Luminescence and Energy Transfer

133 Pages·1980·6.992 MB·English

Table of Contents
The Luminescence of Closed-Shell Transition-Metal Complexes. New
Developments
G. Blasse . . . . . . . . . . . . . . . . . . . . . . . . . . .
Energy Transfer in Concentrated Systems
R. C. Powell, G. Blasse . . . . . . . . . . . . . . . . . . . . . . 43
Luminescence Properties of Uranate Centres in Solids
K. C. Bleijenberg . . . . . . . . . . . . . . . . . . . . . . . 97
Author-Index Volumes 1-42 . . . . . . . . . . . . . . . . . . . . 129
The ecnecsenimuL fo llehS-desolC
lateM-noitisnarT .sexelpmoC
New stnempoleveD
egroeG
Blasse
Physical Laboratory, State University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
This chapter describes new developments in the understanding of the luminescence of complexes
consisting of a central highly-charged transition metal ion without d electrons surrounded by a
number of oxygen ions (usually 4 or 6 in a tetrahedral or octahedral arrangement, respectively).
Examples are titanates, vanadates, molybdates and tungstates. Recently results from low-tempera-
ture measurements have become available. On the other hand reliable molecular-orbital calcula-
tions have been performed. These are compared with each other. It is possible to obtain from the
luminescence data information on the splitting of energy levels due to crystal-field and spin-
orbit interactions. It is stressed that still a lot of data are lacking and that considerable effort has
to be made to obtain a complete picture. Also the influence of ions with s 2 configuration (Pb 2+,
Bi 3+) on these complexes is considered.
Table of Contents
1 Introduction ........................................... 3
2 The Electronic Structure of Closed-Shell Transition-Metal Complexes .......... 4
2.1 Tetrahedral Complexes ..................................... 4
2.2 Octahedral Complexes ...................................... 6
3 The Luminescence of Closed-Shell Transition-Metal Complexes .............. 10
3.1 The Permanganate and the Chromate Complex (MnO~ and CRO42-) ........... 10
3.2 The Vanadate Complex VOW- • ................................ 11
3.3 The Titanate Complexes ..................................... 17
3.3.1 The TiO~- Complex ....................................... 17
3.3.2 The TiO 8- Complex ....................................... 17
3.3.3 The TiO~- Complex ....................................... 21
3.4 The Molybdate Complexes ................................... 22
3.4.1 TheMoO~- Complex ...................................... 22
3.4.2 The MoO~- Complex ...................................... 24
3.5 The Niobate Complexes ..................................... 25
3.5.1 The NbO~- Complex ...................................... 25
3.5.2 The NbO67- Complex ...................................... 25
3.6 The ZrO~- Complex ...................................... 25
3.7 The Tungstate Complexes .................................... 26
3.7.1 The Tungstate Complex WO 2- . ................................ 26
3.7.2 The Tungstate Complex WO66-. ................................ 27
3.8 The Tantalate Complexes .................................... 30
3.8.1 The Tantalate Complex TaO~- . ............................... 30
3.8.2 The Tantalate Complex TaO67- . ............................... 30
G. Blasse
3.8.3 The Tantalate Complex TaO~ 1- . ............................... 31
3.9 General Conclusions from Sect. 3 ........... . .................... 31
4 The Influence of Surrounding Cations with Low-Lying Energy Levels .......... 32
4.1 The Vanadate-Bismuth Complex (VO34--Bi 3+) ........................ 3 3
4.2 The Molybdate-Lead Complex (MoO2--pb 2+) ........................ 34
4.3 The Tur gstate-Lead Complex (WO2--pb 2+) ......................... 37
4.4 The Tungstate-Bismuth Complex (WO66--Bi3+) ....................... 39
5 Conclusions and Proposals for Further Research ....................... 39
6 References ............................................ 40
The Luminescence of llehS-desolC lateM-noitisnarT sexelpmoC
1 Introduction
In this chapter we wish to describe new developments in the understanding of the
luminescence of complexes consisting of a central highly-charged transition metal ion
without d electrons surrounded by a number of oxygen ions (usually 4 or 6 in a
tetrahedral or octahedral arrangement, respectively). In this chapter we will denote
these complexes generally by nPOM - , where n represents the number of oxygen
neighbours and p the formal charge of the complex. In Table 1 on page 31 we have listed
a number of complexes of this type following the periodic table. From a point of
luminescence intensity and phosphor applicability the WO 2- and the VO 3- group
are the best-known examples and have been studied extensively. From the point of
absorption spectroscopy and energy level calculations the MnO~ and CrO~- are well-
known representatives on which a very large number of papers have been published.
