NOTES

Pressure-Induced d- to./=Orbital Energy Crossover in Sm 2+ Ion in CsSmI3: A Luminescence Study

GANG CHEN, SHIHUA WANG, RICHARD GEOFFREY HAIRE, and JOSEPH RICHARD PETERSON* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, U.S.A. (G. C., J.R.P.); Department of Chemistry, Beifing Normal University, Beijing 100875, China (S. W.); and Transuranium Re- search Laboratory (Chemical and Analytical Sciences Division), Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6375, U.S.A. (R.G.H., J.R.P.)

Index Headings: Orbital energy crossover; Pressure; Luminescence; CsSmI3.

INTRODUCTION The divalent samarium ion has a 4f 6 configuration. Its

energy level diagram resembles that of its isoelectronic ion Eu 3+. On the other hand, the lowest excited state of the 4f55d I configuration with energy analogous to the lowest excited state of the 4f 6 configuration, 5Do, also exists. This factor alters the luminescence properties of the Sm 2+ ion in host crystals. Broad luminescence bands in the visible and/or the near-infrared regions should be observed for the transition from the 4f55d I configuration to the ground state. It is well known that if the excited level 5D 0 in the 4f 6 configuration is lower in energy than the lowest level of the 4f 55d I configuration, then the sharp luminescence bands from the f - f transitions can be ob- served, l As reported earlier from studies with alkaline- earth fluorides, the luminescence lines of Sm 2÷ are very sensitive to the host crystal for the transition between the 4f55d I and 4f 6 configurations but not for the f - f transi- tions within the 4f 6 configuration. 2 Pressure effects on the different optical transitions ofSm z÷-doped SrF2 have also been studied recently, and it was reported that pressure has a larger effect on the transition between the 4f55d l and 4f 6 configurations than on those within the 4f 6 con- figuration?

Received 12 November 1993; accepted 29 April 1994. * Author to whom correspondence should be sent.

The luminescence spectrum from CsSmI 3 at ambient conditions has been reported. 4 It exhibited a characteristic different from the spectra recorded from Sm2+-doped al- kaline-earth halide crystals. The beginning of the 4f55d I

4f 6 transitions (red edge), at about 12,500 cm -l, is very low compared to the red edge in other host crystals (e.g., see Ref 1). Because the lowest excited state, 5D0, in the 4f 6 configuration is usually much higher (> 14,400 cm -l) than this value, the luminescence from transitions within the 4f 6 configuration cannot be observed in the spectra from CsSmI3 at ambient conditions. We report here our investigation of the pressure effect on the lu- minescence spectrum from CsSmI 3 in order to examine whether the transitions within the 4f 6 configuration can be observed at elevated pressure.

EXPERIMENTAL

Preparation of the CsSmI3 sample has been described elsewhere? CsSmI3 powder was loaded into a diamond anvil cell (DAC) in a helium environment. Our DAC is similar to the Merrill and Bassett-type DAC and has been described previously. 6 Slight modifications made our DAC compatible with the microscope attachment to our Ra- man spectrometer. Pressure was measured via the stan- dard ruby fluorescence technique, y Dry silicone oil was used as the pressure-transmitting medium.

Luminescence spectra were recorded at room temper- ature and at various applied pressures with a Ramanor Model HG.2S spectrophotometer (Jobin-Yvon/Instru- ments SA). A Nicolet 1170 signal averager was used to accumulate the spectra from multiple scans. The 488-nm line of an argon-ion laser was used as the excitation source. The power on the sample was less than 6 mW.

RESULTS AND DISCUSSION

The luminescence spectrum from CsSmI3 at room tem- perature has been reported previously; 4 it is characterized by a broad, intense emission starting at about 14,250 cm-l and extending to lower energies. We first measured the spectrum at 16.4 GPa, at which pressure we observed a change in the color of the sample from dark green to black. The luminescence spectrum from CsSmI 3 at 16.4 GPa (see Fig. 1) exhibits additional peaks appearing on the top of a broad band. The very strong peak just above 14,000 cm-1 stems mostly from the ruby used as a pres- sure marker. The peaks at 14,583 and 13,795 cm -l are new spectral characteristics at 16.4 GPa which did not appear at ambient conditions.

1026 Volume 48, Number 8, 1994 0003-702S/94/4S0S-102652.00/0 APPLIED SPECTROSCOPY © 1994 Society for Applied Spectroscopy

~ P(t)= 1 8 . 4 GPo 7

1 5 6 o o ~ 4 6 o 0 1 ~6oo W a v e n u m b e r s ( c m -1)

FIG. 1. Luminescence spectra from CsSmI3 at 16.4 GPa on increasing pressure and at 7 GPa on decreasing pressure.

