J. Phys. Chem. Solids Pergamon Press 1970. Vol. 3 1, pp. 453-46% Printed in Great Britain.

SOME OPTICAL PROPERTIES OF Hg,CI,

E. WDER Institute of Technology, Department of Solid State Physics, Lund, Sweden

(Received 30 June 1969)

Abstract - Measurements of diffuse reflexion, absorption, emission and excitation spectra have been made on powders and single crystals of Hg,Cl,. The diffuse reflexion and absorption measurements give an absorption edge at 3.5 eV at room temperature and 3.8 eV at 10°K. The fluorescence spectrum consists of four bands at 1.5, 2.2, 2.5 and 3.1 eV with half-widths of 0.2-0.3 eV at 10°K. The excitation spectra have different peaks for the different emissions but also one common excitation region for all emissions (shifted 0.1 eV for one of them). The common region consists of at least three partly over- lapping peaks. No dark or photo conductivity was detected. The semiconductor model is rejected and a model which emphasizes the molecular crystal character of Hg,Cl, is suggested. The Seitz-Mott model for the quenching of luminescence is used also to explain the more complicated temperature dependence of the emission intensity. The emissions are believed to be due to energy transfer without mobile electrons and holes after absorption bv the host lattice (or a sensitizer) to centres 01 different kinds.

1. INTRODUCTION

IT HAS been known for a long time that mercurous chloride, Hg,_L&, gives a strong orange-red fluorescence under U.V. excitation at room temperature[ 1,2]. A few other in- vestigations have been made of the optical properties of Hg,CI,. Absorption spectra in the infrared have been measured [3-71. Suchan and Drickamer[8] studied the effect of pres- sure on the absorption edge and found the value 23 250 cm-l (4300 A) when no pressure was applied. This is surprising because Hg,Cl, is a white powder. Pringsheim[9] reports that a strong orange-red fluorescence is obtained when mercurous chloride is excited by near U.V. light and that this fluorescence is a general property of the salt to such a degree, that it cannot be ascribed to some accidental im- purity. On the other hand it is said that none of the other colourless mercurous salts (sulfate, nitrate, acetate) shows a similar fluorescence. The fluorescence mechanism is unknown and it is not even known whether it is a property of the pure Hg,Cl, crystal or if it is due to impurities. Further it seems that authors who mention the fluorescence have not noticed that Hg,C& is one of the rather rare examples of inorganic luminescent mole-

cular crystals. Other examples of such crystals are the solid halogens [ 10, 111. The reasons for this investigation have therefore been the scarcity of information on Hg,Cl, and the partly contradictory results.

2. EXPERIMENTAL The optical measurements described in this paper have

been made on several different types of Hg,Cl, samples. Powders from the Merck and BDH Companies with different grain sizes were used as well as small single crystals obtained by sublimation in high vacuum ( 10d5 Torr.) of different powders. The powders were all of reagent grade. No differences in any respect were noted when different samples were used for the measurements.

The low temperature measurements were made mainly in a Sulfrian stainless steel helium cryostat. This was equipped with four windows on quartz (Suprasil) and the samples were mounted on a copper block which was screwed into a copper rod cooled by a surrounding spiral tube through which liquid helium was pumped. By regula- ting the helium flow rate temperatures between 10°K and 80°K could be obtained at the sample and with liquid nitrogen in the cryostat, the temperature range between 70°K and 300°K was covered. The temperature measure- ments were made with thermocouples. A Zeiss PMQ I1 spectrometer consisting of a double monochromator MM 12 with exchangeable glass and quartz prisms. together with a lP28 photomultiplier or a PbS uhoto- conductor, was used for measuring the fluorescence spectra. The light entering the monochromator was chopped and the amplified signal from the detector was registered by a Moseley 710 IBM recorder. The calibra- tion of the spectral response of the spectrometer and the focusing elements (lenses, mirrors) was made by means of

453

454 E. ElDER

a calibrated tungsten ribbon lamp. The same equipment as for fluorescence investigations was used for the measurement of the spectra of diffuse reflexion and absorption. For absorption and diffuse reflexion spectra, a 30 W H, lamp was used and this lamp in conjunction with another Zeiss MM 12 double monochromator was also used for the excitation spectra. The fluorescence was excited by a 90 W long arc Hg lamp with interference filters or a 250 W Xe lamp in conjunction with a Leiss prism monochromator.

