High prcssurc synthcsis of matcrials: from H to HTSC

High prcssurc synthcsis of matcrials: from H to HTSC

REVISTA MEXICANA DE FíSICA"¡S SllI'LE:\IENTO
High prcssurc synthcsis of matcrials:
Miguel Ángel Alario-Franco
1. 6-10
JUNIO 1999
from H to HTSC
anJ Emilio Mor<lll
LlI!Jomtorio de Química del Estado Sólido, Facu//(ld de Ciencias QUÍmic(I", Universidad Complutense
280.JO Madrid, Spaill.
Recihido el 511e marzo de 1995; ,\('cplado e15 de mayo de 1998
In (hc prescnl paper. a descriplion is made of the mnin experimental techniqucs tha!. Ihanks 10 the combined use of high prcssure and
high tempcratllfe. are of relevance in the synlhesis of malerials. The ("hosen cxarnplcs cncompass a wide range of useful solids including.
among others. Hydrogcn. rhe High TcmpcratuH: Superconducrors. diamond ami sume relevant examples of silicatcs thar abotllllJ in the quile
elahoratcu mineral kingdom.
Kl'.nl'(mls: Matcrinls; high pressure; sllperconulIclors;
solids
Se ahorda en este artículo una dcscripci(lIl somera de 1Js técnicas experimelltales que utilizan conjuntamente. Alta presión y alta temperatura
cn procesos relevantes en síntesis de materiales. Los ejemplos recogidos aharcan un amplio espectro de sóliuos lÍti1cs y van uesde el simple
hidrógeno al complejo mundo de los superconductores de alta temperatura pasando por el diamante y el no menos clahorado reino mineral
de los silicatos.
/){'scril'/orl's:
Materiales; alta presión; superconductores;
sólidos
PAes: 07.35 +k: 62.50.+p; 74.70.Vy; 81.20.-n
I. Introduction
2. Historical account
Although lhe full understanding
of a chelllical equilihriulll
rc(]uircs Ihe knowledgc of the three classical Iherlllodynalllic
variables: cOlllposilion,
tempcrature
and pressure, and with
Ihe obviotls exception of lhe reaclions laking place in the gas
rhase, il is not very comlllon to take i!lIO accounl lhe last of
Ihese thn.'e magnitudes,
since most chernical reactions takc
place al room prcssurc. Yet lhe use 01" pressure as a con.
trollahle para meter in chcrnislry has a great interesl in Ihe
sense thal, eXlcnding lile span of Ihe field lo a real rhird di-
The earliesl experiments using pressure in the ehernieallabo.
ratory are prohah!y Ihose of Hannay r 11 relaled, as most ofthe
earlier sllhseqllellt work, to (he synlhesis of diamonJ [2]. one
01' (he 1110["(:interesting u.<;(1itlsolid.\'. i.c. IIU1t('r;als f1], Inspired hy lhe discovery of primary diamonds in 50ulh Al"rica,
which suggesled the importance 01' pressure in lhe geological
allows one 10 increase the possihiJities of enlarging
the numher of chemical srccies lhrough chemical reaclions
under prcssure or, at the very Ieasl, Illodifying their properlies Ihrough rhase Iransitions lhal are, more often Ihan not,
accompanicd
of importanl changcs in lhe physicochemical
properties of Ihe solid. Thc syn(hcsis 01' m:lIcrials can Ihen
Illllch hencfll of lhe use of high pressures, a capacity Ihal ur
lo relalively rcccntly was severely limiled ror lechnologica[
lhe e.xperilllenls endee! IIp in dangerous
explosions,
in one
01"ihern sorne diamond crystals were found. However, these
diamonds
which are still shown in a l1l11SeUl1lin LonJon
(England). \Vere later shov.'n (() be natural diamonds and cerl¡¡inly nol the product 01' a rllan made process 1.11. Moissan,
reasons. Fortunalely, il is possihle nowadays to cOllltllercially
purchase. key in hand, high pressure equipment capahle of
heing used in a relatively simple way. In (he presenl communicalion. afler a hrief historical account of lhe use of high
presslln::s in Matcrials Science, a cursory description 01' lhe
expcrimcntal
tcchniqlles more oftcn cmployed will he givcn;
Ihis will thcn he followcd hy an accounl 01"some interesting
silicon carhide or 1Il0iss:lIliIC. a very harJ malerial indeed,
ando the other. the use of liquid iron as a solvenl 01"carbono
v.'hose rapid cryslalisation
he expected 10 givc hirn lhe hard-
11ll'I/.ÚOII.
