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Structural investigation of the phase transitions of Tribromo ammonium mercurate (II) monohydrate, NH4HgBr3·H2O
Journal of Alloys and Compounds 463 (2008) 100–106
Structural investigation of the phase transitions of Tribromo ammonium mercurate (II) monohydrate, NH4HgBr3·H2O M. Loukil a,b,∗ , A. Kabadou b , A. Ben Salah b , H. Fuess a a b
Institute for Materials Science, Darmstadt University of Technology, Petersensstrasse 23, D-64287, Germany Laboratoire des Sciences des Mat´eriaux et d’Environnement, Facult´e des Sciences de Sfax, Sfax 3018, Tunisie Received 19 July 2007; received in revised form 25 August 2007; accepted 27 August 2007 Available online 6 September 2007
Abstract Single crystal diffraction, Infrared spectroscopy and differential scanning calorimetry DSC techniques have been used to investigate the different phases of NH4 HgBr3 ·H2 O, Tribromo ammonium mercurate (II) monohydrate, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 198 and 340 K, by DSC experiment. X-ray diffraction measurements confirm the presence of the first anomaly. At ˚ b = 17.226(2) A, ˚ room temperature NH4 HgBr3 ·H2 O, crystallizes in the orthorhombic space group Cmc21 , with the lattice constants a = 4.475(1) A, ˚ and Z = 4. Below 200 K the structure is monoclinic P21 with: a = 4.379(4) A, ˚ b = 17.220(10) A, ˚ c = 10.103(2) A, ˚ β = 90.023(9)◦ and c = 10.240(2) A Z = 2 (T = 120 K). The mercury atom is surrounded by four bromine atoms in an irregular tetrahedron. The tetrahedra are linked at two corners, resulting in infinite (HgBr3 )n− n chains along the “a” axis. The ammonium groups are located between the chains ensuring the stability of the structure by hydrogen bonding contacts: N H· · ·Br, O H· · ·Br, N H· · ·O. The structural phase transformation was attributed to an orientational disorder of ammonium groups. © 2007 Elsevier B.V. All rights reserved. Keywords: IR; DSC; Ammonium orientational disorder; Single crystal structure; Low-temperature
1. Introduction The ternary compounds of general formulae AHgX3 (where A is a monovalent cation and X a halide) exhibit interesting structural and physical properties. The coordination of HgII in this family of compounds seems to depend upon the complexity of the A cation and the nature of the halogen. Where A is a simple cation, e.g. an alkali metal (Na, K, Rb) such as in KHgBr3 ·H2 O [1], the characteristic coordination of mercury is tetrahedral with two short Hg–Br bond lengths, always close to the sum of the covalent radii, the additional two Br atoms are significantly further away but at distances which are less than the sum of the van der Waals radii. However, in CsHgBr3 material [2] the mercury atoms are octahedrally coordinated. Partly to study new structural types and partly to search for new physical proper∗
Corresponding author. Tel.: +216 9663 9332; fax: +216 7427 4437. E-mail address: [email protected] (M. Loukil).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.086
ties, we surveyed the NH4 Br–HgBr2 phase diagrams. This work continues our X-ray studies of mercury salts of general formulas NH4 HgX3 and (NH4 )2 HgX4 (where X is a halide ion). The authors are interested in this compounds because they show a rich variety of phase transitions and undergo interesting physical properties related with hydrogen bonds, such as ␣-NH4 HgCl3 [3–4], Cs0.7 (NH4 )0.3 HgCl3 [5] and Cs0.92 (NH4 )0.08 HgBr4 [6]. In previous papers we reported the crystal structure of (NH4 )2 HgCl4 ·H2 O [7] and NH4 HgI3 ·H2 O [8]. The present paper is concerned with the crystal structure of the new title compound at two different temperatures (120 and 299 K). 2. Experimental details 2.1. Synthesis Thin colourless plates of NH4 HgBr3 ·H2 O were obtained at room temperature by slow evaporation from water hot solution of HgBr2 and excess of NH4 Br [2].
