REVISTA MEXICANA DE FíSICA"¡S SllI'LE:\IENTO 1. 6-10

High prcssurc synthcsis of matcrials: from H to HTSC

Miguel Ángel Alario-Franco anJ Emilio Mor<lllLlI!Jomtorio de Química del Estado Sólido, Facu//(ld de Ciencias QUÍmic(I", Universidad Complutense

280.JOMadrid, Spaill.

Recihido el 511e marzo de 1995; ,\('cplado e15 de mayo de 1998

JUNIO 1999

In (hc prescnl paper. a descriplion is made of the mnin experimental techniqucs tha!. Ihanks 10 the combined use of high prcssure andhigh 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 quileelahoratcu 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 temperaturacn procesos relevantes en síntesis de materiales. Los ejemplos recogidos aharcan un amplio espectro de sóliuos lÍti1cs y van uesde el simplehidrógeno al complejo mundo de los superconductores de alta temperatura pasando por el diamante y el no menos clahorado reino mineralde 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

Although lhe full understanding of a chelllical equilihriulllrc(]uircs Ihe knowledgc of the three classical Iherlllodynalllicvariables: cOlllposilion, tempcrature and pressure, and withIhe obviotls exception of lhe reaclions laking place in the gasrhase, il is not very comlllon to take i!lIO accounl lhe last ofIhese thn.'e magnitudes, since most chernical reactions takcplace al room prcssurc. Yet lhe use 01" pressure as a con.trollahle para meter in chcrnislry has a great interesl in Ihesense thal, eXlcnding lile span of Ihe field lo a real rhird di-11ll'I/.ÚOII. allows one 10 increase the possihiJities of enlargingthe numher of chemical srccies lhrough chemical reaclionsunder prcssure or, at the very Ieasl, Illodifying their proper-lies Ihrough rhase Iransitions lhal are, more often Ihan not,accompanicd of importanl changcs in lhe physicochemicalproperties of Ihe solid. Thc syn(hcsis 01' m:lIcrials can IhenIllllch hencfll of lhe use of high pressures, a capacity Ihal urlo relalively rcccntly was severely limiled ror lechnologica[reasons. Fortunalely, il is possihle nowadays to cOllltllerciallypurchase. key in hand, high pressure equipment capahle ofheing used in a relatively simple way. In (he presenl com-municalion. afler a hrief historical account of lhe use of highpresslln::s in Matcrials Science, a cursory description 01' lheexpcrimcntal tcchniqlles more oftcn cmployed will he givcn;Ihis will thcn he followcd hy an accounl 01"some interestingchelllical processes in which lhe use of high pressure has heeninslrumental in achieving the proposcd goals and Vo•...hich are01' relcvance lO Chemistry, Physics, Geology anJ even Aslro-physics.

2. Historical account

The earliesl experiments using pressure in the ehernieallabo.ratory are prohah!y Ihose of Hannay r 11 relaled, as most oftheearlier sllhseqllellt work, to (he synlhesis of diamonJ [2]. one01' (he 1110["(:interesting u.<;(1itlsolid.\'. i.c. IIU1t('r;als f1], In-spired hy lhe discovery of primary diamonds in 50ulh Al"rica,which suggesled the importance 01' pressure in lhe geologicalsynthesis rrocess, Hannay lried, in the lasl third 01' lhe nine-Iccnlh ccntury lo ernulate nalure by hcaling oil enclosed in¡ron pipes closed by screwing at one cm!. Although most oflhe e.xperilllenls endee! IIp in dangerous explosions, in one01"ihern sorne diamond crystals were found. However, thesediamonds which are still shown in a l1l11SeUl1lin LonJon(England). \Vere later shov.'n (() be natural diamonds and cer-l¡¡inly nol the product 01' a rllan made process 1.11. Moissan,Chelllislry Nubel Prilc in 1906, also tried to make diamondsand can be credited fOl" (wo intcresling llndings in tlle un-successlúl altelllpl. One was Ihe discovcry of carborundum,silicon carhide or 1Il0iss:lIliIC. a very harJ malerial indeed,ando the other. the use of liquid iron as a solvenl 01"carbonov.'hose rapid cryslalisation he expected 10 givc hirn lhe hard-ness Holy Grail. The name oí" Bridgman. also Nohcl Prize,in Physics in 1946. precisely for his illllTlCnSC(.'onlribution loIhe use 01"high pressures in the lahoratory, is ccrtainly, up to1l0W, Ihe more relevant one in Ihe lield. Bridgman devclopedIhe opposed allvillhal hear his Ilallle ami that allowed him toachievc pressurcs of the onler nI' lOO 000 ¡¡tm (Tahle 1) lhalhe 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

