930

Factors Affecting the Resistance and Performance of Fluoride Ion-Selec tive Electrodes Wei Shen,' Xue D. Wung,' Robert W. tkttrulE,*' Gvueme L. Nyberg,' and John Liesegung+'

+ School of Chemistry, Faculty of Science and Technology, La Trobe University, Melbourne, Victoria 3083, Australia "School of Physics, Faculty of Science and Technology, La Trobe University, Melbourne, Victoria 3083, Australia

Received: August 1, 1994 Final version: September 26, 1994

Abstract Some lanthanide fluoride (LnF,, Ln = Ce, Nd, Sm) crystals have been grown, and fluoride ion-selective electrodes (F-ISE) have been constructed using these and some commercial LaF, single crystal membranes. Aspects of crystal growing, particularly the high-temperature dimorphism of SmF,, are addressed, :and the effects on the electrode resistance and response characteristics of crystal quality, doping and membrane geometric dimensions are studied. The results show that, while doping is effective in increasing the membrane conductivity, membrane geometric dimensions also affect the electrical resistance substantiaUy. Undoped membranes have noise-free Nernstian response to F- solutions, provided appropriate geometric dimensions are chosen. The crystal orientation of the membrane does not have noticeable effect on the performance. The crystal quality also affects the membrane resistance and performance. Single crystal membranes with minor and localized visual defects show little: difference in conductivity and no difference in performance compared with a 'perfect' single crystal membrane. Membranes grown from lower grade starting materials do not show poorer performance, although these membranes were slightly cloudy. Membranes with severe visual defects have much lower conductivity, and deteriorated performance.

Keywords: Fluoride ion-selective electrode, Cerium fluoride, Neodymium fluoride, Samarium fluoride, Membrane resistance

1. Introduction

The fluoride ion-selective electrode (F-ISE) [ 1,2] provides a relatively fast and reliable way of determining the F- concentration in aqueous solution. At the same time, it has stimulated a tremendous research effort [3--71, both to improve its performance and to elucidate its response mechanism. Although most of the research has concentrated on the F- ion transference at the membrane surface, some bulk properties such as the membrane electrical resistance and crystallinity have been studied and have been found to affect the performance of the electrode.

The most widely used sensing element of the F-ISE is the LaF, single crystal. A few other lanthanide fluorides (CeF3, PrF,, NdF3 and SmF3) have also been studied or proposed as suitable membrane materials [2,3,8,9]. This is because these fluorides are thought to have the tysonite (LaF,) crystal structure, and therefore are also likely to be good F- conductors. There are further speculations in the F-ISE literature that NdF, and SmF3 could have higher selectivity than LaF, [3]. So far, however, there have been no reports on the use of SmF3 in F-ISE membranes.

LaF3, as well as some other lanthanide fluorides, is a good ionic conductor, and at room temperature has an ionic conductivity of around to lO-'S cm-' (depending on the conditions under which the crystal is formed) [1,4-6,9]. It is generally thought that the conductivity of pure LaF3 single crystals is too low for use in the F-ISE, and that it is necessary to dope the crystal with divalent ions such as 1 5 ~ ' ~ and/or Ca+' to increase the bulk conductivity, thereby reducing noise and improving the reproducibility of potential readings [ 1,2,6,8,9]. Although it has been indicated that, with the advent of high input-impedance millivoltmeters, the problem of noisy and inconsistent signals due to the high membrane bulk resistance is virtually overcome [ 101, thlere still have been conflicting reports over the resistance and the performance of undoped LaF, membranes [4,6,9]. This suggests that there may

be other factors which affect the electrical resistance of the membrane.

It has been generally believed that polycrystalline and other nonsingle crystal materials are not suitable for the F-ISE [8]. Early studies of these materials have shown that they lead to nonreproducible response, a low detection limit and a short lifetime [2,1 I]. Among these, the short lifetime may be related to the poor chemical stability of these materials. A more recent study [I21 has shown that F-ISEs made from polycrystalline LaF3 membranes have a much higher resistance and a sub- Nernstian response to F-. On the other hand, F-ISFETs constructed using polycrystalline LaF3 thin films as the gate material have been reported to have a good Nernstian response to F- [13], with a detection limit identical to that of conventional F-ISEs with single crystal membranes. This result indicates that the poor response characteristics of the polycrystalline F-ISE membrane are not due to any difference in the formation of the surface potential, but are more likely to be due to the high electrical resistance of the membrane. This is a characteristic which has not previously been given much explicit consideration, and so merits some investigation in its own right.

