Article
Reference
Synchrotron powder diffraction study of the low‐temperature lattice distortion of PbMo
6S
8FRANÇOIS, M., et al.
FRANÇOIS, M., et al . Synchrotron powder diffraction study of the low‐temperature lattice distortion of PbMo
6S
8. Journal of Applied Physics , 1994, vol. 75, no. 1, p. 423-430
DOI : 10.1063/1.355868
Available at:
http://archive-ouverte.unige.ch/unige:129470
Disclaimer: layout of this document may differ from the published version.
1 / 1
Synchrotron powder diffraction study of the low-temperature lattice distortion of PbMo
6S
8M. Frangoisa) and K. Yvon
Laboratoire de Criytallographie. 24 quai Ernest Anserniet, UniversitM de Geneve, CII-1211 Gnetve, Switzerland
D. Cattani and M. Decroux
Departinent de Phrsique de la Matiere Condensee, 24 quai Ernest Ansermet, UniversitM de Genlve,
CHW1211 Generve, Switzerland
R. Chevrel, M. Sergent, and S. Boudjada
Laboratoire du Solide Inorganique Moleculaire, URA CNRS 1495, Unikersite de Rennes I, Avenue du Genjral Leclerc, F-35042 Rennes Cedex, France
Th. Wroblewski
Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY ANotkestrasse 85, D-2000 Hainbwg, 52, Germany
(Received 8 March 1993; accepted for publication 27 September 1993)
Synchrotron and Guinier x-ray powder diffraction data as a function of temperature on a superconducting PbMo6S8 sample (T,= 14.7 K) showed evidence for a subtle structural phase transition at T,= 140 K, leading from a rhombohedral high-temperature to a triclinic low-temperature modification. The transition is incomplete and presents a hysteresis of about 110 K. The triclinic cell parameters obtained at 130 K, a=6.534(9) A, b=6.532(9) i, c=6.529(9) A, a-89.27(8), t3=89.12(8)0, y=88.97(8)Y, show that the distortion affects mainly the angles (ba/ar=0.3%) and less the edges (6a/a,<O.1%). This result contrasts with previous neutron diffraction work [Phys. Rev. 35, 5365 (1987)] where the distortion was found to be much greater and to affect both the angles and the edges (ba/a,= 1.2%, ba/ar 1.2%), and to be of the same order of magnitude than in the MMo6S8 (M=Sr,Ba,Eu) analogues (MiBa: 6a/ar= l.2%,6a/ar= 3.5%). Guinier data on an oxygen containing sample having a lower superconducting transition temperature (T,= 10.5 K) showed a smaller distortion that started at lower temperature (T1= 100 K).
1. INTRODUCTION
PbMo6S5 (called PMS hereafter) is of interest for the development of superconducting wires because of its large upper critical fields.' However, up to now the critical cur- rents Jo obtained have not yet reached values large enough for practical applications. The mechanisms limiting J, in these wires are not yet understood. Recent investigations on the homogeneity of the critical temperature Tc have revealed a rather broad range from about T,= 15 K to T,. 8 K.2 Generally, low Ta's in PMS are attributed to oxygen contamination,3 but some low-T, samples exam- ined were not significantly contaminated by oxygen.4 An- other reason for the degradation of To (and hence Jc) could be the occurrence of a structural phase transition at low temperature. Such transitions are known to occur in MMo(Ss analogues containing divalent metal cations such as M.= Ca,SrBa,Eu. They lead from a rhombohedral room-temperature (RT) modification to a triclinic low- temperature (LT) modification, and suppress superconductivity.5',6 Diffraction evidence for a similar transition in PMS was first reported from high-resolution neutron powder data by Jorgensen and Hinks7 and by Jor-
'3Permanent address, Laboratoire de Chimie Minerale, Laboratoire as- soci6 au CNRS 158, Universit& de Nancy 1, B. P. 239, 54506, Vandoeu- vres les Nancy Cedex, France.
gensen et al. 8 The samples were superconducting at T= 14.5 K, and their triclinic lattice distortion at 10 K [a= 6.5759(9) A, b=6.5383(9) A, c=6.4948(8) A, a=88.516(l1)', /3=89.604(11)0, Y=89.298(13)Y; ba/ar
= 1.2%; 6a/a,=o 1.2%] was as large as that in the nonsu- perconducting analogues MMo6S8 (M=Ba: 5a/ar= 1.2%,
/alar=3.5% at 173 K9). The transition was sample de- pendent, and up to 30% of the RT phase remained un- transformed at 10 K. According to the authors the lattice transformation coincided with a minimum of the hexago- nal lattice parameter, Chex, as a function of temperature, which occurred at Tjz,110 K. X-ray diffraction experi- ments have so far failed to confirm these results, presum- ably because of a lack of resolution.
