Optical detection of paramagnetic centres: from crystals to glass-ceramics
An unambiguous attribution of the absorption spectra to definite paramagnetic centres identified by the EPR techniques in the most cases is problematic. This problem may be solved by applying of a direct measurement techniques—the EPR detected via the magnetic circular dichroism, or briefly MCD–EPR....
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Uldis Rogulis 2018-01-16T17:29:09Z 2018-01-16T17:29:09Z 2016 Optical detection of paramagnetic centres: from crystals to glass-ceramics / Uldis Rogulis // Физика низких температур. — 2016. — Т. 42, № 7. — С. 689-693. — Бібліогр.: 21 назв. — англ. 0132-6414 PACS: 76.30 Da, 76.70 Hb https://nasplib.isofts.kiev.ua/handle/123456789/129165 An unambiguous attribution of the absorption spectra to definite paramagnetic centres identified by the EPR techniques in the most cases is problematic. This problem may be solved by applying of a direct measurement techniques—the EPR detected via the magnetic circular dichroism, or briefly MCD–EPR. The present survey reports on the advantages and disadvantages applying the MCD–EPR techniques to simple and complex paramagnetic centres in crystals as well as glasses and glass-ceramics. This work was supported by Latvian Science Council project No 302/2012. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Low-Temperature Radiation Effects in Wide Gap Materials Optical detection of paramagnetic centres: from crystals to glass-ceramics Article published earlier |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics Uldis Rogulis Low-Temperature Radiation Effects in Wide Gap Materials |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics |
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optical detection of paramagnetic centres: from crystals to glass-ceramics |
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Uldis Rogulis |
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Uldis Rogulis |
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Low-Temperature Radiation Effects in Wide Gap Materials |
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Low-Temperature Radiation Effects in Wide Gap Materials |
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2016 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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An unambiguous attribution of the absorption spectra to definite paramagnetic centres identified by the EPR techniques in the most cases is problematic. This problem may be solved by applying of a direct measurement techniques—the EPR detected via the magnetic circular dichroism, or briefly MCD–EPR. The present survey reports on the advantages and disadvantages applying the MCD–EPR techniques to simple and complex paramagnetic centres in crystals as well as glasses and glass-ceramics.
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0132-6414 |
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Optical detection of paramagnetic centres: from crystals to glass-ceramics / Uldis Rogulis // Физика низких температур. — 2016. — Т. 42, № 7. — С. 689-693. — Бібліогр.: 21 назв. — англ. |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7, pp. 689–693
Optical detection of paramagnetic centres: from crystals
to glass-ceramics
Uldis Rogulis
Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV-1063, Latvia
E-mail: rogulis@latnet.lv
Received January 18, 2016, published online May 25, 2016
An unambiguous attribution of the absorption spectra to definite paramagnetic centres identified by the EPR
techniques in the most cases is problematic. This problem may be solved by applying of a direct measurement
techniques — the EPR detected via the magnetic circular dichroism, or briefly MCD–EPR. The present survey
reports on the advantages and disadvantages applying the MCD–EPR techniques to simple and complex para-
magnetic centres in crystals as well as glasses and glass-ceramics.
PACS: 76.30 Da Ions and impurities: general;
76.70 Hb Optically detected magnetic resonance (ODMR).
Keywords: EPR, ODMR, magnetic circular dichroism, paramagnetic centres, point defects
1. Introduction
Magnetic resonance technique, particularly electron
paramagnetic resonance (EPR) (or alternatively, electron
spin resonance, ESR) spectroscopy is the powerful method
for structural characterization of paramagnetic point de-
fects in solids [1–10] which allows the determination (1)
nature and valence of the impurity; (2) nature and number
of ligands; (3) symmetry of the complex; (4) possible pres-
ence of nearby defects; (5) true metal-ligand distance and
its dependence on pressure and temperature etc. The opti-
cally detected magnetic resonance (ODMR) allows the
investigation of the structure of luminescence and colour
centres [11].
