Incommensurate phases in biphenyl

Solid State Communications, Vol.31, pp. 521—524. Pergamon Press Ltd. 1979. Printed in Great Britain. INCOMMENSURATE PHASES IN BIPHENYL H. Cailleau Groupe de Physique Cristalline, ERA au CNRS No. 015, Université de Rennes, Campus de Beaulieu, 35042 Rennes-Cedex, France and F. Moussa and J. Mons Laboratoire Leon Brillouin, CEA Saclay, BP 2, 91190 Gif sur Yvette, France (Received 30 March 1979 by E.F. Bertaut) Successive phase transitions in deuterated biphenyl are re.examined with higher resolution elastic neutron scattering. The two low temperature phases were found to be incommensurate. The wave vector characterizing 6b)b* in phase II existing between the reflections are:rq = 21 K and T 8 = ~ + ~(1 — 1 = 38K, and q,, = ~(l — ~b)b* in phase III below T11. The variations of &, and ~ b with temperature are small. The deviations of satellite reflections exhibit clearly a jump at TT1. FOR BIPHENYL in the gaseous state, the orthohydrogen repulsion introduces a non.planar configuration with respect to a torsional angle between the planes of phenyl rings. In the crystal, intermolecular forces, which would give a planar configuration are of the same order than some of the intramolecular ones (torsional field) and molecules are non.rigid. So, at room temperature, the space group is P2 i/a with two planar molecules on inversion sites, but the librational ampli. tude around long molecular is large (~ Raman 10°)[II. Recently, softthemodes have been axis observed with spectroscopy [2] showing that a “displacive” phase transition takes place around 40K. It has been suggested that this phase transition corresponds to the stabilization of the non.planar configuration [2], similarly to the “order—disorder” phase transition of p.terphenyl [3, 4]. In a neutron diffraction study [5], with A = 1.26 A, this structural phase transition has been considered as resulting the appearance at the Moreover, B (0, ~,0) the recipro. cal point from of superlattice reflections. corresponding zone boundary mode showed a pronounced softening [6]. On the other hand, Cullick and Gerkin observed this phase transition around 42K in an EPR experiment and reported the observation of a possible second phase transition around 15 K [7]. Bree and Edelson confirmed the existence of two phase tran. sitions near 40 and 16K (38 and 24K for a deuterated compound) [8]. In this letter, we report the observation, with higher resolution elastic neutron scattering, of two incommensurate phases below 38 and 21 K respectively (deuterated crystal). Elastic neutron scattering measurements were per. formed on the cold source triple-axis spectrometer at

the EL3 Reactor in SACLAY. Fixed incident neutron wavelength of 4.1 A was used with pyrolitic graphite double monochromator. Higher order contamination was suppressed by the use of cooled Be filter and germanium analyser. Deuterated biphenyl (99.4%) was obtained from Merck, Sharp and Dohme, Ltd. This compound was purified by zone refining and single crystals were grown from the melt by the Bridgman technic. The 3, chosen with a crystal had dimensions 2.5 sample x 1.5 xwas 0.5mounted mm mosaic spread of 40’. The in a standard liquid helium cryostat, with a stability better than 0.1 K. Figure 1 shows scans along (~, 1.461, 0) and (1.957, ,~,0) directions in the intermediate phase (phase II), existing below T 1 = 38K. Peaks located at the same place are observed in other Brillouin zones. The reflections are not characterized by the wave5bvector q~=with changing but by qs = &za + ~(1 —~b)b, ~ an temperature. They correspond to satellite reflections near the B point, as represented in Fig. 2. Below T 11 21K the phase III is not incommensurate along a*, but only along b*. The sateffite reflections are characterized by wave vector q~= ~(1 — ~b)b*(Fig. 2). The evolution of intensities and deviations ~a and ~b for satellite reflections with the temperature are shown in Fig. 3. The change of~~ with temperature is small. It is more important for ~ which becomes zero at T11 and remains zero in the phase III. Deviations exhibit clearly a jump at T11. At this temperature we observed also the coexistence of the two phases II and III shown by the presence of the three peaks along a*. These observations establish that the two phases

521

522

INCOMMENSURATE PHASES IN BIPHENYL 1500—

KII::~~—KIII::~

Vol. 31, No.7

/

I

f~I

T=28K

1000

—

~ 500

-

In S.-

1.4 61, 0) —

I.-

= 1.95

2.00

2.05

1.50 (b)

1.55

5o0~/~/\

1.45

Fig. 1. Scans taken along a* and along b* at 28 K. Full lines are guides to the eye only.

