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Phase equilibria in the HgSe–HgBr2–HgI2 system and crystal structure of Hg3Se2Br2 and Hg3Se2I2
Journal of Alloys and Compounds 365 (2004) 121–125
Phase equilibria in the HgSe–HgBr2 –HgI2 system and crystal structure of Hg3Se2 Br2 and Hg3Se2 I2 Yu.V. Minets∗ , Yu.V. Voroshilov, V.V. Pan’ko, V.A. Khudolii Department of Chemistry, Uzhgorod National University, 46, Pidhirna Str., 88 000 Uzhgorod, Ukraine Received 18 June 2003; accepted 20 June 2003
Abstract The phase diagrams of the Hg3 Se2 Br2 and Hg3 Se2 I2 pseudo-binary system and the isothermal section of the HgSe–HgBr2 –HgI2 ternary system at 470 K have been built using X-ray and differential thermal analyses. The crystal structures of Hg3 Se2 Br2 (I) and Hg3 Se2 I2 (II) were determined. Compound I is monoclinic [space group C2/m, a = 17.529(6), b = 9.408(4), c = 9.775(4) Å, β = 89.51(3)◦ ], compound II is also monoclinic [space group C2/m, a = 19.392(7), b = 9.658(7), c = 10.918(3) Å, β = 116.64(7)◦ ]. The main motive of both structures are [SeHg3 ] pyramids which form endless chains in I and frameworks of cubo-octahedra [IHg12 Se8 ] in II. © 2003 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Crystal structure; Phase diagram
1. Introduction This work is a continuation of systematic investigations of the pseudo-ternary systems HgX1 –HgX2 –HgHal2 and HgX–HgHal12 –HgHal22 (X = S, Se, Te, Hal = Cl, Br, I) [1–6]. The HgSe–HgBr2 system contains one ternary compound Hg3 Se2 Br2 which melts congruently at 833 K. There is a similar compound Hg3 Se2 I2 in the HgSe–HgI2 system. It is formed by a peritectic reaction at 693 K [7]. There are continuous series of solid solutions with a minimum in the HgBr2 –HgI2 system. HgI2 has a polymorphous transformation at 400 K [8]. Of the crystal structure of Hg3 Se2 Br2 it was reported [8] that its powder pattern can be indexed in the orthorhombic system with lattice parameters a = 9.42, b = 9.74, c = 8.78 Å. The structure of Hg3 Se2 I2 was not solved before the start of our investigation.
2. Experimental In order to determine the phase equilibria in the HgSe– HgBr2 –HgI2 ternary system, the vertical section of the phase ∗
Corresponding author. Tel./fax: +38-31-223-4157. E-mail address: [email protected] (Yu.V. Minets).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00656-X
diagram of Hg3 Se2 Br2 –Hg3 Se2 I2 and a series of additional alloys were investigated. The samples were prepared from binary compounds (HgSe, HgBr2 and HgI2 ) in evacuated silica ampoules. The synthesis was carried out in a vertical one-temperature furnace with maximal temperatures 820–960 K for different compositions. The alloys were annealed at 620 and 470 K for 400 h. The samples were studied by differential thermal analysis (NTR-62M low frequency thermal analyzer) and X-ray diffraction (Dron-3 diffractometer, CuK␣ radiation). The crystal structure of Hg3 Se2 Br2 was investigated on a DARCH-1 diffractometer (MoK␣ radiation). The crystals turned out to be monoclinic, pseudo-rhombic with lattice parameters similar to those found in [9] but one parameter was doubled (Table 1). Space