In this survey we report on the advantages and disad-
vantages when applying the EPR detected via the mag-
netic circular dichroism (MCD–EPR) techniques to sim-
ple and complex paramagnetic centres in crystals as well
as glasses and glass-ceramics. The MCD–EPR is one of
the variations of the optically-detected magnetic reso-
nance techniques [11].
The main advantages of the MCD–EPR are:
— the linkage between the paramagnetic and optical
properties of the colour centre could be estimated directly;
— if several optical bands overlap, they could be sepa-
rated;
— in several cases, it is possible to identify the struc-
ture of the paramagnetic (PM) centre by the MCD–EPR
angular dependencies;
— after the magnetic circular dichroism is identified, it is
possible to follow the changes of the defect concentration
involved in definite processes via the MCD changes. It is es-
pecially useful, if the absorption bands overlap, but the MCD
bands have very characteristic features.
There are some disadvantages and restrictions:
— the MCD–EPR linewidths is, as a rule, larger than
the corresponding EPR lines, mainly due to the lower ho-
mogeneity of the ODMR magnets;
— not all the absorption bands possess an intense MCD
signal;
— if the MCD–EPR is structureless or its angular de-
pendencies are not pronounced, a further characterisation
is possible only if the PM centre has a well parametrised
EPR spectrum.
We will discuss the experimental aspects of the MCD–
EPR and then give several examples each characterising
the information available, when applying the MCD–EPR
to the optical detection of the paramagnetic centres in crys-
tals, glasses and glass-ceramics.
2. Experimental aspects
The MCD–EPR spectrometer is, as a rule, custom-built
low-temperature one. The details are described elsewhere
[11]. It consists of the following main units: magneto-
optical cryostat with a sample cavity, microwave accesso-
ries and a circular polarisation unit. While the EPR spec-
trometers usually work using the X-band microwaves, the
MCD–EPR spectrometers are, as a rule, equipped with at
© Uldis Rogulis, 2016
Uldis Rogulis
least 24 GHz microwave source, often also with 36 GHz to
45 GHz or even 72 or 95 GHz sources. The higher micro-
wave frequencies for the MCD–EPR techniques are neces-
sary to reach higher magnetic fields for the MCD and a
better resolution for the MCD–EPR spectra. The necessary
magnetic fields are from at least 2 T for the 24 GHz mi-
crowave band up to at least 4 T for the 93 GHz microwave
range. Therefore, the superconducting magnets at liquid
helium temperatures should be used.
At first, the PM part of the MCD should be separated
[11]. It could be reached by comparing the MCD at two
different temperatures, for example, 4.2 and 1.5 K. The
PM MCD part at 1.5 K is about of 2 times stronger com-
pared to the MCD at 4.2 K.
The MCD–EPR measurement is performed by scanning
the magnetic field at some of PM MCD wavelengths and
applied microwave power [11].
So-called “tagged MCD” spectra [11] could be meas-
ured, by switching on-off microwave power at different
MCD wavelengths.
The most complicated version of the techniques is the
so-called optically-detected ENDOR [12], however, rarely
applicable due to its technical complexity.
3. Results and discussion
To demonstrate the examples of the MCD–EPR appli-
cation and the information available, we separated them in
subsections 3.1.–3.4.
3.1. Estimation of the EPR parameters through the
MCD–EPR
At first, we discuss the possibilities to estimate some of
the EPR parameters through the MCD–EPR by the exam-
ples of Cd-centres in BaF2.
Optical absorption and MCD spectra (shown in Fig. 1.)
of γ-irradiated BaF2 single crystals of type I and II are dif-
ferent (see details in [13]).
To find out the nature of the centres responsible for
these spectra, the MCD–EPR techniques has been applied.
