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ESR studies of Ba(BrO3)2· H2O single crystals γ-irradiated at 300 K and 77 K
of hfolecular Structure, 112 (1984) 71-83 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Journal
ESR STUDIES AT300XAND77K
D. L. SASTRY
OF Ba(BrG3)2=H20
SINGLE
CRYSTALS
-y-IRRADIATED
and K. V. S. RAMA RAO
Department of Physics,
Indian
Institute
of Technotogy,
Madms
600 036 (India)
(Received 23 May 1983)
ABSTRACT The paramagnetic defect centers observed in Ba(BrO,), KO single crystals +rradiated at 300 K were identified as 0; and 0: There are two other chemically inequivalent O- centers which could be observed at 77 K. 0; centers do not exhibit any superhyperfine (SHF) interaction, but all the O- centers show SHF interaction due to the protons of waters of hydration. The 0; center observed in the r-irradiated dehydrated polycrystalline samples was found to be undergoing rapid molecular rotation. After -/-irradiation at 77 K, a complex pammagnetic center identified as BrO;--H,O was observed. The stabilities of these centers and their orientations in the present lattice are discussed. The present results also indicate differences in the nature of radiation damage mechanisms and the possibIe hydrogen bondings in the isostructura! Ba(BrO,), .H,O, Ba(ClO,)> H,O and Sr(BrO,), l&O compounds. l
INTRODUCTION
A programme of electron spin resonance (ESR) investigations of the nature of paramagnetic defect centers produced in T-irradiated single crystals of divalent hydrated bromates has been initiated in an attempt to identify new paramagnetic centers [l] . As part of this programme earlier investigations on single crystals of Sr(Br03)1.H20 after r-irradiation at 77 K revealed the formation of pseudo V, centers containing two bromine nuclei [l] _ Another comparable compound for which ESR studies have been reported is Sr(C103)2 [2]. Only C102 radicals were identified in this lattice after X-irradiation at 300 K. The crystal structure of Sr(C103)2 is unknown and further more, these crystals were anhydrous, so no appropriate comparison could be carried out with the results obtained in Sr(BrO&-H20. Ba(Br0,)2 -Hz0 is isostructural with Ba(ClO,)* - Hz0 [ 3 3 and Sr(Br03)2 0 Hz0 [l]. Therefore ESR investigations have been carried out on these single crystals and the results obtained show interesting differences with those already reported in Ba(ClO&-Hz0 143 and Sr(Br03)2.H20 Cl]. The present investigations are expected to help in not only understanding the nature of paramagnetic defect centers produced in this lattice, but also the differences in radiation damage mechanisms in isostructural alkaline earth halates containing water of hydration. 0022-2860/84/$03_OQ
0 1984 Elsevier Science Publishers B.V.
72 CRYSTAL
STRUCTURE
Ba(BrO& -Hz0 is monoclinic with four molecules per unit cell and the space group is I,,, 133. The unit cell parameters are (z = 0.906 m-n, b = 0.792 nm, c = 0.966 nm and /3 = 93.1”. The unit cell projection of this cryti is shown in F’ig. I_ This compound is isostructural with Ba(C103)2-H20 in which the proton positions were determined by neutron diffraction [ 53. In l&se studies it was found that water oxygens were hydrogen bonded to one of the oxygen atoms of the ClO; group and the average hydrogen bond length is 0.2891 nm 151. Proton positions in Ba(BrO& -Hz0 are not known. Recent X-ray diffraction studies on single crystals of Sr(Br03)2-Hz0 have revealed that this compound is also isostructural with Ba(BrO&-Hz0 111. EXPERIMENTAL
Ba(BrO,), =H,O was prepared in this laboratory by mixing hot aqueous solutions of “AnalaIL” grade BaCl, - 2H20 and KBr&. Ba(BrO,),- Hz0 obtained as a precipitate was thoroughly washed with warm distilled water and recrystahised from hot aqueous solution several times. Since the solubility of the substance is very low (= 5 g in 1000 ml of water at 100°C [S] ), single crystals were grown by slow evaporation of the saturated solution maintained at 50°C. Thermogravimetric analysis has shown that this substance loses all water of hydration around 170°C and decomposes above 280°C. Thcugh the seed crystals were transparent, large single crystals invariably developed a cloudy appearance. But the single-crystal nature of the large crystals (5 X 4 X 2 mm) was conbrmed and their crystallographic faces have been identified by X-ray diffraction [7f. A 6oCo r-ray source was used for irradiation at 300 K and 77 K. The ESR spectra were recorded
5a 5r OW 0
Fig. 1. Projection diagram of the unit cell of 52(BrO,),
lH,O
on the (OOl)*
plane. .
73 a Varian E-4 X-band spectrometer employing a 100 KHz field modulation. A Varian E-257 variable temperature accessory was used for temperature-variation studies. A vacuum oven with f 2°C accuracy was employed for thermal bleaching. The ESR spectra were recorded in steps of 5” or 10” in three mutually orthogonal planes (i) (loo), (ii) (010) and (iii) (OOl)* which is the same as the ab plane.
on
RESULTS
hadiationat300R No ESR spectra could be recorded at 300 K or 77 K in the unirradiated crystals. But after r-irradiation for 10-24 h, the crystals (dimensions =4 X 3 X 2 mm) developed a slight greyish indigo colouration and ESR spectra spread over 5 to 10 mT, revealing the presence of different pammagnetic centers, could be recorded. Two centers denoted as A and B could be observed at 300 K, as can be seen from Fig. 2(a). When these crystals were cooled to 77 K two other defect centers denoted as C and D, were observed. These centers are shown in Fig. 2(b). The centers A, B, C and D do not exhibit the hyperfine interaction which is characteristic of bromine nuclei. However B, C and D exhibit the superhyperfine interaction which is characteristic of the protons (I = +) of water of hydration. The ESR spectra of B, C and D centers show greater anisotropy than those of A. Since there is no other suitable alternative, all the four radicals can be assumed to be oxygen species. No bromine-containing radicals could be detected even after prolonged irradiation for 30-40 h using large (-8 X 5 X 4 nm) single crystals. Irradiationat 77K Single crystals of dimensions 3 X 2 X 1 mm were irradiated for 6-7 h at 77 K. The crystals developed a deep bluish colouration and ESR spectra
Fig. 2. E_SR spectra recorded in Ba(BrO,& -IIt0single crystals after T-irradiation 300 K. H is 10” from the (100) axis in the (OOl)* plane. (a) Spectrum recorded 300 K. (b) Spectrum recorded at 77 K.
