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EPR of V centres in plastically deformed CaS
J Phys. Chem Solids Vol57,
Pergamon
No. 9. pp. 1329-1335.1996 Copyright0 1996EhevierScienaLtd Pnnted III Great Britain. AU riglm reserved 0022-3697/% SI5.00 + 0.00
0022~3697(95)003274
EPR OF V CENTRES V. SEEMAN, M. DANILKIN,
IN PLASTICALLY
DEFORMED
CaS
M. MUST, A. OTS, E. PEDAK, L. PUNG and E. PARNOJA Tartu University, EE2400 Tartu, Estonia
(Received I June 1995; accepted 20 November 1995)
Abstract-Several types of hole V centres have been observed by the EPR method in plastically deformed CaS polycrystals after the X-irradiation at 77 K. Dominating V -, VsH, and V:(,,) centres are formed when a hole is trapped by a sulphur anion near the isolated cation vacancy or near the cation vacancy associated with one or two (SH)- groups. Holes are released from the V -, VSH, and V&,) centres at 120-285, 160210, and 1lo-160 K, respectively. Hole release in the case of V - centres takes place in an unusually wide temperature range. The possible mechanisms responsible for this process are discussed. The isolated cation vacancies are easily transformed into associates with one or two (SH)- groups by heating the samples up to 450-670 K. These associates disappear completely after the samples are heated above 800 K. Keywords: A. chalcogenides, D. defects, D. electron paramagnetic resonance (EPR).
1. INTRODUCTION Calcium sulphide is a well-known base for a variety of luminophors. Both impurity defects (activators, coactivators) and also intrinsic lattice defects (cation and anion vacancies) influence the properties of CaSbased luminophors. Yet there are no reliable data available on cation vacancies or their associates with impurities in CaS, although point defects in alkaline earth sulphides have been studied for years. This can be explained by a deficit in sulliciently large single crystals of CaS. Also, magnetic resonance methods often yield no comprehensive information about point defects in powder samples. However, a family of V centres was reported in our previous paper [ 11.The V centres were studied in CaS polycrystals activated with halogen ions or alkaline metals after X-raying samples at 77K. Structural defects present in the samples become paramagnetic in case a hole is captured by a sulphide anion next to the cation vacancy associated with a haloid ion (Cl or F) or by a sulphide anion next to Na+ or Li+ substituting Ca2+ [l]. The centres observed have an axial structure: S--w,-Halor S-M+-S2-, where Hal stands for Cl or F, and M stands for Na or Li. We named these defects Vcl, VF or pa]‘, [Li]’ on the analogy of the well-known hole V centres in alkaline earth oxides. However, it is important to point out here that no isolated cation vacancies with captured holes (i.e. V - centres) can be detected in CaS polycrystals after any chemical or thermal treatment. Yet, in the course of our experiment we could find an easy way of producing cation vacancies by the plastic deformation (pressing) of CaS polycrystals. Plastic deformation produces,
besides isolated cation vacancies, some associates of defects in which cation vacancies are involved. These defects become paramagnetic due to the capturing of holes after the excitation of the samples. The Vcentres are prevailing among different paramagnetic centres thus formed. Here we report the results of the EPR study of hole V centres in plastically deformed samples and some properties of cation vacancies.
2. OBJECTS, INSTRUMENTATION, EXPERIMENTAL METHODS Calcium sulphide was prepared CaS04
with
flowing
purified
AND
by the reduction hydrogen
at
of
1150-
1170 K, according to the method described in [2]. Initial CaS04 was precipitated from high-puritygrade Ca(N03)2 and (NH&SO, solutions. Transition and heavy metals were specially removed before the precipitation by organic extracting agents. Alkaline metals were removed by repeated washing of precipitated CaS04 with distilled water. The resulting CaS contained very small quantities (less than 10m5at.%) of Cr, Fe, Pb, Mn, etc. Also, it contained small quantities of Na, Li, and F, Cl (less than 10V4at.%). The product contained up to 99.4% of the theoretical quantity of sulphide anions. Some quantities of CaS were doped with halogen ions or alkaline metals. The required amounts of dopants were added and the samples were heated with 2-20% of sulphur under flowing argon at 1170-1420K for 30-45min. Then the samples were cooled under flowing argon to room temperature outside the furnace.
1329
1330
V. SEEMAN et al.
An alternative method of direct synthesis was also developed to prepare some of the CaS samples. The calcium metal of high purity grade and high-purity elementary sulphur were heated together in a preevacuated sealed quartz tube. The reaction proceeded at 900-1050K. In order to obtain a more homogeneous product, it was annealed up to 1200 K. A small excess of sulphur was used to prevent a casual oxidation of the product. Nevertheless, it contained about 93% of the theoretical quantity of sulphide anions due to nonuniform combustion of calcium in sulphur vapours. This method of direct synthesis was designed in order to obtain CaS with a low hydrogen content. The results obtained with two different calcium sulphides (with and without hydrogen) were compared in order to prove the models of Vs, and V$,,) centres. For the same purpose samples activated with oxygen and with (OH)- groups were prepared. The plastic deformation of the samples was performed by a hydraulic press at room temperature. The applied pressure varied from 100 to 2500MPa. The pellets produced by pressing were crushed and ground into powder before carrying out EPR measurements. The EPR spectra were measured with an X-band (8.875 GHz) spectrometer of 100 KHz magnetic field modulation. A continuous-flow helium cryostat (Oxford Instruments ESR-9) was used to keep the samples at necessary temperatures. Pulse annealing of the samples was performed to determine thermal stabilities of V centres and cation vacancies. To make defects paramagnetic the samples were X-irradiated at 77 K before annealing (50KV, 18 mA, W anode). Pulse annealing was carried out in the cryostat at temperatures below 300K. A special separately mounted electric furnace was employed for pulse annealing at higher temperatures (300- 1120 K). On pulse annealing the samples were kept at the required temperature for a certain time (2 min) after fast heating up, and then they were fast cooled down to the temperature chosen for measurements (35K for V centres). To investigate ionic processes at temperatures above 300K, at which holes escape from Vcentres, the samples were X-rayed at 77 K after each step of annealing to populate the cation vacancies with holes. The X-irradiation of the samples was continued until the EPR signal intensity stopped increasing (usually 1 h).