(. )Thc fllost frcqucntly lIsed lInils in UP work are undcrlincd

TAIH.E 1. SOllle prcssure llnits ano Ihcir cquivalcncc(.)

~I Kg/cm:.! = O.967S alm

1 har = O.9X69 alm

I KbJf ~ 10' Pa

aUn1 .1Im = 1.0321 har

I atm = 1032.1 mhar

10 Kbar = J(fl Pa = I GPa

har

1 har = 1.0197 Kglcm:!

I bar ~ 10197.35 Pa

1M.!:@r= 1011 Pa

SI: Pase,I(Pa)

J Pu= I N/m2 = Kg.m/s2/m2

1 Pa = 9.678 x 105 atm

I I..P.1! = 1012Pa

df'll~i t y(p:,j C111:l)

he couhJ nol lIlatcrialize his drealll 01' offeri1lg (/ symheticd¡alllo"d 10 Al.\'. B,.¡dgl1lo11 (5). DialllollJs were f1nally 00-taincd in DccclIlocr 1955, in a reproduciole way [GI, althc GEC Laooratorics oy Trac)' Hall in lhe so calkJ Bclt-typc apparatus, which, hesidcs two opposeJ pistons, ineludesa challlocr-or Bclt-Ihat confines lhe sUlllple cell in ilspcrimcler; fmm lhis syslelll, induslrial equiprncnt has heendcveloped thal allows lhe manufacturing, nowadays, 01' moreIhan RO Tons uf diamonds annually: Ihis actually rcpresenls80% 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 difrer-ent cell shapes anJ uses, in parlicular lhe synlhesis 01' maleri-als as \vill he secn helow.

01" particular imporl<mce is the denolllinalcd DiamondAnvil Cell or DAC, in which two lapcred dimllonds are uscdas pistons. This has thc very imporlanl consequcllcc Ihat, dia-mond hcing insulating ano transparenl to electromagnelic ra-dialioll, cither IR or X-rays, as well as I1culrons, can he llsedso as lo lakc at.lvantage oflhe speclroscopic (Jr dilTraclomelricmcthods of struclure and chcmical characteril.alioll, spcciallywilh the more hrillianl and besl resolved synchrolron radi.¿¡Iion. Besides, electricnl 171. magnetic and other Illcasme-lllcnls as Müsshallcr spcclroscopy 181, are being uscd will1Ihe DAC, in ¡¡ wide rage of lemperatures, cnOfmollsly cnlarg-ing lhe sludy 01"pllase Iransilions.

Silllultancously, shock wave cxperimcnls, have been pro-duced in exlrelllely cOlllplex equiplllcnt. In these, a lanlalulIIimpactor is acceleraled in a gun, up to velocities nI' the ordcr01' 7 Km/s, IOwards a plalc thal Iransmits Ihe impulsion lo asample encloscd wilhin two alumina single cryslal discs 191;with this (eehnique, rressmes ovcr 2 Illillioll alln and tcmpcr-atmes 01' Ihe ordcr 01' 4000 K have ocen produccd.

3. Thc influellcc uf prcssure (amI tcmpcraturc)in solids

As a rcsult of Ihe prcssure applicd lo a solid, scvcral Illodifi-calions lakc normal1y placc. Sorne 01' Ihesc are:

- The \'olume decrcnses, so lhal, ir Ihe mass is constanl,lhe densily increases.

- Packing cflkiency inereases.

- Since anions are comlllonly more polarisable (eolll-pressihlc = sorl) lhan eations: re/f'a, inereases, so thatcoordinalion indexes (often) incrcases.

- ul1usllal (unslahle?) nxidalion states for differentcalions can hc achic\lcJ e.g. Cr4+, Fc5+, elc.