at at
74
Fig. 3. (a) ESR .spectr~r~ recorded at 77 K in Ba(BrO,), -H,O single crystals after -y-irradiation ar 77 K (b) Proton SHF interaction of E, and E, centers.
spread over 200 mT were recorded. Two paramagnetic defect centers denoted as El and E, could be detected as is shown in Fig. 3(a). These centers have been identified as brominecontsining radicals from the characteristic hyperfine interaction from bromine nuclei. The ‘lBr and “Br transitions of these centers are further split due to the pronounced SHF interaction with protons of water of hydration. There seem to be two inequivalent protons involved in this interaction as can be seen from Fig. 3(b). The ESR spectra of E, and Ez are highly anisotropic and are accompanied by large secondorder effects which arise because of the interaction of the unpaired electron with the large magnetic and quadrupole moments of the bromine nuclei. CALL!ULA?XON
OF ESR PARAME’I’ERS
Center B apparently interacts with a single proton, whereas the C and D exhibit SHF interaction with two inequivalent protons. However, the involvement of a second proton, causing splitting less than the line width (~0.5 mT), cannot be ruled out in the case of center B. The ESR spectra recorded at 77 K in the single crystals 7-irradiated at 300 K were extremely complex, due to severely overlapping Linesfrom several paramagnetic centers which exhibit proton SHF interaction_ E’urthermore the proton SHF splitting of center 3 could not be followed for a sufficient number of orientations because of overlapping lines from center A as well as associated broadening arising from ‘;he dipoiar interaction with the protons of water of hydration. Hence proton SHF interaction was neglected and analysis was confined to the A and B centers. The ESR spectra of A and B centers could be described by the Hamiltonian centers
3%= BH.g.S
75
205
0
135
90
45
Angle
0
180
L5 Angle
Degrees
I”
m
90 Degrees
135
180
Fig. 4. Angular variation at 300 Ir of the g-valuesof the A center observed in BY&B~O,)~I-I10 singie crystals
y-irradiated
at 300
K.
H lies in the
(i) (OOl)*
plane
(ii) (010)
plane
and (iii) (100)
plane.
Fig.
variation at 300 K of the g-values of the B center observed in Ba(BrO,), yirradiated at 300 K. H lies in the (i) (OOl)* plane (ii) (010) plane plane.
5. Angular
H20 single crystals and (iii) (100)
in which terms involving proton SHF interaction were omitted. The angular variations of the g-values of A and B centers are shown in Figs. 4 and 5 respectively_ It can be seen that both the centers occur at two magnetically inequivalent sites. The observed g-values were least-squares fitted to obtain the maxima and minima. The principal g-values and their direction cosines were obtained using Schonland’s procedure 181 and these values are given in Table 1. The ESR. spectra of the E, and E2 centers can be described by the following Hamiltonian in which the bromine nuclear Zeeman term and the terms involving proton SHF interaction are neglected.
X = pH.g.S + S.A.I+
I.&J
A preliminary estimate of the principal g and A values and their direction cosines was obtained by Schonland’s procedure [S] . These parameters were used in the second-order perturbation formulae given by Keijzers et al. [9] to obtain refined principal g and A values. An estimate of the quadrupole coupling constants was ako made by introducing these values to improve the agreement between the observed and calculated positions of the allowed
76
TABLE
1
Principal g-values of 0; and O- centers observed in some lattices with the principal gvalues and their direction cosines of A, B, C and D observed in Ba(BrO,)l - H,O 7-k radiated at 300 K Radical
Latticea
Principal g-values
0;
Ba(ClO,), -H,O (298) Sr(BrO,), -H,Ob (300) Ba(BrO,),*Hz0 (300) Ba(BrO,),b (after dehydration) (300) Ba(BrO,),b (after dehydration) (77) 3C+(PO,), -CaF, (77) MgO (surface) (77) Ca(CIO,), (300) Ba(BrC,),-H,O (300) Ba(BrO,), -H,O (77) Ba(BrO,X .H,O (77)
2.001 2.0023 2.0011
2.0143 2.0139 2.0163
2.0111 2.010 2.0101 2.011
2.002
2.014
2.011
This wor
2.0516
2.0516
2.0012
21
2.0385
2.0385
2.0032
2.0561 2.064 2.094 2.109
2.029 2.035
1.9941 1.991
22
0; A(%) 0; 0; OOO-
E(C) C(C_) D(C) Pr;,nCipal g-values
Direction cosines wzt. A(0;)
[ 1001,
center
g, gYY
0.459 0.714
0.882 -0.300
&z
0.528
-0.362
0.768
2.088 2.0087 2.0092
4 1 This wor This wor
12 This wor This wor This wor
[OlO] and (OOl)* directions B(O-)
0.099 -0.633
Referent
-6.005 0.998
-0.005
center 0.936 0.004
-0.936
-0.345
-0.004 0.346
=Temperature (K) of study is indicated in parentheses- bPolycrystalline samples.
hypefie transitions. The angular variation of the field positions corresponding to the bromine hyperfine interaction is shown in Fig. 6. El and Ez centers can be identified as the same center trapped at two magnetically inequivalent sites. The parameters obtained for these radicals are presented in Table 2. IDENTIFICATION
OF CENTERS
Center A
This center has been identified as an 0; radical from the nature of its principal g-values. These are in good agreement with those previously reported for this center in other lattices, as shown in Table 1. Center
B
This center exhibits much larger g-anisotiopy than 0; and also shows proton SHF interaction which is neariy 0.7 mT at its maximum. The pos-
77
O-2310 O-271 0
Angle
in Degrees
F_ig. 6. Adar variation of the ESR transitions of ‘IBr and 79Br of the E, and El centers. H lies in the (OOl)* plane.