The MCD–EPR spectra shown in the inset of the Fig. 1,
reveal two different hyperfine structure (HFS) lines in both
types of BaF2. The first one belong to the Cdc
+ centre and
the second one, with slightly smaller HFS – to a perturbed
Cd-related centre [13]. According to the EPR data, this
centre has a superhyperfine (SHF) interaction with only 7
fluorine, i.e., one fluorine would be substituted by an ion
without resolved SHF structure [14].
3.2. Estimation of the optical bands of centres through the
“tagged MCD”
The second line of experimental possibilities of the
MCD–EPR techniques is the estimation of the optical
bands of centres through the “tagged MCD” on the exam-
ple of Ga2+ hole centres in RbBr.
Ga-related centres, especially hole centres, have been
widely investigated. However, only absorption or even
MCD measurements alone didn’t allowed to identify dif-
ferent Ga2+ hole centres [15]. The MCD–EPR spectra
shown in Fig. 2 (based on [15]) allowed estimate the HFS
parameters of two different Ga2+ hole centres, which MCD
spectra significantly differ.
These different MCD spectra could be the most clearly
resolved by so-called “tagged MCD” techniques, switching
on-off microwaves at a fixed resonance field and recording
Fig. 1. MCD of the BaF2 crystals of two types measured at B = 2 T;
inset: MCD–EPR of the Cdc
+ centre measured at 310 nm MCD
(curve 1) and of the Cdc
+ perturbed centre measured at 300 nm
MCD (curve 2) in the 53 GHz microwave range. Cd hyperfine split-
ting for the perturbed centre is smaller as for the unperturbed centre.
Fig. 2. “Tagged MCD” of two Ga2+ centres in an x-irradiated
RbBr:Ga crystal measured at 1.5 K; inset: MCD–EPR of the
(Ga2+)’ centre measured at 370 nm (upper curve) and (Ga2+)”
centre detected at 289 nm at the 23.9 GHz microwave frequency.
690 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7
Optical detection of paramagnetic centres: from crystals to glass-ceramics
the microwave-induced changes for all the MCD wave-
lengths. These knowledge of the MCD bands allowed to
follow the recharging processes in the x-irradiated RbBr:Ga
crystals [15]. Similar research was successful in CsBr:Ga
and RbI:Tl crystals as well [16].
3.3. Structural identification of centres through the
MCD–EPR (CsI-Tl)
In CsI-Tl crystals, after x- or γ-irradiation, the induced
absorption and MCD spectra have been obtained, however,
the conventional EPR technique failed to give results.
Therefore, the MCD–EPR technique is the only one allow-
ing to get magnetic resonance spectra and to offer the pos-
sibility for the defect structure identification. The inset in
Fig. 3 (see details in [17]) shows the MCD–EPR spectrum
taken at the MCD-wavelength of 425 nm and B || [100].
Analysing the angular dependencies of the spectrum,
the model of the Tl-related centre consisting of three adja-
cent Tl ions has been estimated [17].
3.4. Estimation of the optical transition range for the
centre with known EPR
The first example is concerned with the optical transi-
tion range of the F-type centres in LiBaF3 crystals.
EPR measurements on LiBaF3 crystals, x-irradiated at
room temperature (RT) and recombination luminescence
detected EPR measurements on the samples x-irradiated at
4.2 K (see details in [18]) showed the presence of several
F-type centres, each with different EPR parameters. The
examination of this question by the MCD and MCD–EPR
techniques (Fig. 4), showed that the MCD bands created
after x-irradiation at RT and liquid-helium temperature are
different as well.
The MCD–EPR showed different broad resonance lines,
while the characteristic symmetry of the F-type centres re-
mains [18]. Therefore, the presence of these F-type centres
in LiBaF3 has been stated and the corresponding MCD and
absorption spectral regions have been estimated [18].
The second case is concerned with the optical transition
range of the phosphate radical centres in phosphate glasses.
Phosphate glasses could be coloured by x-irradiation.