group C2/m was chosen from three possible groups of established diffraction classes according to negative results of piezotests. The starting model of the structure was found by direct methods. Further refinement was carried out in the anisotropic approximation taking absorption correction into account. The powder pattern of Hg3 Se2 I2 was essentially similar to ␣-Hg3 S2 Br2 , so its structure was solved in this structure type [6]. The pattern was obtained on a Dron-3 diffractometer (CuK␣ radiation) and was processed by the profan program (all calculations were carried out by using the csd program package [10]) (Fig. 1). The specification of the structure of Hg3 Se2 I2 was carried out using equivalent
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Table 1 Crystallographic parameters of Hg3 Se2 Br2 and Hg3 Se2 I2
Table 2 Atomic parameters of Hg3 Se2 Br2 and Hg3 Se2 I2
Compound
Hg3 Se2 Br2
Hg3 Se2 I2
Atom
Site
x/a
y/b
z/c
Beq (Å2 )
Diffractometer Space group a (Å) b (Å) c (Å) β (◦ ) Z dcalc (g/cm3 ) Number of reflections Absorption coefficient (cm−1 ) R
Single crystal C2/m 17.529(6) 9.408(4) 9.775(4) 89.51(3) 8 7.577(9) 684 782.48 RF = 0.0517
Powder C2/m 19.392(7) 9.652(7) 10.918(3) 116.64(7) 8 7.37(1) 285 1592.78 RI = 0.0749
Hg3 Se2 Br2 Hg(1) Hg(2) Hg(3) Hg(4) Hg(5) Se(1) Se(2) Br(1) Br(2) Br(3) Br(4)
4i 4i 4h 4f 8j 8j 8j 4i 4i 4i 4i
0.2270(3) 0.5249(3) 0 0.75 0.8760(2) 0.0182(4) 0.7347(4) 0.3769(8) 0.1259(7) 0.1176(7) 0.3757(8)
0 0 0.2354(6) 0.25 0.2236(4) 0.2355(9) 0.2360(8) 0 0 0 0
0.2145(6) 0.2295(6) 0.5 0.5 0.2012(4) 0.2451(7) 0.2460(8) 0.475(2) 0.5224(14) 0.9900(14) 0.0217(13)
2.74(15) 2.9(2) 2.75(12) 2.61(12) 2.72(8) 1.8(2) 1.7(2) 2.3(7) 1.3(5) 2.0(6) 2.4(6)
Hg3 Se2 I2 Hg(1) Hg(2) Hg(3) Hg(4) Se(1) Se(2) I(1) I(2) I(3) I(4) I(5)
4i 4i 8j 8j 8j 8j 2a 2b 4i 4i 4i
0.038(4) 0.223(4) 0.117(3) 0.078(3) 0.202(7) 0.055(8) 0 0 0.120(7) 0.118(6) 0.244(5)
0 0 0.292(4) 0.251(3) 0.263(7) 0.236(8) 0 0.5 0.5 0 0.5
0.674(11) 0.295(8) 0.995(8) 0.295(6) 0.264(22) 0.735(21) 0 0 0.481(10) 0.516(10) 0.009(13)
1.827*
isotropic temperature correction B = 1.83 Å2 and taking account of the presence texture in the powder [11]. The direction of predominant orientation was (010) with texture parameter τ = 3.2(2). The results of the structure determination for both compounds are presented in Tables 1–3.
3. Results and discussion The main elements of the crystal structure of Hg3 Se2 Br2 , like in all Hg3 X2 Hal2 compounds, are [SeHg3 ] pyramids with Hg–Se–Hg angles close to 90◦ . They are connected by Hg atoms into parallel endless zigzag chains (two per unit cell) which are extending along the xz plane and form a complex cation ([Hg3 Se2 ]2+ )∞ . The Br− anions are situated inside neighboring loops of these chains and between
*, Beq = 1/3[B11 a ∗2 a2 + . . . + 2B23 b ∗ c ∗ bccosα]. Bis/eq is presented for Hg3 Se2 I2 .
the chains. All mercury atoms in the structure have the coordination of a distorted octahedron [HgSe2 Br4 ] (Table 3). The unit cell of the structure of Hg3 Se2 Br2 is shown in Fig. 2.
Fig. 1. Experimental X-ray diffraction pattern, calculated and difference diffraction profiles for Hg3 Se2 I2 .
