Spectroscopic studies of PrCl3 and PrBr3 in the solid state

Journal

of the Less-Common

SPECTROSCOPIC SOLID STATE*

Metals,

STUDIES

148

(1989)

193

- 200

193

OF PrCl, AND PrBr, IN THE

W. R. WILMARTH Chemistry

Department,

University

of Tennessee,

Knoxville,

TN 37996-l

600

(U.S.A.)

G. M. BEGUN Transuranium Research Laboratory (Chemistry Division), Oak Ridge Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6375 (U.S.A.)

National

J. R. PETERSON+ Chemistry Department, University of Tennessee, Knoxville, TN 37996-l 600 (U.S.A.) and Transuranium Research Laboratory (Chemistry Division), Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6375 (U.S.A.) (Received

June

3, 1988;

in revised form September

13, 1988)

Summary PrCl, and PrBr, have been investigated by X-ray powder diffraction analysis, solid state absorption spectrophotometry, and Raman spectroscopy. The X-ray diffraction results indicated that PrCl, exhibited the UClstype hexagonal crystal structure, whereas PrBr, was dimorphic. The lowtemperature form of PrBr, exhibited this hexagonal structure, while the form prepared by quenching molten PrBrs exhibited the PuBr,-type orthorhombic crystal structure. Solid state absorption spectra were recorded for the three materials and were consistent with expected crystal field effects. Phonon Raman spectra were obtained and are comparable with the Raman spectra of isostructural compounds.

1. Introduction X-ray diffraction and solid state absorption spectrophotometry have been used by the authors in the characterization of lanthanide and actinide compounds for a number of years [l - 31. Recently, phonon Raman spectroscopy has been used to identify the crystal structure exhibited by some lanthanide trihalides [ 4, 51. The purpose of this work was to extend the use *Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. +Author to whom correspondence should be addressed. 0022-5088/89/$3.50

@ Elsevier

Sequoia/Printed

Lake

Geneva,

WI,

in The Netherlands

194

of these techniques to PrCI, and PrBr,. Other workers have studied the structure of PrCl, under a variety of experimental conditions utilizing X-ray diffraction, fluorescence, or absorption spectroscopic techniques [6 - 91. It seems that, to date, PrBr, has only been characterized structurally and found to exhibit the UCl,-type hexagonal structure [lo]. We wanted to investigate the possible existence of a second crystallographic phase of PrBrs.

2. Experimental

details

2.1. Sample preparation The anhydrous praseodymium trichloride and tribromide samples were prepared by the reaction of the appropriate hydrogen halide gas with commercial praseodymium oxide (purity, 99.8%) obtained from Research Chemicals, Inc., Burbank, CA. A detailed description of our preparation technique has been published previously [ 11. 2.2. Sample

characterization

The anhydrous PrCl, and PrBr, samples were examined by X-ray powder diffraction, absorption spectrophotometry, and Raman spectroscopy. X-ray powder diffraction patterns were obtained using Debye-Scherrer cameras (57.3 and 114.6 mm), Cu Ka and MO Ko radiation, and techniques described elsewhere [ll]. The lattice parameters were refined using the LCR-2 program [ 121. Errors are reported here as one standard deviation. The solid state absorption spectra were obtained at room temperature with a microscope spectrophotometer of local design. This single-beam instrument, with a useful wavelength range of 300 - 1100 nm, has been described previously [ 11. Raman spectra were obtained with a Ramanor HG-2S spectrophotometer (Jobin Yvon-Instruments S.A.). The 514.5 nm line of an argon-ion laser was used as the excitation source. The Raman scattered light was collected at 90” from the excitation beam.