The creation of 2
4PO − and 2
3PO − radicals has been esti-
mated by the EPR [19]. However, the correlation between PM
radicals and induced absorption bands has not been estimated
by direct methods. The MCD spectra shown in Fig. 5 (based
on [20]), alone also do not allow to do conclusions, how-
ever, measuring the MCD–EPR (see inset), the observed
HFS is the same as that estimated earlier by the EPR.
Therefore, it was concluded that both 2
4PO − and
2
3PO − radicals have very similar absorption shape and
are indeed responsible for the colouring of the phos-
phate glasses by x-rays.
The last example is concerned with the optical transi-
tion range of the Gd3+ centres in CaF2 crystallites in the
oxyfluoride glass-ceramics.
The analysis of the nature of absorption bands of impu-
rity/defect centres is complicated, especially if these bands
are broad. It is even more difficult to decide which of the
bands belong to the glasses and which to the crystallites in
the glass-ceramics. As we observed by the example of
Gd3+-doped oxyfluorides, the MCD spectra of the glass
Fig. 3. MCD of a γ-irradiated at 295 K CsI:Tl crystal measured at
T = 1.5 K and B = 2 T; inset: MCD–EPR of the Tl-trimer centre
measured at 425 nm MCD and 24.32 GHz microwave frequency
at B || [100].
Fig. 4. MCD of a LiBaF3 crystal (1) after x-irradiation at 4.2 K
and (2) after x-irradiation at RT measured at 1.5 K and B = 1 T;
inset: MCD–EPR of the F-type centre in LiBaF3 x-irradiated at
4.2 K, measured at 500 nm MCD for different magnetic field
orientations and microwave frequency of 24.42 GHz.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7 691
Uldis Rogulis
and glass-ceramics seemed to be very similar. The solution
has again been found by the MCD–EPR techniques. In
glasses, no MCD–EPR signal could be observed. The EPR
spectra of the Gd3+ centres in glasses are typical of low-
symmetry centres and the EPR is therefore broadened over a
whole magnetic field range. On the contrary, the EPR of
glass-ceramics showed spectra of the cubic Gd3+ centres with
well resolved fine structure [21]. The same Gd3+ spectrum but
without so good resolved fine structure has been detected also
in the oxyfluoride glass-ceramics (see Fig. 6, based on [21]).
Therefore, the MCD–EPR technique allows to find out
a direct correlation between the cubic Gd3+ centre and its
MCD, but a similar MCD in the glasses could be attributed
to the low-symmetry Gd3+ centres [21].
4. Conclusions
It is shown how the MCD–EPR techniques could be
successfully applied for optical characterization and identi-
fication of many paramagnetic centres in crystals, glasses
and glass-ceramics.
The main advantages are: the possibility of direct attri-
bution of the optical (MCD) transitions to certain para-
magnetic centres; the possibility to resolve the overlapped
absorption (MCD) bands of several centres and to follow
the behaviour of the PM centres during the processes; the
possibility to identify the structure of a PM centre if its
identifying via EPR is not possible.
The following disadvantages should be taken into ac-
count: the lower resolution of the MCD–EPR spectra; non-
availability of the measurements if the MCD is very weak
or absent at all; difficulties to identify the MCD–EPR spec-
tra if they are poorly resolved and the EPR data are princi-
pally not available.
Nevertheless, the MCD–EPR with described above re-
strictions has positioned itself as a powerful tool for direct
identification of the optical transition range of paramag-
netic centres. Its application field includes structural identi-
fication of complex paramagnetic centres in crystals as
well as amorphous structures–glasses and glass–ceramics.
This work was supported by Latvian Science Council
project No 302/2012.
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1. Introduction
2. Experimental aspects
3. Results and discussion
3.1. Estimation of the EPR parameters through the MCD–EPR
3.2. Estimation of the optical bands of centres through the “tagged MCD”
3.3. Structural identification of centres through the MCD–EPR (CsI-Tl)
3.4. Estimation of the optical transition range for the centre with known EPR
4. Conclusions
|