We thus decided to perform x-ray powder diffraction experiments of highest resolution on two PMS samples, one having a relatively low oxygen content and a relatively high critical temperature (T..= 14.7 K), and the other hav- ing a relatively high oxygen content and a relatively low critical temperature (T,= 10.5 K). Preliminary results were presented elsewhere. 10
II. EXPERIMENTAL DETAILS
A. Synthesis and characteristics of the samples Samples of nominal composition Pbo.95Mo6S7 (called S7) and PbMo6S8 (called S8) were prepared by sintering
A 1994 American Institute of Physics 423 J. Appl, Phys. 75 (1), 1 January 1994 0021 -8979/94/75(1 )/423/8/$6.00
TABLE 1. Characteristics and refined lattice and profile parameters of PbMo6S8 . samples S8 and S7.
Sample code S8 S7
Nominal composition PbMo6Sg Pbo,95Mo6S7
Reaction temperature (°C) 1600 1460
Oxygen content, x -0.02(2)a 0.30(2)'
T. (K) 14.7 10.5
ATE (K) 1 0.3
Vha (A3) at 290 K 840.48(2) 835.32(2)
a4, (A) at 290 K 6.5440(2) 6.5302(3)
arh () at 290 K 89.221(5) 89.553(5)
Chbahex at 295 K 1.2498(2) 1.2391(2)
FWHM (0 6 0),ex at 295 K 0.160 0.238
'Estimated from the lattice parameters on Fig. 1 of Ref. 3.
element mixtures as described before." The sulphur- deficient sample S7 was synthesized at 1460'C in a quartz tube where it was allowed to pick up oxygen, while the stoichiometric sample S8 was synthesized at 1600'C in an oxygen-free environment in a boron nitride crucible sealed by thermal compression. These different treatments re- sulted in samples having different superconducting transi- tion temperatures and different cell parameters as shown in previous work,12and also other different properties.13 Both samples were single phase as judged from x-ray Guinier powder diffraction. Their oxygen contents x were esti- mated by linear interpolation of their hexagonal cell pa- rameter ratios c/a as a function of oxygen content (see Fig.
I in Ref. 3). The values found, x=0.0 (S8), and x=0.3 (S7) were normalized according to the chemical formula PbMo6S8-xOx by assuming that oxygen was partially sub- stituting sulfur in the structure. For sample, S7, this lead to the composition PbMo6S7700.3 which differed somewhat from its nominal composition Pbo 95Mo6S7. Auger spectroscopy12 showed the presence of oxygen in sample S7 but not in sample S8. The superconducting critical temper- atures were determined by specific-heat measurements, and found to be lower for the oxygen-contaminated sample S7
(T,= 10.5 K) than for the oxygen-free sample S8 (T,= 14.7 K). The width of the specific heat anomaly, however, was larger in the oxygen-free sample S8 (AT,= 1 K) than in the oxygen-contaminated sample S7 (AT,= 0. 3 K). The main characteristics of these samples are summa- rized in Table I.
B. X-ray Guinler diffraction
Preliminary powder diffraction patterns of samples S7 and S8 were recorded on a high-resolution Guinier diffrac- tometer equipped with a closed-cycle He refrigerator1 4 in the temperature range 298-10 K, in steps of 10 K, during about 2 h for each temperature, in the theta range 14W-50', and in steps of 0.005' in 0, by using monochromated
[Ge(l111l)] Cu Ka, radiation (A = 1.54056 A) and silicon as an internal standard.
