DEFECTS IN PbFCl

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DEFECTS IN PbFCl

A. Halff, J. Schoonman, A. Eijkelenkamp

To cite this version:

A. Halff, J. Schoonman, A. Eijkelenkamp. DEFECTS IN PbFCl. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-471-C9-473. �10.1051/jphyscol:1973977�. �jpa-00215453�

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JOURNAL DE P H Y S I Q U E Colloque C9, supplktne~zt au no 11-12, Totlle 34, Nouetnbre-Dkcembre 1973, page C9-471

DEFECTS IN PbFCI

A. F. HALFF, J. SCHOONMAN a n d A. J. H. EIJKELENKAMP Solid State Department, Physical Laboratory,

University of Utrecht, Sorbonnelaan 4, Utrecht, The Netherlands

RbumC. -La conductivite ionique de monocristaux de PbFCI pur et dope a Cte mesurCe.

La conductivitk perpendiculaire a I'axe c est superieure a la conductivite parallele a cet axe d'envi- ron trois puissances de 10 a la temperature ordinaire. Les nombres de transport ont etC determines entre 510 et 550 K. Les experiences rnontrent que t1.1, est inferieur a 0,03, ce qui signifie que

I F 4- tcl = I. La conductivite de PbFCl a etk comparee a la conductivite de PbC12 (defauts de Schottky) et de D-PbFl (defauts anioniques de Frenkel). Cette comparaison, la conductivite des cristaux dopes et des considerations de structure sont en faveur d'un desordre thermique de Schottky.

Abstract. - The ionic conductivity of uiidoped and doped PbFCl single crystals has been measured. The conductivity of PbFCl l c-axis exceeds at room temperature the conductivity

11

c-axis by about three powers of ten. Transference numbers have been determined in the temperature region 510-550 K. The experiments revealed that t1.1, is less than 0.03, which nieans that t~

+

^tcl⁼^{1 .}

The conductivity of PbFCl has been compared to the conductivity of PbClz (Schottky defects), and D-PbFz (anion Frenkel defects). This comparison, the conductivity of the doped crystals and structural considerations favour a thermal disorder of the Schottky-type.

1. Introduction. - In literature scarce attention has been devoted t o the ionic conductivity of mixed halides of the type P b F X ( X = Cl, Br). These crystals have a layered tetragonal structure. The unit cell consists of plane sheets perpendicular to the c-axis with layer sequence

F, Pb, C1, CI. Pb, F .

The great similarity in the mass transport processes it1

PbCI, and PbBr, [I], [2] suggests identical mass transport processes in PbFCl and PbFBr. Since only transference n u ~ n b e r s for anions in PbFBr are known [3], viz. t , = 1 - t,, = 0.87 (520-570 K), it has been assumed [4] that PbFCl is a pure anionic conductor too. In order t o test this assumption we have measured transference numbers for the anions in undoped and doped PbFCl samples. Moreover, ionic conductivity experiments on single crystals doped with thallium, o r bismuth have been performed. The experimental results and a critical examination of the crystal structure lead t o the conclusion that Schottky defects prevail in PbFCI.

2. Experimental. - The preparation of ~ ~ n d o p e d and doped PbFCl cryst:~ls and a description of the conductivity equipment has been publisl~cd elsewhere

PI,

[51, [61.

Transference numbers of the ions in undoped and doped PbFCl were measured in the temperature region 510-550 K in a cell of the type

where X i = PbFCI. The right-hand part of this cell is used as a coulometer. In this cell-arrangement pressed tablets were used, whereas the ionic conductivity was studied along and perpendicular to the c-axis, using single crystals.

3. Defect chemistry of PbFCI. - In a previous paper we have introduced the following scheme for a Schottky-type of disorder in PbFCl [4],

with

Ks = ( Kscl K ~ ~ ) ~ ' ~

.

(4) Here 0 denotes the perfect lattice, Vli, a lead ion

\,acnncy (erective charge - 2 g), V; a fluoride- and V:, a chloride-ion vacancy (effective charge

+

^(1).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973977

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C9-472 A. F. HALFF, J. SCHOONMAN AND A. J. H. EIJKELENKAMP The incorporation of monovalent cations at lead

ion sites Me;,, (effective charge - q ) affects the concen- tration of the lattice defects in all three sublattices, MeCl -+ Me',,

+

^xV,,

+

(1 - x) V i

+

Cl,", (5)

~ 1 ; denotes a normal occupied chloride ion site (effective charge zero).

The various equilibrium constants and the total electroneutrality condition can be represented by

K~~ = CV;bl

C V ~ I ~ ((9

Kscl= CV;bI cV,II (7)

K, ⁼CV;bI CV,I CVEIl (8) and

From eq. (4) and these relations we can derive

and for the transference numbers

and

Relation (12) holds for the region where mixed anionic conduction prevails.

