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Abstract

This paper presents the results of investigations of the radiation effects in NH 2 (CH 3 ) 2 Al(SO 4 ) 2 x6H 2 O crystals doped with chromium. On the basis of absorption spectroscopy and resonance Raman scattering study it has been shown that comparatively low doses of radiation first of all causes recharging of Cr 3+ on Cr 4+ ions. Besides, the processes of dehydrogenation and the changes in arrangement of the hydrogen bond network were observed. These changes are followed by distortion of the complexes bonded by such a type of bonds.
Vol. 104 ( 2003) ACT A P HY SIC A POLON IC A A No . 6
Radiati on E˜ ect s in D M AA S:C r
Ferro electric Crystal
V . K apust ian ik a, Z. Czapla b, Yu. El iyachevs kyy a,
Y u. K orcha ka, B. Ku l y k aand V . Rudy k a
aFerroics Physics Laboratory, Electro nics D epart ment
Lviv I. Franko Nati onal Universi ty, Tarna vskogo st., 107, 79017 Lvi v, Ukraine
bInsti t ute of Exp erim ental Physi cs, Uni versi ty of W roaw
pl . M. Borna 9, 50-204 Wroaw, Poland
(Received September 11, 2003)
This paper presents the results of investi gations of the radiation e˜e cts
in NH2(CH3)2A l(SO4)2È6H2O crystals doped with chromium . On the basis
of absorption spectroscopy and resonance Raman scattering study it has been
shown that comparatively low doses of radiation Ùrst of all causes recharging
of Cr3 + on Cr 4+ ions. Besides, the processes of dehydrogenation and the
changes in arrangement of the hydrogen bond netw ork w ere observed. T hese
changes are follow ed by distortion of the comple xes bonded by such a typ e
of bonds.
PACS numb ers: 78.40.Ha, 64.70.K b, 77.84.{s, 78.30.Hv
1. I nt rod uct io n
Crysta l s of di m ethylamm onium alum inum sulfate hexahydra te
NH2(CH3)2Al (SO4)2È6H2O (DMAAS) have been wi dely studi ed recentl y using
di ˜erent experim ental m etho ds [1{ 7].
In parti cular a detailed structura l investigatio n was underta ken in order
to expl ain the occurrence of macroscopic polari zati on in thi s compound [8, 9].
It has been ascertained tha t D MAAS in the paraelectri c phase is monoclinic,
P 2 =n wi th the latti ce constants a = 6:408 ¡
A, b = 10:752 ¡
A, c = 1 1 : 13 4 ¡
A
and a = 100 : 4 4 [8]. In thi s structure, aluminum atoms are coordi nated octahe-
dra lly by six water molecules whi ch form strong hydro gen bonds O{ H. . .O (l ength
2.55{ 2.70 ¡
A) with the oxyg ensof the sulfate anion, and dimethyla mmoni um cations
are disordered [9]. At TC1 =152 K the crysta l undergoes a phase tra nsiti on in-
volvi nga reorienta ti on of dimethylam monium cations, the structure becomesfully
(571)
572 V. K apusti anik et al.
ordered and macroscopic polari zati on appears [6, 9]. In the works [3{ 5] anomali es
of the dielectri c and opti cal parameters at TC 2 =110:5 K and T1= 7 5 K were
found. The low tem perature anomaly wassupposed to berelated to the transition
into the anti ferroelectri c phase, whereas the temperature region between TC 2 and
T1was pro posed to be characteri sed by a co-exi stence of ferro electri c and anti f er-
roelectri c phases [5]. An addi ti onal phase transition of thi s system at 390 K was
suggested based on the analysis of opti cal absorpti on edge in UV [10].
