X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES

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X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES

G. Albertini, B. Dubini, S. Melone, M. Ponzi-Bossi, F. Rustichelli, G. Torquati

To cite this version:

G. Albertini, B. Dubini, S. Melone, M. Ponzi-Bossi, F. Rustichelli, et al.. X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES. Journal de Physique Colloques, 1979, 40 (C3), pp.C3-384-C3-388. �10.1051/jphyscol:1979376�. �jpa-00218772�

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X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES

(*) G. ALBERTINI, B. DUBINI, S. MELONE, M. G. PONZI-BOSS1 Istituto di Fisica Medica, Facolth di Medicina, Universith di Ancona, Italia

F. RUSTICHELLI (**) and G . TORQUATI

Dipartimento di Scienze Fisiche, Facoltri di Ingegneria, Universiti di Ancona, Italia

RCsum6. - La technique de diffraction des rayons X a tt6 utiliste pour ktudier les transitions de phase suivantes qui concernent les phases ordonntes biaxiales du TBBA : solide-smectique B,, smectique B,-smectique VI, smectique VI-phase VII, phase VII-solide. La transition de phase solide-smectique B, apparait Etre dependante de l'histoire thermique de I'khantillon, en accord avec des mesures recentes de microcalorimttrie. Une coexistence de phases a tt6 observee aux transitions smectique VI-phase V11 et solide-smectique B, et un effet d'hysttrtsis A la transition smectique VI-phase VII.

Abstract. - X-ray diffraction was used to investigate the following transitions involving ordered biaxial phases of TBBA : solid-smectic B,, smectic B,-smectic VI, smectic V1-phase VII, phase VII-solid. A dependence on the thermal history of the sample was observed at the solid-smectic B, phase transition, in agreement with recent DSc data. A phase coexistence was observed at the smectic VI-phase V11 and at the solid-smectic B, phase transitions. An hysteresis effect was observed at the smectic VI-phase V11 phase transition.

1. Introduction. - X-ray (or neutron) diffraction constitutes a valuable tool in the investigation of h a s e transitions of liquid crystal materials. However these techniques were applied so far mainly to study high temperature phase transitions, i.e. nematic (or cho- 1esteric)-smectic A phase transitions [l-51, nematic (or cho1esteric)-smectic C phase transitions [6] and smectic C-smectic A phase transitions [7].

This kind of phase transitions, which were largely investigated by other experimental techniques, were theoretically treated in detail [g-101.

Many uncertainties however still exist concerning these transitions. On the other hand very few investi- gations by X-ray (or neutron) diffraction exist concerning phase transitions involving the solid or the more ordered low temperature smectic phases.

One of the few studies of this kind was performed by Levelut, Doucet and Lambert 111, 121, who investigated by X-ray diffraction the solid-smectic B phase transition of several compounds including the typical substance terephtal-bis-butylaniline (TBBA).

This paper presents the results of a study by X-ray diffraction of the following low temperature phase transitions of TBBA : solid-smectic B,, smectic B,-smectic VI, smectic VI-phase VII, phase VII-solid.

(*) This work was supported by C.N.R.

(**) JRC EURATOM lspra, Italia.

In particular, due to the dependence on the thermal history of TBBA [13], the solid-smectic B, phase transition was investigated for both the virgin and non virgin samples. Different results were found in the two cases. Furthermore an hysteresis effect concerning the smectic VI-smectic V11 phase transition recently observed by differential scanning calorimetry [l31 was confirmed during this study.

After a brief description of the experimental details in the next section, the results will be presented and discussed.

2. Description of the experiment. - The experimental apparatus and technique are similar to those described in reference [14].

The experiment consisted in the observation of X-ray intensity evolution for several diffraction peaks as a function of temperature, with particular regard to the phenomena occurring at the phase transition.

The temperature control was achieved with a precision of

+

0.1 OC. The sample was considered to be a powder from a crystallographic point of view.

However we could not verify the validity of this assumption.

The denomination virgin sample has been adopted for a freshly recrystallized sample from the solvent, never submitted to the transition towards the smectic

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979376

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X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES C3-385

B, phase. The denomination non virgin sample is assigned to a sample submitted at least once to the transition and then cooled down to room temperature.

