LOW TEMPERATURE MÖSSBAUER STUDIES OF THULIUM COMPOUNDS

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LOW TEMPERATURE MÖSSBAUER STUDIES OF THULIUM COMPOUNDS

B. Triplett, N. Dixon, P. Boolchand, S. Hanna, E. Bucher

To cite this version:

B. Triplett, N. Dixon, P. Boolchand, S. Hanna, E. Bucher. LOW TEMPERATURE MÖSSBAUER

STUDIES OF THULIUM COMPOUNDS. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-653-

C6-657. �10.1051/jphyscol:19746144�. �jpa-00215759�

JOURNAL D E PHYSIQUE Colloque C6, supplement au n° 12, Tome 35, Decembre 1974, page C6-6!

LOW TEMPERATURE MOSSBAUER STUDIES OF THULIUM COMPOUNDS

B. B. TRIPLETT, N. S. DIXON, P. BOOLCHAND (*) and S. S. HANNA Department of Physics, Stanford University, Stanford, Ca. 94305, U. S. A. (**)

and E. BUCHER

Bell Telephone Laboratories, Inc., Murray Hill, New Jersey 07947, U. S. A.

Resume. — Nous presentons une etude de TmVC>4, TmF2, TmF3 et TmSe a l'aide de la reso- nance Mossbauer de 8,4 keV de "'Tm. Cette etude couvre un domaine de temperature allant de 0,050 K a 365 K et des informations Mossbauer concernant les phenomenes suivants sont obte- nues : 1) la distorsion Jahn-Teller dans TmVCU a 2,1 K ; 2) la levee de la degenerescence de 1'etat fondamental de TmF2 malgre I'environnement cubique et l'absence d'ordre magnetique ; 3) la variation avec la temperature de 1'interaction quadrupolaire dans TmF3 et 4) le caractere d'etat de valence mixte et l'ordre magnetique observe dans TmSe.

Abstract. — We discuss studies of TmV0

, TmF

, TmF

and TmSe utilizing the 8.4 keV Moss- bauer resonance in

Tm. The studies cover a temperature range from 0.050 K to 365 K and Mossbauer information is derived which pertains to the following phenomena : 1) the Jahn-Teller distortion in TmVCU at 2.1 K ; 2) the removal of the ground state degeneracy in TmF2 in spite of cubic environment and the absence of magnetic order ; 3) the temperature dependence of the qua- druple interaction in TmF

, and 4) the mixed valence state character and magnetic ordering observed in TmSe.

1. Introduction. — The 8.4 keV Mossbauer reso- nance in

Tm is potentially a very powerful tool for exploring solid state phenomena in the rare earth region. Tm compounds themselves possess some highly interesting and varied physical phenomena, including properties that suggest the possibility of the observation of electronic-nuclear cooperative inter- actions. In spite of these possibilities the

Tm resonance is one of the least studied Mossbauer reso- nances. The reasons for this situation are clear. The 8.4 keV y-ray has an energy only slightly larger than the L X-ray edges for Tm and Er. The source half- life is only 9.4 days, and the narrowest linewidths observed for the best source-absorber combinations are still more than a factor of two larger than the natural linewidth. Moreover, the very small crystal field split- tings in Tm ^compounds mean that the phenomena of interest occur only at low temperature. Similarly, sources no longer emit narrow single lines at these temperatures and must generally be maintained far above the absorber temperature in order to obtain high quality spectra. Lastly, the rather large hyper- fme splittings in

Tm require drive velocities up

(*) Permanent address : Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, U. S. A.

(**) Supported in part by the National Science Foundation.

to ~ 70 cm/s and techniques for calibrating the drive at these velocities.

However, these problems are only technical and previous work has already shown that very low temperature studies with

Tm are feasible [1, 2].

We report here three studies on Tm absorbers.