The luminescence of CaWO4 has been known for a long time. This phosphor
finds application since many decades (nowadays only as an X-ray phosphor in inten-
sifying screens). As a luminescent mineral (scheelite) this compound has also been
known to mineralogists. The luminescence which can be excited by short-wavelength
ultraviolet radiation, X-rays, cathode rays etc., shows an emission spectrum consist-
ing of a very broad band peaking at about 420 nm )1 as shown in Fig. 1. This emission
band corresponds to a broad excitation band peaking at about 240 nm, sothat the
Stokes shift of this luminescence is very large (some 16000 cm-1 ). Figure 1 is an
outspoken example of the luminescence spectra of the complexes under considera-
tion, although the Stokes shift may be smaller in other cases.
The first fundamental approach to the luminescence of the tungstate group was
made by Kr6ger t) who concluded from the study of a large number of tungstates
that the lattice group WO42- itself was responsible for the luminescence. At that time
it was considered unusual that luminescence should originate from a lattice con-
stituent itself. It was generally thought that the introduction of an activator was
necessary to obtain luminescence. Compounds in which lattice constituents are the
luminescent species were then called "self-activated phosphors" during many years.
Later it appeared that they are not a special class of luminescent materials. Their
properties are due to the inefficiency of energy migration through the host lattice as
will be discussed in Chap. 2 of this volume.
CX
Fig. .1 Emission dna excita-
tion spectrum of the tung-
state luminescence of 200 40O I I OO6 I
OWaC 4 at room temperature .X (nm)
G. Blasse
New interest in the luminescence of complexes of this type was generated in the
sixties when it was realized that rare-earth activated tungstates and vandates were very
suitable as laser materials and phosphors (the most well-known example being
YVOa-Eu 2)). Research profited from the extensive work by J¢rgensen on electron
transfer spectra of many types of complexes a) and it was soon realized that the
emission and absorption spectra of the complexes MOP - had to be ascribed to charge-
transfer transitions. Using this assignment it became also possible to understand the
thermal luminescence quenching somewhat better4).
New developments have occurred in the seventies. These were mainly due to
three factors:
a) use of laser excitation yielding time-resolved spectra. These were especially of
importance for an understanding of energy transfer phenomena (see Chap. 2).
b) the performance of low-temperature measurements. This aspect was neglected along
time probably because the spectra do not show vibrational structure. The life time
of the luminescence, however, may vary strongly in the helium temperature range.
c) advanced calculations on the energy level scheme of the MO p- complexes became
available. These related especially to the non-luminescent MnO 2 and CrO,-. It
was realized, however, that these groups are isoelectronic with, for example, the
VO4 a- and the 42OW - complexes.
The organization of this chapter is as follows. In Sect. 2 we will review some
calculations on complexes MOP - with a special stress upon the results. In the follow-
ing sections the recent experimental developments will be discussed and compared
with the results of calculations. For each metal M there is a separate section. Finally
we will try to give an outlook and to suggest research topics which are necessary to
complete our knowledge of the luminescence of these systems.
2 The Electronic Structure of Closed-Shell Transition-Metal Complexes
2.1 Tetrahedral Complexes
The most famous ion in this class is undoubtedly the permanganate ion. Wolfsberg and
Helrnholz s) were the first to actually calculate a M.O. level scheme for the MnO~ ion.