3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Models of V centres The EPR measurements of CaS before deformation demonstrated that there was a minute quantity of the ions Mn2+ and Cr3+ within a cubic symmetry
I
0 29
v=8676
0 30
0
2 MHz
31
B. T
Fig. 1. EPR spectra of V centres in plastically deformed CaS polycrystals: full-scale spectrum at 35 K, averaged spectrum of V - centre at 145 K. surrounding. After X-irradiation at 77K, weak signals of [Na]’ and [Li]’ centres could also be observed, which indicates some contamination of CaS with alkaline metals. VF and V,-, centres were observed in the samples doped with proper halogen ions. After mechanical deformation (grinding, smashing, pressing) F + centres become detectable (F’ stands for the electron trapped at the anion vacancy). F; centres could also be detected but only after the deformed samples had been X-rayed (FL stands for the electron trapped at the associate Q-V,, of anion and cation vacancies). Sharp single isotropic lines with g = 2.0031 (for F+) and g = 2.0017 (for FL), corresponding to these centres, could be observed in EPR spectra at low level of microwave power, provided that the temperature was not too low [3, 41. Several anisotropic centres with similar properties are also formed in plastically deformed CaS. These centres are observable after X-irradiation of the deformed samples at 77K. The quantity of these
;
e)
1
0.2933
0.2960
0 2990
B,S
Fig. 2. EPR spectra of V centres (gL-region, T = 35K) in CaS polycrystals plastically deformed at 300K: before annealing (a) and after annealing at 673 K (b), 710 K (c), 823 K (d) and 932 K (e).
EPR of V centres in plastically deformed CaS
.z
/
0
I
iO0
5c
‘I,mir,
Fig. 3. The EPR g,-line intensity (a) and width (b) of V eentres (T = 35 K) versus the amount of the pressure applied; the line intensity versus the time of X-irradiation at 77 K (c). centres
increases
steadily
crystals is increased
when
pressure
on poly-
(see Fig. 3(a)). Several centres of
an axial symmetry and one centre of a lower symmetry of the g-tensor were revealed by an analysis of EPR spectra after the excited samples had been annealed to different temperatures. The lines of these centres, corresponding to the parallel components of the gtensor, are weak and overlap strongly with the F+ centre signal and with signals of some unknown defects within the range of g g 2.002. The more intensive lines connected with the perpendicular gtensor components overlap partly (see Figs 1 and 2). The g-tensor components and other parameters for these defects designated as V-, V.n, and V&u) are given in Table 1. The g-values of these centres are very close to those of Vcl and Vr centres. All the centres are
1331
observable at a high level of microwave power and at the same optimal temperatures of 25-35 K (except in the case of [Na]’ and [Li]’ centres). Hence, the V-, and Vc/~l,VP centres have a similar VSHt V&H)’ structure involving the cation vacancy. Below we shall discuss the model of each centre studied. 3.1.1. V - centre. 1. The V - centre is characterized by the highest temperature of hole release of the group studied (see Fig. 4). It disappears fully at 285 K, while the V,-l and VF centres are destroyed at the temperature as low as 215K. Therefore, there is no charge compensating defect in the close vicinity of the cation vacancy in comparison with the Vcl and V, centres. 2. The gl-value of the V- centre is the smallest one in the whole family of V centres. The following expression for g, is known to be true for the analogous axial centres [S]: gl = 2.0023 - (1 - 2X/A), where X is the spin-orbit interaction constant (for S- ion X < 0), and A is the energetic interval between the ground state Pz and the excited states Px and Py which are split by the axial crystalline field. The maximum A-value, and hence the minimum value of g,, can be expected in the case of the V- centre. 3. The motional averaging of the EPR spectrum of the V- centre can be observed. With the temperature rising up to lOOK, the axial spectrum lines broaden and disappear, and thereafter the wide isotropic line (ABpp=35-10e4T at Torn= 145K) withg= (2g, + g11)/3 = 2.084 becomes detectable in the temperature range 1lo-170K (see Fig. 1). The spectrum transformation described above indicates hole jumping between the sulphur anions surrounding the cation vacancy at temperatures above 110 K. Hence, there are no defects in the close vicinity of the vacancy. No motional averaging could be observed in the case of VSH, V&,), Vck, VF centres containing impurity defects near the cation vacancy. The EPR lines of these centres disappear due to broadening nearly at the temperature of thermal destruction (hole
Table 1. The parameters of V eentres in Cast Centre
vF
Tooptlm.
PK
8-11 14-18 20-35 20-35 20-35
VVW
G&I,
135-155 20-35 20-35
g,
gll
2.2345 f 0.0015 x2.003 2.1700 f 0.0005 z2.002 2.1315 f0.0005 c2.002 2.1312 f 0.0005 %2.002 2.1240 f 0.0010 x2.002 the averaged spectrum: g = 2.0840 f 0.0020 2.1330 f 0.0010 x2.002 g, = 2.1464 f 0.0010 gr = 2.1351 f 0.0015 gs 0 2.002 2.1407 f 0.0100 zz2.002
Thole K escape,
120-160 120-160 180-215 160-210 up to 285 160-210
1lo-160
20-35 not measured VM t g,-values of [Nap, [Li]‘, V,, VF have been corrected (cf. Ref. [I]).
