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Z2 centres in the triplet state
K. Strohm, H. Paus
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 7, Tome 41, Juillet 1980, page C6-119
Z
2centres in the triplet state
K. M. Strohm and H. J. Paus
Physikalisches lnstitut, Teil 2, Universitat Stuttgart, Pfaffenwaldring 57, (7000) Stuttgart-80, W-Germany
Résumé. — Un aperçu est donné d'une série d'expériences EPR sur l'état triplet des centres Z2 dans KCI. Elles démontrent une multiplicité considérable de configurations atomistiques pour cette sorte de centres colorés dépendant de l'impureté cationique.
Abstract. — A survey is given of a series of EPR experiments on the triplet state of Z2 centres in KC1. They demonstrate a considerable variety of atomistic configurations for this sort of colour centre depending upon the impurity cation.
1. Introduction. — Certain divalent cations (Ca,
Sr, Ba, Eu, Yb) in alkali halides combine with F centres forming Z2 centres. The absence of an EPR signal [1] and of a paramagnetic MCD effect in the ground state [2] have led to the opinion, that they are two-electron centres. Trie-possibility of their optical ionization under formation of paramagnetic Z2 ( = Z3) centres [3] using F centres as electron traps [4] supported this idea. Further confirmation was given by EPR experiments on their triplet state [5] created by optical irradiation. EPR and ENDOR investigations using the paramagnetic E u+ + impurity as a probe in the Z2 centre have been interpretable by a single configurational model containing a (200) complex and two F centres [6]. Experiments on the formation kinetics in Sr-doped KC1 [7-9] have not yet given clear-cut results, partly because of the strongly overlapping absorption bands. Newer triplet EPR investigations on Z2(Ca), Z2(Sr), Z2(Ba) and Z2(Yb) centres in KC1 have led to a further insight into the problem and have revealed a surprising complexity of the atomic arrangement in the centres [10]. They shall be reviewed in this paper. The experiments are performed at liquid nitrogen temperature (LNT) on a conventional X band spectrometer using a double lock-in technique with field modulation at 100 kHz and light modulation at a few hertz.
2. Results and discussion. — 2.1 THE Z2(Ca) CEN-TRE. — The results on the Z2(Ca) centres fully confirm the published data [5], but establish additio-nally a connexion to their optical absorption pro-perties. The angular dependence of the EPR spectrum reveals a pure (100) axial symmetry of the centre with a fine structure (FS) parameter
D/hc = 265 x l O ^ c m "1 .
In a classical approximation the D value can be
inter-preted by a dipole-dipole interaction of two electronic magnetic moments being 1.5 lattice spacings apart. This rough approximation has formerly successfully been used in the interpretation of the corresponding data of the triplet F2 centre [11] : there, this distance (5.57 A) happened to be slightly larger than a^^Jl, as expected for the ground state F2 centre configura-tion. Similarly, the mean electron distance in the Z2(Ca) centre ground state is inferred to be roughly one lattice spacing a0. Optical experiments [10] have
shown, that Z2(Ca) centres reorient themselves in the crystal under polarized irradiation at ~ — 100 °C. Reorientation occurs also at LNT under unpolarized white light excitation. This is detectable by changes in relative line intensities in the EPR spectrum. Quantitative analysis allows the determination of the oscillator strength ratio for parallel and perpendicular optical transitions relative to the centre axis :
./||/A = 1-6 •
Optical orientability and mean electron distance suggest a particularly simple configurational model for the Z2(Ca) centre, earlier proposed in a slightly different form on the basis of entirely different
argu-ments [4] : an anion vacancy and a Ca+ + ion in (100) nearest neighbourhood bind two electrons.
2.2 Z2(Yb) CENTRES. — The situation with the Z2(Yb) centre is slightly more difficult. The EPR spectrum shows altogether five different types of triplet centres all supposed to be Z2(Yb) centres, since created by optical excitation in the same spectral region. Four of them exhibit strict (100) axial sym-metry as the Z2(Ca) centre. Only the data of the most frequent type are given in table I. The others have axial FS parameters of 5 980, 8 410 and 9 350, all in units of 1 0- 4 c m- 1. It is known from optical experi-ments, that these (100) centres are alignable mostly like the Z2(Ca) centres.