sibility of center B being a OH radical can thus be ruled out, since OH radicals are characterised by a larger hyperfine splitting of the order of 2.2 mT [lo] . Hence the possibility of this center being 0; or O- radicals weakly %&acting with a water proton is considered_ 0; is a 2pIIg* radical and the unpaired electron resides in a IIg*(f,) or lIg*(P,) orbital both of which are generally degenerate. O- has 2p5 electronic structure with the unpaired electron residing in a pr orbital. The px and pr orbitals are degenerate in weak axially symmetric crystal fields. Both the radicals show axial symmetry and gll > gl for 0; and gL > glI for O- [ 111. Because of the nearly degenerate ground state, gll in 0; and g, in O- will be greater than the free spin g-value. Due to the effect of strong orthorhombic crystal fields the degeneracy (between px and py in O- and lIg*(p,) and lIg”(p,) in 0;) in both the radicals will be lifted and orthorhombic g-values can be observed for these centers. However, in an earlier report, a center with principal g-values similar to those observed in the present case (Table 1) has been identified as O- in Xirradiated Ca(ClO,), single crystals [12]. Hence center B is also identified as an O- radical weakly interacting with a water proton. This identification can be confirmed by Of7 substitution and the possibility of a second proton interacting with this center can be established by ENDOR studies.
0,358 0.032 0,934
t1001
-0.892 0,943 0,294
ml
'Thetemperature of study ie Indicated in pnrothoscs.
&z
Bxx $YY
2,041 2,038 2.038 2,036
Dir&ion eoelnee w,r.C,
2.007 Sr(BrO,),*H,O(77) 2.003 ICBrO, (77) 2.008 ~~~~r~:~~~~~~(77~ 2,007 Ba(BrO,), ‘H,O{‘77)
Principal g-value8of J3, centor
BrOi% E,
BrO:’
0,231 0.934 -0.278
[ool]*
2,010 2,000 2.006 2,008
440 430 486 480
440 480 411 410
700 800 667 640
420 670 620 610
0.4 0.26 0.3 0.3
1 16 This work Thie work
&in H~rnilto~i~n pQr~et5~ for “Br of E, aid E, &er~t5reo~~orved in Bn(BrO,), l H,Owith those reported for “BrO:- in curlier studiee
TABLE 2
79
C and D
Centers
These centers are also assumed to be O- radicals interacting with two inequivalent protons of water of hydration. The maximum splittings of the two inequivalent protons are nearly 0.9 mT and 0.5 mT. These radicals are obviously trapped in environments which are chemically different from that of B. One of the g-values of these centers obtained from polycrystalline spectra is given in Table 1. These centers are characterized by greater spinorbit interactions as can be seen by the large deviation of g from the freespin value. This in turn leads to faster spin-lattice relaxation, thus making these radicals observable only at 77 K. E, and E,
Centers
These centers are character&d by highly anisotropic principal g and A tensors. The A tensor also contains a large isotropic component. The asymmetry parameter (q ~0.3) indicates that this radical deviates considerably from axial symmetry_ The parameters given in Table 2 indicate that these radicals are BrO$ centers trapped at two magnetically inequivdient sites. These centers are expected to be similar to CIOZ; radicals observed in yirradiated powder samples of KCIO,; CaC03 [13]. The unpaired electron spin densities in bromine 3s (a:) and 3p (a: ) orbitals for BrOZ; centers are given in Table 3 along with unpaired electron spin densities in chlorine 2s (a:) and 2p (a: ) orbit&. It can be seen from Table 3 that the p/s ratio (ai/az) of Clot- is very different from the value obtained for Br-02; radicals. This discrepancy is probably due to the fact that ClOi- is in fact an O- fragment interacting with ClO, whereas BrOf- is a genuinely double-charged ion, as indicated by the recent ESR investigations in r-irradiated KC103 by Byberg [14] _ TABLE
3
Unpaired electron spin densities in chlorine (“Cl) 3s and 3p orbitals and theirp/s ratios
2s and 2p orbitals and bromine (“Br)
Radical
Lattice
Unpaired electron spin densities
Reference
Cl0:BrO’,BrO:E, or E2 (BrO:3
KClO, KBrO,= Sr(BrO,)? *I&O
0.071 0.063 0.082 0.062
13 16 This work This work
Ba(BrO,),- H,Ob
0.086 0.407 0.41 0.38
0.157 0.47 0.492 0.442
1.2 6.08 5.5 6.1
%alculated from ref. 16. bAverage values for E, and E2 which are the two mqnetically inequivalent BrO’;-sites.
SO
DISCUSSION
Orientation of the centers: Center A 0; is a 19 valence electron radical with Cz, symmetry like CIQz [ 111. The maximum principal g-value, i.e. g,, in these radicals, lies along the 00 00 direction. The intermediate g-value, i.e. g,, lies along the Cfv direction and the minimum g-value lies along a direction perpendicular to the molecular plane. The g,, directio n cosines (0.714, -0.30, -0.633) observed in the present case are in reasonably good agreement with the direction cosines (0.660, -0.353, -0.626) calculated from the crystal structure for one of the 0 directions of the Br-0; ion. This indicates that this radical is formed 0 at the Br0; site. 0; was also produced in the isostructural Ba(C103)2-H,0 [a]. But the orientation of this radical is not the same with respect to the a, b, and c* axes in these two lattices. Although g,, has nearly the same direction cosines, the directions of Efvvfor this center in these two lattices are at nearly 90” to each other. l
l
l
l