It hasbeenderi ved from X- ray investigati on tha t in both paraelectri c and fer-
roelectri c phasesthe six Al { O distances are no t equa l and the di ˜erences are m ore
pro nounced in the ferroelectri c phase, where the Al { O di stances rang e from 1.864
to 1.898 ¡
A [9]. The expl anati on of these results is no t straightf orw ard, theref ore
chro mium dop ed DMAAS crysta l s were prepa red and thei r spectral pro perti es
were inv estigated [11]. The tri valent chrom ium i ons, wi th thei r stro ng coupl ing
to the surroundi ng ligands appeared as the conveni ent probes of symmetry and
streng th of the crysta l Ùeld. As it follows from the data of previ ous spectral in-
vestigati ons [11, 12] the symm etry and the structure of the compl ex around Cr 3 +
in DMAAS :Cr are very similar to those in ruby crysta l [13]. In both cases one
can observe the di storted o ctahedra l coordi nati on with only di ˜erence tha t the
oxyg en atom s serving as the ligands in the ruby are replaced by water m olecules
in the i nvesti gated crysta l. Under such circum stances the latter crysta l s should
be considered not only as the model ferroics but also as the model materi als of
quantum electroni cs. Fro m thi s poi nt of vi ew study of the ioni zing i rra diati on of
these ob jects woul d be interesti ng both f or the funda menta l solid sta te physi cs and
for the practi cal appl icati ons.
As it has been shown in [14] the high doses of Û-ra y irra diati on ( 106À
5È107R, Co-60 isotope) are followed by a considerable diminishi ng of the dielec-
tri c constant as well as the dielectri c dispersion of the undo ped DMAAS crysta ls.
Unf ortuna tel y, the physical and chemical nature of thi s phenom enon was not ex-
pl ained. Neverthel ess, on the basis of avai labl e data one can conclude tha t the
observed considerable di mini shi ng of the low frequency di spersion shoul d b e con-
nected wi th loweri ng the dom ain mobi lit y under the i nÛuence of irradi ati on. In
the sam e ti me, the decrease in di electri c constant v alues i n thi s case i s possibl y
connected wi th i rreversi bl e ra dio chemical changes of the sampl e. Unf ortuna tel y,
the autho rs of the abo ve-mentio ned pa per di d not investi gate the reversibi lity of
the radiati on defects, altho ugh it is known tha t such a typ e of compounds with
organi c cati on is hardl y stabl e at the used hi gh val ues of irra diati on doses. On
the other hand, it is known tha t such materi als with hydro gen bonds app ear to
be very sensiti ve to m uch lower dosesof irradi ation [15, 16] m anifesting reversible
radi ati on e˜ects. Under such circum stances it woul d be conveni ent to study the
inÛuence of low dosesof irradi ati on on the properti es of DMAAS: Cr crysta ls.
It is necessary to note tha t the spectral metho ds are very inf ormati ve and
e˜ecti ve in resolvi ng the above-m entioned pro blem. Moreover, as it was shown
Radiation E˜e cts in DMA AS:Cr Ferroelectri c Cr ystal 573
in the previ ous inv estigatio ns [11], the D MAAS crysta ls doped wi th chro mi um
app eared to be conveni ent for a study of small changes of the metal coordi nation
since there is a possibility to investigate the resonance Raman scatteri ng. Indeed,
around Ñ = 636 nm the absorpti on band in the nonpolari zed electroni c spectrum
was observed. It m eans tha t for excita ti on of the resonance spectra one could use
the helium { neon laser ( Ñ = 6 3 3: 8 nm ). Mo reover, in thi s caseit has been found tha t
the bands correspondi ng to the vi brati ons invol ving the Cr 3 + ion and its ligands
app eared to be the m ost i ntensi ve. Under such circum sta nces the com pari son of the
resonance spectra of the doped sample with the Raman spectra for pure DMAAS
crysta ls would give worth inf ormati on about the structura l changesincludi ng those
connected wi th the radiati on e˜ects.
2. E xper i m en tal
Single crysta ls of NH2(CH3)2Al (SO4)2È6H2O:Cr 3+(DMAAS :Cr )were
grown from a soluti on conta ini ng alum in um sulfate, excess of di methyl ammoni um
sulfate, and chromium sul fate at a constant tem perature of 303 K by a slow evap-
orati on m ethod. The molar rati o of Al 3 + :Cr3 + in soluti on was equal to 1:0.065.
The crysta lline platel ets of a-cut of di ˜erent thi cknesses (about 1.5{ 3 m m) were
used in m easurements of the absorpti on spectra. T o avoid the inÛuence of absorp-
ti on water the sampleswere previ ously annealed at the temperatures hi gher than
320 K.
The absorpti on spectra of D MAAS: Cr in a vi sible region were recorded at
T = 293 K with an SF-26 absorpti on spectrophotometer.
The Ram an scatteri ng spectra were m easured using a com puteri zed D FS-52 M
spectrom eter empl oying a phonon counti ng system . As the source of excitati on the
He{ Ne-laser radiati on was used. The spectral resoluti ons of the spectrom eter was
1 { 2 cmÀ1.