3. Results and discussion. - 3 . 1 SOLID-SMECTIC B, PHASE TRANSITION. - The solid-smectic B, phase transition was at first investigated.

The phase diagram of terephtal-bis-butylaniline (TBBA) can be found in reference [13].

The monoclinic structure of the solid phase of TBBA was recently determined and a comparison performed with the smectic B, phase [15]. The solid-smectic B, transition and in particular the smectic B, phase were investigated in detail by X-ray diffraction [16,17].

All the phases investigated in this study show a monoclinic structure.

A nearly hexagonal packing exists inside each smectic B, layer. The molecules can rotate around their long axis in this phase, as indicated by inelastic neutron scattering experiment [l 81. Moreover fluc- tuations of the molecules of large amplitude about the mean positions exist.

Phonons corresponding to transverse modes, with a wave-vector perpendicular to molecular direction, were observed also in smectic B, and smectic V1 phases, whereas no shear-wave, with wave-vector normal to layer, was observed in the smectic B, phase; only an increase of the quasi-elastic scattering was detect- ed [19].

A recent investigation by differential scanning calorimetry (DSc) showed that the characteristics of the solid-smectic B, phase transition depend on the thermal history of the sample [13]. As a consequence three different sets of observations were performed concerning this phase transition.

3.1 . l Complete transition for the virgin sample. -

During the heating process of a freshly recrystallized sample from the solvent a single peak was observed at the phase transition by DSc.

Figure 1 reports the heights of some diffraction peaks corresponding to the solid and smectic B, phases as a function of the temperature.

These results will be discussed below at 3.1.4.

3.1 .2 Complete transition for the non virgin sample.

- Figure 2a reports the diffraction peak heights for a non virgin sample after a loop defined in reference [l31 and figure 2b the corresponding data for a non virgin sample after five loops.

A coexistence of both phases appears from figures 2a and 2b and will be discussed below. Figure 3 reports a diffraction pattern at 107.8 OC, showing directly this coexistence.

3.1.3 Unachieved transition for the non virgin sample. - This set of measurements is based on the observation of a double peak in the D S c during the heating process of a non virgin sample, and of a peculiar behaviour during the cooling process after the appearance of the first peak. In particular, during this

? l I I I I I

3

0 SOLID

3

SMECTIC BC

TEMPERATURE ("G) --c

FIG. 1. - Evolution of the X-ray diffraction peak heights at the solid-smectic B, phase transition for a virgin sample of TBBA.

SOLID

: SMEGTIC BC.

I

102" 104' 106' 108' 110"

TEMPERATURE (OG) ⁴

FIG. 2a. - Solid-smectic B, phase transition for a non virgin sample of TBBA after a loop.

cooling process no evidence exists for the high enthalpy crystallization peak.

Therefore in the present experiment after the peak intensity corresponding to the solid phase disappeared at the same temperature (110 O C ) for all the reflections

26

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160 -(l121

T

₀_SOLID

SMECTIC B,

104' ' 106' ' 1080 ' 110' ' 112~ ' ' 1;4' TEMPERATURE ('C)

-

FIG. 26. - Solid-smectic B, phase transition for a non virgin sample of TBBA after five loops.

TAKE OFF ANGLE 2 f3

-

FIG. 3. - Phase coexistence at the solid-smectic B, transition for a non virgin sample (X-ray diffraction pattern).

(Fig. 4a), the sample was cooled till the peaks of the solid phase appeared again (Fig. 4b).

The hysteresis deduced from figures 4a and 4b for the (01 1) reflection is reported in the insert of figure 4b.

. 3.1.4 Discussion. - Some remarks can be made concerning the data related to the solid-smectic B, transition.

a) The virgin sample behaves differently from the non virgin sample. In particular a phase coexistence appears in the non virgin sample (Fig. 2a, 2b, 3), whereas the melting process in the virgin sample is associated with the solid peak disappearance followed by the appearance of the smectic B, peaks.