V. SEEMAN et al.
70
170
270
T,K
Fig. 4. Electron-hole processes on pulse annealing of plastically deformed CaS (pressure 2500MPa). To populate defects with holes or electrons samples were X-irradiated at 77 K before annealing. The intensity of EPR signals is given
versus the pulse annealing temperature. V:(,,) centres are studied after annealing the samples at 66OK. release). Therefore, the V- centre model may be schematically represented as S--v,-S2-, and the hole is really captured near the isolated cation vacancy (v,). This V- centre is analogous to Vcentre of a similar structure (O--II,-02-) observed in alkaline earth oxides (e.g. [7]). 3.1.2. Vsn centre. F’S, centres have an axial symmetry, and the g-tensor value of these centres is close to that of Vet and Vr centres. Moreover, the thermal stabilities of Vsu, Fct, V, centres are practically alike (see Table 1). Hence, the structure of the Vsu centre involves some defect with an effective charge +l at the anion position nearest to v,. We suppose this defect to be the molecular ion (SH)-, as hydrogen may become incorporated in the CaS lattice in the course of the material synthesis (Part 2). We tried to detect the superhypetline structure (SHFS) caused by the proton in the EPR spectrum of the Vsn centre. However, we did not detect any SHFS even in the samples with the Vsn centres prevailing in number (ionic processes at 650 K yield the maximum Vsu centre concentration, see the text below). This was probably due to the circumstance that the width of EPR line is too large in comparison with the SHF splitting value. The same happens when Vr centres in plastically deformed samples are studied and the SHFS from “F becomes unobservable because of the broadening of EPR lines due to multiple defects generated under deformation. However, there is indirect evidence that (SH)- ion is present near the cation vacancy in the case of VSH centres. The annealing of the deformed samples at 450-650K results in a complete transformation of isolated cation vacancies into associates (see the text below). This transformation of defects and the low temperature at which it takes place can be explained by the migration of small light-weighted ions, most likely protons. The protons are captured by cation
vacancies and thus v,-(SH)- associates are formed. The quantity of these associates should depend on the concentration of hydrogen impurity. The samples with a low hydrogen content were prepared by a direct reaction between elementary Ca and S (see the text above). After these samples had been plastically deformed and X-irradiated at 77K, the number of VSH centres was found to be about a tenth of that measured in samples of ordinary synthesis (the reduction of CaS04 with hydrogen). Moreover, the transformation of isolated cation vacancies on annealing (500-675K) proceeds in another mode in samples with low hydrogen content. Only a small part of isolated v, are transformed into v,-(SH)- associates, while the rest are transformed into w,-Hal. Another model of this centre may also be proposed in case the (OH)- group, instead of the (SH)- group, were associated with the cation vacancy (S--v,(OH)-). To check this model, samples with different oxygen content were prepared and studied. Oxygen was introduced by annealing the samples at high temperature (1170-1370K) either with a limited oxygen access or with the admixture of (NH4)2 SO4 and other oxygen sources. The samples obtained were chemically analysed for their S2- content which varied from 74% to 99.4%. After the samples have been pressed and X-irradiated at 77K, the relative quantities of VSH and V- centres were evaluated. The relative quantity of VSH centres does not seem to depend on the level of the oxygen concentration, as the V-/Vs, ratio is constant regardless of S2- and 02- contents. Consequently, oxygen is not involved in VSH centre. Thus, the model of VsH centres can be schematically represented as S-v,-(SH)-. These centres are similar to VO, centres in alkaline earth oxides. 3.1.3. V&u) centre. The EPR spectrum of this centre is shown in Fig. 2. One of the lines of the centre overlaps partly with the gl-line of V&l-I) VSH. The other line of the V GSHj centre is less intensive and lies at a lower magnetic field. The presence of this additional low-field line indicates that the symmetry of the centre is lower than the axial symmetry. The widths of the EPR lines of I-‘:(,,) and VSH centres are almost the same. Hence, V~~,,~ is not a VSH centre lying at the surface of the microcrystal. The thermal stability of V:(,,) (temperature of hole release) is lower than that of P’S, centres (see Fig. 4). The centres and VSH centres quantity of both V&) increases after annealing the deformed samples in the temperature interval from 450K to 650K. Also, the quantity of V:(,,) centres (as well as that of VSH) is much smaller in the samples prepared by the direct Ca + S reaction in comparison with ordinary CaS. All the facts considered enable us to represent the
EPR
of V centres in plastically deformed CaS
model of the V&n) centre as a hole captured at a sulphide anion next to the cation vacancy associated with two (SH)) groups, which is schematically presented as s(SH)--&SH)-. A similar centre was found by EPR in SrO single crystals [6]: 0(OH)--
&OH)-.
Two other defects, each involving a cation vacancy, were observed after the annealing of deformed samples. One of these, Gnat centre, appears after heating the samples up to 600-740 K. It is similar to V~Zor Vet centres studied in undeformed samples doped with halogens. Thermal stability and the g,-value are very close to those of V, or Vet centres. However, the line width exceeds that of VHal in undeformed samples. That is why we could neither observe any SHFS in EPR spectra nor decide which halogen is involved in the structure of the centre. Another centre, l’,, is formed after heating the deformed samples up to 700800K. The axial structure of the Vhl centre can be inferred from the EPR spectrum. The g,-line of I’M (Fig. 2) lies at lower magnetic fields than those of other axial centres involving v,. Large g,-value of this centre (see Table 1) probably indicates that the negative charge of the cation vacancy is compensated better than in the case of VHalor VS* centres. An impurity metal with a high charge (perhaps M4+) can be associated with the vacancy: S--v,-S2--M4’. 3.2. Formation of V centres and cation vacancies in CaS All the centres discussed above involve a cation vacancy in their structure. It should be stressed that cation vacancies cannot be produced by X-rays. We have never seen V-, VSH, and V$,,) centres in Xirradiated but undeformed samples. The same conclusion can be inferred from the single-stage character of the accumulation of V-, VsH, and I’,+(,,) centres under X-irradiation at 77K (see Fig. 3(c)). We consider that cation vacancies are created in CaS under plastic deformation. The intensity of EPR lines of V centres increases proportionally with the increase in the pressure applied (Fig. 3(a)). The quantity of w,-(SH)and (SH)--v,-(SH)associates also increases with pressure but shows a tendency to saturation. One could suppose that the saturation reflects the deficiency of hydrogen in the samples. However, on annealing the deformed samples at
1333
temperatures of 450-700K all the isolated v, are transformed into v,-(SH)) and (SH)--v,-(SH)associates. Such full transformation is observed even in the samples deformed with the maximum pressure, in which the quantity of v, is very large. We suppose that cation vacancies are preferentially generated at low deformations in the imperfect regions of crystals. Hydrogen is possibly located in the same regions, and the associates are easily formed in the process of deformation. In the case of increased deformations the generation of vacancies involves other crystal regions where hydrogen can appear only on annealing. Thus, isolated v, (and hence, V centres) dominate after the deformation of CaS (before annealing). The imperfect regions mentioned are possibly the boundaries of blocks, dislocations, splits or surfaces of microcrystals. One of the possible mechanisms of the creation of vacancies under deformation is the interaction of moving dislocations. 3.3. Thermal stability of V centres. Electronhole processes Thermal electronic and thermal ionic processes with V centres fortunately proceed at different temperature ranges in CaS. Figure 4 illustrates the results of the pulse annealing of V centres (electron-hole processes). The quantity of V - centres decreases steadily with the temperature rises from 120K to 285 K. VsH and centres are destroyed when temperatures V&l+) reach 160K and 2lOK, respectively. The thermal destruction of V _ centres within the temperature range of 120-285 K is accompanied by a decrease in the number of FF centres, although F, centres are stable even at room temperature. The quantity of the F’ centres is practically unchanged within the whole temperature range of the thermal destruction of V -, VsH,and V,+(,,) centres (Fig. 4). The results presented can be easily interpreted if the thermal destruction of centres is associated with hole V-, VSH, and V&q release from the centres. It should be pointed out that the probability of electron-hole recombination at V centres is very low due to their effective negative charge. The thermal destruction of V - centres taking place within the unusually wide temperature range is probably due to the presence of some disturbing defects in the vicinity of V- centres in plastically deformed samples (dislocations, boundaries of blocks, cation and anion vacancies, interstitials, impurities, etc.). These defects can affect the potential barrier for the transition of a trapped hole to the valence band, the barrier being decreased or increased depending on the charge and location of the disturbing defect. The assumption of the disturbing defects is in agreement
