vi ABSTRACTSOF PAPERSTO APPEAR IN J. PHYS. CHEM. SOLIDS Vol. 18, No. 4

To discussthe FaradayandVoigt effectsin the Eu electronicstructureof the silver iodide polymorphs.Thechalcogenideswe derivea generalexpressionfor the four phaseswhich we haveinvestigatedare thea,J3 andpolarizability of a systemin which the importantelec- y modificationsand thehigh pressurepolymorphhavingtronic transitionsare from localized to itinerantorbitals. the NaC1structure.The ionicity valuesobtainedforThisexpressionis appliedto a model systemin which thesedifferentphasesagreewell with the valuesoftheelectronictransitionstakeplace from a 4f orbital to Phillips derivedfrom the dielectricdataand the corn-a Sd t2g Wannierorbital centeredat the samecell. We putedbandstructuresarein good agreementwith inde-treatin detail the matrix elementsof the electricdipole pendentpseudopotentialcalculations.operatortaking into accountthemany-bodycharacter Received25 July 1975of the intermediatestatelocalized hole is includedex- Revised31 October1975plicitly within a simpleapproximation.We presentanumericalcalculationof the frequencydependent 7. THERMAL EXPANSION OFCADMIUMVerdet constantandVoigt phaseshift for the paramag- FLUORIDEnetic andferromagneticphasesof EuSe.

D. Shaharabany,M. Wolf and D. Gerlich,Received30 December1974 Departmentof Physicsand Astronomy,Tel-Revised31 October1975 Aviv University,RamatAviv, Israel.

The thermalexpansionof CdF2 singlecrystal has5. A TIGHT-BINDING APPROACHTO THE

ELECTRONICSTRUCTUREOFTHE SILVER beenmeasuredover the temperaturerange80 300°K.HALIDES I. SILVER CHLORIDE From thelatter data,the thermalGruneisenconstantas

a function of temperaturehasbeenevaluated,andcom-P.V. Smith,Departmentof Physics,University paredwith the elasticGruneisenconstant.The corre-of New England,Armidale,N.S.W.2350, lation betweenthe two setsof data,and with possibleAustralia.

latticeinteractionsis discussed.In this paperwe presenta modifiedversionof the Received19 June1975

tight-bindingmethodfor determiningthe electronicstructureof thesilver halides.The main advantageof

8. FORCEFIELD OF FeS2WITH PYRITEthis approachis that it enablesthebandstructureof STRUCTUREAT ZERO WAVE VECTORthesematerialsto be derivedfor all energiesadjacentto

H. A. LauwersandM.A. Herman,Laboratoriumthe bandgap.This is achievedby includingan s functioninto the tight-bindingbasisset of atomicorbitals. In voor AnorganischeScheikunde,Rijksuniversi-additionto thebasicsimplicity of the conventionaltight- tair CentrumAntwerpen,Goenenborgerlaan

171 2020 Antwerpen,Belgium.binding methodthis approachallows theionicity of thepotentialsto be determinedempiricallyby fitting the ex- Publishedexperimentalfrequenciesare usedto cal-perimentalband-gapand incorporatesinto theband- culateforce field and effectivechargeof FeS2withstructurecalculations p ands d interactionswhich pyrite structure.Thevaluesobtainedare discussed.Somemay well be important. preliminarycalculationson MnS2 are reported.

Thevalidity of theapproachis examinedfor theReceived21 July 1975caseof silver chlorideandgood agreementis obtainedRevised31 October1975

with previouscalculations.

Received25 July 1975 9. LINDEMANN’S CRITERION AND THERevised31 October1975 MELTING OF SOLIDS AT HIGH PRESSURES

V.V. Palciauskas,Departmentof Geology,6. A TIGHT-BINDING APPROACHTO THE University of Illinois, Urbana,IL 61801,U.S.A.ELECTRONIC STRUCTUREOF THE SILVER

HALIDES II. THE SILVER IODIDE POLY- ThegeneralizedLindemann’scriterion is utilized toMORPHS derivethe variationof themelting temperatureTm with

P.V. Smith, Departmentof Physics,University compression.The result incorporatesthe basicfeaturesof NewEngland,Armidale,N.S.W.2351, of the lattice potentialas well asthevolumedependence

of the frequencydistributionthroughthe first twoAustralia.momentsof the modeGruneisenparameter.This melt-

Wehaveappliedthemodified tight bindingapproach ing relationpredictsthat when Tm is plottedagainstthedevelopedin thefirst paperof this seriesto calculatethe volume of themelting solid, the fusion curvesof solids