They started with M.O.'s which transform as irreducible representations of Td. Making
the simplifying effective-core approximation and using a Hiickel-type calculation
(no ~. 1 in the Hamiltonian) they arrived at the following one-electron level scheme
i<j rij
for the ground state of the MnO 4 ion: (1 al)2 (1 t2) 6 (1 e) 4 (2 t2) 6 (ti)6, the se-
quence of the excited orbitals being 3 t2 < 2 al < 2 e < 4 t2. Here the filled orbitals
are concentrated on the oxygen ions and the empty ones on the manganese ion,
sothat the lowest optical transitions can be considered as 2 p (oxygen)-3 d (manganese)
charge-transfer transitions.
However, in 1958 Ballhausen and Liehr )6 proposed a level scheme in which the
first empty orbital was assumed to have e symmetry, this being the order predicted
4
The Luminescence of Closed-Shell Transition-Metal Complexes
also by crystal-field theory. At this state there did not seem to be any argument to
make a definite choice between these alternatives, until ESR spectra of the manganate
ion convincingly showed that the unpaired electron was in the antibonding e level
rather than in the t2 level .)7 Several experiments have confirmed this.
Using the ordering of Ballhausen and Liehr it is easily seen that the first orbital
promotion gives rise to four states, viz. 1T1, 1T2, 3T1,3T 2 . A simple calculation
indicates that the order of these levels is E(1T2) > EOT1) > E(aT1) ~ E(ST2)8).
With respect to the excited states, it is often assumed that their nature can be under-
stood on the basis of a simple MO level diagram. These simple virtual-orbital ap-
proaches, however, do not take into account the orbital relaxation during the excita-
tion process. Assignments made on this basis must thus be considered with great
caution; below we will come back to this point. We may summarize the assignments
of the first four bands in the absorption spectrum of MnO~ as follows:
I: 1A 1 ~ 1Tt(t t ~ 2e); II: 1A 1 -+ 1T2(t 1 ~ 2e);
III: 1A 1 -+ 1TI(?)? ; IV: 1A 1 -+ 1T2(? ).
"Have the triplets ever been observed?", one may ask. As far as we are aware only
one author )9 comments on this point stating that, "there seems to be a broad band
whose centre is at about 10.000 cm- 1 ,,, immediately followed by: "but it may only be a
base-line effect." We now turn to the later developments in this field. In Fig. 2 we
present the level schemes resulting from advanced calculations (i.e. direct excited
state calculations involving some kind of orbital relaxation). These calculations are
non-relativistic. One should realize that in the literature there are as many different
level schemes as there are different theoretical methods lo). A criterium to judge the
quality of a calculated level scheme is of course its correspondence with the experi-
MnO~ CrO42" VO34" averaged states
~TTT~f'.1 (3 t2--~2el
(eV) E 7 "~'~ ."pk ~.. x 1 2T (3ac-~St2)
6 ~:.;i--" , ,. "'. ITI.'T2.' E .'A1(4t2-,5t =
\%.
5 .~$'f" ' "" 'TI"T2 'IE "A2(t,--~Stz
~:x"'~14t2 -Ib2e)
4 - - i-S: ......
3 -' ...... ' '\ '" 2Tl'IT1 (tl--lb2e)
/"/"
2 ...... " 2TJvT~;X (t 1 -~2e)
'%
Fig. 2. Electromc • energy level schemes of MnO -4 - , CrO 4 2-- and VO 4 3--- calculated by Zlegler .
11) 1
et al. . The energy E is given with respect to the A 1 ground state. The method used is the
HFSDVM, a version of the Xa method in which one averages over all states within one con-
figuration with the same spin-multiplicity. The excited state levels resulted from transition
state calculations
G, Blasse
Fig. 3a, b. A contour map of the difference
in electron density between the ground
state of MnO 4 and its t 1 -+ 5 t 2 excited
singlet state. The plot is in one of the
O-Mn-O planes. The dashes contours rep-
resent regions where the electron density
of the excited state is higher than that of
the ground state, the solid contours where
there is less electron density in the excited
then in the ground state: (a) separate SCF
calculations on the ground state and
t I +-- 5 t 2 excited state; (b) t 1 ~ 5 t 2
excited state constructed from ground state
occupied and virtual orbitals. Because
relaxation of the bonding orbitals is lacking,
the result is an erroneous build-up of charge
on the metal (change-transfer). After Ref. 11
mental results. The calculation of Ziegler et al. (Fig. 2)11) can be considered success-
ful in this sense. In addition to MnO~-, CrO- and VO4 3- Ziegler et al. calculated also
MoO~-, RuO 4 and TcO~ and came to a uniform level ordering for all calculated ions;
except for the third controversial band of MnO~- the calculated singlet-singlet transi-
tion energies are in good agreement with the experimental values.
