Naphthalene probe behaviour in normal phase I and incommensurate phase III of biphenyl

J. Phys. Chem.

00223697#4MO131-6 . ,

Solids Vol. 56, No. I, pp. 51-59, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rinhts nserwd OOZZ-3697195 $9.50 + 0.00

NAPHTHALENE PROBE BEHAVIOUR IN NORMAL PHASE I AND INCOMMENSURATE PHASE III OF BIPHENYL A. VERON,?

and F. LARI-GUILLETS

J. EMERY?

tUR.A CNRS 807, Equipe de Physique de l’Etat Condense Universiti du Maine, Boulevard 0. Messiaen, 72017 Le Mans Cedex, France SLaboratoire de Physique Cristalline-Institut des MatCriaux de Nantes, 2 Rue de la Houssinitre, 44042 Nantes Cedex 03. France (Received 22 March 1994; accepted 26 July 1994)

Abstract-A comparative analysis between low field (3 GHz) and X band (9.5 GHz) electron paramagnetic resonance (EPR) spectroscopy in photo-excited naphthalene dilutly substituted in biphenyl molecular crystal is developed. X band EPR results obtained in the three phases of biphenyl are presented; the spin Hamiltonian parameters in normal phase give the orientation of the naphthalene molecular probe in the host crystal. In the incommensurate phase III, the EPR analysis shows that the naphthalene molecules rotate and do not follow the displacement field of the biphenyl molecules. Further, they rotate around a direction which is perpendicular to the long axis while the biphenyl molecules twist around it. Keywords: D. critical phenomena,

D. defects, D. electron paramagnetic resonance (EPR), D. phase

transitions.

EPR in a photo-excited electronic triplet state molecule of naphthalene-d, and phenanthrene-d,, which dilutely substituted the host molecules in pure biphenyl-d,,. The most important features of their results are the splittings of the EPR lines which occur at the phase transitions. In a first step each line splits in two “lines” at T,, then two other “lines” appear rapidly. At the second phase transition only two “lines” remain. The authors determine the spin Hamiltonian in the high temperature phase and in the other phases to account for the splittings, because they work in zero-magnetic field detection, only the B! and B: spin Hamiltonian parameters were investigated. In this paper we report some new results for naphthalene-d, spin Hamiltonian parameters in the photo-excited electronic triplet, investigated by using X (9.5 GHz) EPR band spectrometers with the magnetic field varying between 0 and 5000 G. Such studies are interesting for two reasons. Firstly, the spectra being sensitive to the orientation of the magnetic field in the spin Hamiltonian frame, we can define the orientation of the EPR molecular probe in the pure biphenyl host crystal. Secondly, in the incommensurate phase the spectra are sensitive to spin Hamiltonian parameters other than B; and B!, with different symmetry (B:, B;‘, B;‘). These parameters are more appropriate for describing the symmetry breaking which characterizes the phase transitions and the movement of the EPR probe in

1. INTRODUCTION

The biphenyl molecular crystal with the chemical formula C,,H,, and consisting of two phenyl rings connected by a single C-C bond (Fig. 1) has been the subject of intense interest during the last few years. When it is cooled from the high temperature phase (phase 1) it exhibits two structural phase transitions: a second-order one at T, = 40 K and a first-order one at T,, = 17 K. The transition temperatures in the deuterated compound are similar, r, = 38 K, r,, = 21 K. These transitions are characterized by a twist of the two phenyl rings around their molecular axis, whose amplitudes are modulated with the wave vector in a genera1 direction in the first incommensurate phase and along the b* axis in the lower phase; so the order parameter dimension changes from n = 4 in the first incommensurate phase, named phase II, to n = 2 in the second, namely phase III. The high temperature phase is named phase I. These phase transitions were studied extensively by different methods in the deuterated or hydrogenated compounds: electronic absorption and emission in pure biphenyl-d,, [l], electron paramagnetic resonance (EPR) in low field (34GHz) [2,3], neutron scattering [4-81, X-ray [9], Raman scattering [10-141 and nuclear magnetic resonance [15, 161;nevertheless, the first evidence of these phase transitions by EPR [2,3] was interpreted without any reference to their incommensurate characteristics. The authors used 51