2. Experimental. — The sources used in this series of experiments were

Er dissolved in Al. The

Er was prepared by neutron irradiation of an alloy contai- ning 10 % by weight

Er dissolved in Al. Rare earth contaminants were not found to be a problem. Howe- ver, our original reduction and alloying procedure was found to introduce troublesome Th and Ta contamina- tion. An alternate method for making these sources is currently under investigation.

Measurements in the temperature range 18 K < T < 365 K were made in a cold-finger cryostat described previously [3, 4]. Two running configurations were commonly used. Configuration A allowed two absorbers to be run simultaneously with the two sources and two absorbers at nearly the same temperature. This situation was used for the experi- ments on TmF

and TmF

where the linewidth broa- dening produced by the absorbers was always much larger than that from the source. In configuration B a single stationary source was kept at room tempera-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746144

C6-654 B. B. TRIPLETT, N. S. DIXON, P. BOOLCHAND AND S. S. HANNA ture and a single absorber was driven at low (high)

temperature. This configuration was used for TmVO, where the temperature dependence of the linewidth was felt to be interesting.

For measurements between 0.050 K and 18 K a He3-He4 dilution refrigerator was used. The techniques for coupling the Mossbauer drive to the refrigerator will be described elsewhere. The essential improvement over previous configurations is that the source mount extends through superinsulated radiation shields to within 2.5 cm of the absorber in a common vacuum space. The source mount is a thin wall stainless steel tube 28 cm long. It does not go through any vacuum seals nor does it touch any part of the dewar assembly except the velocity drive. The source is generally not heated and cools to about 120 K where it still has a relatively narrow linewidth. The y-rays emitted by the source pass through three Be windows, the absorber mounted beneath the dilution refrigerator mixing chamber, and four more Be windows before entering the Xe proportional counter outside the dewar.

The seven Be windows have a 0.68 mm total thickness and three are vacuum seals. The total y-ray flight path is 10 cm. The Mossbauer confi- guration produces no significant additional heat load on the mixing chamber (special considerations preven- ted a refrigerator design with sufficient thermal radia- tion shielding to allow lower temperatures outside the mixing chamber).

Lack of space prohibits descriptions of thermo- metry, source preparation, or the preparation of absorber compounds. However, several reports have appeared in the literature concerning changes in the absorber properties depending on absorber prepara- tion techniques and specifically the use of glues or rigid binders with powdered absorbers. This problem of making good thermal contact at very low tempera- tures while avoiding sample strains is a standard low temperature problem, and we wish to comment briefly.

The problem is to find a binder which is both che- mically inert with respect to the powdered absorber and flexible enough so that thermal contraction can occur without seriously straining or fracturing either the contact medium or absorber powder. The problem is different depending on the thickness of the absorber.

Flexible varnishes will work well for very thin absor- bers whereas greases, oils, of materials which become solid only at low temperatures are required for thicker samples. The absorbers used in these studies were powdered materials dispersed on 0.08 mm Be windows and generally bonded with a small amount of Leconal varnish. GE-7031 varnish diluted either with chloro- form or a 50-50 mixture of methanol and toluene is often a suitable binder substitute. Absorbers prepared in this (or any other) fashion should be repeatedly dunked in liquid nitrogen and alternately warmed to room temperature to verify the thermal stability of the bond.

We have had excellent results with absorbers pre- pared with the above prescription, but we cannot yet discount the possibility that there may be small absorber preparation dependent effects which we have missed. However, there are two important cases where absorber preparation dependent effects are observed for reasons which are, in retrospect, obvious. These absor- bers are finely powdered and we have studied in this work only compounds which are extraordinarily che- mically stable. In unpublished work we have observed instances where more reactive Tm compounds have reacted with the solvent in the binder. Similarly, plastic deformation has been observed to affect the spectra of TmMg and TmCd. These CsCl structure inter- metallics are difficult to powder and have a structure that is almost unstable when they are prepared without a Tm deficiency. The Mossbauer spectra obtained from these materials will change after the plastically deformed powder is annealed in vacuum.