1334
V. SEEMAN et al.
with a very large width of the EPR line observed for gl-components of V- centre (A&r r~ 5 - 10m4T at T = 35 K). The width of EPR line increases with the increase of applied pressure (Fig. 3(b)) and exceeds that of VF and Vc, in undeformed samples almost by an order of magnitude. The formation of the vacancy pairs Q-V, detected by EPR as F, centres is indirect evidence of the possibility that anion vacancies are located not far from cation vacancies. Nevertheless, it is important to note that the character of the thermal destruction of Vr, Vc, and [Na]‘, [Li]’ centres is only slightly affected by the deformation of CaS samples. Holes are released from most of these centres at the same temperatures both in deformed and undeformed samples. Thus, we can suggest one more mechanism to explain the fact that the thermal destruction of Vcentres takes place in a wide temperature range. This may be due to hole jumping between cation vacancies situated not far from each other. As we observed the motional averaging of the EPR signal above 110 K, we suppose that the hole rotating around the cation vacancy is very likely to be retrapped by another cation vacancy located in the vicinity of the former one. The probability of hole transfer to a more distant cation vacancy increases with the rise in temperature. As a result, a hole may be transported to some recombination centre where it is annihilated together with a trapped electron. Hole jumping between cation vacancies probably takes place in the region of high defect density. This mechanism should be responsible for V- centre destruction at least within the temperature range of 120-215K (the upper limit is taken approximately equal to the temperature of Vcl and VF destruction). Ordinary hole release into the
300
450
600
valence band is likely to take place at temperatures as high as 240-285 K. 3.4. Cation vacancies in ionic processes The defects created under plastic deformation in CaS can be transformed or annealed completely by heating the deformed samples. We determined the thermal stability of isolated cation vacancies and their associates with (SH)- groups or anion vacancies. For this purpose the relative number of each of the and FL, F+ was measured centres v-, VSH, v&H), in the course of pulse annealing of the samples. The results are shown in Fig. 5. The isolated cation vacancies disappear at temperatures between 450K and 670K. This process is accompanied by the formation of associates v,-(SH)) and (SH)--v,(SH)-. The sum of the number of u, and w,-(SH)-, (SH)--v,-(SH)is approximately constant between 450 K and 575 K. Therefore, the transformation of v, into associates w,-(SH)- and (SH)--w,-(SH)is likely to occur. The transformation can result both from the capture of migrating cation vacancies by the (SH)- impurity and from the capture of migrating hydrogen (protons) by cation vacancies. If the moving entities had been cation vacancies, the formation of new vacancy pairs v,-V, could have been observed as anion vacancies existing up to 750K. However, this transformation does not take place, at least not in the temperature range of 450-575K within which the number of vacancy pairs u,-v, remains constant (see Fig. 5). Hence, the transformation of V, into v,-(SH)) and (SH))-v,(SH)- associates is due to the capture of migrating
750
900
T.
K
Fig. 5. Ionic processeson pulse annealing of plastically deformed CaS (pressure 2500 MPa). F+ and F, centres measured after X-irradiation at 300 K, V centres measured after X-irradiation at 77 K. The X-raying time of 1h is sufficient to populate the maximum possible number of defects with holes (electrons).
EPR of V centres in plastically deformed CaS
by v,. The isolated cation vacancies v, decrease essentially in number and disappear completely in the temperature range of 575-670 K. In the same temperature range, cation and anion vacancy pairs vc-va are formed. v,-Hal associates (Hal = F or Cl) are also formed almost in the same temperature range (600-750 K). As the two different associates are formed at close temperatures, one can suppose that isolated v, are likely to move at temperatures exceeding 575 K. The associates of v, with (SH)- groups still increase in number at these temperatures (v,-(SH)up to 6.50K and (SH)--v,-(SH)) up to 700 K). This is due to the fact that protons are located in regions with a different extent of lattice damage, and hence, they are released in a very wide temperature range. Associates v,-(SH)- decrease in number and disappear at temperatures exceeding 660K. However, more complex associates (SH)--v,-(SH)are still formed up to 700K. This fact can be explained by the capture of one more proton by v,-(SH)-. Most of the defects involving a cation vacancy v,-(SH)), (SH)--v,-(SH)-) dis(Ilc-%* v,-Hal-, appear in the temperature range of 700-830K. All these defects are quite different in their structure and effective charge. Hence, one could suppose that these defects disappear due to the annihilation of v,, together with migrating Ca interstitials. However, some new defects involving v, (S--w,-S2--M4+, see the text above) are formed almost at the same temperatures (700-800K). These defects are stable up to 900 K. Therefore, Ca interstitials are not responsible for the decay of the defects involving a cation vacancy. The vacancies are likely to disappear due to their migration towards the surface of crystals or blocks. The ionic processes in plastically deformed CaS
protons
1335
proceed at remarkably lower temperatures than the temperature at which ionic conductivity in undeformed CaS was observed (above lOOOK) [8]. The difference arises from high concentrations of mutually-affecting defects produced by plastic deformation. Among these defects a noticeable role is probably played by dislocations. Migration of cation vacancies can be involved (directly or indirectly) in the process of annealing of dislocations. The defects are nonuniformly distributed in the destroyed crystal lattice. The process of the restoration of the crystal lattice in different defect regions proceeds at different temperatures. This is why the temperature of the formation of various associates is different (Fig. 5). acknowledge the financial support of the Estonian Science Foundation. Acknowledgement-We
REFERENCES 1. Danilkin M. I., Riiv 1. R., Seeman V. O., Belskiy A. N. and Soldatov S. N., Trans. Inst. Phys., Estonian Acad. Sci. 67,97 (1990) (in Russian). 2. Pedak E. J.. Allsalu M.-L. J. and Kanter M. J.. J. Ad Chem. (USSR) 45,2619 (1972) (in Russian). -A 3. Kuznetsov A., Jaek I., Seeman V. and Pung L., Proc. Estonian Acad. Sci., Phys.-Math. 23, 33 (1974) (in Russian). 4. Seeman V., Danilkin M., Lepist M., Must M., Ots A., Pedak E., Pung L., Parnoja E. and Rammo I., Intern. Symposium LUMDETR’94, Tallinn, 25-29 Sept. 1994. Abstracts, p. 100. 5. Schirmer 0. F., J. Phys. Chem. Solids 32,499 (1971). 6. Haldre ii., Seeman V. and Lehto T., Intern. Conf. Defects in Znsul. Cryst., Riga, 18-23 May 1981. Abstracts, p. 317. I. Abraham M. M., Chen Y., Boatner L. A. and Reynolds R. W., Solid State Commun. 16,1209 (1975). 8. Egami A., Onoye T. and Narita K., Trans. Jap. Inst. Metals 22, 399 (1981).