M. Loukil et al. / Journal of Alloys and Compounds 463 (2008) 100–106
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Table 1 Crystal data and structure refinement for NH4 HgBr3 ·H2 O at room and lowtemperatures Temperature (K) Formula weight (g mol−1 ) Crystal system, space group ˚ a (A) ˚ b (A) ˚ c (A) β (◦ ) ˚ 3) V (A Z Crystal size (mm3 ) F(0 0 0) ˚ Wavelength (A) Diffractometer
Scan mode θ range for data collection (◦ ) Index ranges
Independent reflections Reflections with Iobs > 2σ(Iobs ) Absorption correction Transmission factors Rint and Rσ Reliability factor (R1 )a Reliability factor (WR2 )b Flack x
120(2) 476.38
299(2) 476.38
Monoclinic, P21 (no. 4) 4.3792(4) 17.2201(10) 10.103(2) 90.023(9) 761.86(17) 4 0.30 × 0.18 × 0.08 824 0.71073 (Mo K␣) OxfordSapphire CCD 4ω 9.34–26.37
Orthorhombic, Cmc21 (no. 36)
−5 ≤ h ≤ 5 −14 ≤ k ≤ 21 −12 ≤ l ≤ 12
−6 ≤ h ≤ 6 −23 ≤ k ≤ 24 −12 ≤ l ≤ 14
2263 1309
936 517
Numeric analytical Tmin = 0.017; Tmax = 0.244 0.067 and 0.053 0.048 0.101
Numeric analytical Tmin = 0.034; Tmax = 0.168 0.096 and 0.0718 0.055 0.146
0.05(4)
0.04(6)
4.4755(10) 17.226(2) 10.240(2) – 789.5(3) 4 0.30 × 0.18 × 0.08 824 0.71073 (Mo K␣) Oxford-Sapphire CCD
4ω 9.87–30
2 (w|Fo |2 −|Fc |2 ) Where a R1 = |Fo | − |Fc |/|Fo | and b WR2 = . 2 2 (w|Fo | )
Fig. 1. The DSC curve obtained from NH4 HgBr3 ·H2 O on both cooling and heating.
2.3. Thermal and spectroscopic analysis Thermal analysis techniques were employed to characterize phase transitions of the title compound. Differential scanning calorimetry (DSC) measurements were carried out using a Setaram DSC121 calorimeter in the temperature range 170–520 K at a heating rate of 5 K min−1 using a polycrystalline sample in a flowing nitrogen atmosphere. Infrared absorption spectra of suspensions of crystalline powders in KBr were recorded on a PERKIN-ELMER 1750 spectrophotometer in the 400–4000 cm−1 range. The heating of the sample was performed in an air-atmosphere SPECTAC heating cell. A thermocouple EUROTHERM REGLER was used for the temperature measurements.
3. Results and discussion
2.2. X-ray diffraction and data collection at different temperatures
3.1. DSC measurements
The crystal structure of ammonium tribromide monohydrate has been determined at different temperatures by single crystal X-ray methods. A thin transparent plate crystal with dimensions (0.30 × 0.18 × 0.08) was chosen from the preparation. The intensity data were collected on an OXFORD DIFFRACTION XCALIBUR four-circle diffractometer using ˚ and equipped with graphite monochromatized Mo K␣ radiation (λ = 0.7173 A), a SAPPHIR CCD two-dimensional detector. The temperature range from 300 to 120 K was achieved using cold-nitrogen gas controlled by OXFORD Cryojet controller. Lorentz and polarizing effect corrections were carried out before the refinement. A numeric analytical absorption correction was carried out with the program CrysAlis RED [9]. Mercury and bromine atoms were located using the SHELXS-97 program [10], whereas N atoms were deduced from a Fourier-difference map during the refinement of the structure with an adapted version of the SHELXL-97 program [11]. The non-hydrogen atoms were refined anisotropically. The H atoms were located geometrically, and attributed isotropic thermal factors equal to those of the atoms to which they are linked. Crystal data and conditions of intensity collections for both room and low-temperatures are given in Table 1. The atomic coordinates and equivalent thermal parameters are given in Table 2, the anisotropic displacement parameters in Table 3 and the bond lengths and angles in Table 4. The structural graphics were created with the DIAMOND program [12].
