Nuclear Instruments and Methods in Physics Research A283 (1989) 111-118North-Holland, Amsterdam

MERCURIC IODIDE MATERIALS RESEARCH: SOME RECENT DEVELOPMENTSAND OPEN PROBLEMS

M. PIECHOTKA and E. KALDISLaboratorium Ar Festkörperphysik, ETH H6nggerberg, 8093 Zurich, Switzerland

Received 26 May 1989

Although,the last several years brought appreciable progress in mercuric iodide materials research, there are still some openproblems to be solved . Structural homogeneity, synthesis reaction, nonstoichiometry, purity of mercuric iodide with respect tohydrocarbons - these are the main items as far as the preparation of the starting material is concerned . In the growth of largemercuric iodide crystals, the fundamental relationships between the growth rate, crystal shape, its structural perfection on the oneside and undercooling as the driving force on the other have still to be found. Such investigations are in progress in our laboratory asa part of a space expenment on crystal growth of mercuric iodide planned for the middle of the 90's .

1 . Introduction

The aim of this paper is not to review the extensivework done on the preparation of HgI z at EG&G,LETI, Hebrew University of Jerusalem, UniversityClermont-Ferrant, ETH and other laboratories, but tofocus on the most important problems to be solved.These are, from our point of view :(1) structural homogeneity,(2) synthesis reaction and nonstoichiometry,(3) hydrocarbons in HgIz,(4) kinetics of crystal growth of HgIz .

2. Structural homogeneity

What do we mean by structural homogeneity? Let ushave a look at the phase diagram in the Hg-I system inits original layout proposed by Dworsky and Komarek[1] in 1970 (fig. 1) . There are only two phases of HgI 2indicated in the diagram: a-HgI 2 and ß-HgI 2 , withtransition temperature at 131.2 ° C. However, already atthat time it was known that two other metastable phasesexist within the stability range of a-HgI2: a white phase(reported by Tamman [2] and then by Kleber andcoworkers [3]) and an orange phase discovered byKohlschuetter [4] and thoroughly investigated bySchwarzenbach [5]. The latter phase deserves specialattention due to its structural similarity to a-HgI2 whichmay have some consequences as far as impurities inHgI2 are concerned (see section 3).

The last several years provided experimental evi-dence for two other phases . In 1983, Long and co-workers [6] observed a slow transformation from a-1-1g12

0168-9002/89/$03 .50 © Elsevier Science Publishers B.V .(North-Holland Physics Publishing Division)

to ß-HgI2 on freshly ground HgI 2 powders exposed toair within 24 h. The X-ray diffraction reflexes of thea-phase gradually disappeared and those of the ß-phaseemerged. Since the sample remained red over the entireexperiment, the authors suggest that the 0-phase ap-peared only on the surface of the sample. In 1985 [7,8]we have established a large change in the sublimationenthalpy of HgI z near 70'C . Preliminary measure-ments by DSC did not show any bulk heating effects atthis temperature. However, this temperature roughlycoincides with the temperature of around 60-75'Creported for the transition from the orange phase to thered phase [4,9] and quite well with a local decrease inmagnetic susceptibility of the red HgI 2 reported byStrakhov [10] and confirmed by Mikhail and coworkers[11] (fig . 2). Our present opinion is that the change insublimation enthalpy should be attributed to theorange-red transition on the surface of mercuric iodide :the change in sublimation enthalpy is accompanied byan increase of the entropy of sublimation, which indi-cates the appearance of a complicated structure, likethat of the orange phase. This, of course, in the casethat the enthalpy change has a structural origin anddoes not result, for instance, from the changes of thechemical composition .

When and why do or may appear all these phases onthe way from preparing the starting material to manu-facturing the detectors? The answer to this question -although not possible now - is very important becauseof the probably detrimental effects of these transitionson the detector performance. Several years of experi-ence with growing HgI 2 crystals at EG&G EnergyMeasurements Inc. show [12] that the surfaces of theas-grown HgI 2 crystals exhibit a much higher con-

112

M. Piechotka, E . Kaldis / Mercuric iodide materials research

UO

W

HQ

111

wH

300

200

100

L . rt-H912

12 " (t -Fig' 2

L " /1 - 11912

131,2 °C

101,3 -C

3 . Nonstoichiometry and synthesis reaction

256,7 °C

L

L- Îi H9 1 2

a-Hg12

" Hg212

centration of defects than their bulk and it must be cutaway in order to get high quality detectors . Surfacetransformations such as those already discussed mightwell be responsible for such effects .