The conductivity measurements were made with crystals of the approximate size of 0.5 x 0.5 x 0.5 mm. The contacts were made of painted colloidal silver, indium amalgam or pressure contacts of brass and of indium. The crystal was supported by a Teflon sample holder which also contained a small 25 V battery. A Keithley 602 electrometer was used to detect the current.

3. RESULTS

(a) Diffuse reflection

The diffuse reflection spectrum of Hg,Cl, powder was measured between 300°K and 10°K. The spectrum of MgO as a standard substance was measured under the same experimental circumstances. This method of determining the absorption edge has the important advantage of being applicable when a substance is available only as a powder. It has also been shown that the diffuse reflection spectra are not very sensitive to impurity contents [ 121. However, there are difficulties in correlating a measured spectrum with the

spectral variation of the absorption coefficient obtained in other ways. Different means to define the absorption edge have been sugges- ted. Here lolog (intensity with MgO/intensity with Hg,Cl,) vs. photon energy has been drawn. The point where the linear part of the curve bends upwards on the low energy side has been used as absorption edge (Fig. 1). These values of the absorption edge are plotted vs. temperature in Fig. 2. The temperature shift - dE/dT is 17 X 10m4 eV/K at 300°K.

At low temperatures there is apparently a strong increase of the reflectivity around 3.15 eV. However, this effect is caused by a luminescence band which appears at low temperatures and which has its peak at 3.1 eV. This is reported in section 3 (c).

(b) Absorption

The crystals were so brittle that they could not be ground and polished to the thickness wanted. However, absorption measurements have been made on crystals which had natur- ally grown to small plates.

lolog lo/Z is plotted vs. photon energy for an approximately 50~ thick slice (Fig. 3). Reflexion losses are neglected.

The absorption edge was defined by extra-

1 35 M

PHOTON ENERGY WI

Fig. 1. Diffuse reflexion spectra of Hg,Cl, powder. The strong decrease of the curve for 10°K is due to fluorescence emission 4 which has its peak at

3.1 eV.

OPTICAL PROPERTIES OF H&l, 455

T IKI

Fig. 2. Absorption edge energy as a function of tempera- ture. Values have been taken from absorption 0 and

diffuse reflexion measurements +.

PNOTON EPERGY I l V I

Fig. 3. Absorption spectra of a Hg,CI, crystal of thickness 50~. The absorption edge values are plotted in Fig. 2.

polating the linear part of the curve to (Y = 0. The values obtained for the edge are also plotted in Fig. 2. The luminescence emissions (Section 3 (c)) did not influence the absorption measurements. A filter with strong absorption in the visible and good transmission in the U.V. part of the spectrum was inserted in the light beam behind the sample. Corrections were made with regard to the U.V. transmission curve of the filter.

(c) Emission spectra

The emissions appear at different tempera- tures, are excited in partially different bands and two of the emissions appear only under certain circumstances. For orientation all of them have been collected in Fig. 4 and de- noted from 1 to 4. It should be noted that such a spectrum with all emissions cannot be recorded under constant temperature and excitation wavelength.

Figure 5 shows the spectral distribution of emission 1 at different temperatures. The peak at 1.5 eV (830 nm) has a considerable width especially at high temperatures (300”K), and the emission is therefore seen as orange- red by the eye at this temperature and as red at lower temperatures where the short wave- length tail decreases. The existence of this emission of Hg,Cl, has been known for a long time but no results of more thorough investi- gations have been published.

I.0 I.5 243 2.5 34 3.5 PHOTON ENERGY IeVl

Fig. 4. All fluorescence emissions of Hg,CI,. The emissions are excited under different conditions and have been plotted with the same maximum

intensity.

456 E. ELlDER

l.0 1.5 2.0 PHOTON ENERGY I eV I

Fig. 5. Emission I at different temperatures.

Emission 2 has the peak at 2.2 eV (569 nm), number 3 at 2.5 eV (500 nm) and number 4 at 3-l eV (397 nm).