The sampl ewas irra diated by Co-60 isotope with an acti vi ty of 1.5 MBk.
3. Resu l ts an d d iscu ssio n
The nonp ol arized absorpti on spectra of the vi rgin and i rradi ated sampl es
of DMAAS: Cr obta ined at room temperature (a paraelectri c phase) are shown in
Fi g. 1. These spectra include comparati vely wide asymmetri c bands tha t testi fy
to thei r compl ex structure. The deconvolutio n of the bands onto the elementa ry
conto urs of Gaussian shape was perform ed using a standard computer program
package [17] tha t considerably simpliÙed the interpreta ti on of the spectra. The
low intensity of most bands is caused by low chro mium content as well as by the
selecti on rul es.
From X-ray investigatio ns data [9] it is kno wn tha t the Al3 + ions are coor-
di nated octahedrally by six water m oleculesin the paraelectri c phase of DMAAS
crysta l. The o ctahedra l sym metry Ohis accepted as the initi al site symmetry for
the Cr3 + ions substituti ng the Al ions. Neverthel ess, as it appears f rom our
574 V. Kapust i anik et al.
Fig. 1. Optical absorption spectra of DMA A S:Cr crystals obtained at ro om temp er-
ature in nonp olari zed light for the virgin sample (a) and the samples irradia ted by
di˜erent doses of radiation (b, c). T he solid lines depict the Ùtting curves resulting from
the Gaussian s 1{8 shown by dots.
experimental data , the Cr(H 2O)3 +
6octahedron is at least tetra gonally distorted
and the site symmetry in thi s case should be D4 h . The correspondi ng correlati on
di agram for the energy levels of Cr 3 + ions has been presented in [11]. The electron
tra nsiti ons 4A2g(4F ) !4Tg( F ) ; A g(F)!Tg( F ) and Ag( F ) !Tg( P )
are parity forbi dden but spin allowed. The latest transiti on should correspond to
the comparati vely intensi ve band in the spectra overl apped wi th a charge tra nsfer
band form ing the opti cal absorpti on edge. In the same ti me one can note that the
num b er of bands is hi gher tha n it is expected f rom the ab ove-m enti oned di agra m
for the allowed electro n transiti ons. Under such circumstances it is necessary to
consider also the spin forbi dden tra nsiti ons between the term s of di˜erent multi -
pl icity . In the T anabe{ Sugano di agram for dions in the octahedral Ùeld at the
certa in values of the crysta l Ùeld D q (m ore exactl y D q =B , where Bis R acah pa-
Radi ation E˜e cts in DMA AS:Cr Ferroelectri c Cr ystal 575
rameter) one can observe the cro ssing points of the 4T2g (4F ); 4T1g(4F ) term s with
the 2Eg(2G ) ; 2T1g(2G ) , and Tg(G)term s [18]. The electron tra nsiti ons f rom
the ini ti al state on the latter doubl et levels are spin forbi dden. The results of the
band identi Ùcati on obtained using both above-mentioned diagrams are presented
in T able I.
TABLE I
It has been found that even comparati vely low dosesof irra diati on incl uding
both Û-quanta and Ù-parti cles cause the considerabl e changes of the spectra in
visibl e and UV regions. In result of irradiati on the new band 8 at a high energy
part of the spectra is clearly separated from the edge of the intensi ve band cor-
respondi ng to the spin allowed tra nsiti on. Ano ther interesti ng consequence of the
irra diati on is suppressing of the almost all bands correspondi ng to the interna l
tra nsiti ons of Cr ion. Thi s tendency is most clearl y seen for the hi gher values of
the irra di ation doses. It is also importa nt to note tha t these e˜ects of irradiation
pra ctically disappear after keeping of the sampl e at room tem perature duri ng at
least 48 hours (see Fig. 2).
Taki ng into account tha t the energy diagram of Cr appeared to be very
close to tho sefor the ruby crystals one could suggest tha t irra diati on woul d lead to
the sim ilar results. The ioni zati on under the inÛuence of Û-rays would be followed
Ùrst of all by recharging of the chromium ions according to the scheme:
576 V. Kapust i anik et al.
Fig. 2. Comparison of the optical absorption spectra of DMA AS:Cr crystals obtained
at ro om temp erature in nonp ola rized light for the virgin and irradia ted samples and the
sample kept during 48 hours after irradia tion . The numb er 8 denotes the new bands
arising af ter irradiati on.