chelllical processes in which lhe use of high pressure has heen
inslrumental
in achieving the proposcd goals and Vo•...hich are
01' relcvance lO Chemistry, Physics, Geology anJ even Aslrophysics.
synthesis rrocess, Hannay lried, in the lasl third 01' lhe nineIccnlh ccntury lo ernulate nalure by hcaling oil enclosed in
¡ron pipes closed by screwing at one cm!. Although most of
Chelllislry Nubel Prilc in 1906, also tried to make diamonds
and can be credited fOl" (wo intcresling
llndings in tlle unsuccesslúl altelllpl. One was Ihe discovcry of carborundum,
ness Holy Grail. The name oí" Bridgman. also Nohcl Prize,
in Physics in 1946. precisely for his illllTlCnSC (.'onlribution lo
Ihe use 01"high pressures in the lahoratory, is ccrtainly, up to
1l0W, Ihe more relevant one in Ihe lield. Bridgman devcloped
Ihe opposed allvillhal hear his Ilallle ami that allowed him to
achievc pressurcs of the onler nI' lOO 000 ¡¡tm (Tahle 1) lhal
he applied lo graphile. But, wilhoul Ihe cOllcourse 01"lemrcr.
ature. tI/e sister \'nriahle in Ihe process. and wilh no catalysl,
IIIGIII'RESSURE
SYNTIIESIS OF MATERIALS: FI{OM 11Tú IITSC
7
TAIH.E 1. SOllle prcssure llnits ano Ihcir cquivalcncc(.)
~
aUn
har
SI: Pase,I(Pa)
I Kg/cm:.! = O.967S alm
1 .1Im = 1.0321 har
1 har = 1.0197 Kglcm:!
J
1 har = O.9X69 alm
I atm = 1032.1 mhar
I bar ~ 10197.35 Pa
1 Pa = 9.678 x 105 atm
I KbJf ~ 10' Pa
10 Kbar
=
=
J(fl Pa
I GPa
11
1 M.!:@r= 10
Pa
Pu= I N/m2 = Kg.m/s2/m2
=
I I..P.1!
1012Pa
(. )Thc fllost frcqucntly lIsed lInils in UP work are undcrlincd
he couhJ nol lIlatcrialize
his drealll
01' offeri1lg
(/ symhetic
- Since
10 Al.\'. B,.¡dgl1lo11 (5).
a challlocr-or
Bclt-Ihat
confines lhe sUlllple cell in ils
pcrimcler;
fmm lhis syslelll, induslrial equiprncnt has heen
dcveloped thal allows lhe manufacturing,
nowadays, 01' more
Ihan RO Tons uf diamonds annually: Ihis actually rcpresenls
80% of the tOlal diamolld allllual trade in lhe whole wmld.
Suhsequently,
other multianvil de vices have hccll devel.
opcd, such as the lelrahcdral, cuoic ami octahcdral anvils, lha!
with a difrcrclll, and more elahorale, gcolllclry, allow difrerent cell shapes anJ uses, in parlicular lhe synlhesis 01' malerials as \vill he secn helow.
01" particular
imporl<mce
is the denolllinalcd
Anvil Cell or DAC, in which two lapcred
dimllonds
Diamond
are uscd
as pistons. This has thc very imporlanl consequcllcc Ihat, diamond hcing insulating ano transparenl to electromagnelic
radialioll, cither IR or X-rays, as well as I1culrons, can he llsed
so as lo lakc at.lvantage oflhe speclroscopic
(Jr dilTraclomelric
mcthods of struclure and chcmical characteril.alioll,
spccially
wilh the more hrillianl and besl resolved synchrolron
radi.
¿¡Iion. Besides, electricnl 171. magnetic and other Illcasmelllcnls as Müsshallcr spcclroscopy
181, are being uscd will1
Ihe DAC, in ¡¡ wide rage of lemperatures,
cnOfmollsly cnlarging lhe sludy 01"pllase Iransilions.
Silllultancously,
shock wave cxperimcnls,
have been produced in exlrelllely cOlllplex equiplllcnt.