Inlcralomic dislances decrcase until a phase lransfor-mal ion 01' a dccomposition reaclion lakes place.

\Ve will sho\\', in \Vh;]1 I"ollows, some examplcs of highpressurc processcs lhal conform lo some of lhe above consid-crations.

4. Sumc t)'pical high pressure/high tempcra-turc proccsscs reactiulIs alld/or phase trall-sitinns [10-141

1: Incrcase in coordination and change in Ihe stacking se-qucnce:

ABABAn (h"p) '* AnCAI3C (eep)

j\' ZII I\' () wllrt,zit.p ~ vi ZIl vi () sodium chloride(15]

[Zn - O,j (1(21('1) '* [ZIl- OH](alloc!a)

c1zlI_o ill<T('aSl'S »; do_o dccrcascs «Ir pressurc (slill) incrcases, coordinalion (stHI) incrcases

(of¡en)

viNa ivel _ t.;nH' :} viiiCS viiiCI - type

face c(,lltn'(1 Cllhic sinll>lc Cllbic (e.y. CaG GOO Kbar)

11: Change in the stacking and not in the coordinationa) "closc-packcd" slrueturcs

ABABAI3 (hep) '* AI3CABC (eep)

Olivill(, :} Spine1

(~Ig. Fe)SiO.'¡[(~lg, Fe) - Ool, [SiO,]

h) non c10se packed slruclures:

i\'SiO:.! - 21\har -+ iVSiO:z - 30 Kbar --+(1llartz tridimite

i\.SiO:.! - lOOKbar --+ viSiO:z

('ocsite stishovitc (rutile - trpe)295 4.28(D.J ~ 45%)'.

Re¡'. Me.\". 1-"1,\."¡5SI (1999) 6-10

MIGUEL ÁNGEL ALARI(HRANCO ANIl EMILIO MORÁN

AIlCAIlC -> AIlACAB -> AIlAI3AIl

POI a..,...•illlll fel(!spar K ¡"Al Í\Si30H - 120 Kbar + 900 e --+

lIollanditp - tYlw 1"':"iAl "Si:iOH (ViAl "Si3 randolll)

It is intcrcsting to note that the prcssurc al which the COOf-

dination incrcascs shows an, almost, linear rclation with thetemperalure [161.

A similar cxamplc which is also 01' rclcvam:c in thegcophysical \\'orlJ is lhal of fcldspars which can transform¡nln Ihe hollalH.litc.typc slructurc wheTe hoth silicoll and alu.minulll are in octahcdral positions:

1I is worth Illclllioning lhat. tiue 10 lhe si/e ¡ntlllcnec. irlhe Ictrahcdral (,lIion is gcrmaniuITI, Ihe transformalion lakcsplace al Ihe l1luch lowcr prcssurc 01'30 Khar; Ihis has allowcdlO mimic in the Iahoratory lhe Illore scvcrc natural proccsscsin which silicalcs intcrvcne.

Therc are also many interesling processcs taking place. athigh pressure. around Ihe corundum type slructure; ror cxam-pie:

TAHLE 11. Synthesis <lnd sorne properties of corundum IikcCo,O, [131

This work. also rcflccls the effect of pressure in lhe crys-lal f¡eld. Ohviously. under the cffeet of pressure. Iigands-oxygcn 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 morestahIe in the low spin configuration. In fact. Ihis cffect isroulinely used lo cafihrate high prcssure experiments byanalysing Ihe inftuenee of pressure on Ihe frequeney of thelight emilteo by a ruby Laser.

V) Somelimes. the HP phase is unstablc (i.e. 1101 qllellch-ab/e) al room prcssure, l'.g.

a) Synthcsis:2 Col', + 3 Nn.,O, - GO Kh3f170()C -> 6 NaF + 3/200 + COoO,

Matcrial <1(1\) c(A) ionic radi(A)

AI,O, 4763 13.()() 0.53Co,O, 4.78 12.96 0.54 -> 3d6 L.S.b) Spin transformaríon

L.S-C0:20j -400 C & I "tm <liT-+ C0203Co,O, 14.883 113.38 0.61 -> 3d6 H.S.L.S -+ 1I.S. Yolume Changc + 6.7 %

(t.Ga'203cofllndum-typc

(Ga-O¡;J 213 (>crup.