In this investigation, undoped and doped CeF, and NdF3 crystals were also grown and evaluated as F-ISE membranes. We have also attempted to make SmF3 F-ISE membranes and, in this article, we address difficulties in growing single crystals of SmF3. As an alternative approach to evaluating the ISE properties of SmF3, single crystals of SmF3/NdF, mixtures were grown, and their resistivity and response characteristics were studied, and compared with those of pure NdF, single crystal membranes. We have also examined the effect of the geometric dimensions of the membrane on its resistance and performance. Finally, the effect of the crystal orientation and crystal quality on the membrane resistance have been examined, and the resistance and performance of low quality membranes are compared with high quality membranes. The deteriorated performance of low quality membranes is interpreted as the poor bulk F- transport properties of these crystals.

Electrounalysis 1995, 7, No. 10 0 VCH Verlugsgesellschaft mbH, 0-69469 Weinheim. 1995 lO40-0397/95/lOl0-930 $ 5.00+.25jO

Factors Affecting the Resistance and Performance of Fluoride Ion-Selective Electrodes 93 1

2. Experimental

2.1. Source and Quality of the Crystal Membranes and Electrodes

Potential measurements were carried out utilizing the Orion 94-09 electrode (#1) and electrodes fabricated (by the authors) from the following crystal membranes (see Table 1):

0 New commercial electrode membrane disks from two manufacturers (#2 and #3). These membrane disks were transparent, apparently flawless and had a light yellow color.

0 A membrane disk dismantled from a failed commercial electrode (#4). It was transparent, and had a light yellow color. The thickness of this membrane was modified (see Table 1).

0 Disks of pure LaF3 (#5) and LaF, doped with 1% w/w EuF2 (#6) obtained from the Shanghai Institute of Optical Instruments (P.R. China). Both types were of optical quality, with one surface highly polished. The undoped crystal was colorless and the doped crystal had a light yellow color.

0 Four NdF3 membranes cut from NdF3 single crystals grown by the authors using a Crystalox (UK) induction heater furnace. Two (#7 and #8) were cut from the same crystal and were transparent, apparently flawless, and bright purple in color. One NdF3 membrane (#9) was cut from another crystal and contained minor and localized visual defects, although it was also transparent. The other NdF3 membrane (#lo) was doped with 1% w/w CaF2.

0 Three CeF, membranes grown by the authors. Two were cut from two parts of a CeF3 single crystal separated by a crack which was formed due to a power failure during growth. One part of the crystal was transparent, and X-ray diffraction has confirmed its single crystallinity (#I 1). The second part was not transparent and had a metallic tinge (#12). The other CeF3 membrane (#13) was doped with 1% w/w CaF,.

0 Two Ndlp,Sm,F3 (#14 and #15, x=O.O5 and 0.14, respectively) membranes were cut from crystals grown from mixtures of NdF3 and SmF3. Both were dark-red and shiny. X-ray diffraction confirmed their single crystallinity.

2.2. Crystal Growth and Electrode Preparation

The Bridgman method was used for growing single crystals of

fluoride salts. An induction heater (Crystalox, UK) was employed. The induction coil was housed in a high vacuum chamber hPa), which was modified to also allow a continuous flow of He through the chamber. Thus the crystal could be grown either under a high vacuum or under a He atmosphere. A glassy carbon crucible (SIGRI) of 10 mm diameter was used to contain the melt. Since the Bridgman growth in essence is a process of controlled homogeneous nucleation, it is important for the heater to have an appropriate temperature gradient. In this laboratory, a cylindrical graphite susceptor was used to couple the RF energy and to heat the crucible. By utilizing a smaller wall thickness in the crystal- lization section of the susceptor, a temperature gradient of 60 "C/cm was generated. This temperature gradient was found suitable for growing CeF3, NdF3 and Ndlp,Sm,F3 single crystals. For growing CeF3 crystals a He atmosphere is required to reduce the evaporative loss of the melt.

