Electron paramagnetic resonance study of the warwickites Mg1+xTi1−xBO4

Solid State Communications,Vol. 104, No. I, pp.35-38, 1998 8 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/98 $19.00+.00

ELECTRON PARAMAGNETIC RESONANCE STUDY OF THE WARWICKITES Mg ,+,Ti I-rBOd J. Dumas,a’* M. Continentino,’ J.J. Capponi’ and J.L. Tholence’ “Laboratoire d’Etudes des Proprietes Electroniques des Solides, CNRS, B.P. 166, 38042 Grenoble Cedex 9, France qnstituto de Fisica, Universidade Federal ~uminense Campus da Praia Vermelha, Niteroi, 242 10-340, R.J.Brazil ‘Laboratoire de Cristallographie, CNRS, B.P. 166, 38042 Grenoble Cedex 9, France (Received

12 November 1997; accepted 25 November 1997 by P. Burlet)

We have performed EPR measurements between 10 K and 300 K at 9.4 GHz on crystals of the warwickites Mg I_xTi,+BO.+ These compounds belong to the family of quasi-one dimensional disordered magnetic systems. The warwickites are characterized by the presence of ribbons along the c axis where the metallic ions are r~domly located. The EPR spectra consist of two lines attributed to Ti3+ paramagnetic centers. A narrowing of the linewidths is observed upon increasing the temperature. A large change in the relative amplitudes of the lines is observed as a function of temperature. We suggest that the observed behaviour reflects intra and inter-ribbons interactions between Ti3+ (S = l/2) ions. A hopping mechanism is proposed for the line narrowing. 0 1998 Elsevier Science Ltd Keywords: A. disordered systems, D. exchange and superexchange, D. spin dynamics, E. electron p~amagnetic resonance.

1. INTRODUCTION

different concentrations of Ti3+. As x is increased, some of the Ti ions have the configuration Ti4’ in order to maintain charge neutrality. This allows to study the evolution of the magnetic properties as the number of Ti3’ Ions * is varied. The temperature dependence of the magnetic susceptibility and of the specific heat of the series Mg ,+,Ti ,+B04 [2] at low temperatures has been well described within the spin-112 random exchange Heisenberg antiferromagnetic chain model. In this model, the Ti3+ ions interact with random exchange antiferromagnetic interactions due to superexchange mediated by oxygens. In order to provide further insight into the exchange interactions in the warwickites, we report EPR measurements at X-band (9 GHz) on Mg,+,Ti I-rB04 crystals. Except for Mijssbauer studies on MgFeB04 [ 11, no local measurements have been performed so far on the warwickites. We show that the observed temperature dependence of the widths and amplitudes of the EPR lines reflects the intra- and inter-ribbons interactions.

The warwickites have received widespread attention in recent years [l-6]. These insulating compounds are mixed borates with general formula M2’M3’BO~. They are characterized by a quasi-one dimensional structure due to the presence of ribbons extending along the c axis of an orthorhombic crystallographic structure and weakly connected to each other from the magnetic point of view. These ribbons are made of four columns of edge-sharing oxygen octahedra in the center of which the divalent and trivalent metallic ions are located. Partial ordering of M*’ and M3” cations in the ribbons is observed. M2+ stands for Mg2+ or Ca2+ and M3+ for Ti3+, V3+, Cr3+, Fe3+ or Sc3+. A systematic study of their c~stallographic structure was made by Capponi et al. [6]. In Fig. 1, a projection in the ab plane is shown. In MgTiB04, the Ti ions are in the 3d’ (S = l/2) magnetic configuration, with a single unpaired spin, which is responsible for the magnetic behavior. In Mg ,+,Ti ,_-+ B04, different x values correspond to

2. EXPERIMENTAL RESULTS The crystals used in this study were grown at Laboratory of Cristallography, CNRS, Grenoble using

* Corresponding author. E-mail: [email protected] 35

STUDY OF THE WARWICKITES Mg ,+,Ti I_xB04

36 l

0 02

01

A

Mgl.,3Tio.77B0,

0T2

.Tl

Vol. 106, No. 1

AB2

ABl _-

_-

_-

.O I @

a

RG: 10’ P=lmW

25K

RG: 4x10 P=lmW

160 K

RG: 8x10’ P= 1OmW

300K P= 1OmW

_

a

I

1

,

1

2

3

v

I

I

4

5

H( kG)

Fig. 1. Projection in the ab plane of the structure of the warwickites. The ribbons are shown by dashed lines. Full and empty symbols correspond to different coordinates along the c axis. The metals (T = Mg, Ti) lie in the centre of oxygen octahedra. From [ 11.

