b2”( = 3&‘), bP( = to be the same as that for the Am&G&8H2G. where 608.“) and be”’ (= 126OBa”)will be used.
Tabk I. Uoit-ceJJ parameters a, b. c (ia AU), atomicradiaa(AU),pad B for the various RAs0&8&0 Crystals as listed in mJtd npto ZktJJmtrotfor T&&J. 3rd Bdn, Vol. II, 1973(Editedby J. D. B. Donnay and B. M. OndiL: Na&ml Bamaa of Saata& aad JointCommiaeeson Pow&rDitbactionstandards) R Pt-
8-T‘
Radius
c/b
6
a
b
t
1.013
2.0439
102052'
18.45
6.83
13.960
Md
0.995
2.0082
102O38'
18.426
6.80
13.656
sm
0.964
2.0015
102021'
18.30
6.76
13.53
Eu
0.950
2.0095
102O15'
18.317
6.75
13.564
Yb
0.858
2.0168
lOlO56'
18.103
6.65
13.412
Y
0.896
2.0106
101059'
18.20
6.700
13.471
Systematics
of EPR spectra of Gd”
fROOM-ltJREDATA,TANAL.TSl&-TK.SOR
The samples were mounted on a rotatable disc at a distance of 3AJ4 from the bottom in the middle of the side wall of smaller dimension of a 775, cavity in the fashion described by Weger and Low[22], and the XYZ axes as defined in Section 2 were determined. The data were obtained at every 10” interval in the ZX plane for all hosts at all temperatures, except for the Eu host at all three temperatures and for the Y host at liquid helium temperature, for which they were obtained for every 5 interval in ZX plane. The resonant line positions for the former group of hosts were obtained with the use of a nuclear fluxmeter and a gaussmeter while for the latter group of hosts they were computed using the position of the DPPH marker and the klystron frequency, assuming that th : magnetic geld changes linearly with time during the sweeping of the magnetic geld (these measurements are accurate to better than 0.1% as verified later using a nuclear fluxmeter and a frequency counter).
479
The angular variation of the data (in the ZX plane) clearly indicates two distinct spectra corresponding to two inequivalent sites, the pattern being the same for all the hosts investigated, as well as at all temperatures. For the case of Sm the room temperature angular variation of the data is displayed in Fg. 1. The spectra obtained for the two sites can be fitted to the same set of spin Hamiltonian parameters. The spin Hamiltonian parameters, as evaluated from the room temperature data for the various hosts are listed in Table 2. The plots of the room temperature parameters b2”and bz2as functions of the host-ion radius are displayed in Figs. 2 and 3 respectively. It is found that while the parameter b22 can be fitted approximately to a straight line when plotted against the host-ion radius, such is not the case for b2’: 62’ can only be fitted to separate straight lines, within the subgroups (Pr, Nd, Sm, Eu) and (Yb, Y). As discussed in Ref. [4], using the point-charge model, a linear relation should hold between 6,‘” and the host-ion radius. However, it appears that the considerations leading to this conclusion were rather
I-
b-
I
-20
I
I
2
I
I
20 ANGLE
I
I
I,
40 IN DEGREES
I
60
,
,
20
x
,
100
,
S.
480
K. MISMand P.
MIKOLAICUK
Table 2. Values of room-temperaturespin Hamiltonianparametersfor Gb+-doped R&W&+Wfi hosts. where R = Pr, Nd, Sm. Eu, Y and Yb. The same errorsare found in all hosts, and the errorsfor all b,” are the same. All b,“’are in GHz. SMD (in GHzT=Z(IAEil- ~YY where AE, is the calculated energy ditTerencebetween levels resonance for the jk resonant magneticvalue, hv, is the correspondingenergy of the microwave quanta, and 51 is the numberof lines used in each Etting.For R = Nd and Yb (not investigatedat liquid helium temperatures)a negative sign has been chosen for !I*”in accordancewith that for R = Pr.Nd andY, the absoluatc signs for which are determinedfrom liquidheliumdata.Notethattheabsolutesign for the Sm host. as determined from liquidhelium temperaturedata is opposite to those for R = Pr. Nd and Y
participating in
R 9
zz
Pr
1.991
Nd
Sm
1.991
1.907
1.995
1.995
1.992
-1.863
-1.898
1.866
EU
2.005
Y
1.977
Yb
1
1.976
5.001 9 b;'
1.985 -1.907
1.998
1.999
-1.808
-1.820
2.003 b:
1.135
1.127
-1.129
1.082
1.118
1.084
b:
0.035
0.036
-0.036
0.036
0.037
0.040
b:
0.012
-0.024
-0.007
-0.005
0.021
0.021
b:
-0.016
-0.036
0.049
-0.045
-0.032
-0.012
b:
0.003
0.001
-0.001
0.003
0.001
-0.001
b2
0.014
-0.006
-0.018
0.015
0.040
-0.002
b4 6
-0.008
0.005
o.')oo
-0.018
0.038
0.003
b6 6
-0.001
0.004
0.003
-0.032
0.036
0.009
n sno
60 0.184
53 0.037
53 0.030
idealized. If the changes in the dimension of the [Gd2(H&),$’ complex, when substituted for a [R2(H20)sr complex, are not linear in the RW-ion radius, such a linear behavior cannot be expected[7]. Zero-field
splitting
