V4+ in SrTiO3: A Jahn-Teller impurity

Solid State Communications, Vol. 32, pp. 1099—1101. Pergamon Press Ltd. 1979. Printed in Great Britain. V4’~IN SrTIO3: A JAHN—TELLER IMPURITY Th.W. Kool and M. Glasbeek Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127, Amsterdam, The Netherlands (Received 10 July 1979 by A.R. Miedema) EPR results of V’~’,with S = ~, in SrTiO3 are reported. The tetragonal local symmetry of the impurity ion is related to strong T2g x e~coupling as4’evidenced by intensity variations in the presence of stress. At 4.2 K the EPR behaviour is related to the intrinsic local strain in SrTiO V 3. A LINEAR Jahn—Teller interaction of the type 27r2, X e~ in the strong coupling regime has recently been 5’ substitutional for Ti4~in SrTiO invoked for Cr 3 [1,2]. Nonlinear electron—lattice couplings were strongly enhanced when a modified heat treatment was applied and as a result the question of local dynamics on the EPR time scale could be addressed [3]. As a natural extension of this work we report here on the4~. first EPR results of measurements another d’ impurity in SrTiO3,atV9.16 GHz EPR were performed using SrTiO 3 single crystals doped with 60 ppm V (as purchased from Semi-Elements Inc.). At 4K the spectra showed, in addition to the lines from a number of well known iron centers, the response of an S = ~ center with a characteristic hyperfine splitting due to an I = isotope. After irradiation with UV light the latter spectrum was enhanced by a factor of five. Figure 1 shows the angular dependence when the magnetic field H is rotated in a (001) plane. Evidently the center has tetragonal local symmetry with the main axes51inisotope the {100} crystallographic Since the Vabundance, has a nuclear spin I = directions. in a 99.76% natural one readily attributes the S = ~ spectrum to V4, as argued further below. An accurate fit with the experimental angular variation is obtained when we take 7C = ~W.g .S + S .A .1., i.e. negligible nuclear quadrupole interactions, and coinciding principal axes of the g and hyperfme tensors. It appeared sufficient to consider the hyperfine interaction up to second order [4] and we findg ±0.005 IA 11 = 1.9420 ±0.005,g1 = 1.8945 1 and JA 11 I = (146.78 ±0.05) x l0~cm 1I = (44.04 ±0.05) x I0~cm~.Theg- and A-values showed 4’~ no changesdisappeared, with temperature up due to 35toKfast where the V spectrum probably spin—lattice relaxation processes. Additional splittings in the V4~ spectrum, as expected on account of the rotation of the oxygen octahedra in the SrTiO 3 host crystal below Tc [5], could be resolved only when monodomain crystals were used. The rotation angle ~ was measured to be

400C

~H(G)

3900 3800 ______

3700 ___________________________ 3600 _~._-~

~

350(3 _____________________________ 3400 —~

3300 _________________________ _____________________________ 3200 3100 3000 2900 2800

______________________________

[100]

100

20°

30°

40°[1i0]

4’ in Fig. 1. Angular dependence of the EPR lines of V SrTiO3 when B is rotated in the (001) plane, T= 4.2 K. 2.0 ±0.10 at 4.2K, i.e. ~ complies with the intrinsic rotation angle [6]. The EPR results clearly show that V~is substitutional for Ti4~whereas the experimental value of ip

1099

very likely excludes a nearby defect ion as the cause for the tetragonal site symmetry. Localized symmetry lowering can, of course, also occur spontaneously,

1100

V~IN SrTiO3: A JAHN—TELLER IMPURiTY

Vol. 32, No. 11

Hooit~

[oio]

\

100

\~110 ‘1

1

2962

3309

4’ in SrTiO 3 for HIl [0011.Zero external

Fig. 2. Influence of [110] stress on the intensities of theM1 = ~lines of V stress: dotted lines;P110 = 5.42 x 10~dyne cm’: solid lines. 4

In 21fl

2)

~

1

2

3

4

5

(10~dynes cm-

6

7

8

9

Fig. 3. Plot ofin (I~/2I~) vs external uniaxial stress (P

110) at 4.2 K, H II [001].

namely in the presence of off-center potentials [7] or, in case the system possesses orbital degeneracy, Jahn— Teller interactions [8]. Since the ionic radius of V~ 4~(~0.64 A) it is (- 0.61 A) is almost equal to that of Ti

hard to visualize that off-center displacements are of relevance here. On the other hand,the electronic configurationlocalized would permit electron—lattice couplings produce distortions in close analogy to to