Dehydrated CeF,, NdF3 and SmF3 (99.9% pure, Johnson Matthey) were used for growing CeF3, NdF3, SmF, and Ndl-,Sm,F3 crystals. Prior to melting, the fluoride powder was heated at 200 "C under high vacuum (lo-' to hPa) for 5 h to remove any adsorbed water [7,14]. The fluoride, in the glassy carbon crucible, was then heated to 30 "C above its melting point for 2 h to thoroughly remove any trapped gas bubbles, and was then lowered through the crystallization section of the susceptor at a rate of 2.25mm/h. After the crystallization was complete, the temperature was slowly lowered to 100 "C below the melting point of the fluoride, and the crystal was allowed to be annealed at this temperature for 2 h. Then the susceptor temperature was lowered at a rate of 25 "C/h to 200 "C, and finally, the crystal was cooled naturally to room temperature. Lower grade fluoride starting materials were also used for growing single crystals. These result in slightly cloudy crystals due presumably to the presence of trace amounts of oxide impurity. These crystals showed the same electro- chemical performance, however, as the highly transparent crystals. The membrane disks were cut from the crystals into the desired size and shape, and then polished to a mirror finish with diamond paste and 0.25 pm alumina.

Electrodes were made by gluing either these or the commercial membranes onto PVC barrels with silicone rubber sealant (silastic 738, Dow Corning). The electrodes were filled with an internal solution of 0.1 M NaF and 0.1 M NaC1, and an

Tdble 1 The geometry factor, resistance and conductivity of some membranes A membrane area, t membrane thickness, R electrode resistance, v membrane conductivity

No. Membrane

Commercial Commercial Commercial Commercial LaF, (Shanghai) LaF, (EuF2, I % ) (Shanghai) NdFj NdF, NdFt NdF; (CaF2, 1%) CeF, CeF; CeF, (CaF2, 1%) Nd0.95Sm0.05F3 Nd0.86Sm0.14F3

0.80 0.20 0.20 0.30 0.50 0.50

0.79 0.16 0.23 0.79 0.13 0.21 0.72 0.38 0.35

0.10 0.50 0.80 0.068 0.14 0.14

0.17 0.033 0.19 0.16 0.1 0.16 0.13 0.15 0.15

0.20 2.5 2.5 0.22 [b] 0.28 0.28

0.21 0.21 0.83 0.20 0.77 0.76 0.18 0.43 0.43

1.0 x 10'

5.8 105 5.7 lo4

7.7 104

3.0 lo5 3.3 x 10' 2.0 x 106 3.0 lo4

2.0 x 108 3.0 104 2.0 x 10'

5.8 x 10'

7.8 x 10'

7.0 x 10'

3.0 x lo6

2.0 x 10-6

3.9 x 10-6 3.6

7.0 6.3 I O - ~ 4.1

1.1 x 10-6 2.6 7.3 x 10-6 2.1 x 10-6 1.4 x

4.2 x 4.2 x

3.6 x

6.8 x

88 57 58 88 88 87

59 57 58 58 58 53 59 58 55

[a] The resistance values are the average of 8 determinations (RSD =0.73%, n = 5), except for 14 and 15, which represent the average of 2 determinations. [b] Membrane thickness is modified.

Electroanalysis 1995, 7, No. 10

932 W. Shen et al.

Ag/AgCl electrode was used as the internal reference. The electrodes were conditioned (briefly polished using 0.25 p alumina, and soaked in lOP7M NaF solution for one hour [ 151) before being electrochemically tested.

2.3. Electrochemical Measurement and Membrane Crystallinity Analysis

All reagents used in the electrochemical measurements were analytical reagent grade. The potentiometriic response curves for the home-made electrodes using doped and undoped mem- branes were generated using the following electrochemical cell:

AglAgClIKCl(sat.)lll.3 M NH4N03 /ITest solutionlMembranel (1)

The reference electrode was an Orion double-junction sleeve- type, and potential measurements of the TISAB-buffered NaF solutions of known concentration were made using an Orion Ionanalyser (Model 901). The potential reading was defined as stable if the potential variation was smaller than 0.1 mV over 2min. All electrodes (except for CeF,, #12) exhibited very good Nernstian behavior in the concentration range of lo-' to

M, with slight curvature down to M. This behavior was also observed with the Orion F-ISE (##l).

