Distribution of hydroxide ions in doped KCl crystals

J. Phys. Chrm. Solids. 1974, Vol. 35. pp. 600-603.

DISTRIBUTION

Pergamon Press.

Printed in Greal Brilain

OF HYDROXIDE

IONS IN DOPED

KCI CRYSTALS

T. G. STOEBE Institute of Physi&, University of Sao Paulo, Sao Paulo, Brazil and Department

of Mining, Metallurgical

& Ceramic Engineering *, University Washington 98 195, U.S.A. (Received

INTRODUCTION In a previous publication[ I], possible distributions of hydroxide impurity ions in alkali halides have been discussed based on available data[2-61. Additional information now allows a further delineation of defect structures present in KCI crystals containing divalent cation and hydroxide ion impurities. This is based primarily on the work of Jannuzzi[7], who studied the ionic thermocurrents (ITC) soectrum of KCl:Ca:OH and KCI : Ca crystals containing several concentrations of each-impurity; the results were also checked in KCI: Sr: OH and KCI : Sr crystals. In these experiments Jannuzi observed that several sets of ITC peaks appear in certain of these samples, as summarized in Table I. The 220°K ITC peak has been established as being due to divalent impurity-vacancy dipoles[8]. and the drastic decrease in the intensity of this ITC peak in crystal A, which contains a I : 2 ratio of Ca to OH, has been reported elsewhere by Jannuzzi et a/.[91 as confirming a reaction to form a Ca(OH),-type of complex in this material. Cappelletti and Fieschi[lO] have examined the ITC spectrum in similar crystals, observing no ITC peaks at all after annealing atbOO”C followed by a rapid cool; they were able to see an ITC spectrum only if these samples were fast cooled from 600°C. as is also noted in Table 1. Of primary interest here is the appearance of additional ITC peaks in certain OH-doped samples. In particular, the 247°K peak, as reported by Jannuzzi, is present primarily in crystals which contain divalent impurityvacancy dipoles in addition to OH ion impurities, but where the I : 2 ratio of Ca to OH has not yet been reached. Hence, this peak is most intense in crystals E and C, while in crystal A its intensity decreases, and in crystal D it disappears (as does the dipole peak), presumably due to Ca(OH), precipitation in the lattice. In crystals E and C, the intensity of this 247°K peak decreases with increasing time of thermal treatment at 400°C. Januzzi reports the reorientation energy of this peak as 0.91 eV in KCI: Ca: OH and 1.04eV in KCl:Sr:OH, with relaxation time pre-exponential factors of the order of 10-17-10-1Bsec; his dielectric loss experiments in the KCI : Sr: OH crystals showed an activation energy of 0.90eV with a pre*Permanent

address.

l4May

of Washington.

Seattle,

1973)

exponential factor of 5 x lo-“sec. These values may be contrasted with Januzzi’s divalent impurity-vacancy dipole reorientation values of 0.64eV using ITC in KCI : Ca and of 0.65 eV using both ITC and dielectric loss in KCI: Sr, values which agree well with those in the literature[l, IO. 1 I]. Jannuzzi notes that the energies and preexponential factors in the OH-doped crystals correspond well with the anion vacancy motion energy of 0.95 eV and pre-exponential factor of IO-“set in KCI [l2]. DEFECT

EQUILIBRIA

These results may be interpreted using the defect reaction equilibria developed earlier [ I]. In general, the overall defect reaction which occurs in crystals containing such impurities will involve divalent impurity ions, F,,,+, cation vacancies, V,,,- and substitutional hydroxide ions, (OH),rO. where the subscripts indicate the cation (M) or anion (X) site of the species and the superscripts indicate the specie’s charge with respect to the neutral M+Xlattice. Lacking hydroxide impurities, the primary defect reaction may be written as F.\r++ h-

* (Fuh,)”

(1)

where the product divalent impurity-vacancy dipoles give rise to the 220°K ITC peak. When the reaction involves hydroxide ions, further complexes are formed according to F,,,++v,,,-+2(OH)x” or

* {F,,,(OH),v.,,}“+

(OH).?