In these, a lanlalulII
impactor is acceleraled
in a gun, up to velocities nI' the ordcr
01' 7 Km/s, IOwards a plalc thal Iransmits Ihe impulsion lo a
sample encloscd wilhin two alumina single cryslal discs 191;
with this (eehnique, rressmes ovcr 2 Illillioll alln and tcmpcratmes 01' Ihe ordcr 01' 4000 K have ocen produccd.
polarisable
(eolll-
pressihlc
sorl) lhan eations: re/f'a, inereases,
coordinalion
indexes (often) incrcases.
anions
are comlllonly
more
so that
=
DialllollJs were f1nally 00taincd in DccclIlocr
1955, in a reproduciole
way [GI, al
thc GEC Laooratorics
oy Trac)' Hall in lhe so calkJ Bclttypc apparatus, which, hesidcs two opposeJ pistons, ineludes
d¡alllo"d
- ul1usllal (unslahle?)
nxidalion
states for different
calions can hc achic\lcJ e.g. Cr4+, Fc5+, elc.
Inlcralomic dislances decrcase until a phase lransformal ion 01' a dccomposition
reaclion lakes place.
\Ve will sho\\', in \Vh;]1 I"ollows, some examplcs of high
pressurc processcs lhal conform lo some of lhe above considcrations.
4. Sumc t)'pical high pressure/high
tempcraturc proccsscs reactiulIs alld/or phase trallsitinns [10-141
1: Incrcase
qucnce:
in coordination
ABABAn
j\'
ZII
I\' ()
(h"p)
wllrt,zit.p
[Zn - O,j (1(21('1)
c1
o
ill<T('aSl'S
zlI_
Ir pressurc
(of¡en)
viNa ivel
_
and change
'*
AnCAI3C (eep)
~
vi
'*
[ZIl- OH](alloc!a)
»;
face c(,lltn'(1 Cllhic
ZIl
do_o
(slill) incrcases,
t.;nH'
in Ihe stacking
:}
vi ()
sodium chloride(15]
dccrcascs
coordinalion
viiiCS viiiCI
se-
-
«
(stHI) incrcases
type
sinll>lc Cllbic (e.y.
CaG GOO Kbar)
11: Change in the stacking and not in the coordination
a) "closc-packcd"
slrueturcs
ABABAI3 (hep)
'*
AI3CABC (eep)
Olivill(, :} Spine1
3. Thc influellcc uf prcssure (amI tcmpcraturc)
in solids
(~Ig. Fe)SiO.'¡[(~lg, Fe) - Ool, [SiO,]
h) non c10se packed slruclures:
As a rcsult of Ihe prcssure applicd lo a solid, scvcral Illodificalions lakc normal1y placc. Sorne 01' Ihesc are:
- The \'olume decrcnses,
lhe densily increases.
- Packing
cflkiency
i\'SiO:.! - 21\har -+ iVSiO:z - 30 Kbar
(1llartz
tridimite
so lhal, ir Ihe mass is constanl,
inereases.
--+
i\.SiO:.! - lOOKbar
df'll~i
t
y(p:,j C111:l)
Re¡'. Me.\". 1-"1,\. "¡5SI (1999) 6-10
('ocsite
295
--+
viSiO:z
stishovitc (rutile - trpe)
4.28(D.J ~ 45%)'.
MIGUEL ÁNGEL ALARI(HRANCO
It is intcrcsting to note that the prcssurc al which the COOfdination incrcascs shows an, almost, linear rclation with the
temperalure [ 161.
A similar cxamplc which is also 01' rclcvam:c in the
gcophysical \\'orlJ is lhal of fcldspars which can transform
¡nln Ihe hollalH.litc.typc slructurc wheTe hoth silicoll and alu.
minulll are in octahcdral positions:
POI
a..,...•
illlll fel(!spar K ¡"Al Í\Si30H
lIollanditp
-
120 Kbar
+ 900
e --+
- tYlw 1"':"iAl "Si:iOH (ViAl "Si3 randolll)
1I is worth Illclllioning lhat. tiue 10 lhe si/e ¡ntlllcnec. ir
lhe Ictrahcdral (,lIion is gcrmaniuITI, Ihe transformalion lakcs
place al Ihe l1luch lowcr prcssurc 01' 30 Khar; Ihis has allowcd
lO mimic in the Iahoratory lhe Illore scvcrc natural proccsscs
in which silicalcs intcrvcne.