The electrode resistance R was measured using the method reported by Newman et al. [12]. The electrochemical cell above was first connected to a high impedance potentiometer (Keithley 614 electrometer, input impedance = 10'20), and the cell potential, V I , was measured in the TISAB-buffered 0.1M NaF. The cell was then shunted with a suitable high resistor, R 1 , and the new potential, V2, measured. The electrode resistance was then calculated from

0.1 M NaF, 0.1 M NaCIIAgCIIAg

= R,(Vl - V2)IVz (2) The conductivity (a) and the resistivity (61) of the membranes were calculated using

cT = t / ( R A ) (3)

p = l /u (4)

where t is the membrane thickness and A is the membrane area. The same electrochemical tests were also carried out using

some of the electrodes with reduced solution contact area. This was done by covering the external membrane surface with insulating film (water proof scotch tape), leaving ca. 1 mm2 of conditioned membrane surface for solution contact.

The crystallinity of the membranes was analyzed by Laue diffraction using a Cu X-ray source on a standard Philips X-ray generator and a Polaroid film cassette.

3. Results and Discussion

3.1. Dimorphism of SmF3 and the Response Characteristics of Ndl-,Sm,F3 Single Crystal Membranes

There has been some speculation in the IF-ISE literature that NdF3 and SmF, crystals could also be suitable membrane materials, since they are thought to have the same crystal structure as LaF, [3,8]. Some results [3] suggest that, because of their smaller unit cell dimensions, NdF3 and SmF3 might be less susceptible to interference from other ions, possibly including the hydroxide ion [16]. We have attempteal to grow crystals of CeF,, NdF3 and SmF3. Although we have successfully grown

Electroanulysis 1995, 7, No. 10

CeF, and NdF3 single crystals (both undoped and doped with CaF2), we were unable to produce SmF3 single crystals due to its high-temperature dimorphism.

The trifluorides of La, Ce, Pr and Nd occur only in hexagonal form and do not undergo structural change with temperature. Other lanthanide trifluorides may have hexagonal and ortho- rhombic forms at different temperatures [17]. The hexagonal is the stable form at high temperature and the orthorhombic form at low temperature. For SmF3, the inversion from the hexagonal to orthorhombic forms occurs at 555°C [17], well below the temperature of crystallization from the molten state (1 306 "C). Since the unit cell volume of the orthorhombic form is larger than that of the hexagonal form [3], the crystal will expand substantially upon inversion. We found this occurred in both the vacuum and the helium protected growth, resulting in crystals that were badly strained and cracked. In an attempt to preserve the hexagonal form, we first cooled the crystal down to ca. 750 "C, and then quickly moved the crystal out of the hot-zone while quenching it through the inversion temperature with fast flowing helium. This procedure was ineffective, due presumably to the low thermal conductivity of the crystal. Our experience has shown that it is extremely difficult to grow large size SmF, single crystals of the hexagonal form suitable for an F-ISE membrane under normal growing conditions (i.e., under vacuum or under low pressure of inert gas atmosphere). This is consistent with findings of other research groups [17,18].

X-ray diffraction analysis indicates that the SmF, crystal obtained by quenching is polycrystalline. SEM imaging reveals numerous microcracks. Membranes made using disks cut from the SmF, polycrystal were found either to have leaks or to be too resistive to be used in F-ISE.

To gain some knowledge of the response properties of SmF,, we decided to grow crystals from NdF, and SmF, mixtures. Since the unit cell dimensions of hexagonal SmF, are very close to those of NdF3, it is possible that mixtures of NdF, and SmF, could be grown into single crystals of a hexagonal form, thereby overcoming the dimorphism of SmF3 alone.

Ndl_,Sm,F3 single crystals (x = 0.05 and 0.14) were successfully grown. Electrochemical tests show that both compositions have the suitable conductivity, and both respond to the buffered F- solutions in a Nernstian manner (with Ndo 86Smo,14F3 slightly sub-Nernstian) and have a similar

I 1 , , 1

1 2 3 4 5 6

PF Fig. 1. The potentiometric response curves ( A E vs. pF) for Nd,-,Sm,F, electrodes (#14 and #15). The response curve of NdF3 (#7) is also presented for comparison. Nernstian slopes are given in Table I .

Factors Affecting the Resistance and Performance of Fluoride Ton-Selective Electrodes - 933

detection limit to other lanthanide fluoride membranes (Fig. 1). These crystals, however, failed to show superiority over undoped NdF3 and LaF3 in either selectivity against OH- or response time [19].