In the RRS hosts, the parameter b: is remarkably linear as a function of the host-ion radius; this is in contrast to the behavior of the parameters bzOand b2* in the other hosts-RTF, RCH, RNH and RSH for which these parameters do not exhibit such remarkable linearity. Now for the RRS hosts, the. site symmetry dictates that b2* = 0, which means that for the RIB hosts the zero-held sphtting is proporknal to bz”. Thus for these hosts, the linearity of bz” with respect to the host-ion radius also implies that the zero-field splitting is linear with respect to the host-ion radius. On the other hand, for the RTF, RCH, RNH and RSH hosts the site symmetry is such that the parameter 4’ for these hosts is non-zero. Hence, for these hosts, the zero-field splitting is not proportional to bzo, as the parameter bz* contributes to the zero-field splitting in a “quadratic” fashion (as found using perturb&on theory), while b2’ contributes lineatly. It would, then, be interesting to see how the zerofield splitting behaves as a function of the host-ion radius. Indeed, for the R!3H hosts, it is found that the zero-field splitting is a linear functioo of the host-ion radius as exhibited in Fg. 4. In order to compare this behavior of the zero-kid splitting in the RSH hosts with those in the other hosts for which
80 0.150
50 0.118
51 0.165
b: # 0, the zero-geld spliings for the RTF, RCH and RNH hosts are plotted as functions of the host-ion radius in Fm. 5-7 respectively. An inspection of these ftgures shows that for the RTF and RCH hosts the zero-6eld splitting is again a linear function of the host-ion radius. As for the RNH hosts, the behavior is also linear (with the exclusion of the Yb host; as discussed in Ref. [9] this departure may be due to somewhat diflerent unit-cell parameters for the Yb host, as compared to those for the other RNH hosts). 4.Low-TKhoBIuTupEREsuL.Ts
spin Hamiltonian parameters as evahtated from the data obtained at liquid nitrogen and liquid helium temperatures are listed in Table 3 (for R = Pr, Sm, Eu and Y). As shown in Fii. 8 and 9 the zero-field splittings at each of these temperatures are linear in the host-ion radius. Further, the overall splitting, for each host, increases as the temperature is lowered (see, e.g. Fu. 10 for the Sm host). All the lines obtained for the’ various hosts remained unsplit at ail temperatures, except for the Sm host for which, below WK, splittings into doublets occur for the three highest field lines (while the remaining lines are found only to broaden), with a separation of about 216 G for the magnetic field orientation along the. 2 axis. This spliking is found only for the magneticlkklorientation in a region of +W about the Z axis. For magnetic field orientations at 10 and IS” from the Z axis, only the highest two lines are split into doublets, the average The
0.04
Yb .
0.88
0.92
0.96
.
Sm
IONIC RADIUS ( A 1
.
Y
1.00
Nd .
0
?r
1.04
Fig 2. Graph showing the spin Hamiltonian parameter b:, describing the EPR data for Gd’+ doped Rt(SG4),~8Hfi samples plotted as a function of the host-ion R.” radius. The host-ion radii are as listed by Misra et al.[B]. Note that opposite sign to that given in Table 2 has been chosen for b,O foi the Sm host. The sines of the circles represent the error.
-1.8
; 0
II
0.9 -
1.0 -
l.l-
0.80
11
0.84
f
’
11 0.92
IONIC RADIUS
1
0.88
( A)
‘1 0.96
“1
1.00
1.04
Fig. 3. Graph showing the spin Hamiltonian parameter bz2 describing the EPR data for Gd” doped R#G,),.BHfi samples plotted as a function of the host-ion R’* radius. The host-ion radii are as listed by Misra et a/.[B]. The solid line is as found using the least-squares fitting procedure. Note that opposite sign to that given in Table 2 has been chosen for b2 for the Sm host. The sizes of the circles represent the error.
nn 0
-
h
1.2 -
1.3 -
s. K.hk%A
(‘HO)
(XHO)
ONillll~S
ONIIlIl~S
and P.
MIKOLAICZAK
al314-0132
aWd-011Z
Systcmatics of EPR spectra of Gd’+
( rHo)
Ill 2
I n
I
9NIllIldS
aim
-0132
II,,,,
0
I,, t
(‘“o)
* I
0
?)NIllIl~S
allll
I,, ‘;
-
OUlZ
y
P I
484
S. K. MISRA and P. MIKOLAICZAK
8
( x)16 )
bNl1111dS
alsid - ou32
-8 r: -8
d
-:
-t
6
6
-2
(‘H9)
9NIllIldS
a13ld-OUPZ
d
Systematics of EPR spextra of Gd”
0
N
rc
0
0
* I
(ZHO)
3NllllldS
alaId-01132
0
I
485
N
9
i
i
4&i
S.
K. MIW.Aand P. Mxowcw
G respectively. Such splittings accordance with the observation’ of Bogie and Symmons[ll]. (For the evaluation of the spin Hamiltonian parameters, the average line positions of the doublets were used [ 111.)The splitting cannot be due to a change in the crystal ,structure of the Sm host, but rather due to the magnetic moments of the Sm3’ ions. For, as explained by Bogle and Symmons[l I], if a change in the crystal structure takes place it should be manifested in the EPR spectra in the other RSH hosts as well, but this latter is not found to be so experimentally. Further, a structure change in the crystal should result in a splitting of at least the three high field lines (as observed for the magnetic tieId orientation along the 2 axis) for all orientations of the magnetic field.