The compounds studied in this work have stable crystal structures and are very stable chemically (with a possible exception for TmF, at high tempe- rature). They powder easily and have been chosen because the absorber preparation problem is minimal.

Additional evidence that this is indeed the case may be found in the narrow linewidths observed for TmVO, and TmAsO, (the latter material is not discussed in this publication) below their Jahn-Teller distortions and for the inner lines of TmSe.

3. Results and discussion. - A. TmVO,. - The nonmetallic compound TmVO, has the tetragonal D,, space group above the cooperative Sahn-Teller distor- tion at 2.1 K and the orthorhombic D, space group below this temperature. TmV04 has a doublet crystal field (CEF) ground state above the distortion, and this degeneracy must be removed in the zero temperature limit. Studies of Jahn-Teller distortions in rare earth systems have been greatly complicated by the fact that the distortion and magnetic ordering or paramag- netic hyperfine structure generally appear at nearly the same temperature. However, TmVO, does not show any evidence of magnetic ordering (at least above 0.063 K) and thus is a considerably simpler system.

Figure 1 shows typical Mossbauer spectra taken with a TmVO, absorber (6 mg/cm2 of Tm) above and below the distortion. Figure 2 shows the tempera- ture dependence of the quadrupole splitting A , the half-linewidths r/2 of the two quadrupole split lines, and the asymmetry (E,/E,) - 1 of the two lines, where E, and E, are the percentage effects observed for the lines.

A particularly striking aspect of the data is the near- continuity of A through the Jahn-Teller distortion as schematically indicated by the solid line through the data. The quadrupole splitting, in the axially symme- tric case, may be written [ 5 ]

< AE >, ⁼ f p1 < 3 J; - ^j2

+ p2 C ; (1)

VELOCITY [cm/s)

FIG. 1. - Mossbauer absorption spectra observed at various temperatures for an absorber of TmV04. The upper two spectra were taken at temperatures above the Jahn-Teller distortion

which occurs at 2.1 K.

FIG. 2. - The quadrupole splitting A, the half-linewidths r/2, and the effect asymmetry (El/Ez) - 1 plotted against tempera-

ture for an absorber of TmV04.

where p, and p2 are proportional to the 4f electronic and the lattice contributions to the electric field gra-

dient, respectively. Barnes et al., attempted to separate

p , and p2 using a two parameter fit to the temperature

dependence of the quadrupole interaction observed for thulium ethyl sulfate (TmES) and Tm20,. Such a sepa- ration yields important information on quadrupole shielding in Tm. The data in figure 1 suggest that TmVO, offers several major advantages to an attack on this problem. First, the doublet ground state electronic wave function for TmVO, is known from optical Zeeman studies to consist prediminately of I Jz ⁼ f 5 >,

thus eliminating uncertainties introduced by fitting an optical spectrum to determine the crystalline elec- tric field (CEF) parameters. Second, TmVO,, in contrast to TmES, is stable to temperatures much higher than the total ground multiplet CEF splitting, thus allowing a direct measurement of the lattice contribution in a temperature region where

< 3 J: - J2 > = 0. Third, the change in the lattice contribution to < AE >,=, at the Jahn-Teller dis- tortion can also be measured, in principle, since the 4f electronic ground state wavefunction remains unchanged and the additional quadrupole component from the non-axially symmetric part of the electric field gradient can be calculated.

The value of < 3 52 - J2 > is assumed to be nearly constant through the distortion because of the symmetric nature of the doublet ground state which is the only state populated at the distortion temperature.

Consequently, the near continuity of A through the distortion is taken as qualitative evidence that either a) the change in the lattice contribution to < ^AE >

at the distortion compensates the non-axially symme- tric 4f component or 6) that both these terms are small. This situation may be contrasted with TmAsO, where a singlet~is populated in addition to the doublet CEF ground state at the distortion temperature.