Pergamon
No. 9. pp. 1329-1335.1996 Copyright0 1996EhevierScienaLtd Pnnted III Great Britain. AU riglm reserved 0022-3697/% SI5.00 + 0.00
0022~3697(95)003274
EPR OF V CENTRES V. SEEMAN, M. DANILKIN,
IN PLASTICALLY
DEFORMED
CaS
M. MUST, A. OTS, E. PEDAK, L. PUNG and E. PARNOJA Tartu University, EE2400 Tartu, Estonia
(Received I June 1995; accepted 20 November 1995)
Abstract-Several types of hole V centres have been observed by the EPR method in plastically deformed CaS polycrystals after the X-irradiation at 77 K. Dominating V -, VsH, and V:(,,) centres are formed when a hole is trapped by a sulphur anion near the isolated cation vacancy or near the cation vacancy associated with one or two (SH)- groups. Holes are released from the V -, VSH, and V&,) centres at 120-285, 160210, and 1lo-160 K, respectively. Hole release in the case of V - centres takes place in an unusually wide temperature range. The possible mechanisms responsible for this process are discussed. The isolated cation vacancies are easily transformed into associates with one or two (SH)- groups by heating the samples up to 450-670 K. These associates disappear completely after the samples are heated above 800 K. Keywords: A. chalcogenides, D. defects, D. electron paramagnetic resonance (EPR).
1. INTRODUCTION Calcium sulphide is a well-known base for a variety of luminophors. Both impurity defects (activators, coactivators) and also intrinsic lattice defects (cation and anion vacancies) influence the properties of CaSbased luminophors. Yet there are no reliable data available on cation vacancies or their associates with impurities in CaS, although point defects in alkaline earth sulphides have been studied for years. This can be explained by a deficit in sulliciently large single crystals of CaS. Also, magnetic resonance methods often yield no comprehensive information about point defects in powder samples. However, a family of V centres was reported in our previous paper [ 11.The V centres were studied in CaS polycrystals activated with halogen ions or alkaline metals after X-raying samples at 77K. Structural defects present in the samples become paramagnetic in case a hole is captured by a sulphide anion next to the cation vacancy associated with a haloid ion (Cl or F) or by a sulphide anion next to Na+ or Li+ substituting Ca2+ [l]. The centres observed have an axial structure: S--w,-Halor S-M+-S2-, where Hal stands for Cl or F, and M stands for Na or Li. We named these defects Vcl, VF or pa]‘, [Li]’ on the analogy of the well-known hole V centres in alkaline earth oxides. However, it is important to point out here that no isolated cation vacancies with captured holes (i.e. V - centres) can be detected in CaS polycrystals after any chemical or thermal treatment. Yet, in the course of our experiment we could find an easy way of producing cation vacancies by the plastic deformation (pressing) of CaS polycrystals. Plastic deformation produces,
besides isolated cation vacancies, some associates of defects in which cation vacancies are involved. These defects become paramagnetic due to the capturing of holes after the excitation of the samples. The Vcentres are prevailing among different paramagnetic centres thus formed. Here we report the results of the EPR study of hole V centres in plastically deformed samples and some properties of cation vacancies.
2. OBJECTS, INSTRUMENTATION, EXPERIMENTAL METHODS Calcium sulphide was prepared CaS04
with
flowing
purified
AND
by the reduction hydrogen
at
of
1150-
1170 K, according to the method described in [2]. Initial CaS04 was precipitated from high-puritygrade Ca(N03)2 and (NH&SO, solutions. Transition and heavy metals were specially removed before the precipitation by organic extracting agents. Alkaline metals were removed by repeated washing of precipitated CaS04 with distilled water. The resulting CaS contained very small quantities (less than 10m5at.%) of Cr, Fe, Pb, Mn, etc. Also, it contained small quantities of Na, Li, and F, Cl (less than 10V4at.%). The product contained up to 99.4% of the theoretical quantity of sulphide anions. Some quantities of CaS were doped with halogen ions or alkaline metals. The required amounts of dopants were added and the samples were heated with 2-20% of sulphur under flowing argon at 1170-1420K for 30-45min. Then the samples were cooled under flowing argon to room temperature outside the furnace.
1329
1330
V. SEEMAN et al.
An alternative method of direct synthesis was also developed to prepare some of the CaS samples. The calcium metal of high purity grade and high-purity elementary sulphur were heated together in a preevacuated sealed quartz tube. The reaction proceeded at 900-1050K. In order to obtain a more homogeneous product, it was annealed up to 1200 K. A small excess of sulphur was used to prevent a casual oxidation of the product. Nevertheless, it contained about 93% of the theoretical quantity of sulphide anions due to nonuniform combustion of calcium in sulphur vapours. This method of direct synthesis was designed in order to obtain CaS with a low hydrogen content. The results obtained with two different calcium sulphides (with and without hydrogen) were compared in order to prove the models of Vs, and V$,,) centres. For the same purpose samples activated with oxygen and with (OH)- groups were prepared. The plastic deformation of the samples was performed by a hydraulic press at room temperature. The applied pressure varied from 100 to 2500MPa. The pellets produced by pressing were crushed and ground into powder before carrying out EPR measurements. The EPR spectra were measured with an X-band (8.875 GHz) spectrometer of 100 KHz magnetic field modulation. A continuous-flow helium cryostat (Oxford Instruments ESR-9) was used to keep the samples at necessary temperatures. Pulse annealing of the samples was performed to determine thermal stabilities of V centres and cation vacancies. To make defects paramagnetic the samples were X-irradiated at 77 K before annealing (50KV, 18 mA, W anode). Pulse annealing was carried out in the cryostat at temperatures below 300K. A special separately mounted electric furnace was employed for pulse annealing at higher temperatures (300- 1120 K). On pulse annealing the samples were kept at the required temperature for a certain time (2 min) after fast heating up, and then they were fast cooled down to the temperature chosen for measurements (35K for V centres). To investigate ionic processes at temperatures above 300K, at which holes escape from Vcentres, the samples were X-rayed at 77 K after each step of annealing to populate the cation vacancies with holes. The X-irradiation of the samples was continued until the EPR signal intensity stopped increasing (usually 1 h).