Centers B, C and D Center B is formed with its minimum g-value nearly coinciding with the c* direction. The g,, value for this center nearly coincides with the [OlO] direction. The separation between a Ba*’ ion and an 0, atom both of which lie along the [OlO] axis is 2.62 A according to the crystal structure. Hence the interstitial space between these two atoms is presumably the most suitable trapping site for this radical. Centers C and D might have formed at different distances from the Ba” ion, which probably contributes most to the orthorhombic crystal field strength at the O- ion sites. The fact that the B, C and D centers exhibit different proton SHF interactions also supports this observation. Thus it is possible that C and D centers are also formed in the interstitial space, but being further away from the Ba*+ion and closer to the 0, gives rise to their increased proton SHF splitting. Besides, since the radicals C and D might be formed farther from the Ba2’ ion the strength of the crystal field at these radicals may be less than that at center B. Thus the C and D centers possess faster spin-lattice relaxation and are hence observable only at 77 K_ Centers E, and El These centers are obviously trapped at the BrO; site. The direction cosines of the minimum principal g-value agree within 20” with respect to the line passing through the center of mass of the BrO; ion and the Br atom. Furthermore, these centers exhibit proton SHF interaction which is shown in Fig. 3(b). There are two inequivalent protons, one “major” proton giving rise to a maximum splitting of 6.5 mT and a “minor” proton giving
81
rise to a maximum splitting of 2.2 mT. Though BrOS- radicals were observed in Sr(BrO&.HZO such SHF interaction could not be detected Cl]. The unpaired electron spin density residing in the bromine 4s(az) and 4p(ai) orbitals has decreased from the values observed for this radical in Sr(BrO& Hz0 and KBr03 as can be seen from Table 3. These differences can be accounted for by assuming a transfer of unpaired electron spin density from the bromine atom in BrO$- to the protons of water molecules in the case of Ba(BrOs)z - H,O. This observation suggests a major difference between the hydrogen bonding in the two isostructural compounds Sr(BrO&. Hz0 and Ba(BrO&.H,O. But such differences do not seem to exist hetween Ba(Br03)2 *Hz0 and Ba(ClO,), Hz0 as can be seen from the following considerations. The proton SHF coupling is mostly dipolar in nature and its maximum value is proportional to lrw3), where r is the distance between the unpaired electron and the interacting nucleus [ 151. Hence the ratio of the maximum splittings of the “minor” and “major” protons, which is found to be 0.34 for BrO$- radicals in the present case, is assumed to be equal to the ratio O’l/r,>3, where rl and rz are the distances of the nearer and farther protons of the water molecule nearest to rhe BrO; group. The ratio of the distances of the two protons of the water molecule nearest to the chlorine atom in Ba(C103)2- Hz0 single crystals can be calculated as 0’Jr,13 = (2.98/4.23j3 = 0.35 from neutron diffraction data [5] . This excellent agreement suggests that the hydrogen bonding in Ba(CIO,),. H,O and Ba(BrO&.H20 crystals will be quite similar and the ratio of the maximum and minimum hydrogen bond lengths in the two crystals the same. To account for the large proton interaction, E 1 and E, can best be described as BrO$--H20 complex centers. l
Stability
of the centers
The 0; ion was stable up to 120°C in Ba(Br03)z - Hz0 single crystals r-irradiated at 300 K. Above this temperature the ESR signals due to this center staked decreasing in intensity. The B, C and D centers were found to be stable even after the irradiated crystals were thermally bleached at 170°C for several hours. In the hydrated polycrystalline samples y-irradiated at 300 K, the 0; ESR spectrum exhibits well-resolved components of the three principal g-values. But in the r-irradiated dehydrated polycrystallinc samples only the ESR spectrum corresponding to the isotropic g-value of 2.010 could be observed. When these samples were cooled to 77 K the features corresponding to the three principal g-values (Table 1) were observed. This suggests that probably 0; undergoes a rapid molecular rotation about its C,, axis in the dehydrated samples. E I and E, centers were not stable above 200 K.
82
Radiation
damage mechanisms
It has been seen that the primary products are essentially the same in Sr(BrOs)z=I&O and Ba(BrOs)a*H20 except for the associated proton SHF splitting in the latter. But the secondary products in these two lattices are very different. The following radiation damage mechanisms are tentatively proposed for ~-irradiated Ba(BrO& - Hz0 single crystals. I. r-irradiation at ‘77 K */-rap Bl0; * BrO,+e-
(1)
BrO; + e- -
BrOi-
(2)
Br03-
BrOtO,
(3)
No lines that could be attributed to the Br03 radical were observed in the present case, hence it is proposed that this radical decomposes into BrO and O1 as in (3). BrO, even if present, cannot be observed because of fast spinlattice relaxation arising from a degenerate ground state [ ll] . II. +rradiation
at 300 K
BrOZ;
* Br09 + e-
BrO*3 -
Bf
BrO;- -
Bra; -I- o-
(4)
+ O-3
(5) (6)
since no BrOz radicals were observed it is assumed that BrOs-, which is a primary product, dissociates only into O;, and probably O- radicals. Since BrO; (reaction 6) is diamagnetic, it cannot be observed in the ESR studies. The present results suggest that the radiation damage mechanisms are different fiorn those which proposed for Sr(BrO&-HZ0 [l] , Ba(C10,)2 Hz0 143 and KBQ [lS] _ The water of hydration is expected to influence not only the radical stabilities but also the possible radiation damage mechanisms [17, 18-201. T?Cs can be understood in the case of KBr03 which does not contain water of hydration. But Sr(BrO&*H20 and Ba(ClO&* Hz0 are isostructural with Ba(Br03)* Hz0 and contain water of llydration. Hence the difference in the radiation damage mechanisms in Ba(Br03)z* Hz0 and Sr(Br03)2.Hz0 can be attributed to the differences in the nature of hydrogen bonding in these twc lattices, whereas the differences from Bzx(CIO~)~* H,O may be predominantly influenced by the differences in the anions. l
ACKNCWLEDGEMENT
One of the authors (D. L. S.) is grateful to the Department of Atomic Energy for financial a&stance.
