Thermal properties of silver pyrophosphate-anomalously high glass heat capacities

Mat. Res. Bull., Vol. 21, pp. 1369-1374, 1986. Printed in the USA. 0025-5408/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals Ltd.

THERMAL PROPERTIES OF SILVER PYROPHOSPH/~TE-ANOMALOUSLY HIGH GLASS HEAT CAPACITIES S. Ananthraj +, K.B.R. Varma and IK.3. Rao Materials Research Laboratory Indian Institute of Science Bangalore 560012, India

(Received August 15, 1986; Communicated by C.N.R. Rao)

ABSTRACT Heat capacity behaviour of silver pyrophosphate has been investigated in the glassy and crystalline phases. Heat capacity of the glass is unusually large which may be of configurational origin. It is proposed that an equilibrium of complex phosphate anionic species gives rise to the observed high configurational entropies and also accounts for glass stability. Infrared spectra of the glasses which contain additional features have been used to support the presence of such equilibrium. MATERIALS INDEX : Silver pyrophosphate, anomalous heat capacity.

Introduction Several systems based on silver salts form important fast ion conducting glasses (1-5). They can serve both as model and practical materials. Silver pyrophosphate has thus been investigated in literature (2) as a part of AgI-Ag4P207 systems. Phosphates are in general good glass formers. Hence we have considered that silver pyrophosphate glass would be an interesting model system to investigate in its glassy state. In this communication we present our studies of the heat capacity behaviour of crystalline and glassy silver pyrophosphate. Infrared spectra of silver pyrophosphate in both crystalline and glassy states have also been studied in this context. 5ome of the additional absorption features appearing in the infrared spectra of silver pyrophosphate glass have been used to provide structure based explanation of the unusually high values of the glass heat capacity.

Communication No.60 from Materials Research Laboratory

+Teacher fellow at the

Department of Chemistry, Central College, Bangalore. 1369

1370

S. ANANTHRAJ, et a l .

Vol. 21, No. 11

Experimental Silver pyrophosphate was prepared starting from silver nitrate (Analar grade) and sodium pyrophosphate (Reagent grade) - by double displacement reaction in solution. Silver pyrophosphate was then dried at r v 3 9 3 K for about 6 hours. The X-ray diffraction pattern of silver pyrophosphate thus prepared (Fig.l) was found to compare well with literature data (4). Silver pyrophosphate powder was melted in a platinum crucible [m.p. 843 K (5)]. The temperature of the melt was raised to about 1080 K and the melt was maintained at this temperature for about an hour. It was poured on to a polished brass plate and quickly pressed using a similar polished brass block. The brass plate was maintained at 320 K to prevent the resulting glass discs from cracking. This type of quenching procedure gives rise to initial cooling rates of the order of 1000 K per sec (6). Resulting glasses were transparent, slightly yellow in colour and gave rise to a typical amorphous x-ray diffraction pattern (F-ig.1).

Ag{P~ 01 (fl) CrystoI (b) Gloss

FIG. 1 Powder Diffraction Patterns for AgaP20 7 (a) Crystal (b) Glass

50

40

30

20

10

2e

Heat capacities of silver pyrophosphate glasses were determined using Perkin Elmer DSC-2 differential scanning caloriemeter employing sapphire as standard. A uniform heating rate of 10 K min-1 was used throughout the measurements. T~ • Lj was evaluated as the intersection point of the extended linear regions ]n the C_ vs T plots around the glass transition elbow. Infrared spectra of both crystalline~and glassy samples were obtained in KBr pellets and recorded at laboratory temperature on a Perkin Elmer 590 I.R. Spectrometer between 380 cm-1 and 400 cm -1 . Results and Discussion Heat capacities of the glassy and crystalline silver pyrophosphate are presented in fig.2. Measurements were carried out repeatedly on various glass samples obtained from different batches. Three such representative measurements are shown in the inset of fig.2. Glassy silver pyrophosphate crystallizes immediately after its glass transition which corresponds to 453 K. This value of Tq may be compared with the corresponding melting temperature at 843 K which-gives a Tg/T m = 0.53. It was found very d i f f i c u l t to make stable heat capacity measurements beyond Tg.The

Vol. 21, No.

ii

SILVER P Y R O P HO SPH A TE

1371

heat capacities of crystalline silver pyrophosphate were quite reproducible and substantially lower than the corresponding glass heat capacities. Between laboratory temperature and T_ the heat capacity of silver pyrophosphate glass increases by about 40 3 K -! tool-S.I The most intriguing aspect of the glass heat capacity is that it is unusally high and still keeps increasing towards its T~ . The value of 320 3 K - l mol - l may be compared with Dulong-Petit heat capacity" of 324 3 K - I mol -l, calculated assuming that silver pyrophosphate consists of 13 particles. It is very unusual that all authors are not aware of any report of such large heat capacity values in phosphate glasses below their glass transition temperature. The large heat capacity in the glass could be of configurational origin. Silver pyrophosphate glass is reasonably good ionic conductor with conductivities of the order of 10-5 S cm-l at 430 K (7). Hence Ag + ions are quite mobile in the glassy phase and the glass should therefore be able to explore a large number of configurations giving rise to the observed heat capacities. However, when the Ag + ions are present in such essentially 'quasimolten' state, roughly constant (though large) heat capacities are generally observed (3) unlike the rising value of the heat capacities noted in Fig.2. Such rising heat capacities may therefore indicate the continuously increasing vibrational excitations associated with P20~-group in the glass. It appears that other explanations are possibly particularly when we compare glassy and crystalline heat capacities. There is substantial difference in the heat capacities of the glass and the crystal of silver pyrophosphate (Fig.2) as noticed

61C

A~P201GLOSS vSAM~E [ 6SAMPLE11 o SAMPLEIII

&O0

Ag~ P207

~'380~ zut

570

Tg =453K

530

3~0[ /

I 490 7

GLASS o CRYSTAL

o

i ~,

u 320I , 300~

• •~ °• ~' v~v

,o

°

450 7.

