Journal of Alloys and Compounds, 181 (1992) 463-468 463 JAL 8038

Preparation of the metastable high pressure T-R2S3 phase (R--Er, Tm, Yb and Lu) by mechanical milling

S. H. Han, K. A. Gschneidner, Jr., and B. J. Beaudry Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011-3020 (USA)

Abstract

The preparation of the metastable crystalline high pressure polymorphs of R2Sa, where R-=Er, Tm, Yb and Lu, was investigated at room temperature by mechanical milling (MM). For Er2S3 and Yb2S3 the pure metastable high pressure T-phases were obtained by MM whereas for the TmzS3 and Lu2S3 samples the metastable high pressure T-phases coexisted with the corresponding equilibrium ambient polymorphic phase.

1. Introduct ion

Mechanical alloying (MA) is a dry, high energy ball milling, non-equilibrium process which was developed by scientists and engineers in the late 1960s for the product ion of oxide-dispersion-strengthened alloys [1, 2 ]. MA allows two materials with widely different melting points to be alloyed. More recently MA has been used to synthesize amorphous alloys [3, 4]. Mechanical milling (MM) is a similar process, except that a single component phase is used to form nanocrystalline or polymorphic materials. Recently, we found that the metastable crystalline high temperature polymorph of Dy2S3 or metastable crystalline high pressure polymorph of Y2Sa can be prepared by MM [5]. The discovery opened a new door for the synthesis of metastable crystalline polymorphs.

The objective of this paper is to repor t on the preparation of metastable crystalline high pressure polymorphs of the R2S3 phases, where R--Er, Tin, Yb and Lu, by MM.

2. E x p e r i m e n t a l de ta i l s

The R2Sa compounds, where R~Er , Tm, Yb and Lu, were prepared by the direct combination of stoichiometric amounts of each rare earth metal and sulfur in a quartz ampoule at 8 5 0 ~ 9 0 0 °C [6]. These sesquisulfides were placed in a tungsten carbide (WC) vial and sealed with rubber O-rings in a helium a tmosphere using a helium-filled glove-box. The powders were mechanically milled in a Spex 8000 mixer-mil l using three 10 mm diameter

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464

tungsten carbide balls with milling times ranging from 10 min to 10 h. The weight ratio of the balls to the powders was 5 to 1. The mechanically milled powders were characterized by X-ray diffraction using Cu Ka radiation.

3. R e s u l t s an d d i s c u s s i o n

Erbium sesquisulfide (5-Er2S3) and thulium sesquisulfide (6-Tm2Sa) are isostructural with 6-Y2Sa and have the monoclinic 6 phase type structure [7]. For Er2S3, the metastable crystalline high pressure polymorph, y-Er2Sa, was obtained by MM for 2 h (Fig. 1), whereas for Tm2S3, even after 40 h of MM, the equilibrium monoclinic 6 phase was still present together with the metastable high pressure cubic T phase (Fig. 2). In this pattern, impurity peaks from tungsten carbide (WC) vial and balls are observed, suggesting that about 5 vol.% of the sample consists of WC, the rest being R2Sa. This is consistent with the weight of WC lost by the balls during MM from the weight change observed before and after MM. The room temperature equi- librium ytterbium and lutetium sesquisulfides (e-Yb2Sa and •-Lu2S3) have the rhombohedral • phase type structure [7]. Figure 3 shows the phase trans- formation of •-Yb2Sa as a function of milling time. As can be seen, the intensity of the main peaks from the • phase drops rapidly and metastable high pressure cubic T phase lines appear simultaneously within an hour, but it takes 2 - 4 h for the • phase to be converted to the y phase. In this pattern, impurity peaks due to the tungsten carbide (WC) are" also observed. In the case of Lu2Sa, even though it underwent MM for 40 h, only a small fraction of • phase was transformed into the metastable high pressure y polymorph (Fig. 4). Although only partially successful in the cases of Tm3Sa and Lu2Sa, we have shown for four more materials that MA or MM can cause the

