Calorimetric study of lead oxide Pb3O4 phase transition

Mat. Res. Bull. Vol. 14, pp. 1275-1279, 1979. Printed in the USA. 0025-5408/79/101275-05502.00/0 Copyright (c) 1979 Pergamon P r e s s Ltd.

CALORIMETRIC

STUDY OF LEAD OXIDE Pb304

PHASE TRANSITION

P. GARNIER, J.F. BERAR, G. CALVARIN Laboratoire de Chimie-Physique du Solide (E.R.A. au C.N.R.S.) Ecole Centrale des Arts et Manufactures Grande Voie des Vignes, 92 290, Ch~tenay-Malabry, France

(Received August 2, 1979; Communicated by E. F. Bertaut)

ABSTRACT The heat capacity of Pb304 has been measured in the range 80 - 280 K. No anomaly has been observed at the temperature of the crystallographic transition (170 K), between the tetragonal and orthorhombic phases. However a deviation related to a pretransitional effect occurs in the range 150 - 220 K. The results are compared to those obtained for other physical properties.

Introduction A ferroelastic phase transition has been observed for Pb304 at 170 K by X-ray powder diffraction (I). The cell is tetragonal at high temperature and orthorhombic below 170 K. The evolution of the a and b cell parameters versus temperature is shown on figure I. The lattice distortion of the low temperature phase is considerable : the rate I00 (a-b)/a is equal to 1.2 percent at 164 K, 4 at 130 K and 7.4 at 30 K (2). The transition is continuous and is preceded by a pretransitional effect, which is characterized by a gradual decrease of the volumic thermal expansion and by a continuous broadening of some particular diffraction peaks (2). This effect appears at a temperature which varies, according to the crystallisation state of the sample, in the range 220-250 K. (3). As early as 1929 , R.W. Millar has measured the heat capacity of Pb304 at 15 temperatures in the range 71-273 K ; no anomaly was observed then

(4) (figure 2).

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P. GARNIER, et al.

Vol. 14, No. 10

Our recent crystallographic results and the significant distortion which occurs at ]70 K suggested to us, a new study of the behavior of the heat capacity of Pb304 at low temperature in order to compare it to known variations of other physical properties

.

Experimental results We used a commercial product (Merck) annealed at 775 K during I0 days. The crystallites of the product we obtained, always exhibit some domains in which the lattice is slightly distorted, whatever the duration of annealing (3). A preliminary differential thermal analysis does not reveal any emission of heat at the temperature of the crystallographic transition . The heat capacity was measured on an adiabatic calorimeter at the C.E.N. of Fontenay-aux-Roses (5,6) . About ]40 measures of heat capacity were carried out in the range 80 - 280 K . We have calculated the mean value of C by a P last square method over small ranges of 5 K. The results are written out on figure 2 and table ] Cp ( J. K-1. moleJ)

9,1 140

120

100

8,6 80,

~5 T(K) 100

150

200

250

FIG. I Evolution of the cell parameters (a and b) of Pb304 at low temperature

306

'-,I

I

100

i

I

140

'

|

i

180

I

220

i

I

~0

I

T (K)

FIG. 2 Heat capacity of Pb304 (e Millar's results ; o : Present work)

Taking into account the rather bad thermal conductivity of Pb304 the relative accuracy varies from I per cent at 80 K to 2 per cent at 300 K. Our C values are lower than Millar's by about I0 per cent . The principal thermodynamic quantities are listed in table 1.

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L E A D OXIDE

TABLE

1277

1

Thermodynamic quantities for Pb304

T

Cp j . K -1 m o l . - I

80 85 90 95 00 05 I0 15 20 25 30 35 40 45 50 55 60 65 70 75 80 185 190 195 200 205 210 215 220 225 230 235 240 245 25O 255 260 265 270 275 280

67.3 70.8 73.9 76.8 79.8 82.6 85.3 88.1 90.7 93.3 95.1 97.4 100.5 102.3 104.9 106.7 109.6 11.5 13.1 14.8 16.2 17.6 18.7 19.6 20.4 21.3 22.1 22.6 23.2 24.2 24.7 25.4 26.5 26.7 27.6 27.8 28.2 28.3 29.0 29.2 29.5

S - S80 j . K -1 m o l . -1

0 4.18 8.32 12.4 16.4 20.4 24.3 28.1 31.9 35.7 39.4 43.0 46.6 50.2 53.7 57.2 60.6 64.0 67.3 70.7 73.9 77.1 80.3 83.4 86.4 89.4 92.3 95.2 98 0 100 8 103 5 106 2 108 8 111 5 114 0 i16 6 119 0 121.5 123.9 126.3 128.6

i03 (H - H 80) J. mol.