83 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
D. L. S&q and K. V. S. Rama Rao, J. Chem. Phys.. (1983) in press. D. Suryanarayana and J. Sobhanadri, J. Phys C, 7 (1974) 3770. G. Kartha, Proc Indian Acad. Sic., Sect. A, 38 (1953) 1. D. Suryanarayana and J. Sobhanadri, J. Chem. Phys., 61(1974) 2827. S. K. Sikka, S. N. Momin, H. Rajagopal and R. Chidambaram, J. Chem. Phys., 48 (1968) 1883. Hand Book of Physics and Chemistry, N. A. Lange (Ed.), McGraw-Hill, New York, 1966,225 pp. A. VaUi, Ph.D. Thesis, I.I.T., Madras, 1981. D. S. SchonIand, Proc. Phys. Sot. London, 73 (1959) 788. C. P. Keijzers, G. F. M. Pauluasen and E. de Boer, Mol. Phys., 26 (1973) 973. P. Removic and 0. Gal, J. Magn. Reson., 13 (1974) 177. P. W. Atkins and M. C. R. Symons, The Structure of Inorganic Radicals, Eisevier, Amsterdam, 1967. D. Suryanarayana and J. Sohhanadri, J. Magn. Reson., 16 (1974) 274. R. S. Eachus and M. C. R. Symons, J. Chem. Sot. A, (1968) 2433. J. R. Byberg, J. Chem. Phys., 75 (1981) 2667. A. Carrington and A. D. McLachian, Introduction to Magnetic Resonance, Harper and Row, New York, 1967. J. R. Byberg and B. S. Kirkegaard, J. Chem. Phys., 60 (1974) 2594. R. C. Cotton and M. C. R. Symons, J. Chem. Sot. A, (1969) 446. D. L. Sastry and K V. S. Rama Rao, Phys. Status. Solidi. B, 75 (1982) 113. D. L. Sastry and K V. S. Rama Rao, J. Chem. Phys., (1983) in press. D. L. Sastry and K. V. S. Rama Rao, J. Phys. C., 16 (1983) 2961. B. Segall. G. W. Ludwig, H. H. Woodbury and P. D. Johnson, Phys. Rev., 128 (1962) 76. L. E. Vannotti and J. R. Morton, Phys. Rev., 174 (1968) 445.
Journal
ESR STUDIES AT300XAND77K
D. L. SASTRY
OF Ba(BrG3)2=H20
SINGLE
CRYSTALS
-y-IRRADIATED
and K. V. S. RAMA RAO
Department of Physics,
Indian
Institute
of Technotogy,
Madms
600 036 (India)
(Received 23 May 1983)
ABSTRACT The paramagnetic defect centers observed in Ba(BrO,), KO single crystals +rradiated at 300 K were identified as 0; and 0: There are two other chemically inequivalent O- centers which could be observed at 77 K. 0; centers do not exhibit any superhyperfine (SHF) interaction, but all the O- centers show SHF interaction due to the protons of waters of hydration. The 0; center observed in the r-irradiated dehydrated polycrystalline samples was found to be undergoing rapid molecular rotation. After -/-irradiation at 77 K, a complex pammagnetic center identified as BrO;--H,O was observed. The stabilities of these centers and their orientations in the present lattice are discussed. The present results also indicate differences in the nature of radiation damage mechanisms and the possibIe hydrogen bondings in the isostructura! Ba(BrO,), .H,O, Ba(ClO,)> H,O and Sr(BrO,), l&O compounds. l
INTRODUCTION
A programme of electron spin resonance (ESR) investigations of the nature of paramagnetic defect centers produced in T-irradiated single crystals of divalent hydrated bromates has been initiated in an attempt to identify new paramagnetic centers [l] . As part of this programme earlier investigations on single crystals of Sr(Br03)1.H20 after r-irradiation at 77 K revealed the formation of pseudo V, centers containing two bromine nuclei [l] _ Another comparable compound for which ESR studies have been reported is Sr(C103)2 [2]. Only C102 radicals were identified in this lattice after X-irradiation at 300 K. The crystal structure of Sr(C103)2 is unknown and further more, these crystals were anhydrous, so no appropriate comparison could be carried out with the results obtained in Sr(BrO&-H20. Ba(Br0,)2 -Hz0 is isostructural with Ba(ClO,)* - Hz0 [ 3 3 and Sr(Br03)2 0 Hz0 [l]. Therefore ESR investigations have been carried out on these single crystals and the results obtained show interesting differences with those already reported in Ba(ClO&-Hz0 143 and Sr(Br03)2.H20 Cl]. The present investigations are expected to help in not only understanding the nature of paramagnetic defect centers produced in this lattice, but also the differences in radiation damage mechanisms in isostructural alkaline earth halates containing water of hydration. 0022-2860/84/$03_OQ
0 1984 Elsevier Science Publishers B.V.
72 CRYSTAL
STRUCTURE
Ba(BrO& -Hz0 is monoclinic with four molecules per unit cell and the space group is I,,, 133. The unit cell parameters are (z = 0.906 m-n, b = 0.792 nm, c = 0.966 nm and /3 = 93.1”. The unit cell projection of this cryti is shown in F’ig. I_ This compound is isostructural with Ba(C103)2-H20 in which the proton positions were determined by neutron diffraction [ 53. In l&se studies it was found that water oxygens were hydrogen bonded to one of the oxygen atoms of the ClO; group and the average hydrogen bond length is 0.2891 nm 151. Proton positions in Ba(BrO& -Hz0 are not known. Recent X-ray diffraction studies on single crystals of Sr(Br03)2-Hz0 have revealed that this compound is also isostructural with Ba(BrO&-Hz0 111. EXPERIMENTAL
Ba(BrO,), =H,O was prepared in this laboratory by mixing hot aqueous solutions of “AnalaIL” grade BaCl, - 2H20 and KBr&. Ba(BrO,),- Hz0 obtained as a precipitate was thoroughly washed with warm distilled water and recrystahised from hot aqueous solution several times. Since the solubility of the substance is very low (= 5 g in 1000 ml of water at 100°C [S] ), single crystals were grown by slow evaporation of the saturated solution maintained at 50°C. Thermogravimetric analysis has shown that this substance loses all water of hydration around 170°C and decomposes above 280°C. Thcugh the seed crystals were transparent, large single crystals invariably developed a cloudy appearance. But the single-crystal nature of the large crystals (5 X 4 X 2 mm) was conbrmed and their crystallographic faces have been identified by X-ray diffraction [7f. A 6oCo r-ray source was used for irradiation at 300 K and 77 K. The ESR spectra were recorded
5a 5r OW 0
Fig. 1. Projection diagram of the unit cell of 52(BrO,),
lH,O
on the (OOl)*
plane. .