magnetic field. A listing is given in Table 4 of the average linewidths as measured for the magnetic tield orientations along the 2 and X axes. The increased linewidths at liquid helium temperature for the -tic Sm host may be explained by the long spin-lattice relaxation time of the Sm ion at that temperature. For, as explained by Pines and Slicher[23], the linewidth contributed by Gd3+- R-” interaction is 6H or gfl(SH)*&, according as 7 is greater or less than g@H/h respectively. (Here 6H is the average interaction field produced by the R’+ ion, g is the g factor for the Gd3+ ion. 6 is the Bohr magneton and T is the correlation time for R” ion, i.e. the average time for which the R3’ ion stays in any one spin state.) The narrow lines obtained for the diamagnetic hosts Y and Eu support this explanation. b ion Y3+ has an inert-gas structure while Eu3+ has the spectroscopic ground state 7,. which has no magnetic moment.] For the Pr host (which is paramagnetic), most likely the spin-lattice relaxation time of the P?’ ions is sufficiently short at liquid helium temperature not to cause appreciable broadening of lines. This is analogous to the wellknown narrowing of nuclear resonance lines in liquids, as discussed by Andrew[ll, 241. The narrowing effect may be described qualitatively in terms of the ‘%nging time” of the resonating Gd3* ions regarded as tuned circuits; the ringing time is IlAv where Au is the liiwidth in cycleslsec. At all temperatures, for ail the paramagnetic hosts investigated, except for the Sm host at liquid helium temperature, the host-ion spin-lattice relaxation time, and equivalently the correlation time of the localfield fluctuations, is short compared to the ringing time: the Gd” resonant frequency cannot follow the fluctuations and effectively responds to their average value, which is zero. On the other hand for the Sm host at liquid helium temperature, the spin-lattice relaxation time is larger than the ringing time; the Gd3’ ions respond to the local field, which is manifested in the linewidths. For the Yb host (paramagnetic), investigated only at room temperature, for the magnetic field orientation along the Z axis, the linewidth of the transitions increases monotonically with the magnetic field. Such behavior has be-en reported in the EPR of Gd3’ in RCH
splitting being 230 G and 200 are in
Intmsities The intensities of the high-field lines at liquid helium temperature relative to the low-field lines are found to decrease for the Pr, Y and Eu hosts, while these increase for the Sm host. Thus the absolute sign of tbe parameter bzo should IX negative for the Pr, Y and Eu hosts, and positive for the Sm host. The sign of bP for the Sm host is the same as that determined indirectly from specific heat data by Symmons and Bogle[ll]. but it is in disagreement with their determination for the Y host. Since, at liquid-helium temperatures, the intensities of EPR lines provide an unequivocal determination of the absolute sign of bzo: the bz” for those hosts considered at liquid helium temperatures are given with their absolute signs. The occurrence of different signs in diierent hosts for the parameter bzo has also been found for the RCH hosts[ll]. where the sign of b2” was found to be negative for the Eu host, whereas it was positive for the other hosts.
5. LINWlmW For each host, the linewidths are found to be different for different orientations of the external magnetic field. Further, they are slightly different for Werent transitions corresponding to the same orientation of the
Tabk 4. Average peak-to-Peak liiwidths of tirstdcrivative lineshape in Gauss (average of all transitions observed)for the various hostsat varioustemperatures,as obtaiwd for the magneti fieldorientationsalong the Z and X axes. [RT = rwm temperature, LNT = liquid nitorgen te-mperaturc,LHT = liquid helium ~empcrature:for exact temperaturessez Tabks 2 and 31 Twperature
(OK)
RT
Hi(Z
R = Pr
20
Nd
Sm
Eu
Yb
Y
20
12
14
37
17
49
13
RT
HllX
16
8
19
39
LNT
H(IZ
16
a
22
9
LNT LHT LHT
HI/X HIIZ HI/X
'Not measured
23 23 14
a a a
31 111 76
11 18 1.1
a Cl a a
20 23 27 21
Systematicsof EPR spectraof Gd’+
hosts for R = Yb, Ho and Er, as well as for some nickel salts doped with Mn2’[2.5]. It should be further noted that for both the diamagnetic and paramagnetichosts, the interaction of the Gd3+ ion with the protons of the surrounding water molecules can also contribute to the line broadening. The slight differences in the linewidths for different transitions corresponding to the same orientation of the external magnetic field probably arise due to the exchange interaction with the surrounding ions. The exchange interaction has unequal effects on different transitions, while the dipolar interaction affects different transitions equally[6]. (.CowCuIDMcpEMARI(s
The main points of the experimental EPR study on the Gd.‘+ ion doping the various isostructural RSH hosts may be summarized as follows: (i) At all temperatures, the magnitudes of the zerofield splitting increase linearly as the host-ion radius increases. Furthermore, for each host, this splitting increases linearly as the temperature is decreased. (The parameter b2”does not exhibit a clear-cut linear dependence on the host-ion radius.) (ii) Below 15°K. for HUZ, the splitting of the highest three field lines into doublets for the Sm host is most likely due to the magnetic moments of the Sm” ions, rather than to a structure change. (iii) The increase in the linewidths at liquid helium temperature in the paramagnetic Sm host, as compared with those in the other paramagnetic hosts, is mainly caused by the slow spin-flip experienced by the Sm3’ ions in their interaction with the lattice. Acknowkdgemmts-We are grateful to the National Research Council of Canada for finaocialsupport(GrantNo. A4485) and to Dr. B. Frank for a critical readinn of the manuscriot. Mrs. Barbara Mikolajcxak’s assistanceindrawing the variousfigures is also thankfully acknowledged.
487
1. Wybourne R G.. Phys. Reu. 148,317 (1966). 2. Abraham hf. M.. Clark G. W., Finch C. B., Reynokl R. W. and Zeldes H., 1. Chem. Phys. 50,2057 (1%9). 3. Newman D. J. and Urban W.. 1. Phvs. CS. 3101 (1972). 4. Sharma V. K., 1. Chan..Phys. 54.4% (1971). 5. BuckmasterH. A.. Chatterjee R. and Shing Y., Cwt. 1. Phys. so, 991 (1972). 6. Viswanathan C. R. and Wong E. Y., 1. Chem. Phys. 49,966
VW.
7. Gerkin R E. and Thorsell D. L., J. Chem. Phys. 57. 2665 (1972). 8. Misra S. K. and Sharp G. R.. J. Phys. CIO. 897 (1977): Malhotra V. hf., Bist H. D. and Upreti G. C.. 1. Magtr. Reson.
27,439 (1977).