-IP+IIT ->$ .Ga'20,\[Ga-O,;]1/3 oeeup.[Ga-O,] 1/6oeup.

$ - InGaO, -> InGaOr II -> InCaOr III (Disorderedllllll'nite = cortllldlllIl)

[In - 0(;]

[Ga - O.,]

[In - 061[Ca - O.,]

(In - 06]

[Ga - 061

III - vil ZrCh - 40 Kbar + 25 e --+ t - viii Zr02

v;~I(\F, (rutile) - lOOEhar + 25 e -> viii~I(\F2 (Fluorite)

~1 Vi!\lnF2 ('. _ Pb02 type)

\"iiiY3 Í\'Fe3012 - HP&HT --+ 3viiíy viFe03 + vIFe203ganwt perovskitc + corundum

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 soof speciaI relevance to Geophysics.

a) e.g. the spinels:

ln hoth cases, Ihe comhined densities of lhe resulling prod-ucts are higher than that of the reaclanl and Ihis plays an im-port,Hl role in driving Ihe reaction.

VII) In i.\ 10\Vcr range of prcssure, <lnd using un aqueousl11ediun, it is ohscrved that at moderatcly high temperatures,water hecolllcs a ver)' powcrful solvcnt, much more so thanundel" amoienl conditions. Those conditions, known as hy-Jrolhcrmal are oftcn encountcrcd in !\1ineralogy and havesuccessfuly heen used to synthcsise, even al Ihe industrialscaIe, materiaIs as importan as quartz [19]. er02. or Zeolites.

IV) Phase transformationsVery often. phase Iransformations produce changcs in Ihe

Physkal-Chcmical propcrlies, ~md the paradigmatic cxampleis. indc(.'d, Ihc graphitc (G) 10 diamond (D) transronnalinn,~'herc the change in cnordination, and hyhridisation. is ac-companicd hy very drastic changes in the propertics; in par-ticular. dcnsity (g!cm:i): Graphitc-avcragc-2.266 I Dia-mono 3.514; haroness (Mohs) G: < 1; D: 1()-llIaximumin Naturc; rcsistivity (ohm cm) G: hasal OA-5x 10-.1; par-allel to e O.2-I,-scmimctaIlD: 1014-16-insulator; magncticpropcrties (G: Pauli paramagneticl D: diamagnctic), ThcnnalcunouelivilY (Wm Ik () G: ~ I0'/1) ~ I()'. ele.

Interestingly cnough, the reccntly found form 01' carhon,Cijo. buckl1llf/lSlef.fllllerene ahhreviated lofullcrcl1c-, can alsohe transformed lo diamond al high pressures and moderaleIcmperaturcs [171.

In other occasions. howevcr, the changcs are much moresuhllc. This is the case. for example, of (...orundumli"c Co20J

which, \vhCIl synlhcsised undcr pressurc is ohtaincd JoS Ihelo\\' spin !"orm and Ih;lt in suhscquent Ihennal lrcalmenl doesundergo a LS lo HS Iransformation, as shown [18) hy Iheslructural data in Tahle 11; in Ihis tahlc, a good examplc ofa High-Pressurcl High Temperaturc synthesis proccss is .:lIsoglven.

M2TiO. - HP &: HT -> viMO +spinel rack salt +

h) Ihe gamels

vir..ilviTi03ilmenite

Rt'l', Mi'X. Fú. 45S1 (1999)6-10

HIGH PRESSURE SYNTHESIS OF ~lATERIALS FRO~l H TO HTSC 9

lhe presenl aUlhors have used it lo prepare sornemelastahle malerias sueh as RhO, [20J and sificon-free hy-drogarnels 121J.

