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THE EFFECTS OF ADDITIVES ON THE PYROELECTRIC PROPERTIES OF T. G. S.
E. Keve, K. Bye, P. Whipps, A. Annis
To cite this version:
E. Keve, K. Bye, P. Whipps, A. Annis. THE EFFECTS OF ADDITIVES ON THE PYROELEC- TRIC PROPERTIES OF T. G. S.. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-229-C2-231.
�10.1051/jphyscol:1972279�. �jpa-00215015�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page C2-229
THE EFFECTS OF ADDITIVES
ON THE PYROELECTRIC PROPERTIES OF T. G. S.
E. T. KEVE, K. L. BYE, P. W. WHIPPS and A. D. ANNIS Mullard Research Laboratories, Redhill, Surrey, England
R6sum6. - Nous resumons l'effet polaire permanent dti a la substitution d'alanine L sur les proprietes pyroelectriques de T. G. S. Nous expliquons les effets observes au moyen d'un modkle dans lequel le (( switching )) ferroelectrique est g&n6 par la perte de symktrie due B la substitution d'une molecule. D'autres additifs actifs et passifs peuvent &re proposes a partir de ce modkle.
Abstract. - The permanent poling effects of L-alanine substitution on the pyroelectric properties of T. G. S. are summarized. The observations are explained in terms of a structural model in which ferroelectric switching is impeded by the lack of the required prototypic symmetry in the substituent molecule. Other active and passive additives may be proposed from the model.
Much of the recent interest in T. G. S. is related to use in radiation detectors using the pyroelectric effect.
The combination of moderately high pyroelectric coefficient and low dielectric constant at room tempe- rature, together with the ease with which large optical quality single crystals can be grown from solution, are some of the properties that make T. G. S. superior to other materials for many detector applications. The effective pyroelectric output of any ferroelectric crystal depends on its degree of poling. The major disadvan- tage of using ferroelectric crystals as pyroelect~ics is the possibility of a single domain crystal becoming rnulti- domain or depoling in use.
However, it was found recently by Lock [I] that T. G. S. crystals grown from a solution containing between 2 and 35 % alanine (a) show a permanent self bias. Dielectric hysteresis loops of such crystals are distinctive in that they are partially or completely displaced along the field axis (see Fig. la). It was found that the crystals containing alanine retained their self bias even after prolonged heating above the Curie
(a)
FIG. la. - 50 Hz dielectric hysteresis loop of A. T. G. S.
temperature and after application of large reverse bias electric fields. Results on more detailed studies of these phenomena are presented here and an explanation is proposed in terms of the structural inhibition by the alanine molecules of the ferroelectric switching in T. G. S. An extension of this explanation has enabled the proposal of other dopants which can be predicted to give or not to give permanent self bias. Furthermore, it allows the prediction of the optimum way of intro- ducing the alanine substituent.
The crystals used for the measurements were grown [2] below T, by slow cooling a solution contain- ing 10 % L-alanine. Analysis of single crystals showed them to contain approximately 0.1 % alanine. These crystals are referred to as A. T. G. S. subsequently.
Table I lists the quantities P,, E, and Eb (see Fig. l a ) which represent the reversible polarization, the relative coercive field associated with the reversal and the self bias of the crystal respectively. Also listed are dP/dT, resistivity and dielectric constant for T. G. S. and A. T. G. S. A. T. G. S. shows, typically, a large self bias with Eb x 2 E,. Smaller bias can occasionally be found in normal T. G. S., but in good quality crystals it is usually absent. A. T. G. S. requires much higher fields than T. G. S. to approach saturation. Figure la shows that in zero applied field, there is only one stable orientation of polarization for A. T. G. S. whereas there would be two for T. G. S. The dynamic pyro- electric outputlapplied field hysteresis loop in figure 16 indicates that the crystal is in its maximum polarization state at zero applied field. On reverse bias, the signal reduces to zero and saturates at a value which is about 80 % of the initial spontaneous polarization. Thus the polarity of a fraction of the crystal is fixed even in large reverse bias fields. On reducing the field, the signal returns to its initial value with hysteresis which is equivalent to that shown at 50 Hz in figure la.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972279
E. T. KEVE, K. L. BYE, P. W. WHIPPS AND A. D. ANNIS
Frequency of Measurement
A
Eb(*) 50 Hz
PI(*> 50 Hz
El(*) 50 Hz
dP/dT d. c.
P 1.6 kHz
E' 1.6 kHz
(*) See Figure 1.
Some electrical properties of T.G.S. and L-alanine substituted T. G. S. ( A . T. G . S . ) at 20 OC
T. G. S. +
L-alanine (A. T. G. S.)
A
5.0 3.6 2.6 3.5 1.1 x 1O1O
23
t( Normal )) poled - T. G. S.
-
0.04 3.1 0.5 3.5 1.2 x lo9
30
T. G. S. f L-alanine with reverse
bias = E,,
A
0 3.6 2.6 3.5 5.5 x lo8
29
t( Normal )) T. G. S. with bias Eb = 3 E,
-
1.5 kV/cm
3.1 pC/cm2
0.5 kV/cm
3.5 x lo-' pC/cm2/0K 3.7 x lo9 sz x cm
29
FIG. lb. - Dynamic pyroelectric output/applied field loop of A. T. G. S.