Let us now return to the phase diagram in the Hg-Isystem. As a result of our mass spectrometric investiga-tions we may introduce a tentative line at 70 ° C corre-sponding to the apparent surface transformation dis-cussed above. Another new phase just below the meltingpoint of HgIz has also been reported, but its existenceis discussed in another paper of this issue [13] .

There are two compounds in the system Hg-1: HgIzand Hg2I2, both indicated as strictly stoichiometricover the entire temperature range of their stability.Nowadays, however, the existence of the nonstoichiom-etry of HgI z is generally accepted . But what is thedirection and range of the deviation from stoichiome-try? An attempt to draw conclusions from the availabledata fails to give a consistent picture. The results of thechemical analyses are shown in fig. 3 . They were per-formed by two various methods represented by the left-and right-hand side of the picture. The width of each

H g 2 12

/5+C2 " H9212

231,4 °C

L

MAX 288'C

L - Hg(L)

H9212 "Hg

(L)

241,5 °C

10

20

30 }40 - .

f -

60

70

80

~-

90 -100

Hgl2

Hg212 Hg

MOL HgFig . 1 . Phase diagram of the Hg-1 system as proposed by Dworsky and Komarek [1]. The diagram is supplemented with a dashedhorizontal line at 70° C indicating an apparent surface transformation of a-H91 2 (cf . fig . 2) according to Piechotka and Kaldis [7,8] .

hatched area corresponds to the number of samplesanalysed . The results lead to somewhat contradictoryconclusions : one method shows an almost exclusivelyI-rich, whereas the other only Hg-rich HgI 2 samples .Obviously, there must be a chemical reason behindthese differences . Therefore we have developed anothermethod, in which a molecular beam of mercuric iodidevapors effusing from a Knudsen cell is analysed by aquadrupole mass spectrometer so that the chemicalcomposition of the vapors can be determined [14] . If asample of HgI 2 is nonstoichiometric then the excesscomponent is present in the molecular beam in additionto the HgI z vapor . Its presence can be evidenced byclastograms . In this way we have unambiguously foundthat both an excess of mercury as well as an excess ofiodine exist in HgIz [14]. An indirect proof of this hasalso been given by the lattice constant and densitymeasurements made by Nicolau and Rolland [15] .

The mass spectrometric investigations [14] also re-vealed two other important aspects of the non-stoichiometry of HgI2 . First of all, rather unexpectedly,it was found that the nonstoichiometry is fixed byhydrocarbons, i.e . the excess component cannot be re-moved by sublimation if the sample is contaminatedwith HC. Fig . 4 shows that the hydrocarbon-free sampleabruptly changes its (I/Hg) ratio towards the stoichio-

0

0

63

M. Piechotka, E. Kaldis / Mercuric iodide materials research

TEMPERATURE OC100

50

A

2.5

3.01000/T , K

Fig. 2. Vapor pressure curve of mercuric iodide [7,8] . Thechange in sublimation enthalpy at approximately 70 ° C coin-cides with the stability limit (60-75 o C) of the orange phasereported by Kohlschuetter [4] and Gorsky [9] (A) as well aswith the local minimum in the magnetic susceptibility reported

by Strakhov [10] and Mikhail et al . [111(B).

Fig. 3 . Nonstoichiometry range of mercuric iodide determinedwith wet chemical analysis methods. The width of each hatchedarea is proportional to the number of samples analysed . (1)Ref. [17] ; (2) ref. [38]; (3) ref. [39] ; (4) ref. [401 ; (5) ref . [41] .