• I 211 ~ 310

Er2S3 / l [ t 321 420

I . -

z ~- ~ (~

J,_, . . . . "~ . . . . ~=o . . . . }s . . . . 40 . . . . 4~ . . . . 5o

20 (degree) Fig. 1. X-ray diffraction pat terns of Er2S3 as a function of milling time: pat tern a, 0 rain; pat tern b, 10 rain; pat tern c, 20 rain; pattern d, 40 rain; pat tern e, 1 h; pattern f, 2 h. The (hkl) values for the T-Er2S3 phase are shown on the X-ray pattern for the 2 h MM sample (pattern O-

465

l,-

Z LU I-- Z

/ / Tm2 S3 I

¢'4

2e (degree)

Fig. 2. X-ray diffraction patterns of Tm2S 3 after 40 h of MM. The lines marked "&" correspond to the $-Tm2S a phase, while the "7(hkl)" lines indicate the y-Tm2S 3 phase lines.

Yb2 $3

9"~ "~ ,~,~ . ¢~

I-.-

_z o (c)

2e (degree)

Fig. 3. X-ray detraction patterns of Yb2S a as a function of mil l ing time: pattern a, 0 mLn; pattern b, 10 min; pattern c, 20 min; pat tern d, 40 rain; pattern e, 1 h; pat tern f, 2 h; pattern g, 4 h; pattern h, 10 h. The (hkl) values of the y-Yb2Sa phase are shown on the X-ray pattern for the 10 h MM sample (pattern h).

equilibrium crystalline phase to transform at least partially into a metastable crystalline polymorph.

Figure 5 shows the lattice parameters of the 7-R2S3 prepared by MM (as milled) as a function of the atomic number of the rare earth element. It is seen that the lattice parameters for the MM y-R2Sa phases increase with increasing atomic number, whereas the lattice parameters for the y-R2S3 prepared by a high temperature-high pressure decompression technique [8] exhibit the normal lanthanide contraction. The anomalous lanthanide ex- pansion for the MM samples is not understood, especially in view of the normal behavior observed by Eatough et al. [8]. In this case the lattice parameters of mechanically milled R2S3 powders could be affected by the

466

mm I Lu2S3 m

>" (d)

~. . .~ ~..,...~ (c) _z

A:~t AliLlll,k 20 (degree)

Fig. 4. X-ray diffraction patterns of Lu2S 3 as a function of milling time: pattern a, 0 h; pattern b, 1 h; pattern c, I0 h; pattern d, 20 h; pattern e, 40 h. The (hkl) values for the 7-Lu2S3 phase are shown on the 40 h X-ray pattern (pattern e).

8.5 "-,,

W @ 4J ! W 8.3-

l 0 .a

8.2

8.1

R2S 3

o

* 0

o This study * Eatough ct al

o

39 66 67 68 69 70 71 ' I ~ I ' I ' ! ' I ' I ' I

y Dy Ho Ez Tm Yb Lu

Atomic Number

F i g . 5 . L a t t i c e p a r a m e t e r s o f " a s - m i l l e d " 7 p h a s e r a r e e a r t h s e s q u i s u U i d e s .

impurities (tungsten, carbon and/or oxygen), but it is doubtful that either defects or impurities could cause the lattice parameters of the MM samples to be 0.1-0.25/1, larger than those obtained on the high pressure processing samples. Lattice and point defects might cause a shift in the lattice parameters of about 0.01 /~ at the most, an order of magnitude smaller than what is observed. Furthermore, the impurity elements that might be incorporated into the R2Sa matrix during MM are all smaller than their ionic counterparts in the R2S8 phase (i.e. tungsten is smaller than the rare earth, and carbon

467

and oxygen are smaller than sulfur). Therefore, it is difficult to rationalize a lattice expansion due to impurities. Table 1 lists the volumes before [9] and after MM for all the R2S3 phases studied to date. Maximum volume change ( - 8 . 4 5 % ) was observed for e-~/phase transformation of Yb2S3.