The electron is transferred into the conducti on band over the 2Estate of Cr 3+ ion.
In thi s case the bands corresponding to the interna l tra nsiti ons should be bleached
tha t is observed in the experiments. Besides, one could expect an addi ti onal ab-
sorpti on connected either with Cr2 + or wi th Cr4 + ions charge tra nsfer tra nsiti ons.
Ano ther possibility would be connected with the form ation of the localized excito n
state incl udi ng Cr 4 + ion and electron trapped by the ammonium group. Besides,
one can consider another typ eof the excito n states ari sing when the electrons from
the valent orbi tals of the oxyg enato msof water molecul eare localized on the Cr3+
recharging it to Cr 2 + . In thi s case one could also expect the appearance of OÀ
ions. Neverthel ess, such a possibilit y wa s excl uded at least for the ruby crysta ls
on the basis of the deta iled investigatio ns [19]. Mo reover, accordi ng to [19] the
menti oned new band in the region of high energies 4.23 and 3.86 eV (resp ectively,
Ñ = 29 3 and 323 nm, depending on the dose of irradi ation) would be related to
the absorpti on of Cr4 + . The addi ti onal absorpti on connected wi th Cr 2 + in such
a typ e of structures is expected at the energies of 5.07, 3.18{ 3.55, and 2.59 eV
(or at the wavelengths of 245, 350{390, and 480 nm , respecti vely) and woul d be
hardl y di vided from the bands of Cr ion. In any case there was not observed
any growth of absorpti on at the above-m enti oned energies.
On the other hand, one can note the considerable shift of the new band 8 to-
ward the lower energiesobserved at growth of the irradi ation dose. Such behavi our
is characteri sti c exactl y of the excitonic mechanism of the radiati on e˜ect [20]. In
thi s caseit is clearly seen that the new band \ borro ws" intensi ty from the band cor-
respondi ng to the spin allowed electro n transi tion A ( F ) T ( P ) for Cr .
In the same time one can suppose tha t for the used very low doses of ir-
radi ation the f ormati on of displacement defects and correspondi ng F-centres is
expected with a much lower probability tha n the above-m enti oned processes.
Radi ation E˜e cts in DMA AS:Cr Ferroelectri c Cr ystal 577
TABLE I I
IdentiÙcation of the bands in the Raman spectra of DMA AS:C r crystal.
Geometry of the experiment: Z ( Y Y ) X + Z ( Y Z )X .
Frequencies (w ave numb ers), cmÀ1
V irgin sample I rradiated Band
sample identiÙcation
126
139 142 Lattice vibratio ns
181 183
226,263 216,242,279 § ( CH3)
314 316 ¡6(C r{H 2O)
348 ¡6(C r{H 2O)
362 372 £(C{N {C )
393 395 (C{N {C )
463 462
513 513 5(C r{H 2O)
542 531 5(Al{H 2O )
{ 551 2(C r{H 2O)
574 574 2(Cr{H 2O)
617 613 2(A l{H 2O) or
654 636,654 (H 2O)
676,696,737,751,773,796,810 681,712,756,787,807 (H 2O)
833 832 1(C r{H 2O)
871 870 1(A l{H 2O )
892,915,938 895,923,943 (C{N )
986 988
1015 1032 3(Me{H 2O)
{ 1075
1090 1110
1128 1131 (CH ) + (NH )
| denotes the vibration s of sulfate groups.
Addi ti onal inform ati onconcerning radi ati one˜ects and correspondi ng struc-
tura l changes would be dra wn from the Raman spectroscopy whi ch depending on
the geometry of experiment possessesresonance or so-called pre-resonance char-
acter. In Fi g. 3 the Ram an spectra for the virgin and irra diated samplesare com-
pared. These spectra for the chosengeometry possessthe pre-resonance character.