3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Models of V centres The EPR measurements of CaS before deformation demonstrated that there was a minute quantity of the ions Mn2+ and Cr3+ within a cubic symmetry
I
0 29
v=8676
0 30
0
2 MHz
31
B. T
Fig. 1. EPR spectra of V centres in plastically deformed CaS polycrystals: full-scale spectrum at 35 K, averaged spectrum of V - centre at 145 K. surrounding. After X-irradiation at 77K, weak signals of [Na]’ and [Li]’ centres could also be observed, which indicates some contamination of CaS with alkaline metals. VF and V,-, centres were observed in the samples doped with proper halogen ions. After mechanical deformation (grinding, smashing, pressing) F + centres become detectable (F’ stands for the electron trapped at the anion vacancy). F; centres could also be detected but only after the deformed samples had been X-rayed (FL stands for the electron trapped at the associate Q-V,, of anion and cation vacancies). Sharp single isotropic lines with g = 2.0031 (for F+) and g = 2.0017 (for FL), corresponding to these centres, could be observed in EPR spectra at low level of microwave power, provided that the temperature was not too low [3, 41. Several anisotropic centres with similar properties are also formed in plastically deformed CaS. These centres are observable after X-irradiation of the deformed samples at 77K. The quantity of these
;
e)
1
0.2933
0.2960
0 2990
B,S
Fig. 2. EPR spectra of V centres (gL-region, T = 35K) in CaS polycrystals plastically deformed at 300K: before annealing (a) and after annealing at 673 K (b), 710 K (c), 823 K (d) and 932 K (e).
EPR of V centres in plastically deformed CaS
.z
/
0
I
iO0
5c
‘I,mir,
Fig. 3. The EPR g,-line intensity (a) and width (b) of V eentres (T = 35 K) versus the amount of the pressure applied; the line intensity versus the time of X-irradiation at 77 K (c). centres
increases
steadily
crystals is increased
when
pressure
on poly-
(see Fig. 3(a)). Several centres of
an axial symmetry and one centre of a lower symmetry of the g-tensor were revealed by an analysis of EPR spectra after the excited samples had been annealed to different temperatures. The lines of these centres, corresponding to the parallel components of the gtensor, are weak and overlap strongly with the F+ centre signal and with signals of some unknown defects within the range of g g 2.002. The more intensive lines connected with the perpendicular gtensor components overlap partly (see Figs 1 and 2). The g-tensor components and other parameters for these defects designated as V-, V.n, and V&u) are given in Table 1. The g-values of these centres are very close to those of Vcl and Vr centres. All the centres are
1331
observable at a high level of microwave power and at the same optimal temperatures of 25-35 K (except in the case of [Na]’ and [Li]’ centres). Hence, the V-, and Vc/~l,VP centres have a similar VSHt V&H)’ structure involving the cation vacancy. Below we shall discuss the model of each centre studied. 3.1.1. V - centre. 1. The V - centre is characterized by the highest temperature of hole release of the group studied (see Fig. 4). It disappears fully at 285 K, while the V,-l and VF centres are destroyed at the temperature as low as 215K. Therefore, there is no charge compensating defect in the close vicinity of the cation vacancy in comparison with the Vcl and V, centres. 2. The gl-value of the V- centre is the smallest one in the whole family of V centres. The following expression for g, is known to be true for the analogous axial centres [S]: gl = 2.0023 - (1 - 2X/A), where X is the spin-orbit interaction constant (for S- ion X < 0), and A is the energetic interval between the ground state Pz and the excited states Px and Py which are split by the axial crystalline field. The maximum A-value, and hence the minimum value of g,, can be expected in the case of the V- centre. 3. The motional averaging of the EPR spectrum of the V- centre can be observed. With the temperature rising up to lOOK, the axial spectrum lines broaden and disappear, and thereafter the wide isotropic line (ABpp=35-10e4T at Torn= 145K) withg= (2g, + g11)/3 = 2.084 becomes detectable in the temperature range 1lo-170K (see Fig. 1). The spectrum transformation described above indicates hole jumping between the sulphur anions surrounding the cation vacancy at temperatures above 110 K. Hence, there are no defects in the close vicinity of the vacancy. No motional averaging could be observed in the case of VSH, V&,), Vck, VF centres containing impurity defects near the cation vacancy. The EPR lines of these centres disappear due to broadening nearly at the temperature of thermal destruction (hole
Table 1. The parameters of V eentres in Cast Centre
vF
Tooptlm.
PK
8-11 14-18 20-35 20-35 20-35
VVW
G&I,
135-155 20-35 20-35
g,
gll
2.2345 f 0.0015 x2.003 2.1700 f 0.0005 z2.002 2.1315 f0.0005 c2.002 2.1312 f 0.0005 %2.002 2.1240 f 0.0010 x2.002 the averaged spectrum: g = 2.0840 f 0.0020 2.1330 f 0.0010 x2.002 g, = 2.1464 f 0.0010 gr = 2.1351 f 0.0015 gs 0 2.002 2.1407 f 0.0100 zz2.002
Thole K escape,
120-160 120-160 180-215 160-210 up to 285 160-210
1lo-160
20-35 not measured VM t g,-values of [Nap, [Li]‘, V,, VF have been corrected (cf. Ref. [I]).
