Excited state spectroscopy of chromium ions in various valence states in glasses

Journal of Alloys and Compounds 341 (2002) 19–27

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Excited state spectroscopy of chromium ions in various valence states in glasses a, a b Cz. Koepke *, K. Wisniewski , M. Grinberg a

b

Institute of Physics, N. Copernicus University, Grudzia¸dzka 5 /7, 87 -100 Torun´ , Poland ´ , Wita Stwosza 57, 80 -952 Gdansk ´ , Poland Institute of Experimental Physics, University of Gdansk

Abstract The spectral properties of different types of chromium-activated glasses are shown, discussed and interpreted. Chromium in four valence states, Cr 31 , Cr 41 , Cr 51 and Cr 61 , is seen in the spectral characteristics of these glasses where Cr 31 , Cr 51 and Cr 61 play an important role. Trivalent chromium in octahedral coordination reveals the spectra and decays that are typical for low-field materials with a substantial participation of the site-to-site disorder. The characteristics of the Cr 51 and Cr 61 ions appeared to be more interesting. Absorption, excitation and luminescence of the pentavalent chromium (d 1 system) in the octahedral coordination are severely affected by the Jahn–Teller effect, along with the nuances associated with that effect. The hexavalent chromium (d 0 system) in the four-fold coordination, forming the [CrO 4 ] 22 group, is seen mostly in the excited state absorption (ESA) spectra that can be interpreted in terms of the transitions between crystal field split terms of the Cr 51 O 2 centre, which forms after the charge transfer (CT) transition, and between one of those terms and another double-electron state of larger electron lattice coupling, which forms after two consecutive transitions of electrons via the CT transition.  2002 Elsevier Science B.V. All rights reserved. Keywords: Disordered systems; Luminescence; Light absorption and reflection

1. Introduction Glasses activated by transition metal ions are still of interest from the point of view of possible laser applications. There are several transition metal activators attractive in this context, especially ions that gave rise to several successful lasers based on crystalline matrices. They are first of all: chromium and titanium. Since so far the efforts addressed to obtaining the titanium (Ti 31 ) activated glass of similar spectroscopic properties as Ti–sapphire medium are rather unsuccessful, we focused our attention on the chromium activated glasses. This is because the chromium ions of various ionic radii can be easily embedded in the glass host owing to a much looser and relaxed glass structure than that of crystals. In several glass matrices chromium ions can occur in different valence states simultaneously and Cr 31 , Cr 41 , Cr 51 and Cr 61 are most likely to be active and observed in those matrices. In this contribution we limit our considerations to three kinds of glasses: ZAS glass being locally of the ZrO 2 –A1 2 O 3 – SiO 2 composition, where the trivalent chromium ion is most active [1], the LBO glass of the local composition: *Corresponding author.

Li 2 B 4 O 7 , where we could observe the trivalent as well as hexavalent chromium ions [2], and finally the silica sol– gel glass of much more complicated local composition [3] where, although we could observe the tetra-, penta- and hexavalent chromium ions, but only two latter revealing activity of spectroscopically attractive features [4]. We have performed investigations typical for spectroscopic characterisation of the material i.e. measurements of the absorption spectra, luminescence spectra, luminescence decays, time-resolved luminescence spectra, luminescence excitation spectra and excited state absorption (ESA) spectra. We put a special stress upon the ESA measurements since these experiments are of greatest and most critical importance in determining whether the material is a good candidate to be a laser gain medium. In this paper we provide a short survey of the results obtained for the aforementioned glasses along with interpretations.

2. Material and experimental A detailed description of the material preparation is given in Ref. [1] for the ZAS glass, in [2] for the LBO

