Temperature dependence of EPR spectra of light-induced Cr3+ centers in silver chloride crystals

JOURNAL

OF MAGNETIC

RESONANCE

16,454-462

(1974)

TemperatureDependenceof EPR Spectraof Light-Induced Cr3+ Centersin Silver Chloride Crystals W. JOHN Sektion Chemie AND W. WINDSCH Karl-Marx-Universitd

Sektion Physik, Leipzig, German Democratic Republic

Received December 21, 1973 ; revision received July 8, 1974 In chromium-doped AgCl crystals, the conversion from CrZf ions to Cr3+ ions by illumination with blue light below 178 K has been detected by means of EPR. Three. kinds of spectra could be identified. The angular dependence of the EPR signals and the temperature dependence of signal intensities indicate two [lOOI-axial centers of the point groups Cd0 and Ddh and an orthorhombic center of the point group D2,,. The centers are interpreted as Cr 3+ ions associated with one or two Ag+ vacancies. At temperatures above 178 K, the recharging to Cr*+ takes place. The interaction between Cr3+ and vacancies is estimated. The mechanism of charge transfer between Crz+ and Cr3+ is discussed. INTRODUCTION

Investigations of the dielectric loss factor (I), measurements of optical absorption and ionic conductivity (2, 3), and experiments according to the ionic thermo-current method (4) show that C?+ ions incorporated into AgCl at cation sites can be converted to Cr3+ ions at cation sites by heating in chlorine atmosphere at about 400°C. Such Cr3+ ions are associated at lower temperatures with one or two Ag+ vacancies. In (5) we gave a report on the EPR detection of a Cr3+ associated with Agf vacancies in AgCI, generated only by illumination with blue light and not by heating in chlorine atmosphere. We found an orthorhombic spectrum; however, we were not able to make an unambiguous selection from the possible models. In this paper we shall give such an assignment together with the interpretation of new results. EXPERIMENTAL

RESULTS

The single crystals were grown by a modified Bridgman-Stockbarger technique from zone-refined AgCl and CrCI, introduced into the melt under vacuum. The crystal had a concentration of about 0.25 mol% chromium. The samples for EPR measurements, cubes of about 4 mm, were prepared from a larger crystal cylinder of 9 mm diameter. We used a JES-3BQ spectrometer inX-band. The magnetic field was measured by means of proton resonance and a g-marker of DPPH. Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

454

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In chromium-doped AgCl crystals without thermal treatment, we did not find EPR signals before illumination in the relevant region apart from some very weak lines at g z 2. Our spectrometer could detect Cr3+ concentrations in excess of 10e5 mol%. We conclude chromium to be incorporated as C?+ corresponding to the results of (2, 3).

FIG.

1. EPR

spectrum during illumination

at 135 K in the position

H

parallel to [lOO].

FIG. 2. Angular dependence of spectra I, II, and III during rotation of the crystal around an [lOOI axis (without the transitions l/2 * -l/2).

Under illumination with blue light (A z 405 nm) or light of a mercury lamp at temperatures below 178 K, various spectra were obtained. The EPR spectrum in Fig. 1 recorded during illumination at 135 K shows, in addition to the lines known from (5) and designated as spectrum I, two other kinds of spectra labeled II and III.

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JOHN

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However, after switching off the lamp, only spectrum I is stable. The lines of II and III disappear and the intensity of spectrum I increases. At temperatures above 178 K. this spectrum vanishes also. All three kinds of spectra can be reproduced by a renewed illumination at lower temperatures. At about 183 K, spectrum I appears only during illumination, yet it is unstable. During illumination with green or red light, we did not find any effect. From the number of lines and intensities of the three spectra, we assume a 3d3 configuration of chromium, which implies the presence of Cr3+ ions (S = 3/2). The comparison of the EPR intensities of another AgCl crystal, heated in chlorine with the intensities of the signals generated by illumination of the samples without thermal treatment reveals that only one out of about five hundred CrZ+ ions is converted into a Cr3+ ion by illumination. The angular dependence of the spectra (Fig. 2) has been obtained by rotating the crystal attached to a polystyrene pin in the static magnetic field. The analysis of the angular dependence of the spectra furnishes the parameters given in Table 1. A hyperfine pattern of the isotope s3Cr (I = 3/2, natural abundance 9.5 %) was not found. The orthorhombic spectrum I can be described by a spin-Hamiltonian of the form S’ = g/!IHs + O[s; - +S(S + I)] + E&