Energy-level diagrams of Sm 2+ ion in alkaline-earth halides show that its 5Do state lies between 14,500 and 14,650 cm -1 (see Ref. 1) and that its 7F2 state is located about 650 to 1050 cm -~ above the ground state. The 4f energy levels are less sensitive to changes in the lanthanide ion environment because they are core states, and the crystal field acts only in the form of a perturbation on the free ion's energy levels. Therefore the 5D o state in the Sm 2÷ ion in CsSmI 3 should be at an energy comparable to the values reported in other host crystals.' On the basis of energy considerations, we would thus assign the two peaks (14,583 and 13,795 cm -~ in Fig. 1) to the emissions within the 4f 6 configuration from the 5D o ~ 7F o and 5D o

7F 2 f - f transitions, respectively. Both peaks appear quite broad. As discussed by Wood and Kaiser, 2 the line- width increases rapidly with temperature in some crystals. Extrapolating their temperature dependence of the Sm 2+ ion linewidth to room temperature confirms that our ob- served linewidths in CsSmI 3 are very reasonable, if we assign them to the 5D 0 ~ 7F 0 and 5D o ~ 7 F 2 f - f transi- tions. Therefore, on the basis of energy considerations and the spectral similarity between our observations and those of others, ~,2 we assume that the peaks at 14,583 and 13,795 cm -1 in Fig. 1 are from the 5D o ~ 7F 0 and 5D 0 7F2 f - f transitions within the 4f 6 configuration, respec- tively.

The new spectral peaks at 16.4 GPa from t h e f - f tran- sitions can be observed only if the 5Do state of the 4 f 6

configuration is lower in energy than the lowest energy level of the 4f55d I configuration. 1,3 Thus, the lowest en- ergy level of the 4f55d ~ configuration is higher in energy than the 5D 0 state of the 4f 6 configuration at this pressure. Therefore, compared to the energy-level structure of the Sm 2+ ion in CsSmI 3 at ambient conditions, there is a crossover of the d- and f - orbital energies at 16.4 GPa. The lowest excited state of the 4 f 6 configuration, the 5D 0 state, is now, at 16.4 GPa, lower in energy than the lowest energy level of the 4f55d ] configuration. Schematic en- ergy-level diagrams in Fig. 2 illustrate this orbital energy c r o s s o v e r .

The spectral character of the 16.4 GPa spectrum was retained when the pressure was released to 7 GPa (see Fig. 1). The color of the sample at 7 GPa was still black.

A

i

E O

0 3

O

X

>- 13E UJ Z UJ

21

18

15

12

9

6

3

0

4f55d 1 ~ J 4f55d 1

• 5D 0 5D 0

7F# _ 7F4

7F 0 7F 0

Ambient Pressure At 16.4 GPa

(a) (b) • 2+ FIG. 2. Schematic energy-level diagrams of the Sm ion in CsSmI 3 at

(a) ambient pressure and (h) 16.4 GPa.

This hysteresis in optical properties is very likely con- nected with a reversible first-order phase transition. The high-pressure phase of CsSmI3 exhibits a black color and has a lower excited electronic 5D 0 state within the 4f 6 configuration than the lowest energy level within the 4f 5 5d 1 configuration. When the pressure was released to 7 GPa, the high-pressure phase was retained, and thus its optical properties were still observed.

Usually the high-pressure phase of a material possesses a higher density than its ambient-pressure phase, which gives rise to shorter interatomic distances in the high- pressure phase. Because the 5d orbitals of the central metal ion extend out into the ligand coordination space, the effect of the ligands on the 5d orbitals of the Sm 2+ ion can be large. 2 The 5d orbitals of the Sm 2÷ ion have a larger overlap with the orbitals of the coordinating li- gands since increasing pressure produces shorter inter- atomic distances. This increase in the orbital overlap in the high-pressure phase results in an increase in the energy of the 5d orbitals in accord with Pauli's exclusion prin- ciple. In contrast, the energy change in the 4f orbitals with increasing pressure is small, because the 4f orbitals are in an inner shell. It is this energy increase in the 5d orbitals in the high-pressure phase which drives the cross- over in relative energies of the 5d and 4f orbitals.

A change in the color of the CsSmI3 sample may reflect many physical changes, such as a shift in the charge- transfer band, a pressure-induced decomposition to gen- erate a very small amount of iodine in the sample, etc. 8 Further studies using other techniques (e.g, X-ray dif- fraction and absorption edge determination) will be nec- essary to obtain more information about the origin of the

APPLIED SPECTROSCOPY 1027

observed color change and to confirm whether there is neutral iodine present in CsSmI3 under high pressure.