Emission 1 is not symmetric, especially not at high temperatures, but emissions 2-4 are symmet~c and can be we11 described by Gaussian curves.

The temperature dependence of the half- widths of the emissions is shown in Fig. 6.

(d) Excitation spectra

For each emission the excitation spectrum has been measured over a large temperature interval. A comparison between the excitation

3 ! j I

,o

?azU= EMISSION 2 -..(I-- EMISSION 3

T lK1

Fig. 6. Half-widths of emissions l-4 as a function of

spectra of different emissions is given in Fig. 7(a). All curves refer to constant number of photons falling on the specimen per unit energy interval all over the spectral region studied. The excitation intensity was measured with a calibrated spectrometer.

Emission 1 has, at the lowest temperatures, an excitation spectrum with three distinct

/

EXClTATlON PHOTON ENERGY IeV)

Fig. 7(a). Excitation spectra of all fluorescence emissions. Observe that different temperatures have been used.

4-o fv5 41

EXCITATION PHOTON ENERGY IeV )

Fie. 7(b). Excitation spectrum of emission 1. I I temperature.

OPTICAL PROPERTIES OF Hg2CI, 457

EXCITATION PHOTON ENERGY leV1

Fig. 7(c). Excitation spectrum of emission 3.

3.5 4.0 4.5 50 EXCITATION PHOTON ENERGY k&‘)

Fig. 7(d). Excitation spectrum of emission 4. I I

peaks at 4.00, 4.25 and 4.55 eV (Fig. 7(b)). If the peaks are separated, the values of the last two will obviously change slightly. At higher temperatures the structure is less marked and at 290°K there is only a trace of a peak at 4- 15 eV. Possibly a new peak appears at about 3.8 eV on the curve for 178°K and it might be this peak which is shifted to 3.65 eV at 290°K. The excitation spectrum of emission 3 (Fig. 7(c)) shows a still more distinct structure at low temperatures. Peaks are found at 4.12 and 4.35 eV. A slight trace of a peak is seen at 4.65 eV. There is also a separate excitation band with the peak at 3.75 eV. Emission 2 has

that of number 3 but due to some special properties of emission 2, the spectrum could be measured only at 80°K (Fig. 7(a)). A separate excitation band is seen with the peak at 3.45 eV. Excitation spectrum 4 (Fig. 7(d)) also has great similarities with No. 3. Peaks are found at 4*13,4+38 and 4.66 eV, all without correction for the influence of the adjacent peaks.

(e) Te~per~t~~~ dependence of e~i~siun intensity

The temperature dependence of the inte- grated emission 1 is shown in Fig. 8. The decrease at low temperatures is due to narrow- ing down of the band. The peak height is approximately constant at low temperatures.

For emission 2 and 3 different results are obtained if the material is excited with hv < 3-9 eV (Fig. 9) or hv > 3.9eV (Fig. 10). Emission 2 has about the same temperature dependence as 3 for excitation energies larger than 3.9 eV, Further investigations of tempera- ture dependence of emission 2 have not been possible for reasons explained in Section 3 (f).

In Fig. 10 emissions 3 and 4 are shown when excited in a broad band between 4.1 and 4.6 eV. These measurements are made under

0 I I

Fig. 8. Temperature dependence ofthe integrated emission at 80°K a spectrum which is very similar to 1 for excitation at 4-O eV.

458 E. EJDER

‘+\

I ” 25 50 75

T IKI

Fig. 9. Temperature dependence of the integrated emission 3 for excitation at 3.75 eV.

L 0

-\ I \ EMISSION 4

I \I

60 60 T (Kl

Fig. 10. Temperature dependence of the integrated emissions 3 and 4 for excitation at 4.1 eV.

circumstances when emission 2 is negligible. (No pre-exposure of the sample before cooling.)

