The crystallization of vitreous and metastable Pb5Ge3O11

Journal of Crystal Growth 42 (1977) 574—5 78 ‘0 North-Holland Publishing Company

THE CRYSTALLIZATION OF VITREOUS AND METASTABLE Pb5Ge3O11 K. NASSAU, J.W. SHIEVER, D.C. JOY and A.M. GLASS Bell Laboratories, Murray Hill, New Jersey 07974, USA

Vitreous Pb5Ge3O1 1 was prepared by water quenching and by roller quenching. The crystallization was studied by DTA, X-ray diffraction, and TEM. After a glass transition at 325°C,meta-stable Pb5Ge3O1 1 crystallized exothermically at 370°C.This was subsequently followed by the formation of the stable phase of the same composition, again exothermicafly at 490°C.

a = 10.271, c = 10.718 A, Z = 3,hexagonal R3C; two forms (not further specified) were said to occur. Gouju et al. [3] gave orthorhombic parameters for Pb3Ge2O7 (a = 4.265, b = 13.350, and c = 5.950 Am the text; a 8.815, b = 3.665, c = 7.830 A in the

1. Introduction The formation of glasses and metastable phases in the PbO—Ge02 system is so common that it has proved difficult to establish the equilibrium phase diagram and the composition of the compounds occurring. We report a study of the crystallization of vitreous Pb5Ge3O11, which we find to pass via a metastable phase to the well-known ferroelectric stable phase. In fig. 1 are included parts of the proposed PbO—~ Ge02 phase diagrams of Speranskaya [1], Philips and Scroger [21, and Gouju, Fournier and Kohimuller [3]; it can be seen that apart from the existence of the congruently melting PbGeO3, there is no agreement in the region shown. Speranskaya [1] also reported Pb6GeO8 which the others did not observe. In fact, only Speranskaya [1] notes even the existence of Pb5Ge3O11, a compound which can be readily grown in single crystal form from the melt. The other two phase diagrams show instead Pb3Ge2O7, one congruently melting [2], the other mncongruently melting [3]. Mydlar, Nowotny and Seifert [5] obtained a twinned crystal of uncertain composition (in the Pb2GeO4 to Pb3Ge2O7 range) and determined it to be hexagonal with a = 10.23 and c = 10.61 A, possibly C6/mmm or C6/m. The powder pattern agreed with the PbGe2O4 of Merker and Wondratscheck [6] and of Argyl and Hummel [7], and the compound was also assumed to be identical with the previous reported Pb5Ge3O1 1 [1] and Pb3Ge2O7 [2]. Bordeaux and Lajzerowicz [8] reported Pb5Ge3O1 with

Pbo: Ge02

SYSTEM

800

—1800

]

700 1:1

5:3

3:1

60040 ~ SPERANSKAYA 1959

1.1

700 4:1

3.2

40 60 80 PHILLIPS AND SCROGER 1965

800

800

T 700~—

700 ~

I

31

5:3 5:3 600k 60 3~2 3:1 STABLE 600 GOUJU, FOURNIER, AND KOHLMULLER 1968 HASEGAWA ‘ SHIMADA, AND ~—... 32 KOIZUMI 1973 ~ETASTABLE ORDINATES ~o 80 e~ ~o 400 ABSCISSAE - Mo~% Pbo THIS WORK Fig. 1. Various PbO—Ge02 phase diagrams showing the Pb5Ge3O1 region. 574

K. Nassau et al.

I Crystallization of vitreous and metastable

abstract) and Hasegawa, Shimada, and Koizumi [9] gave hexagonal a = 10.16 and c = 19.37 A for the same composition. There does appear to be general agreement (e.g., refs. [4,9,10]) on the structure of the melt-grown form of Pb5Ge3O11, with lattice parameters near a = 10.25 and c = 10.7 A, hexagonal P3, also consistent with the parameters of others [5,8]. Finally, Hasegawa et al. [9] have examined the glass to crystal transformation near the Pb5Ge3O11 composition as discussed in detail below.