A. VERON et al.

52

the incommensurate phases. In relation to the first partial results [2,3], they will give a new insight into the behaviour of the diluted naphthalene molecules in the incommensurate phases; in particular we shall see the probe does not follow the twist of biphenyl which means the naphthalene probe behaves as a defect. This paper is arranged as follows. In Section 2 we detail the experimental procedure for the use of the EPR. Experimental results are presented in Section 3. The results in the high temperature phase will be given to confirm the low field ones and also to show the competition between the molecular symmetry and the site symmetry in the determination of the spin Hamiltonian: the main results being the anisotropy curves (line positions vs the orientation of the static magnetic field) of the naphthalene probe and their orientations in pure biphenyl. The second part of this section is devoted to the general features of the spectrum in the incommensurate phases and experimental results; qualitative comments about the spectra are also presented. The first detailed analysis of experimental results in incommensurate phase III, which is the main subject of this paper, is presented in Section 4: we compare “zero field” and X band experiments and determine the magnitude of spin Hamiltonian parameters. We deduce that the naphthalene field displacement cannot be a twist, thus we have been obliged to study the relations between spin Hamiltonian parameters and diplacement field. We conclude this paper with the more probable naphthalene-d, displacement field.

2. EXPERIMENTAL

The EPR experiments performed on the naphthalene-d,, molecules dilutely substitute the biphenyl ones with a concentration equal to 0.5%. The biphenyl doped crystal was grown by lowering the melt through a temperature gradient (Bridgman method). The products, biphenyl-h,, and naphthalene-d,, were purchased from Aldrich and were purified before use. The X band spectrometer (frequency around 9.5 GHz and static magnetic field varying from 0 to 5000 G) is equipped with a cryostat. The crystal is cooled down by helium gas flowing in a Dewar inside the cavity. The precise temperature of the sample may differ because of the temperature gradient of the flow. The exact conditions of the current l-low may vary from one experiment to another, but in one experiment the conditions are held constant, and in the whole set of experiments the temperatures are recalibrated by using the sample itself (the splitting between edge singularities is an exact measure of the

temperature). This system gives us a temperature measurement at about f 0.1 K. The fundamental state of naphthalene-d, is not a paramagnetic one. Thus we must work in an excited triplet state [17]. Some studies [18] have shown that the lowest triplet state responsible for the paramagnetism is also a phosphorescent state: the molecule is optically excited to a higher singlet state before it falls into the phosphorescent state. The decay time measured from the EPR signal is at about 17 s, in agreement with that obtained from the phosphorescence emission. This state is reached in the single biphenyl crystal by irradiating the naphthalene-d, with a high pressure mercury arc lamp. The light is focused with a quartz lens and a mirror at the top of the quartz switch in the cavity with an altuglass hood as shown in Fig. 1. The lamp output was filtered by a 5 cm pathlength of solution containing a CoSo, and NiSo, mixture which avoids heating the crystal with non-pertinent radiations. 3. EXPERIMENTAL

RESULTS

3.1. Experimental results in the commensurate phase We prefer to use naphthalene-d, to study incommensurate phases of biphenyl because naphthalene-h, shows a hypertine structure which prevents a precise study of the phase transitions [19]. An EPR signal is not observed at room temperature, probably because the triplet state is depopulated by thermal effects, the signal beginning to appear below 150 K. Along arbitrary orientations of the magnetic field four sharp lines were found around 3200 G and two other near 1600 G (Fig. 2). When illumination ceased all these resonance lines decayed and as no resonance was observable in pure biphenyl, probably due to the

C

L B b

a

Fig. 1. Crystal structure of biphenyl in high temperature phaseI. a =8.12 A, b = 5.63 A, c =9.51 A,8 =9~.1”,from Ref. [4].

Naphthalene probe behaviour

I

(a)

I--

-u-+

(b)

Extremum

(cl

53

to describe the crystalline field; and the molecular axis system (Fig. 4) which is especially well adapted to describe the molecular field (the x-axis is the long axis of the molecule, the y-axis is the short axis and the z-axis is perpendicular to the plane of molecule). These axes coincide with the three twofold axis of the molecule. The fine structure has two different origins: (i) The spin-spin Hamiltonian

interaction

described

by the

(2)

160

Pr~-A-I

I

,I200

2200

3200

4200

1

0

5200

loo0

Fig. 2. X band EPR spectra in the high temperature phase I with a static magnetic field H: (a) H along Ox; (b) and (c) H in the (a, b) plane.