1010)

(0101

3

C

‘S

r

A

r

A

(1001

PHASE II

(100)

PHASE Iii

Fig. 2. Locations of satellite reflections in the (Ii k 0) scattering plane for phase II and phase III. Dotted lines coriesponds to the limits of the first Brillouin zone.

Vol. 31, No.7

INCOMMENSURATE PHASES IN BIPHENYL I

ç- 0.075

-

523 I

_.~._._-~__..—,-----~-—.—

-

0

0.050

-

-

0.025

-

-

0.05 0

-

.0

(0

I

I ~_.___._-,-.-.-.

-

C

=

~0.025 Cd

-

-

~0

10

~

TEMPERATURE 4’O (K)

Fig. 3. Temperature dependence intensity and deviations 6~and 8b for satellite reflections. In phase II, the intensity of the two peaks found of along a* is added. appearing on lowering temperature are incommensurate. In the early neutron diffraction study [5], with a shorter wavelength, intensities of satellite reflections near the B point were probably integrated. Also, the exact modulation of the torsional angle has not been taken into account. However, the refinement of the average structure corresponding to main reflections, as considered by de Pater for another modulated structure [9], is satisfying, the average torsional angle being about 10°.It will be interesting to measure the intensity of different satellite reflections and particularly possible harmonics. On the other hand, this structural phase transition is driven by a soft mode instability [2, 6]. From symmetry arguments, the mode at the B point in the high temperature phase is doubly degenerate. The occurrence of incommensurate phases can thus be explained by the existence in the lower phonon branch of a minimum away from the B point [10, 11]. There is no mcommensurability along c” and along a* the incommensurability exists only in the intermediate phase. In polyphenyls, the coupling responsible for the torsional motions is

anisotropic. It is stronger along b*, a* and c’ respect. ively, as expected from the packing of molecules [6]. These features seem to indicate a correspondence between strength of coupling and occurrence of incommensurability in a certain direction. Another indication for this would be the small temperature dependence of ~b [12]. A full account of the inelastic neutron scattering study will be published elsewhere. Acknowledgements We wish to thank B. Hennion for efficient suggestions and S. Aubry, R. Pick and C.M.E. Zeyen for helpful discussions. —

REFERENCES 1. 2. 3.

G-P. Charbonneau & Y. Delugeard, Acta Cryst. A. Bree & M. Ed~sonChem. Phys. Lett. 46 500 (1977). J.-L. Baudour, Y. Delugeard & H. Cailleau, Acta Cryst. B32, 150 (1976).

524 4. 5. 6. 7.

INCOMMENSURATE PHASES IN BIPHENYL J.-L. Baudour, H. Cailleau & W.-B. Yelon,Acta Cryst. B33, 1773 (1977). H. Cailleau, J.-L. Baudour & C..M.-E. Zeyen,Acta C,yst. B35, 426 (1979). H. Cailleau, A. Ghard, F. Moussa & C.-M.-E. Zeyen,Solid State Commun. 28,259(1979). A. Cullick & R.-E. Gerkin, Chem. Phys. 23, 217 (1977).

8. 9. 10. 11. 12.

Vol. 31, No.7

A. Bree & M. Edelson, Chem. Phys. Lett. 55, 319 (1978). C.J. de Pater,Acta Cryst. B35, 299 (1979). R. Pick (private communication). V. Dvorak & Y. Ishibashi, J. Phys. Soc. Japan 45, 775 (1978). S. Aubry (private communication).