Pre- and postmelting of cadmium telluride

A stepwise character of cadmium telluride melting is shown by using differential thermal analysis and conductivity measurements in the 1323 to 1473 K temperature range. According to the results of differential thermal analysis the parameters of CdTe melting are determined by the premelting processes...

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Veröffentlicht in:Semiconductor Physics Quantum Electronics & Optoelectronics
Datum:1999
Hauptverfasser: Shcherbak, L.P., Feichouk, P.I., Plevachouk, Yu.A., Kopach, O.V., Turyanska, L.T.
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Sprache:English
Veröffentlicht: Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України 1999
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Zitieren:Pre- and postmelting of cadmium telluride / L.P. Shcherbak, P.I. Feichouk, Yu.A. Plevachouk, O.V. Kopach, L.T. Turyanska // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 4. — С. 76-80. — Бібліогр.: 21 назв. — англ.

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Digital Library of Periodicals of National Academy of Sciences of Ukraine
id nasplib_isofts_kiev_ua-123456789-120258
record_format dspace
spelling Shcherbak, L.P.
Feichouk, P.I.
Plevachouk, Yu.A.
Kopach, O.V.
Turyanska, L.T.
2017-06-11T14:09:57Z
2017-06-11T14:09:57Z
1999
Pre- and postmelting of cadmium telluride / L.P. Shcherbak, P.I. Feichouk, Yu.A. Plevachouk, O.V. Kopach, L.T. Turyanska // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 4. — С. 76-80. — Бібліогр.: 21 назв. — англ.
1560-8034
PACS: 72.20, 72.80.P
https://nasplib.isofts.kiev.ua/handle/123456789/120258
A stepwise character of cadmium telluride melting is shown by using differential thermal analysis and conductivity measurements in the 1323 to 1473 K temperature range. According to the results of differential thermal analysis the parameters of CdTe melting are determined by the premelting processes that are related to defect production in crystal lattice. The crystallization processes are controlled with the melt state (structure) that depends on its maximum temperature.
en
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
Semiconductor Physics Quantum Electronics & Optoelectronics
Pre- and postmelting of cadmium telluride
Article
published earlier
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
title Pre- and postmelting of cadmium telluride
spellingShingle Pre- and postmelting of cadmium telluride
Shcherbak, L.P.
Feichouk, P.I.
Plevachouk, Yu.A.
Kopach, O.V.
Turyanska, L.T.
title_short Pre- and postmelting of cadmium telluride
title_full Pre- and postmelting of cadmium telluride
title_fullStr Pre- and postmelting of cadmium telluride
title_full_unstemmed Pre- and postmelting of cadmium telluride
title_sort pre- and postmelting of cadmium telluride
author Shcherbak, L.P.
Feichouk, P.I.
Plevachouk, Yu.A.
Kopach, O.V.
Turyanska, L.T.
author_facet Shcherbak, L.P.
Feichouk, P.I.
Plevachouk, Yu.A.
Kopach, O.V.
Turyanska, L.T.
publishDate 1999
language English
container_title Semiconductor Physics Quantum Electronics & Optoelectronics
publisher Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
format Article
description A stepwise character of cadmium telluride melting is shown by using differential thermal analysis and conductivity measurements in the 1323 to 1473 K temperature range. According to the results of differential thermal analysis the parameters of CdTe melting are determined by the premelting processes that are related to defect production in crystal lattice. The crystallization processes are controlled with the melt state (structure) that depends on its maximum temperature.
issn 1560-8034
url https://nasplib.isofts.kiev.ua/handle/123456789/120258
citation_txt Pre- and postmelting of cadmium telluride / L.P. Shcherbak, P.I. Feichouk, Yu.A. Plevachouk, O.V. Kopach, L.T. Turyanska // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 4. — С. 76-80. — Бібліогр.: 21 назв. — англ.
