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Single-crystal absorption and emission spectra of CsVCl3 and CsMgCl3:V2+
Journal of Luminescence 27 (1982) 249—256 North-Holland Publishing Company
249
SINGLE-CRYSTAL ABSORPTION AND EMISSION SPECTRA OF CsVCI3 AND CsMgCI3:V2~ Andreas HAUSER and Hans U. GUDEL Institut fur Anorganische Chemie, Universität Bern, CH-3000 Bern 9, Switzerland
Received 6 April 1982
Single-crystal absorption and emission spectra of CsVCI 2~ in the region of the first excited state were measured at temperatures 3down and toCsMgCl3:V 6 K. The 4A2g ‘—‘4T 4T2g state and surprisingly well-resolved 25 transition fine structure shows which a large can be trigonal analyzed splitting in terms of of theelectronic origins and vibronic side bands.
1. Introduction CsVC1 3 and CsMgC13 crystallize in the CsNiC13 type structure, space group P63/mmc [1,2], the prominent feature of which is its quasi-one-dimensional character with chains of trigonally distorted, face-sharing MCi6 octahedra running parallel to the c-axis. Absorption spectra of CsVX3 (X = Cl, Br, I) single crystals in the region of d—d bands have been reported previously [3]. It was shown that the intensity of the spin-forbidden transitions is strongly enhanced by the large exchange coupling between neighbouring vanadium(II) ions along the chains [4],whereas the spin-allowed transitions are not affected. Up to now2~,where emissionthe from 4T vanadium(II) compounds has been measured in KMgF3:V 2g state to states a dynamic Jahn—Teller 2~,where the is subject and 2Tjg are lower in energyeffect than [5,6],and in MgO:V the 4T2g [7,8]. In the present communication we report single-crystal absorption and emission spectra of both pure CsVC1 2~ in the 3 and CsMgCl3:V region of the 4A2g ~4T 2g transition at 6 K. The fine structure is fully analyzed.
2. Experimental All the crystals grown by themeasurements Bridgeman technique [2].mol% The 2~crystalwere used for absorption contained 0.3 CsMgCl3:V vanadium(II), the one used for emission less than 0.1%. The crystals cleave easily along the c-axis. Absorption spectra were obtained on a Cary 17 0022-231 3/82/0000—0000/$02.75
©
1982 North-Holland
250
A. Hauser, H. U. Güdel
/
Single-crystal absorption and emission spectra
spectrometer using a matched pair of Glan—Taylor prisms for polarization. Emission spectra were recorded with a 3/4 m Czerny—Turner monochromator (Spex 1702) with a grating blazed at 750 nm and a lead sulfide detector cooled to liquid nitrogen temperature. In order to eliminate the dark current of the detector a 325 Hz chopper and a lock-in amplifier (PAR 186) were used. For the excitation a sealed beam xenon lamp (Varian R-l50-7A) in conjunction with a broad band filter (Schott KG3) was used. Low temperatures were achieved with a helium gas flow cooling technique.
3. Theoretical background Vanadium(II) is a d3 system with a 4A2g ground state. The site symmetry of vanadium(II) in CsVCI 3 as well as in the doped material is D3d, i.e. the 4T inversion center is retained. The first excited state is a 26 (O~~ notation). The transition from the ground state is therefore spin allowed and magnetic dipole (MD) allowed, but it remains electric dipole (ED) forbidden. 3.1. Trigonal splitting and spin—orbit interaction
In fig. 1 the effects of the trigonal field and the spin—orbit interaction on the ground state and the first excited state are shown schematically. For a large 4A trigonal splitting ~ the two Kramer’s doublets of the 15 (D3d notation) are nearly 4E degenerate (splitting < 1 cm I) whereas the four Kramer’s doublets of the 5 are split by 10 to 20 cm~[9]. 3.2. Selection rules
In D3d true electronic origins are only observable they are MD 4A2g if ground state to allowed. the two The selection rules for the transitions from the 03d
spin - orbit interact on
4
~
//___
Eg~~
3[~,’ ~‘+~
splitting 10 -20cm1
~(
-1
2g
[j
.~
~‘
splitting <1cm for large ~ zero field
~
A
1 24
~
splitting<01 cm
Fig. 1. The effects of a trigonal field and spin—orbit interaction on the 4A 2~ground state and the 2g first excited state of vanadium(11) in CsVCI3 (schematic representation).