The pyroelectric coefficients are comparable as long as the T. G. S. sample is fully poled, whereas the doped crystal requires no poling. The resistivity measured with 40 V/cm r. m. s. a. c. field is much higher for A. T. G. S. than T. G. S. In general, it was found that dP/dT, p and E were much more reprodu- cible from crystal to crystal for A. T. G. S. than they were for T. G. S.
The self bias Eb shown by all A. T. G. S. crystals is equivalent in the absence of an applied field to perma- nent polarization. This self bias remained unchanged even after several hours of annealing at 60 OC. The crystals were found to repole themselves on cooling through the Curie point and attain the initial values of all parameters at room temperature. Figure 2 shows that some polarization persists above the Curie tempe- rature and that the dielectric anomaly is depressed to about 2 % of T. G. S.
The above results indicate the A. T. G. S. is sponta- neously poled and cannot be depoled. This implies that the crystals are always single domain. Scanning electron microscope photographs of A. T. G. S. and T. G. S. under comparable conditions were found to show the usual lenticular domains for T. G. S., but no
T ° C
FIG. 2. - Variation of polarization and dielectric constant with temperature for A.T.G.S.
domains could be seen in A. T. G. S., thus supporting the other evidence.
The model proposed to explain these effects postu- lates that the alanine molecule (b), is sufficiently similar to glycine (a) to partially substitute for it in the structure of T. G. S. Considerations of the ferro- electric switching mechanism based on the structure reported by Hoshino et al. [3] shows that the glycine I molecule, which lies on a mirror plane in the proto- type and on one or other side of it in the ferroelectric phase, constitute the main reversible dipole. Switching entails the change of the distorted glycine I molecule into its mirror image.
H H
(a) NH:-C-COO- I I
(b) NH:-C-COO-
I I
When a glycine I molecule is replaced by an alanine molecule, ferroelectric switching must be affected locally. The lack of mirror plane symmetry in the free alanine molecule prevents it from changing into its mirror image inthe solid.
THE EFFECTS OF ADDITIVES ON THE PYROELECTRIC PROPERTIES OF T . G . S. C2-23 1
Local strains must be introduced in the lattice where glycine is replaced by a larger molecule. In the case of alanine, the strain field must be asymmetric or polar due to the asymmetry of the deviation of alanine from glycine. Furthermore, the polarity of the strain field cannot be inverted as the substituent molecule cannot change into its mirror image. The molecule and its associated polar strain field are equivalent to a dipole with relatively long range influence. This dipole is both irreversible and permanent as long as the substituent molecule does not break down. When the T. G. S.
crystal contains only one optically active form of the additive e. g. L-alanine, all the molecules substituting for glycine I must lie in the same sense, with respect to the b axis, if the stereochemistry and hydrogen bonding is to be preserved. All the permanent dipoles, therefore have the same polar sense and a sufficient concentra- tion of them will result in a self bias on the crystal which can polarize the intervening matrix of cr pure )) T. G. S. A spontaneously poled single crystal is then produced.
The above model suggests that any molecule able to replace some glycine I molecules, but which lacks the required mirror plane, will behave similarly to A. T. G. S. Examples of such (( active )) substituents are serine (c) and a-amino-n-butyric acid (d).
H H
(c) NH:-C-COO- I I
( d ) NH:-C-COO-
C H 2 0 H I I
C2H5
It would be expected that a dopant such as a-amino- iso-butyric acid (e) will induce symmetrical or non- polar strains which need not preferentially polarize the crystal. A distorted a-amino-iso-butyric acid molecule can change into its mirror image by displace- ments of atoms and by the interchange of roles of the two CH3 groups on the a-carbon, just as glycine I can.
Replacement of glycine I b y such a molecule would not, therefore, affect the ferroelectric properties of T. G. S.
7H3
T. G. S. crystals have been prepared from solutions containing the above additives. The hysteresis loops of T. G. S. (serine) and T. G. S. (a-amino-n-butyric acid) were highly displaced ; that of a-amino-iso-butyric acid substituted T. G. S, was symmetrical, as predicted.
Using DL-alanine in solution gives similar effects to L-alanine, as long as the crystal is grown below Tc, since large permanently poled regions of each polarity are obtained. Crystals grown above Tc give symmetrical loops. However, if only one optically active form is used, it must be taken up in the same polar sense even in the non-polar phase. Thus crystals of T. G. S.
(L-alanine) grown above T, become spontaneously, fully poled when cooled below T,. Furthermore this method gives crystals which are more uniform and have larger self bias, than those grown below the Curie temperature.
References
[I] LOCK (P. J.), Appl. Phys. Letters, 1971,19,390.
[2] BARTLETT (B. E.), Mullard Southampton.
[3] HOSHINO (S.), OKAYA (Y.) and PEPINSKY (R.), Phys.
Rev., 1959, 115, 323.