F.\,++~,\,-+22(OH)x”

* IF,,,(OH).~(OH).rV,,,}”

(2) (3)

where some of the possible lattice forms of the product complexes have been presented earlier[ I]. A further reaction has been suggested[3]. involving the formation of an (OH),- ion on one anion site, which is accompanied by the formation of an anion vacancy 2(OH),“ * [(OH)&-+ V,‘. This results in a further reaction which gives finally F.\,++h,-+2(OH)x”

* {F,\,[(OH),I,YI~+

(4)

(V,,,V,,.)“. (5)

Technical Notes

601

Table I. ITC peaks observed in KCI crystals doped with divalent cations (F) and hydroxide ion impurities. levels given are those concentrations added to the melt in mole fraction Investigator Jannuzzi, fast cooled from 400°C

F:OH ratio

Crystal

Composition

c B

I X IO-” CaCI,, I X IO+ CaCI,.

0.5 x lO-3 KOH 1.0 x IO-” KOH

2: I I:1

A

I X 10e3 CaCI,,

2.0 x IO-:’ KOH

I:2

D

I X IO-:’ CaCI,,

3.0 x IO-” KOH

I:3

Cappelletti t-101.. fast cooled from 400°C heavily OH doped* Cappelletti ef al., fast cooled from 600°C

Doping

ITC peaks observed 220°K 247°K - 200°K

Yes ( 197-204”) Yes (190”) No No Yes (200.209”)

Yes Yes

Yes Yes

Yes*

Yes*

Not

No

No Yes

No No

*Intensity small. ?Reappears after room temperature aging for one month. *Inferred from Jannuzzi’s results. This involves the formation of a cation-anion vacancy pair. which may or may not be bound to the divalent impurity-(OH),ion pair. Other possible reactions giving free cation vacancies have been discarded since such reactions would lead to an increase in the ionic conductivity, the opposite to what is observed[2,3]. The experiments of Jannuzzi were performed after a 400°C anneal followed by a fast cool to room temperature. Ionic conductivity results[3] indicate that impurityvacancy-OH reactions occur in the range 300-400” for KCI crystals containing the impurity concentrations used by Jannuzzi and the reactions given above probably proceed in equilibrium at the annealing temperature, although they may also occur during fast cooling. The formation of the final reaction product given in equation (5) is supported by the precipitation reaction that occurs when the I : 2 ratio of Ca to OH is reached, a conclusion that is valid at least in relatively heavily doped samples such as these. The product in reaction 5 is also the only one of the reaction products involving defect species with possible reorientation energies such as those observed by Jannuzzi, as discussed below. DISCUSSION

The similarity of the observed reorientation energy and the frequency factor of the 247°K peak with those of free anion motion is surprising, since such an anion would have to be associated with a dipole to give rise to an ITC peak. The defect involved cannot be the anion in a cationanion vacancy pair, as the resultant reorientation energy should be higher (a theoretically determined energy for a Cl- ion iumo into a vacancy pair is 1.15 eV, while that for a K+ ion jump into the pair is i.30 eV [ 131; experimentally. the value may be even higher[ 121). Similarly, it is unlikely that this reorientation involves cation vacancies, since the motion energy of the cation vacancy is close to 0.7 eV [ I41 while that of cation vacancies bound to divalent impurities in KCI (forexample, the divalent impurity-vacancy dipole) is even smaller[7.8]. Further, there is no reason to expect that the cation vacancy in the products of reactions 2 or 3 would encounter any additional difficulty in re-orientation. since there is more than enough room in the KCI lattice to

accommodate the OH- impurity ion as it substitutes for a single Cl- ion, and motion of the cation vacancy should be essentially independent of the presence of OH impurities in such complexes[ I]. For the case of reaction 5. the analysis is somewhat more complex. The (OH),- ion, located on one Cl- ion site, is probably somewhat too large for that site[ I]. This may be partially balanced by the electrostatic binding expected between the (OH),- ion and the divalent impurity F+. The F+ ion will still provide binding for the cation vacancy, V.,,-, and this binding may in fact be strengthened by the need to provide more space for the (OH),- ion in the lattice. This may be especially favored since the cation vacancy can assume a nearest neighbor position to the (OH),- anion. providing directly the extra lattice space required by the (OH),- ion. If the anion vacancy is also closely bound a possible lattice configuration of the complex is shown in Fig. I(a). Here, the actual charges of each species. rather than the effective charges, are shown for clarity. The anion vacancy position is shown corresponding to that of a recent Z, center model[ 151, the center being modified in this case by the presence of the (OH),- ion; other possible nearest neighbor cation and anion vacancy positions are also shown. If the anion vacancy is tightly bound as in Fig. l(a). it would probably give reorientation energy values similar to those of a vacancy pair, which are higher than the observed values. However, the strohg binding of the F+-(OH),--I/,,,- part of the complex, mentioned earlier, could in fact lead to loose binding of the anion vacancy. In this case, the Vx+ ion could move about in next-nearest and next-next-nearest neighbor positions of the (OH),ion and/or the cation vacancy V,w-, some of which are shown in Fig. I(b). This would give relatively free anion vacancy motion, which would give rise to a reorientation energy approximating that of a free anion vacancy, as is observed. Another possibility here is that the observed reorientation takes place by the motion of the strongly bound cation vacancy in nearest neighbor positions of the (OH),ions, which are also indicated in Fig. l(a). This reorientation could be more difficult than that for dipole reorienta-