(f) Special properties of emission 2 and 3

If a sample has not been exposed to U.V. radiation at room temperature, emission 3 is very weak at 80°K when excited around 3.75 eV. If, however, the sample is pre-exposed to U.V. light at room temperature, emission 3 is much stronger after cooling. The emission intensity at 80°K is approximately proportion- al to the pre-exposure time when moderate

times and intensities are used. After cooling, the intensity obtained is constant over long periods for constant excitation. If long times and high U.V. intensities are used at room temperature, appreciable photodecomposition of the Hg,Cl,-molecules is observed and probably free Hg is formed which can be inferred from the fact that the sample darkens. (Compare with the silver halides AgCl, AgBr, AgI [ 13, 141.) At low temperatures the process is slower and e.g. at 20°K no darkening has been observed even after several hours with the highest attainable U.V. intensity. This is also in accordance with experiences from silver halides.

Emission 2 shows the same phenomenon but with one important difference; when the sample is excited after pre-exposure at room temperature and cooling, the intensity of emission 2 depends on the pre-exposure time, but decreases exponentially with the time of excitation (Fig. 11). The decay constant is approximately proportional to the excitation intensity. The emission can be recovered only by warming the sample, exposing it to U.V. light, cooling and exciting it again. If the sample is kept for 1 hr at room tempera- ture in darkness after pre-exposure, emission 2 is considerably weaker after subsequent cooling, but still stronger than without pre- exposure. Keeping the sample at 80°K half

0 20 40 60 TIME (Si

Fig. 11. Intensity of emission 2 as a function of excitation time. The sample was pre-exposed at room temperature and cooled in darkness to 80°K before the excitation. The

initial intensity is denoted I,.

OPTICAL PROPERTIES OF H&I, 459

an hour in darkness after exposure at room temperature, did not influence the emission intensity. It has not been possible to make measurements of these phenomena at lower temperatures than 80”K, as no cryostat was available which could cool the sample from room temperature to temperatures below 80°K and up to room temperature again, in reason- able time.

Emissions 1 and 4 are not influenced by pre-exposure.

(g) Conductivity measurements

No dark conductivity or photoconductivity has been found. With 25 V over the crystal, the dark current was < lo-r4 A. With a 250 W small arc Xe-lamp carefully focused with a big quartz lens, no detectable current was found (~1 O-l4 A).

Measurements of transient photoconducti- vity with blocking electrodes of Mylar and integrated current measurement was tried, but no photoconductivity was detected.

4. DISCUSSION

Molecular crystals are examples of crystals where the tight-binding model of excitons is applicable. In molecular crystals the binding forces within the molecule is strong in com- parison with the van der Waals forces which connect the molecules to a crystal. This means that in such crystals one cannot start with the usual semiconductor model where it is as- sumed that the electrons are nearly free and then make corrections. Instead we have to start with the opposite point of view and consider the energy levels of a free molecule. The electron levels in the free molecule will appear as excitons in the crystal. The shift of positions and the broadening of the electron levels is due to the crystal binding. Examples of crystals, where at low temperatures we get sharp exciton lines with very little shift in comparison to the free molecule, are antra- cene and benzene. On the other hand there are substances where an appreciable broaden-

ing and shift in position occur, e.g. solid iodine[lO, 111.

The excitation spectra of Hg,CI, begin with a peak, which is rather sharp and the rest evidently also consists of overlapping peaks. These peaks can be interpreted as tight-bound excitons as in antracene or solid iodine[ 10, 111. Some investigations have been made in order to determine whether we have energy transfer by bound excitons or transport by indepen- dently mobile electrons and holes in conducti- vity and valence bands.

It was found that the temperature depen- dence curve of emission 4, i.e. emission intensity vs. temperature, is independent of excitation intensity. However, for a bimolecul- ar process it has been shown that the curve is shifted to higher temperatures with higher excitation intensities. The investigation of emission 4 of Hg,C& (Fig. 12) shows that there is a shift of only 0.8”K towards lower tempera- ture when the excitation intensity is increased 166 times. 0.8”K is within the limits of measure- ment error and the curve can be considered independent of excitation intensity. There- fore, it is concluded that emission 4 is not due to recombination of free charge carriers via a recombination centre. The results of two other experiments also fit into this picture.

No photoconductivity could be detected. This can be explained by the exciton model with no free electrons and holes. However, also with mobile electrons and holes the photoconductivity could be small due to low mobility or short carrier lifetime. Furthermore no influence by secondary light during the excitation or immediately after was found.