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fulltext 76 © 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 4. P. 76-80. 1. Introduction In recent years an extensive literature has evolved that indi- cates at a multistage character of melting for materials with different types (ionic, covalent, metallic) of chemical bon- ding. Distinct differences exist between stages of premelting, melting, and postmelting [1-5]. The above processes affect the crystallization parameters, as well as structural charac- teristics of melt-grown crystals. This serves as an evidence that information on features of pre- and postmelting phe- nomena during interconversions between the contacting condensed phases is of both theoretical and practical value. One of matters for scientific enquiry that are of theoreti- cal, as well as practical, interest is cadmium telluride. It be- longs to a few materials that retain semiconducting proper- ties even at temperatures over that of melting (Tm) [6,7]. Unfortunately, the existing information on the structure-sen- sitive properties of CdTe near Tm is few in number and con- tradictory. Further still, when systematizing the literature data on the high-temperature physico-chemical parameters of CdTe [8], it was found that a considerable (from 1323 to 1379 K) spread exists even for the maximum Tm values. There- fore accumulation of knowledge of a correlation between the CdTe melt state and crystallization temperature Tcr is an urgent problem. A specific character of CdTe melting (manifestation of an extra endothermic effect (EEE) in melt) has been first de- monstrated in [9] using precise differential thermal analysis (DTA). This fact, however, has not received proper atten- tion. More recently, using DTA, for bulky (40 to 100 g) CdTe samples, it was found [10-12] that the EEE temperature TEEE = Tm + 10 K = 1373 K is the same as a crytical tempera- ture T*. The latter is defined in the following way. An ecess over it is accompanied by melt supercooling, ∆T - = Tm – Tcr > 0, while at melt cooling down from T < T* the ∆T - = 0. According to [10,11], EEE appears when a bistructural melt (containing microaggregates (clusters) inherited from a crys- tal) transforms into a single-structural one. DTA of melting made for CdTe samples whose masses ranged from 1 to 5 g [13] enabled to raise the EEE temperature up to a value of TEEE = Tm ± (25 - 30) K. Since in some cases EEE in CdTe melts is of pulsed type [13], it might be of interest to monitor changes in CdTe conductivity in the vicinity of Tm in a quasi- continuous mode. The aim of this paper is to study how kinetics of CdTe heating and cooling affects the features of solid-to-liquid and liquid-to-solid phase transitions. For this we used DTA and measurements of melt conductivity at temperatures up to 1473 K. 2. Experimental For our studies we took p-CdTe single crystals whose con- ductivity, σ, was 10-2Ω-1⋅cm-1, hole concentration, p, was 1014 cm-3, and hole mobility at room temperature, µ298, was from 60 to 80 cm2/V⋅s. These crystals have been Bridgman- grown from Cd and Te of high purity. After evacuation down to a pressure of 10-2 Pa in ungraphitized quartz ampoules of diameter 8 and height 25 mm, the CdTe blend (mass 0.5 g) has been melt. Then a free space over the compact sample was minimized using a quartz plug. DTA was performed using the following two units: (i) DTA 402 unit (Netsch, Germany), and (ii) a nonstandard modified unit that enables to record time-based thermograms. In the latter case Ar served as both heat-transfer agent and inert atmosphere in the furnace space. In both experimental runs temperature was recorded using Pt-Pt/Rh thermocou- PACS: 72.20, 72.80.P Pre- and postmelting of cadmium telluride L.P. Shcherbak, P.I. Feichouk, Yu.A. Plevachouk*, O.V. Kopach, L.T. Turyanska Chernivtsi Yu.Fed’kovich State University, Chernivtsi, Ukraine tel.: 380(37) 2298485; E-mail: petro@sacura.chernovtsy.ua *Institute for Applied Physics, Lviv I. Franko State University, Lviv, Ukraine Abstract. A stepwise character of cadmium telluride melting is shown by using differential thermal analysis and conductivity