The differential scanning calorimetry (DSC) measurements were used to identify the phase transition temperatures. A typical DSC curve obtained from NH4 HgBr3 ·H2 O crystals on both cooling and heating is shown in Fig. 1. We observe two endothermic anomalies, at T1 = 198 K and at T2 = 340 K. The corresponding enthalpy changes are H1 = 5.4 J/g and H2 = 16.7 J/g, respectively. 3.2. Structure description (at T = 299 K) At room temperature (299 K), NH4 HgBr3 ·H2 O crystallizes in Cmc21 , orthorhombic space group (Fig. 2). The structure of NH4 HgBr3 ·H2 O at room temperature consists of heavily distorted HgBr4 tetrahedra which are linked at tow corners, resulting in infinite (HgBr3 )n− n chains along the aaxis. The other bromine atoms are held by hydrogen bonds of ammonium and water molecules. A comparison between the arrangements of the two salts NH4 HgBr3 ·H2 O and KHgBr3 ·H2 O [1] shows that they are
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Table 2 Comparison of atomic coordinates and equivalent thermal parameters Site x
y
z
Ueq
Room-temperature (299 K) Hg Br(1) Br(2) Br(3) N O
8b 4a 8b 8b 4a 8b
−1/2 0 −1/2 −1/2 0 −1/2
0.99901(6) 0.99879(12) 1.13756(14) 0.85915(14) 1.2087(15) 1.3010(7)
−0.0425(16) 0.1305(16) −0.1085(15) −0.1160(15) 0.1437(12) 0.1385(18)
0.0790(7) 0.0429(6) 0.0480(10) 0.0386(7) 0.106(17) 0.038(4)
Low -temperature (120 K) Hg(1) Br(1) Br(2) Br(3) N(1) O(1) Hg(2) Br(4) Br(5) Br(6) N(2) O(2)
2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a
0.25011(18) 0.2482(7) 0.7436(3) 0.2527(7) 0.741(3) −0.259(4) 0.25029(18) −0.2412(5) 0.2484(7) 0.2507(7) 1.236(4) 0.243(4)
0.9260(5) 0.7884(5) 0.9275(6) 1.0659(5) 0.7291(7) 0.7254(6) 0.9291(5) 0.9287(6) 0.7899(5) 1.0683(6) 0.6259(10) 0.6244(10)
0.18445(9) 0.2646(4) 0.00973(19) 0.2595(3) 0.0054(16) −0.4920(16) −0.31543(9) −0.4848(3) −0.2386(4) −0.2351(3) −0.023(2) −0.5229(19)
0.0253(4) 0.0191(8) 0.0060(4) 0.0116(7) 0.004(4) 0.024(5) 0.0254(4) 0.0230(6) 0.0222(8) 0.0174(8) 0.024(6) 0.018(5)
Ueq = 13 Σi Σj Uij ai∗ aj∗ ai aj .
isostructural and they crystallize in the same orthorhombic space group Cmc21 . However, we observe that the substitution of K+ by NH4 + leads to an increase of the unit cell parameters and volume. 3.2.1. Mercury cations environment The Hg atoms are surrounded by four bromine atoms in the form an irregular tetrahedron with distances between 2.481(3) ˚ The two shortest distances correspond to covaand 2.854(3) A. lent symmetric bonds Hg Br(2) and Hg Br(3) with an angle of
99.82(7)◦ . The other four distances correspond to the coordination bonds Hg Br(1) with an angle of 146.83(15)◦ (Fig. 3). 3.2.2. Ammonium cations environment The ammonium cations are located between the HgBr4 tetrahedra ensuring the stability of the structure by hydrogen bonding contacts: N H· · ·Br, O H· · ·Br, N H· · ·O. At room temperature, the ammonium groups are coordinated by six bromine atoms [three Br(2) and three Br(3)] belonging to the HgBr4 tetrahedra and two oxygen atoms at N Br distances between
Table 3 ˚ 2 ) at different temperatures Anisotropic displacement parameters (in 10−3 A Atoms
U11
U22
U33
U23
Room-temperature (299 K) Hg 0.0681(8) Br(1) 0.0382(11) Br(2) 0.0646(15) Br(3) 0.0382(11) N 0.048(16) O 0.024(7)
0.0326(6) 0.0304(10) 0.0303(11) 0.0377(12) 0.023(5) 0.007(5)
0.1364(15) 0.0600(13) 0.049(3) 0.0398(19) 0.043(12) 0.081(9)
−0.0027(8) −0.0010(12) 0.0060(12) −0.0086(12) 0.07(3) −0.010(7)
Low-temperature (120 K) Hg(1) 0.0222(4) Hg(2) 0.0231(4) Br(1) 0.0180(16) Br(2) 0.0031(6) Br(3) 0.0215(15) Br(4) 0.0232(9) Br(5) 0.0159(15) Br(6) 0.0225(16) O(1) 0.046(15) O(2) 0.010(8)
0.0118(11) 0.0117(11) 0.0179(18) 0.0047(10) 0.0046(15) 0.0190(15) 0.025(2) 0.0109(18) 0.053(17) 0.012(9)
0.0420(6) 0.0415(6) 0.0214(17) 0.0103(8) 0.0086(12) 0.0269(13) 0.0258(18) 0.0189(15) 0.000(8) 0.013(10)
0.0002(9) −0.0009(9) 0.0041(19) −0.0003(18) −0.0005(15) 0.005(2) 0.0069(19) −0.0034(18) −0.002(10) 0.016(8)
Uij = exp[−2π2 (h2a* 2U11 + · · · + 2hka* b* U22 + · · ·)].
U13
U12
0 0 0 0 0 0
0 0 0 0 0 0
0.0006(4) 0.0006(4) 0.0073(12) 0.0024(6) −0.0060(12) −0.0057(8) 0.0112(13) −0.0102(13) 0.000(8) −0.002(7)
0.0063(6) 0.0048(6) 0.0025(11) 0.0022(12) 0.0004(10) 0.0034(18) 0.0013(12) 0.0038(11) −0.021(12) 0.008(7)
M. Loukil et al. / Journal of Alloys and Compounds 463 (2008) 100–106
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Fig. 2. Perspective view of the structure of NH4 HgBr3 ·H2 O at 120 and 299 K.