I -

~

~ +

cally with boiled distilled water at room temperature.

3. Results Both roller quenched and water quenched samples gave essentially identical results. A typical DTA curve

~325

-

~,

r

VITREOUS Pb5Ge3O1j

-

~

cryst

T

~49O METASTABLE Pb5Ge3O11

2. Experimental Samples were prepared by grinding and sintering stoich.iometric amounts of reagent grade oxides. The powder was melted in a platinum crucible and immersed into cold water to produce bulk vitreous material over 10 mm across and several mm in thickness. Some melt was also quenched in a roller apparatus as described by Chen and Miller [11]; this is estimated to give cooling rates of over l0~°C/secand yielded 1/2 X 1 cm flakes 10—20 pm thick. Differential thermal analysis was performed on a Dupont 990 system at 10°C/mm in flowing N2. Powder X-ray diffraction was performed on a Philips Norelco diffractometer using Ni-filtered Cu radiation. Transmission Electron Microscope (TEM) examination was performed on samples which had been chemically thinned in HF—HNO3. A Philips EM300 machine was operated at 100 kV. Low incident beam current and long exposure times were used to avoid charging of the uncoated specimens. Dark field micrographs werewere taken to obtain a measurewith of the grain size; these found to be consistent the angular width of the spots and rings of the electron diffraction patterns. Several samples were prepared for each annealing schedule and each was thinned. Dielectric studies were performed on each specimen to confirm the state of the material before TEM examination. Densities were determined pycnometri.

5Ge3011

-

0

DTA I0~C/min,N2 I I 100

200

~

Tm° 720

STABLE CRYSTALLINE

M L

Pb5Ge3O11 I

I

I

I

I

300

400

500

600

TOO

TEMPERATURE

800

1°c)

Fig. 2. DTA curve of vitreous Pb5Ge3O1 ~.

is shown in fig. 2. After a glass transition Tg = 325°C there are two exotherms at ~ = 370°C and Ttrans = 490°C, followed by the melting endotherm at Tm = 720°C.After the solidification exotherm on cooling, only the Pb5Ge3O11 ferroelectric transition Tf = 178°Cremained. The diffraction traces of fig. 3 were taken on the as-prepared glass, after heating to 450°Cto obtain the metastable crystalline phase, and after heating to 550°Cfor the stable crystalline phase. The stable Pb5Ge3O11 powder pattern could be fully indexed on the basis of the previous published data a = of 10.25 and c = 10.685 A, V° 972.2 3. Inwith the case the1 metastable phase, indexing on a A hexagonal cell with a = 10.19 and c = 19.34 A as shown in table 1 was achieved without reference to other phases. The density of the vitreous and metastable phases were measured to be 7.15 and 7.39 g/cm3, respectively, both better than ±0.10,close to the density of 7.33 g/cm~for stable Pb 5Ge3O1 1 reported by Hasegawa [9]. Our dielectric constants measurements had shown [12] that the low frequency dielectric constant rises sharply from about 30 in the glassy phase to over 300 at the transition to the metastable phase and then drops rapidly to below 100. The glassy material becomes instantly opaque at the first transition, mdi-

576

K. Nassau et a!.

/ Crystallization of vitreous and metastable Pb5Ge3O i Table 1 Metastable Pb5Ge3O11 (hexagonal, 3) a A,c/a1.898, V= 1739 A

U

kPb STABLE 5Ge3OII~3OI1

H

METASTABLE Pb5Ge3O11

/:EOU~~ Pb5 Ge3o11

I 27

I 29

I

35 DEGREES 28 Fig. 3. Diffractometer traces of vitreous, metastable, and stable Pb5Ge3Oi 1~ 25

31

33

cating a very rapid growth of the crystallites to sizes comparable to the wavelength of light (-~0.2pm). The metastable phase is not ferroelectric. The dielectric constant again rose sharply to about 10,000 at the transition to the stable ferroelectric Pb 5Ge3011. The development of ferroelectricity with increasing gram size was studied in some detail [12]. TEM examination of these samples was also performed and the results are shown in fig. 4. After heating for some minutes at 450°C, the metastable phase showed a wide range of crystallite sizes from less than 100 A to several pm in size, as judged from dark field micrographs (not shown) and the electron diffraction pattern of fig. 4a. After only one minute at 520°C, essentially all

=

10.19 A, c

=

19.34

hk~1

d (caic.)

d (obs.)