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00091-9

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glass and in [3] for the silica sol–gel glass. For the sake of this paper it is only important that from the point of view of Cr 31 ions the ZAS glass is a low-field material whereas the LBO glass is an interesting example of a medium-field material where the R-lines as well as broad quartet emission band are observed. In both those glasses one can see a substantial influence of the site-to-site disorder on the spectroscopic characteristics of the sixfold coordinated Cr 31 ions. Apart of the Cr 31 ions the LBO glass contains also a substantial amount of the Cr 61 ions of the four-fold coordination, which is seen in the absorption as well as in the ESA spectra. The silica sol–gel glass does not contain the Cr 31 ions at all because of the effective oxidation reactions during hydrolysis and dealcoholation that favour the formation of the Cr 41 , Cr 51 and Cr 61 ions [3] and all of them are seen in the spectroscopic characteristics of this material. Measurements of the absorption spectra and emission spectra were performed with the AVIV 14DS spectrometer (slits: 0.5 mm) and a 0.5-m grating monochromator combined with photomultiplier tube (PMT), respectively. The decays were detected by a PMT coupled to a SR430 photon counter. As the sources of excitation the Ar-ion, dye, Ti–sapphire lasers and the N 2 laser-pumped dye laser (for decays) were used. A Perkin-Elmer LS50B luminescence spectrometer registered the luminescence excitation spectra. The excited state absorption (ESA) spectra were measured using two alternate set-ups. One of them was similar to that described in [5] utilising a CW source of excitation (Ar ion laser), a tungsten lamp as the source of the probe beam, and an optical chopper (lock-in technique) and a 0.4-m grating monochromator plus PMT in the detection branch. The second set-up utilised a RD-EXC150 / 25 XeCl excimer laser (308 nm) as a source of excitation, a Hamamatsu Xe flash lamp as the source of the probe beam and an Ocean Optics S-1000 system or Oriel InstaSpec II photodiode array detector coupled to a MultiSpec 1 / 8 m spectrograph in the detection branch. The first ESA set-up worked in the CW regime and longitudinal geometry of beams passing through the sample, whereas the latter operated in pulsed regime and transverse geometry (as in [6]).

3. Results and interpretations We shall describe our results comparing the behaviour of the ions of a given valence state in different glass hosts, wherever it is possible.

3.1. Cr 31 ions The trivalent chromium ions are present in the ZAS and LBO glasses. In the ZAS glass the Cr 31 ions occupy the low-field sites of | octahedral symmetry. The ground state absorp-

tion spectra for both glasses are shown in Fig. 1a and c. Features characteristic for the octahedrally coordinated Cr 31 manifest mostly in the form of the quartet–quartet absorption bands [ 4 A 2g → 4 T 2g , 4 A 2g → 4 T 1g (a) and 4 A 2g → 4 T 1g (b) transitions]. These transitions are much more pronounced in Fig. 1b and d where the respective luminescence excitation spectra are depicted. According to the luminescence spectra, we observed in both glasses behaviour typical for materials with a strong site-to-site disorder, namely the dependence of the emission spectrum positions on the excitation wavelengths (Fig. 2b), which can serve to determination of the distribution of the sites over the energy [1,7]. The crystal field-induced sites distribution can be also seen in the excitation spectra for the LBO glass (Fig. 1d), where the differences in shape (the blue shift of the 4 T 2g peak when monitoring at 688 nm compared to that at 750 nm) is explained in [2] by means of this distribution. The exemplary low temperature (LT, |10 K) and room temperature (RT) luminescence spectra for the ZAS and LBO glasses are presented in Fig. 2a–d. The RT spectrum of the ZAS glass is a typical broad band corresponding to the 4 T 2g → 4 A 2g transitions in the low crystal field. The spectra for LBO glass are more interesting, displaying the 4 T 2g → 4 A 2g as well as 2 E g → 4 A 2g transitions of a different strength at different temperatures. The detailed descriptions and explanations of the phenomena associated with the emission in the Cr:LBO glass are given in several papers [7–9]. The domination of the 4 T 2g → 4 A 2g or 2 E g → 4 A 2g transitions depending on the temperature reflects in the emission decays seen in Fig. 3a–d for the ZAS and LBO glasses. The multiexponential decay is a manifestation of the siteto-site disorder and can be expressed by a histogram [1,10]: amplitude versus time constant. The most important results in view of possible laser applications are the ESA spectra of the Cr 31 ions shown in Fig. 4a for the ZAS and LBO glasses, the former (the excitation wavelength lexc 5308 nm) expressed in terms of the excited state difference cross section, the latter ( lexc 5 610 nm) in terms of the excited state transmission [2,11]. In the case of the Cr 31 :ZAS glass one can expect a gain in the region 7000–11 300 cm 21 which is likely in view of a lack of substantial ground state absorption in this region of the emission. The expected gain spectrum has been calculated using the McCumber theory [1,12,13]. The measurements utilising much more sensitive detector than that in the presented experiment are in progress. In the presented excited state difference spectrum one can distinguish also the ESA region (11 300–15 000 cm 21 ) and the region where the ground state absorption bleaching is dominant (15 000–27 000 cm 21 ). Using the data obtained from the absorption, emission and luminescence excitation spectra and the procedure described in Ref. [11] we have created a single configuration coordinate (SCC) diagram [1] which can serve to