- 9;)

VI

with S = 312, where the z-axis of the coordinate system is a cubic axis and the x- and y-axes are [lOOI-type axes. The angular dependence of the line positions of the transition M t+ M + 1 can be expressed by the following second-order perturbation formula (6) : H,H,-2Ep - &

[D(3 cos2 0 - 1) + 3Ecos 2cpsin2 01 [(D - Ecos 24~)’ sin2 0 cos’ 8 + E2 sin’ 8 sin22q]

x [24’M(M

-&W x [2&+

+ 1) - 4S(S + 1) + g]

sin2 8 + E cos 2q( 1 + I)-6M(M+

I)-31.

COS'

@I2 + 4E2 cos2 tl sin’ 2qj

PI

M is the component of the electron spin. Ho = hv/gB and the values of D and E are reduced by a factor of g/I from those in Eq. [I]. The magnetic field lies in the (B,cp)direction in an (x,y,z)-coordinate system. Since the rhombic deformation of the crystal field takes place along the [I IO]-axes, these centers appear sixfold with the same probability corresponding to the six [ 1 IO]type axes of the cubic AgCl lattice. Spectrum I can be interpreted by models corresponding to the point groups C2, or D2,,. Models of lower symmetry do not agree with the superposition behavior of the lines of the six centers with the same probability. Axial spectra II and III can be described by spin-Hamiltonians of the same shape as in Eq. [I] but without the term in E. The axes of the coordinate system are identical with the cubic axes. Spectra II and III are caused by axial centers along the [lOOI-axes. Such centers appear in cubic lattices threefold with the same probability. They can be explained by models corresponding to the point groups C,, or D4,,.

LIGHT-INDUCED

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The comparison of the parameters D of spectra II and III (Table 1) shows that the centers of spectrum II have a greater tetragonal deformation of the crystal field than those of spectrum III. TABLE PARAMETERS

1

OF THE SPIN-HAMILTONIAN

Cr3+

Spectrum

g

I II III

1.987 1.987 1.987

FOR THE THREE

KINDS

OF

SPECTRA

IDI (cm-‘)

f 0.001 f. 0.002 f 0.002

0.0152 0.0492 0.0241

INTERPRETATION

IEl (cm-7

+ 0.0001 + 0.0003 k 0.0003

OF

0.0007

+ 0.0001 -

SPECTRA

Isolated Cr3+ ions at cation sites would give an isotropic EPR spectrum due to the cubic environment. Such a spectrum has not been found. A Jahn-Teller distortion is not expected owing to the nonorbitally degenerate ground state 4A,,. Since the Cr3+ ion at a cation site has two surplus charges relative to the monovalent AgCl lattice and because of the local charge compensation, we assume t.hat at lower temperatures the Cr3+ ion forms centers with one or two associated Ag+ vacancies. We rule out an association with vacancies outside of the second cation sphere because of the overlow stability of the centers. The final assignment from among possible models is made by means of thermodynamic considerations. ESTIMATION

OF

THE

INTERACTION

BETWEEN

Cr3+

IONS

AND

Ag+

VACANCIES

For this estimate we assume that all cation sites are equal in energy. The Cr3+ centers are distinguished only by the number of vacancies and not by their structure. We use the following symbols: The index k means a cation site. V stands for vacancy. The symbol * is the positive and ’ the negative surplus charge relative to the idealized lattice. The quantities xc, x1, x2 are respectively the concentrations of isolated Cr3+ ions and of centers of such ions with one or two vacancies, and y is the concentration of all Cr3+ ions, assumed to be constant. We assume that the Cr3+ ions are not interacting with the remaining Cr2+ ions. K, and K2 are constants at a given concentration y. El is the energy required to remove a vacancy from Cr;. E2 is the energy required to remove a vacancy from Cr, VL. The Cr3+ ions are in statistical equilibrium with the vacancies Crt f V; *

Cr; V;,

[31

and at lower temperatures Cr; V; + V; 22 Cr; VLV;.

[41

According to the law of mass action, we can write from [3] xc -= X” Xl

Kl

&%IkT

=

(Pl,

151

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JOHN

AND

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and from [4] X1X" x2

z K2 e-Wk=

=: cp2.