Yu.V. Minets et al. / Journal of Alloys and Compounds 365 (2004) 121–125
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Table 3 Interatomic distances and some bond angles in the structures of Hg3 Se2 Br2 and Hg3 Se2 I2 Hg3 Se2 Br2 Hg(1) Hg(2) Hg(3) Hg(4) Hg(5) Se Br(1) Br(2) Br(3) Br(4)
Bond angle Hg–Se(1)–Hg Hg–Se(2)–Hg
–2Se –4Br –2Se −4Br –2Se –4Br –2Se –4Br –2Se –4Br –3Hg –4Hg –4Hg –4Hg –4Hg
Distance (Å)
Hg3 Se2 I2
2.507(9) 2.927(15)–3.68(2) 2.496(9) 2.998(15)–3.52(2) 2.509(8) 3.135(11)–3.305(11) 2.503(9) 3.21(1)–3.244(11) 2.516(9)–2.536(10) 2.815(10)–3.734(13) 2.496(9)–2.536(10) 3.244(11)–3.305(11) 3.210(10)–3.135(11) 2.815(10) 2.998(15)–3.20(2)
Hg(1)
ω (◦ ) 92.9(3)–94.6(3) 93.5(3)–94.5(3)
Distance (Å)
I(4) I(5)
–2Se –4I –2Se –4I –2Se –4I –2Se –4I –3Hg –4Hg –4Hg –4Hg –4Hg –1Hg –2Hg –5Hg –6Hg
Bond angle Hg–Se(1)–Hg Hg–Se(2)–Hg
ω (◦ ) 85.7(3)–108.4(4) 90.7(5)–93.7(5)
Hg(2) Hg(3) Hg(4) Se I(1) I(2) I(3)
2.36(9) 2.79(16)–4.09(16) 2.56(8) 2.94(14)–4.07(8) 2.60(21)–2.66(23) 3.04(6)–3.92(10) 2.45(19)–2.59(19) 3.01(7)–3.77(5) 2.36(9)–2.66(23) 3.63(5) 3.77(5) 3.04(6) 3.75(5) 2.94(14) 3.01(7) 2.79(16)–3.78(15) 3.14(10)–4.09(16)
In the structure of Hg3 Se2 I2 which belongs to the ␣-Hg3 S2 Br2 structure type, eight [SeHg3 ] pyramids are united with a [IHg12 ] cubo-octahedron forming the main element of this structure. All triangular faces of this cubo-octahedron are centered by Se atoms like in the -Hg3 S2 Cl2 structure type [5]. The [XHg3 ] pyramids in
Fig. 2. Crystal structure of Hg3 Se2 Br2 : (a) projection on xz plane; (b) unit cell.
Fig. 3. Crystal structure of Hg3 Se2 I2 : (a) projection of unit cell on xz plane; (b) coordination polyhedra: 1–4, [HgSe2 I4 ], 5,6, [SeHg3 ], 7, [ISe8 ]; (c) cube-octahedron [IHg12 ].
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Fig. 5. Isothermal section of HgSe–HgBr2 –HgI2 system at 470 K (1, single-phase; 2, two-phase; 3, three-phase alloys).
Fig. 4. Phase diagram of Hg3 Se2 Br2 –Hg3 Se2 I2 polythermal section and the changes of unit cell parameters of solid solutions.
both these structure types do not form endless molecules like in Hg3 Se2 Br2 but they are united into a framework of cubo-octahedra [HalHg12 X8 ] the vacancies of which are occupied by halogen atoms. The projection of the structure of Hg3 Se2 I2 and the coordination polyhedra are presented in Fig. 3. The structure of mercury selenoiodide like ␣-Hg3 S2 Br2 can be presented by volume-centered space group I2/m with lattice transformation matrix C2/m → I2/m: 001/101/010 (a = 10.918, b = 17.401, c = 9.652 Å, β = 86.6◦ ). The vertical section Hg3 Se2 Br2 –Hg3 Se2 I2 in the phase diagram of the HgSe–HgBr2 –HgI2 pseudo-ternary system is characterized by a peritectic reaction and large ranges of solid solutions. The peritectic coordinates are 95 mol% Hg3 Se2 I2 and 703 K. The solubility of the components at 620 K are 0–74 mol% Hg3 Se2 I2 (␣-solid solution) and 0–18 mol% Hg3 Se2 Br2 (-solid solution). The part of the phase diagram near the melting point of Hg3 Se2 I2 is complicated because of the peritectic reaction involving the three-phase region (L + HgSe + ) and the region of primary crystallization of mercury selenide. The changes of the lattice parameters of the ␣-and -solid solutions are shown in the bottom part of Fig. 4. The substitution of I for Br in Hg3 Se2 Br2 leads to increasing unit cell parameters within the limits: a = 17.53–18.02, b = 9.41–9.66, c = 9.78–10.07 Å, β = 89.5–89.7◦ . The lattice parameters of Hg3 Se2 I2
decrease with substitution of Br for I: a = 19.39–19.14, b = 9.67–9.64, c = 10.92–10.77 Å, β = 116.6–114.7◦ . The obtained data and X-ray analysis of additional alloys resulted in the isothermal section of the HgSe–HgBr2 –HgI2 ternary system at 470 K as shown in Fig. 5. The phase equilibria in this system are determined by the solid solutions in the Hg3 Se2 Br2 –Hg3 Se2 I2 section. In the top part of the diagram there are two two-phase regions, involving the equilibria of HgSe with Hg3 Se2 Br2 and Hg3 Se2 I2 , respectively. They are separated by the three-phase field (HgSe + Hg3 Se2 Br2 + Hg3 Se2 I2 ). A similar situation exists in the bottom part: the two two-phase fields formed by HgBr2 and HgI2 with the corresponding selenide halides are separated by a narrow three-phase region based on the two-phase equilibrium in the Hg3 Se2 Br2 –Hg3 Se2 I2 section.