V. SEEMAN et al.
70
170
270
T,K
Fig. 4. Electron-hole processes on pulse annealing of plastically deformed CaS (pressure 2500MPa). To populate defects with holes or electrons samples were X-irradiated at 77 K before annealing. The intensity of EPR signals is given
versus the pulse annealing temperature. V:(,,) centres are studied after annealing the samples at 66OK. release). Therefore, the V- centre model may be schematically represented as S--v,-S2-, and the hole is really captured near the isolated cation vacancy (v,). This V- centre is analogous to Vcentre of a similar structure (O--II,-02-) observed in alkaline earth oxides (e.g. [7]). 3.1.2. Vsn centre. F’S, centres have an axial symmetry, and the g-tensor value of these centres is close to that of Vet and Vr centres. Moreover, the thermal stabilities of Vsu, Fct, V, centres are practically alike (see Table 1). Hence, the structure of the Vsu centre involves some defect with an effective charge +l at the anion position nearest to v,. We suppose this defect to be the molecular ion (SH)-, as hydrogen may become incorporated in the CaS lattice in the course of the material synthesis (Part 2). We tried to detect the superhypetline structure (SHFS) caused by the proton in the EPR spectrum of the Vsn centre. However, we did not detect any SHFS even in the samples with the Vsn centres prevailing in number (ionic processes at 650 K yield the maximum Vsu centre concentration, see the text below). This was probably due to the circumstance that the width of EPR line is too large in comparison with the SHF splitting value. The same happens when Vr centres in plastically deformed samples are studied and the SHFS from “F becomes unobservable because of the broadening of EPR lines due to multiple defects generated under deformation. However, there is indirect evidence that (SH)- ion is present near the cation vacancy in the case of VSH centres. The annealing of the deformed samples at 450-650K results in a complete transformation of isolated cation vacancies into associates (see the text below). This transformation of defects and the low temperature at which it takes place can be explained by the migration of small light-weighted ions, most likely protons. The protons are captured by cation
vacancies and thus v,-(SH)- associates are formed. The quantity of these associates should depend on the concentration of hydrogen impurity. The samples with a low hydrogen content were prepared by a direct reaction between elementary Ca and S (see the text above). After these samples had been plastically deformed and X-irradiated at 77K, the number of VSH centres was found to be about a tenth of that measured in samples of ordinary synthesis (the reduction of CaS04 with hydrogen). Moreover, the transformation of isolated cation vacancies on annealing (500-675K) proceeds in another mode in samples with low hydrogen content. Only a small part of isolated v, are transformed into v,-(SH)- associates, while the rest are transformed into w,-Hal. Another model of this centre may also be proposed in case the (OH)- group, instead of the (SH)- group, were associated with the cation vacancy (S--v,(OH)-). To check this model, samples with different oxygen content were prepared and studied. Oxygen was introduced by annealing the samples at high temperature (1170-1370K) either with a limited oxygen access or with the admixture of (NH4)2 SO4 and other oxygen sources. The samples obtained were chemically analysed for their S2- content which varied from 74% to 99.4%. After the samples have been pressed and X-irradiated at 77K, the relative quantities of VSH and V- centres were evaluated. The relative quantity of VSH centres does not seem to depend on the level of the oxygen concentration, as the V-/Vs, ratio is constant regardless of S2- and 02- contents. Consequently, oxygen is not involved in VSH centre. Thus, the model of VsH centres can be schematically represented as S-v,-(SH)-. These centres are similar to VO, centres in alkaline earth oxides. 3.1.3. V&u) centre. The EPR spectrum of this centre is shown in Fig. 2. One of the lines of the centre overlaps partly with the gl-line of V&l-I) VSH. The other line of the V GSHj centre is less intensive and lies at a lower magnetic field. The presence of this additional low-field line indicates that the symmetry of the centre is lower than the axial symmetry. The widths of the EPR lines of I-‘:(,,) and VSH centres are almost the same. Hence, V~~,,~ is not a VSH centre lying at the surface of the microcrystal. The thermal stability of V:(,,) (temperature of hole release) is lower than that of P’S, centres (see Fig. 4). The centres and VSH centres quantity of both V&) increases after annealing the deformed samples in the temperature interval from 450K to 650K. Also, the quantity of V:(,,) centres (as well as that of VSH) is much smaller in the samples prepared by the direct Ca + S reaction in comparison with ordinary CaS. All the facts considered enable us to represent the
EPR
of V centres in plastically deformed CaS
model of the V&n) centre as a hole captured at a sulphide anion next to the cation vacancy associated with two (SH)) groups, which is schematically presented as s(SH)--&SH)-. A similar centre was found by EPR in SrO single crystals [6]: 0(OH)--
&OH)-.
Two other defects, each involving a cation vacancy, were observed after the annealing of deformed samples. One of these, Gnat centre, appears after heating the samples up to 600-740 K. It is similar to V~Zor Vet centres studied in undeformed samples doped with halogens. Thermal stability and the g,-value are very close to those of V, or Vet centres. However, the line width exceeds that of VHal in undeformed samples. That is why we could neither observe any SHFS in EPR spectra nor decide which halogen is involved in the structure of the centre. Another centre, l’,, is formed after heating the deformed samples up to 700800K. The axial structure of the Vhl centre can be inferred from the EPR spectrum. The g,-line of I’M (Fig. 2) lies at lower magnetic fields than those of other axial centres involving v,. Large g,-value of this centre (see Table 1) probably indicates that the negative charge of the cation vacancy is compensated better than in the case of VHalor VS* centres. An impurity metal with a high charge (perhaps M4+) can be associated with the vacancy: S--v,-S2--M4’. 3.2. Formation of V centres and cation vacancies in CaS All the centres discussed above involve a cation vacancy in their structure. It should be stressed that cation vacancies cannot be produced by X-rays. We have never seen V-, VSH, and V$,,) centres in Xirradiated but undeformed samples. The same conclusion can be inferred from the single-stage character of the accumulation of V-, VsH, and I’,+(,,) centres under X-irradiation at 77K (see Fig. 3(c)). We consider that cation vacancies are created in CaS under plastic deformation. The intensity of EPR lines of V centres increases proportionally with the increase in the pressure applied (Fig. 3(a)). The quantity of w,-(SH)and (SH)--v,-(SH)associates also increases with pressure but shows a tendency to saturation. One could suppose that the saturation reflects the deficiency of hydrogen in the samples. However, on annealing the deformed samples at
1333
temperatures of 450-700K all the isolated v, are transformed into v,-(SH)) and (SH)--v,-(SH)associates. Such full transformation is observed even in the samples deformed with the maximum pressure, in which the quantity of v, is very large. We suppose that cation vacancies are preferentially generated at low deformations in the imperfect regions of crystals. Hydrogen is possibly located in the same regions, and the associates are easily formed in the process of deformation. In the case of increased deformations the generation of vacancies involves other crystal regions where hydrogen can appear only on annealing. Thus, isolated v, (and hence, V centres) dominate after the deformation of CaS (before annealing). The imperfect regions mentioned are possibly the boundaries of blocks, dislocations, splits or surfaces of microcrystals. One of the possible mechanisms of the creation of vacancies under deformation is the interaction of moving dislocations. 3.3. Thermal stability of V centres. Electronhole processes Thermal electronic and thermal ionic processes with V centres fortunately proceed at different temperature ranges in CaS. Figure 4 illustrates the results of the pulse annealing of V centres (electron-hole processes). The quantity of V - centres decreases steadily with the temperature rises from 120K to 285 K. VsH and centres are destroyed when temperatures V&l+) reach 160K and 2lOK, respectively. The thermal destruction of V _ centres within the temperature range of 120-285 K is accompanied by a decrease in the number of FF centres, although F, centres are stable even at room temperature. The quantity of the F’ centres is practically unchanged within the whole temperature range of the thermal destruction of V -, VsH,and V,+(,,) centres (Fig. 4). The results presented can be easily interpreted if the thermal destruction of centres is associated with hole V-, VSH, and V&q release from the centres. It should be pointed out that the probability of electron-hole recombination at V centres is very low due to their effective negative charge. The thermal destruction of V - centres taking place within the unusually wide temperature range is probably due to the presence of some disturbing defects in the vicinity of V- centres in plastically deformed samples (dislocations, boundaries of blocks, cation and anion vacancies, interstitials, impurities, etc.). These defects can affect the potential barrier for the transition of a trapped hole to the valence band, the barrier being decreased or increased depending on the charge and location of the disturbing defect. The assumption of the disturbing defects is in agreement