Twenty parameters (2 scale factors, 1 zero position, 5 background, 2 cell, 8 profile, 2 overall temperature factors) were refined by using the Rietveld method'5 and the DBWS program. 16 The diffraction profile parameters were defined by assuming a pseudo-Voigt function S of type
32 3
0 *< ;Gumimer data
24 cooling
4 0
0 40 80 120 160 200 240 280 T(K)
FIG. 1. Average line broadening (Lb, for definition see text) of the x-ray Guinier patterns of PMS as a function of temperature for oxygen-rich (Tr= 10.5 K) sample S7 (filled circles) and oxygen-free (T,= 14.5 K) sample S8 (open triangles) during cooling.
S= 7L(FWHMk,20k) + ( 1-77)G(FWHMk,20k), (1) where L and G are Lorentzian and Gaussian functions, respectively, 77 is a mixing parameter, and FWHMk the full width at half maximum of the Bragg reflection at the po- sition 20k, as expressed by the empirical formula
(FWHMk)2=Utan 2ok+V tan Ok+ W. (2) From the refined profile parameters i, U, V, W, an av- erage line broadening, Lb(T), at the temperature T was defined as
Lb(T)=[ I; FWHM,(T)-I FWHM,(290 K)
| /
E FWHMk(290 K), (3)
where 7 is the summation over k= 155 Bragg reflections.
A plot of Lb as a function of temperature is shown in
Fig. 1.
Lattice parameters as a function of temperature as re- fined by the Rietveld method are shown for both the rhom- bohedral and the hexagonal setting in Fig. 2. The limited resolution of these data did not allow us to refine triclinic lattice parameters below the structural phase transition (see Sees. II C and III B). Nevertheless, the accuracy of the values obtained (1.0X 10-4), was higher by a factor of 2 compared to that estimated for the previous neutron dif- fraction data,7 which were also based on a rhombohedral cell model.
C. Synchrotron radiation experiments
In order to increase the experimental resolution, sam- ple S8 was studied by synchrotron radiation as a function of temperature [powder diffractometer at HASYLAB beamline B2;17 0-20 geometry; ,t=1.21218(2) A;
Si( 511) monochromator, instrumental resolution Ad/d>6.0X 10-4 between sin 0/A=0.2 and 0.8 A--;
(Ad/d)max-8X 10-5 at sin 0/X-'0.33
A-l].
The temper- ature was decreased, first rapidly from 295 to 200 K at a rate of 100 K per hour, and then slowly from 200 to 25 K at a rate of 5 K per hour. In order to economize measuring424 J. Appl. Phys., Vol. 75, No. 1, 1 January 1994 Francois et al.
(a)_
s8
100 200
(A) 6.54
6.53
6.52 V
,n --
~
(I)a) E0
80)
a 89.6
0
89.4
89.2 F
1 89.0 L
300 0
T(K) T(K)
FIG. 2. (a) Hexagonal ahe,, chh and (b) rhombohedral arh, arh, cell parameters of PMS as a function of temperature for samples S7 and S8 (from Guinier data). Solid line: approximation by quadratic function. Note that below 140 K the data approximate a triclinic lattice.
time, only selected diffraction regions were investigated.
The intensity profiles of the following reflections were mea- sured: [(0 6 O)hex, (3 2 4)hex, (2 3 2)hex, (4 1 O)hex, and (3 2 l)hex] which should split, and (0 0 3)hex and (0 0 6) hex, which should not split during a structural phase transition leading from R3 to P1 symmetry. As expected, the former reflections were found to broaden while the latter remained essentially unchanged. The sample was then heated from 25 to 295 K at a rate of 10 K per hour by measuring the hexagonal line profiles (0 0 3)hex,
(0 0 6)hex, (1 1 0)hex, and (0 6 0)hex, In addition the (0 2 l)hex, (2 1 O)hex, and (3 2 )hex lines were record- ed at 40 K and the (3 2 l)hex line at 80, 130, 160, 180, and 295 K.