3.2. Insolubility

An important property that a F-ISE membrane material must have is extremely low solubility in water. The solubilities of CeF3, NdF3, Ndi-xSm,F3 single crystals and the quenched polycrystalline SmF3 were evaluated. This was done by immersing these crystals in distilled water for two days, and determining the F- level in the aqueous phase using a commercial F-ISE. In all cases there was no measurable change in the electrode potential within experimental error, indicating that the amount of F- released to the distilled water is very small, below the detecting limit of the F-ISE. These materials therefore meet the solubility requirement of the ISE membrane.

3.3. Effect of Geometry on Electrode Performance

When immersed in fluoride standard solutions, all electrodes made from single crystals of LaF3, CeF3 (except for #12) and NdF3 produced stable potential readings. The slope values obtained for the linear section of each of the calibration plots are listed in Table 1. Electrodes made with doped and undoped LaF3 membranes showed Nernstian behavior and gave stable potential readings even though their bulk conductivities differed by a factor of 10 (Table 1). This is in excellent agreement with the results reported by Takashi [4], but is in contradiction to those reported by Koksband and Rasmussen [9].

It is clear from Equation 3, that the membrane resistance may be affected by changing its dimensions (i.e., thickness and surface area). The present results (Table 1) show that the geometry factor ( t / A ) of the membranes of some commercial electrodes differs by a factor of more than 10, with the Orion membrane ( t / A = 0.2) being the most favorable in terms of the electrode resistance.

The resistance measurements of NdF3 membranes (#7 and #8) show that, for membranes of convenient surface area and different thickness, Equation 3 is roughly obeyed, i.e., the conductivity values calculated using Equation 3 are similar (for membranes of the same quality [6]). The marginally lower conductivity of NdF3 #11 is attributable to a minor visual defect within the membrane, and is discussed below. These results show that, for membranes of the same quality (i.e., assuming constant conductivity), the membrane resistance may be varied by a factor of up to 10 or more by controlling the t / A value.

In relation to the observation (or otherwise) of noise with the undoped LaF3 membrane, it is worth noting that the geometry factor reported by Koksband and Rasmussen [9] was 25.6cm-' (solution-contact area = 0.0078 cm', average thickness = 0.2 cm), which IS almost 2 orders of magnitude less favorable than that of the present sample, or that reported by Takashi [4] (0.35cm-'; solution-contact area = 0.283 cm2; thickness = 0.1 cm). It seems likely that this is the origin of the noise problem observed by Koksband.

Electrodes with such a high resistance [in the range of lo7 0, if a conductivity of 3.6 x 10-7Scm-' is assumed (see Table l)] may not function well even with a high-impedance laboratory millivoltmeter. To investigate this, some electrochemical measurements were made on electrodes with a reduced solution-contact area, on the assumption that this would

Table 2. The resistance and performance of electrodes with a reduced solution-contact area of 1 mm2. Original areas are given in Table 1.

Membrane Resistance f R J Area reduction Resistance and stability factor increase ,factor

# I Commercial 5.3 x lo5 1/50 1.4

#5 LaF3 4.3 x 106 1/50 5.5

Stable

(Shanghai) Stable

(Shanghai) Drifting and

#6 LaF3:EuF2 1.5 x lo5 1/50 1.9

unstable

Very noisy #9 NdF3 est. 2.5 x lo7 1/23 ca. 12.5

increase their resistance. The results (Table 2) show that Equation 3 is no longer obeyed. Presumably the charge carriers no longer travel within the cylindrical volume defined by the reduced solution-contact area and the thickness of the membrane. When the solution contact-area is reduced to 1/50, the resistance of the doped LaF, (Shanghai, #6) and the Orion electrode (#l, Table 2) increases by factors of only 1.9 and 1.4 respectively. For the undoped membranes, on the other hand, the resistance increases considerably more for the same or even smaller relative area reductions, though still not by the geometric reduction factor (Table 2). For the Orion and the doped LaF3 membranes, the smaller solution-contact area did not result in unstable potential readings, although it did take longer to establish the equilibrium potential compared with the situation where the full membrane surface is allowed to make contact with the solution. In contrast to this, the (area reduced) undoped membranes of higher resistance did produce unstable (long term drifting) and noisy potential readings.