1.50a

~â1 .ox

0.5

0

5

10TIME , HRS

Fig. 4. Change in stoichlometry of mercuric iodide vaporsduring total evaporation of two mercury-rich Hg1 2 samples.The hydrocarbon-doped sample remains nonstoichiometric till

the very end of evaporation .

metric value during evaporation whereas the con-taminated sample stays mercury-rich till the very end ofevaporation . Furthermore, the equilibrium vapor pres-sure curves of various Hg12 samples measured by massspectrometry showed a variation of the sublimationenthalpy upon stoichiometry and the presence of hydro-carbons [14]. This is a thermodynamic proof that we areindeed dealing with solid solutions of the excess compo-nents and/or hydrocarbons in HgI 2 and not with thecorresponding mixtures of phases .

In which way are I and Hg excesses accommodatedin the a-HgI 2 crystals? Electrical measurements whichwould help to answer this question are very difficult dueto the high resistivity of the crystals and problems withobtaining nonrectifying contacts. However, from thelattice constant and density measurements published byNicolau and Rolland [15] one can calculate at leastthree values of the mass of the unit cell (fig . 5) : one atthe exact stoichiometric composition, the other two onboth sides of the stoichiometric point, both with theirunit cell mass higher than that at the stoichiometricpoint, which indicates that Frenkel disorder dominatesin both cases and that antistructural defects are notexcluded on the Hg-rich side.

An important practical question concerning the non-stoichiometry is whether strictly stoichiometric Hg1 2can be prepared and under which conditions? Unfor-tunately, direct synthesis from the elements in a closedsystem does not lead to the stoichiometric material,instead, as reported by several authors [16-18],mercury-rich HgI 2 is obtained . This drawback has athermodynamic and a kinetic origin . Three reactionsproceed when we heat a mixture of 12 with Hg: (1)formation of Hg1 2, (2) formation of H9212 and (3)oxidation of Hg2 I 2 to HgI 2. Reaction (1), leading toHgI2, is thermodynamically favored at higher tempera-tures . Thus a prolonged heating at lower temperatures -

2 0.0

2030 .

2020

0r

aeo

a~o

t tlóo

t aso

m

N0

WUL-ZWSHuOyas

192

1.98 2.00

2.040/Hg)-

HgI2

Fig. 5. Mass of the umt cell of a-HgI 2 calculated from thedensity data published by Nicolau and Rolland [15] . The massof the nonstoichiometric unit cells is higher than that of thestoicluometric material, indicating that either Frenkel (Hg, or1,) or antistructural defects (Hg 1) are responsible for the

nonstoichiometry of mercuric iodide.

which is usually applied in order to avoid the hazards ofheating the unreacted iodine - leads to a mixture ofboth iodides. Subsequent attempts to oxidise H9212 inthe solid state are reportedly hindered by diffusion ofiodine or mercury towards the surface [19] . This can bethe reason why melting sometimes improves thestoichiometry of HgI 2 . The other synthesis methods, forinstance in DMSO-methyl alcohol solution [15], alsogive a mercury-rich product. Decomposition of Hg 2 I 2,

M Piechotka, E. Kalde / Mercuric iodide materials research

as an alternative method used by Gospodinov andShvestarov [20], does not help either . Recently, precipi-tation of Hg1 2 from a water solution of Hg(N03 ) 2 byKI has been applied at EG&G [21] . However, from thepoint of view of purity of the HgI 2 we believe that thedirect synthesis from the elements under pure condi-tions is the best way to obtain Hg12 provided that theabove mentioned difficulties are overcome .

4. Purity of mercuric iodide

How pure or, more relevantly, how impure is HgI 2?Purification from inorganic impurities with standardmethods and chemical analysis of inorganic impuritiesby spectrochemical or SSMS (spark source mass spec-trometry) methods is relatively easy to achieve. What westill do not know, however, is the behavior and role ofparticular inorganic impurities in HgI 2 . One of ourrecent results shows the behaviour of water in mercuriciodide . Fig. 6 is a plot of the change in H2O+ signaltogether with the other main ions during two subse-quent sublimation-condensation runs of HgI 2 moni-tored by a quadrupole mass spectrometer . Not onlydoes the H2O+ signal not disappear during sublimation,but it remains at approximately the same level. Thus,repeated sublimations do not seem to remove water andother methods have to be used (see ref . [22]) .