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Fig. 1. Measured absorption (a, c, e) and luminescence excitation spectra (b, d, f). For Cr:LBO glass (d) the excitation spectrum monitored at different wavelengths alters due to the site-to-site disorder [2]. For Cr:sol–gel glass (e, f) the spectra are deconvoluted into Gaussian bands. Note the different wave number scale for the Cr:sol–gel glass.

predict the shape of the ESA spectrum. The SCC diagram utilises a Hamiltonian typical for the d 3 systems [11], the crystal field parameter 10 Dq, the Racah parameters B and C and the strength of the electron–lattice coupling, the four latter values taken from the obtained experimental data. The complete SCC diagram for the Cr 31 :ZAS glass is presented in Fig. 4b together with the provided shapes of the doublet–doublet and quartet–quartet ESA bands. Having in mind the degeneracy of initial states and the Boltzmann factors we have composed the doublet–doublet and quartet–quartet ESA bands into one theoretically predicted band [1] which can be compared to the experimental. The result, along with the influence of the ground state absorption bleaching is presented in Fig. 4a (ZAS glass) as a curve fitting the experimentally obtained excited state difference spectrum. The ESA spectrum (Fig. 4a, expressed in terms of the excited state transmission) connected with the Cr 31 ions in

the LBO glass is less complex, it does not exhibit any influence of the ground state bleaching, showing just simple ESA band peaking around 18 400 cm 21 . Using the above mentioned SCC procedure for the Cr:LBO glass we could ascribe the obtained ESA spectrum neither to the doublet–doublet nor to the quartet–quartet transitions. This suggests that in the LBO glass the Cr 31 -related ESA transitions occur from the states (both quartets and doublets) of the Cr 31 ions to the conduction band of the glass host [2].

3.2. Cr 41 ions There is not too much to say about the Cr 41 ions in the materials examined. We observed just ground state absorption features in the sol–gel glasses, but excitation into their wavelengths did not result in any luminescence in the

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Fig. 2. Low (a, c, e) and room temperature (b, d, f) emission spectra. For the Cr:ZAS glass the dependence of the band position on the excitation photon energy proves the site-to-site disorder. Spectra of the Cr:sol–gel glass are decomposed into Gaussian bands. Note the different wave number scale for the Cr:sol–gel glass.

region of interest, up to 1300 nm. This forced us to limit our considerations to Cr 31 , Cr 51 and Cr 61 ions.

3.3. Cr 51 ions The pentavalent chromium ions are distinctly seen in the spectral characteristics of our sol–gel glasses. They are responsible for a local maximum around 21 500 cm 21 (465 nm) in the ground state absorption spectra and luminescence excitation spectra (see also [14]) seen in Fig. 1e and f. Excitation into this wavelength region yields the emission which, however, is not simple. Fig. 2e and f presents the LT and RT emission spectra corrected for the spectral response of the experimental set-up and fitted by the least possible number of Gaussians. At RT one of the emission bands peaks around 14 600 cm 21 and at LT this band splits into two bands peaking around 14 100 and 15 350 cm 21 , whereas another, broader band, peaks around