Thei r identi Ùcation for the virgin sample was performed in [11]. Al tho ugh the ir-
radi ation do not change drasti cally the above-m entioned spectra (T able II) one
can note the followi ng changes:
578 V. Kapust i anik et al.
1. clear splitti ng of the bands correspondi ng to the to rsional vibra tion of CH3
group;
2. the band correspondi ng to ¡6(Cr{ H2O) m ode i s shifted toward hi gher fre-
quencies;
3. the ¡s
2mode of sulfate complex is broadened and shi fted to ward hi gher fre-
quencies;
4. the vi brati on of water molecules (w- and £-typ e) are shifted toward higher
frequenci es;
5. the ¡s
1m ode of sul fate com pl ex is bro adened;
6. the valence vi bra ti ons ¡3(Cr{ H2O) are shifted to ward higher frequenci es;
7. more pro nounced splitti ng of ¡s
3m ode of sulfate compl ex;
8. practi cally all modes correspondi ng to the skeletal C{ N{ C vi brati ons are
slightl y shi fted toward higher frequencies.
The observed e˜ects pra cti call y are reversi ble. Thi s concl usion i s conÙrm ed
by Fi g. 3b whi ch as an example presents the low frequency part of the Raman
spectra for vi rgin and irra di ated sampl es as well as the spectra obta ined af ter
48 hours after irra diati on.
Fig. 3. Raman spectra of the virgin and irradiated samples of DMA A S:Cr obtained
at room temp erature (a) and the low frequency part of the same spectra includi ng also
those for the sample kept during 48 hours af ter irradiatio n (b). Orientations and the
corresp ondi ng mo des: Z (Y Y ) X + Z ( ) .
The above-menti oned experim ental facts (the item s 2, 4, and 6) fairl y well
correl ate with the model based on the recharging of the chromium ion. One could
expect that the bonds of Cr ion with ligands would be stronger than for Cr .
In such a case the frequency of the m etal{ hydra te complexes interna l vibra tions
(Ùrst of al l val ence vi brati ons) are expected to be hi gher.
On the other hand, under the inÛuence of comparati vely low doses of ir-
radi ati on Ùrst of all one coul d observe the dehydro genati on pro cesswhi ch woul d
inÛuences the vi brati ons of m ethyl groups and water molecule (i tems 1, 4). As it
Radi ation E˜e cts in DMA AS:Cr Ferroelectri c Cr ystal 579
is clearly seen from Fig. 3b the e˜ect is reversible. It is not odd taki ng into ac-
count a hi gh mobi l ity of pro to ns especiall y i n such compounds wi th comparati vely
hi gh proto nic conducti vi ty [11]. Unf ortuna tel y i t wa s im possibl e to observe the
dehydro genati on e˜ect for other typ es of organic groups vibrati ons due to com-
parati vely intensi ve pho sphorescence observed at hi gher frequenci es. One can also
suppose tha t the observed frequency shi ft of both deform ati on and valence skele-
ta l vibrati ons of the di methyl ammo nium group (item 8) woul d be related to the
breaking of the hydro gen bonds (i ncludi ng N{ H. . .O bonds) with their following
relaxati on to a certain new structure. It would change the degree of disorderi ng of
the di methyla mmonium complexes that are onl y pa rti ally ordered at ro om tem -
perature.
Ano ther interesti ng e˜ect of irradi ati on concerns the sulfate ion. Accordi ng
to the items 3, 5, 7 thi s complex is considerabl y distorted obvi ously thro ugh its
hydro gen bonds wi th the meta l{ hydra te compl exeswhi ch are stro ngly a˜ected by
irra diati on.
4. Con cl u si on
As a resul t of performed investigati ons it has been shown that Cr3 + ions
app eared as a convenient pro be for the study of structura l changesin the DMAAS
crysta ls arising under the i nÛuence of di ˜erent externa l f actors. On the basis of
absorpti on spectroscopy and the Raman scatteri ng study it was found that com-
parativel y low dosesof radi ati on Ùrst of all cause recharging of Cr3 + on Cr 4 + ob-
viously with the form atio n of the localised exciton states. On the other hand, the
pro cessesof dehydro genati on occur in result of irra diati on. Mo reover, the changes
in the arrangement of the hydro gen bond netwo rk are observed tha t leads to the
di storti on of the compl exes bo nded by such a typ e of bonds. Al l observed e˜ects
are found to be practi cally reversible altho ugh for the ful l returni ng of the sampl e
toward the pre-radiati on state at least thi rty days are necessary.
Ac kn o wl ed gm en t
Thi s work was supported by J§zef Mi anowski Fund (Republ ic of Poland).