9. Misra S. K. and Mikolajcxak P., /. C/tern.Phys.693093 (1978). 10. Bogle G. S. and SymmonsH. F., Aust. J. HIYS. 12. I (1959): Bogle G. S. and Symmons H. F., Pmt. Phis. Sot. Al& 75 (1961). II. Bogk G. S. and Symmons H. F., f+vc. Phys. SIC. AT% 77s (1962). 12. Malhotra V. M.. Solid SlcrreCommun. IS.499 (1976). 13. Malhotra V. hf., Bist H. D. and Upreti G. C., 1. Msg. Reson. 21. 173 (1976): Kumaraswamy A., Suranarayana D. and SobhandariJ., Iruf. J. Fun and App. Phys. 14,360 (1976). 14. Malhotra V. M., Bist H. D. and Upreti G. C.. Chem. Phys. Lctf. 48,334 (1977). IS. Misra S. K.. J. Mug. Reson. 23,403 (1976). 16. Misra S. K. and Mikolajcxak P., J. Mrrgn. Reson. (To be published). 17. Fitzwater D. R. and Rundlc R. E., Ames. Lob. Report. No. I SC-241,Ames, Iowa (1952). 18. Bums J. H. and Baybarx R. D., Inow. C/arm. 11,2233 (1972). 19. CRC Handbook of Chemistryand Physics, 56th Edn. p. 1385. CRC Press Inc.. Ckveland (1977X 20. Misra S. K. and Sharp G. R., Phys. Status So&ii (b) 75.607 (1976). 21. Abragam A. and Bleaney B., Ekctmn Pammagnetic Resonance of Tmnstiion Ions. Clarendon, Oxford (1970). 22. Weger M. and Low W., Phys. Rm. 111. 1526(1958). 23. Pines D. and Slichter C. P., Phys. Rcu. 10, 1014 (1955): Daniels J. M. and FarachH. A., Can. 1. Phys. 38, I51 (1960). 24. Andrew E. R., Nuclear MagneticResonance p. 118.University Press, Cambridge(1955). 2.5. Janakimman R. and Uureti G. C.. 1. Chem. Phvs. 54. 2336 (1971): JanakiramanR.-and Upreti G. C., Phys. &alas Solidi (b)47,679(1971): UpretiG. C.,I. Magn.Reson. 14,274(1974).
Department
of Physics,
SUSHIL K. hfIsru and P. hfIKOLAJCZAK ConcordiaUniversitySir GeorgeWilliamsCampus,Montreal, Quebec, Canada,H3GME
(Received 28 September 1978: accepted I5 November 1978) Msbrt--X-band data on Gd” ions doping some &structural sin& crystals of rare-earth trMphate octahydmte [R&SO&.8H20] single crystals (RSH) have been obtained at room temperature for the hosts characterized by R = Pr. Nd, Sm. Eu, Yb and Y, and at liquid nitrogen and liquid helium temperatures for the hosts characterized by R = Pr. Sm. Eu and Y. The data are analyzed using a rigorous least-squans fitting procedure in which all resonant line positions obtained for various orientations of the external magnetic field are fitted simultaneously to the same spin Hamiltonian parameters. The intensities of the lines obtained at liquid helium temperatures indicate the absolute S&II of the parameter b2’ to be negative for R = Pr, f31 and Y, whereas for the Sm host the sign of bzO is found to be positive. While the parameter b2’ does not exhibit a ckar cut linear dependence upon the host ion radius, the parameter b: is found to behave approximately linearly as a function of the host-ion radius. On the other hand, the zero-field splitting is foundto be a linearfunctionof the hostion radius at all three temperatures--room,liquid nibpgen and liquid helium. A comparison is presented of the systematics of the EPR spectra for the RSH hosts with those for some other isostruct&al rare-e&h hosts.
1. INTRODUClION The systematics of EPR spectra for the Gd3’ ion embedded in homologous isostructural single crystals of rareearth salts are of interest from the Point of view of understanding the interaction of the Gd” ion with its environment. It is clear that the theoretical attempts have not bee.n successful in predicting correctly the zero&Id splittings of the S-state Gd’+ ion. Wyboume( 11 computed the zero-field splitting and found it to be double in magnitude and opposite in sign to the observed value for the lanthanum ethyl sulphate host. Abraham et d[2] computed the values for the spin Hamiltonian parameter b2’ on the basis of the point charge model: these values were not in agreement with the experimental values. Newman and Urban’s[3] computation yielded zero-field splittings in the Sc, Y and Lu hosts of half the magnitude, and of the same sign, as the experimental values. The theoretical computation of the zero-field splitting of the Gd” ion thus remains an outstanding problem, and as new and more accurate experimental results become available, it is hoped that a better insight into the problem can be obtained. Host-lattice effects in the EPR spectra of the Gd3+ion have previously ken studied for the rareearth trifluotide (RTF)[4), rareearth ethyl sulfate (RES)[5-71, rare-earth trichloride hexahydrate (RCH)[8] and rareearth trinitrate hexahydrate (RNH)[9] hosts. For the RTF hosts [4], semiquantitative theoretical considerations based upon the point charge model led to the conclusion that the spin Hamiltonian Parametersdepend linearly upon the host-ion radius: this behavior was indeed found to be in accordance with the experimental results as far as the parameter b2“ was concerned, but
not for the. parameter bz2. For the. RES hosts[5-71 a linear dependence was found experimentally for all the parameters (b2’, b.‘, bh’, be”, upon the host-ion radius. (The sign of the b2’ vs host-ion radius slope was, however, found to be opposite in the RES hosts as compared with that in the RTF hosts.) Insofar as the RCH hosts are concerned, both b2’ and b2’ were found to be approximately linear functions of the host-ion radius. For the RNH hosts also, linear behaviorof both b2’, bz2 was observed as a function of the host-ion radius. The EPR of the Gd”+ ion embedded in isostructural single crystals of rare-earth trisulphate octahydrate, R#O&~8H~O (RSH), hosts is important not only from the point of view of understanding the interaction of the S-state GdN ion with its environment, but also because these host materials, when doped with Gd3‘ ions, constitute interesting materials for the purpose of constructing zero-field masers[lO]. BogJe and Symmons( I 11 studied the ground-state splitting of the Gd” ion in the RSH hosts for R = Y, Sm and Nd by directly measuring the zero-field resonance frequencies at room, liquid nitrogen and liquid helium temperatures. From their data they evaluated the absolute signs and magnitudes of the Parameters b2’, b22 and b,“. They found a dramatic change of line-shapes for the samarium salt on cooling from 80 to 4°K; as well, they found that all the lines broadened considerably a1 4°K with the two highest lines splitting into doublets. The room-temperature X-band EPR studies for the Pr host[l2] by Malhotra (at room temperature),for the Nd host[ 131by Malhotra et 01,and by Kumaraswamy et d (at 335, 300 and 80°K). and for the Sm host by Malhotra et ol.[ 141 have recently been reported; the values of all the spin Hamiltonian