3. Results and discussion 3.1. X-ray diffraction The room-temperature (RT) lattice parameters obtained from our X-ray powder diffraction studies of PrCl, and PrBr, are compiled in Table 1 along with the values for other selected lanthanide trihalides. Our results for the UCl,-type hexagonal (space group C& - P6,/m) form of PrC13 and PrBr, are in agreement with the literature values. As for the previously unreported, PuBr,-type orthorhombic (space group 0:: - Cmcm) form of PrBrs, which was obtained by quenching from the melt, the lattice parameters and calculated formula volume are in accord with the corresponding

195 TABLE

1

Room-temperature

Compound

LaC13 CeC13 PrC13

lattice

parameters

Structure type

Lattice

UC13

0.7478 0.7451 0.7422 0.740(2)

UC13

UC13 UC13 UC13

PrBr3 NdBr3 SmBr3

PuBr3 PuBr3 PuBr3

lanthanide

trihalides

(nm)

Parameters

Formula Volume (nm3 )

Ref.

0.106 0.104 0.102

13

-

0.4375 0.4313 0.4279 0.425(6)

0.7967 0.7952 0.794 0.793(5)

-

0.4510 0.4444 0.439 0.440(3)

0.123 0.122 0.120 0.120

0.397(2) 0.411 0.4042

1.258(5) 1.265

0.957(5)

0.119 0.119 0.117

a0

UC13

LaBr3 CeBr3 PrBr3

of selected

bo

CO

-

1.2706

aNumbers taken from ref. 10 were converted bThis work.

0.917 0.9124

10a

6 b

0.101

10 10 10 b b

10 14

from kX units.

values for the isostructural tribromides of neodymium to samarium. The RT lattice parameters for the orthorhombic form of PrBr, obtained in this work are a, = 0.397(2), b, = 1.258(5), and ca = 0.957(5) nm. The calculated formula volumes of the hexagonal and orthorhombic forms of PrBr, are almost identical and are 0.120 and 0.119 nm3 respectively. 3.2. Raman spectra The RT phonon Raman spectra of the UCls-type hexagonal (h-) form of PrCl, and PrBrs, and the PuBr,-type orthorhombic (o-) form of PrBr, are shown in Fig. 1. Nuclear site symmetry analysis of the hexagonal structure predicts the following irreducible representation for the normal modes of the crystal: r

Raman

=zA,+E,,+

3E2,

Similarly,

the irreducible

r

4-4, + 3B1, + B, + 4B3,

Raman

=

representation

for the orthorhombic

structure

is

The corresponding Raman spectral data (along with tentative symmetry assignments) for the hexagonal and orthorhombic forms of these trihalides are compiled in Tables 2 and 3 respectively. The observed band frequencies for h-PrCl, are in good agreement with those previously obtained at low temperature by Damen et al. [ 151 for PrCl, and by Schaack and Koningstein for LaCl, and CeCl, [5]. The bands observed from h-PrBr, are shifted to lower frequencies from those of h-PrCl, due to the increase in formula mass, but the characteristic Raman band pattern remains evident. The band frequencies observed in the spectrum of

h-PrC13 214 185

‘:

176

101

138

h-PrBr3

I 250

200

1

150

A WAVENUMBER

Fig. 1. Room-temperature wavelength = 514.5 nm).

I 100

50

(CI?-‘1

Raman spectra of h-PrC13, h-PrBr3, and o-PrBr3 (excitation

h-PrBrs are in agreement with those reported by Asawa [17] for h-LaBr,. In his work on single crystals of LaBrs, Asawa made symmetry assignments that conflict with those made for h-LaCl, [5, 181. The tentative symmetry assignments we made for h-PrBr, are based on analogy with those made for the hexagonal lanthanide trichlorides [ 5,15,X3]. In the case of o-PrBrs, our observed phonon band frequencies are in agreement with those observed for the isostructural tribromides of neodymium to samarium [4,16]. The different characteristic Raman band patterns for o-PrBr, and h-PrBr, provide an efficient method for the identification of the crystal structure exhibited by a particular sample of this compound.

197 TABLE Raman

2 spectral

data (cm-‘)

from UCla-type

hexagonal

lanthanide

trihalides

Mode

LaC13a

CeC13 a

PrCl, b

PrCl, c

PrBr3 c

A,

181.6 210.9 187.0 108.0 179.8 217.6

216.8 194.8 108.4 183.1 221.5

183 214 198 106 179 217

185d 214d 185 101 176 214

138 120d 120 74 109 -

A, El, EQ E% Eu

aData at 80 K from reference 5. bData at 10 K from reference 15. CThis work. dAssigned to two vibrational modes.