˚ and N O distances about 2.745(4) A ˚ 3.526(2) and 3.713(3) A (Fig. 3). 3.3. Comparison of the structures at room and low-temperatures At low-temperature (120 K), the title compound crystallizes in the P21 monoclinic space group. It follows that the decrease
of temperature does not affect the structural arrangement, the main feature of the structure remains based on (HgBr3 )n− n chains running along a-axis (Fig. 2). 3.3.1. Temperature effect on the lattice parameters The temperature dependence of all lattice parameters and the cell volume was determined from single crystal data collected in the temperature range between 120 and 299 K.
Fig. 3. Environment of mercury and nitrogen atoms at 120 and 299 K.
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Table 4 ˚ and angles (◦ ) at different temperatures Comparison of bond distances (A) T = 120 K
T = 299 K
Bonds in HgBr4 tetrahedra ˚ Hg Br(1) (A) ˚ Hg Br(2) (A)
2.749(12)–2.834(13) 2.504(12)–2.520(12)
2.854(3) 2.481(3)–2.524(3)
Angles in HgBr4 tetrahedra Bri Hg Brj (min) (◦ ) Bri Hg Brj (max) (◦ )
100.23(14)–100.76(16) 143.34(8)–143.68(8)
99.82(7) 146.83(15)
(c) However, symmetry decrease lead to large changes in the environment of the nitrogen atoms: at 120 K the nitrogen site divides into two crystallographic independent sites (Fig. 4). Decreasing temperature from 299 to 120 K leads to a relative displacement of nitrogen atoms accompanied by a large
Nitrogen Bromine bonds ˚ N Br (A)
3.416(3)–3.614(8) for N(1) 3.37(5)–3.57(2) for N(2)
3.526(4)–3.713(9)
The net behaviour is an increase of the unit cell volume as well as the “a” and “c” parameters especially with increasing temperature (Fig. 4). This corresponds to the increase of the Hg Br(1) distances, directed along [1 0 1] direction (Table 4). On the other hand, “b” and “β” parameters remain practically constant with temperature. However, “b”, “c” and “V” parameters especially exhibit one anomaly at 200 K which coincides with the exothermic peak already observed in the DSC curve. Albeit weak evidence, these results strongly suggest that there are changes in the crystal structure at this temperature. Furthermore, from Fig. 4, one notices that the monoclinic “β” angle decreases and approaches 90◦ with increasing temperature. This behaviour indicates a trend of NH4 HgBr3 ·H2 O to higher symmetry in the room-temperature phase. 3.3.2. Temperature effect on the crystal structure Decreasing temperature gives the following results: (a) NH4 HgBr3 ·H2 O crystals present two different phases on cooling from room temperature. The structural phase transformation can be attributed to an orientational disorder of ammonium groups as also observed in the majority of ammonium mercury compounds such as ␣-NH4 HgCl3 [3–4], Cs0.7 (NH4 )0.3 HgCl3 [5], (NH4 )2 Hg3 Cl8 ·2H2 O [14] and Cs0.92 (NH4 )0.08 HgBr4 [6]. According to the single crystal X-ray diffraction and DSC results already presented here the phase transition sequence for NH4 HgBr3 ·H2 O would be summarized as follows:
In general, no significant modifications in the internal structure of a tetrahedron could be noted in the two phases (120 and 299 K). (b) The symmetry decreases when the temperature decreases and the agreement factor decreased from 0.055 to 0.048 and the thermal motion of Hg and Br is reduced. Thus, the mercury atom at low temperature tends to have more ordered and stable tetrahedral coordination.
Fig. 4. Temperature dependence of volume “V” and the lattice parameters “a”, “b”, “c”, and “β” for NH4 HgBr3 ·H2 O.
M. Loukil et al. / Journal of Alloys and Compounds 463 (2008) 100–106
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Table 5 Infrared frequencies (cm−1 ) and band assignments at room and high temperature T = 300 K IR frequencies 450 1408 1614 3012 3120 3588
T = 540 K (cm−1 )
Assignment
Intensity
IR frequencies
m vs m sh s mb
– 1392 – 2998 3158 –
(cm−1 )
Intensity – vs – sh s –
H2 O libration νNH4 + H O H deformation νNH4 + νNH4 + O H stretching
Relative intensities: sh, shoulder; m, medium; mb, medium broad; s, strong; vs, very strong.