short lifetime of excitons, this proved that the observed signal arose from the excited state of naphthalene. Two systems of peaks are expected associated with the two crystallographically inequivalent naphthalene molecules. Three lines for each molecule are typical of an S = 1 spin: the forbidden transition with the M, variation A, = 2 in low field and the two A, = 1 transitions in high field. The anisotropy curves in the three crystallographic planes are shown in Fig. 3. In the (a, c) plane the two systems of lines are confused accordingly with the plane symmetry of crystal which makes the two paramagnetism defects equivalent. The general form of the fine structure spin Hamiltonian for an S = 1 spin is:

2000

3ooo

4@Oo

5ooo

Magnetic field (Gauss) 180

iorn

2000

3000

4ooo

Magnetic field (Gauss) 180

In=2

Xsf = 1 BT’OT’, In=

(1)

-2

where 0: are the Stevens [20] operators which are defined in a particular axis system. Generally, we use an axis system in which the spin Hamiltonian takes the more simple and general form according to the symmetry of the probe environment. Here we have the choice between two axis systems: the crystallographic axis system which is especially well adapted

1000

2000

3OKl

4oOa

Magnetic field (Gauss)

Fig. 3. Line positions vs the static magnetic field orientations: (a) in the (a, b) plane; (b) in the (a, c) plane; and (c) in the (b, c) plane. (0) Experimental results; (-) theoretical results.

A. VERON et al.

54

Fig. 4. Spin Hamiltonian molecular axis in the crystal frame (a, b, c). where the spin Hamiltonian is constructed by averaging .#,, in the orbital state (‘po]%‘pd ]p, ), and has the symmetry of the molecule (L$, for naphthalene):

in the molecular axis system. (ii) The crystalline field which has the site symis realized through metry; this contribution spin-orbital contributions. We have two reasons to think that the second contribution can be neglected: firstly, the crystal field is relatively weak in molecular crystal and, secondly, spin-orbital interaction is generally weak in the triplet state involved in the paramagnetism because its lifetime is very long and the g value is very close to that of the free electron. If a crystalline field contribution exists, the crystallographic axis system is of no particular interest because the symmetry site is very low (only an inversion) thus the spin Hamiltonian has the general form of eqn (1). When H is along a molecular axis, the splitting of the Amr= 1 lines is extreme and the Amr= 2 line intensity is zero; this characteristic is used to mark experimentally the molecular axis. Thus we have verified that naphthalene molecule takes approximately the same orientation as biphenyl molecule (see Table 1). The line position curves vs the orientations of the static magnetic field are described correctly by the spin Hamiltonian Z=ggSS.H+B;O;+B:O;

(4)

where Bi = (340 x 10e4 f 5) cm-‘, B: = (- 150 + 5) x lO-4 cm-’ and g = 2.0023, with the spin HamilTable 1. Orientation of the spin Hamiltonian axis (@a, 0,) and of the biphenyl molecular axis (Q,, (PM)in the crystal frame (a, b, c*)

Ox

axis Oy axis 02 axis

18.2 93.6 108.1

7.1 - 63.2 38.0

17.3 loo.7 103.1

- 4.6 - 57.7 35.4

tonian axis given in Table 1. These results mean the EPR naphthalene probe, when it is substituted to the biphenyl molecules, takes nearly the same orientation. The values of the spin Hamiltonian parameters are in agreement with those obtained by Cullick and Gerkin [2,3] in “zero-field” experiment. In this type of experiment the frequency is chosen to obtain the EPR line between 0 and lOOG, and consequently the Zeeman effect is a perturbation of the fine structure Hamiltonian. If we take a Hamiltonian of the form (3), the energy levels depend only on the second order of perturbation of the square of the static magnetic field. In practise Cullick and Gerkin [2,3] measured the line positions for several frequencies and they fit the curve v =f(H) with a parabola; after extrapolation in H = 0 they obtained the energy splitting between the two energy levels involved in the transition. They measured the three transitions between the three levels and they deduced the parameters Bi and Bi. This method is limited because it is impossible to access a general Hamiltonian (five parameters) with only three measures; it does not permit a complete study of the incommensurate phase where a more general Hamiltonian could appear. The agreement with our own results confirms the form of Hamiltonian because other parameters in Hamiltonian (4) would give insignificant values for Bi and B: by the “zero-field” method without affinity with our own values of these parameters. The analysis of experimental results in the high temperature phase permits us to determine the origin of the spin Hamiltonian: the principle contribution comes from spin-spin interaction which depends on molecular geometry; no crystalline field contribution is detected. This result means the phase transitions would be observed through indirect geometry deformation and/or disorientation of a molecular probe induced by a change in the crystalline field. The g value is very close to the free electron one and confirms these results. 3.2. Magnetic resonance line shape in the incommensurate phase [21] In this section we recall what we need to understand incommensurate spectra. In a commensurate system the number of magnetic lines in the spectrum is small because only a few paramagnetic sites are non-equivalent. Conversely, in an incommensurate system where the translation lattice periodicity is lost there is an essentially infinite number of nonequivalent paramagnetic sites which contribute to the magnetic resonance spectra. Thus the resonance line shape is characteristic of the local nature of the incommensurate modulation.

