Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

1B /2003G027

Pressure Effects on an Organic Radical Ferromagnet γ-BBDTA•GaCl4
M. Mito*1, M. Fujino1, T Tajiri1, H. Deguchi1, S. Takagi1, W. Fujita2, K. Awaga3
Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
2
Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan
3
Graduste School of Science, Nagoya University, Nagoya 464-8602, Japan

1

Introduction
A radical cation salt, BBDTA(= benzo[1,2-d:4,5d’]bis[1,3,2]dithiazole)•GaCl4, has three polymorphs,labeled α, β and γ. The γ-phase with orthogonal threedimensional network shows the ferromagnetic ordering at
7.0 K, in contrast to diamagnetic properties of the α- andβ-phases with one-dimensional stacking [1]. The very
high TC of 7.0 K in this salt originates from compact
intermolecular linkages through S•••S or S•••N
interatomic contacts, and largely exceeds the previousrecord, 1.5 K [2], in the organic radical system. The
further increase of TC might be realized by making thesecontacts shorter. In this study, the pressure effects on theorganic ferromagnet, the γ-phase BBDTA•GaCl4, havebeen investigated through the ac magnetic susceptibility
measurements and structural analysis experiments in thepressure region up to 17.8 kbar.

BBDTA•GaCl4 arrives at 14.5 K at P = 16.2 kbar, andnearly approaches 16 K for TDAE-C60 [4].
Figure 1 shows the diffraction pattern of the γ-phaseof BBDTA•GaCl4 in the pressure region up to 17.8 kbar,
with that of the β-phase for the reference. The date for P≤ 2.9 kbar represents that the diffraction peaks of the γphase, labeled with the C2/c system, shift toward higher
angles with increasing pressure. However, for P ≥ 6.6kbar, a steady graze makes us find two pronounced
anomalies related to a triclinic system at around 2θ = 8.59.5 degree, which can be seen clearly in the α and βphases. Thus, we have come to interpret the results of ac
susceptibility as the influence of two factors, (1) thestructural transformation from the ferromagnetic γ-phaseto the nonmagnetic α- and/or β-phases and (2) theincrease of ferromagnetic interaction in the surviving γphase.

ExperimentThe thiazyl organic crystal, BBDTA•GaCl4, wasprepared according to the procedure elsewhere [1]. The
γ-phase crystallizes in the monoclinic system C2/c with a= 11.260(5) Å, b = 10.851(6) Å, c = 13.209(7) Å, β =114.853(2) º and V = 1464.44(13) Å3. On the other hand,the diamagnetic α- and β-phases crystallize in the triclinicsystem. Measurements of the ac magnetic susceptibilities
were performed with the ac field of amplitude 2 Oe(peak-to-peak) and frequency 100 Hz in the region up to16.2 kbar using a piston cylinder type of pressure cell.
The SR powder structural analysis experiments at room
temperature were carried out in the pressure region up to17.8 kbar using a diamond anvil cell at the beamline 1B.
Results and Discussion
First we have observed that the magnitude of ac
susceptibility is remarkably suppressed with increasingpressure, and decreases down to about a half of the initial
by little pressure of 0.10 kbar [3]. At around P = 2.0 kbar,
there remains only the magnetic signal for the 3.5 % of
the initial, and the amount is finally suppressed down to
0.07 % of the initial at P = 16.2 kbar.
However, in spite of the suppression of the ac
susceptibility, the transition temperature TC itself isenhanced by pressure. The TC increases with linear
fashion against pressure in the considered pressure region.
The amount of the gradient, dTC / dP, is estimated to be +5.1×10-1 K/kbar. The normalized gradient with the valueof TC(P = 0), is + 7.3×10-2 /kbar, which is larger thanthose of a few ferromagnets. Finally, the TC of the γ-phase

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Users' Report

Fig.1. Powder diffraction pattern of the γ-phase
BBDTA•GaCl4 in the pressure region up to 17.8 kbar..
References[1] W. Fujita and K. Awaga, Chem. Phys. Lett. 388(2004) 186.
[2] R. Chiarelli, et al., Nature 363 (1993) 147.
[3] M. Mito, et al., Polyhedron in press.
[4] B. Narymbetov, et al.,Nature, 407 (2000) 883-885.
* mitoh@elcs.kyutech.ac.jp