Organic impurities (hydrocarbons, hereafter HC) aremuch more troublesome . Their presence in HgI 2 hasbeen experimentally shown in 1982 in our laboratory[23,24] and also in LETI [25] . From then on they havebeen recognized as the main purification problem to

Fig . 6 . Two subsequent sublimation runs of mercuric iodide monitored with a quadrupole mass spectrometer. Note that the H 2O+signal nether disappears nor decreases in the second run.

wá

1b810310410 5ORGANIC IMPURITY CONCENTRATION, ppm at

Fig. 7. Average mass transport rate as a function of concentra-tion of organic impurities in mercuric iodide . Samples A and Bwere sublimed in the presence of oil for roughing vacuumpumps (kept at room temperature). Sample C was evacuated

with a roughing vacuum pump without a cold trap.

cope with . The main questions concerning hydro-carbons in mercuric iodide are:(1) How do HC get into HgI2?(2) Which sites do they occupy in the lattice?(3) What is their concentration?(4) Why is it difficult to remove them from the lattice?HC get into HgI2 mostly from iodine . Even in the

purest reagent available (I2 Suprapur from Merck) theycan be seen as a forest of peaks detected by acombinedgas chromatography-mass spectrometry technique [26].

U

12,50

12,40

12,30

M. Plechotka, E. Kaldis / Mercuric iodide materials research

Another source of contamination is oil in vacuum sys-tems for the evacuation of HgI 2 samples. However, weare not dealing with a rather common case of physicaladsorption of HC traces from the vacuum backgroundto the sample, but with a strong chemical interaction . Avery simple experiment shows that HgI2 sublimed inthe presence of vacuum oil with a vapor pressure below10-6 Ton absorbs a vast amount of hydrocarbons asdetermined by SSMS, which also reduces the vaportransport rate of such a material (fig. 7). Furthermore,some of our on-line mass spectrometric experimentsrevealed a steep increase in CH 3I+ signal at the mo-ment the ampoule containing HgI 2 was being sealed offunder vacuum by a propane-oxygen torch, a processroutinely used in material preparation of mercuriciodide . Thus, not only purification methods but alsoproper handling procedures have to be developed.

As far as the mechanism of the HC accommodationinto HgI2 is concerned, it seems reasonable to firstconsider intercalation due to the layer structure of a-HgI 2 . However, if we look at the lattice constant c ofa-H91 2 crystals and powders as measured by variousauthors (fig . 8) we will find only a ± 0.5% variation,which neither excludes nor firmly confirms intercala-tion . Therefore, two other possibilities should be consid-ered . Huth [27] proposed that HC are incorporated witha simultaneous distortion of the [HgI,] layers at placeswhere impurities agglomerate. Indeed, the ion micro-probe as well as optical and SEM observation of HgI2crystals reveal a nonuniform distribution of impurities[27] . The mosaic structure [281 which is inherent toa-H912 crystals might also result from this mechanismof incorporation . However, the a-HgI2 lattice does notneed to be distorted in such a way, because the largechannels along the a- and b-axes offer free room forimpurities, especially for long HC molecules. If this is

POWDERS SOLUTION GROWN

VAPOR GROWN UNKNOWNCRYSTALS CRYSTALS ORIGIN

Fig. 8. Variation in the lattice constant c of a-HgI 2 as reported by various authors . For the detailed references (indicated by smallletters) see ref . [261 .

16NV)Es I0Ewá

0a 167zaF A : HgI 2 + oil vaporNa B : HgI 2 + oil vapor + iodinewá C : HgI2

10

Eá102z0a

z

z10100

lop

SSMS ANALYSES OF MERCURIC IODIDE

J MUHEIM (ETH ZURICH)

L NICOLAU, A. ANDREANI (LET[, GRENOBLE)

C O AI Ca Mn LI Mp P CI NI Zn Ag

C2Hn C4HO C..14J

N

Na

K

Cr

Fe

B

SI

S

TI

Cu

Br

C,H,

C3HpCsH,

IMPURITY (ELEMENTS AND HYDROCARBON FRAGMENTS)

Fig. 9 . Concentration of impurities in the same batch ofmercuric iodide as found by Muhe)m et al. [23] at ETH Zurichand Nicolau and Andreani [29] at LETI Grenoble. Both groupsused the spark source mass spectrometry method ; under differ-

ent sparking conditions, however .

true, the orange phase, in which the channels are muchlarger, will be much more feasible to incorporate HC.At the same time, organic impurities might play the roleof a stabilizing factor for this metastable structure,which would explain why long-life crystals of the orangephase have been grown successfully from organic solu-tions .