measurements in the 1323 to 1473 K temperature range. According to the results of differential thermal analysis the parameters of CdTe melting are determined by the premelting processes that are related to defect production in crystal lattice. The crystallization processes are controlled with the melt state (structure) that depends on its maximum temperature. Keywords: cadmium telluride, postmelting, conductivity, melt structure, polymorphic transformations. Paper received 17.09.99; revised manuscript received 15.12.99; accepted for publication 17.12.99. L.P. Shcherbak et al: Pre- and postmelting of cadmium telluride 77SQO, 2(4), 1999 ples that provided an accuracy of ± 1K. The heating (Vh) and cooling (Vc) rates were controlled within 1 to 10 K/min range. The high-temperature conductivity of melts was meas- ured using a four-probe technique in BN cells; the argon pressure, PAr, was 13.2 MPa. After heating the sample stu- died (whose mass was 3 g) up to 1473 K we recorded the change in conductivity of the melt that was cooled with a rate of 0.5 to 1 K/min. After the sample has been cooled 50 K below the crystallization temperature, it was reheated (with the same rate) up to 1473 K; its conductivity, σ(T) , was checked at time intervals of 1 min. 3. Results and discussion 3.1. Differential thermal analysis After performing a run of experiments for the same sample we have concluded that both heating rate and «previous thermal history» of a sample affect the parameters of solid- to-liquid and liquid-to-solid phase transitions in CdTe. At rapid (during 3 min) sample heating from room temperature up to 1273 K followed by heating with a rate of 10 K/min a differential thermocouple notes only one abrupt kink of the zero line. This kink is related to the endothermic effect of melting at a melting temperature value Tm(CdTe) = 1363 ± 2 K that is given in a number of handbooks. At slower (Vh = 2 K/min) heating from 1273 K, however, the readings of a differential thermocouple begin to depart from the da- tum line at temperatures that are 10 to 50 K below that at which the endothermic effect of melting at 1363 ± 2 K is recorded. In this case a weak EEE appears in a melt at 1373 K (Fig. 1, a). The features of CdTe mealting are considerably changed when performing heating-cooling cycles (temperature cy- cling) in the 1420 – 1310 K range and keeping the melt at 1420 K for 10 – 30 min. At early stages of temperature cy- cling with Vh/c = 10 K/min only EEE1 at 1373 ± 1 K is ob- served. Then, at further melting processes EEE2 and EEE3 appear at 1379 ± 1K and 1391 ± 1K, respectively (Fig. 1, b). Besides, in a number of cases EEE at 1413 ± 1 K is regis- tered. Temperature cycling with a rate Vh/c = 2 K/min is ac- companied with appearance of EEE2 and EEE3 even at the first repetition of melting. At further stages both EEE1 and EEE2 enthalpies increase. This fact is reflected in an in- crease of the corresponding EEE surface in the thermogram. Thus a character of postmelting that is observed in CdTe melts is controlled with the crystal heating kinetics. At a definite state of crystal defects it occurs not by a single, but by several stages. By varying kinetics of CdTe sample heating one can obtain one more, unexpected effect. At slow (Vh < 1 K/min) heating from room temperature or at multiple temperature cycling in the high-temperature region the sample mealting temperature, Tm, increases up to T a value of 1379 ± 1 K that coincides with that of EEE2. In this case only EEE3 is ob- served. It is of a pronounced pulsed character (Fig. 1,c). So a spread of data on CdTe Tm values that exists in the litera- ture [8] may result from the fact that the samples studied by different authors were of different defect contents. Judging from the results of investigation of correlations between overheating ( mTTT −=+ max melt˜ ) and cooling ∆T - of a melt after keeping it at a maximum temperature for 10 min (Fig. 2, a), its «thermal past history» affects also the crystal- lization parameters. Contrary to the data given in [5,6], the initial portion of the curve shown in Fig. 2, a at T < Tm +10 K is characterized by negative ∆T - values, i.e. a melt is sponta- neously crystallized at a temperature exceeding Tm. In this case the ∆T- versus ∆T + curve passes through a minimum. The fact that the crystallization temperature of a material is over Tm is an evidence that some structural units that de- crease the activation energy of nucleation have retained in a melt. They seem to be crystal-like lattice fragments (clus- ters) that retain in a melt up to Tm +10 K. The same conclu- sion can be drawn from an analysis of how the surface of the T, K a b c T, K 1373 K 1373 K 1373 K 1363 K 1363 K 1363 K 1490 K 1320 K 1379 K 1393 K 1334 K 1379 K 1391 K Fig.1. Typical thermograms of CdTe melting: a – heating from temperature Troom up to 1273 K during 3 min, then heating rate Vh = 10 and 2 K/min, respectively; b – temperature cycling in the 1420 to 1310 K range at Vh/c = 2 and 10 K/min, respectively; c – heating from Troom at Vh < 1 K/min. L.P. Shcherbak et al: Pre- and postmelting of cadmium telluride 78 SQO, 2(4), 1999 melting heat effect, Sendoeff (that is registered in a thermo- gram when the melt temperature is kept constant after it has reached Tmelt = Tm + ∆T, Fig. 2, b) depends on melt super- heating. From Fig. 2,b it follows that Sendoeff and, accord- ingly, the melting enthalpy, peak (at a given heating rate) only when having reached Tmelt = Tm + 10 K. Further in- creasing of melt superheating does not change both Sendoeff and melting enthalpy values. 3.2. High-temperature conductivity of CdTe The temperature dependence of CdTe melt conductivity for the case of Vh/c = 1 K/min is given in Fig. 3. Unlike the y (T) curve at cooling that is monotonous (close to exponential) – see the inset in Fig. 3, the y(T) curve taken at heating has a number of kinks at temperatures coinciding with those of EEE1 – EEE3. However, no conductivity jumps were ob- served. For the sake of simplicity the experimental data on y drop at melt cooling down from 1473 K are shown in Fig. 3 near Tm only. Here, as in [7], the melt conductivity hyster- esis is most pronounced. This indicates at supercooling of a highly superheated melt. Shown in Fig. 3 are also the data on y(T) for a CdTe melt whose composition differs from that studied by us (one can see this from the lower value of Tm). However, as may be seen from Fig.3, at temperatures over 1410 K both y(T) curves completely coincide. This fact may point to stability of content and/or structure of the melt structural units over the temperature range studied, what- ever the starting composition of the crystal and heating ki- netics. It should be noted that according to Ioffe’s empirical rule [14], a melt retain semiconducting features (dy/dT > 0) only if a short-range order (inherent in a crystalline phase) holds. For CdTe melt one should conclude that orientation of cova- lent bonding between the crystal components retains up to 1473 K (though, probably, it is predominantly associative). 3.3. Premelting Since EEE2 and EEE3 are observable only after high-tem- perature treatment of the solid phase, the reasons for their appearance may be premelting effects. This statement im- plies usually processes that are related to an intense pro- duction of point and linear defects. At a definite defect con- tent in a crystal the temperature dependencies of a number of structural-sensitive parameters (specific enthalpy, spe- cific heat Cp, molar volume, etc.) demonstrate extrema. Their presence is viewed as a proof of the premelting effects that are, on frequent occasion, related to jump-type changes in crystal structure [1, 2]. The temperature dependence of Cp for CdTe based on a totality of existing literature data may be approximated by two lines that are almost parallel. This does not form the basis for a conclusion that an extremum exists near Tm. How- ever, a departure of the datum line immediately before Tm (that is observed when performing temperature cycling) may Т,К -1 · - 1 heating cooling [6] -1 1310 1330 1350 1370 1390 1410 1430 1450 1470 140 120 100 80 60 40 20 0 2.15 2.05 1.95 1.85 1.75 0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 1000/T, K σ, O hm cm -1 · -1 σ , O hm cm lg Fig. 3. Temperature