55

Naphthalene probe behaviour

In the one-dimensional displacement field is:

“plane

wave” limit the

corresponds to the multi-soliton regime in which we introduce a probability distribution of phase P(4): 1

u(r) = A cos (d(r) + &)

(9)

PG$)K s2 +

& = cste,

(5)

where A is the parameter order amplitude, 4(r) = k - r is the local phase and k is the incommensurate wave vector. The line position can be expanded in powers of the displacement field u: H =H,+a,u

+fa,u2+...,

cos*P(4

-

2

J(

.

f#+J

>

p is an integer which characterizes the periodicity in the locking phase. This local field splitting arises through the local spin Hamiltonian whose BT parameters are modulated in the incommensurate phase. It means the spin Hamiltonian takes the form [22]:

(6) m=2

where HOis the line position in the high temperature phase. Inserting eqn (5) into eqn (6) we find the local line splitting:

AZ(G)=

c &‘(u’)O~, In-2

(10)

where Br are functions of the displacement field component ti. The same treatment as before permits us to write Br as a function of the phase 4 [22]:

AH(d) = H(4) - HO= hl cos r$ + h2(1 + cos 24). In=2

(7)

AZ(d)=

1 &‘(4)&‘.

(11)

In= -2

By taking into account the non-local contributions we obtain a more general form of the local field splitting [20]: AH(4) = h, cos(4 - 4,) + h2 + h; cos 2(4 - &). (8) The spectrum is characterized by singularities located at a position defined by: aAH 7’

=o

Such a spectrum is displayed in Fig. 5. We can see from this figure that the edge singularities are not symmetrical, i.e. some phases are privileged. This

Consequently, the spin Hamiltonian parameters of the naphthalene probe in incommensurate phase are dependent only on the displacement field of the naphthalene molecule itself. Neutron scattering investigations of incommensurate biphenyl [4,5] have shown that the static wave modulation is essentially characterized by a torsion of biphenyl molecule along its long axis. Consequently we can imagine that an analogous displacement field occurs for the naphthalene probe. Using symmetry properties and the preceding results we can construct the spin Hamiltonian from this hypothesis. We remark that the three twofold axes of the planar molecule are conserved in the presence of this displacement field, which means that the modulated spin Hamiltonian takes the same form as the high temperature spin Hamiltonian

The amplitude of modulation of this parameter will be deduced by comparing the results obtained in “zero-field” and in X band experiments. 2630

2670

2710

Magnetic field (Gauss) Fig. 5. Naphthalene typical line in incommensurate phase III of biphenyl in the (a, c) plane.

3.3. Experimental phases

results

in incommensurate

An EPR measurement with naphthalene-d, as the probe shows the two phase transitions of biphenyl around 40 and 17 K. Typical spectra are shown in

56

A. VERON er al.

Fig. 6. The first transition

is characterized by a splitting of the line into four singularities; the four singularities are an important characteristic of this phase because they are independent of orientation. This seems to be in contradiction to the general behaviour of first order and second order being out of phase in the development of AH(r$), which would be orientation dependent [21]. An incommensurate spectrum with four singularities has also been observed in the “zero-field” experiment by Cullick and Gerkin [2,3]. In this phase (phase II) the spectra are characterized by disymmetries about the shift and the relative

intensities between singularities. Generally the splitting of the line is more importnat at one side of high temperature position than the other and two singularities are weak at the side where the shifting is more important. In the vicinity of 15 K an evolution of the spectrum occurs: two singularities decrease and simultaneously the two other increase in the same proportion (total intensities before and after transiton are equal). During this transition the positions are unchanged, i.e. the amplitude of the order parameter remains constant. The intermediate spectrum can be reconstructed as the superposition of a spectrum of phase II and the one of phase III with a proportion which is temperature dependent [19]. This result is interpreted as the phase coexistence characteristic of a first-order transition and is observed by neutron scattering.