A profile analysis based on the pseudo-Voigt function (1) was made by using the W20 pattern deconvolution program, 8 which yielded angles, half-width parameters and integrated intensities for "deconvoluted" diffraction profiles. The profiles were found to be nearly Lorentzian (77 close to 1) as expected for a high-resolution instrument, and their superposition was regarded as a fit to the exper-
imental data (called "fitted" profile). The contribution of the sample to the half-widths, FWHMpMS, was deduced from that of the instrument, FWHMi.,t, (measured with a silicon standard) and the experimental values, FWHMeXp, as defined by
FWHMpMs=FWHMexp-FWHMinst. (4)
The line positions obtained were subsequently used to calculate rhombohedral and triclinic lattice parameters at five different temperatures, T=80, 130, 160, 180, 295 K, by a least-squares program (LATCON19) based on either 5 reflections (80 and 295 K) or 14 reflections (130, 160, and 180 K). Their integrated intensities were used to esti- mate the relative amount of rhombohedral and triclinic
phases present in the sample (see Sec. III D).
111. RESULTS AND DISCUSSION A. Guinier data
The room-temperature powder pattern of sample S7 is more diffuse than that of sample S8. Selected half-width
J. Appl. Phys., Vol. 75, No. 1, 1 January 1994
11.492 -
11.490 [
11.488 I
11,486~
E0
8.
0
11.402
11.400
11.398
(b2 -
S8
arh
9.19
9.18
9.17
9.16 _ 0
S7
(Crh
4,~ ~ 0 1 r
S8
100 200 300
z le
S7
I
Francois et al. 425
parameters are shown for the reflection (O 6 O)hex in Table I. They suggest that sample S7 is less homogeneous than sample S8, in agreement with the results of the specific- heat measurements. 11As the temperature is decreased, the average line broadening of the diffraction patterns of both samples first remains constant and then increases abruptly (see also Fig. 1). This abrupt increase indicates a lattice distortion due to a structural instability and/or strain. The transformation temperatures estimated from Fig. 1 are T.= 140(5) K for sample S8 and T,= 100(10) K for sam- ple S7. Interestingly, the line broadening at 10 K is greater in sample S8 (Lb=32%) than in sample S7 (Lb=20%), which suggests that the lattice distortion is sample depen- dent and decreases as a function of oxygen content.
The thermal lattice expansion based on a hexagonal model as shown in Fig. 2 is qualitatively similar to that reported previously.7 As the temperature is decreased, Chex goes through a shallow minimum and then increases. The data suggest that the lattice expansion is sample dependent.
The expansion along chex is stronger for S8 chex(20 0 K) 15cheX/5T=-0.27(2) l0- K-' than for S7
CheC(2 0 0 K) '5Chex/, T-,5 -0.22(3)l0-5 K-', in contrast to the expansion along ahe which is weaker for S8
ahex(3 0 0 K) 'bahe,/5T=1.19(2)I 50 K--l than for S7
ahex(30 0 K)-'6ahex/6T=1.43(3)i0-5 K-'.
B. Synchrotron data
The diffraction line shapes of sample S8 during heating undergo characteristic changes, as can be seen from the measured, deconvoluted, and fitted profiles of the hexago- nal reflections (O 6 O)hex and (O 0 6)hex at various temper- atures shown in Figs. 3(a)-3(e). The profiles measured at 80 K [Fig. 3(a)I are representative for those measured between 25 and 120 K. In that temperature range, the line broadening of (O 6 Q)hex is about 200%, while that of (O 0 6)hex is about zero, as expected for a triclinic lattice distortion which splits (O 6 O)hex into triclinic (2 2 4)tn, (2 4 2)tri, and (4 2 2) t and leaves (O 0 6)hex unsplit. Note that the experimental profiles at these temperatures show
neither splitting nor shoulders despite the very high reso- lution of the instrument. Thus it was not possible to define triclinic but only rhombohedral line positions and FWHMpMs parameters in this temperature interval. The patterns measured at 130, 160, and 180 K [Figs. 3(b)- 3(d)] are representative for those measured between 130 and 250 K. Both (O 6 O)hex and (O 0 6)hej, have complex shapes and shoulders. In that temperature interval only triclinic line positions (but no FWHMpMS parameters) could be defined. The profiles measured at 295 K [Fig.
3(e)] are representative for those measured between 270 and 295 K. The diffraction profiles in that temperature interval showed no shoulders, and thus allowed rhombo-
hedral line positions and FWHMpMS parameters to be de- fined. In view of these findings two different structure mod- els had to be used for profile simulation in three temperature domains as shown in Fig. 4. In the tempera- ture intervals 40 K<TC120 K and 270 K<T<295 K (called "R3") the FWHMpMS values for (O 6 0)hex, (00 6)hex ( Ii O)hex, and (O 0 3)he, were used to define the profile parameters in terms of a single-phase aniso- tropic strain model (see Sec. III C), whereas in the tem- perature range 130 KE<T<250 K (called -R3+PP"), a strain-free two-phase model was used (see Sec. III D).