13 300 cm 21 at LT and 13 700 cm 21 at RT. The RT band with the maximum around 14 600 cm 21 , can be ascribed to the octahedrally coordinated Cr 51 ions that are typical examples of the d 1 system along with all the consequences of such an assignment. The most important consequence is the Jahn–Teller (J–T) effect, quite natural in these systems [15,16]. This effect, as a result of the coupling to the two-dimensional vibrational mode eg , splits the ground manifolds into three intersecting paraboloids ( 2 T 2g ^ eg ) and the excited state into a Mexican hat-like hypersurface ( 2 E g ^ eg ) [17]. The J–T splitting in the excited state is responsible for two bands (peaking at |22 000 and |25 300 cm 21 ) seen in the absorption spectra (Fig. 1e) and, even more distinctly, in the luminescence excitation spectra (Fig. 1f). The J–T splitting in the ground state is seen in the LT luminescence spectra (Fig. 2e) in form of the aforementioned bands at |14 100 and 15 350 cm 21 . The existence of these two bands is proof that there is an

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Fig. 3. Low (a, c, e) and room temperature (b, d, f) decay profiles under different excitation wavelengths and observed at various emission wavelengths. Note the different time scales for each row.

admixture of anharmonic vibrations [17,18] in the overall process. In such a situation the bottom trough of the Mexican hat-like 2 E g state becomes more structured having three shallow (#200 cm 21 ) wells with saddles between them. Hence at LT the population is frozen in the wells and only from these points the transitions can occur to the ground state (only two paraboloids of the 2 T 2g state are accessible for transitions from the wells). This is why the 14 600 cm 21 emission band splits at LT into two bands (14 100 and 15 350 cm 21 ). At RT the situation is simpler: the emission probability from the bottom of the ‘Mexican hat’ to the ground state tends to be the same, regardless of the current position in the trough of the 2 E g state. Because of the different splitting and positions of the manifolds constituting the 2 T 2g state, seen from the trough of the 2 E g state, the transitions from the 2 E g state end-up in the rich variety of the 2 T 2g state energies. Such an averaging of the transitions energies causes the structureless luminescence

spectrum. This behaviour is well recognised in case of the spectrum of Ti–sapphire lasers [15,19]. We did not observe any ESA characteristics that could be connected with the Cr 51 ions. This is justified theoretically since the d 1 system has just one excited state 2 E g (not counting the J–T splitting). The Cr 51 -induced ESA, if it exists, could be situated in the infrared range at least in the case of transitions between the J–T split 2 E g state. In the decay time characteristics we observe a component of |ms time constant either at LT and RT. It is believed to be a typical decay time for the Cr 51 -related emission [14,20].

3.4. Cr 61 ions The most interesting from spectroscopic point of view are the Cr 61 ions that occur in the Cr:LBO glass and silica sol–gel glass:Cr. They exist in the form of the Cr 61 O 22

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Fig. 4. Measured ESA spectra (a) and corresponding SCC diagrams as interpretation (b,d). Smooth lines in the ESA spectra are theoretical fits. Right hand sides of the SCC diagrams represent the shapes of theoretically predicted ESA spectra that fit the experimental ones. Note that the spectra for the Cr:ZAS glass are expressed in terms of cross-sections whereas the remaining ESA spectra in terms of the ESA absorption coefficients.

centres in the frame of the [CrO 4 ] 2 groups [20]. The [CrO 4 ] 22 group is a typical example of the closed shell molecular complex of tetrahedral coordination, namely a typical d 0 system. As a d 0 system the Cr 61 ion does not have electrons on the d shell, so the only absorption transitions can be via the charge transfer: from ligands to the central ion. Under excitation into the vicinity of 360 nm the Cr 61 O 22 centre transforms into the Cr 51 O 2 centre undergoing the following charge transfer (CT) in the framework of the [CrO 4 ] 22 group [4,20] Cr O (3d 2p ) → Cr O (3d 2p ) 61

22

0

6

51

2

1

5

in other words the p electron constituting one of the ligands bonds of the t 1 p symmetry transfers to the central ion d-shell [21,22]. The resulting state of the 3d 1 2p 5

electronic configuration splits in the crystal field into two states of the symmetries: 2e (lower, two-fold degenerated) and 4t 2 (higher, three-fold degenerated). Hence the ground state absorption spectrum has two CT bands corresponding to the following transitions CT 1

Cr 61 O 22 , (3d 0 2p 6 ) → Cr 51 O 2 , (3d 1 2p 5 )(t 15 , 2e) (1)

CT 2

Cr 61 O 22 , (3d 0 2p 6 ) → Cr 51 O 2 , (3d 1 2p 5 )(t 15 , 4t 2 ) (2)