477
418
S. K. h4rsru and P.
parameters were evaluated from the data obtained only for the external magnetic field orientation along the 2 and X axes. Since no data were obtained at liquid helium temperatures the absolute signs of the parameters could not be determined. It is the purpose of this paper to report the X-band EPR measurements on the Gd3+ ion in the RSH hosts, for R = Pr, Nd, Sm, Eu, Yb and Y at room temperature, and for R = Pr, Sm. Eu and Y at both liquid nitrogen and liquid helium temperatures. It is possible to obtain EPR spectra in the. parpmrypleticlattices here as the host spin-lattice relaxation times are short enough to enable one to observe the Gd3’ resonances. The data are analyzed using a rigorous least-squares fitting procedure[IS] in which all the resonant* line positions obtained for several orientations of the magnetic field in the 2X plane are fitted simultaneously. This decreases the probable error, for it is well knowa[161 that the mean square error is inversely proportional to the diflerence between the number of data points fitted and the number of parameters. The details of the crystal structure, sample preparation and the spin Hamiltonian for the various hosts are provided in Section 2. The room-temperature data, thei analysis and the resulting systematics are presented in Section 3. Section 4 deals with the details of the IOWtemperature.data. A general discussion of the linewidth is give.1 in Section 5. Some concluding remarks are made in Section 6.
MIKOLAJCZAK
the Am3’ ion is coordinated to eight oxygens: four belonging to (SG#- and the other four to the water molecules. Further, according to them the symmetry of the polyhedron formed by the eight oxygen ions is an intermediate case between’s distorted antiprism (&) and a distorted dodecahedron (023) implying that the site symmetry of the Am’+ ion will be one of the subgroups Ci, C2, or Ci of the spacegroup CL.
Samplepnparation None of the RSH salts is very soluble in water. Moreover, the water solubilities decrease with increasing tempetature[l9]. The samples were prepared by crystallion by slow evaporation of aqueous solutions containing small amounts of Gd#0,)3~8H&J at room temperature. The RIGd ratio was chosen to be 500 for all samples except for Eu, for which it was 100. It took from about two to eight ‘weeks for crystals of reasonable size to form. The crystals are not deliquescent.
Spin Hamiltonian The site symmetry seen by a Gd3+ ion in the RSH hosts is most probably monoclinic, which is the same as that seen in the RCH and RNH hosts. This is further supported by the symmetry of the angular variation of the data. It is ‘found that the spectra obtained for the magnetic field orientation at angle 8 = l&UPto 90’ from theZaxisintheZXplanearethemirrorimaees(about the X-axis) of those obtained for B = do to 90” in the samepIane.[TbeXYZaxesarechoaentobethose directions’of the external magnetic field for which the BMQLTONUN overall splittings of the EFR lines are maximum: of these crystal stractllre the overall splitting is maximum along the 2 axis and All the host crystals studied in this paper are monosmallest along the Y axis.] The data are thus fitted to the clinic and their unikell parameters are listed in Tabk 1. following spin Hamiltonian (for further justification see There are eight metafhc ions per unit ceU and the space Ref. [20]), expressed in the notation of Abragam and group is A2/a(Ch). Thus tbere are only two magneticBleaney[21]. ally inequivalent sites for the RM ion: these are related by the twofold symmetry axis of the crystal[ll]. Ac- a! = jig *R *s + &QO + &Q2 + BP040 + B:o,2 cording to the partial determination of Fttzwater and t B,'O,' t Be”O,”t &,'O,' t &,*a4 t Bc.“O,“. (1) RundIe[l’l] nine oxygen atoms are coordmated to the rare-earth ion for the Nd host. However, Burns and Baybarz[lS] found the crystal structure for the Nd host In the following, the parameters b2”( = 3&‘), bP( = to be the same as that for the Am&G&8H2G. where 608.“) and be”’ (= 126OBa”)will be used.
Tabk I. Uoit-ceJJ parameters a, b. c (ia AU), atomicradiaa(AU),pad B for the various RAs0&8&0 Crystals as listed in mJtd npto ZktJJmtrotfor T&&J. 3rd Bdn, Vol. II, 1973(Editedby J. D. B. Donnay and B. M. OndiL: Na&ml Bamaa of Saata& aad JointCommiaeeson Pow&rDitbactionstandards) R Pt-
8-T‘
Radius
c/b
6
a
b
t
1.013
2.0439
102052'
18.45
6.83
13.960
Md
0.995
2.0082
102O38'
18.426
6.80
13.656
sm
0.964
2.0015
102021'
18.30
6.76
13.53
Eu
0.950
2.0095
102O15'
18.317
6.75
13.564
Yb
0.858
2.0168
lOlO56'
18.103
6.65
13.412
Y
0.896
2.0106
101059'
18.20
6.700
13.471
Systematics
of EPR spectra of Gd”
fROOM-ltJREDATA,TANAL.TSl&-TK.SOR
The samples were mounted on a rotatable disc at a distance of 3AJ4 from the bottom in the middle of the side wall of smaller dimension of a 775, cavity in the fashion described by Weger and Low[22], and the XYZ axes as defined in Section 2 were determined. The data were obtained at every 10” interval in the ZX plane for all hosts at all temperatures, except for the Eu host at all three temperatures and for the Y host at liquid helium temperature, for which they were obtained for every 5 interval in ZX plane. The resonant line positions for the former group of hosts were obtained with the use of a nuclear fluxmeter and a gaussmeter while for the latter group of hosts they were computed using the position of the DPPH marker and the klystron frequency, assuming that th : magnetic geld changes linearly with time during the sweeping of the magnetic geld (these measurements are accurate to better than 0.1% as verified later using a nuclear fluxmeter and a frequency counter).