73 a Varian E-4 X-band spectrometer employing a 100 KHz field modulation. A Varian E-257 variable temperature accessory was used for temperature-variation studies. A vacuum oven with f 2°C accuracy was employed for thermal bleaching. The ESR spectra were recorded in steps of 5” or 10” in three mutually orthogonal planes (i) (loo), (ii) (010) and (iii) (OOl)* which is the same as the ab plane.
on
RESULTS
hadiationat300R No ESR spectra could be recorded at 300 K or 77 K in the unirradiated crystals. But after r-irradiation for 10-24 h, the crystals (dimensions =4 X 3 X 2 mm) developed a slight greyish indigo colouration and ESR spectra spread over 5 to 10 mT, revealing the presence of different pammagnetic centers, could be recorded. Two centers denoted as A and B could be observed at 300 K, as can be seen from Fig. 2(a). When these crystals were cooled to 77 K two other defect centers denoted as C and D, were observed. These centers are shown in Fig. 2(b). The centers A, B, C and D do not exhibit the hyperfine interaction which is characteristic of bromine nuclei. However B, C and D exhibit the superhyperfine interaction which is characteristic of the protons (I = +) of water of hydration. The ESR spectra of B, C and D centers show greater anisotropy than those of A. Since there is no other suitable alternative, all the four radicals can be assumed to be oxygen species. No bromine-containing radicals could be detected even after prolonged irradiation for 30-40 h using large (-8 X 5 X 4 nm) single crystals. Irradiationat 77K Single crystals of dimensions 3 X 2 X 1 mm were irradiated for 6-7 h at 77 K. The crystals developed a deep bluish colouration and ESR spectra
Fig. 2. E_SR spectra recorded in Ba(BrO,& -IIt0single crystals after T-irradiation 300 K. H is 10” from the (100) axis in the (OOl)* plane. (a) Spectrum recorded 300 K. (b) Spectrum recorded at 77 K.
at at
74
Fig. 3. (a) ESR .spectr~r~ recorded at 77 K in Ba(BrO,), -H,O single crystals after -y-irradiation ar 77 K (b) Proton SHF interaction of E, and E, centers.
spread over 200 mT were recorded. Two paramagnetic defect centers denoted as El and E, could be detected as is shown in Fig. 3(a). These centers have been identified as brominecontsining radicals from the characteristic hyperfine interaction from bromine nuclei. The ‘lBr and “Br transitions of these centers are further split due to the pronounced SHF interaction with protons of water of hydration. There seem to be two inequivalent protons involved in this interaction as can be seen from Fig. 3(b). The ESR spectra of E, and Ez are highly anisotropic and are accompanied by large secondorder effects which arise because of the interaction of the unpaired electron with the large magnetic and quadrupole moments of the bromine nuclei. CALL!ULA?XON
OF ESR PARAME’I’ERS
Center B apparently interacts with a single proton, whereas the C and D exhibit SHF interaction with two inequivalent protons. However, the involvement of a second proton, causing splitting less than the line width (~0.5 mT), cannot be ruled out in the case of center B. The ESR spectra recorded at 77 K in the single crystals 7-irradiated at 300 K were extremely complex, due to severely overlapping Linesfrom several paramagnetic centers which exhibit proton SHF interaction_ E’urthermore the proton SHF splitting of center 3 could not be followed for a sufficient number of orientations because of overlapping lines from center A as well as associated broadening arising from ‘;he dipoiar interaction with the protons of water of hydration. Hence proton SHF interaction was neglected and analysis was confined to the A and B centers. The ESR spectra of A and B centers could be described by the Hamiltonian centers
3%= BH.g.S
75
205
0
135
90
45
Angle
0
180
L5 Angle
Degrees
I”
m
90 Degrees
135
180
Fig. 4. Angular variation at 300 Ir of the g-valuesof the A center observed in BY&B~O,)~I-I10 singie crystals
y-irradiated
at 300
K.
H lies in the
(i) (OOl)*
plane
(ii) (010)
plane
and (iii) (100)
plane.
Fig.
variation at 300 K of the g-values of the B center observed in Ba(BrO,), yirradiated at 300 K. H lies in the (i) (OOl)* plane (ii) (010) plane plane.
5. Angular
H20 single crystals and (iii) (100)
in which terms involving proton SHF interaction were omitted. The angular variations of the g-values of A and B centers are shown in Figs. 4 and 5 respectively_ It can be seen that both the centers occur at two magnetically inequivalent sites. The observed g-values were least-squares fitted to obtain the maxima and minima. The principal g-values and their direction cosines were obtained using Schonland’s procedure 181 and these values are given in Table 1. The ESR. spectra of the E, and E2 centers can be described by the following Hamiltonian in which the bromine nuclear Zeeman term and the terms involving proton SHF interaction are neglected.
X = pH.g.S + S.A.I+
I.&J
A preliminary estimate of the principal g and A values and their direction cosines was obtained by Schonland’s procedure [S] . These parameters were used in the second-order perturbation formulae given by Keijzers et al. [9] to obtain refined principal g and A values. An estimate of the quadrupole coupling constants was ako made by introducing these values to improve the agreement between the observed and calculated positions of the allowed
76
TABLE
1
Principal g-values of 0; and O- centers observed in some lattices with the principal gvalues and their direction cosines of A, B, C and D observed in Ba(BrO,)l - H,O 7-k radiated at 300 K Radical
Latticea
Principal g-values
0;
Ba(ClO,), -H,O (298) Sr(BrO,), -H,Ob (300) Ba(BrO,),*Hz0 (300) Ba(BrO,),b (after dehydration) (300) Ba(BrO,),b (after dehydration) (77) 3C+(PO,), -CaF, (77) MgO (surface) (77) Ca(CIO,), (300) Ba(BrC,),-H,O (300) Ba(BrO,), -H,O (77) Ba(BrO,X .H,O (77)
2.001 2.0023 2.0011
2.0143 2.0139 2.0163
2.0111 2.010 2.0101 2.011
2.002
2.014
2.011
This wor
2.0516
2.0516
2.0012
21
2.0385
2.0385
2.0032
2.0561 2.064 2.094 2.109
2.029 2.035
1.9941 1.991
22
0; A(%) 0; 0; OOO-
E(C) C(C_) D(C) Pr;,nCipal g-values
Direction cosines wzt. A(0;)
[ 1001,
center
g, gYY
0.459 0.714
0.882 -0.300
&z
0.528
-0.362
0.768
2.088 2.0087 2.0092
4 1 This wor This wor
12 This wor This wor This wor
[OlO] and (OOl)* directions B(O-)
0.099 -0.633
Referent
-6.005 0.998
-0.005
center 0.936 0.004
-0.936
-0.345
-0.004 0.346
=Temperature (K) of study is indicated in parentheses- bPolycrystalline samples.