I (obs.)

10.0 20.0 00.5 20.3 21.0 21.2

8.825 4.412 3.868 3.641 3.335 3.153

8.84 4.41 3.855 3.65 1 3.340 3.160

m m m m ms w

21.3 30.0 40.0 10.9, 40.3, 31.5 32.0 32.2 41.0 40.6 50.3 33.0 42.0 11.11 10.12,51.0 41.7, 51.1

2.962 2.942 2.206 2.087 2.025 1.982 1.926 1.821 1.702 1.698 1.668 1.662 1.585 1.580

2.960 2.950 2.202 2.088 2.026 1.984 1.924 1.819 1.701

vs vs vw vw vw m m m

1.665

vw

1.582

vw

traces of the metastable phase had disappeared; from the dark field image the crystallite size is less than 30 nm, while from the electron diffraction ring widths of fig. 4b it is larger the 5 nm. Clearly multiple (presumably homogeneous) nucleation has occurred within the large metastable phase crystallites, with a nucleation rate large compared with the growth rate. After a total of 4 mm at 523°C,micrographs and fIg. 4c indicate coarse crystallites in the pm range once again, this time of the stable phase.

4. Discussion Hasegawa et al. [9] had in fact, obtained a very similar DTA curve to that of fig. 2 and made the novel suggestion that during the crystallization of glassy compositions near PbsGe3Oii, the compound Pb3Ge2O7 first forms from the glass and then converts near 500°C back to PbsGe3Oii, also shown in fig. 1. Their primary reason for this attribution was the identification of the metastable phase from the identity of its powder pattern compared to that of their own Pb3Ge2O7. However, since essentially

K. Nassau

~

Ct al.

/ Crystallization of vitreous and metastable Pb5Ge3O1

~

__________________________________________________

identical powder patterns can be obtained for Pb2GeO4, Pb 3Ge2O7, and Pb 5Ge3011 compositions, such an identification has little significance. The X-ray data of table I for metastable Pb5Ge3O11 were in essential agreement with the powder patterns of the Pb3Ge2O7 of Hasegawa et al. [9] and of Philips and Scroger [2] as well as the Pb2GeO4 of Argyle and Hummel [7] and of Merker and Wondercheck [6], and also the unit cell parameters given by Hasegawa 3et density al. [9] (10.16, 19.37 and our unitA). cell Using theor7.39 g/cmof Hasegawa for metastable parameters those Pb 5Ge3O11 gives the rather unsatisfactory calculated Z = 5.4 for the number of molecules per unit cell; using Hasegawa’s “Pb3Ge2O7”, this gives a caclulated Z = 8.8, but now his segregation of PbO into the glassy phase would result in the presence of a much denser component and would significantly reduce the Z = 8.8 value, which is far away from a plausible Z = 6. Gouju’s [3] unit cell parameters for “Pb3Ge2O7” with 3, Z =depending 1 give a on calculated density of 4.31 or 5.77 which set of parameters is used. g/cm of these values of Z or calculated densities are None satisfactory, and single crystals may be needed to resolve the inconsistencies. The mechanism proposed by Hasegawa et al. [9] would require the formation of the metastable phase to involve the disproportionation:

I,

2Pb 5Ge3O1 i (glass)

3Pb3Ge2O7 (cryst)

—~

Fig. 4. TEM diffraction patterns of Pb5Ge3O1 1: (a) metastable phase; (b) stable phase after 1 mm at 520°C;(c) stable phase after 4 mm at 523°C.