479
The angular variation of the data (in the ZX plane) clearly indicates two distinct spectra corresponding to two inequivalent sites, the pattern being the same for all the hosts investigated, as well as at all temperatures. For the case of Sm the room temperature angular variation of the data is displayed in Fg. 1. The spectra obtained for the two sites can be fitted to the same set of spin Hamiltonian parameters. The spin Hamiltonian parameters, as evaluated from the room temperature data for the various hosts are listed in Table 2. The plots of the room temperature parameters b2”and bz2as functions of the host-ion radius are displayed in Figs. 2 and 3 respectively. It is found that while the parameter b22 can be fitted approximately to a straight line when plotted against the host-ion radius, such is not the case for b2’: 62’ can only be fitted to separate straight lines, within the subgroups (Pr, Nd, Sm, Eu) and (Yb, Y). As discussed in Ref. [4], using the point-charge model, a linear relation should hold between 6,‘” and the host-ion radius. However, it appears that the considerations leading to this conclusion were rather
I-
b-
I
-20
I
I
2
I
I
20 ANGLE
I
I
I,
40 IN DEGREES
I
60
,
,
20
x
,
100
,
S.
480
K. MISMand P.
MIKOLAICUK
Table 2. Values of room-temperaturespin Hamiltonianparametersfor Gb+-doped R&W&+Wfi hosts. where R = Pr, Nd, Sm. Eu, Y and Yb. The same errorsare found in all hosts, and the errorsfor all b,” are the same. All b,“’are in GHz. SMD (in GHzT=Z(IAEil- ~YY where AE, is the calculated energy ditTerencebetween levels resonance for the jk resonant magneticvalue, hv, is the correspondingenergy of the microwave quanta, and 51 is the numberof lines used in each Etting.For R = Nd and Yb (not investigatedat liquid helium temperatures)a negative sign has been chosen for !I*”in accordancewith that for R = Pr.Nd andY, the absoluatc signs for which are determinedfrom liquidheliumdata.Notethattheabsolutesign for the Sm host. as determined from liquidhelium temperaturedata is opposite to those for R = Pr. Nd and Y
participating in
R 9
zz
Pr
1.991
Nd
Sm
1.991
1.907
1.995
1.995
1.992
-1.863
-1.898
1.866
EU
2.005
Y
1.977
Yb
1
1.976
5.001 9 b;'
1.985 -1.907
1.998
1.999
-1.808
-1.820
2.003 b:
1.135
1.127
-1.129
1.082
1.118
1.084
b:
0.035
0.036
-0.036
0.036
0.037
0.040
b:
0.012
-0.024
-0.007
-0.005
0.021
0.021
b:
-0.016
-0.036
0.049
-0.045
-0.032
-0.012
b:
0.003
0.001
-0.001
0.003
0.001
-0.001
b2
0.014
-0.006
-0.018
0.015
0.040
-0.002
b4 6
-0.008
0.005
o.')oo
-0.018
0.038
0.003
b6 6
-0.001
0.004
0.003
-0.032
0.036
0.009
n sno
60 0.184
53 0.037
53 0.030
idealized. If the changes in the dimension of the [Gd2(H&),$’ complex, when substituted for a [R2(H20)sr complex, are not linear in the RW-ion radius, such a linear behavior cannot be expected[7]. Zero-field
splitting
In the RRS hosts, the parameter b: is remarkably linear as a function of the host-ion radius; this is in contrast to the behavior of the parameters bzOand b2* in the other hosts-RTF, RCH, RNH and RSH for which these parameters do not exhibit such remarkable linearity. Now for the RRS hosts, the. site symmetry dictates that b2* = 0, which means that for the RIB hosts the zero-held sphtting is proporknal to bz”. Thus for these hosts, the linearity of bz” with respect to the host-ion radius also implies that the zero-field splitting is linear with respect to the host-ion radius. On the other hand, for the RTF, RCH, RNH and RSH hosts the site symmetry is such that the parameter 4’ for these hosts is non-zero. Hence, for these hosts, the zero-field splitting is not proportional to bzo, as the parameter bz* contributes to the zero-field splitting in a “quadratic” fashion (as found using perturb&on theory), while b2’ contributes lineatly. It would, then, be interesting to see how the zerofield splitting behaves as a function of the host-ion radius. Indeed, for the R!3H hosts, it is found that the zero-field splitting is a linear functioo of the host-ion radius as exhibited in Fg. 4. In order to compare this behavior of the zero-kid splitting in the RSH hosts with those in the other hosts for which
80 0.150
50 0.118
51 0.165
b: # 0, the zero-geld spliings for the RTF, RCH and RNH hosts are plotted as functions of the host-ion radius in Fm. 5-7 respectively. An inspection of these ftgures shows that for the RTF and RCH hosts the zero-6eld splitting is again a linear function of the host-ion radius. As for the RNH hosts, the behavior is also linear (with the exclusion of the Yb host; as discussed in Ref. [9] this departure may be due to somewhat diflerent unit-cell parameters for the Yb host, as compared to those for the other RNH hosts). 4.Low-TKhoBIuTupEREsuL.Ts
spin Hamiltonian parameters as evahtated from the data obtained at liquid nitrogen and liquid helium temperatures are listed in Table 3 (for R = Pr, Sm, Eu and Y). As shown in Fii. 8 and 9 the zero-field splittings at each of these temperatures are linear in the host-ion radius. Further, the overall splitting, for each host, increases as the temperature is lowered (see, e.g. Fu. 10 for the Sm host). All the lines obtained for the’ various hosts remained unsplit at ail temperatures, except for the Sm host for which, below WK, splittings into doublets occur for the three highest field lines (while the remaining lines are found only to broaden), with a separation of about 216 G for the magnetic field orientation along the. 2 axis. This spliking is found only for the magneticlkklorientation in a region of +W about the Z axis. For magnetic field orientations at 10 and IS” from the Z axis, only the highest two lines are split into doublets, the average The
0.04
Yb .