Therc are also many interesling processcs taking place. at
high pressure. around Ihe corundum type slructure; ror cxampie:
-IP+IIT ->
$ .Ga'20,\
AIlCAIlC
$ - InGaO,
->
(Ga-O¡;J 213 (>crup.
AIlACAB
-> InGaOr
(t.Ga'203
cofllndum-typc
[Ga-O,;] 1/3 oeeup.
[Ga-O,] 1/6oeup.
-> AIlAI3AIl
II -> InCaOr
III (Disordered
llllll'nite = cortllldlllIl)
[In - 0(;]
[In - 061
(In - 06]
[Ga - O.,]
[Ca - O.,]
[Ga - 061
ANIl EMILIO MORÁN
TAHLE
11. Synthesis
<lnd sorne
glven.
of corundum
Iikc
a) Synthcsis:
2 Col',
+ 3 Nn.,O, -
GO Kh3f170()C -> 6 NaF + 3/200 + COoO,
c(A)
ionic radi(A)
Matcrial
<1(1\)
AI,O,
Co,O,
4763
4.78
13.()()
12.96
0.53
0.54 -> 3d6 L.S.
b) Spin transformaríon
-400 C & I "tm <liT-+ C0203
14.883
113.38
0.61 -> 3d6 H.S.
L.S-C0:20j
Co,O,
L.S -+ 1I.S. Yolume Changc
+ 6.7
%
This work. also rcflccls the effect of pressure in lhe cryslal f¡eld. Ohviously. under the cffeet of pressure. Iigandsoxygcn ion s in this case-gcI
cIoser to the central ion, cobalt,
amI ~
10 Dq. the ligano licio parameter. beco mes higher;
as a consequence,
Ihe Co3+ ion. a d6 ion, hecomes more
stahIe in the low spin configuration. In fact. Ihis cffect is
roulinely used lo cafihrate high prcssure experiments
by
analysing Ihe inftuenee of pressure on Ihe frequeney of the
light emilteo by a ruby Laser.
V) Somelimes. the HP phase is unstablc (i.e. 1101 qllellchab/e) al room prcssure, l'.g.
=
III -
+ 25 e --+
vil ZrCh - 40 Kbar
+ 25 e
v;~I(\F, (rutile) - lOOEhar
~1
IV) Phase transformations
Very often. phase Iransformations produce changcs in Ihe
Physkal-Chcmical
propcrlies, ~md the paradigmatic cxample
is. indc(.'d, Ihc graphitc (G) 10 diamond (D) transronnalinn,
~'herc the change in cnordination, and hyhridisation. is accompanicd hy very drastic changes in the propertics; in particular. dcnsity (g!cm:i): Graphitc-avcragc-2.266
I Diamono 3.514; haroness (Mohs) G: < 1; D: 1()-llIaximum
in Naturc; rcsistivity (ohm cm) G: hasal OA-5x 10-.1; parallel to e O.2-I,-scmimctaIlD:
1014-16-insulator;
magnctic
propcrties (G: Pauli paramagneticl D: diamagnctic), Thcnnal
cunouelivilY (Wm Ik () G: ~ I0'/1) ~ I()'. ele.
Interestingly cnough, the reccntly found form 01' carhon,
Cijo. buckl1llf/lSlef.fllllerene ahhreviated lofullcrcl1c-, can also
he transformed lo diamond al high pressures and moderale
Icmperaturcs [171.
In other occasions. howevcr, the changcs are much more
suhllc. This is the case. for example, of (...orundumli"c Co20J
which, \vhCIl synlhcsised undcr pressurc is ohtaincd JoS Ihe
lo\\' spin !"orm and Ih;lt in suhscquent Ihennal lrcalmenl does
undergo a LS lo HS Iransformation,
as shown [18) hy Ihe
slructural data in Tahle 11; in Ihis tahlc, a good examplc of
a High-Pressurcl High Temperaturc synthesis proccss is .:lIso
properties
Co,O, [131
t -
viii Zr02
-> viii~I(\F2 (Fluorite)
Vi!\lnF2 ('. _ Pb02 type)
in spilc 01' hOlh transformations heing reconstructivc.
VI) AlIematively. the comhined effeet of pressure and
temperature results in a Chemical (decomposition)
Reaction.