Technical

(a)

Notes temperature will produce an excess concentration of lattice vacancies. Hence after the quench. some free dipoles will remain. as will the simple products of reactions 2 and 3 which will have been formed during the quench. The complex formation in reaction 5 probably would not form under these circumstances, since there is insufficient time during the quench for the formation of the required anion vacancy (equation (4). In the products of reactions 2 and 3 more lattice space is available and the cation vacancy may be able to move more freely than in an impurity-vacancy dipole. Under these conditions. the energy of reorientation will be less and the ITC peak temperature lower[S]. as observed. These new peaks anneal out rapidly, however, as precipitation is highly favored in such heavily OH-doped crystals.

SUMMARY AND CONCLUSIONS

(b)

Fig. 1. Possible defect models for the product of reaction [5]. Actual charges shown: divalent impurity F*+, double hydroxide ion (OH)L-, cation vacancy V, and anion vacancy V.Y. (a) Tightly bound positions for V.r, the primary position indicates as and other similar posibound positions for V,,, shown as Ic’,,I ions as [*j; tightly and similar positions asO. (b) Loosely bound positions for Vs. others farther away not shown. Similar positions for-mare shown as:+:.

q

tion, although the available results are not sufficient to allow a choice between this possibility and that of loosely bound anion vacancy motion. This general model for the defect species responsible for the 247°K peak is supported by related evidence. The intensity of the 247°K peak decreases with annealing time, which probably indicates the migration of these F+(OH),vacancy complexes through the lattice (entailing separate cation and anion vacancy-assisted motion) to form a larger F(OH), precipitate with no ITC signal. As larger complexes form. there is less need for the bound vacancies to provide lattice space, and the vacancy pair can dissociate from the complex: these vacancy pairs probably anneal out either during heating or during the subsequent quench, since no ITC signal for vacancy pairs has ever been observed. The precipitation process is accelerated by higher concentrations of OH ions in the lattice and by higher concentrations of the complex, leading to the observed decrease of the 247°K peak in the crystal containing the I : 2 doping ratio (crystal A) and its absence for the I : 3 doping ratio (crystal D). The other, simpler products of reactions 2 and 3 may also form initially, as evidenced by the appearance of ITC peaks in the range l90-210°K (Table I). In particular, in crystals containing an excess of OH ions, such as those of Cappelletti et al., annealing at 600°C will break up some of the OH ion precipitates, while quenching from that

The observations of additional ITC peaks in OH-doped samples are unique and have been shown above to provide indirect evidence for the formation (during quenching) of simple impurity-OH-vacancy complexes (reactions 2 and 3). as well as for the formation of the larger complexes represented by reaction 5. Hence this data provides further experimental confirmation of the defect reaction models presented earlier [ I]. The details regarding specific defect placement and relative defect complex binding are not clear, however, since the information available from the ITC experiment alone is insufficient for the develop“model”. The defect placements ment of one specific noted in Fig. I encompass only the most likely ones; a further delineation would require additional data from other types of experiments capable of determining the immediate environments of ions in this complex, as was utilized in the earlier analysis [ I]. Furthermore, the above results apply only to KCI doubly doped with OH and Sr or Ca, and it is not clear if they may be applied more widely. No similar peaks were observed by Laj[4] in LiF or NaF containing divalent cation impurities and OH ions, a result which may be explained by the smaller size of the Fluorine ion compared to (OH),-, and the resultant inability of reaction 4 to proceed in fluorides. ITC studies in other OH-doped materials would be of interest to provide further information in this area. Acktlolc,ledgements-This work was partially supported by the U.S. Food and Drug Administration, Bureau of Radiological Health, under research grant RLOO 125-05.

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