From figures 2a and 2b it seems that the phase coexistence should be correlated to the precocious appearance of smectic B, peaks (already at T ⁼104 OC in figure 2a).

b) Figures 4a and 4b show that the solid-smectic B, transition becomes reversible with an hysteresis effect of about 10 OCwhen the heating process is reversed after the solid peak intensity disappears. This phe-

102 104 106 108 110 TEMPERATURE ('6) -+

FIG. 4a. - Unachieved solid-smectic B, transition for a non virgin sample.

I I I I I I I I I I

o SOLID T

92' 94O 96* 98' 100' 102' 104' 106' 1 0 8 ~ 110' TEMPERATURE ("C)

-

FIG. 4b. - Smectic B,-solid transition for a non virgin sample.

nomenon explains the broad peak observed in the DSc in figure 3 of reference 1131 at about 13 OC below the first of the two peaks appearing by heating the sample.

On the other hand when the non virgin sample is heated above 113 0C and then cooled again the diffraction peak of smectic V1 and V11 phases appears, in agreement with the DSc data [13].

c) No equivalence appears in the X-ray diffraction (Fig. 2b) of the double peak appearing in the DSc for a complete phase transition of the non virgin sample.

It seems that the enthalpy associated to the second DSc peak does not influence the lattice order.

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X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES C3-387

Other experimental techniques are probably neces- sary to clear up this mysterious fact and to make understand the physical reasons for the different behaviour of the virgin and non virgin samples.

A recent determination of the temperature depen- dence of lattice cell parameters was not able to show that two different phases correspond to the virgin and non virgin state [14].

3 . 2 SMECTIC BC-SMECTIC V1 TRANSITION. - Figu- res 5a and 5b report the peak intensities and lattice

spacings at the smectic BC-smectic V1 phase transition.

The first order character of the transition appears merely from figure 5b in agreement with the sharp and intense peak observed by DSc. No hysteresis effect was observed by reversing the direction of the

. transition.

3 . 3 SMECTIC VI-PHASE V11 TRANSITION. - Figure 6 shows the data concerning the smectic VI-phase V11 transition. A phase coexistence is found : it appears explicitly from figure 7.

FIG. 5a. - X-ray diffraction peak heights at the smectic B,-smectic YI transition.

TEMPERATURE CC) t

FIG. 6. - Smectic VI-phase V11 and phase V1I:solid transitions.

TBBA T= 71 "G PHASE \'I1

--(111)

l : ; : : : ; . : ; 2 . . ,

15 20 , S 2,s ,

TAKE OFF ANGLE 2 B

-

FIG. 7. - Phase coexistence at the smectic VI-phase V11 transition (X-ray diffraction pattern).

In addition an hysteresis effect is observed for the reflection (011) of the smectic V1 phase and for the reflection (111) of the phase V11 as it appears from figures 8a and 8b.

This fact is in agreement with a similar hysteresis effect found by D S c 1131.

I

I I 1 1 I I 1 I I

78" 80' 82' 84' 86' 88' 90' 92' 94' 3 . 4 PHASE VII-SOLID PHASE TRANSITION. - Figure 6 TEMPERATURE ('G)

-

shows the data concerning the phase VII-solid tran- FIG. 56. - Lattice parameters at the smectic B,-smectic VI tran- sition, which appears to be first order in agreement

sition. with the sharp and intense peak of the DSc.

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25 0 I I I I I I I I

(111) PHASE VII

FIG. 8a. - Hysteresis for the reflection (011) of the smectic V1 FIG. 8b. - Hysteresis for the reflection (111) of the phase VII.

phase.

4. Conclusion. - Three main facts emerge from iii) the hysteresis at the smectic VI-phase V11 tran-

this work : sition and at the smectic B,-solid transition under par-

ticular conditions.

i) the different behaviour of a virgin sample as

compared to a non virgin one ; Acknowledgments. - It is a pleasure to thank ii) the coexistence of the two phases at the solid- ~ r . . M. Bartozzi for helping in data elaboration and smectic B, transition for a non virgin sample and at the Mrs. L. Ferrantini, S. Polenta and E. Santoni for the smectic VI-phase V11 transition ; valuable technical assistance.

References

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