decrease of the thermal motion, resulting in a contraction of the cavity around the ammonium groups (Table 4): this decrease is induced by the hydrogen bond network between N atoms of the ammonium groups and Br atoms. (d) By examination of Table 4, one observes especially slightly ˚ at 299 K instead of shorter distances Hg-Br (2.481(3) A ˚ 2.504(12) A at 120 K). At 120 K we also notice, amongst the HgBr4 groups, that the largest bond Hg-Br increases ˚ relatively from 2.834(13) to 2.854(3) A. An obvious explanation for the phase transitions is the assumption of an “order-disorder” reorientation of the NH4 + group. As a conclusion, based on the present results, we can propose that the ammonium group (NH4 + ) exhibits a dynamic orientational disorder at room temperature, which might be similar to that observed in many ammonium compounds such as NH4 HgI3 ·H2 O [13] and (NH4 )2 Hg3 Cl8 ·2H2 O [14]. 3.4. IR spectroscopy investigation We have undertaken an IR study between 300 and 550 K, in order to gain more information on the crystal dynamics and to determine the nature of the high temperature phase transi-
tion (Fig. 5). The frequencies of the observed two phases are listed in Table 5. The IR bands associated with the NH4 + group were assigned by comparison with the spectra of NH4 HgI3 ·H2 O [13]. From these curves, it could be seen that up to about 340 K, the intensities of the bands characteristic of water molecules at about 492, 1594 and 3550 cm−1 decrease gradually and vanish totally at about 550 K. These spectroscopic results show that the title compound loses water molecules of crystallization gradually from 340 to 550 K. 4. Summary The ammonium mercury compound NH4 HgBr3 ·H2 O crystallises in the orthorhombic space group Cmc21 , and in the monoclinic P21 space group at 299 and 120 K temperatures, respectively. The main building blocks of the title compound, are strongly distorted [HgBr4 ] tetrahedra connected via common bromine atoms, which result in a one dimensional arrangement of (HgBr3 )n− n channels propagating parallel to the a-axis. The crystalline building stability is ensured by hydrogen bonding contacts: N H· · ·Br, O H· · ·Br and N H· · ·O which provide a linkage between ammonium groups, water molecules, and (HgBr3 )n− n chains. It follows that the decrease of temperature does not affect the structural arrangement, but it leads to a decrease of lattice cell dimensions. The ammonium disorder at room temperature influences the distortion of the tetrahedral [HgBr4 ] resulting from the increase of the electrostatic interaction between N and Br atoms. The single crystal X-ray diffraction and DSC results show that at low-temperature NH4 HgBr3 ·H2 O plates present a phase transition from monoclinic through orthorhombic room temperature structure. IR-spectroscopy shows that this compound loses crystallization water gradually in the temperature range [340–550 K]. This phase transformation can be explained by an orientational “order-disorder” phase transition of NH4 tetrahedra at low-temperature. Acknowledgements
Fig. 5. IR spectra at different temperatures of NH4 HgBr3 ·H2 O.
M. Loukil would like to thank the Deutscher Akademischer Austausch-Dienst (DAAD) for support in enabling the visit to
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Darmstadt (Germany). This study has been supported by the Tunisian Ministry of Scientific Research and Technology. References [1] V.M. Padmanabham, V.S. Yadava, Acta Cryst. B25 (1969) 647. [2] G. Natta, L. Passerini, Gazz. Chim. Itali 58 (1928) 472. [3] K. Negita, N. Nakamura, H. Chihara, Chem. Phys. Lett. 63 (N1) (1979) 187. [4] T. Asaji, T. Ishizaka, Z. Naturforsch. 55 (a) (2000) 83. [5] A. Kabadou, R. Ben Hassan, A. Ben Salah, T. Jouini, Phys. Stat. Sol. 208 (b) (1998) 387. [6] S. Walha, A. Kabadou, R. Ben Hassen, A. Madani, J. Jaud, R. Abdelhedi, Ben A. Salah, Ann. Chim. Sci. Mat. 26 (5) (2001) 43. [7] M. Loukil, A. Kabadou, I. Svoboda, H. Ehrenberg, A. Ben Salah, H. Fuess, Z. Kristallogr, NCS 218 (2003) 29.
[8] M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah, H. Fuess, Z. Kristallogr, NCS 218 (2003) 269. [9] R.C. Clark, J.S. Reid, CrysAlis RED, Oxford Diffraction Ltd., Program for analytical numeric absorption correction, Version 170.17 (2003). [10] G.M. Sheldrick, SHELXS-97, program for the solution of crystal structures, University of G¨ottingen, Germany, 1990. [11] G.M. Sheldrick, SHELXL-97, program for crystal structure determination, University of G¨ottingen, Germany, 1997. [12] K. Brandenburg, DIAMOND, Crystal Impact GbR, Version 2.1 e, Bonn, Germany, 2001. [13] M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah, H. Fuess, J. Alloys Comp. 386 (2005) 107. [14] M. Loukil, A. Kabadou, A. Ben Salah, H. Fuess, J. Alloys Comp. 428 (2007) 65.