An interesting feature of the calculations of Ziegler et al. is the following. In the
early LCAO-MO work it was recognized that in the transitions t 1 -+ 2 e and
3 t2 ~ 2 e an electron is transferred from MO's with no (or practically no) contribu-
tion of the metal orbitals (tl, 3 t2) to an orbital with significant metal participation
(2 e). For this reason these transitions were called "charge-transfer" transitions, in-
dicating a transfer of charge from the ligands to the metal upon excitation. Ziegler
et al. show that this result is inherent to the virtual orbital approach: when relaxation
of the ground state orbitals is taken into account, population analysis shows that
electronic excitation consists of a rearrangement of electron density both at the
ligand and at the metal instead of a build-up of charge on the metal at the expense of
the ligands (see Fig. 3).
Finally we emphasize the fact that the calculations involving triplet levels show
that the triplets due to the t 1 -+ 2 e orbital promotion are always below the first
singlet transition. We shall see later that these triplets are the levels from which
luminescence in closed-shell transition-metal complexes originates.
2.20ctahedralComplexes
The number of theoretical studies on octahedral complexes is considerably less than
on tetrahedral complexes. This is probably related to the fact that the number of
The Luminescence of Closed-Shell Transition-Metal Complexes
systems which contain isolated octahedral complexes seems to be lower than that for
tetrahedral complexes. In solids the octahedra usually share corners, edges or even
faces. A good exception to this rule is the ordered perovskite structure. Isolated
octahedral complexes in this crystal structure have been extensively studied in our
laboratory as will be shown in this and the companion chapters.
Richardson has reviewed his work on octahedral complexes in Ref. 12. Here we
are interested in his work on the isolated TiO6 8- complex, which was used to calculate
the energy band structure of SrTiO3 where the octahedra share corners. The position
of the lowest optical absorption band was found from AE = Ae - Ehp -- ER, where
Ae is the difference between the SCF orbital energies of the highest occupied O 2 p
orbitals and the lowest unoccupied Ti 3 d orbitals, phE is the hole-particle interaction
between the vacated O 2 p MO and the populated Ti dt2g MO and E R is the electronic
relaxation energy. For the isolated complex Ae was calculated to be 14.8 eV and Ehp
7.9 eV. Further ER is 2.7 eV. This yields for AE 4.2 eV. This is 0.9 eV too large if
compared with the optical absorption band gap of SrTiOa. It seems more realistic,
however, to compare this value with the results for La2MgTiO 6. This is an ordered
perovskite which contains isolated TiO6 octahedra. Macke has reported 4.1 eV for the
optical band gap t3). This is a very good agreement. Nevertheless this calculation does
not yield detailed information about the excited state, necessary to explain the results
of luminescence measurements.
Orbital enercjies
£lajC
O.51 2tlg
6eg 3t2u
2hg
3t2u
10.0
4t2g
9alg
5.0 lObu
4t2g
6eg
0 £ :-41.52 +23.52 +0.47 -22.96eV
ltlg
-5.0 ltlg~
~gtlu
9flu--..
~'2t2u
2t2u I
8alg 8alg
-10.0
ge5
ge5 .__.
3t2gf i
A ÷5'1 A 025+ WO 6- BI÷ B2 ÷
Fig. 4. Orbital energies of the valence orbitals as a function of the charge on the A or B sites.
1 tlg is taken as the reference level. After Ref. 37
.G essalB
/
~-* ~%-% ~o7 /
.--. o~,t -% woeA2")#2-)
,~-%
.