Indeed, two such bands are seen in various chromium activated glasses [23]; usually lower CT 1 band distinctly seen in the absorption spectrum as a local maximum

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peaking at |360 nm (like in the LBO glass) or partially immersed in the lattice band-to-band absorption edge (like in the sol–gel glass). The higher, CT 2 band, when in glasses, is usually entirely immersed in the lattice absorption [23]. Thus, the evidence of the Cr 61 ions in the LBO and sol–gel glasses is seen in the absorption spectra (Fig. 1c and e; resolved Gaussian band peaking at |27 550 cm 21 ) and, for the sol–gel glasses, in the luminescence excitation spectra (Fig. 1f; peak around the same position as in the absorption spectra). The Cr 61 ions in the LBO glass do not give rise to the emission [2] and the emission there is entirely due to the Cr 31 ions. In the sol–gel glasses however, apart from the emission due to the Cr 51 ions, we can see another band in the emission peaking at |13 260 cm 21 at LT and 13 690 21 cm at RT (Fig. 2e and f). We believe that this emission band is due to the Cr 61 ions i.e. from a term of the Cr 51 O 2 centre of the 3d 1 2p 5 , (t 15 , 2e) configuration to the ground state of the Cr 61 O 22 centre of the 3d 0 2p, (t 16 ) configuration. Additional proofs of such an origin of this emission are the luminescence excitation spectra where distinct band associated with the Cr 61 ions, centred around |27 600 cm 21 , is seen (as in Fig. 1f). This is rather surprising conclusion—that the [CrO 4 ] 22 group sometimes (like in the LBO glass) do not give rise to the luminescence and sometimes do (like in the sol–gel glass)—in view of common belief [24] that [CrO 4 ] 22 group rather does not produce any luminescence 1 . The investigations of the excited state absorption in both mentioned glasses allowed us to verify this conclusion. The ESA spectra of the Cr:LBO and Cr:sol–gel glasses obtained with the 308 nm excitation are presented in Fig. 4a. Both spectra are composed of two well-defined bands that can be fitted by Gaussians. This fact suggests that the ESA transitions occur rather not from the Cr-related states to the lattice conduction band (then they could be rather single, structureless bands), but instead, the terminal ESA states are two discrete, well-defined states. We have attempted to create the SCC diagrams describing the [CrO 4 ] 22 group using exclusively the obtained GSA and ESA spectra and the procedure explained in detail in [2]. The resulting diagrams are depicted in Fig. 4c and d for the LBO and sol–gel glass, respectively. The right hand sides of the diagrams show the Gaussian bands that fit the ESA spectra, the bands consistent with the presented SCC diagrams. The most important difference between these two diagrams is the energy barrier between 3d 1 2p 5 , (t 15 , 2e) excited state and 3d 0 2p 6 , (t 61 ) ground state. The lower barrier for the LBO glass and the much higher one for the sol–gel glasses can explain the lack of the [CrO 4 ] 2 -related luminescence in the former (because of the radiationless 1 Despite the common belief that the luminescence cannot occur in the frame of the [CrO 4 ] 2 group, Dalhoeven and Blasse in Ref. [25] show that it can take place in some matrices.