dependence of CdTe conductivity at CdTe melt heating and cooling; ▲ – data of [6]. Fig.2. Supercooling ∆T- (a) and endothermic effect area (b) versus CdTe melt superheating curves. Superheating ∆T + , K Su pe rc oo lin g ∆ T - ,K a bc d a 50 40 30 20 10 10 20 30 40 -10 0 Superheating ∆T+ b S , a. u. , K 0 10 20 30 40 20 10 10 5 0 en do ef f L.P. Shcherbak et al: Pre- and postmelting of cadmium telluride 79SQO, 2(4), 1999 be attributed to a change in the Cp (T) curve slope due to an intense defect production in a crystal. Indeed, according to [16] at T ≥ Tm – 50 K congruent sublimation in the CdTe lattice is enchanced. Due to a growth of the point defect (vacancies in both sublattices) content in a crystal, some prerequisites to the structural reconstruction (either local or over the whole volume) appear. There exist some infe- rences in the literature about occurrence of one or several polymorphic transformations in the CdTe lattice. These in- ferences were made basing on circumstantial evidence on the temperature dependence of a number of structural-sen- sitive properties [17]. But experimental determination of a nature of structural transitions in CdTe made by using di- rect diffraction techniques is hindered due to component volatility and chemical aggressiveness. At the same time a considerable Tm increase after the sample has been ther- mally treated at premelting temperatures for a rather long time (the effect observed in the present paper) serves as a weighty circumstantial evidence that a structural transfor- mation has occurred in the crystal studied. Taking into ac- count an occurrence of a polymorphic modifications in the crystal lattices of II – VI compounds [18], one can assume that the same process occurs also in the lattice of CdTe at high temperatures. Indeed, according to [19], a hexagonal modification was recorded, at room temperature, in the CdTe crystal that was quenched after long-term keeping at 1323 K. 3.4. Postmelting As was shown in [20], the CdTe molar volume Vm changes under melting slightly (∆Vm = 0.6%). This correlates with the evidence (given in Fig. 2, b) that melting is not com- pleted at Tm. Since the enthalpy of CdTe melting peaks only at Tm + 10 K, one should consider that below that tempera- ture some rather big fragments are present in the melt. They inherit the crystal lattice structure. Different terms (grains, aggregates, conglomerates) are used in the literature to de- note such fragments. This fact characterizes most likely dis- tinctions in size for these particles, as well as their ability to grow or disintegrate, depending on thermal fluctuations in a melt. It is most convenient to explain the superheating-super- cooling correlations (presented in Fig. 2, a) in terms of the cluster model for melting [21]. Within this model a transfor- mation of a two-structure melt with crystal-like clusters to a uniform medium may occur in two ways: through fragmen- tation (breaking up) or dissolving individual surface atoms (defect drain). In the first case the cluster sizes decrease, while the cluster content grows. Dissolution from the sur- face is accompanied by a change of cluster content that occurs in agreement with their size decreasing. According to Fig. 2,a the fragmentation mechanism is predominant near Tm and an excess of Tcryst over Tm is related to a presence in a melt of many retained crystallization centers. Over the temperature corresponding to the minimum on the ∆T- = f (∆T+) curve the defect drain mechanism becomes predomi- nant, and content of existing crystallization centers goes down. Thus presence of a minimum near Tm (Fig. 2, a) indi- cates at a change of the mechanism of structural transforma- tions in a melt under the heating conditions used. Following the line of these considerations one has to assume that at the inflection point on the ∆T- = f (∆T+) curve (that coincides with the EEE temperature) the existing clusters or smaller structural units (say, tetrahedrons) still serve as the equilib- rium crystallization centers during further cooling. At T > T (EEE1) such nuclei are absent or their mutual coordination becomes unfavorable