161

Moreover, the following balance equations are still valid : y=x,+xl+x,

]71

and x, =2x, + Xl. Equations [5]-[8] give the concentration variable T and of the parameter y :

K31

of the free vacancies x, as a function of the

]9] The solution of [9] forms a family of curves with y as parameter. The exact course of this curve family can be obtained only if the constants K,, K2, El, and E2 are known. In principle, it is possible to determine these constants from a large number of measurements of the temperature dependence of the EPR intensities on crystals with different Cr3+ concentrations if a computed analysis is used. However, for a qualitative discussion of the solution of [9], we can make approximations in some ranges near particular points : ~,3+~2~,z+~2~(P~-~Y)~"-~rp~(PzY=~.

Range 1. The temperature is so high that Cr’+ centers containing vacancies were not yet formed (x1 = x2 = 0). That means that Eqs. [7] and [8] reduce to

x, = 2y

DOI

and all Cr3+ ions of the isolated type. Range ZZ. There exist already just as many Cr;’ I’; centers as isolated Crk ions z x1) and the Cr, VLVL are negligible (x2 k 0). With [9] then Eq. [5] becomes (xc ‘pi =: x, and Eq. [9] gives

Range 111. The temperature is so low that isolated Cr, are practically no longer present and Cr, VLVL are not yet present (xc %x2 z 0, x, M x1). From [9], there follows

WI Range IV. In this range the centers Cr;’ VL and Cr, VL VL have nearly the same

concentration gives

(x, z x2) and isolated Cr; ions are negligible (xc M 0). Equation [9] x, M gy - cpl +
[131

Range V. Only the centers Cr, VLVi are still present (xc = x1 = 0, y = x2). Equation [9] becomes x, =o. 1141

The use of these five approximations and extrapolation procedures leads to the qualitative course of x, = f (1 /T) shown in Fig. 3. The extent of each range is arbitrary. If one

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459

does not consider that these approximations are made in ranges near particular points, then mathematically speaking they are incompatible with each other. However, they do show the overall qualitative behavior.

I

II

Ill

a

?7

.

FIG. 3. Theoretically estimated variation of the concentrations x, as a function of temperature. The numbers mark the portions estimated directly from the approximation. TEMPERATURE

DEPENDENCE

OF THE EPR SIGNAL

INTENSITIES

Figure 4 shows the temperature dependence of the intensities of the signals of spectra I-III under illumination. The temperature-dependent course of the EPR intensities depends on the thermic history of the sample. As illustrated in Fig. 4a, the signals are more intense when the sample is kept for a longer time at lower temperatures before illumination. In contrast, Fig. 4b exhibits only relatively small signal intensities because the sample was kept before illumination for a longer time above 183 K. EXPLANATION

OF THE SPECTRA

BY MODELS

The experimental temperature dependence (Fig. 4) for spectra II and III shows a behavior similar to the approximation given in Fig. 3. These spectra can be described only by [lOO]-axial models with the point groups C,, or D4,, (Fig. 5). We supposed the Ag’ vacancies to associate in the first or second cation sphere. Under this assumption the vacancies can associate only at equivalent sites on a [ lOO]-axis. Interpreting spectrum II by a center consisting of a Cr3’- ion and two vacancies along the same [lOO]-axis (Fig. 5b) and spectrum III by the analogue model with only one vacancy associated along the [loo]-axis (Fig. 5a), we obtain an agreement with the approximation in Fig. 3. A rise of temperature increases the mobility of vacancies. One of them is separated from center II (Fig. 5b), and the center III (Fig. 5a) with only one vacancy remains. Now we must still explain spectrum I. This can be done by models of the point group C,, with a [ 1 lo]-reference axis or DZh with a [lOO]-reference axis (Fig. 6). The model of Fig. 6a is excluded because of Coulomb repulsion of the vacancies. Even for the case when the interaction between the vacancies has no influence, there would exist no reason for the vacancies of the stretched associate II to change into an angular arrangement when the lamp is switched off. The remaining models are centers with one or two vacancies on the same [l lo]-axis. The temperature-dependent behavior permits now the assertion that in the system AgCl : Cr3’ at the same number of vacancy centers along [l lo], are more stable than

460

JOHN AND WINDSCH intensity

50

(orb/l

spectrum I I

units)

I I m

Ix,) fx,)

-

-

T CKI

0 190

180

170

160

ml

lL0

160

150

ILO

133

0

0

N’

.’