Vol. 32, No. 11 V4’ IN SrTiO3: A JAHN—TELLER IMPURITY 1101 5~’:SrTiO Cr 3 [1,2]. In view of this we investigated the pseudo-cubic elastic stiffness constants. With V2 = behaviour of the system under uniaxially applied stress. ~kT(C112 and C12)~0 C in (11/2111), C11 = 2.96 x2 10~ [lO],we As is wellaknown [8J,a strong linearand vibronic coupling dynecm 12 = 1.12 x 31012 between triply degenerate T state a localized e~ obtain from the slope in Fig. I V dynecm 2 I = 2 x i0~cm~’.The 5~ mode in a cubicaround crystal the leads to tetragonal value was found inthe theclose case similarity of tetragonal the octahedron impurity ion. Indistortions the strong of very sites same [1] thereby expressing in Cr coupling limit, externally induced strain couples through electron—lattice coupling of the two transition metal its e 0 and e~components leading to a lifting of the ions. When the plot in Fig. 3 is extrapolated to zero degeneracy among the three lowest vibronic levels, external stress a residual local strain is found of Thermal relaxation then produces an alignment among e = 8 x lO~at 4.2 K. This value is almost equal to the the tetragonal distortions. With the crystal mounted in a intrinsic local strain of 2 x l0”~at 4.2 K, as formerly quartz tail of2the He-cryostate, stresses to exposed 8.5 x to measured forthe SrTiO3 neutron back scattering [11]. were applied. The crystalupwas Apparently, strainbyexperienced by V4~ is nearly corn108 light dyne shone cm through the optical transmission cavity UV pletely due to the slight tetragonal deformation of the for optimum signal to noise ratio. With the external surrounding oxygen octahedron [12] rather than the stress directed along the pseudo-cubic [110] axis the intrinsic macroscopic strain. In summary, SrTiO 4 is following qualitative changes are observed: (i) lines due subjected to strong vibronic coupling of the type3 : V to sites in the [100] and [010] domains become supT 25 x e, which,for 5~. transition ions, so far was only pressed are and no longer F> 2.5 of x the known for Cr 108 dyneand cm2, (ii) thedetected relativefor intensities lines due to the remaining [0011 domain sites also change with applied stress. The latter effect is illustrated REFERENCES in Fig. 2 in which the intensities for the hyperfine lines with m 1 = ~, for H II [001], are compared. Sites with 1. H.J. de Jong & M. Glasbeek, Solid State Commun. main axes perpendicular to the [110] direction (i.e. with 19, 1197 (1976). g(,ff = g11) become suppressed whereas their high field 2. HJ. de Jong & M. Glasbeek, Solid State Commun. 28, 683 (1978). components (with ~ ~ = gj) become stronger. The 3. M. Glasbeek, HJ. de Jong & W.E. Koopmans observed phenomena reflect (i) the well known sup(submitted for publication elsewhere). pression of the [1001 and [0101 domains under [1101 4. A. Abragam & B. Bleaney, Electron Paramagnetic stress [9] and (ii) preferential alignment among the Resonance of Transition Ions, p. 167. Clarendon tetragonal distortions in the remaining [001] domain. In Press,(1967). Oxford (1970). 2I~~) is plotted as a function of the [110] 5. 546 H. Unoki & T. Sakudo,J. Phys. Soc. Japan 23, Fig. 3 In (I±/ stress magnitude, where I~and I~refer to the integrated 6. K.A. Muller, Solid State (‘ommun. 9, 373 (1971). intensities of the g 1- and g11-lines, respectively. The linear 7. M.F. Deigen & M.D. Glinchuk, Soy. Phys. Usp. 17, dependence is characteristic of a level splitting in a multi691 (1975). well potential, the splitting being linearly dependent on 8. F.S. Ham, Paramagnetic Resonance (Edited byElectron S. Geschwind), p. 1. Plenum, New the applied stress. Specifically, when i,(lt(~,~i~® and York (1972). ~ denote the vibronic ground levels in the Tx e~ 9. K.A. Muller, W. Berlinger, M. Capizzi & H. scheme, [110] stress splits these states into an upper Gränicher, Solid State Commun. 8, 549 (1970). lying ~ state and a doubly degenerate ground state ~ W. Rehwald, Solid State Com?nun. 8, 1483 (1970). (~‘~ and ~i,~®) with an energy separation of L~E= 11. A. Heidemann & H. Wettengel, Z. Physik 258,429 E~—Et,~= j(c11 C12)~V2P110. Here V2 defmes the (1974). strain coupling coefficient and C11 and C12 denote the 12. B. Alefeld, Z. Physik 222, 155 (1969). —

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