What is the concentration of hydrocarbons in HgI 2?The data on the concentration of HC, especially at thelower limit, are somewhat contradictory, although theyhave been obtained with the same type of spark sourcemass spectrometer . Fig. 9 compares the SSMS analysesperformed on one of the purest HgI 2 samples at ETHZurich [23] and LETI Grenoble [29]. The total amountof HC reported by Nicolau and Andreani [29] is morethan two orders of magnitude lower than that measuredby Muheim [23] . The disparity is due to different spark-ing conditions : a cooled Hg12 electrode was used byNicolau and Andreani, whereas no cooling was appliedby Muheim . Furthermore, both groups worked with oilvacuum pumps, which, as shown earlier, may introducean additional uncertainty to the results on HC content .The other methods include total carbon determination(in the form of CO2 from combustion of HgI 2) and gaschromatography combined with mass spectrometry [30] .They show phtalate esters, silicone oil and normal by-

M. Piechotka, E. Kaldis / Mercuric iodide materials research

drocarbons in addition to several hundreds atomic ppmof carbon in mercuric iodide.

It is clear that HC get into HgI 2 because of theirchemical affinity to both I and Hg, as well as to HgI 2itself . Consequently, only chemical methods of gettingrid of them seem to be effective. One of our earlyexperiments showed that after heating HgI 2 in a closedampoule containing 100 Ton of oxygen with the sourceof HgI 2 at 600'C and vapors at 1000'C, no blackskins are formed on Hg, 2 anylonger . A safer modifica-tion of this method has been proposed by Whelan [31],in which OZ is added to an Ar stream saturated withHgI 2 vapors . It is still not known whether, and to whatextent, mercuric iodide is contaminated with oxides andhydroxides by such treatments .

Another method, which we call lattice filtering, hasbeen tested in our laboratory . Iodine, which is the mainsource of HC, is introduced via direct synthesis fromthe elements into a compound with a close packedstructure, for instance Cul, with no place available forlong chains of HC . Then the compound is decomposedback to iodine [26] . The purest Cul contained almostone order of magnitude less HC than the purest HgI 2 .However, the method is difficult to upscale for theproduction of a mercuric iodide starting material .

5 . Kinetics of the vapor growth of HgI 2 crystals

The continuous trend to large and homogeneoussingle crystals of mercuric iodide for high energy y-raydetectors gave the impact to vapor growth experimentsunder microgravity conditions on the one hand [32,33]and enhanced interest for growth kinetics on the other.The most important open problems in.this field are [33] :

(1) Why does the growth rate of the Hg12 crystalsdecrease with growth time although the temperaturedifference is kept constant? This effect was originallyobserved by Kobayashi et a] . [24] and recently shownalso with H92C12 crystals by Singh et al . [34] . A de-crease of the mass transport rate due to a continuousincrease of the partial pressure of gaseous impurities inthe growth ampoule ; poisoning of the growth steps forthe same reason ; increasing heat resistance * of thegrowing crystal (some calculations have been done inref. [33]) - these are possible explanations of the phe-nomenon, which has to be further investigated . Espe-cially so, because in a very few cases very good HgI 2crystals have been grown with a high growth rate (4 X10-6 cm/s) [12] .

* Recent work in our laboratory (Isshik) et al. [37]) after thecompletion of this paper clearly indicates that the reason forthis phenomenon is the very low thermal conductivity of thelayer structures of HgI2 and Hg2C12 .

(2) What is the dependence of the growth rate onthe undercooling? This fundamental relationship is stillnot known for mercuric iodide. In the preliminary phaseof our planned space experiment with HgI2 [33] we aredeveloping an apparatus for a continuous high resolu-tion (several micrometers) measurement of the transportrate during vapor transport of mercuric iodide. Trans-port rate vs undercooling curves will be measured for avariety of HgI 2 starting materials on earth and in spaceso that possible convective contributions can be esti-mated.