C. Single-phase anisotropic strain model
In the temperature intervals 40-120 K and 270-295 K the line broadening of sample S8 was interpreted in terms of anisotropic strain on the rhombohedral lattice. The (O 0 6)hex and (O 6 0)hex reflection profiles were used to define two separate sets of profile parameters U. V, W, be- cause the program used'6 did not allow us to parametrize the profile anisotropy. A strain parameter ehk1 was defined from the so-called Williamson-Hall plot,2 0 following Eq. (5)
FWHMpMsXcos 01A=KhkI/DhkI+4e hkl1XSin 01/, (5) where the strain ehkl, the domain size D(A), and the Scherrer constant Khkl depend on the crystallographic di- rection [hkld. Results for the 40 K data are plotted on Fig.
5 for the reflection families (hkO), (001), and (hkl). The slope of the dashed lines is proportional to the strain ehkl in the various [hkl directions, i.e., ehko=el > ehk1> eO1=el,
Thus, the strain is strongest perpendicular to the threefold axis [el (40 K)=0.0500 or 0.00087 radian] and decreases as one goes away from this direction (el[ =0.020 or 0.00035 radian). It does not appear to significantly change between 40 and 120 K, as suggested by the relatively constant FWHMpMs values shown in Fig. 4. The fact that the var- ious lines in Fig. 5 tend to intersect at zero excludes a possible size effect responsible for the line broadening. The domain size D estimated from Eq. (5) is greater than 1000 A.
D. Two-phase model
The patterns of sample S8 as measured in the interme- diate temperature region [130, 160, and 180 K; see Figs.
3(b)-3(d)] were deconvoluted in terms of a two-phase model which assumed the coexistence of a rhombohedral and a triclinic phase. The lattice parameters of both phases and their relative abundance did not change significantly in this temperature region. At 130 K the refined parameters of the rhombohedral phase are ath=6.5 3 3(1) A, arh=89.111(5)0, while those of the triclinic phase and its relative abundance (t) are summarized in Table II.
E. Hysteresis
The Guinier data on sample S8 show that the transi- tion during cooling (as determined by the line broadening) starts at about T11 = 140 K (Fig. 1), while the synchrotron data show that the transition during heating (as deter- mined by the disappearance of the triclinic phase) finishes
426 J. Appl. Phys., Vol. 75, No. 1, 1 January 1994 Francois et al.
(O 0
) hex T=80K 11.0
0.5
0
1.5 ((06)hex T=130K (2 22)
1.0
0-5
0 ___ ___
1.5 t ~hex T~1hex
0.5 I
k5 (0 61hex T=1 80K~
(2 22) tr
1.5. r=295K
k 6)hex 1.0-
D.5.
C
2.0-
3.0
0
3.0-
2.0.
0 -
3.0
2.0-
1.0-
0-
(a)
(b,
(IC)
(d2
(e,)
FIG. 3. Synchrotron powder diffraction profiles of hexagonal (hex) and triclinic (tri) reflections of PMS sample S8 at 80, 130, 160, 180, and 295 K 0.-- 1.21218 A). Crosses indicate experimental data. solid and dashed lines "fitted" and "deconvoluted" profiles, respectively.
J. Appl. Phys., Vol. 75, No. 1, 1 January 1994 Francois et al. 427
T--SOK
0O 6 0) hex
_-j-/v~~~~~~~~~~~~~~~~~~~}>~~~~
T=130K (0 60)
-
hex
(2 2
~)
/0 Q
>21
T=180KI
(6 )hex (2 42) tri-
I I I I I
I
I
C.
1.
1.
367 Ms~t 369 37.0 371
0.12 0.08
TN EQ.