. 190

18J

-TtK3 170

130

b

FIG. 4. Temperature dependence of signal intensities during illumination. The arrows point at the sequence of the thermal treatment. (a) Before illumination, the sample was stored for a longer time at 133 K. At first it was illuminated at 133 K, then the temperature was raised. (b) The sample was stored above 183 K before illumination. Afterwards the temperature was lowered and the sample illuminated. mofl,

0 cr3. on-

0 voconcy

0

CL” a.

b

FIG. 5. The axial models.

5”

0

5”

%h b.

c

FIG. 6. The rhombic models.

LIGHT-INDUCED

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461

centers along [lOO]. That means, for a vacancy it is energetically more favorable to associate on an [l lo]-axis than on a [lOO]-axis. The further choice of the models is made from a thermodynamic viewpoint : We assume that spectrum I would belong to a [l IO]-center with only one vacancy (Fig. 6b). Then a fourth type of spectrum corresponding to a [llO]-center with two vacancies ought to appear at lower temperatures. On the other hand, since [IlO]-centers are more stable than [lOO]-centers as shown above, this spectrum ought to appear at the same or at higher temperatures than spectrum II of the [lOO]-center with two vacancies. Hence the temperature interval in question is fixed. In spite of a careful search, such a fourth spectrum was not found and the assumption made above must be wrong. The conclusion follows that spectrum I belongs to a [l lo]-center with two vacancies (Fig. SC). DISCUSSION

The question concerning the conversion mechanism from Cr2+ to Cr3+ can be answered as follows: The effect takes place only with blue light (x 3 eV) or with light of a shorter wave length, but not with green or red light. According to Brown (7), the band gap in pure AgCl is 3.1-3.2 eV in the temperature range under consideration. Chromium doped into AgCl produces a disruption of the lattice. That means that the band gap will be smaller. For this reason it is possible that an electron passes over to the conduction band. The presence of the electron in the conduction band permits its transfer into an electron trap which is slightly below the conduction band in energy. We suppose that an interstitial silver ion which is present in the lattice due to a Frenkel defect provides such an electron trap. Similar electron traps have already been described in pure silver halides in (8, 9). The neutral silver atom produced by an electron trapped on an interstitial silver ion should possess a 5s’ electron configuration and ought be paramagnetic. However, we did not find corresponding EPR signals. For this reason we suppose that silver aggregates are generated according to the following mechanism : Ag+ + e- --f Ago + Ag+ --f Agf + e- -+ * * * -+ Ag,.

[I51

A value of n > 2 would explain the disappearance of paramagnetism. The theory of photography (10, 21) says that such silver aggregates are stable at room temperature provided that n >/ 4. In our experiments these supposed aggregates decay already at temperatures above 178 K. That means n must be 2 or 3. Raising the temperature diminishes the band gap (7). If we assume that the energy level of the electron trap is constant or only slightly dependent on the temperature, then this energy level will touch the gap edge at a determined temperature of, say, 178 K. The electron leaves the trap and effects the recharging to Cr2+. The EPR signals disappear as observed. ACKNOWLEDGMENTS The authors thank Prof. E. Hoyer for suggesting the subjects of this study.

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REFERENCES 1. H. B~~TTGER,Whys. Stat. Sol. 4,669 (1964). 2. H. D. KOSWIG AND I. KUNZE, Phys. Stat. Sol. 9,451 (1965). 3. W. ULRICI, Phys. Stat. Sol. 27, 333,489 (1968). 4. I. KUNZE AND P. M~~LLER, Phys. Stat. Sol. 38,271 (1970). 5. W. JOHN AND W. WINDSCH, Phys. Srut. Sol. (b) 57, K139 (1973). 6. K. MORIGAKI, M. FUIIMOTO, AND J. ITOH, J. Phys. Sot. Japan 13,1174 (1958). 7. F. C. BROWN, J. Phys. Chem. 66,2368 (1962). 8. K. KOBAYASHI AND F. C. BROWN, Phys. Rev. 113,507 (1959). 9. R. VAN HEYNINGEN, Phys. Rev. 128,2112 (1962). 10. J. W. MITCHELL, Phot. Korr. 1. Sonderheft (1957). 11, F. TRAUTWEILER, Phot. Sci. Eng. 12(3), 138 (1968).