Among the R2S3, where R--Y, Dy, Er, Tm, Yb and Lu, only 6-Tm2S 3 and e-Lu2S3 could not be transformed entirely into high pressure 7 phase. Consideration of the enthalpies of formation of the ambient equilibrium R2S3 phases may be helpful in understanding the observed behaviors. The enthalpies of formation of Tm2S~ and Lu2Sa appear to be the most negative ( - 297 + 30 kcal mo1-1) of the R2S3 phases [10], but because of the large error limits associated with these and the heats of formation values of the other R2S3 phases it is difficult to know whether this is an important factor. What would be more important and significant is to know the free energy of formation values of both the equilibrium and the corresponding metastable ~/-R2S3 phase.

For rare earth sesquisulfides, both the high pressure orthorhombic v}- polymorphs [11] and high pressure cubic y polymorphs [8] have been reported. Higher temperatures and pressures are needed to form the 7 polymorphs (about 2000 °C and 7.7 GPa) than for the ~ polymorphs (about 900 °C and 2 GPa). However, during MM the ~? phase was not observed in the R2Sa materials. Therefore the MM conditions used in this study caused the 6 phase (or a or • phase) to be transformed into the ~/phase directly. Less severe MM conditions will be studied in the future to see whether the ~7 phase can be formed.

The impact of the balls on the powders causes local temperature and pressure increases in the MA or MM process. In metallic solids the maximum temperature rise is about 350 °C [12] and in the case of ionic compounds (poor thermal conductors) it may reach 500 °C, and the maximum pressure rise is estimated to be about 1 GPa [13]. However, these local maximum temperature and pressure rises cannot explain the preparation of metastable high temperature or high pressure polymorphs by MM. The MA (or MM)

TABLE 1

Lat t ice p a r a m e t e r s o f "as-mi l led" s amp le s and vo lume changes be t ween before and af ter mechan ica l mil l ing

R2Sa a Volume of Volume of Volume (/~) "/-R2Sa r o o m change

(/~a a t o m - 1) t e m p e r a t u r e (%) phase (/~3 a t o m - 1)

y-Y2Sa 8 .340 21.75 23.60 (6) - 7.84 -y-Dy2S~ 8 .308 21.50 21.36 (a) ÷ 0.66 -/-Er2S a 8 .338 21.74 23.02 (6) -- 5.56 y-TmzSa 8 .349 21.82 22.74 (6) - 4.05 ~Yb2Sa 8 .358 21.89 23.91 (e) - 8.45 ~Lu2Sa 8.435 22.51 23.69 (e) - 4.98

468

p r o c e s s g e n e r a t e s s eve re p la s t i c d e f o r m a t i o n a n d this p l a s t i c d e f o r m a t i o n i n t r o d u c e s a h igh dens i t y o f d e f e c t s (po in t de f e c t s a n d d i s l oca t i ons ) [12l . These de f ec t s d i s to r t the la t t i ce and c o n s e q u e n t l y i n c r e a s e the f ree e n e r g y o f t he l a t t i ce [4]. B e c a u s e o f th is f ree e n e r g y inc rease , the l a t t i ce g r adua l l y b e c o m e s uns t ab l e . W h e n th is de s t ab i l i z a t i o n r e a c h e s a ce r t a in cr i t ica l po in t , the tt ( o r a o r e) p h a s e of R2Sa m a y be t r a n s f o r m e d to the m e t a s t a b l e T p o l y m o r p h wh ich is m o r e s tab le , i .e . h a s a l o w e r f ree e n e r g y t han its or ig ina l phase .

Acknowledgment

The a u t h o r s w o u l d l ike to t h a n k Nile B e y m e r for p r e p a r i n g p u r e ra re e a r t h me ta l s . W o r k a t the A m e s L a b o r a t o r y was c a r r i e d out u n d e r the s u p p o r t o f t he Office o f Spec ia l A p p l i c a t i o n s Divis ion, US D e p a r t m e n t of Energy , b y Iowa S ta te Univers i ty , u n d e r C o n t r a c t W - 7 4 0 5 - E n g - 8 2 .

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