hypefie transitions. The angular variation of the field positions corresponding to the bromine hyperfine interaction is shown in Fig. 6. El and Ez centers can be identified as the same center trapped at two magnetically inequivalent sites. The parameters obtained for these radicals are presented in Table 2. IDENTIFICATION
OF CENTERS
Center A
This center has been identified as an 0; radical from the nature of its principal g-values. These are in good agreement with those previously reported for this center in other lattices, as shown in Table 1. Center
B
This center exhibits much larger g-anisotiopy than 0; and also shows proton SHF interaction which is neariy 0.7 mT at its maximum. The pos-
77
O-2310 O-271 0
Angle
in Degrees
F_ig. 6. Adar variation of the ESR transitions of ‘IBr and 79Br of the E, and El centers. H lies in the (OOl)* plane.
sibility of center B being a OH radical can thus be ruled out, since OH radicals are characterised by a larger hyperfine splitting of the order of 2.2 mT [lo] . Hence the possibility of this center being 0; or O- radicals weakly %&acting with a water proton is considered_ 0; is a 2pIIg* radical and the unpaired electron resides in a IIg*(f,) or lIg*(P,) orbital both of which are generally degenerate. O- has 2p5 electronic structure with the unpaired electron residing in a pr orbital. The px and pr orbitals are degenerate in weak axially symmetric crystal fields. Both the radicals show axial symmetry and gll > gl for 0; and gL > glI for O- [ 111. Because of the nearly degenerate ground state, gll in 0; and g, in O- will be greater than the free spin g-value. Due to the effect of strong orthorhombic crystal fields the degeneracy (between px and py in O- and lIg*(p,) and lIg”(p,) in 0;) in both the radicals will be lifted and orthorhombic g-values can be observed for these centers. However, in an earlier report, a center with principal g-values similar to those observed in the present case (Table 1) has been identified as O- in Xirradiated Ca(ClO,), single crystals [12]. Hence center B is also identified as an O- radical weakly interacting with a water proton. This identification can be confirmed by Of7 substitution and the possibility of a second proton interacting with this center can be established by ENDOR studies.
0,358 0.032 0,934
t1001
-0.892 0,943 0,294
ml
'Thetemperature of study ie Indicated in pnrothoscs.
&z
Bxx $YY
2,041 2,038 2.038 2,036
Dir&ion eoelnee w,r.C,
2.007 Sr(BrO,),*H,O(77) 2.003 ICBrO, (77) 2.008 ~~~~r~:~~~~~~(77~ 2,007 Ba(BrO,), ‘H,O{‘77)
Principal g-value8of J3, centor
BrOi% E,
BrO:’
0,231 0.934 -0.278
[ool]*
2,010 2,000 2.006 2,008
440 430 486 480
440 480 411 410
700 800 667 640
420 670 620 610
0.4 0.26 0.3 0.3
1 16 This work Thie work
&in H~rnilto~i~n pQr~et5~ for “Br of E, aid E, &er~t5reo~~orved in Bn(BrO,), l H,Owith those reported for “BrO:- in curlier studiee
TABLE 2
79
C and D
Centers
These centers are also assumed to be O- radicals interacting with two inequivalent protons of water of hydration. The maximum splittings of the two inequivalent protons are nearly 0.9 mT and 0.5 mT. These radicals are obviously trapped in environments which are chemically different from that of B. One of the g-values of these centers obtained from polycrystalline spectra is given in Table 1. These centers are characterized by greater spinorbit interactions as can be seen by the large deviation of g from the freespin value. This in turn leads to faster spin-lattice relaxation, thus making these radicals observable only at 77 K. E, and E,
Centers
These centers are character&d by highly anisotropic principal g and A tensors. The A tensor also contains a large isotropic component. The asymmetry parameter (q ~0.3) indicates that this radical deviates considerably from axial symmetry_ The parameters given in Table 2 indicate that these radicals are BrO$ centers trapped at two magnetically inequivdient sites. These centers are expected to be similar to CIOZ; radicals observed in yirradiated powder samples of KCIO,; CaC03 [13]. The unpaired electron spin densities in bromine 3s (a:) and 3p (a: ) orbitals for BrOZ; centers are given in Table 3 along with unpaired electron spin densities in chlorine 2s (a:) and 2p (a: ) orbit&. It can be seen from Table 3 that the p/s ratio (ai/az) of Clot- is very different from the value obtained for Br-02; radicals. This discrepancy is probably due to the fact that ClOi- is in fact an O- fragment interacting with ClO, whereas BrOf- is a genuinely double-charged ion, as indicated by the recent ESR investigations in r-irradiated KC103 by Byberg [14] _ TABLE
3
Unpaired electron spin densities in chlorine (“Cl) 3s and 3p orbitals and theirp/s ratios
2s and 2p orbitals and bromine (“Br)
Radical
Lattice
Unpaired electron spin densities
Reference
Cl0:BrO’,BrO:E, or E2 (BrO:3
KClO, KBrO,= Sr(BrO,)? *I&O
0.071 0.063 0.082 0.062
13 16 This work This work
Ba(BrO,),- H,Ob
0.086 0.407 0.41 0.38
0.157 0.47 0.492 0.442
1.2 6.08 5.5 6.1
%alculated from ref. 16. bAverage values for E, and E2 which are the two mqnetically inequivalent BrO’;-sites.