Structural investigation of the phase transitions of Tribromo ammonium mercurate (II) monohydrate, NH4HgBr3·H2O M. Loukil a,b,∗ , A. Kabadou b , A. Ben Salah b , H. Fuess a a b
Institute for Materials Science, Darmstadt University of Technology, Petersensstrasse 23, D-64287, Germany Laboratoire des Sciences des Mat´eriaux et d’Environnement, Facult´e des Sciences de Sfax, Sfax 3018, Tunisie Received 19 July 2007; received in revised form 25 August 2007; accepted 27 August 2007 Available online 6 September 2007
Abstract Single crystal diffraction, Infrared spectroscopy and differential scanning calorimetry DSC techniques have been used to investigate the different phases of NH4 HgBr3 ·H2 O, Tribromo ammonium mercurate (II) monohydrate, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 198 and 340 K, by DSC experiment. X-ray diffraction measurements confirm the presence of the first anomaly. At ˚ b = 17.226(2) A, ˚ room temperature NH4 HgBr3 ·H2 O, crystallizes in the orthorhombic space group Cmc21 , with the lattice constants a = 4.475(1) A, ˚ and Z = 4. Below 200 K the structure is monoclinic P21 with: a = 4.379(4) A, ˚ b = 17.220(10) A, ˚ c = 10.103(2) A, ˚ β = 90.023(9)◦ and c = 10.240(2) A Z = 2 (T = 120 K). The mercury atom is surrounded by four bromine atoms in an irregular tetrahedron. The tetrahedra are linked at two corners, resulting in infinite (HgBr3 )n− n chains along the “a” axis. The ammonium groups are located between the chains ensuring the stability of the structure by hydrogen bonding contacts: N H· · ·Br, O H· · ·Br, N H· · ·O. The structural phase transformation was attributed to an orientational disorder of ammonium groups. © 2007 Elsevier B.V. All rights reserved. Keywords: IR; DSC; Ammonium orientational disorder; Single crystal structure; Low-temperature
1. Introduction The ternary compounds of general formulae AHgX3 (where A is a monovalent cation and X a halide) exhibit interesting structural and physical properties. The coordination of HgII in this family of compounds seems to depend upon the complexity of the A cation and the nature of the halogen. Where A is a simple cation, e.g. an alkali metal (Na, K, Rb) such as in KHgBr3 ·H2 O [1], the characteristic coordination of mercury is tetrahedral with two short Hg–Br bond lengths, always close to the sum of the covalent radii, the additional two Br atoms are significantly further away but at distances which are less than the sum of the van der Waals radii. However, in CsHgBr3 material [2] the mercury atoms are octahedrally coordinated. Partly to study new structural types and partly to search for new physical proper∗
Corresponding author. Tel.: +216 9663 9332; fax: +216 7427 4437. E-mail address: [email protected] (M. Loukil).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.086
ties, we surveyed the NH4 Br–HgBr2 phase diagrams. This work continues our X-ray studies of mercury salts of general formulas NH4 HgX3 and (NH4 )2 HgX4 (where X is a halide ion). The authors are interested in this compounds because they show a rich variety of phase transitions and undergo interesting physical properties related with hydrogen bonds, such as ␣-NH4 HgCl3 [3–4], Cs0.7 (NH4 )0.3 HgCl3 [5] and Cs0.92 (NH4 )0.08 HgBr4 [6]. In previous papers we reported the crystal structure of (NH4 )2 HgCl4 ·H2 O [7] and NH4 HgI3 ·H2 O [8]. The present paper is concerned with the crystal structure of the new title compound at two different temperatures (120 and 299 K). 2. Experimental details 2.1. Synthesis Thin colourless plates of NH4 HgBr3 ·H2 O were obtained at room temperature by slow evaporation from water hot solution of HgBr2 and excess of NH4 Br [2].