I
0.04 -
0.03 I- 0.0 1-
0.01 I
0 ~
C
1Rj
I
I
0-
40 80 120 160 200 240 280 T(K)
FIG. 4. Full width at half maximum (FWHMpMs) of the (0 6 O)he,
(0 0 6)hex, (1I |)hx, and (10 3)h, reflectionsofsampleS8asafunction of temperature, measured during heating by synchrotron radiation.
at about TIT=250 K (Fig. 4). Although these two tem- peratures were determined by two different experiments, we can conclude that the transformation has a very broad hysteresis of about 110 K, and thus is of first order, in agreement with the change in structural symmetry. The hysteresis presumably depends on the cooling rate and per- haps also on the oxygen content. In our experiments we took care to decrease the temperature very slowly, because in preliminary experiments, not reported here, patterns of fast cooled samples showed no evidence at all for a struc- tural phase transition.
F. Comparison with previous work
Amnplitude of the lattice distortion: The triclinic cell parameters, average distortion of the cell edges (ba/ar) and angles (5alar), and relative quantity of triclinic phase of sample S8 as measured at 130 K are compared in Table II with the corresponding values reported for triclinic PMS as measured at 10 K by neutron diffraction,8 and with those of the triclinic analogues MMo6S8 containing diva- lent metal cations M-=Ca,21 Sr,22IEu, and Ba9 as measured by x-ray diffraction. Clearly, the lattice distortion in our PMS sample is the weakest among them all. The cell edges are practically undistorted (S/ar=0.0% at 130 K), while the cell angles (6a/ar=0.3% at 130 K) distort about four times more weakly than in the PMS sample investigated by neutron diffraction (&/ar= 1.2% 6a,= 1.2% at 10 K8).
Notice that the lattice distortion in our PMS sample as measured at 40 K (the lowest temperature in our synchro- tron experiment) is smaller than that measured at 130 K (during heating). It even excluded the definition of tri- clinic cell parameters (see Sec. III B). This fact is surpris- ing and presumably related to hysteresis (see Sec. III E).
A distortion as great as that reported for the PMS sample investigated in Ref. 8 can be excluded for our sample be-
0,1 0.0 0.0 0.0' 0.X
0~!
0.1~
03.
Y,t 0'.0t
(060)
0-1~~~~~~~~~~~~~~~~-
0 -
~~~~~~~~(321)
0 . .(211)
(110)
.02- "I1 (003) 06
0 00 0 1e 0'4 03 0.40
(Sut(O) /%
(hk%.,
(000k..
FIG. 5. Williamson-Hall plot at 40 K for sample S8.
cause it would have led to a separation of the triclinic (2 2
4)tri and (4 2 2)t,, reflection profiles by more than 0.70, in contrast to the observed separation which is less than 0.29 in our experiment [see Figs. 3(b)-(d)]. The reasons for these different results are unknown to us.
Thermal expansion: The lattice expansion of PMS is rather anisotropic. Based on a hexagonal lattice approxi- mation, ab,, decreases continuously as a function of tem- perature upon cooling, whereas Chex goes through a mini- mum at about 220 K and then increases (Fig. 2). The change in sign of the expansion coefficient along Chex was observed before and interpreted7 as an anomaly indicating a structural phase transition. Our analysis indicates that this interpretation is not necessarily correct. In fact, a de- scription in terms of a rhombohedral lattice approximation shows no such anomaly. Both arh and arl, decrease smoothly as a function of temperature, and follow a qua- dratic function as shown in Fig. 2. This is also true for the data shown in Ref. 7, if plotted in the rhombohedral set- ting. A structural transition clearly exists also in the sam- ple investigated in Ref. 7, as shown by the anomalies in the neutron diffraction profiles at low temperature, but the transition occurs at a temperature that presumably differs from that at which the thermal expansion coefficient along
Chex changes sign. Such a sign change should not be con- sidered to be unusual for this compound in view of the particular thermal vibrational behavior of the linearly co- ordinated Pb atom.2 3
IV. CONCLUSIONS
Our study confirms the existence of a structural phase transition in PMS at low temperature. In agreement with previous reports, the transition is of first order and leads to a two-phase region with strong hysteresis. The hysteresis is presumably time dependent and explains why the triclinic lattice distortion at 40 K is smaller than that measured at 130 K during heating. The change from a positive to a negative expansion coefficient along Chex is a consequence of a simultaneous decrease of both the rhombohedral cell edges and the rhombohedral angle, and thus does not nec- essarily coincide with the discontinuity of these parameters
428 J. Appl. Phys., Vol. 75, No. 1, 1 January 1994 4#*
(060)hex
I II (003)hl. :
hex..00
-1
I
, I
Francois et al.