-1

0 0,345 0.707 I.II 1.54 2.00 2.49 3.02 3.56 4.15 4.75 5.39 6.05 6.74 7.45 8.20 8.95 9.75 10.6 11.4 12.2 13.1 14.0 15.0 15.9 16.8 17.8 18.8 19.8 20.8 21.8 22.9 23.9 25.0 26.1 27.2 28.3 29.4 30.5 31.7 32.9

So, unlike crystallographic study, thermodynamic results do not reveal with certainty the existence of the phase transition. No peak appears on the heat capacity curve. Neverthele~ a small deviation from a "normal" variation is observed in the range 150-220 K. Though the deviation between the experimental points and the "normal" variation (in doted line on figure 2) is small (= 2.5 per cent) this deviation is significant.

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Vol. 14, No. 10

This anomaly was not observed by Hillar (4) but we confirm it, qualitatively, by a second experiment carried out on a microcalorimeter M.C.B. (Arion, France), a flux-meter type working as a differential scanning calorimeter . The enthalpy associated to this anomaly has been evaluated from figure 2 by a planimetric method

: AH = (85 ~ 10) J. mol. -I i.e. an entropy of AS = (0.46 ±

0.07) J. K -I mol. -I . These values are extremely small for a phase transition .

Discussion The lack of latent heat and the quasiregular variation of the heat capacity confirm that the phase transition of Pb304 is continuous and diffuse . We must notice that the interval of the deviation of the capacity (150-220 K) coincide with the interval of transition and pretransition observed by X-ray diffraction. The volume curve of Pb304 exhibits a similar deviation in the same range of temperature as shown on figure 3 . The evolution of scalar quantities like V(T) and C (T) does not reveal, with certainty,Pthe existence of the phase transition; on the other hand the variation of tensorial quantities (2) (strain, thermal expansion) proves its existence without doubt.

~3 510

5o9,5 509 t / I /

508

5O7,5

Thermodynamical results indicate however that the difference in lattice energy between the two phases (tetragonal and orthorhombic) is small; this can explain that a slight deviation at the stoichiometry, or the presence of impurities could generate a slight lattice distortion of the tetragonal phase as we shown it previously (4,7).

Let's now compare our crystallographic and thermodynamic results with those obtained by other technics. 507 J.P. Huvenne (8) notice at low temT (K) perature the appearance of new liI I I I I 0 50 100 150 200 250 30O nes on Raman spectrum, corresponding to degeneracy lifting during the teFIG. 3 tragonal-orthorhombic transition. They do not observe any notable Evolution of the cell change of the vibration frequencies. volume of Pb304 . We must compare the latter point with the lack of important anomaly concerning the heat capacity. Hence the transition of Pb304 is probably not associated to a soft mode . According to J.R. GAVARRI and al (9) the transition is due to a problem of steric hindrance between the lone pairs of electrons of the bivalent lead atoms; when the temperature decreases this steric hindrance involves a rotation of the chains (parallel to the ~ crystallographic axis)

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LEAD OXIDE

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of octahedra -~[PbO6] and a strain of the lattice. This steric effect which can modify the polarisability of some atoms, probably originates in the intensity decrease of some particular lines of Raman diffusion at about 220 K (8) . We can also consider that this effect involves the loss of inversion centre in the point group of symmetry and accounts perhaps for the existence of the pyroelectric current, reported by ~ v o z c h i k o v (IO) below 220 K with a maximum at 195 K In conclusion it is remarkable to graphic transition takes place at sical properties from about 220 K (2), appearance of a pyroelectric ticular lines of Raman scattering pacity curve .

notice that though most important crystallo170 K we begin to observe anomalies in phy: decrease of thermal expansion coefficient current (IO), decrease of intensity of par(8) and at last the deviation of the heat ca-

The authours thank M. J. DANAN (C.E.A. de Fontenay-aux-Roses) and K° CHHOR (Laboratoire de Chimie-Physique du Solide - Universit~ Paris XIII) for their assistance during the experiments

Bibliographie 1

P. GARNIER, G. CALVARIN, D. WEIGEL,

C.R. Acad. Sc. Paris C. 275, 211, (1972)

2

P. GARNIER, G. CALVARIN, D. WEIGEL, J. Solid State Chem. 16, 55 (1976).

3

R.W. MILLAR, J. Amer. Chem. Soc., 51, 207 (1929)

4

P. GARNIER, G. CALVARIN, D. WEIGEL, J. Solid State Chem., 26, 357 (1978).

5

J. DANAN, R.R. CONTE, Note C.E.A., n ° 954 (1968)

6

J. DANAN, Th~se Nancy (1972)

7

P. GARN!ER, Thgse Paris

8

J.P. HUVENNE, G. VERGOTEN, B. BONIFACE, F. FLEURY, P. LEGRAND, Phys. status solidi (a), 48, 417 (1978).

(1978)

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S.A. POTACHOV,

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(a),