Fig. 2. EPR absorption derivative lines of a Mg 1.23Ti0.~7B04 crystal at different temperatures. Microwave magnetic field parallel to the c axis. Receiver gain (R.G.) and microwave power (P) are indicated. Modulation field amplitude: 8 G.

an electrocrystallization method as described by Blum and Bozon [7]. The electrolysis of sodium borates solutions of titanium and magnesium oxides at a constant temperature T = 850°C allows to control the Mgmi ratio in the samples. The crystals appear as thin needles, a few millimeters long and a few tenths of a millimeter thick. They are elongated along the c axis. Crystals with four different compositions (see Table l), as determined by energy-dispersive analysis of X-rays (EDX) were prepared. X-ray diffraction patterns of crushed crystals reveal strong diffraction peaks, indicating a well defined composition of the sample, since the unit cell parameters vary with x. EPR measurements were carried out at X-band in the temperature range 10 K-300 K using a ESP 300 Bruker spectrometer equipped with an Oxford ESR9 continuous helium flow system and a TE102 rectangular cavity. The

crystal was mounted with the c axis parallel to the microwave magnetic field. The EPR spectra of Mg ,+,Ti I_,BO, consist of two asymmetric lines A, B with g-factors gA = 1.85 and ga = 2.06. The temperature dependence of these two lines are very different, as illustrated in Fig. 2 for a sample. At low temperatures Mg t.23’h.+04 (T= 10 K) only the line A is clearly visible with a derivative peak-to-peak linewidth &HA = 266 G. Upon increasing temperature, the two lines become apparent and they overlap each other. At room temperature, the amplitude of the line A becomes much smaller than that of the line B. The peak-to-peak linewidth of the line A clearly decreases with increasing the temperature, as shown in Fig. 3. Below -25 K, AHA seems temperature independent. The linewidth AHa of the line B, is much larger than MA as shown in Fig. 2. Due to the overlapping of the two lines, we define us as AH0 = 26H~,

Table 1. Characteristics of the Mg ,+,Ti I-+B04 warwickites and parameters of the EPR lines A and B Compound

Color

Mg 1.NTi0.60B04 Black Mg 1.23Ti0.77BG4Black MgTiB04 Black Yellowish Mg I.sTio.&h

Charge (from PI) Mg :&TiijTi$ Mg:f3Ti:fTi,$7 Mg 2+Ti3+ Mgf$Ti$

&?A

gB

AHA

MB

MA

VB/~A)

(IOK)

(300K)

(300K)

(300K)

(10K)

at 300K

1.85

2.06 2.06

23 G 23 G

930 G 333 G

150G 266 G 750 G

3.3 7

1.85

1.85 No EPR signal

37

STUDY OF THE WARWICKITES Mg i+,Ti ,_,BO,

Vol. 106, No. I

AHA 0

600 t’ “““““““““““““‘ii 10

Fig. 3. Peak-to-peak linewidths AHA and AHa as a function of temperature. as indicated in Fig. 2. AHa decreases with temperature. The temperature dependence of the peak-to-peak amplitudes IA and Za of the two lines is shown in Fig. 4. While IA decreases with tem~mture, Ia defined as In = 2&n (see Fig. 2) increases with temperature. While IA > Zn at low temperature, one finds la > Z,+,at room temperature. Similar results are obtained for Mg ,.40Tio.&O.+ While the linewidth AZ!ZA of samples Mg 1.23Ti0.77B04 and Mgi,40Ti0.&04 are nearly identical at room temperature, the linewidth AZ!?a of Mg,,~Ti~.~O~ is much larger than that observed in Mg i.2sTie.rrB04 at room temperature. (See Table 1). In MgTiB04, containing only Ti3’ ions, a single asymetric broad line A is observed also with gA = 1.85. However, the linewidth AHA is much larger than that of Mg l.t3Ti0.7fB04 or of Mg,,~Ti~.~O~; we find AHA = 750 C at 10 K. Moreover, AHA increases with temperature, as shown in Fig. 5, in variance with the behavior found in Mg,,23Ti0.77B04or Mg1.40Ti0.&04. The peak-to-peak amplitude IA decreases with temperature and vanishes at -60 K.

Mgt.k’%.77BO4

_! :

u

0

~,I~~,

? c4 ,bt n R,R

0

50

100

150

1

/

i

R,

200

I

/

I / / /l

250

0

300

-f'(K)

Fig. 4. Amplitude peak-to-peak of the lines A and B as a function of temperature.