4T
A. Hauser, H. U. Gudel
/
Single-crystal absorption and emission spectra
25 1
trigonal components of the first excited state are as follows: 2g —a4A Ig MD-allowed with El c, 4A2g
-.4E 6
MD-allowed with EH c.
Herzberg and Teller [10] postulated an intensity mechanism for ED allowed vibronic side bands [11,12]. Factor group analysis for AMX3 compounds [13,14] gives 15 odd parity vibrations at k 0 which belong to the following representations in D6h: 2a2~+ 3e1~+ 2b1~+ b2~+ 2e2~. The frequencies of the a2~and e1~modes are known from IR spectroscopy [15] for both CsVC13 and CsMgC13 (see table 1). According to Johnstone et a!. [13] the low-energy a25 mode and the ~ modes involve movements of A atoms only. In4Afirst order they are not expected contribute to of thethe vibronic structure 4A 4Eg transition. Undertothe operations site group D of the 26 ~~ 1g, 3d the remaining modes transform as 2a25 + 2a1~+ 4ev. The vibronic selection rules for electric dipole allowed transitions in D3d are summarized in table 2 together with the factor group-site group correlation. 4A 4Eg We thusfor expect 6 vibronic origins for thebe 15 transition and 8 forspectrum the transition k 0. There will, however, contributions to the
Table I Ground state vibrational frequencies at k =0 for CsMCI
CsMgCI3 ~
CsVCI3
255 82 174 317
265 d) 83 162 312
a)
3 (in cm 1)~ 2~l~ CsMgC13:V
Atoms involved in normal coordinates C)
a 15 ~ ei’.,, ~
~ b~0 b~ ~
b) 0 d)
0
—
—
—
—
49 250
52 256
—
—
—
—
Ref. [15]. This work. Refs. [13,14]. Ref. [16].
258 170 311
Cl only largely Cs largely Cl M,CI
38 232
MCI M,C1
—
— —
123 205
largely Cs largely M. Cl M,CI M.d
252
A. Hauser, H. U. Gudel
/
Single-crystal absorption and emission spectra
Table 2 Factor group—site group correlation for odd-parity vibrations and vibronic selection rules for 2g Ag~4E 4 T 5( 25) electronic transitions. D3d (enabling mode)
D65
aio~~=~a
Electronic transition
Polarization
4A25=4A,S
ElI
=
~
~ 4A~
=4E 5 ~ 5
ela~
e2~~
forbidden Elc
EIIc. Elc
involving vibrational functions with k ~ 0 [12]. Providing that the Cl—M interaction is much stronger than the Cl—Cs interaction this will merely result in a broadening of lines, so that the unit cell analysis can explain most of the intense features.
4. Results Figs. 2 and 3 show the main features of the of absorption 2~in the region the 4A and 4T emission spectra of CsVC13 and CsMgCl3:V 25—a 75 (O~notation) transition. The nearly equal extinction coefficients per vanadium in the pure and diluted crystals indicate that their intensity is due to a single-ion mecha-
Emission
Absorption
/~
orb. units
4
A19
—
~
/
~
L/molcm
\E//c
/tII/\~
7.0
8.0
90
1O~O
Fig. 2. Absorption and emission spectra of the crystals of CsVCI1 at 6K. The electronic origin ~
4A110 4T 25 25 (Oh notation) transition in single = 8299 cm
A. Hauser, H. U. GQdel Emission
/
Single-crystal absorption and emission spectra
253
Absorption
I
S
orb units
Amolcm —
f\EIC
“
E.Lc
~/i
80
cm~xi~~~
100 110 2t The vanadium (II) concentration in absorption is 0.3 Fig. 3. As for fig. 2 but for CsMgCl mole%, in emission less than 0.1 mole%. 3:V The electronic origins 0 = 8179 cm~. 70
90
pr ogressr Cci
76788082
2~emission spectrum at 6 K.