References [1] V.V. Panko, V.A. Khudolii, M.Yu. Stetsovitch, Yu.V. Voroshilov, Ukr. Khim. Zh. 55 (1989) 232. [2] V.A. Khudolii, V.V. Panko, Yu.V. Voroshilov, M.S. Shelemba, Neorgan. Mater. 26 (1990) 2425. [3] V.A. Khudolii, V.V. Panko, M.S. Shelemba, L.I. Lopit, A.S. Fedor, Yu.V. Voroshilov, Zh. Neorg. Khim. 38 (1993) 1584. [4] V.A. Khudolii, V.V. Panko, Yu.V. Voroshilov, Neorgan. Mater. 31 (1995) 396. [5] Yu.V. Voroshilov, V.A. Khudolii, V.V. Panko, Zh. Neorg. Khim. 41 (1996) 287. [6] Yu.V. Voroshilov, V.A. Khudolii, V.V. Panko, Yu.V. Minets, Neorgan. Mater. 32 (1996) 1461. [7] V.V. Panko, V.A. Khudolii, Yu.V. Voroshilov, Zh. Neorg. Khim. 34 (1989) 2331.
Yu.V. Minets et al. / Journal of Alloys and Compounds 365 (2004) 121–125 [8] B.G. Korshunov, V.V. Safonov, D.V. Drobot, Fusion Diagrams For Halide Systems of Transition Metals: A Handbook, Metallurgia, Moscow, 1977. [9] H. Puff, R. Kohlschmidt, Naturwissenschaften 49 (1962) 299.
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[10] L.G. Akselrud, Yu.N. Grin, P.Yu. Zavalii, V.K. Pecharskii, V.S. Fundamenskii, Proc. XII Eur. Crystallogr. Meeting, Moscow 3 (1989) 155. [11] V.K. Pecharskii, L.G. Akselrud, P.Yu. Zavalii, Kristallography 32 (1989) 155.