Consequently, a change in the value of < ³ J: - J2 >

is expected to occur below T,. This expectation is in agreement with the experimental results which will b e

presented elsewhere.

FIG. 3. -Apparent line position observed for an absorber of

TmV04 plotted against temperature.

C6-656 B. B. TRIPLETT, N. S . DIXON, P. BOOLCHAND AND S. S. HANNA

The effect asymmetry (E,/E,) - 1 shown in figure 2 is of considerable interest, showing an apparent pre- cursor effect to the Jahn-Teller distortion as it decreases continuously to zero as T decreases to T,. The tempera- ture dependence of the asymmetry and the similar behavior of the apparent line position shown in figure 3 are not yet fully understood, but suggest a more complicated situation for 2.1 K < T < 90 K, such as a quadrupole splitting with an admixture of a magnetic hyperfine interaction as in TmF, (see below).

B. TmF, AND TmF,. - Figures 4 and 5 show results for TmF, and TmF, spectra simultaneously accumu-

FIG. 4. - Half-linewidths r/2 and the hyperline splittings A observed for the two line spectra of TmF2 and TmF3 plotted against temperature. The TmF3 spectra have two equal intensity quadrupole split lines. The TmF2 spectra show two lines with an intensity ratio which is strongly temperature dependent

(see Fig. 5).

FIG. 5. - Effect asymmetry (El/Ez) - 1 observed for absorbers of TmFz and TmF3 plotted against temperature.

lated at various temperatures. Anhydrous TmF, has the orthorhombic crystal structure and exhibits the usual quadrupole splitting of low symmetry Tm compounds, and provided verification that the TmF, was not contaminated with any significant amount of TmF,. Anhydrous TmF, has the cubic fluorite crystal structure in which the Tm2+ ion is isoelectronic to Yb3'. The 'F7,, ground multiplet of these ions is split by the cubic CEF into the T,, T7 doublets and the

r, quartet. The T7 ground state in Tm2+ is magnetic, and our spectra show a strongly asymmetric doublet characteristic of other ~ m ' + compounds in cubic symmetry. These compounds also show proportiona- lity between this asymmetry and the apparent line position as was also observed for TmVO,.

We do not see a resolved effective spin Hamiltonian hyperfine structure. I t is conceivable that such a struc- ture may be resolved a) by magnetically diluting TmF, in a nonmagnetic fluorite structure (such as CaF,) to increase the relaxation time, or b) by applying a strong longitudinal magnetic field as Nowik, Dunlap and Kalvius [6] have recently done for Yb3 *.

C. TmSe. - Figure 6 shows a typical spectrum taken with the TmSe absorber at 0.051 K. To our

VELOCITY (cm/sl

FIG. 6. -Absorption spectrum observed for a TmSe absorber at T

0.051 K. The asymmetry results from the ground state

nuclear alignment.

knowledge, this is the first 16'Tm Mossbauer spectrum to show a fully resolved magnetic hyperfine structure in the absence of large quadupole interactions (TmSe has the simple cubic NaCl structure). Figure 7 shows the effective magnetic field at the 169Tm nucleus, and compares this to the usual Brillouin function for J ⁼ 712. We have divided figure 7 into three regions according to the type of spect~a observed in each region. In region A, roughly corresponding to the tem- perature region T < 1.8 K, only the simple six line spectrum corresponding to the average hyperfine field He,, = 1.93 MOe is observed. In region C, roughly above the ordering temperature TN = 2.8 K observed in recent experiments [7], only a rather narrow single line spectrum (TI2 = 1.85 mm/s) has been resolved.

In the intermediate region B, relaxation effects are

important and this region will not be discussed here.

TABLE I

Measuvements (") of the Tm2+ character in TmSe

% Tm2+ Temperature (K) Experimental measurement Source

- - -

53 0.051 < T < 1.85 Mossbauer hyperfine structure This work 41 One measurement based on the Curie- Magnetic susceptibility Ref. [7]

Weiss behavior observed for 50 < T < 300

20 300 Interpolation of X-ray lattice Ref. [7]

constants

(")These measurements sample different physical observables at different temperatures. It is not yet clear that these observables are directly comparable in this unusual situation.