Fig. 4. Side band structure analysis of the CsMgCl3:V 5
253—
252——
8:
235—
88
239—
90
Fig. 5. Side band structure analysis of the 4A2g
cmxi~~ 4A
4T 15(
25) absorption in CsVCI3 at 6 K.
254
A. Hauser, H. U Gudel / Single-crystal absorption and emission spectra
nism. In contrast to spin-forbidden transitions spin-allowed d—d transitions are not expected to be enhanced by the strong exchange coupling between neighbouring vanadium ions in the chains [17]. The trigonal splitting ~ of the 4T2~state is large: 1400 cm I for CsVCI3 and somewhat smaller for CsMgCl3:V~.The sign can be determined from an analysis of the fine structure, which is reproduced in figs. 4 and 5.
5. Analysis of the fine structure The coincidence in absorption and emission the sharp at 8299 cm an t in CsMgCl 2~ of allows us to line identify it as in CsVC13 and 8178 cm 3:V electronic origin. The band shape is symmetrical with a half-width of 5.5 cm There is no indication of a second absorption band or a hot band in the emission spectrum, which could be related to another electronic origin. We therefore assign the first trigona! component to the 4A 10 state. This assignment is supported by the observed dichroic ratio I(E I c)/i(EU c) 2. The nonvanishing iT intensity can 4Eg character into thebe4Aattributed to spin—orbit coupling which mixes some 15 state. We now turn to a discussion of the vibrational fine structure. The outstanding feature of the side band structure in fig. 4 is the progression on the MD-allowed origin. The first member has a frequency of 258 cm’. which compares well with the values of 265 cm -l in CsVCI3 and 255 cm ‘ in CsMgCl3 from Raman spectroscopy for the totally symmetrical vibrational mode [13,14,16]. Starting from the MD origin we find 6 ED-allowed vibronic origins 38 cm~, 123 cm’. 170 cm~, 205 cm 232 cm I and 311 cm’ lower in energy, each of which shows the same a,g progression as the MD origin. The side bands at v0 170 cm and ~‘0 311 cm can easily be associated with the two IR active modes e~5and e~(see table I). The side bands at v0 —205 cm and p0 123 cm are more intense with EU c. Referring to table 2 we deduce that the corresponding vibrations must transform either as a,~or b15 in the factor group. As there are no a15 vibrational modes in AMX3 compounds, we can assign these side bands to the two IR inactive b1~modes. The remaining two side bands at V0 38 cm and p0 —232 cm’ must be associated with the two IR inactive modes e~5and e,h11, 4A 4A - ~.
——
—
—
—
because a2~and b25 vibrations cannot act as enabling modes for the
25 -=
14
transition. The good agreement of the a10, e~5and e~5vibrational frequencies with results from IR and Raman spectroscopy show that a unit cell analysis may he valid. For a confirmation of the IR inactive vibrational frequencies neutron diffraction data would be useful. 4A 4Alg intensity is interesting note plus that about one third of the total 20 ~C hasItMD character to (origin a 10 progression) whereas in the very similar case
A. Hauser, H. U. Güdel / Single-crystal absorption and emission spectra
255
of Cs2NaInC16:Cr3~less than 10% of the emission intensity is MD [18]. The intensities of the members of an aig progression are related by the following approximate formula [19]: 2iT2vcm (~r)2 1 h
n—I’
where nis the number of quanta of a12 and ~r is the difference in equilibrium I/ 1)6 K)
0.5
i~0
200
IlK]
2~(•).