5. The (somewhat special) case oChydrogen

Hydrogen, the lighlesl. and indeed lhe simplesl. 01' all ele-I11cnls. has occn the subject of much thoughts and expcri-Illcnts 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], lheslory slarted sorne seventy years ago \Vhen Bernal 124] sug-gested lhal, al a suffieienlly high pressure. all malter. and inparticular hydrogen. \Vould beeame a kind 01' sponge \Viththe clectrons free lo mave. more or Icss like in the c1assi-cal Drude-Lorenlz mode! 01'a melal. Sometime lateroWignerand Hunlinglon [25J predieled lhal molecular solid hydrogenwould hccame a metal under a prcssurc of a quartcr of a mil-lion almospheres: Ho\Vever. op lo no\V, the simplesl 01' allchcmical spccies has dcficd metallisation in the salid state.According to a recent theoretical study 01'Edwards and Ah-seroft [26J a reason for lhis apparently odd behavioor eouldreside in n spontaneous electronic polarisation that wouldlake place in solid hydrogen at very high pressures. 01' lheorder 01' 1.5 mi Ilion almospheres. and 10\Vlemperalures. lhisis in agrccment with the observed infrarcd absorption of salidhydrogen al very high pressures. that can only be explaned onlhe hasis 01'dipolc speeies.

Yct. in dynamic high pressure experiinents. as inshock wavc compression of hydrogen. j.e. al high pres-sures (I~O ePa = 1.4 ~Ibar) and high temperalurcs(_ 3(XX) K) [27J. lhere is a marked inerease in the eonduetiv-ity 01'lhe fluid hydrogen thal can be inlerpretad as a lransitionfrom lhe insulating lo the melal slale. lhis metallie fluid \ViIIthen he responsable uf the magnetic field generated in starsand gianl planels from a dynamo effeel [28].

6. High pressure and HTSC materials

In lhe elassieal BCS model 01'supereonduelivily. \Vhere lal-tice vibrations-phonons-play a major role in lhe pairing

1. J.W. Mellor.ln Colltl'rehellsil'e TrealÜI! 011 Orgallic, Itlorgallic{Ifld TheorelÍcal Chemislry, Vol. V. (London. 1924).

2. An c'lcellent and lively aCCOllnt of these matters is containedin (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)' in /J,.itaill. (Royal So-C¡CIV (JI"Chemistry, London. Eng1and. 1988) p. 462.

meehanism. il is lo be expeeled that lhe influenee 01'pressurein the superconducling properties has to be 01' much impor-tance. Indeed, there are mao)' examples 01' lhe inftuence ofpressure 011Te. such as in lhe case 01' silicon which, underpressure, cxperiences nol less (han four phase transitiol1s. allorlhem lo supereondueling phases [10J.

As soon as the so-called High Temperature Superconduc-lors \Vere discovercd by Bednorz and Muller [30], il \Vasob-ser ved the beneficial effect of pressure in Te, whieh in thecase 01'La'_rSrr.CuO.¡ inereased [31J from -35 lo -50 K.

But the use 01' pressure has also been beneficial in the syn-thcsis 01' HTSC materials, in particular when the search for ahigher Te led Antipov and co-workers to the use 01' mercuryas an ingredienl 01'these lype 01'materials [32, 31J. Mereurybeen a ralher noble metal, its oxide is rather unstable exceplat rclativcly low tempcrature aod at 600 K decomposes toits componenls.

By using a precursor 01' composilion: 8a2Can_l CunOy•

lhal \Vas healed under pressure (- 20 Kbar and - 1000 K)wirh mereury ox ideoThese workers wcre able ro prepare lhefamily Hg Ba,Can_,CunOy [n = 2-9(?)J. lhat sho\Vs lhehighcst values of Te ever recorded: 135 K at room prcssurefor thc 11 = 3 member when optimally doped. But these mate-rial, does also show the highesl evcr critica tempcrature underprcssure: 164 K al25 Gpa [34J.

By similar melhods, the modifiealion 01' the so-ealledeharge reservoir layer [351 has produeed a family [36,37Jofnon-loxie malerias (Cu/C)Ba,Can_,CunOy [n = 3-7(?)Jwith quite higl-¡ values 01' Te. j.e. 117.5 K at room prcssurefor lhe n = 4 member. 120.0 for n = 4 and 134 under pres-sure [38], for a not-yet well eharaeteriscd sample_ lhe ahoyeexamples givc a brief, yet suffieicntly represcntative overview01' the interest of pressurc as a driving force in Materials Sci-coec.

Acknowledgments

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Re\'. Mex. rú. 45 SI (1999) 6-10

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