This is shown in the following two examples which are al so
of speciaI relevance to Geophysics.
a) e.g. the spinels:
M2TiO. - HP &: HT -> viMO
spinel
rack salt
+ vir..ilviTi03
+ ilmenite
h) Ihe gamels
\"iiiY3 Í\'Fe3012 - HP&HT
ganwt
--+ 3viiíy viFe03
perovskitc
+
vIFe203
+ corundum
ln hoth cases, Ihe comhined densities of lhe resulling products are higher than that of the reaclanl and Ihis plays an import,Hl role in driving Ihe reaction.
VII) In
i.\
10\Vcr range of prcssure,
<lnd using un aqueous
l11ediun, it is ohscrved that at moderatcly high temperatures,
water hecolllcs a ver)' powcrful solvcnt, much more so than
undel" amoienl conditions. Those conditions, known as hyJrolhcrmal are oftcn encountcrcd in !\1ineralogy and have
successfuly heen used to synthcsise, even al Ihe industrial
scaIe, materiaIs as importan as quartz [19]. er02. or Zeolites.
Rt'l', Mi'X. Fú. 45S1 (1999)6-10
HIGH PRESSURE SYNTHESIS OF ~lATERIALS
lhe presenl aUlhors have used it lo prepare sorne
melastahle malerias sueh as RhO, [20J and sificon-free hydrogarnels 121J.
5. The (somewhat special) case oChydrogen
Hydrogen, the lighlesl. and indeed lhe simplesl. 01' all eleI11cnls. has occn the subject of much thoughts and expcriIllcnts undcr prcssure and sorne controversy cxists-perhaps
\Vc ,ao now say cxisted--concerning what will il became
\Vhen suhjeeled to high pressures 122J. Apparently [23], lhe
slory slarted sorne seventy years ago \Vhen Bernal 124] suggested lhal, al a suffieienlly high pressure. all malter. and in
particular hydrogen. \Vould beeame a kind 01' sponge \Vith
the clectrons free lo mave. more or Icss like in the c1assical Drude-Lorenlz mode! 01'a melal. Sometime latero Wigner
and Hunlinglon [25J predieled lhal molecular solid hydrogen
would hccame a metal under a prcssurc of a quartcr of a million almospheres: Ho\Vever. op lo no\V, the simplesl 01' all
chcmical spccies has dcficd metallisation in the salid state.
According to a recent theoretical study 01' Edwards and Ahseroft [26J a reason for lhis apparently odd behavioor eould
reside in n spontaneous electronic polarisation that would
lake place in solid hydrogen at very high pressures. 01' lhe
order 01' 1.5 mi Ilion almospheres. and 10\Vlemperalures. lhis
is in agrccment with the observed infrarcd absorption of salid
hydrogen al very high pressures. that can only be explaned on
lhe hasis 01'dipolc speeies.
Yct. in dynamic high pressure experiinents. as in
shock wavc compression of hydrogen. j.e. al high pressures (I~O ePa
= 1.4 ~Ibar) and high temperalurcs
(_ 3(XX) K) [27J. lhere is a marked inerease in the eonduetivity 01'lhe fluid hydrogen thal can be inlerpretad as a lransition
from lhe insulating lo the melal slale. lhis metallie fluid \ViII
then he responsable uf the magnetic field generated in stars
and gianl planels from a dynamo effeel [28].
6. High pressure and HTSC materials
In lhe elassieal BCS model 01' supereonduelivily. \Vhere laltice vibrations-phonons-play
a major role in lhe pairing
1. J.W. Mellor.ln
{Ifld TheorelÍcal
Colltl'rehellsil'e
Chemislry,
TrealÜI!
011
Orgallic,
Itlorgallic
Vol. V. (London. 1924).
2. An c'lcellent and lively aCCOllnt of these matters is contained
in (hc book The flew aJchemislJ, ediled by Robert r-.1. Hazen,
(lImes Books. New York, 1993).
3. M.A. Alado-Franco. De superconductores)'
orros materiales_
(Real Academia de Ciencias E'laClas. Fhicas y Naturales,
~ladrid. 1993).