Fig. 6. Temperature dependence of the emission intensity of CsVCI3 (x) and CsMgCl3:V
distance between ground and excited state. In the emission spectrum of C5MgCI 2+ the a, 9:V 0 progression is well enough resolved to allow an estimate of ~r, using the above formula and the observed relative intensities: ~r 0.17 ± 0.02 A. 2~the vanadium(II) emission intensity is temperature indeIn CsMgCl3:V pendent within experimental accuracy between 10 K and 300 K (see fig. 6). Multiphonon relaxation processes do not appear to be active in this temperature range. From the absorption spectrum (see fig. 3) to the first excited state of vanadium(II) in CsMgCl 2~only the a,g vibrational mode can be extracted. With 255 cm’ it is only 3:V slightly lower than the ground-state frequency of 258 cm Considerably more fine structure can be resolved in the corresponding absorption spectrum of pure CsVC1 3 (see fig. 5). The MD electronics origin at 8299 cm’ shows four distinct side bands 53 cm’, 130 cm~, 188 cm’ and 228 cm ‘to the high-energy side which are assigned to vibronic origins. Again there is a progression in the a,5 mode on the MD origin and on the vibronic origins. Its first interval is 253 cm ‘, somewhat lower than the ground-state frequency of 265 cm ‘. —
256
A. Hauser, H. U. Gudel
/
Single-crystal absorption and emission spectra
6. Discussion of the CsVCI3 emission The CsVCI3 emission is very weak even at 6K. The pattern of the a,9 progression can be followed, however, the first interval being 262 cm ‘. This compares well with the value of 265 cm’ from a Raman scattering experiment [16]. In pure CsVC13 the emission intensity decreases as the temperature is raised (see fig. 6). Multiphonon relaxation can be ruled out as a ‘deactivation process by comparison with the doped compound. The large exchange interaction (—2J= 160 cm’ [4]) between neighbouring vanadium(II) ions makes an efficient excitation transfer along the chains to quenching traps possible [20]. According to Dexter [20] the excitation energy transfer rate is proportional to the spectral overlap of absorption and emission. Because of the observed Stokes’ shift of 1200 cm’ and the corresponding large difference in equilibrium position of the nuclei between ground and excited state, the spectral overlap increases with temperature, leading to a decreasing emission intensity. in good agreement with the observed temperature dependence. At 6 K the overlap is still relatively high, because of the high intensity of the MD origin. The resulting transfer rate is several orders of magnitude larger than l/TR, where the radiative lifetime TR 700 ~es as estimated from the absorption intensity [21], because the total emission intensity at 6 K is orders of magnitude 2~. weaker for CsVC13 than for CsMgCl3:V References [1] H.J. Seifert and P. Ehrlich, Z. Anorg. AlIg. Chem. 302 (1969) 284. [2] G.L. McPherson, T.J. Kistenmacher and GD. Stocky, J. Chem. Phys. 52 (1970) 815. [3] A. Hauser and H.U. Gudel, Chern. Phys. Lett. 82 (1981) 72. [4] M. Niel, C. Cros, M. Vlasse, M. Pouchard et P. Hagenmuller, Mat. Res. Bull. 11(1976) 827. [5] M.D. Sturge, Solid State Com. 9 (1921) 899. [6] M.D. Sturge, Phys. Rev. BI (1970) 1005. [7] M.D. Sturge, Phys. Rev. 130 (1963) 639. [8] G. Viliani, 0. Pilla, M. Montagna and A. Boyrivent, Phys. Rev. B23 (1981) 18. [9] U. Geiser. Lizentiat Univ. Bern (1980). [10] G. Herzberg and E. Teller, Z. Physik. CHern. B21 (1933) 410. [II] J. Ferguson, Progr. Inorg. Chern. 12 (1970) 205. [12] CD. Flint, Coord. Chern. Rev. 14 (1974) 47. [13] lW. Johnstone, GD. Jones and D.J. Lockwood, Solid State Corn. 39(1981) 395. [14] W. Breitling, W. Lehmann, T.P. Srinivasan and R. Weber, Solid State Corn. 20 (1976) 525. [15] G.L. McPherson and JR. Chang, lnorg. Chem. 12 (1973) 1196. [16] A. Hauser, unpublished work. [17]Y. Tanabe, T. Moriya and S. Sugano, Phys. Rev. Letters 15 (1965) 1023. [18]H.U. Gudel and T.R. Snellgrove, Inorg. Chern. 17 (1978) 1617. [19] C.J. Ballhausen, Theor. Chim. Acta 1(1963) 285. [20] D.L. Dexter, J. Chern. Phys. 21(1953) 836. [21]G.F. Irnbusch, in: Luminescence of Inorganic Solids, B. Di Bartolo, ed. (Plenum Press. New York, 1948) p. 115.