4. SPIN HAMILTONIAN PARAMETERS IN THE INCOMMENSURATE PHASE HI

4.1. Comparison band

2380

2410

2440

magneticfield (Gauss)

2370

2405 magneticfield (Gauss)

Fig. 6. Phase transitions shown through the naphthalene molecular probe behaviour. Note the change in the number of singularities between the two incommensurate phases (T = 20 K and 7’ = I5 K).

between “zero-jield”

and X

If we assume the Hamiltonian has the form (12), it is easy to determine the modulation amplitude from “zero-field” splitting because energy levels are simply related to parameters B!j and B:; it is the method Cullick and Gerkin [2, 31used to associate parameters B! and B: to each singularity (considered at this time as commensurate lines). We can deduce from their results that the order of the modulated parameters is less than 5 x 10-4cm-‘. Now if you use these values to simulate our X band spectra we conclude there is an incoherence. Effectively, in the X band experiment, the splittings are very important in orientations where parameters Bi and B: have weak effects. On the contrary along the molecular axis (H/l Ox, HllOy) where B’j and B: have the most important effects no splitting is observed: it is impossible to describe X band results with a modulated Hamiltonian containing only Bt and B:. At this point we conclude the modulated parameters Bi and B: can be neglected before the other modulated parameters BT~, B;’ and Bi which are probably one order greater. This result means that torsion is not the main displacement field of naphthalene, contrary to one hypothesis we made at the beginning. This is understandable when we know the geometry of this molecule. This is an important result which will have severe consequences: firstly, the naphthalene probe does not account for the displacement field. It seems to be normal when we recall the molecular geometry which prevents the twist between the two rings; secondly, this defect will act on the modulation phase

Naphthalene probe behaviour

57

which will be pinned; and thirdly, the EPR naphthalene spectra are those of a modulated structure, so the defects are partially mobile [23] in the sense that it “can follow” the modulation. The apparent incoherence with “zero-field” interpretation arises from difficulties accessing all the parameters by this method, in particular the parameters BF~, B;’ and B:. A perturbation treatment of a general modulated Hamiltonian shows that modulated parameters Bi and Bi occur at the first order and modulated parameters BF~, B; ’ and B: at the second order. Thus parameters B;‘, B;’ and Bl modulated with an amplitude of 100 x 10d4 cm-’ produce the same splitting as parameters By and Bi modulated with an amplitude of 5 x 10m4cm-‘. Consequently the splittings in the “zero-field” and in the X band become coherent. In the (a, b) plane where fine structure has its maximum splitting the high field line position is double the low field line position. If incommensurate splitting arose from the g tensor, there would be a factor of two between the high field and low field splittings; we do not observe this factor, which means the splitting arises from modulated Br parameters.

There are three rotations along the Ox, Oy and Oz axes which correspond to b,*, bza and b,, symmetry, respectively. There are 24 modes of deformation, and applying group theory we can count the different vibrational modes. We obtain:

4.2. Origin of spin Hamiltonian

The operator T(7) transforms the spin operator 0: in T(7)OT T+(7) which may be developed as a linear combination of 07, thus it is equivalent to say it is the B:(C) parameters which are transformed. Knowing the 0: operator we deduce that: B! and Bi are transformed as ug representation; B;* are transformed as b,, representation; B;’ are transformed as b, representation; and Bi are transformed as b, representation. From this we deduce a simplified form of eqn (14) because first order and second order must transform as the parameter. First order: Bt and Bi contain u(u,); B;* contains u(b,,); B;’ contains u(b,); and Bi contains u(bzg). Second order: Bi and B: contain all the squares;

We know the spin Hamiltonian arises from the naphthalene displacement field and we know it is not a torsional mode but we do not know what exactly the nature of this field displacement is. To determine this, we are trying to associate a modulated spin Hamiltonian form with each particular symmetric field displacement; for this we use group theory. The point group of the naphthalene molecule is

In the general case the field displacement is a superposition of field displacements which are transformed as an irreducible representation of point group of molecule. The field displacement G is defined by its components:

rvib = 5~, + 2~” + 46, + 26” + b, + 4b” + 2b, + 4b,. (13)

Let us develop B:(C)

B;(C) = c B$“u’(cr) + 1

i.a

u’(L);

U2(b,“)i. . . >

associated to each particular symmetry. There are three families of displacement field: an overall translation of molecule; an overall rotation of molecule; and deformations. The translations have no effect on an EPR experiment; so we forget them.