1 1
• o-,,'T,o---'_.?-y / / ..---C. t A2u,A lu
~-.'~,o-- - / / / / /r3A2u
.o--.',,,,,,,----..,/ / / /
-¢ ?r,u
,T3~-w u - --- / / /
- - 1,rl u
A3 3 x
"-''"-'~, 3Tt u
/ / /
__'Aagl, Tlg
--3Ttg
3A2g
b
I i I
Fig. 5. a Energy difference between the excited state and the ground state for different elec-
tronic configurations versus the charge on A and B sites; b These energy differences extrapolat-
ed to 2+ charges on A and B sites. After Ref. 37
Van Oosterhout )41 has performed a SCF-MO study on the isolated tungstate
group (W066-) 14). This study is of special importance for our goal, since the author
made the calculation in order to explain his luminescence measurements, whereas all
earlier calculations aimed to explain absorption spectra only. Since the isolated W066-
complex can be suitably studied in ordered perovskites A + B +2 WO 6 (A = Ca, Sr, Ba;
B = Mg, Ca, Sr, Ba) the influence of point charges on the A and B sites around the
tungstate complex was also taken into account.
Figure 4 shows the orbital energies of the valence orbitals as a function of the
charge on the A or B ions. It should be realized that the highest filled MO's are 1 tlg,
9 tlu and 2 t2u which are concentrated on the oxygen ion: the lowest unoccupied
M.O. is the 4 t2g (5 d) MO which is concentrated on tungsten. The energy level
structure of the excited state is given in Fig. 5.
The change in charge distribution relative to the ground state is illustrated in
Fig. 6 for the 1A2g (t~g - t~g) excited state in W 0 66 - as a representative example. We
observe the hole in the MO of tlg symmetry localized on the oxygen ions. The electron
is excited to the MO of t2g symmetry but this orbital is very much delocalized. Forty
percent of the charge is donated to the tungsten, 60% stays on the oxygen ions. We
also observe that there is an important shift of negative charge back to the oxygen in
The Luminescence of Closed-Shell Transition-Metal Complexes
0.5
t2g
t2g t tlul
ha ~'~,%
.'C" . \.
alg ~- eg eg alg
v /
-0.5 tlg
Fig. 6. Difference in total gross
atomic populations between the
ground state and the 1A2g excited -1.0 . . . . .
, s p d d f f ~ s p~ p~/
state in OW 6-. Aq = fi(1A2g ) - ~f y
fi(1Alg). After Ref. 37 w o
the o bonding molecular orbital of eg symmetry. A small amount of negative charge
is donated back to the oxygen in the molecular orbitals of alg and tlu symmetry. The
net charge transfer from oxygen to tungsten is 0.1 7 units of negative charge relative
to the ground state. This result parallels those obtained by Ziegler et al. mentioned
above (see Fig. 3).
The allowed absorption transition in the WO 6- complex is ascribed to the
1Alg - ITtu transition. The 1Tlu levels are at 5.19 and 5.03 eV above the ground
state for the t~u - g12t and t~u - tlg electronic configuration, respectively. This is
well within the experimental absorption band which extends from 4.0 to 5.5 eV.
The emission is ascribed to a different state, viz. the two 3Tlu states 0% -tg
and t% - t~g). In view of the observed value of the transition probability, spin-orbit
coupling must be important, which is not surprising in view of the nature of the
central ion. We may expect that spin-orbit coupling does not split the states signifi-
cantly due to the strong delocalization in the excited state (see Fig. 6). We will show
later on that this is in fact the case.
From Fig. 5a we infer that an increasing field from the B ions (higher charge on
the B site) leads to an increase in the energy difference between the ground state and
all excited states. This can be interpreted as a stabilization of the charge on the oxygen
ions. From Fig. 5 we also see that an increasing field from the A ions (higher charge
on the A site) leads to a lowering in energy difference between the ground state and
all excited states. From Fig. 4 we see that this is due to a stabilization of the tungsten
t2g orbital. If it is realized that the B ions are along the W-O directions and the A
ions in between these (see Fig. 7), this result is seen to be in agreement with simple
ionic arguments as presented earlier ,3 is).
We now turn to a consideration of the experimental data on the luminescence of
closed-shell transition-metal complexes. It will be shown that the results of the calcula-
tions mentioned in this chapter are very helpful in the interpretation of the experimen-
tal results.