249
SINGLE-CRYSTAL ABSORPTION AND EMISSION SPECTRA OF CsVCI3 AND CsMgCI3:V2~ Andreas HAUSER and Hans U. GUDEL Institut fur Anorganische Chemie, Universität Bern, CH-3000 Bern 9, Switzerland
Received 6 April 1982
Single-crystal absorption and emission spectra of CsVCI 2~ in the region of the first excited state were measured at temperatures 3down and toCsMgCl3:V 6 K. The 4A2g ‘—‘4T 4T2g state and surprisingly well-resolved 25 transition fine structure shows which a large can be trigonal analyzed splitting in terms of of theelectronic origins and vibronic side bands.
1. Introduction CsVC1 3 and CsMgC13 crystallize in the CsNiC13 type structure, space group P63/mmc [1,2], the prominent feature of which is its quasi-one-dimensional character with chains of trigonally distorted, face-sharing MCi6 octahedra running parallel to the c-axis. Absorption spectra of CsVX3 (X = Cl, Br, I) single crystals in the region of d—d bands have been reported previously [3]. It was shown that the intensity of the spin-forbidden transitions is strongly enhanced by the large exchange coupling between neighbouring vanadium(II) ions along the chains [4],whereas the spin-allowed transitions are not affected. Up to now2~,where emissionthe from 4T vanadium(II) compounds has been measured in KMgF3:V 2g state to states a dynamic Jahn—Teller 2~,where the is subject and 2Tjg are lower in energyeffect than [5,6],and in MgO:V the 4T2g [7,8]. In the present communication we report single-crystal absorption and emission spectra of both pure CsVC1 2~ in the 3 and CsMgCl3:V region of the 4A2g ~4T 2g transition at 6 K. The fine structure is fully analyzed.
2. Experimental All the crystals grown by themeasurements Bridgeman technique [2].mol% The 2~crystalwere used for absorption contained 0.3 CsMgCl3:V vanadium(II), the one used for emission less than 0.1%. The crystals cleave easily along the c-axis. Absorption spectra were obtained on a Cary 17 0022-231 3/82/0000—0000/$02.75
©
1982 North-Holland
250
A. Hauser, H. U. Güdel
/
Single-crystal absorption and emission spectra
spectrometer using a matched pair of Glan—Taylor prisms for polarization. Emission spectra were recorded with a 3/4 m Czerny—Turner monochromator (Spex 1702) with a grating blazed at 750 nm and a lead sulfide detector cooled to liquid nitrogen temperature. In order to eliminate the dark current of the detector a 325 Hz chopper and a lock-in amplifier (PAR 186) were used. For the excitation a sealed beam xenon lamp (Varian R-l50-7A) in conjunction with a broad band filter (Schott KG3) was used. Low temperatures were achieved with a helium gas flow cooling technique.
3. Theoretical background Vanadium(II) is a d3 system with a 4A2g ground state. The site symmetry of vanadium(II) in CsVCI 3 as well as in the doped material is D3d, i.e. the 4T inversion center is retained. The first excited state is a 26 (O~~ notation). The transition from the ground state is therefore spin allowed and magnetic dipole (MD) allowed, but it remains electric dipole (ED) forbidden. 3.1. Trigonal splitting and spin—orbit interaction
In fig. 1 the effects of the trigonal field and the spin—orbit interaction on the ground state and the first excited state are shown schematically. For a large 4A trigonal splitting ~ the two Kramer’s doublets of the 15 (D3d notation) are nearly 4E degenerate (splitting < 1 cm I) whereas the four Kramer’s doublets of the 5 are split by 10 to 20 cm~[9]. 3.2. Selection rules
In D3d true electronic origins are only observable they are MD 4A2g if ground state to allowed. the two The selection rules for the transitions from the 03d
spin - orbit interact on
4
~
//___
Eg~~
3[~,’ ~‘+~
splitting 10 -20cm1
~(
-1
2g
[j
.~
~‘
splitting <1cm for large ~ zero field
~
A
1 24
~
splitting<01 cm
Fig. 1. The effects of a trigonal field and spin—orbit interaction on the 4A 2~ground state and the 2g first excited state of vanadium(11) in CsVCI3 (schematic representation).