B$2’ui(jl)u’(y).

(14)

UB,?

We analyse the effect of a transformation r on the spin Hamiltonian; there exist two equivalent methods for proceeding: (i) t transforms the field displacement u’ in r(z7), thus spin Hamiltonian Z(r7) transforms in #(r(J)); (ii) we associate to r an operator T(r) which operates in spin space, the spin Hamiltonian x(G) transforms in T(t)Af(i2)T+(r).

B;2 contains rΜ u(b2,)u(bS,); u(a,)u(b,,); u(b2,)u(bJg); B;’ contains u(c,)u(b,,); u(blU)rG2U); u(u,)u(bjg); u(b,,)u(b&); and Bi contains u(u,)u(b,,); u(b,,)u(b,,);

u’(a,); u+z,>; . . . ; u’(a,); u2(a,); . . . ;

up to second order

u@,)u(b&

u(b,,)u(b,).

Anisotropy of splitting gives informations about the modulated spin Hamiltonian form. The spectra are reconstructed by using the local spin Hamiltonian given by eqn (11) with m = f 1, -2, and the resonance condition: E,(H, 4) - E,(H, 4) = hv;

Ei = (il&‘($)li).

The corrected values are calculated up to second order of perturbation. When the resonance condition

A. VERON et al.

58

Table 2. Exnerimental results at 15K

Bi= -87 x IO-‘cm-’ B,’ = 0 x lo-‘cm-’

Distribution P($) Cutphase l$,=lOO

8,2= -7 x IO-‘cm-’

6 =O.ool

Parameters

Rotation parameters Rotational axis u, = 0.00 uY= 0.85 ll, = -0.52 Rotation amplitude a = 2.5

for a rj value 4 E [0, II] is realised a line is associated; the spectrum is a sum of local lines, whose positions depend on the spin Hamiltonian parameters and the intensity is weighted by the p(Cp) distribution:

P(4) = 16+ J(2wos4 -

cos l#l,))]-’

This distribution accounts for the disymmetries in splitting and the singularity intensity and for critical behaviour of the splitting [19,23]. We find modulated parameters amplitudes at T x 15 K B;‘=Ox values B: = -(87 f 5) x lo-‘cm-’ lo-‘cm-’ and B;* = -(7 + 2) x lo-‘cm-’ with parameters given in Table 2.

2370

2405

the the for the the

If we assume it is very difficult to deform the naphthalene molecule then we must consider only rotational modes. We see B: is the most important parameter. In Table 2 these three parameters are converted in terms of rotation with an amplitude equal to 2.5” around an axis perpendicular to the x-axis of the molecule. Figure 7 gives some typical reconstructed spectra.

5. CONCLUSION This paper was devoted to the properties of the naphthalene molecular EPR probe in biphenyl single crystal. The investigations were performed in the X band and the results were compared with the ones obtained in low field experiment. This first step permits an understanding of the probe behaviour in a simple case before beginning the phase II investigations, in which the n = 4 order parameter dimension gives rise to new difficulties in the EPR spectrum analysis. It shows that the EPR probe, which takes nearly the same orientation as the biphenyl molecule (investigation in normal or high temperature phase), does not exhibit the same displacement field but experiences a rotation movement around an axis perpendicular to the long twist axis (long molecular axis). If this result could be expected because of the molecular geometry, we cannot understand why the “multi-soliton” regime is observed in phase II and at the beginning of phase III while a plane wave regime is observed in phase II and the solution regime appears lower in phase III. This result suggests the EPR probe behaves as a defect and could permit discrimination between domains in phase II as the EPR spectra seem to reveal it during the transition between phase II and phase III at 17 K.

2440

Magnetic field (Gauss)

Acknowledgement-We are greatly indebted to M. Spiesser for his help in the crystal growth.

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(1977). 2630

2670

2710

Magnetic field (Gauss)

Fig. 7. Spectrum reconstruction of EPR spectra in phase III with parameters given in Table 2. (0) Experimental results; (-) theoretical results. (a) In the (a, b) plane (&,, a) = 37”, (I&,,a) = 37”; (b) in the (a, c) plane (B,, a) = - 57”.

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