4. Experimental results. ^-In figure 1 we have plotted the ionic conductivity of undoped P-PbF,, PbCI,, PbFCl

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c-axis) and PbFCl (I c-axis). The ionic conductivity of PbFCl is highly anisotropic.

In figure 2 the ionic conductivity of undoped crystals and crystals doped with thallium, or bismuth is presented. Conductivities here referred to are along the c-axis. The magnitude of the conductivity of undoped samples strongly depends on the remainder impurity content. The discrepancy between the results obtained with undoped samples, which exists in figures I and 2 is due to a different amount of remainder impurities.

The conductivity increases with respect to the conductivity of undoped PbFCl by incorporation of monovalent impurities, and decreases upon doping with trivalent impurities.

In the temperature region 490-550 K the transference number for the lead ions is less than 0.03.

In figure 3 the temperature dependence of t,, is plotted. The dopes (TI, Bi, 0 ) are indicated in the figure. In figure 4 the logarithm of t,/r,, is plotted versus reciprocal temperature.

FIG. 1. - The ionic conductivity of undoped PbFCl

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c-axis), PbFCl (1 c-axis), PbClz and B-PbF2.

The experiments show, that t , is not dependent on the type of dope (see eq. (11)). Moreover, the results in figure 4 are in good agreement with eq. (12).

5. Discussion. - The experimental results clearly demonstrate that PbFCl is an anionic conductor.

Monovalent cations on lead ion sites require for charge compensation either halide ion vacancies, or interstitial lead ions. Charge compensation provided by interstitial lead ions can be ruled out in view of

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DEFECTS IN PbFCl

FIG. 2. -The ionic conductivity (// c-axis) of undoped crystals, and crystals doped with thallium, or bismuth.

0 0 PbFCI-TI, 244 ppm (a)

VV PbFCI-TI, 89 ppm (4)

x x undoped PbFCl (I)

++

PbFCI-Bi,

-

¹⁷ ^ppm ^(a)

A A undoped PbFCl (11) PbFCl-Bi,

-

22 ppm (b) (a) PbFCl (I) used as starting material.

(0) PbFCl (11) used as starting material.

FIG. 3. - The temperature dependence of the transference number for chloride ions in undoped tablets of PbFCI, and in tablets doped with thallium, oxygen and bismuth.

the present results. The decrease in the conductivity upon doping with trivalent cations can be understood by charge compensation provided by lead ion vacancies,

FIG. 4. - Transference numbers for anions in undoped and doped PbFCl plotted as log (f~lfcl) versus T-1. The doped samples are indicated in the curve (Bi, 0, TI).

o r both interstitial fluoride and chloride ions. The centres of the largest interstitial sites available in PbFCl are found a t positions oo o,

4 +

^v^with^{v =}^0.038^[4].

These interstitial spaces allow ions with radius of 0.45

A.

The radii of the ions involved are 1.26

A

(Pb2+ in 8 coordination), 1.32

A

( P b 2 + in 9 coordination), 1.36

A

(F-), and I .81

A

(Cl-). In lead chloride (Schottky defects) the niaximum available interstitial space is 0.7

A,

and in p-PbF, (anion Frenkel defects) 0.61

A.

The conduction activation enthalpy, AN,,,, in the region where chloride ion vacancies are responsible for the mass transport in PbFCI : 0.28 eV

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c-axis) is comparable to the activation enthalpy for chloride ion vacancy migration in PbCI, : 0.34 eV. I n the region where fluoride ion vacancy migration pre- dominates in PbFCl, AH,, has the value 0.53 eV.

This value is smaller than the value for AH,, for fluoride ion vacancy migration in /I-PbF, : 0.70 eV.

The conductivity results support the idea that PbFCl exhibits anionic conduction via anion vacancies, whereas structural considerations as compared to PbCI, and /I-PbF, lead to a preference for lead ion vacancies as charge compensating species in PbFCl doped with trivalent cations. Therefore the thermal disorder in PbFCl is assumed to be of the Schottky- type.

References

[I] DE VRIES, K. J., V A N SANTEN, J. H., P/IJJS;C(I 29 (196.7) 482. [4] SCHOONMAN, J., DIRKSEN, G. J., BLASSE, G., J. Solid Stcit~

Clienl. 7 (1973). July : in press.

[2] SCHOONMAN, J., J. SolicI S t o t ~ C I I P I ~ . 4 (1972) 466.

151 . . SCHOONMAN, J., J. Solid Stute Cl~et~r. 5 (1972) 62.

[3] LANDOLT-BORNSTEIN, Zn/lletl~t~~rt(, I ~ I I ( / F i i t ~ k f i o ~ i ~ ~ t ~ )). 6 (61 HALFF, A. F., SCHOONMAN, J., DIRKSEN, G. J., to be Auflage. Teil 6 . Elektrische Eigenschaften I, p. 244. published.