Phase equilibria in the HgSe–HgBr2 –HgI2 system and crystal structure of Hg3Se2 Br2 and Hg3Se2 I2 Yu.V. Minets∗ , Yu.V. Voroshilov, V.V. Pan’ko, V.A. Khudolii Department of Chemistry, Uzhgorod National University, 46, Pidhirna Str., 88 000 Uzhgorod, Ukraine Received 18 June 2003; accepted 20 June 2003
Abstract The phase diagrams of the Hg3 Se2 Br2 and Hg3 Se2 I2 pseudo-binary system and the isothermal section of the HgSe–HgBr2 –HgI2 ternary system at 470 K have been built using X-ray and differential thermal analyses. The crystal structures of Hg3 Se2 Br2 (I) and Hg3 Se2 I2 (II) were determined. Compound I is monoclinic [space group C2/m, a = 17.529(6), b = 9.408(4), c = 9.775(4) Å, β = 89.51(3)◦ ], compound II is also monoclinic [space group C2/m, a = 19.392(7), b = 9.658(7), c = 10.918(3) Å, β = 116.64(7)◦ ]. The main motive of both structures are [SeHg3 ] pyramids which form endless chains in I and frameworks of cubo-octahedra [IHg12 Se8 ] in II. © 2003 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Crystal structure; Phase diagram
1. Introduction This work is a continuation of systematic investigations of the pseudo-ternary systems HgX1 –HgX2 –HgHal2 and HgX–HgHal12 –HgHal22 (X = S, Se, Te, Hal = Cl, Br, I) [1–6]. The HgSe–HgBr2 system contains one ternary compound Hg3 Se2 Br2 which melts congruently at 833 K. There is a similar compound Hg3 Se2 I2 in the HgSe–HgI2 system. It is formed by a peritectic reaction at 693 K [7]. There are continuous series of solid solutions with a minimum in the HgBr2 –HgI2 system. HgI2 has a polymorphous transformation at 400 K [8]. Of the crystal structure of Hg3 Se2 Br2 it was reported [8] that its powder pattern can be indexed in the orthorhombic system with lattice parameters a = 9.42, b = 9.74, c = 8.78 Å. The structure of Hg3 Se2 I2 was not solved before the start of our investigation.
2. Experimental In order to determine the phase equilibria in the HgSe– HgBr2 –HgI2 ternary system, the vertical section of the phase ∗
Corresponding author. Tel./fax: +38-31-223-4157. E-mail address: [email protected] (Yu.V. Minets).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00656-X
diagram of Hg3 Se2 Br2 –Hg3 Se2 I2 and a series of additional alloys were investigated. The samples were prepared from binary compounds (HgSe, HgBr2 and HgI2 ) in evacuated silica ampoules. The synthesis was carried out in a vertical one-temperature furnace with maximal temperatures 820–960 K for different compositions. The alloys were annealed at 620 and 470 K for 400 h. The samples were studied by differential thermal analysis (NTR-62M low frequency thermal analyzer) and X-ray diffraction (Dron-3 diffractometer, CuK␣ radiation). The crystal structure of Hg3 Se2 Br2 was investigated on a DARCH-1 diffractometer (MoK␣ radiation). The crystals turned out to be monoclinic, pseudo-rhombic with lattice parameters similar to those found in [9] but one parameter was doubled (Table 1). Space group C2/m was chosen from three possible groups of established diffraction classes according to negative results of piezotests. The starting model of the structure was found by direct methods. Further refinement was carried out in the anisotropic approximation taking absorption correction into account. The powder pattern of Hg3 Se2 I2 was essentially similar to ␣-Hg3 S2 Br2 , so its structure was solved in this structure type [6]. The pattern was obtained on a Dron-3 diffractometer (CuK␣ radiation) and was processed by the profan program (all calculations were carried out by using the csd program package [10]) (Fig. 1). The specification of the structure of Hg3 Se2 I2 was carried out using equivalent
122
Yu.V. Minets et al. / Journal of Alloys and Compounds 365 (2004) 121–125
Table 1 Crystallographic parameters of Hg3 Se2 Br2 and Hg3 Se2 I2