These results are consistent with the general knowledge of the membrane resistance, which plays an important role in the electrode behavior. Electrodes can tolerate a relatively small increase in resistance without affecting their performance; with a large resistance increase, however, the performance deteriorates. Electrodes with undoped membranes appear to be more dependent upon the geometry factor, particularly the solution- contact area, than doped membranes. Thus, while doping is not essential for an electrode to have a Nernstian behavior and stable potential readings, the electrode resistance must be controlled appropriately by choosing a suitable membrane geometry.

3.4. The Effect of Crystal Orientation and Quality on the Electrode Performance

There have been inconsistent reports about the crystal- lographic anisotropy of the conductivity of the LaF, crystal. Some authors [6,20] believe that the conductivity parallel to the c-axis (all) is higher than that perpendicular thereto ( c T ~ ) , and suggest that the membrane of a F-ISE should be cut with the crystallographic c-axis perpendicular to the solution contacting surfaces [6]. Others [21], however, did not observe any difference. We have made two electrodes with two (LaF,) membranes by cutting from the same crystal, and making the crystallographic c-axis of the two membranes parallel and perpendicular to the solution contacting surfaces, respectively. Electrochemical tests have revealed that the difference in the conductivity of the two membranes was negligibly small, and we did not observe any difference in performance, either in the Nernstian slope or in the detection limit, within experimental

Electroanafysis 1995, 7, No. 10

934 W. Shen et al.

error. This suggests that the crystal orientation does not require special consideration in the F-ISE applicati'on. X-ray diffraction analysis has also shown that membrane orientations of all commercial electrodes (#1-#4) were arbitrary.

To investigate the effect of crystal qualiity on the resistance and the performance of the membrane, a CNeF3 membrane (#12) was chosen. Although Laue diffraction (confirmed its single crystallinity, this membrane contained a high density of defects and had a strong metallic tinge. Polishing the membrane demonstrated that this was not a metal deposit on the surface, and XPS confirmed that the chemical composition was identical to CeF3 (#11). Thus, we do not believe that the visual defect of this membrane was due to any contaminaiion during growth, but was most likely due to a high density of lattice dislocations caused by the sudden change of the temperature gradient resulting from a power failure. Such a crystal structure may therefore be taken as an intermediate form between a single crystal and true polycrystalline. This mernbrane had a much lower conductivity than a single crystal of CeF3 (#I 1) (Table l), and its response to F- was slightly sub-Nernstian and the potential readings were noisy. This response is typical of a high membrane resistance. The slightly sub-Nernstian slope of such a membrane may be associated with the slower response time and the fact that 'steady-state' has not been reached even with the stringent 'two-minute' criterion. A similarly low slope is seen with the low conductivity membrane consists of Nd0,86Sm0,14F3.

The conductivity of the light lanthanide fluorides is strongly dependent on the crystallinity [21,22]. These crystals are good F- conductors, and the mechanism of the electrical conduction is the transport of F- via charged anion vacancies (Frenkel type defects) [21]. Polycrystalline LaF3 has a much lower conductiv- ity (10-'2Scm-' [22]) compared to that of the single crystal ( S cm-'). Schoonman and Huggins [23] indicated that such a reduction in conductivity in the polycrystalline LaF3 is attributable to a grain boundary polarization effect. The grain boundary may be modelled as a thin 'air g:ap' in series with the bulk of the solid electrolyte and shunted by a grain boundary resistance. Thus, the higher resistance at the grain boundary reduces the mobility of F- [23]. Single crystals with visual defects contain dislocations which may have a similar effect in reducing the F- mobility as does the grain boundary in polycrystals. When the visual defects are minor and localized (e.g., NdF3 #9), they have an insignificant effect on the membrane conductivity, and do not require special considera- tion. The lower conductivity of Nd0.86Smo,14F3 compared to Ndo 95Smo,osF3 may perhaps also be explained on this basis, since it is likely to have a higher density of dlefects than the latter. Membranes with severe visual defects, however, tend to have significantly lower conductivity, and are not suitable for F-ISEs of the conventional design.