4T
A. Hauser, H. U. Gudel
/
Single-crystal absorption and emission spectra
25 1
trigonal components of the first excited state are as follows: 2g —a4A Ig MD-allowed with El c, 4A2g
-.4E 6
MD-allowed with EH c.
Herzberg and Teller [10] postulated an intensity mechanism for ED allowed vibronic side bands [11,12]. Factor group analysis for AMX3 compounds [13,14] gives 15 odd parity vibrations at k 0 which belong to the following representations in D6h: 2a2~+ 3e1~+ 2b1~+ b2~+ 2e2~. The frequencies of the a2~and e1~modes are known from IR spectroscopy [15] for both CsVC13 and CsMgC13 (see table 1). According to Johnstone et a!. [13] the low-energy a25 mode and the ~ modes involve movements of A atoms only. In4Afirst order they are not expected contribute to of thethe vibronic structure 4A 4Eg transition. Undertothe operations site group D of the 26 ~~ 1g, 3d the remaining modes transform as 2a25 + 2a1~+ 4ev. The vibronic selection rules for electric dipole allowed transitions in D3d are summarized in table 2 together with the factor group-site group correlation. 4A 4Eg We thusfor expect 6 vibronic origins for thebe 15 transition and 8 forspectrum the transition k 0. There will, however, contributions to the
Table I Ground state vibrational frequencies at k =0 for CsMCI
CsMgCI3 ~
CsVCI3
255 82 174 317
265 d) 83 162 312
a)
3 (in cm 1)~ 2~l~ CsMgC13:V
Atoms involved in normal coordinates C)
a 15 ~ ei’.,, ~
~ b~0 b~ ~
b) 0 d)
0
—
—
—
—
49 250
52 256
—
—
—
—
Ref. [15]. This work. Refs. [13,14]. Ref. [16].
258 170 311
Cl only largely Cs largely Cl M,CI
38 232
MCI M,C1
—
— —
123 205
largely Cs largely M. Cl M,CI M.d
252
A. Hauser, H. U. Gudel
/
Single-crystal absorption and emission spectra
Table 2 Factor group—site group correlation for odd-parity vibrations and vibronic selection rules for 2g Ag~4E 4 T 5( 25) electronic transitions. D3d (enabling mode)
D65
aio~~=~a
Electronic transition
Polarization
4A25=4A,S
ElI
=
~
~ 4A~
=4E 5 ~ 5
ela~
e2~~
forbidden Elc
EIIc. Elc
involving vibrational functions with k ~ 0 [12]. Providing that the Cl—M interaction is much stronger than the Cl—Cs interaction this will merely result in a broadening of lines, so that the unit cell analysis can explain most of the intense features.
4. Results Figs. 2 and 3 show the main features of the of absorption 2~in the region the 4A and 4T emission spectra of CsVC13 and CsMgCl3:V 25—a 75 (O~notation) transition. The nearly equal extinction coefficients per vanadium in the pure and diluted crystals indicate that their intensity is due to a single-ion mecha-
Emission
Absorption
/~
orb. units
4
A19
—
~
/
~
L/molcm
\E//c
/tII/\~
7.0
8.0
90
1O~O
Fig. 2. Absorption and emission spectra of the crystals of CsVCI1 at 6K. The electronic origin ~
4A110 4T 25 25 (Oh notation) transition in single = 8299 cm
A. Hauser, H. U. GQdel Emission
/
Single-crystal absorption and emission spectra
253
Absorption
I
S
orb units
Amolcm —
f\EIC
“
E.Lc
~/i
80
cm~xi~~~
100 110 2t The vanadium (II) concentration in absorption is 0.3 Fig. 3. As for fig. 2 but for CsMgCl mole%, in emission less than 0.1 mole%. 3:V The electronic origins 0 = 8179 cm~. 70
90
pr ogressr Cci
76788082
2~emission spectrum at 6 K.