M. Loukil et al. / Journal of Alloys and Compounds 463 (2008) 100–106
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Table 1 Crystal data and structure refinement for NH4 HgBr3 ·H2 O at room and lowtemperatures Temperature (K) Formula weight (g mol−1 ) Crystal system, space group ˚ a (A) ˚ b (A) ˚ c (A) β (◦ ) ˚ 3) V (A Z Crystal size (mm3 ) F(0 0 0) ˚ Wavelength (A) Diffractometer
Scan mode θ range for data collection (◦ ) Index ranges
Independent reflections Reflections with Iobs > 2σ(Iobs ) Absorption correction Transmission factors Rint and Rσ Reliability factor (R1 )a Reliability factor (WR2 )b Flack x
120(2) 476.38
299(2) 476.38
Monoclinic, P21 (no. 4) 4.3792(4) 17.2201(10) 10.103(2) 90.023(9) 761.86(17) 4 0.30 × 0.18 × 0.08 824 0.71073 (Mo K␣) OxfordSapphire CCD 4ω 9.34–26.37
Orthorhombic, Cmc21 (no. 36)
−5 ≤ h ≤ 5 −14 ≤ k ≤ 21 −12 ≤ l ≤ 12
−6 ≤ h ≤ 6 −23 ≤ k ≤ 24 −12 ≤ l ≤ 14
2263 1309
936 517
Numeric analytical Tmin = 0.017; Tmax = 0.244 0.067 and 0.053 0.048 0.101
Numeric analytical Tmin = 0.034; Tmax = 0.168 0.096 and 0.0718 0.055 0.146
0.05(4)
0.04(6)
4.4755(10) 17.226(2) 10.240(2) – 789.5(3) 4 0.30 × 0.18 × 0.08 824 0.71073 (Mo K␣) Oxford-Sapphire CCD
4ω 9.87–30
2 (w|Fo |2 −|Fc |2 ) Where a R1 = |Fo | − |Fc |/|Fo | and b WR2 = . 2 2 (w|Fo | )
Fig. 1. The DSC curve obtained from NH4 HgBr3 ·H2 O on both cooling and heating.
2.3. Thermal and spectroscopic analysis Thermal analysis techniques were employed to characterize phase transitions of the title compound. Differential scanning calorimetry (DSC) measurements were carried out using a Setaram DSC121 calorimeter in the temperature range 170–520 K at a heating rate of 5 K min−1 using a polycrystalline sample in a flowing nitrogen atmosphere. Infrared absorption spectra of suspensions of crystalline powders in KBr were recorded on a PERKIN-ELMER 1750 spectrophotometer in the 400–4000 cm−1 range. The heating of the sample was performed in an air-atmosphere SPECTAC heating cell. A thermocouple EUROTHERM REGLER was used for the temperature measurements.
3. Results and discussion
2.2. X-ray diffraction and data collection at different temperatures
3.1. DSC measurements
The crystal structure of ammonium tribromide monohydrate has been determined at different temperatures by single crystal X-ray methods. A thin transparent plate crystal with dimensions (0.30 × 0.18 × 0.08) was chosen from the preparation. The intensity data were collected on an OXFORD DIFFRACTION XCALIBUR four-circle diffractometer using ˚ and equipped with graphite monochromatized Mo K␣ radiation (λ = 0.7173 A), a SAPPHIR CCD two-dimensional detector. The temperature range from 300 to 120 K was achieved using cold-nitrogen gas controlled by OXFORD Cryojet controller. Lorentz and polarizing effect corrections were carried out before the refinement. A numeric analytical absorption correction was carried out with the program CrysAlis RED [9]. Mercury and bromine atoms were located using the SHELXS-97 program [10], whereas N atoms were deduced from a Fourier-difference map during the refinement of the structure with an adapted version of the SHELXL-97 program [11]. The non-hydrogen atoms were refined anisotropically. The H atoms were located geometrically, and attributed isotropic thermal factors equal to those of the atoms to which they are linked. Crystal data and conditions of intensity collections for both room and low-temperatures are given in Table 1. The atomic coordinates and equivalent thermal parameters are given in Table 2, the anisotropic displacement parameters in Table 3 and the bond lengths and angles in Table 4. The structural graphics were created with the DIAMOND program [12].
The differential scanning calorimetry (DSC) measurements were used to identify the phase transition temperatures. A typical DSC curve obtained from NH4 HgBr3 ·H2 O crystals on both cooling and heating is shown in Fig. 1. We observe two endothermic anomalies, at T1 = 198 K and at T2 = 340 K. The corresponding enthalpy changes are H1 = 5.4 J/g and H2 = 16.7 J/g, respectively. 3.2. Structure description (at T = 299 K) At room temperature (299 K), NH4 HgBr3 ·H2 O crystallizes in Cmc21 , orthorhombic space group (Fig. 2). The structure of NH4 HgBr3 ·H2 O at room temperature consists of heavily distorted HgBr4 tetrahedra which are linked at tow corners, resulting in infinite (HgBr3 )n− n chains along the aaxis. The other bromine atoms are held by hydrogen bonds of ammonium and water molecules. A comparison between the arrangements of the two salts NH4 HgBr3 ·H2 O and KHgBr3 ·H2 O [1] shows that they are
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Table 2 Comparison of atomic coordinates and equivalent thermal parameters Site x
y
z
Ueq
Room-temperature (299 K) Hg Br(1) Br(2) Br(3) N O
8b 4a 8b 8b 4a 8b
−1/2 0 −1/2 −1/2 0 −1/2
0.99901(6) 0.99879(12) 1.13756(14) 0.85915(14) 1.2087(15) 1.3010(7)
−0.0425(16) 0.1305(16) −0.1085(15) −0.1160(15) 0.1437(12) 0.1385(18)
0.0790(7) 0.0429(6) 0.0480(10) 0.0386(7) 0.106(17) 0.038(4)
Low -temperature (120 K) Hg(1) Br(1) Br(2) Br(3) N(1) O(1) Hg(2) Br(4) Br(5) Br(6) N(2) O(2)
2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a
0.25011(18) 0.2482(7) 0.7436(3) 0.2527(7) 0.741(3) −0.259(4) 0.25029(18) −0.2412(5) 0.2484(7) 0.2507(7) 1.236(4) 0.243(4)
0.9260(5) 0.7884(5) 0.9275(6) 1.0659(5) 0.7291(7) 0.7254(6) 0.9291(5) 0.9287(6) 0.7899(5) 1.0683(6) 0.6259(10) 0.6244(10)
0.18445(9) 0.2646(4) 0.00973(19) 0.2595(3) 0.0054(16) −0.4920(16) −0.31543(9) −0.4848(3) −0.2386(4) −0.2351(3) −0.023(2) −0.5229(19)
0.0253(4) 0.0191(8) 0.0060(4) 0.0116(7) 0.004(4) 0.024(5) 0.0254(4) 0.0230(6) 0.0222(8) 0.0174(8) 0.024(6) 0.018(5)
Ueq = 13 Σi Σj Uij ai∗ aj∗ ai aj .