If we leave out emission 1 for a while, a model for emissions 2, 3 and 4 may be given. Excitation of the host lattice gives the exciton structure in the excitation spectrum of emis- sion 4. Recombination can then occur in the host lattice or over a centre to which the energy is transferred. When the temperature is lowered, the intensity of emission 4 in- creases and reaches a constant value at temperatures lower than 20°K. This indicates

460 E. EJDER

0 10 20 30 40 50 60 T IK)

Fig. 12. Temperature dependence of emission 4 at two different excitation intensities. The curves are plotted as //I, where I, is the

intensity at 0°K (extrapolated).

that no activation energy is needed for emis- sion 4 and it seems probable, that this emission ?5 occurs within the Hg,Cl, molecule. Further, 8% emission 4 cannot be excited for hv < 3.9 eV. Z

In contrast to this, emission 2 and 3 can be excited in one additional band each and when excited there, the intensity increases from the temperature where the emissions first occur down to the lowest temperatures. A process with energy transfer from the host lattice to emission centres seems probable. Examples

CONFIGURATION COORDINATE

of such mechanisms can be found in WOd2- Fig. 13. Schematic drawing showing the potential energy

and MOO,*- compounds. The situation in curve vs. a configuration coordinate for a fluorescent

centre in its ground and excited states. these compounds has been thoroughly ex- amined by Botden [ 151 who applied the Mott- Seitz model. If we apply this model to mer- with emission of a photon or without photon

curous chloride and assume that emission 4 is emission. The quantum efficiency of the

due to the Hg,Cl* molecule, we find, if we luminescence is then

have a crystal without extra centres, the following situation (Fig. 13). I/U = Pla/(P$+Poa) = PJ(PI+Pn)

If U centres (molecules) are excited per unit time interval we have

I = U/(1 +P*/P1).

We have: pa = cl. exp (--E,/kT) where c, is a u = p,a+p,u constant and E, is the activation energy for

thermal quenching. If we assume that p1 is where u is the number of excited states and independent of temperature we have: p1 and pn are the probabilities per unit time for an excited centre to return to ground state Z=U/(l+cexp(-_E,/kT)).

OPTICAL PROPERTIES OF Hg,CI, 461

Taking I = Z. for T = 0 this can be trans- formed to

(Z,/Z) - 1 = c . exp (-EJLT).

There is no influence of excitation intensity on the temperature dependence in this model as both fluorescence and quenching is propor- tional to the excitation intensity.

However, in our case this model is not complete since emission 3 is closely connected to emission 4, as seen in the temperature dependence of the emissions (Fig. 10). This behavior can be incorporated in the Mott- Seitz model by connecting the energy transfer to recombination centres causing emission 3, with a certain thermal activation energy[ 16, 171 (Fig. 14).

The probability pt for energy transfer to an emission centre of type 3 will have the form

cz . exp (-E,/kT)

where Et is the activation energy and cp a constant (Fig. 14).

For the intensity of emission 4, f*, we get

Z,lD = plal (?+a + P,a + Pta 1

(IO/l) - 1 = c3 exp (-E,/kT) + cd exp (--Et/U)

in the same way as in the treatment of simple thermal quenching. If E, and Et are not too

CONFIGURATION COORDINATE

Fig. 14. Schematic drawing showing the potential energy curves vs. a configuration coordinate for the ground and excited states of the Hg& molecule and centre 3.

Differences between the ground states are neglected.

closely equal we can neglect one term at high temperatures and the other at low and keep only one term in each region. From Fig. 10 we see that the transfer of energy starts at lower temperatures than thermal quenching. This means that at low temperatures we have approximately:

‘Olog (Z,/f - I) = “Olog cq - E,/kT lolog e.