1334
V. SEEMAN et al.
with a very large width of the EPR line observed for gl-components of V- centre (A&r r~ 5 - 10m4T at T = 35 K). The width of EPR line increases with the increase of applied pressure (Fig. 3(b)) and exceeds that of VF and Vc, in undeformed samples almost by an order of magnitude. The formation of the vacancy pairs Q-V, detected by EPR as F, centres is indirect evidence of the possibility that anion vacancies are located not far from cation vacancies. Nevertheless, it is important to note that the character of the thermal destruction of Vr, Vc, and [Na]‘, [Li]’ centres is only slightly affected by the deformation of CaS samples. Holes are released from most of these centres at the same temperatures both in deformed and undeformed samples. Thus, we can suggest one more mechanism to explain the fact that the thermal destruction of Vcentres takes place in a wide temperature range. This may be due to hole jumping between cation vacancies situated not far from each other. As we observed the motional averaging of the EPR signal above 110 K, we suppose that the hole rotating around the cation vacancy is very likely to be retrapped by another cation vacancy located in the vicinity of the former one. The probability of hole transfer to a more distant cation vacancy increases with the rise in temperature. As a result, a hole may be transported to some recombination centre where it is annihilated together with a trapped electron. Hole jumping between cation vacancies probably takes place in the region of high defect density. This mechanism should be responsible for V- centre destruction at least within the temperature range of 120-215K (the upper limit is taken approximately equal to the temperature of Vcl and VF destruction). Ordinary hole release into the
300
450
600
valence band is likely to take place at temperatures as high as 240-285 K. 3.4. Cation vacancies in ionic processes The defects created under plastic deformation in CaS can be transformed or annealed completely by heating the deformed samples. We determined the thermal stability of isolated cation vacancies and their associates with (SH)- groups or anion vacancies. For this purpose the relative number of each of the and FL, F+ was measured centres v-, VSH, v&H), in the course of pulse annealing of the samples. The results are shown in Fig. 5. The isolated cation vacancies disappear at temperatures between 450K and 670K. This process is accompanied by the formation of associates v,-(SH)) and (SH)--v,(SH)-. The sum of the number of u, and w,-(SH)-, (SH)--v,-(SH)is approximately constant between 450 K and 575 K. Therefore, the transformation of v, into associates w,-(SH)- and (SH)--w,-(SH)is likely to occur. The transformation can result both from the capture of migrating cation vacancies by the (SH)- impurity and from the capture of migrating hydrogen (protons) by cation vacancies. If the moving entities had been cation vacancies, the formation of new vacancy pairs v,-V, could have been observed as anion vacancies existing up to 750K. However, this transformation does not take place, at least not in the temperature range of 450-575K within which the number of vacancy pairs u,-v, remains constant (see Fig. 5). Hence, the transformation of V, into v,-(SH)) and (SH))-v,(SH)- associates is due to the capture of migrating
750
900
T.
K
Fig. 5. Ionic processeson pulse annealing of plastically deformed CaS (pressure 2500 MPa). F+ and F, centres measured after X-irradiation at 300 K, V centres measured after X-irradiation at 77 K. The X-raying time of 1h is sufficient to populate the maximum possible number of defects with holes (electrons).
EPR of V centres in plastically deformed CaS
by v,. The isolated cation vacancies v, decrease essentially in number and disappear completely in the temperature range of 575-670 K. In the same temperature range, cation and anion vacancy pairs vc-va are formed. v,-Hal associates (Hal = F or Cl) are also formed almost in the same temperature range (600-750 K). As the two different associates are formed at close temperatures, one can suppose that isolated v, are likely to move at temperatures exceeding 575 K. The associates of v, with (SH)- groups still increase in number at these temperatures (v,-(SH)up to 6.50K and (SH)--v,-(SH)) up to 700 K). This is due to the fact that protons are located in regions with a different extent of lattice damage, and hence, they are released in a very wide temperature range. Associates v,-(SH)- decrease in number and disappear at temperatures exceeding 660K. However, more complex associates (SH)--v,-(SH)are still formed up to 700K. This fact can be explained by the capture of one more proton by v,-(SH)-. Most of the defects involving a cation vacancy v,-(SH)), (SH)--v,-(SH)-) dis(Ilc-%* v,-Hal-, appear in the temperature range of 700-830K. All these defects are quite different in their structure and effective charge. Hence, one could suppose that these defects disappear due to the annihilation of v,, together with migrating Ca interstitials. However, some new defects involving v, (S--w,-S2--M4+, see the text above) are formed almost at the same temperatures (700-800K). These defects are stable up to 900 K. Therefore, Ca interstitials are not responsible for the decay of the defects involving a cation vacancy. The vacancies are likely to disappear due to their migration towards the surface of crystals or blocks. The ionic processes in plastically deformed CaS
protons
1335
proceed at remarkably lower temperatures than the temperature at which ionic conductivity in undeformed CaS was observed (above lOOOK) [8]. The difference arises from high concentrations of mutually-affecting defects produced by plastic deformation. Among these defects a noticeable role is probably played by dislocations. Migration of cation vacancies can be involved (directly or indirectly) in the process of annealing of dislocations. The defects are nonuniformly distributed in the destroyed crystal lattice. The process of the restoration of the crystal lattice in different defect regions proceeds at different temperatures. This is why the temperature of the formation of various associates is different (Fig. 5). acknowledge the financial support of the Estonian Science Foundation. Acknowledgement-We
REFERENCES 1. Danilkin M. I., Riiv 1. R., Seeman V. O., Belskiy A. N. and Soldatov S. N., Trans. Inst. Phys., Estonian Acad. Sci. 67,97 (1990) (in Russian). 2. Pedak E. J.. Allsalu M.-L. J. and Kanter M. J.. J. Ad Chem. (USSR) 45,2619 (1972) (in Russian). -A 3. Kuznetsov A., Jaek I., Seeman V. and Pung L., Proc. Estonian Acad. Sci., Phys.-Math. 23, 33 (1974) (in Russian). 4. Seeman V., Danilkin M., Lepist M., Must M., Ots A., Pedak E., Pung L., Parnoja E. and Rammo I., Intern. Symposium LUMDETR’94, Tallinn, 25-29 Sept. 1994. Abstracts, p. 100. 5. Schirmer 0. F., J. Phys. Chem. Solids 32,499 (1971). 6. Haldre ii., Seeman V. and Lehto T., Intern. Conf. Defects in Znsul. Cryst., Riga, 18-23 May 1981. Abstracts, p. 317. I. Abraham M. M., Chen Y., Boatner L. A. and Reynolds R. W., Solid State Commun. 16,1209 (1975). 8. Egami A., Onoye T. and Narita K., Trans. Jap. Inst. Metals 22, 399 (1981).