R efer en ces
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The temperature- and frequency-dependent dielectric susceptibility parallel to the ferroelectric axis of crystals was studied in the ferroelectric phase. The dispersion corresponding to a critical slowing down of the thermal dipole relaxation was observed in the vicinity of in the frequency range 1 - 10 MHz. Besides, a dispersion which is related to the dipole reversal within the domain walls was found at lower frequencies. The latter is much more pronounced in the deuterated sample and is thought to be connected with the dynamics of hydrogen bonds.
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Chromium doped (CH3)2NH2Al(SO4)2·6H2O (DMAAlS) and (CH3)2NH2Ga(SO4)2·6H2O (DMAGaS) single crystals were grown and investigated using the methods of optical spectroscopy. It was found that the Cr3+ ions in the two crystals are situated in a strong crystal field in which the 2E state is the lowest. The single narrow R-line associated with the 2E–4A2 phosphorescence of Cr3+ in DMAAlS in a ferroelectric phase indicates an undistorted octahedral site, whereas important distortion of Oh symmetry and structural disorder was inferred from spectral data obtained with DMAGaS:Cr3+ in a low temperature phase. Results of optical investigation are discussed taking into account the structural data.
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On the basis of absorption spectroscopy data the temperature evolution of Cu2+ ion environment in the layered compounds (CnH2n+1NH3)2CuCl4 (n=2, 3) was studied. For the detailed analysis of experimental data the special program package CrysTool 1.0 based on quantum mechanical models using normalized spherical harmonics (NSH) has been created. It has been found that both crystals contain tetragonally distorted octahedral metal–halogen complexes and that the degree of distortion is less pronounced in the low-temperature phases. The phase transitions (PTs) in both compounds were found to be connected with gradual ordering of the organic cations at cooling. For (C2H5NH3)2CuCl4 the parameters of crystal field, normalized spherical harmonics, and angular overlap model as well as the metal–halogen distances show jump-like anomalies at the points of PTs (T1 = 364 K and T2 = 356 K, at heating), whereas a continuous change was observed at T3 = 330 K.
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This paper presents the results of investigations of the dielectric, birefringent and electrooptic properties of NH2(CH3)2Me(SO4)2 · 6H2O (Me = Al, Ga) crystals. The data obtained are compared with the predictions of a phenomenological theory. In particular, the temperature behaviour of the dielectric susceptibility components and their anomalies in the vicinity of phase transition points were investigated in details. The specific behaviour of the studied parameters in the low temperature region was related to the coexistence of ferroelectric and antiferroelectric ordering of the dimethylammonium groups carrying a dipole moment.
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The dielectric permittivity é and dielectric loss tgδ of NH2(CH3)2AL(SO4)2 · 6H2O (DMAAS) crystals were measured in the low-temperature range at different frequencies of the measuring electric field. The existence of a transition to the ferroelectric phase (TC = 152 K) was comfirmed. In addition, anomalies of both dielectric parameters, characteristic of the phase transition were also observed at T1 = 107 K (on heating) and T2 = 75 K. It has been found that DMAAS crystals possess a considerable dispersion of e' and tgδ in the temperature range of the ferroelectric phase.
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An investigation is made of the absorption spectra, the spectral dependence of the luminescence quantum output, glow curves, and the radiation spectra of the glow curves of ruby crystals. These crystals are coloured by intense light flashes or by γ-rays from Co60. It is found that the absorption is greater in coloured crystals while the luminescence quantum output is smaller. The thermal decoloration of crystals is accompanied by a luminescence in the R-line region. A decrease in the Cr3+ ion concentration is detected in coloured crystals by ESR measurements. The similarity of all the characteristics obtained allows a conclusion about the identity of the colour centres arising under optical and γ-irradiation of the crystals. The results obtained here and published data suggest that Cr2+ and Cr4+ act as colour centres, and that their formation under optical coloration is duo to Cr3+ ion ionization via a two-step process. It is shown that the gradual increase of the ruby generation threshold observed experimentally, which gives laser operation, is due to the formation of colour centres which cause a parasitic absorption of the pumping light. [Russian Text Ignored].
  • A Pietraszko
  • K Êuk Aszewicz
A. Pietraszko, K. Êuk aszewicz, L. F. K irpichniko v a, Po l i sh J. Chem. 6 7, 1877 (1993).
Ac ta Cry stal l ogr
  • N Gale Ç Sic
N. Gale ç sic, V.B. J ordano vsk a, Ac ta Cry stal l ogr. C 48, 256 (1992).