FIG. 7. - Hyperfine fields observed at the 169Tm nucleus in the mixed valence state compound TmSe. The solid curve corres-

ponds to the usuaI Brillouin function for J

712.

The compound TmSe is an unusual mixed valence state material where interconfiguration fluctuation (ICF) between a Tm2+ and a Tm3+ electronic confi- guration are thought to occur on a very rapid time scale [8]. The ~m~~ configuration is expected to have a r , singlet ground state. Accordingly, the reduced effective hyperfine field (- 28 % of the free ion value) could be due to a) hyperfine enhanced singlet ground state ordering of Tm3+ [9] or b) time average mixing

between the Tm2+ magnetic electronic structure and the Tm3+ nonmagnetic structure. The failure to observe a significant enhancement of the effective hyperfine field when the Tm nucleus is aligned at 0.051 K strongly suggests [9] that Tm3+ does not possess an ordered moment. We conclude that Tm2+ : Tm3+ fluctuations are likely to explain the observations and compare in Table I the fraction of Tm2+ character obtained from this work with other estimates [7]. We have assumed He, = 0 for the

~ m configuration and the free ion hyperfine field ~ + of 3.83 MOe for Tm2+ as derived from EPR measure- ments.

It should be emphasized that several attempts to directly observe crystal field levels and long range magnetic order in TmSe have been unsuccessful, possibly because the ICF are faster than the charac- teristic inverse frequency of a thermal neutron (- s). Our measurements sample a time scale of the order of the 16'Tm nuclear precession in the

~ m hyperfine field ~ + (oil - 1.6 x lo-" s). They see only the average of the ~ r n ' + : Tm3+ character and accordingly set a lower limit to the ICF time scale.

Such ICF generally stabilize a nonmagnetic ground state for the mixed valence configuration [lo] and the current work is the first observation of hyperfine structure in such a system.

Acknowledgment. - We wish to thank Profes- sor R. S. Feigelson for single crystal TmVO, grown with the flux method.

References [I] EHNHOLM, G. J., KATILA, T. E., LOUNASMAA, 0. V. and

REIVARI, P., Cryogenics 8 (1968) 136.

[2] KATILA, T. E., SEIDEL, E. R., WORTMANN, G., MOSS- BAUER, R. L., Solidstate Comm~n. 8 (1970) 1025.

[3] RUSSEL, P. B., LATSHAW, G. L., HANNA, S. S. and KAINDL, G., Nucl. Phys A 210 (1973) 133.

[4] SALOMON, D., TRIPLETT, B. B., DLXON, N . S., BOOLCHAND, P.

and HANNA, S. S. (Temperature Dependence of the 6.2 keV Mossbauer Resonance of l8lTa at Low Tempe- rature), this Proceedings.

[5] BARNES, R. G., MOSSBAUER, R. L., KANKELEIT, E. and POINDEXTER, J. M., Phys. Rev. 136A (1964) 175.

[6] NOWIK, I., DUNLAP, B. D . and KALVIUS, G. M., Phys. Rev.

B 6 (1972) 1048.

171 BUCHER, E., ANDRES, K., DISALVO, F. J., MAITA, J. P., GOSSARD, A. C., COOPER, A. S. and HULL, G. W. Jr., to be published in Phys. Rev. B.

[8] CAMPAGNA, M., BUCHER, E., WERTHEIM, G. K., BUCHA- NAN, D . N. E. and LONGINOTTI, L. D., Phys. Rev. Lett.

32 (1974) 885.

[9] TRIPLETT, B. B. and WHITE, R. M., Phys. Rev. B 7 (1973) 4938.

[lo] MAPLE, M. B. and WOHLLEBEN, D., Phys. Rev. Lett. 27 (1971) 511.

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