SO
DISCUSSION
Orientation of the centers: Center A 0; is a 19 valence electron radical with Cz, symmetry like CIQz [ 111. The maximum principal g-value, i.e. g,, in these radicals, lies along the 00 00 direction. The intermediate g-value, i.e. g,, lies along the Cfv direction and the minimum g-value lies along a direction perpendicular to the molecular plane. The g,, directio n cosines (0.714, -0.30, -0.633) observed in the present case are in reasonably good agreement with the direction cosines (0.660, -0.353, -0.626) calculated from the crystal structure for one of the 0 directions of the Br-0; ion. This indicates that this radical is formed 0 at the Br0; site. 0; was also produced in the isostructural Ba(C103)2-H,0 [a]. But the orientation of this radical is not the same with respect to the a, b, and c* axes in these two lattices. Although g,, has nearly the same direction cosines, the directions of Efvvfor this center in these two lattices are at nearly 90” to each other. l
l
l
l
Centers B, C and D Center B is formed with its minimum g-value nearly coinciding with the c* direction. The g,, value for this center nearly coincides with the [OlO] direction. The separation between a Ba*’ ion and an 0, atom both of which lie along the [OlO] axis is 2.62 A according to the crystal structure. Hence the interstitial space between these two atoms is presumably the most suitable trapping site for this radical. Centers C and D might have formed at different distances from the Ba” ion, which probably contributes most to the orthorhombic crystal field strength at the O- ion sites. The fact that the B, C and D centers exhibit different proton SHF interactions also supports this observation. Thus it is possible that C and D centers are also formed in the interstitial space, but being further away from the Ba*+ion and closer to the 0, gives rise to their increased proton SHF splitting. Besides, since the radicals C and D might be formed farther from the Ba2’ ion the strength of the crystal field at these radicals may be less than that at center B. Thus the C and D centers possess faster spin-lattice relaxation and are hence observable only at 77 K_ Centers E, and El These centers are obviously trapped at the BrO; site. The direction cosines of the minimum principal g-value agree within 20” with respect to the line passing through the center of mass of the BrO; ion and the Br atom. Furthermore, these centers exhibit proton SHF interaction which is shown in Fig. 3(b). There are two inequivalent protons, one “major” proton giving rise to a maximum splitting of 6.5 mT and a “minor” proton giving
81
rise to a maximum splitting of 2.2 mT. Though BrOS- radicals were observed in Sr(BrO&.HZO such SHF interaction could not be detected Cl]. The unpaired electron spin density residing in the bromine 4s(az) and 4p(ai) orbitals has decreased from the values observed for this radical in Sr(BrO& Hz0 and KBr03 as can be seen from Table 3. These differences can be accounted for by assuming a transfer of unpaired electron spin density from the bromine atom in BrO$- to the protons of water molecules in the case of Ba(BrOs)z - H,O. This observation suggests a major difference between the hydrogen bonding in the two isostructural compounds Sr(BrO&. Hz0 and Ba(BrO&.H,O. But such differences do not seem to exist hetween Ba(Br03)2 *Hz0 and Ba(ClO,), Hz0 as can be seen from the following considerations. The proton SHF coupling is mostly dipolar in nature and its maximum value is proportional to lrw3), where r is the distance between the unpaired electron and the interacting nucleus [ 151. Hence the ratio of the maximum splittings of the “minor” and “major” protons, which is found to be 0.34 for BrO$- radicals in the present case, is assumed to be equal to the ratio O’l/r,>3, where rl and rz are the distances of the nearer and farther protons of the water molecule nearest to rhe BrO; group. The ratio of the distances of the two protons of the water molecule nearest to the chlorine atom in Ba(C103)2- Hz0 single crystals can be calculated as 0’Jr,13 = (2.98/4.23j3 = 0.35 from neutron diffraction data [5] . This excellent agreement suggests that the hydrogen bonding in Ba(CIO,),. H,O and Ba(BrO&.H20 crystals will be quite similar and the ratio of the maximum and minimum hydrogen bond lengths in the two crystals the same. To account for the large proton interaction, E 1 and E, can best be described as BrO$--H20 complex centers. l
Stability
of the centers
The 0; ion was stable up to 120°C in Ba(Br03)z - Hz0 single crystals r-irradiated at 300 K. Above this temperature the ESR signals due to this center staked decreasing in intensity. The B, C and D centers were found to be stable even after the irradiated crystals were thermally bleached at 170°C for several hours. In the hydrated polycrystalline samples y-irradiated at 300 K, the 0; ESR spectrum exhibits well-resolved components of the three principal g-values. But in the r-irradiated dehydrated polycrystallinc samples only the ESR spectrum corresponding to the isotropic g-value of 2.010 could be observed. When these samples were cooled to 77 K the features corresponding to the three principal g-values (Table 1) were observed. This suggests that probably 0; undergoes a rapid molecular rotation about its C,, axis in the dehydrated samples. E I and E, centers were not stable above 200 K.
82
Radiation
damage mechanisms
It has been seen that the primary products are essentially the same in Sr(BrOs)z=I&O and Ba(BrOs)a*H20 except for the associated proton SHF splitting in the latter. But the secondary products in these two lattices are very different. The following radiation damage mechanisms are tentatively proposed for ~-irradiated Ba(BrO& - Hz0 single crystals. I. r-irradiation at ‘77 K */-rap Bl0; * BrO,+e-
(1)
BrO; + e- -
BrOi-
(2)
Br03-
BrOtO,
(3)
No lines that could be attributed to the Br03 radical were observed in the present case, hence it is proposed that this radical decomposes into BrO and O1 as in (3). BrO, even if present, cannot be observed because of fast spinlattice relaxation arising from a degenerate ground state [ ll] . II. +rradiation
at 300 K
BrOZ;
* Br09 + e-
BrO*3 -
Bf
BrO;- -
Bra; -I- o-
(4)
+ O-3
(5) (6)
since no BrOz radicals were observed it is assumed that BrOs-, which is a primary product, dissociates only into O;, and probably O- radicals. Since BrO; (reaction 6) is diamagnetic, it cannot be observed in the ESR studies. The present results suggest that the radiation damage mechanisms are different fiorn those which proposed for Sr(BrO&-HZ0 [l] , Ba(C10,)2 Hz0 143 and KBQ [lS] _ The water of hydration is expected to influence not only the radical stabilities but also the possible radiation damage mechanisms [17, 18-201. T?Cs can be understood in the case of KBr03 which does not contain water of hydration. But Sr(BrO&*H20 and Ba(ClO&* Hz0 are isostructural with Ba(Br03)* Hz0 and contain water of llydration. Hence the difference in the radiation damage mechanisms in Ba(Br03)z* Hz0 and Sr(Br03)2.Hz0 can be attributed to the differences in the nature of hydrogen bonding in these twc lattices, whereas the differences from Bzx(CIO~)~* H,O may be predominantly influenced by the differences in the anions. l
ACKNCWLEDGEMENT
One of the authors (D. L. S.) is grateful to the Department of Atomic Energy for financial a&stance.
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