0.88
0.92
0.96
.
Sm
IONIC RADIUS ( A 1
.
Y
1.00
Nd .
0
?r
1.04
Fig 2. Graph showing the spin Hamiltonian parameter b:, describing the EPR data for Gd’+ doped Rt(SG4),~8Hfi samples plotted as a function of the host-ion R.” radius. The host-ion radii are as listed by Misra et al.[B]. Note that opposite sign to that given in Table 2 has been chosen for b,O foi the Sm host. The sines of the circles represent the error.
-1.8
; 0
II
0.9 -
1.0 -
l.l-
0.80
11
0.84
f
’
11 0.92
IONIC RADIUS
1
0.88
( A)
‘1 0.96
“1
1.00
1.04
Fig. 3. Graph showing the spin Hamiltonian parameter bz2 describing the EPR data for Gd” doped R#G,),.BHfi samples plotted as a function of the host-ion R’* radius. The host-ion radii are as listed by Misra et a/.[B]. The solid line is as found using the least-squares fitting procedure. Note that opposite sign to that given in Table 2 has been chosen for b2 for the Sm host. The sizes of the circles represent the error.
nn 0
-
h
1.2 -
1.3 -
s. K.hk%A
(‘HO)
(XHO)
ONillll~S
ONIIlIl~S
and P.
MIKOLAICZAK
al314-0132
aWd-011Z
Systcmatics of EPR spectra of Gd’+
( rHo)
Ill 2
I n
I
9NIllIldS
aim
-0132
II,,,,
0
I,, t
(‘“o)
* I
0
?)NIllIl~S
allll
I,, ‘;
-
OUlZ
y
P I
484
S. K. MISRA and P. MIKOLAICZAK
8
( x)16 )
bNl1111dS
alsid - ou32
-8 r: -8
d
-:
-t
6
6
-2
(‘H9)
9NIllIldS
a13ld-OUPZ
d
Systematics of EPR spextra of Gd”
0
N
rc
0
0
* I
(ZHO)
3NllllldS
alaId-01132
0
I
485
N
9
i
i
4&i
S.
K. MIW.Aand P. Mxowcw
G respectively. Such splittings accordance with the observation’ of Bogie and Symmons[ll]. (For the evaluation of the spin Hamiltonian parameters, the average line positions of the doublets were used [ 111.)The splitting cannot be due to a change in the crystal ,structure of the Sm host, but rather due to the magnetic moments of the Sm3’ ions. For, as explained by Bogle and Symmons[l I], if a change in the crystal structure takes place it should be manifested in the EPR spectra in the other RSH hosts as well, but this latter is not found to be so experimentally. Further, a structure change in the crystal should result in a splitting of at least the three high field lines (as observed for the magnetic tieId orientation along the 2 axis) for all orientations of the magnetic field.
magnetic field. A listing is given in Table 4 of the average linewidths as measured for the magnetic tield orientations along the 2 and X axes. The increased linewidths at liquid helium temperature for the -tic Sm host may be explained by the long spin-lattice relaxation time of the Sm ion at that temperature. For, as explained by Pines and Slicher[23], the linewidth contributed by Gd3+- R-” interaction is 6H or gfl(SH)*&, according as 7 is greater or less than g@H/h respectively. (Here 6H is the average interaction field produced by the R’+ ion, g is the g factor for the Gd3+ ion. 6 is the Bohr magneton and T is the correlation time for R” ion, i.e. the average time for which the R3’ ion stays in any one spin state.) The narrow lines obtained for the diamagnetic hosts Y and Eu support this explanation. b ion Y3+ has an inert-gas structure while Eu3+ has the spectroscopic ground state 7,. which has no magnetic moment.] For the Pr host (which is paramagnetic), most likely the spin-lattice relaxation time of the P?’ ions is sufficiently short at liquid helium temperature not to cause appreciable broadening of lines. This is analogous to the wellknown narrowing of nuclear resonance lines in liquids, as discussed by Andrew[ll, 241. The narrowing effect may be described qualitatively in terms of the ‘%nging time” of the resonating Gd3* ions regarded as tuned circuits; the ringing time is IlAv where Au is the liiwidth in cycleslsec. At all temperatures, for ail the paramagnetic hosts investigated, except for the Sm host at liquid helium temperature, the host-ion spin-lattice relaxation time, and equivalently the correlation time of the localfield fluctuations, is short compared to the ringing time: the Gd” resonant frequency cannot follow the fluctuations and effectively responds to their average value, which is zero. On the other hand for the Sm host at liquid helium temperature, the spin-lattice relaxation time is larger than the ringing time; the Gd3’ ions respond to the local field, which is manifested in the linewidths. For the Yb host (paramagnetic), investigated only at room temperature, for the magnetic field orientation along the Z axis, the linewidth of the transitions increases monotonically with the magnetic field. Such behavior has be-en reported in the EPR of Gd3’ in RCH
splitting being 230 G and 200 are in
Intmsities The intensities of the high-field lines at liquid helium temperature relative to the low-field lines are found to decrease for the Pr, Y and Eu hosts, while these increase for the Sm host. Thus the absolute sign of tbe parameter bzo should IX negative for the Pr, Y and Eu hosts, and positive for the Sm host. The sign of bP for the Sm host is the same as that determined indirectly from specific heat data by Symmons and Bogle[ll]. but it is in disagreement with their determination for the Y host. Since, at liquid-helium temperatures, the intensities of EPR lines provide an unequivocal determination of the absolute sign of bzo: the bz” for those hosts considered at liquid helium temperatures are given with their absolute signs. The occurrence of different signs in diierent hosts for the parameter bzo has also been found for the RCH hosts[ll]. where the sign of b2” was found to be negative for the Eu host, whereas it was positive for the other hosts.