Table 2 Atomic parameters of Hg3 Se2 Br2 and Hg3 Se2 I2
Compound
Hg3 Se2 Br2
Hg3 Se2 I2
Atom
Site
x/a
y/b
z/c
Beq (Å2 )
Diffractometer Space group a (Å) b (Å) c (Å) β (◦ ) Z dcalc (g/cm3 ) Number of reflections Absorption coefficient (cm−1 ) R
Single crystal C2/m 17.529(6) 9.408(4) 9.775(4) 89.51(3) 8 7.577(9) 684 782.48 RF = 0.0517
Powder C2/m 19.392(7) 9.652(7) 10.918(3) 116.64(7) 8 7.37(1) 285 1592.78 RI = 0.0749
Hg3 Se2 Br2 Hg(1) Hg(2) Hg(3) Hg(4) Hg(5) Se(1) Se(2) Br(1) Br(2) Br(3) Br(4)
4i 4i 4h 4f 8j 8j 8j 4i 4i 4i 4i
0.2270(3) 0.5249(3) 0 0.75 0.8760(2) 0.0182(4) 0.7347(4) 0.3769(8) 0.1259(7) 0.1176(7) 0.3757(8)
0 0 0.2354(6) 0.25 0.2236(4) 0.2355(9) 0.2360(8) 0 0 0 0
0.2145(6) 0.2295(6) 0.5 0.5 0.2012(4) 0.2451(7) 0.2460(8) 0.475(2) 0.5224(14) 0.9900(14) 0.0217(13)
2.74(15) 2.9(2) 2.75(12) 2.61(12) 2.72(8) 1.8(2) 1.7(2) 2.3(7) 1.3(5) 2.0(6) 2.4(6)
Hg3 Se2 I2 Hg(1) Hg(2) Hg(3) Hg(4) Se(1) Se(2) I(1) I(2) I(3) I(4) I(5)
4i 4i 8j 8j 8j 8j 2a 2b 4i 4i 4i
0.038(4) 0.223(4) 0.117(3) 0.078(3) 0.202(7) 0.055(8) 0 0 0.120(7) 0.118(6) 0.244(5)
0 0 0.292(4) 0.251(3) 0.263(7) 0.236(8) 0 0.5 0.5 0 0.5
0.674(11) 0.295(8) 0.995(8) 0.295(6) 0.264(22) 0.735(21) 0 0 0.481(10) 0.516(10) 0.009(13)
1.827*
isotropic temperature correction B = 1.83 Å2 and taking account of the presence texture in the powder [11]. The direction of predominant orientation was (010) with texture parameter τ = 3.2(2). The results of the structure determination for both compounds are presented in Tables 1–3.
3. Results and discussion The main elements of the crystal structure of Hg3 Se2 Br2 , like in all Hg3 X2 Hal2 compounds, are [SeHg3 ] pyramids with Hg–Se–Hg angles close to 90◦ . They are connected by Hg atoms into parallel endless zigzag chains (two per unit cell) which are extending along the xz plane and form a complex cation ([Hg3 Se2 ]2+ )∞ . The Br− anions are situated inside neighboring loops of these chains and between
*, Beq = 1/3[B11 a ∗2 a2 + . . . + 2B23 b ∗ c ∗ bccosα]. Bis/eq is presented for Hg3 Se2 I2 .
the chains. All mercury atoms in the structure have the coordination of a distorted octahedron [HgSe2 Br4 ] (Table 3). The unit cell of the structure of Hg3 Se2 Br2 is shown in Fig. 2.
Fig. 1. Experimental X-ray diffraction pattern, calculated and difference diffraction profiles for Hg3 Se2 I2 .
Yu.V. Minets et al. / Journal of Alloys and Compounds 365 (2004) 121–125
123
Table 3 Interatomic distances and some bond angles in the structures of Hg3 Se2 Br2 and Hg3 Se2 I2 Hg3 Se2 Br2 Hg(1) Hg(2) Hg(3) Hg(4) Hg(5) Se Br(1) Br(2) Br(3) Br(4)
Bond angle Hg–Se(1)–Hg Hg–Se(2)–Hg
–2Se –4Br –2Se −4Br –2Se –4Br –2Se –4Br –2Se –4Br –3Hg –4Hg –4Hg –4Hg –4Hg
Distance (Å)
Hg3 Se2 I2
2.507(9) 2.927(15)–3.68(2) 2.496(9) 2.998(15)–3.52(2) 2.509(8) 3.135(11)–3.305(11) 2.503(9) 3.21(1)–3.244(11) 2.516(9)–2.536(10) 2.815(10)–3.734(13) 2.496(9)–2.536(10) 3.244(11)–3.305(11) 3.210(10)–3.135(11) 2.815(10) 2.998(15)–3.20(2)
Hg(1)
ω (◦ ) 92.9(3)–94.6(3) 93.5(3)–94.5(3)
Distance (Å)
I(4) I(5)
–2Se –4I –2Se –4I –2Se –4I –2Se –4I –3Hg –4Hg –4Hg –4Hg –4Hg –1Hg –2Hg –5Hg –6Hg
Bond angle Hg–Se(1)–Hg Hg–Se(2)–Hg
ω (◦ ) 85.7(3)–108.4(4) 90.7(5)–93.7(5)
Hg(2) Hg(3) Hg(4) Se I(1) I(2) I(3)
2.36(9) 2.79(16)–4.09(16) 2.56(8) 2.94(14)–4.07(8) 2.60(21)–2.66(23) 3.04(6)–3.92(10) 2.45(19)–2.59(19) 3.01(7)–3.77(5) 2.36(9)–2.66(23) 3.63(5) 3.77(5) 3.04(6) 3.75(5) 2.94(14) 3.01(7) 2.79(16)–3.78(15) 3.14(10)–4.09(16)
In the structure of Hg3 Se2 I2 which belongs to the ␣-Hg3 S2 Br2 structure type, eight [SeHg3 ] pyramids are united with a [IHg12 ] cubo-octahedron forming the main element of this structure. All triangular faces of this cubo-octahedron are centered by Se atoms like in the -Hg3 S2 Cl2 structure type [5]. The [XHg3 ] pyramids in
Fig. 2. Crystal structure of Hg3 Se2 Br2 : (a) projection on xz plane; (b) unit cell.