(3) What is the real undercooling under which thecrystals grow? Because of the relatively low thermalconductivity of HgI2 crystals (0.4 W/m K parallel tothe c-axis at 300 K [35]) a temperature gradient ofseveral K/cm can be expected at a growth rate of 10-6cm/s [33] so that the temperature difference measuredoutside the ampoule may be very different from theactual undercooling . Presently we consider the applica-tion of IR radiation thermometers or thermographiccameras to monitor the surface temperature and to mapthe temperature distribution of the growing HgI2crystals .

(4) Why do the crystals change their habit withincreasing dimensions? Vapor grown large (severalhundreds of grams) HgI 2 crystals have almost exclu-sively a quasi-elliptical, rounded form . However, at theearly stage of their growth, when the dimensions do notexceed several mm, they are usually prismatic. A closeranalysis of the available literature data on the habit ofHgI2 crystals [33] suggests that under certain conditionsno facetting takes place on the crystal surfaces and thecrystals keep their prismatic habit. This apparently hap-pens when (1) no point-like cooling of the crystal isused (e.g . refs . [20] and [24]), (2) a relatively largegrowth rate (several times 10-6 cm/s) is applied. Zale-tin et al . [36] suppose that the habit transition resultsfrom the transition to diffusion limited growth condi-tions, which would well explain the rounded shape ofthe crystals. However, the crystal obtained under purediffusional conditions in space by Van den Berg [32]exhibited no rounded edges. This unclear situationstimulated us to perform systematic investigations ofthe kinetics of the vapor growth of mercuric iodidebefore the planned space experiment is carried out [37] .The way in which the crystals are cooled is also to bechanged. A cold finger with an adjustable cooling areashould preferably be used in our experiment .

6. Summary and conclusions

The reason why mercuric iodide is such a com-plicated material is the specific interrelationship be-tween structure, impurities and nonstoichiometry of thiscompound . It seems that hydrocarbons play the most

M. Piechotka, E. Kaldis / Mercuric iodide materials research

important role because they influence both non-stoichiometry and structure. The nonstoichiometry isfixed by hydrocarbons . On the other hand, incorpora-tion of organic impurities in the lattice of HgI2 will tendto change the mechanical properties of the crystals .However, the direction of these changes as well as of thechanges in other physical properties can be judged onlyby investigating systematically pure crystals with thoseintentionally doped with hydrocarbons . In any case, thehydrocarbon-free starting material and the knowhowfor handling the crystals without causing postcon-tamination by hydrocarbons is necessary for subsequentprogress in HgI2 material preparation and crystalgrowth .

References

[1] R. Dworsky and K.L . Komarek, Monatsch. Chem. 101(1970) 976.

[2] G. Tamman, Z. Anorg. Chem . 109 (1920) 213.[3] W. Kleber, H. Raidt and K.O . Leupold, Krist. Techn. 3

(1969) 65 .[4] H.W . Kohlschuetter, Kolloidchem . Beihefte 24 (1927) 319.[5] D. Schwarzenbach, Z. Krist. 128 (1969) 87 .[6] N.V. Long, K. Kleinstueck, J. Tobisch, P. Klinger, K.

Prokert and V. Schuricht, Cryst. Res. Techn. 18 (1983)K93.M. Piechotka and E. Kaldis, J. Less Common Metals 115(1985) 315.

[8] M. Piechotka and E. Kaldis, Helv . Phys . Acta 58 (1985)797.V.S . Gorsky, Sov. J. Phys. 6 (1934) 515.L.P . Strakhov, Sov. Phys. Sol . Stat . 10 (1969) 1770 and 13(1971) 239.

[111 M. Mikhail, M.A . Ahmed and I. Mekkawy, J. Phys.Chem . Sol. 36 (1975) 1033 .

[12] L. van den Berg, private communication to the authors .[13] A. Burger, S. Morgan, H. Jiang, E. Silberman, M. Schieber,

L. van den Berg, L. Keller and C.N .J. Wagner, this issue,Nucl. Instr . and Meth. A283 (1989) 130.

[14] M. Piechotka and E. Kaldis, J. Electrochem . Soc. 133(1986) 200.