TABLE Il. Triclinic lattice and cell distortions parameters, ba/a, and Ba/ar and relative quantity of triclinic phase at various temperatures I. in structurally unstable Chevrel phases MMo5S8 (M=Pb,Ca,Sr,BaEu), with lattice transition temperatures Tr.
M Pba Pbb Ca' Srd Bae Fue
T 130 K 10 K 27 K 20 K 173 K 40 K
il,; 140 K 110 K 50 K 140 K 175K 112 K
a (A) 6.534(9) 6.4948(8) 6.4912(8) 6.481(1) 6.5896(4) 6.4692(16)
b (A) 6.532(9) 6.5383(9) 6.4977(8) 6.572(1) 6.6500(5) 6.5651(13)
C (A) 6.529(9) 6.5759(9) 6.5060(10) 6.611(l) 6.6899(5) 6.5986(10)
a (-) 89.27(8) 89.298(13) 89.461(13) 89.246(4) 88.731(7) 89.179(15)
,3 (') 89.12(8) 89.604(11) 89.555(15) 89.304(4) 88.818(7) 89.184(16)
y! (') 88.97(8) 88.516(12) 89.393(19) 88.169(4) 88.059(7) 88.009(20)
6a/a,. (%6) 0.0 1.2 0.2 2.2 1.2 2.2
&t/ar, (%) 0.3 1.2 0.2 1.7 3.5 1.6
t (09SU) 30 70 1N0 100 100 100
'This wtork.
bReF. 8.
'Recf 22.
dRef. 23.
'Ref. 9.
(not resolved in our experiments) at the lattice transition temperature. As to the magnitude of the lattice distortion, our work shows that it is very small. In the MTo6S. ana- logues containing divalent metal cations such as M=Sr, Ba., and Eu, the structural distortion at low temperature is much larger and prevents the occurrence of superconduc- tivity. This does not appear to be the case for the PMS compounds measured here. It is tempting to postulate that the small structural transition observed in our PMS sam- ples does not prevent superconductivity, but weakens it in an inhomogeneous way. This may explain why the specific heat anomaly at T, is not sharp, but extends from 14 to almost 15 K even for the best sample we have produced.
Since a fully transforming PMS sample has not yet been achieved, it is difficult to solely attribute the broadening of the specific heat anomaly to a partial structural transition, and to correlate the critical temperature of the transformed phase to the observed value of T,= 14 K.
Another striking aspect of the structural transition is the coexistence of various amounts of transformed and un- transformed phase in different samples. The concentration of the transformed phase in the samples of Refs. 7 and 8 was almost 70%? whereas that in our sample is only 30%.
This difference could be correlated with the different grain size of the samples. In fact the transformed phase is likely to occur at the grain boundaries, either as a consequence of a deviation from ideal stoichiometry, or of strain induced by the anisotropic lattice expansion of the rhombohedral phase at the grain surface. Our samples, which were re- acted at relatively high temperatures, presumably had a larger grain size and thus a smaller surface volume fraction than the samples in Ref. 7 and 8 which were synthesized at a relatively low temperature.
However, independently of the exact mechanism driv- ing the structural transition, the possible occurrence of a degraded superconducting phase at the grain boundaries during cooling indicates that the transition affects the in- tergrain connections and therefore the critical current J,.
Furthermore, the observed dependence of the transition on the cooling rate suggests that the thermal history of the samples has an influence on the physical properties, and in particular on J., if as suggested, the transformed phase
occurs predominantly at the grain boundaries instead of being randomly distributed in the volume. Within such a picture thermal cycling is expected to be detrimental for superconductivity because it tends to increase both the lat- tice distortion and the fraction of triclinic phase. A better understanding of these phenomena appears to be funda- mental for the improvement of Jc of this material.
ACKNOWLEDGMENTS
We thank B. Kuenzler for help with the drawings, and H. Ritter and W. Limper for help with the synchrotron radiation measurements. This work was supported by the Swiss National Science Foundation.
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