Parenthetically, it may be noted that a possible reason why the ISFET is reported to function with a (high resistance) polycrystalline LaF3 thin film is that this film is very much thinner than a regular polycrystalline membrane, and that, for an ISFET, it is the source/drain current which is measured, and this is not influenced by the electrical resistamce of the LaF3 thin film per se. The investigation of an ISFET using a film of SmF3 deposited on the gate would be interesting, and may be the only way to study the F- response of this material.

4. Conclusion

In this study, we have successfully grown single crystals of

CeF3, NdF3 and Ndl-,Sm,F3 (x = 0.05 and 0.14) for use in fluoride ion-selective electrodes. We were unable to grow SmF3 single crystals because of the fracturing caused by its high- temperature dimorphism. The response characteristics of these crystals have been evaluated and compared with LaF3 membranes, and it can be concluded that all materials exhibit suitable conductivity for F-ISE applications, and exhibit good Nernstian response in F- solutions.

The results have further shown that, by variation of the membrane geometry, the resistance of the electrode may be changed over a rather wide range (of a factor of lo), and that this may be used to control the electrode resistance. Electrodes made from undoped membranes of appropriate dimensions perform well. The resistance, and therefore the performance, of electrodes made from undoped membranes rely on the membrane geometry more than those made using doped membranes. Membranes containing minor visual defects show marginally lower conductivity, but do not show either unstable potential readings or non-Nernstian behavior.

The resistance of a single crystal with severe visual defects was found to be much higher than for 'perfect' single crystals. The response characteristics of such a crystal were slightly sub- Nernstian and noisy. A high density of dislocations in a single crystal membrane might play a role similar to the grain boundary effect in polycrystalline membranes in reducing the mobility of F- in the bulk membrane, resulting in a decrease in conductivity.

5. Acknowledgements

The authors thank the Australian Research Council for financial support, and also the Mechanical and Electronic Workshops of the Physics Department, La Trobe University, for their contributions.

6. References

M.S. Frant, J.W. Ross, Science 1966, 154, 1553. M.S. Frant, US Patent 343 182, 1969. J . Anfalt, D. Jagner, Anal. Chim. Acta 1970, 50, 23. A. Takashi, Denki Kagaku oyobi Kogyo Butsuri Kagaku 1979,47,733. Y.G. Vlasov, Y.E. Ermolenko, B.A. Nikolaev, S.V. Chernov, J. Appl. Chrm. USSR 1986,59, 1732. X. Xie, D. Jin, S. Yu, J. Wu, J. Synth. Cryst. 1989, 18, 244. R. De Marco, R.W. Cattrall, J. Liesegang, G.L. Nyberg, 1.C. Hamilton, Surf. Interface Anal. 1989, 14, 463. J. Koryta, K. Stulik, Ion Selective Electrodes, 2nd ed., Cambridge University Press, Cambridge 1983, p. 144. P. Koksband, S.E. Rasmussen, Actn Chem. Scnnd. 1985, A39, 761. W.E. Morf, The Principles of Ion-Selective Electrodes nnd of Membrane Transport, 2nd ed., Elsevier, Amsterdam, 1981, Ch. 3. A.M.G. MacDonald, K. Toth, Anal. Chim. Actn 1968, 41, 99. J.D. Newman, R. Pirzad, D.C. Cowell, A.A. Dwoman, Analyst 1989, 114, 1579. W. Moritz, L. Muller, Analysf 1991, 116, 589. W. Korczak, P. Mikolajczak, J. Cryst. Growth 1983, 61, 601. X.D. Wang, W. Shen, R.W. Cattrall, G.L. Nyberg, J.L. Liesegang, Electronnnlysis, 1995, 7, 22 1. R. De Marco, Private communication. R.E. Thoma, G.D. Brunton, Inorg. Chem. 1966,5, 1937. R.C.C. Ward, Private communication. X.D. Wang, W. Shen, R.W. Cattrall, G.L. Nyberg, J. Liesegang, unpublished. J. Schoonmam, G. Oversluizen, K.E.D. Wapenaar, Solid Stare Ionics 1980, I , 21 1 . A. Sher, R. Solomon, K. Lee, M.W. Muller, Phys. Rev. 1966, 144, 593. T. Mahalingam, M. Radhakrishnan, C. Balasubramanian, Thin Solid Films 1979, 59, 221. J. Schoonman, R.A. Huggins, J. Solid State Chem. 1976, 16, 413.

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