Solid StateCommunications,Vol. 7, pp. 787—789,1969. PergammonPress. Printedin GreatBritain

EXPERIMENTAL CONFIRMATION OF THE PREDICTEDHIGH-PRESSURETRANSFORMATION IN RUBIDIUM CYANIDE

J.B. Clark andCarl W.F.T. Pistorius

ChemicalPhysicsGroupof the NationalPhysicalandNationalChemicalResearchLaboratories,SouthAfrican Council for Scientific and Industrial Research,P.O.Box 395,Pretoria,

SouthAfrica

(Received7 March 1969 by F.R.N.Nabarro)

The previouslypredictedhigh-pressuretransformationof RbCN fromthe NaC1structureto the CsCl structurewasfound at 5.6 ±0.5kbar, 24.5°Cwith a volume changeof at least2.6 cm 3/mole. Thetransitionbecomesmuch sharperat highertemperaturesand occurs6.111±0.039kbarat 105.5°C.The small entropy of transition, viz.,1.6 J/deg-mole,suggeststhat the high-pressurephaseis disordered.

THE POLYMORPHISM at highpressuresof the Rubidium cyanidewas preparedby ion ex-alkali halideshas beenthoroughlystudied. No change.9 The materialwasheavily contaminatedpolymorphictransitionsare known to occur in with rubidiumacetatebut free of rubidium halides.

the lithium halides,while the sodiumhalides It was subsequentlyfreedof acetateby extrac-transformfrom the NaCl structureto the CsC1 tion with ethanol,but still containedseveralperstructureat pressuresnear~ 0.3 Mbar.2’3 The centrubidium carbonate. Rb

2CO3is, however,potassiumhalidesundergothis transitionat known not to haveany high-pressuretransitions

20 kbar,4 and the rubidium halidesat 5 to 40 kbar.’° Both the raw and purified

kbar.4 It appearsthat the transitionpressureis materialswerestudiedby meansof volume-dis-primarily determinedby the natureof the cation, placementtechniquesin a piston-cylinderand that the anion has comparativelylittle apparatus,usingpiston rotation to reducefriction.effect on the transition pressure.5 The apparatusand methodhavebeendescribed

elsewhere.1I

The cyanide ions in the alkali cyanidesatelevatedtemperaturesrotate freely,6’7 and the A typical compression—decompressioncurvecrystal structuresof the high-temperaturephases is shown in Fig. 1. Identical results wereob-of the alkali cyanidescanthereforebe predicted tamedfor the raw and purified materials. Theon the basis of the abovepicture. KCN I with RbCN I/lI transitionhasa volume changeofthe NaC1 structuretransformsto KCN III with 2.6 cm 3/mole. This is a lower limit, as isprobablythe CsCI structureat 20.6kbar.8NaCN invariably the casefor measurementson bulkI with the NaCI structurecan be expectedto samples. Piston-rotationyields transitiontransformto a phasewith the CsCI structure pressuresof.5.6 ±0.5 kbarat 24.5°Cand6.111near“- 0.3 Mbar, while RbCN I, also with the ±0.039kbar at 105.5°C.The transitionis con-NaC1 structure,has beenpredictedto transform siderablyfasterand sharperabove 60°C. Theto a high-pressurephasewith the CsCI structure transitionline is shownin Fig.2. There is noat ~5 kbar.5 The presentstudywas undertaken cleardeviationfrom linearity in the rangestudied,to test this prediction. and the slopeis 165°C/kbar.Using the above

787

788 HIGH-PRESSURETRANSFORMATION IN RbCN Vol. 7, No. 10

tOO0

0

02 0 0z 0Ui 01 0 ~0 0 ,,7,~,/:II77L:::::1~’~0

Ui 0 0 Uio 0 0 ~5o,_10. 0 0

0 0 _______________

Z 0 0 Uio 4-

0

~ 01 0 0 00 50 55 60 6500

0PRESSURE — KILOBAR

2 4 6 8 0PISTON LOAD—KILOBAR FIG. 2. RbCN I/Il transitionline.

FIG. 1. Curvesof pistondisplacementvs. pistonload for RbCN at “~ 80°C,showingthe new The aboveinterpretationwas supportedbyhigh-pressuretransformation. The results are differential thermal analysisof the samplesatnot correctedfor the compressionof the talc 15 kbar, 20—400°C.No order—disorderfillers and the deformationof the pressurevessel, transitioncouldbe observedin this range. It

is, of course,possiblethat the presenceof

valuefor the volume change,the entropy of Rb2CO3supressedor broadenedthe transitiontransitionis found to be 1.6 J/deg-mole. This to the extentthat it could not be observed.low entropy of transitionsuggeststhat the dif- However, KCN containing6 per centK2 CO3 stillferencein entropybetweenRbCN I and II is due shows a sharpand clearorder—disordertransi-

tion.Bonly to a vibrationalentropy contribution,withno differencein disorder. RbCN II is thereforethe counterpartof KCN III, ~and canbe expected Acknowledgements— The authorswould like tothank Drs G. Gafner,J.C.A. Boeyensand R.J.to possess the CsC1structure, At lower tern- Murphy for valuablediscussions. Mr J. Erasmusperatures and high pressuresa transitionto an and his staff and Mr G. O’Grady and his staffordered,or partly ordered, phase similar to KCN kept the equipment in good repairandwerere-IV and CsCN H

1 2 canbe expected. sponsiblefor the manufactureof the furnaceparts.

REFERENCES

1. KLEMENTW. Jr. and JAYARAMANA., Prog. solid Staie Chem.3, 289 (1966).

2. JAMIESONJ.C., in TOMIZUKAC.T. and EMRICKR.M., Physicsof Solidsat High Pressuresp. 444. Academic Press, New York (1965).

3. BASSETT W.A., TAKAHASHI T., MAO H. and WEAVER J., J. appl. Phys.39, 319 (1968).

4. PISTORIUSC.W.F.T. and SNYMAN H.C., Z. phys. Chem. Frankf. Ansg.43, 1(1964); PISTORIUSC.W.F.T., PhysicsChem. Solids25, 1477 (1964); 26, 1003 (1965).

5. PISTORIUSC.W.F.T., CLARK J.B. andRAPOPORTE., J. chem.Phys.48, 5123 (1968).

6. ELLIOTT N. andHASTINGS J., Acta Crystallogr. 14, 1018 (1961).

7. SMITH D., PhysicsChem. Solids29, 525 (1968),

8. BRIDGMAN P.W., Proc. Am. Acad. Arts Sci. 72, 45 (1937),

Vol. 7, No. 10. HIGH-PRESSURETRANSFORMATION IN RbCN 789

9. This work was doneby the Analytical InorganicChemistrysectionof the National ChemicalResearchLaboratory.

10. PISTORIUSC.W.F.T., unpublished.

11. PISTORIUSC.W.F.T., RAPOPORTE. and CLARK J.B., J. chem.Phys.48, 5509 (1968).

12. LELY J.A., Dissertation,Utrecht(1942).

Die bereitsvorausgesagteHochdrucktransformationdes RbCN vonderNaC1-Strukturzur CsC1-Strukturwurdebei 5.6±0.5 Kbar und24.5°Cgefundenmit einerVolumänderungvon wenigstens2.6cm 3/Mol. Der Ubergangwird bei höhererTemperaturwesentlichschérferund findet stattbei 6.111±0.039Kbar und 105.5°C,Aufgrurid derkleinenUbergangsentropievon 1.6 J/°C—Molscheintdie Hochdruckphaseungeordnetzu scm.