TABLE Raman

3 spectral

data (cm-‘)

from PuBr+ype

orthorhombic

lanthanide

tribromides

Mode

PrBrsa

NdBrsb

PmBrsC

SmBr,b

A, A, A, A, Bk

163 150 132 52

165 153 134 53 139

164 154 134 51

-

166 157 136 52

-

Bl, BQ BQ

106 113 90

107 114 90

98 115 93

107 114 89

aThis work. bData from reference CData from reference

139

16. 4.

3.3. Absorption spectra The RT solid state absorption spectra of the three lanthanide materials are shown in Fig. 2. The observed absorption bands arise from Laporteforbidden f-f electronic transitions within the Pr(II1) ion. The differences in the relative intensities and fine structure of the absorption bands are the result of changes in coordination and crystal field splitting in the two crystal structures. Comparison of the absorption spectra obtained from h-PrCl, and h-PrBr, shows the expected shift of each absorption envelope to the red in going from chloride to bromide. This is due to a decrease in the effective nuclear charge attracting the f electrons, brought about by an increase in covalency in going from chloride to bromide. The relative intensities and fine

WAVELENGTH 3

600

Fig. 2. Room-temperature

(rim) 800

1200

solid state absorption spectra of h-PrC13, h-PrBrs and o-PrBr3.

of each band are almost identical. Because the coordination (i.e. crystal structure) is the same, this implies that the crystal field about the Pr(II1) ion in both compounds is very similar. The ability of solid state absorption spectrophotometry to permit the identification of crystal structure is demonstrated by a comparison of the absorption spectra of the two crystallographic forms of PrBr,. In the orthorhombic form, the Pr(II1) ion is eight-fold coordinated and is in a site with CzV symmetry; whereas, in the hexagonal form, the Pr(II1) ion is nine-fold coordinated and is in a site of Csh symmetry. The amount of splitting observed within the various absorption envelopes should increase with a decrease in site symmetry of the Pr(II1) ion. This holds true for the two crystal forms of PrBr, and is most obvious in the absorption band centered at 16.6 X lo5 m-l. This absorption band arises from an electronic transition between the ground electronic state 3H4 and the ‘Dz excited state [ 191. In the lower symmetry (C,,) o-PrBr3, this absorption band appears symmetrical, and at least three components are observed. In contrast, the same absorption structure

199

band in the higher symmetry (C,,) h-PrBr, is more asymmetrical and split into only two components. Another spectral difference is the intensity of the absorption band arising from the transition between the 3H4 ground state and the ‘I, excited state [19] located at 21.3 X 10’ m-‘. In the higher symmetry h-PrBr,, the intensity of this 3H4-11s band, in comparison with the 3H4-3P, abso r p tion band [19] at 20.9 X 10’ m-l, is much greater than the same intensity ratio observed in the spectrum of o-PrBr,. These spectral differences are consistent with a weaker crystal field at the Pr(II1) ion site in higher symmetry h-PrBr,.

4. Conclusions X-ray powder diffraction, solid state absorption spectrophotometry and Raman spectroscopy have been used to characterize PrCls and PrBr,. PrBr, was prepared in the UCl,-type hexagonal and previously unknown PuBr,type orthorhombic crystal modifications. The RT lattice parameters obtained for this orthorhombic form were a0 = 0.397(2), b, = 1.258(5), and co = 0.957(5) nm. Solid state absorption spectra of the hexagonal form of PrCl, and PrBr, were in agreement with the very similar crystal field expected at the Pr(II1) ion site in the two compounds. Differences were observed in the absorption spectra of h-PrBr, and o-PrBrs, and these are consistent with the expected crystal field effects. Phonon Raman spectra were obtained for the three materials and found to be comparable to the Raman spectra of isostructural compounds and should be useful in future crystal structure identifications via spectral analysis.

Acknowledgment Research sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy under grant DE-FG0588ER13865 to the University of Tennessee, Knoxville and contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc.

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