[15] I.F . Nicolau and G. Rolland, Mater. Res. Bull. 16 (1981)759.

[16] G.G. Gospodinov, T.I . Godzhevrgova and D.S. Barkov,Zh . Prikl. Khim . 51 (1978) 530.

[171 G. Dishon, M. Schieber, L. Ben-Dor and L. Halitz, Mater.Res. Bull . 16 (1981) 565.

[18] M. Schieber, I. Beingless, G. Dishon, A. Holzer and G.Yaron, Nucl. Instr . and Meth . 150 (1978) 71 .

[19] R.P . Rastogi and B.L . Dubey, J. Am. Chem . Soc. 89(1967) 200.

[20] M. Gospodinov and P. Shvestarov, J. Cryst . Growth 65(1983) 365.

[21] N.L . Skinner, C. Ortale, M.M. Schieber and L. van denBerg, this issue, Nucl . Instr . and Meth . A283 (1989) 119.

[22] R.B. James, D.K. Ottesen, D. Wong, T.E . Schlesinger,W.F . Schnepple, C. Ortale and L. van den Berg, this issue,Nucl . Instr . and Meth . A283 (1989) 188.

[23] J.T . Muheim, T. Kobayashi and E. Kaldis, Nucl. Instr .and Meth. 213 (1983) 39 .

[24] T. Kobayashi, J.T. Muheim, P. Waegli and E. Kaldis, J.Electrochem. Soc. 130 (1983) 1183.

[25] I.F. Nicolau, Nucl. Instr. and Meth . 213 (1983) 13 .[26] M. Piechotka and E. Kaldis, Monatsh. Chem . 118 (1987)

137.[27] G.C. Huth, Report USC-MISG 400-07, University of

Southern California, Medical Imaging Sci. Group (1979) .[28] W.B . Yelon, R.W . Alkire, M.M . Schieber, L. van den

Berg, S.E. Rasmussen, H. Cristensen and J.R. Schneider,J. Appl . Phys . 52 (1981) 4604 .

[29] I.F. Nicolau and A.M . Andreani, J. Cryst. Growth 87(1988) 117.

[30] S. Steinberg, 1 . Kaplan,M. Schieber, C. Ortale, N. Skinnerand L. van den Berg, this issue, Nucl. Instr. and Meth .A283 (1989) 123.

[31] J.M. Whelan, Detector grade mercuric iodide, US Pat 4,554, 150 (Nov 19, 1985) ; Int . Cl . C01B 9/00, Appl . 542,250 (Oct 14, 1983) .

M. Piechotka, E . Kaldis / Mercuric iodide materials research

[32] L. van den Berg and W.F. Schnepple, this issue, Nucl .Instr . and Meth . A283 (1989) 335.

[331 M. Piechotka and E. Kaldis, Study of Technical Require-ments for Vapor Growth Experiments in MicrogravityEnvironment, Final Report, ESA Contract no . 5943/84/F/FL, ETH Zurich (1986) pp. 240.

[341 N.B . Singh, R.H . Hopkins, R. Mazelsky and M. Gottlieb,J. Cryst. Growth 83 (1987) 334.

[351 S. Lanyi, J. Dikant and M. Ruzicka, Acta Phys. Slov. 28(1978) 210.

[361 V.M . Zaletin, N.V . Lyakh and N.V . Ragozina, Cryst. Res.Technol. 20 (1985) 307.

[371 M. Isshiki, M. Piechotka and E. Kaldis, J. Cryst. Growth,subnutted .

[38] DeLong and Rosenberger, Mater. Res. Bull. 16 (1981)1445 .

[39] Burger et al ., J . Cryst . Growth 56 (1982) 526.[40] Tadjine et al ., Nucl . Instr . and Meth . 213 (1983) 77 .[411 Laskowski et al ., Mater. Res. Bull . 22 (1987) 715.

& METHODS

PHYSICS RESEARCH

Editor-in-Chief: Kai Siegbahn

Section A: accelerators, spectrometers, detectors and associated equipment

Editors: Kai Siegbahn & Erik Karlsson

VOLUME A299 DECEMBER 1990

NORTH-HOLLAND