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transitions) and the presence of such a luminescence in the latter. The luminescence of the Cr 51 O 2 centre can occur from the terms of 3d 1 2p 5 , (t 51 , 2e) configuration. In the d 0 molecular complexes of the described type the luminescent state is triplet term 3 T 1 whereas the ground state is 1 A 1 fully symmetrical singlet term. Such a luminescence caused by spin-forbidden transitions should be of rather long decay time. We observe at LT the decay times of the order of ms and also short components of the order of |200 ns. The longer decay can be connected with the spin-forbidden 3 T 1 → 1 A 1 , transition whereas the short component can originate from the singlets of the 3d 1 2p 5 , (t 51 , 2e) configuration due to e.g. 1 T 1 → 1 A 1 or 1 T 2 → 1 A 1 , transitions. This is so-called ‘hot luminescence’ seen especially for higher energy excitation, which is our case. At RT higher vibrational states are populated and the luminescence decay shortens due to the radiationless transitions going down to microseconds. On the other hand the short component disappears, which can take place when the barrier between 1 T 1 and 1 A 1 or 1 T 2 and 1 A 1 is distinctly lower than between 3 T 1 and 1 A 1 . Such a situation is possible in the case of larger electron–phonon coupling for singlets than for triplets of a given configuration. At any rate at RT we can see a multiexponential decay with time constant distribution centered at |2–3 ms. Summarising our points concerning the double-band ESA we can state as follows: the only centre responsible for the observed ESA spectrum is Cr 51 O 2 which forms via the CT 2 transition in the frame of the [CrO 4 ] 22 group. The [CrO 4 ] 22 group is a typical example of the d 0 complex of | tetrahedral coordination where the highest filled molecular orbital of the ground state is made mostly of ligand 2p orbitals of the t 1 p symmetry [21]. The GSA transition corresponds, simplifying, to the transition of one of the electrons taking part in oxygen bonds to the free d 0 orbital of the central ion, leaving a hole in the bonds. Thus the excited centre, Cr 51 O 2 , is created and in the crystal field its five-fold degenerated 3d 1 2p 5 electronic configuration splits into twofold degenerated state of the e symmetry and threefold degenerated state of the t 2 symmetry. In the crystal field approximation both these states are distanced by the 10 Dq energy. The transitions between terms of this configuration: 3d 1 2p 5 (t 15 , 2e)→3d 1 2p 5 (t 15 , 4t 2 ) are responsible for the higher energy, narrower ESA band (ESA 2 transition in the SCC diagrams). Second ESA band (ESA 1 transition in the SCC diagrams) can be ascribed to the transitions between 3d 1 2p 5 (t 15 , 2e) and a term of different

2

There are several papers treating the CT transitions rather as redistribution of orbitals [26] and / or transitions of rather small part of the charge [27], however, the simple CT model is still successful in describing the optical properties of the system [2,6,22], whether the transfer concerns 100% or several percent of the electron charge.

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configuration: 3d 2 2p 4 which can form after two consecutive transfers of electrons from ligands to the central ion. Such a double-electron state, having substantially different charge distribution, will be more strongly coupled to the lattice and characterised by different energy of phonons. The ESA mechanism described above can be illustrated in terms of single-electron transitions between molecular orbital (MO) energy levels, using a fragment of the MO energy level diagram for the MX 4 complexes after Ballhausen and Gray [21] (Fig. 5a and b). On the other hand,

the GSA and ESA processes can be illustrated by a simple picture in terms of the charge transfer transitions in the frame of the [CrO 4 ] 22 complex (Fig. 5c). Because after each consecutive charge transfer transition the complex lowers its symmetry, also here the J–T effect is inevitable in the first and second excited states. A deeper insight into this mechanism suggests that the transition: Cr 61 O 22 →Cr 51 O 2 can be associated with the coupling to the three dimensional mode t whereas the transition: Cr 61 O 22 →Cr 41 O 22 or Cr 51 O 2 →Cr 41 O 22 with the cou-

Fig. 5. Interpretations of the ground- and excited state absorptions in terms of single-electron transitions between the states of d 0 molecular complexes (a, b), and in terms of charge transfer (CT) transitions (c).

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pling to the two dimensional mode e. This type of the J–T effect [28] can explain the asymmetry of the ESA (especially ESA 1 ) bands in sol–gel glasses (see Fig. 4a).

Acknowledgements This work was supported by the State Committee for Scientific Research (KBN) under grant number: 2 P03B 117 16 and by the Rector of the Nicholas Copernicus ´ who made possible the visit to the University in Torun, University of Strathclyde, Glasgow, for the collaborative work. The authors are grateful to Professor W. Stre¸k of the Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, for supplying the sol–gel samples and fruitful discussions, to Dr. V.I. Gaishun for growing those samples, to Dr. G.H. Beall of Corning Incorporated, Corning, NY, USA, for growing and supplying the ZAS samples, and to Dr. A. Majchrowski of the Institute of Physics, Military University of Technology, Warsaw, for growing and supplying the LBO samples. We are also indebted to Dr. T.P. Han of the University of Strathclyde, Glasgow, UK, for rendering possibility and assistance in measurements performed at the Department of Physics and Applied Physics.

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