The crossover o f the 5d- and 4f-orbi tal energies in the Sm 2÷ ion in CsSmI 3 under pressure can reflect changes in the chemical bonding and crystal structure. Thus, as an applied spectroscopic technique, the spectral proper- ties o f Sm 2+ ion can be used to recognize the occurrence o f a phase transition and to study the changes in chemical bonding accompanying that phase transition.

ACKNOWLEDGMENTS

This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Grant DE-FG05-88ER13865 to the University of Tennessee, Knoxville, and Contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. One of the authors (S.H.W.) gratefully acknowledges support from the National Natural Science Foundation of China.

1. J. Rubio, J. Phys. Chem. Solids 52, 101 (1991). 2. D. L. Wood and W. Kaiser, Phys. Rev. 126, 2079 (1962). 3. C. S. Yoo, H. B. Radousky, N. C. Holmes, and N. M. Edelstein,

Phys. Rev. B 44, 830 (1991). 4. Xinhua Zhao, Shihua Wang, and Baopeng Cao, J. Alloys and Com-

pounds 180, 235 (1992). 5. S. H. Wang, S. M. Luo, and H. A. Eick, J. Less-Common Met. 149,

55 (1989). 6. J. R. Peterson, U. Benedict, J. P. Young, R. G. Haire, and G. M.

Begun, Microchem. J. 34, 76 (1986). 7. H. K. Mao, P. M. Bell, J. W. Shaner, and D. J. Steinberg, J. Appl.

Phys. 49, 3276 (1978). 8. R. G. Haire, U. Benedict, J. P. Young, J. R. Peterson, and G. M.

Begun, J. Phys. C 18, 4595 (1985).

Raman or Fluorescence Spectra? About the Use of FT-Raman Techniques on Inorganic Compounds

E D U A R D O L. V A R E T I ' I * and E N R I Q U E J . B A R A N Programa QUINOR, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115, C. Cor- reo 962, 1900 La Plata, Argentina

Index Headings: Raman spectroscopy; Fluorescence; Luminescence.

I N T R O D U C T I O N

The use of interferometric instruments and excitation o f samples with near-infrared light to obtain Raman spec- tra has established itself as a very useful technique. The main advantage, besides the relative simplicity o f this approach, is the absence o f the intense fluorescence back- ground very often encountered with dispersive instru-

Received 13 December 1993; accepted 5 May 1994. * Author to whom correspondence should be sent. Both authors are

members of the "Carrera del Investigador Cientifico" of CONICET, Argentina.

ments when one is using visible light for excitation, main- ly for organic samples.

However , we found that fluorescence can still be im- portant when exciting the sample with infrared radiation, at least for some inorganic compounds . To our knowl- edge, this effect has been observed before by Hendra and co-workers I and has led other authors to an erroneous interpretat ion o f Raman spectra. 2

E X P E R I M E N T A L

The results were obtained with a Bruker IFS 66 FT- IR inst rument provided with the FRA 106 Raman accessory. Excitation o f samples was accomplished with the 1064- nm light o f a N d : Y A G laser, having a m a x i m u m power o f 350 mW. Raman light was detected with a high-sen- sitivity Ge detector cooled with liquid air. Powder sam- ples were slightly pressed into a small cavity ( ~ 2 - m m diameter) o f the a luminum blocks provided with the ac- cessory.

For emission experiments, the sample was placed di- rectly in front o f one o f the ins t rument standard light inputs, and the aforement ioned detector was also selected.

A H e - N e laser (Uniphase 155SL) was used in some experiments to excite fluorescence, directing the laser beam to the sample center. The nominal power o f the emit ted 632.8-nm light was 1 mW.

R E S U L T S AND D I S C U S S I O N

As part o f a study on substituted apatites, 3 the disper- sive Raman spectra o f several samples were obtained in our laboratory with the use o f visible light for excitation and with results considered as normal. However , when the F T - R a m a n technique was used on some o f these sam- pies, completely different spectra were obtained. Such intriguing results led us to make a series o f measurements on fluoroapatite, Cal0(PO4)6F2--0ne o f the substances which showed such behavior.

A normal Raman spectrum offluoroapati te excited with visible light is reproduced in Fig. 1. It shows the expected bands and is clearly different f rom the F T - R a m a n spec- t rum of Fig. 2, which, on the other hand, does not show

i i i i

1200 I000 800 000 400

c m - I

FIG. 1. A d i s p e r s i v e R a m a n s p e c t r u m o f f luo roapa t i t e , exc i t ed w i t h 5 1 4 . 5 - n m l igh t (Ar - i on laser).

1028 Volume 48, Number 8, 1994 0003-702s/94/4s08-102852.00/0 APPLIED SPECTROSCOPY © 1994 Society for Applied Spectroscopy