20

30

$1

50

60

70

Fig. 5. Peak-to-peak linewidth AZZAfor a MgTiBO,, crystal as a function of tem~rature. The inset shows a typical EPR line recorded at 13 K. In Mg ,.sTio.sB04, containing only Ti4+ (3d”) ions no EPR signal is detected in the whole range of temperature. 3. DISCUSSION In a longitudinal section of a ribbon containing the metal sites, the magnetic cations are randomly located on a triangular lattice with three different distances between the cations. This gives rise to large frustration. Inside a ribbon, the short cation-cation distance is dz = 2.9 A (Fig. 1). Any cations of columns 1,l ’ make the direct d-d interactions the predominant intra-ribbon interactions. The distance d, and the distance d3 along the c axis (see Figs 1 and 2 of [l]) being larger than dz, the superexch~ge mediated by the oxygen in the shared edge of their different octahedra provide a competing mechanism for their interactions. The inter-ribbon interactions are also provided by superexchange, but the distances D,, D2 between cations of different ribbons is larger than d,, d2, d3. We may assume that the Ti ions occupy p~fe~ntially the inner columns 1,l’ in analogy with the results reported by Norrestam [B] for MgScBO,, and for MgFeB04 [l]. In this context, we attribute the EPR lines to Ti3’ centers located in columns 1,l’ and 2,2’ with different neighborh~ds. The Ti 3+ ions on the more symmetric sites in the inner columns 1,l’ would be responsible for the line B with ga = 2.06. The narrow line A would be associated to Ti3+ ions on the sites of the outer columns 2,2’ which have a lower occupancy. Also, since the interactions between Ti 3+on these columns are weaker, this line is int~nsically narrower than line B. We may attribute the decrease of the linewidths AHA, AHa to an exchange narrowing mechanism. In this picture, the linewidth can be written AH - l/w,, with w,, -J/h is a measure of the rate at which electron spin fluctuations take place due to exchange (91. The

38

STUDY OF THE WARWICKI~

Vol. 106, No. 1

since the hopping fr~uency is regally activated 191. In Fig. 6, a logarithmic plot of the temperature dependence of MA is shown. A straight line is found for the temperature range 25-200 K. This plot yields an activation energy W - 5 meV, comparable to that found for the temperature dependence of the EPR linewidth in the quasi-one dimensional compound CuzVz05 [lo] which shows a hopping conductivity.

3 %

55 4

Mg I+XTiI_,BO,

2

q

4

,s~,“‘l’,~ .

4. CONCLUSION

We have presented results of an EPR study of a quasione dimensional disordered system, the warwickites 103/T (K-l) Mg i+,Ti ,_,BO,+ While the two observed lines attributed to Ti3+ centers show a narrowing with increasing the Fig. 6. Logarithmic plot of the linewidth MA as a temperature, the single broad line observed in MgTiBO,, function of reciprocal temperature for a Mg 1.23Ti0,77B04 (X = 0) broadens with temperature. What is surprising is crystal. the change in the relative amplitude of the two lines as a vanishing of EPR line B below -20 K could be due to function of temperature. A mechanism of hopping is the presence of strongly coupled Ti3+ ions, in columns proposed for the observed line narrowing. 0

5

10

1s

20

25

1,l ‘, forming singlet pairs. The broad line B would appear only when the Ti3+ pairs start to dissociate. The increase of the line ~plitude Za with temperature may correspond to a process of diss~iation of these pairs. In MgTiB04 containing only Ti 3+,dipolar broadening and spin lattice relaxation are the dominant mechanisms. The absence of EPR line in Mg 1,5Ti0,5B04can easily be understood since all Ti ions are in the nonmagnetic configuration Ti4+. The large differences between the EPR spectra and their temperature dependence, of the &p&(x = 0.23 and x = 0.40) and u&&(x = 0) Mg I,Ti I-XB04 systems strongly suggest another narrowing mechanism for lines A and B. This is due to charge transport in the doped systems. Such a mechanism would be a motional na~owi~ associated to a variable range hopping between Ti3+ and Ti4+ sites. This mechanism would not be valid in the case of MgTiB04 containing only Ti3+ and no Ti4+ available sites for hopping. In this latter case, a line broadening is in fact observed. However, the assumption of variable range hopping is made difficult to check since the warwickites are insulating. If this latter model is valid, the temperature dependence of the linewidth obeys an activated behavior: AHa exp (W/k&

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