Fig. 3. Crystal structure of Hg3 Se2 I2 : (a) projection of unit cell on xz plane; (b) coordination polyhedra: 1–4, [HgSe2 I4 ], 5,6, [SeHg3 ], 7, [ISe8 ]; (c) cube-octahedron [IHg12 ].
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Fig. 5. Isothermal section of HgSe–HgBr2 –HgI2 system at 470 K (1, single-phase; 2, two-phase; 3, three-phase alloys).
Fig. 4. Phase diagram of Hg3 Se2 Br2 –Hg3 Se2 I2 polythermal section and the changes of unit cell parameters of solid solutions.
both these structure types do not form endless molecules like in Hg3 Se2 Br2 but they are united into a framework of cubo-octahedra [HalHg12 X8 ] the vacancies of which are occupied by halogen atoms. The projection of the structure of Hg3 Se2 I2 and the coordination polyhedra are presented in Fig. 3. The structure of mercury selenoiodide like ␣-Hg3 S2 Br2 can be presented by volume-centered space group I2/m with lattice transformation matrix C2/m → I2/m: 001/101/010 (a = 10.918, b = 17.401, c = 9.652 Å, β = 86.6◦ ). The vertical section Hg3 Se2 Br2 –Hg3 Se2 I2 in the phase diagram of the HgSe–HgBr2 –HgI2 pseudo-ternary system is characterized by a peritectic reaction and large ranges of solid solutions. The peritectic coordinates are 95 mol% Hg3 Se2 I2 and 703 K. The solubility of the components at 620 K are 0–74 mol% Hg3 Se2 I2 (␣-solid solution) and 0–18 mol% Hg3 Se2 Br2 (-solid solution). The part of the phase diagram near the melting point of Hg3 Se2 I2 is complicated because of the peritectic reaction involving the three-phase region (L + HgSe + ) and the region of primary crystallization of mercury selenide. The changes of the lattice parameters of the ␣-and -solid solutions are shown in the bottom part of Fig. 4. The substitution of I for Br in Hg3 Se2 Br2 leads to increasing unit cell parameters within the limits: a = 17.53–18.02, b = 9.41–9.66, c = 9.78–10.07 Å, β = 89.5–89.7◦ . The lattice parameters of Hg3 Se2 I2
decrease with substitution of Br for I: a = 19.39–19.14, b = 9.67–9.64, c = 10.92–10.77 Å, β = 116.6–114.7◦ . The obtained data and X-ray analysis of additional alloys resulted in the isothermal section of the HgSe–HgBr2 –HgI2 ternary system at 470 K as shown in Fig. 5. The phase equilibria in this system are determined by the solid solutions in the Hg3 Se2 Br2 –Hg3 Se2 I2 section. In the top part of the diagram there are two two-phase regions, involving the equilibria of HgSe with Hg3 Se2 Br2 and Hg3 Se2 I2 , respectively. They are separated by the three-phase field (HgSe + Hg3 Se2 Br2 + Hg3 Se2 I2 ). A similar situation exists in the bottom part: the two two-phase fields formed by HgBr2 and HgI2 with the corresponding selenide halides are separated by a narrow three-phase region based on the two-phase equilibrium in the Hg3 Se2 Br2 –Hg3 Se2 I2 section.
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