Figure 15 shows the result of plotting the temperature dependence curve of emission 4 as l”log(Z,/Z- 1) vs. l/T. We find two approxi- mately straight lines corresponding to the two regions of energy transfer and temperature quenching. With corrections with regard to the influence of the small term on the domina- ting one, we find Et = 0.01 eV and E, =

0.03 eV. During these measurements it was arranged so that emission 2 did not influence the result (no pre-exposure of the sample before cooling). Emission 2 has probably a similar temperature dependence as emission 3. It was only seen that when emission 2 was present, its intensity increased when the temperature decreased from 60”K, where the emission first appeared and that emission 2

I IT I K-!IO“l

Fig. 15. Temperature dependence curve of emission 4 plotted as log ((lo/l) - 1) vs. T where lo is the intensity at 0°K (extrapolated). The thermal activation energies for quenching and energy transfer are calculated from

the slopes.

462 E. EIDER

was not present at the lowest temperatures for excitation with hv > 3.9 eV.

All that was said above about energy transfer, is valid for excitation in the region hv > 3-9 eV. Besides this the luminescence of emission 2 and 3 (but not 4) can be excited in specific bands. This corresponds to the transi- tion 2 in Fig. 14. When excited in this band emission 3 increases all the way down to the lowest temperatures used ( 10°K). This is also in agreement with the model used above, since with this excitation there is no energy transfer, but only excitation and emission in the same centre.

3 is simply due to formation of stable emission centres. Emission 2 is also due to the forming of centres, but there is a complication. The emission is not constant with time but de- creases exponentially. Compare bleaching in alkali halides [ 181.

Acknowledgements -The author is deeply indebted to Professor H. G. Grime&s for his guidance and for his continuous interest in this study. He is also grateful to Ch. SLnstrand for the preparation of some of the samples and to Professor B. Aurivillius for valuable discussions.

REFERENCES

Emission 1 has no connection with the other emissions other than through the fact that the excitation spectra have the same structure at low temperatures. However, excitation spec- trum 1 is shifted O-1 eV to the low energy side. The temperature dependence of emission 1 has no connections with that of the other emissions. It has not been possible to find an explanation for this emission from the measurements made and further investigations are necessary.

I. WILKINSON J. A..J. phys. C/tern. 13,691 (1909). 2. RANDALL J. T., Nutitre, Lorzd. 142. I 13 (1938). 3. ADAMS D. IM.. GOLDSTEIN M. and MOONEY

E. F., Trans. Faruduy Sec. 59,2228 (I 963). 4. POULET H. and MATHIEU J. P., J. chim. Phys.

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7. LECOMTE J., Boll. Sci. Fat. Chim. Ind. Bolopnrr 21,51 (1963).

An interesting investigation should be the compa~son of absorption and emission spectra of Hg,Clp in its gas and solid states. However, part of the HgzC1, molecules in the vapor are dissociated to Hg and HgCl, when the crystal is heated. It also seems as if there is no suit- able solvent for mercurous chloride. Other- wise the absorption spectrum of the solution could have been studied.

8. SUCHAN H. L. and DRlCKAMER H. G.,J. Plzys. Chem. Solids 11, I 1 1 ( 1959).

9. PRINGSHEIM P., F/~ore.~ce~ce and Phosphures- cence, p. 51) 1. Interscience. New York (I 963).

10. DUMBLETON M. J., Proc. phys. Sot. BfiS, 53 (1955).

1 I. GARLICK G. F. J., Encyclupediu of Physics, Vol. 26. p. 16. Springer. Berlin (1958).

12. FOCHS P. D., Prof. phys. Sot. B69,70 (1956). 13. BELOUS V. M., BUGRIENKO V. 1. and GOLUB

More investigations are also needed to determine the nature of centres 2 and 3. From what is said in Section 3(f) about the proper- ties of emission 2 and 3, it can be concluded, that the centres are probably formed by dissociation of the Hg,CI, molecule. Emission

S. I.. Opt. Spectry 17,2 16 f 1964). 14. BELOUS V. M., Opt. Spectry. 24,308 (I 968). 15. BOTDEN Th. P. J., Philips Res. Rep. 6,425 ( 195 1). 16. KRiiCER F. A., Some kspecfs of the Luminescence

ofSolids. Ghan. 3. Elsevier. Amsterdam (I 948). 17. BLASSE G. and BRIL A., 1. electrochem. So);. 115,

1067 (1968).

t 1964)

18. COMPTON W. D. and RABIN H., Sulid Stutr Physics, Vol. i 6, p. 12 1. Academic Press, New York