4. A.R. Butler and A.H. Wyatt, Chemistr)'
C¡CIV
in /J,.itaill. (Royal So-
meehanism. il is lo be expeeled that lhe influenee 01'pressure
in the superconducling properties has to be 01' much importance. Indeed, there are mao)' examples 01' lhe inftuence of
pressure 011Te. such as in lhe case 01' silicon which, under
pressure, cxperiences nol less (han four phase transitiol1s. all
orlhem lo supereondueling phases [10J.
As soon as the so-called High Temperature Superconduclors \Vere discovercd by Bednorz and Muller [30], il \Vas obser ved the beneficial effect of pressure in Te, whieh in the
case 01'La'_rSrr.CuO.¡ inereased [31J from -35 lo -50 K.
But the use 01' pressure has also been beneficial in the synthcsis 01' HTSC materials, in particular when the search for a
higher Te led Antipov and co-workers to the use 01' mercury
as an ingredienl 01'these lype 01'materials [32, 31J. Mereury
been a ralher noble metal, its oxide is rather unstable excepl
at rclativcly low tempcrature aod at 600 K decomposes to
its componenls.
By using a precursor 01' composilion: 8a2Can_l CunOy•
lhal \Vas healed under pressure (- 20 Kbar and - 1000 K)
wirh mereury ox ideo These workers wcre able ro prepare lhe
family Hg Ba,Can_,CunOy
[n = 2-9(?)J. lhat sho\Vs lhe
highcst values of Te ever recorded: 135 K at room prcssure
for thc 11
3 member when optimally doped. But these material, does also show the highesl evcr critica tempcrature under
prcssure: 164 K al25 Gpa [34J.
By similar melhods, the modifiealion 01' the so-ealled
eharge reservoir layer [351 has produeed a family [36,37J
ofnon-loxie malerias (Cu/C)Ba,Can_,CunOy
[n = 3-7(?)J
with quite higl-¡ values 01' Te. j.e. 117.5 K at room prcssure
for lhe n
4 member. 120.0 for n
4 and 134 under pressure [38], for a not-yet well eharaeteriscd sample_ lhe ahoye
examples givc a brief, yet suffieicntly represcntative overview
01' the interest of pressurc as a driving force in Materials Scicoec.
=
=
=
Acknowledgments
lhe aulhors \Vish 10 lhank Iheir eolleagues and eollahoralors
in the H.P. \Vork; in particular lhey \Vould Iike lo lhank C.
Chaillout. J.J. Capponi and J. Chenavas from the Laboratoire
de Crislallographie, CNRS Grenoble. Franee.
edited by Robert M. Hazen, (Times Books.
Ncw York, 1993) p. 61.
5. The ll£'wcl1chemiJlS,
6. H.P. Bo\'cnwerk
et al .. N(lture
184 (1959)
1094.
i. M, Núñez Regueiro el al .• ScienCf' 262 (1993) 97.
8. ~1.P. Pasternak el (11., Pltys. Re\'. uu. 79 (1997) 5Q.t6.
9. IV!. NelJis el "l.
J. Chem Phys.
79 (1983) 1480.
10 J. Chcn;was. and J.c. Joubcrt. in Trmlise ill Solid Slate Chelllisr,.)'. Vol. V, (Plenulll. Ncw York, 1975). p. 463.
11. J. Che navas and M. Marezio. J Solid Srale Chem. 27 (1979) 29.
12. G. Dental.call. EIII: 1. Solid Sta/(' /rlOrg. Cltem. 3~ (1997) 759.
(JI"Chemistry, London. Eng1and. 1988) p. 462.
Re\'. Mex.
9
FRO~l H TO HTSC
rú.
45 SI (1999) 6-10
J(I
MIGUEL
13. J.I\1. Lcg:cr amI J. Haincs.
ÁNGEL ALARIO-FRANCO
Ellr. J. Solid Sla/e l/lorg. Cht'lll. 3.t
2G. B. EdwanJs amJ N.W. Ashcrofl.
..
"_
11~~7) 7X5.
1-1. CIl. Bates. \V.B. Whilc. and R. Roy. Scimce 137 (1986) 993.
1[). ROI1l:ln numeral •.•as cxponcnls
rOtlrdinalion numhcr.
al the left of un ion indicate ils
AND EMILIO MORAN
NatIlH'
JKK (1997) 652.
S.T. \Ve;r. AC. rvlitchcll. ami W.J. Nellis. P1Iys. Rel'. Lea. 76
(1996) 1X60.
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