Fig. 4. Side band structure analysis of the CsMgCl3:V 5
253—
252——
8:
235—
88
239—
90
Fig. 5. Side band structure analysis of the 4A2g
cmxi~~ 4A
4T 15(
25) absorption in CsVCI3 at 6 K.
254
A. Hauser, H. U Gudel / Single-crystal absorption and emission spectra
nism. In contrast to spin-forbidden transitions spin-allowed d—d transitions are not expected to be enhanced by the strong exchange coupling between neighbouring vanadium ions in the chains [17]. The trigonal splitting ~ of the 4T2~state is large: 1400 cm I for CsVCI3 and somewhat smaller for CsMgCl3:V~.The sign can be determined from an analysis of the fine structure, which is reproduced in figs. 4 and 5.
5. Analysis of the fine structure The coincidence in absorption and emission the sharp at 8299 cm an t in CsMgCl 2~ of allows us to line identify it as in CsVC13 and 8178 cm 3:V electronic origin. The band shape is symmetrical with a half-width of 5.5 cm There is no indication of a second absorption band or a hot band in the emission spectrum, which could be related to another electronic origin. We therefore assign the first trigona! component to the 4A 10 state. This assignment is supported by the observed dichroic ratio I(E I c)/i(EU c) 2. The nonvanishing iT intensity can 4Eg character into thebe4Aattributed to spin—orbit coupling which mixes some 15 state. We now turn to a discussion of the vibrational fine structure. The outstanding feature of the side band structure in fig. 4 is the progression on the MD-allowed origin. The first member has a frequency of 258 cm’. which compares well with the values of 265 cm -l in CsVCI3 and 255 cm ‘ in CsMgCl3 from Raman spectroscopy for the totally symmetrical vibrational mode [13,14,16]. Starting from the MD origin we find 6 ED-allowed vibronic origins 38 cm~, 123 cm’. 170 cm~, 205 cm 232 cm I and 311 cm’ lower in energy, each of which shows the same a,g progression as the MD origin. The side bands at v0 170 cm and ~‘0 311 cm can easily be associated with the two IR active modes e~5and e~(see table I). The side bands at v0 —205 cm and p0 123 cm are more intense with EU c. Referring to table 2 we deduce that the corresponding vibrations must transform either as a,~or b15 in the factor group. As there are no a15 vibrational modes in AMX3 compounds, we can assign these side bands to the two IR inactive b1~modes. The remaining two side bands at V0 38 cm and p0 —232 cm’ must be associated with the two IR inactive modes e~5and e,h11, 4A 4A - ~.
——
—
—
—
because a2~and b25 vibrations cannot act as enabling modes for the
25 -=
14
transition. The good agreement of the a10, e~5and e~5vibrational frequencies with results from IR and Raman spectroscopy show that a unit cell analysis may he valid. For a confirmation of the IR inactive vibrational frequencies neutron diffraction data would be useful. 4A 4Alg intensity is interesting note plus that about one third of the total 20 ~C hasItMD character to (origin a 10 progression) whereas in the very similar case
A. Hauser, H. U. Güdel / Single-crystal absorption and emission spectra
255
of Cs2NaInC16:Cr3~less than 10% of the emission intensity is MD [18]. The intensities of the members of an aig progression are related by the following approximate formula [19]: 2iT2vcm (~r)2 1 h
n—I’
where nis the number of quanta of a12 and ~r is the difference in equilibrium I/ 1)6 K)
0.5
i~0
200
IlK]
2~(•).