300 340i.EJpSEORA. ~20 r E(K)460 500

"~ 410

/

370 330 o o

290

~

o

330

370

oo L

410

450

I

I

490 530 570 TEMPERATURE(K) FIG, 2

I 610

Heat capacity of Ag4P207 as a

function of temperature.

=

I

650

I

690

I

730

S. A N A N T H R A J , et a l .

1372

Vol. 21, No. I i

earlier. This difference may be attributed to differences in the structures of the glass and the crystal arising from the unique chemistry of phosphates. Since the glass is prepared from its melt held at high temperature, a variety of polymeric phosphate groups are generated due to the dominant polymerisation tendency in phosphates. Typical polymerisation reactions may proceed through disproportionation reactions as follows: _-

In general, p o(n+2}- _ n 3n÷1 "='--

p

[(n-11+21- ~1- p 0{n÷1.21n-1 03(n-1}+1 n÷l 3(n+1)÷1

In such reactions both the total negative charges and the number of particles are conserved. However, longer polymeric phosphate chains can give rise to further cyclization reactions in which the number of particles increase while the total negative charge is still conserved. Thus,

0

0 U

/ 0 / PI ~'0

o=,P. ° o o.P,=o 0

0

d_ I =1~ 0!10- O- -~'/P=O • \ I/0 II

0

Such reactions not in phosphates, but with Ag + ions. We pyrophosphate glass cal entities.

° o +

0 ,

O--P=O I 0-

-

0

0

Jr 2PO~-

I/0olI O=P0P:O ~0~ I/0 / 0

only account for the easy glass formation, generally observed also generate a large variety of configurations in combination feel that the anomalously large heat capacity behaviour of silver is better understood on the basis of the presence of such chemi-

The presence of a variety of anionic species appears to be evidenced in the infrared spectra of the glass shown in fig.].The spectra of the crystalline and the glassy samples bear out some significant differences, particularly in the region of 600-900 cm;1The P-O-P deformation frequency/ at 700 cr~ 1 present in the crystalline pyrophosphate is blue shifted to 717 c m - " in the glass, while the stretching frequency particularly at 950 cm-1 is red shifted to 880 cm -1 . Stretchi_r~g frequencies in the crystalline phase give rise to a narrow band around 1100 em where as this band is significantly broadened in the glass. In addition, a shoulder also appears towards higher frequencies which can be attributed to the formation of additional PO~- groups. Formation of chain fragments and the ring structureslis also indicated by ~ome what pronounced absorption in the low frequency 500 cmregion in the glass. The complex equilibria of variety of phosphate ion species explain the relative ease of the formation of phosphate glasses, it also accounts for the enormously high heat capacities of the glassy phase of silver pyrophosphate. We may add paranthetically that other pyrophosphate glasses may behave similarly. It is significant in this context that the glass is light yellow in eolour, a feature that is normally associated with the presence of orthophosphate groups. Silver pyrophosphate crystals on further heating exhibit a solid state phase transition noted earlier by Takahashi (2). The transformation is associated with significant rise in heat capacity prior

Vo].

21, No.

ii

SILVER PYROPHOSPHATE

1373

Ag4PzO 7

(o) Crystal (b) Gloss

~

'

~

I i 3800

3200

2600

2000

1700

1400

{

~

v

b

I

1100

~

~

(a)

)

I

BO0

I

500

I

200

WAVENUMBER(cm-~) FIG.] I.R. Transmission Spectra of Ag P O7 (a) Crystal (b)b'O~ass/.

to the transition. The rise in C indicates an initial co-operative character of the transition which however culr~inates in a first order like behaviour(8), The cooperative feature in the crystalline heat capacity prior to phase transition may be due to the onset of disorder of Ag + ions in pyrophosphate lattice, The high temperature phase of silver pyrophosphate may therefore be expected to be a good fast ion conductor. Conclusions S i l v e r pyrophosphate forms a single component glass relatively easily. Glasses are characterized by very high heat capacities which can be attributed to configurational origin. The unique chemistry of phosphates enhance this feature. The glass formation in pyrophosphate is likely to involve a complex equilibria of oxyanions and the infrared spectra appear to support such equilibria. Crystalline pyrophoshate exhibits phase transition at 623 IK and the phase transition is preceded by second order like increase in the heat capacity.

Acknowledgement Thanks are due to Professor CoN,R, RaG for his kind encouragement, References 1. T. Minami, Y.

Takuma and M. Tanaka, J. Electrochem Soc., 2/4, 1659 (1977).

2. T. Takahashi, S. Ikeda and O. Yamamoto, J.Electrochem Soc., 119, 477 (1972).

1374

S. ANANTHRAJ, et al.

Vol. 21, No. 11

3. H. Senapathi, (3. Parthasarathy, S.T. Lakshmikumar and K.J. RaG, Philos Mag B., 47, 1, (1983). 4. A.S.T.M. Cards File No. 11-637. 5. T. Yamada and H. Koizumi, Journal of crystal growth., 64, 558 (1983). 6. Unpublished results from this laboratory. 7. S. Ananthraj, K.B.R. Varma and K.J. RaG, in preparation. 8. C.N.R. RaG and K.J. RaG 'Phase Transitions' in Solids. McGraw H i l l International Book Company, New York (1979).