C6-120 K. M. STROHM AND H. J. PAUS
Table I. - Compilation of the characteristic EPR parameters of Z2 centres ;
D
and E fine structure parameters (absolute sign determined only where indicated) ; a,P,
y Euler angles of the FS tensor ;ABw
half wzdth of the E P R lines; r life time of the triplet states.Zz(Yb) Tilted (100)- centre centre I - - 1.990 1.993
+
0.01 0.006 1.970 1.993 0.01+
0.006 2130 7480+
50 - -+
90 130 0 & 10 45O 00There is a further type of Z,(Yb) centre with a tilted FS tensor (Euler angles listed in table I). Its main axis is r ~ t a t e d by 140 out of a (001) crystal direction in a (110) plane. This is compatible with a configurational model as shown in figure
IN
: an anion cation vacancy pair acts as perturbation on the elec- tron cloud bound to the impurity anion vacancy configuration. Also the occurrence of several (100) Z2(Yb) centres can be attributed to such a vacancy pair in various positions but always aligned along (100) in these cases. The altogether very large FS parameters of the Z2(Yb) centres cannot be understood anymoreFig. 1. - Configurational model for 2, centres. The typical 2,
centre molecule consists of the divalent cation impurity and an anion vacancy in (100) nearest neighbourhood. This entity binds two electrons. In some cases the impurity cation vacancy complex remains unbroken and binds a further anion vacancy. The anion cation vacancy pair disturbes the electron cloud that is only sche- matically sketched. c< 20 ms P- centre I - 1.985
+
0.01 1.977+
0.01 6 395+
70 104+
6 450 3.70+
0.50oO
4.0*
0.3 r 20 cc 20 ms B- centre I1 - 1.995+
0.008 1.978+
0.008 5 248 5 70 145+
6 450 40+
0.50 19.0°+
0.50 - 4.0+
0.3 r 20 z ~ ( S r ) cc 20 ms a- centre I11 - 1.983+
0.007 1.965+
0.007 - 3 850+
60 - 229*
10 11.20+
0.50 8.5O+
0.50 OD 4.0+
0.3 r 20 <c 100 ms P- centre - 1.988+
0.008 1.988+
0.008 - 2318 -4 40 355+
10 45O 0"o0
4.0+
0.3 - 104+
5 cr 1.6 s B- centre - 1.995+
0.005 1.995+
0.005 - 3 926+
40 192+
4 45OoO
00 2.7+
0.2 1 1 600on the basis of a classical dipole-dipole interaction alone. Now an appreciable spin orbit interaction due to the heavy Yb ion must be taken into account. 2.3 Z2(Sr) CENTRES.
-
The most embarrassing spectra are found with the Sr impurity. It was not possible to reproduce earlier published results [5]. The spectra contain an enormous number of lines. The measured angular dependence (Fig. 2) gives an idea of the complexity. It has been fully analysed by a computer fit assuming altogether four different types of Z2 centres. All of them exhibit tilted FS tensors, none of them shows (100) axiality, explaining in some respect the non-alignability of Z,(Sr) as stated from optical experiments (reorientations transforming one Z,(Sr) centre type into another one cannot be exclud- ed). All characteristic data of these centres are also listed in table I. They are classified by their triplet life times : three types of cc 20 ms )) centres and a << 100 ms >> centre. All D parameters are remarkably large, perhaps similarly explainable as with the Z2(Yb) centres. For the cc 100 ms )) centre the sign of DZ2 CENTRE!; IN THE TRIPLET STATE C6-121
Fig. 2. - Measured angular dependence of the EPR lines of Z,(Sr) centres in KC1 at LNT. The crystal is rotated around ~ t s (1001 axis I B,. Light chopper frequency 1.5 Hz, microwave frequency 8.9 GHz. The dots are the measured points, the length of the small dashes represenlb the relative transition probability,(computer fit).
trum of figure 2. Both, the tt 100 ms )) and the 1.6 s D
centre arise during the usual optical F - Z, conver- sion together with the t( 20 ms )> centres. In contrast
to these they have optical absorption bands at the low-energy side of the Z2 band : the tr 100 ms )) centre
in the region where Z, centres are reported to absorb, the rt 1.6 s D even farther in the infrared. Detailed
investigations on the Z, centre and related problems shall be published in a forthcoming paper.
2 . 4 Z2(Ba) CENTRES. - In conclusion of this review
we want to emphasize, that all triplet EPR experi- ments have been performed at LNT. Excitation at
the same temperature yields no triplet spectra for the Z,(Ba) centre. In this case obviously ionization occurs upon irradiation. There is only a single structureless line at g = 2 due to an S = 112 centre. On lowering the temperature to 16 K new EPR lines arise. Their angular dependence could not yet be analysed; triplet centres cannot be excluded.
It must be kept in mind, that all conclusions from the presented experiments refer to the excited triplet state of the centres. Conclusions on the ground state may be drawn with care. As with Z2(Ca) also in the other cases atomistic reorientations and redis- tributions may happen during optical excitation.
DISCUSSION
Question. - P. EDEL. this is the most important contribution. We have not
D~ you have evidence that the contributions of spin related it in sign with the value resulting from dipole- orbit interaction and dipole-dipole interaction to the D . dipole interaction. From measurements on z,(s~) value have the same sign ? we infer that most probably all D are < 0.
Reply. - K . M . STROHM. Question. - V . V . RATNAM.
Spin orbit effects anyway give large contributions to (Voir l'article tt The excited state of Z2 and Z,f
K. M. STROHM AND H. J. PAUS
References [ I ] TAKEUCHI, N., MIZUNO, Y., SASAKURA, H., ISHIGURO, M.,
J. Phys. SOC. Japan 18 (1963) 743.
[2] GEHRER, G., BOCK, W., L~~SCHER, E., Phys. Status Solidi 31 (1969) K13.
[3] See paper :
PAUS/SCHEU, The excited state of Z , and Z: centres. [4] OHKURA, H., Phys. Rev. 136 (1964) A446.
[5] ADLHART, W., GEHRER, G., LUSCHER, E., Solid State Commun 9 (1971) 117 and Phys. Status Solidi 66 (1974) 517.
[6] KENNTNER, G., PAUS, H. J., URBAN, W., 2. Phys. B 25 (1976)
219.
[7] CAMAGNI, P., CERESARA, S., CHIAROTTI, G., Phys. Rev. 118
(1959) 1226.
[8] WOLFRAM, G., WITT, V., Phys. Status Solidi 22 (1967) 245. [9] EBERT, G., PAUS, H. J., STROHM, K. M., J. Phys. Chem. Solids
to be published.