for further size growth, and this re- sults in melt supercooling. The appearance of EEE2 and EEE3 in the CdTe melt may be explained as a result of interrelation between pre- and postmelting. On the assumption of local formation of the wurtzite phase under high-temperature treatment of the crys- tal, EEE2 may be related to a dissociation of the 6-link rings into chains; breaking of chains manifests itself in thermograms as EEE3. It is likely that a substantial (up to 1379 K) increase in the sample melting temperature that is observed at low heating rate corresponds to an increase of the high-tempera- ture phase (wurtzite) fraction or to a complete transformation of the whole volume into a high-temperature modification. Conclusions Cadmium telluride melting proceeds nonisothermically and is accompanied with only partial structural modification at Tm. Further increase of the melt temperature is accompanied by a structural transformation that occurs by stages. Every melting stage has its own supercooling depending on the value of melt superheating over Tm. In the premelting tem- perature range polymorphic transformations may occur. They lead to formation of a phase that is melting more difficultly. This results in Tm increase from 1365 to 1379 ± 1 K. References 1. A.R.Ubbelode. Melted State of Matter (Russian translation), Metallurgiya, Moscow (1982). 2. W.Hayes. Premelting // Contemp.Phys. 27(6), pp.519-532 (1986). 3. L.A.Bityutskaya, E.S.Mashkina. Features of pre-and posttransition states at copper melting (in Russian) // Pisma v ZhTF, 21(24), pp.90-93 (1995). 4. L.A. Bityutskaya, E.S.Mashkina. Cooperative effects of pre- and posttransition states at germanium melting (in Russian) // Pisma v ZhTF, 21(18), pp.8-11 (1995). 5. H.J.Koh, P.Rudolph, N.Shafer, K.Umetsu, T.Fukuda. The effect of various thermal treatments on supercooling of PbTe melts // Mater. Sci. Engin. B 34, pp.199-203 (1995). 6. V.M.Glazov, S.N.Chizhevskaya, N.N.Glagoleva. Liquid Semicon- ductors (in Russian). Nauka, Moscow (1967). 7. Yu.V.Rud’, N.V.Sanin. Conductivity of cadmium telluride in the vicinity of melting temperature (in Russian) // Fiz. Tekhn. Poluprov. 5(8), pp.1587-1595 (1971). 8. N.J. Tianrong, A. Silk, A.W. Watson et al. Thermodynamic and phase diagram assessment of the Cd-Te and Hg-Te systems // CALPHAD, (3), pp. 399-414 (1995). 9. H.Shamsuddin, A.Nazar. Thermodynamic properties of cadmium telluride // High Temp. Sci., 28, pp. 245-254 (1990). 10. P.Rudolph, M.Mülberg. Basic problem of vertical Bridgman growth of CdTe // Mater. Sci. Engin., B16, pp. 8-16 (1993). L.P. Shcherbak et al: Pre- and postmelting of cadmium telluride 80 SQO, 2(4), 1999 11. M.Mülberg, P.Rudolph, M.Laasch. Correlation between supercooling and superheating in CdTe during unseeded Bridgman growth // J. Cryst. Growth, 128, pp. 571-575 (1993). 12. P.Rudolph. Fundamental studies of Bridgman growth of CdTe // Progr. Crystal Growth and Charact., 29, pp. 275-281 (1994). 13. O.Panchouk, L.Shcherbak, P.Feichouk, Effect of CdTe «postmel- ting» // J. Cryst. Growth, 161, pp. 16-19 (1996). 14. A.Ioffe, F.Regel. Progress in Semiconductors (London) 4, p.237 (1960). 15. H.Maleki and L.R.Holand. Thermal properties of CdZnTeSe com- pounds // In: Properties of Narrow-gap Cadmium-based Com- pounds. Ed. by Capper. GEC-Marconi Infra-Red Limited. Lon- don. UK (1994). 16. Ya.Kh.Grinberg, V.N.Gus’kov, V.B.Lazarev, M.Ya. Zel’venskii. p-T-x phase equilibria in the Cd-Te system (in Russian) // Neorgan. Mater., 21(12), pp. 1991-2004 (1989). 17. Yu. M.Ivanov. Growth and homogeneity region of CdTe // J.Cryst. Growth, 161, pp.12-15 (1996). 18. Physics and Chemistry of II-VI Compounds, ed. by M.Aven and J.S.Prener. North-Holland Publ. Co., Amsterdam (1967). 19. Yu.P.Gnatenko, M.V.Kurik, V.V.Matlak. Band structure of hex- agonal CdTe //Phys. Letters, 29, pp. 522-523 (1969). 20. N.N.Kolesnikov, M.P.Kulakov, Yu.N.Ivanov. Some properties of melts of A2B6 compounds // J.Cryst. Growth, 125, pp. 576- 582 (1992). 21. V.M.Glazov. Present development of investigations of the postmelting effect in semiconductor melts (in Russian) // Neorgan. Mater. 31 (II), pp. 1287-1305 (1996).