PbO (glass)

and the formation of the stable phase would involve 3Pb3Ge207 (cryst) + PbO (glass) 2Pb5Ge3011 (cryst). Both reactions would thus depend on the diffusion of PbO in the vitreous phase: away from the crystallizing grains in the first case, and back into the grains again in the second case. Against this mechanism must be held the extremely fast nature of the observed transformations. It will be noted that both DTA exotherms in fig. 2 are quite sharp, significantly sharper, in fact, than the melting peaks. Again, the rapid formation of opacity at ~ makes it unlikely that a diffusion process is involved. The definitive evidence, however, lies in fig. 4b and in photomicrographs. Crystallites a few nm in diameter have formed after only one minute at 523°Cwithin pm crystallites of the metastable phase. This would require the completion of -÷

_____

+

578

K. Nassau et al.

/ Crystallization of vitreous and metastable Pb5Ge301 1

diffusion over pm distances in I mm and would imply unreasonably large diffusion coefficients of these large ions at such low temperatures. The simplest and obvious explanation is that the only phases involved are vitreous Pb5Ge3011, metastable Pb5Ge3011, and stable Pb5Ge3011. Since all have the same composition, extremely rapid transformations are not unexpected. One puzzling phenomenon is why essentially the same powder pattern can be obtained over such a wide range of compositions and why so little agreement exists as to the phase diagram. Here the probable answer would be that equilibrium had not been achieved by any of the investigators with the possible exception of Speranskaya [1]. Apparently, the rapid phase transitions observed in the present work with stroichiometric Pb5Ge3011 do not occur at other compositions. On this basis, the last segment of fig. 1 shows our interpretation of both our own and Hasegawa et al.’s [9] results, namely, the existence of Pb5Ge3O1 1 in two forms, metastable and stable. The existence of Pb2GeO4 and Pb3Ge2O7 should accordingly remain “unproven” for the time being. There is little doubt that only with the preparation of metastable single phase will the unit cell and other uncertainties be fully resolved.

subsequent formation of stable ferroelectric Pb5Ge3O11 at 490°C.Both transformations are exothermic, indicating the metastable nature of both the vitreous and the intermediate phases. Some details of the transformationsare given.

Acknowledgements We wish to thank H.S. Chen for the use of his roller quenching apparatus. References [1] El. Speranskaya, Bull. Acad. Sci. USSR, Div. Chem.

Sci. 59 (1959) 145. [2] B. Philips and M.G. Scroger, J. Am. Ceram. Soc. 48 (1965) 398.

[3] D. Gouju, J. Fournier and R. Kohhnuller, Compt. Rend. (Paris) 266 (1968) 1063. [4] K. Sugii, H. Iwasaki and S. Miyazawa, Mater. Res. Bull.

6 (1971) 503.

[5] M. Mydlar, H. Nowotny and K.J. Seifert, Monatsh. Chem. 100 (1969) 191. [6] L. Merker and H. Wondratschek, Glastech. Ber. 30 (1957) 471. [7] J.F. Argyle and F.A. Hummel, J. Am. Ceram. Soc. 46 (1963) 10. [8] D. Bordeaux and J. Lajzerowicz, Bull. Soc. Franc. Mineral. Cryst. 92 (1969) 383.

[9] H. Hasegawa, M. Shimada and M. Koizumi, J. Mater. 5. Conclusions Vitreous Pb5Ge3O11 has been found to crystallize rapidly at 370°Cto an intermediate phase with the

Sci. 8 (1973) 1725. [10] H. Iwasaki, Oyo Butsuri4l (1972) 93. [11] H.S. Chen and C.E. Miller, Rev. Sci. Instr. 41(1970) 1237. [12] A.M. Glass, K. Nassau and J.W. Shiever, to be published.