5. LINWlmW For each host, the linewidths are found to be different for different orientations of the external magnetic field. Further, they are slightly different for Werent transitions corresponding to the same orientation of the
Tabk 4. Average peak-to-Peak liiwidths of tirstdcrivative lineshape in Gauss (average of all transitions observed)for the various hostsat varioustemperatures,as obtaiwd for the magneti fieldorientationsalong the Z and X axes. [RT = rwm temperature, LNT = liquid nitorgen te-mperaturc,LHT = liquid helium ~empcrature:for exact temperaturessez Tabks 2 and 31 Twperature
(OK)
RT
Hi(Z
R = Pr
20
Nd
Sm
Eu
Yb
Y
20
12
14
37
17
49
13
RT
HllX
16
8
19
39
LNT
H(IZ
16
a
22
9
LNT LHT LHT
HI/X HIIZ HI/X
'Not measured
23 23 14
a a a
31 111 76
11 18 1.1
a Cl a a
20 23 27 21
Systematicsof EPR spectraof Gd’+
hosts for R = Yb, Ho and Er, as well as for some nickel salts doped with Mn2’[2.5]. It should be further noted that for both the diamagnetic and paramagnetichosts, the interaction of the Gd3+ ion with the protons of the surrounding water molecules can also contribute to the line broadening. The slight differences in the linewidths for different transitions corresponding to the same orientation of the external magnetic field probably arise due to the exchange interaction with the surrounding ions. The exchange interaction has unequal effects on different transitions, while the dipolar interaction affects different transitions equally[6]. (.CowCuIDMcpEMARI(s
The main points of the experimental EPR study on the Gd.‘+ ion doping the various isostructural RSH hosts may be summarized as follows: (i) At all temperatures, the magnitudes of the zerofield splitting increase linearly as the host-ion radius increases. Furthermore, for each host, this splitting increases linearly as the temperature is decreased. (The parameter b2”does not exhibit a clear-cut linear dependence on the host-ion radius.) (ii) Below 15°K. for HUZ, the splitting of the highest three field lines into doublets for the Sm host is most likely due to the magnetic moments of the Sm” ions, rather than to a structure change. (iii) The increase in the linewidths at liquid helium temperature in the paramagnetic Sm host, as compared with those in the other paramagnetic hosts, is mainly caused by the slow spin-flip experienced by the Sm3’ ions in their interaction with the lattice. Acknowkdgemmts-We are grateful to the National Research Council of Canada for finaocialsupport(GrantNo. A4485) and to Dr. B. Frank for a critical readinn of the manuscriot. Mrs. Barbara Mikolajcxak’s assistanceindrawing the variousfigures is also thankfully acknowledged.
487
1. Wybourne R G.. Phys. Reu. 148,317 (1966). 2. Abraham hf. M.. Clark G. W., Finch C. B., Reynokl R. W. and Zeldes H., 1. Chem. Phys. 50,2057 (1%9). 3. Newman D. J. and Urban W.. 1. Phvs. CS. 3101 (1972). 4. Sharma V. K., 1. Chan..Phys. 54.4% (1971). 5. BuckmasterH. A.. Chatterjee R. and Shing Y., Cwt. 1. Phys. so, 991 (1972). 6. Viswanathan C. R. and Wong E. Y., 1. Chem. Phys. 49,966
VW.
7. Gerkin R E. and Thorsell D. L., J. Chem. Phys. 57. 2665 (1972). 8. Misra S. K. and Sharp G. R.. J. Phys. CIO. 897 (1977): Malhotra V. hf., Bist H. D. and Upreti G. C.. 1. Magtr. Reson.
27,439 (1977).
9. Misra S. K. and Mikolajcxak P., /. C/tern.Phys.693093 (1978). 10. Bogle G. S. and SymmonsH. F., Aust. J. HIYS. 12. I (1959): Bogle G. S. and Symmons H. F., Pmt. Phis. Sot. Al& 75 (1961). II. Bogk G. S. and Symmons H. F., f+vc. Phys. SIC. AT% 77s (1962). 12. Malhotra V. M.. Solid SlcrreCommun. IS.499 (1976). 13. Malhotra V. hf., Bist H. D. and Upreti G. C., 1. Msg. Reson. 21. 173 (1976): Kumaraswamy A., Suranarayana D. and SobhandariJ., Iruf. J. Fun and App. Phys. 14,360 (1976). 14. Malhotra V. M., Bist H. D. and Upreti G. C.. Chem. Phys. Lctf. 48,334 (1977). IS. Misra S. K.. J. Mug. Reson. 23,403 (1976). 16. Misra S. K. and Mikolajcxak P., J. Mrrgn. Reson. (To be published). 17. Fitzwater D. R. and Rundlc R. E., Ames. Lob. Report. No. I SC-241,Ames, Iowa (1952). 18. Bums J. H. and Baybarx R. D., Inow. C/arm. 11,2233 (1972). 19. CRC Handbook of Chemistryand Physics, 56th Edn. p. 1385. CRC Press Inc.. Ckveland (1977X 20. Misra S. K. and Sharp G. R., Phys. Status So&ii (b) 75.607 (1976). 21. Abragam A. and Bleaney B., Ekctmn Pammagnetic Resonance of Tmnstiion Ions. Clarendon, Oxford (1970). 22. Weger M. and Low W., Phys. Rm. 111. 1526(1958). 23. Pines D. and Slichter C. P., Phys. Rcu. 10, 1014 (1955): Daniels J. M. and FarachH. A., Can. 1. Phys. 38, I51 (1960). 24. Andrew E. R., Nuclear MagneticResonance p. 118.University Press, Cambridge(1955). 2.5. Janakimman R. and Uureti G. C.. 1. Chem. Phvs. 54. 2336 (1971): JanakiramanR.-and Upreti G. C., Phys. &alas Solidi (b)47,679(1971): UpretiG. C.,I. Magn.Reson. 14,274(1974).