Fig. 6. Temperature dependence of the emission intensity of CsVCI3 (x) and CsMgCl3:V
distance between ground and excited state. In the emission spectrum of C5MgCI 2+ the a, 9:V 0 progression is well enough resolved to allow an estimate of ~r, using the above formula and the observed relative intensities: ~r 0.17 ± 0.02 A. 2~the vanadium(II) emission intensity is temperature indeIn CsMgCl3:V pendent within experimental accuracy between 10 K and 300 K (see fig. 6). Multiphonon relaxation processes do not appear to be active in this temperature range. From the absorption spectrum (see fig. 3) to the first excited state of vanadium(II) in CsMgCl 2~only the a,g vibrational mode can be extracted. With 255 cm’ it is only 3:V slightly lower than the ground-state frequency of 258 cm Considerably more fine structure can be resolved in the corresponding absorption spectrum of pure CsVC1 3 (see fig. 5). The MD electronics origin at 8299 cm’ shows four distinct side bands 53 cm’, 130 cm~, 188 cm’ and 228 cm ‘to the high-energy side which are assigned to vibronic origins. Again there is a progression in the a,5 mode on the MD origin and on the vibronic origins. Its first interval is 253 cm ‘, somewhat lower than the ground-state frequency of 265 cm ‘. —
256
A. Hauser, H. U. Gudel
/
Single-crystal absorption and emission spectra
6. Discussion of the CsVCI3 emission The CsVCI3 emission is very weak even at 6K. The pattern of the a,9 progression can be followed, however, the first interval being 262 cm ‘. This compares well with the value of 265 cm’ from a Raman scattering experiment [16]. In pure CsVC13 the emission intensity decreases as the temperature is raised (see fig. 6). Multiphonon relaxation can be ruled out as a ‘deactivation process by comparison with the doped compound. The large exchange interaction (—2J= 160 cm’ [4]) between neighbouring vanadium(II) ions makes an efficient excitation transfer along the chains to quenching traps possible [20]. According to Dexter [20] the excitation energy transfer rate is proportional to the spectral overlap of absorption and emission. Because of the observed Stokes’ shift of 1200 cm’ and the corresponding large difference in equilibrium position of the nuclei between ground and excited state, the spectral overlap increases with temperature, leading to a decreasing emission intensity. in good agreement with the observed temperature dependence. At 6 K the overlap is still relatively high, because of the high intensity of the MD origin. The resulting transfer rate is several orders of magnitude larger than l/TR, where the radiative lifetime TR 700 ~es as estimated from the absorption intensity [21], because the total emission intensity at 6 K is orders of magnitude 2~. weaker for CsVC13 than for CsMgCl3:V References [1] H.J. Seifert and P. Ehrlich, Z. Anorg. AlIg. Chem. 302 (1969) 284. [2] G.L. McPherson, T.J. Kistenmacher and GD. Stocky, J. Chem. Phys. 52 (1970) 815. [3] A. Hauser and H.U. Gudel, Chern. Phys. Lett. 82 (1981) 72. [4] M. Niel, C. Cros, M. Vlasse, M. Pouchard et P. Hagenmuller, Mat. Res. Bull. 11(1976) 827. [5] M.D. Sturge, Solid State Com. 9 (1921) 899. [6] M.D. Sturge, Phys. Rev. BI (1970) 1005. [7] M.D. Sturge, Phys. Rev. 130 (1963) 639. [8] G. Viliani, 0. Pilla, M. Montagna and A. Boyrivent, Phys. Rev. B23 (1981) 18. [9] U. Geiser. Lizentiat Univ. Bern (1980). [10] G. Herzberg and E. Teller, Z. Physik. CHern. B21 (1933) 410. [II] J. Ferguson, Progr. Inorg. Chern. 12 (1970) 205. [12] CD. Flint, Coord. Chern. Rev. 14 (1974) 47. [13] lW. Johnstone, GD. Jones and D.J. Lockwood, Solid State Corn. 39(1981) 395. [14] W. Breitling, W. Lehmann, T.P. Srinivasan and R. Weber, Solid State Corn. 20 (1976) 525. [15] G.L. McPherson and JR. Chang, lnorg. Chem. 12 (1973) 1196. [16] A. Hauser, unpublished work. [17]Y. Tanabe, T. Moriya and S. Sugano, Phys. Rev. Letters 15 (1965) 1023. [18]H.U. Gudel and T.R. Snellgrove, Inorg. Chern. 17 (1978) 1617. [19] C.J. Ballhausen, Theor. Chim. Acta 1(1963) 285. [20] D.L. Dexter, J. Chern. Phys. 21(1953) 836. [21]G.F. Irnbusch, in: Luminescence of Inorganic Solids, B. Di Bartolo, ed. (Plenum Press. New York, 1948) p. 115.