isostructural and they crystallize in the same orthorhombic space group Cmc21 . However, we observe that the substitution of K+ by NH4 + leads to an increase of the unit cell parameters and volume. 3.2.1. Mercury cations environment The Hg atoms are surrounded by four bromine atoms in the form an irregular tetrahedron with distances between 2.481(3) ˚ The two shortest distances correspond to covaand 2.854(3) A. lent symmetric bonds Hg Br(2) and Hg Br(3) with an angle of
99.82(7)◦ . The other four distances correspond to the coordination bonds Hg Br(1) with an angle of 146.83(15)◦ (Fig. 3). 3.2.2. Ammonium cations environment The ammonium cations are located between the HgBr4 tetrahedra ensuring the stability of the structure by hydrogen bonding contacts: N H· · ·Br, O H· · ·Br, N H· · ·O. At room temperature, the ammonium groups are coordinated by six bromine atoms [three Br(2) and three Br(3)] belonging to the HgBr4 tetrahedra and two oxygen atoms at N Br distances between
Table 3 ˚ 2 ) at different temperatures Anisotropic displacement parameters (in 10−3 A Atoms
U11
U22
U33
U23
Room-temperature (299 K) Hg 0.0681(8) Br(1) 0.0382(11) Br(2) 0.0646(15) Br(3) 0.0382(11) N 0.048(16) O 0.024(7)
0.0326(6) 0.0304(10) 0.0303(11) 0.0377(12) 0.023(5) 0.007(5)
0.1364(15) 0.0600(13) 0.049(3) 0.0398(19) 0.043(12) 0.081(9)
−0.0027(8) −0.0010(12) 0.0060(12) −0.0086(12) 0.07(3) −0.010(7)
Low-temperature (120 K) Hg(1) 0.0222(4) Hg(2) 0.0231(4) Br(1) 0.0180(16) Br(2) 0.0031(6) Br(3) 0.0215(15) Br(4) 0.0232(9) Br(5) 0.0159(15) Br(6) 0.0225(16) O(1) 0.046(15) O(2) 0.010(8)
0.0118(11) 0.0117(11) 0.0179(18) 0.0047(10) 0.0046(15) 0.0190(15) 0.025(2) 0.0109(18) 0.053(17) 0.012(9)
0.0420(6) 0.0415(6) 0.0214(17) 0.0103(8) 0.0086(12) 0.0269(13) 0.0258(18) 0.0189(15) 0.000(8) 0.013(10)
0.0002(9) −0.0009(9) 0.0041(19) −0.0003(18) −0.0005(15) 0.005(2) 0.0069(19) −0.0034(18) −0.002(10) 0.016(8)
Uij = exp[−2π2 (h2a* 2U11 + · · · + 2hka* b* U22 + · · ·)].
U13
U12
0 0 0 0 0 0
0 0 0 0 0 0
0.0006(4) 0.0006(4) 0.0073(12) 0.0024(6) −0.0060(12) −0.0057(8) 0.0112(13) −0.0102(13) 0.000(8) −0.002(7)
0.0063(6) 0.0048(6) 0.0025(11) 0.0022(12) 0.0004(10) 0.0034(18) 0.0013(12) 0.0038(11) −0.021(12) 0.008(7)
M. Loukil et al. / Journal of Alloys and Compounds 463 (2008) 100–106
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Fig. 2. Perspective view of the structure of NH4 HgBr3 ·H2 O at 120 and 299 K.
˚ and N O distances about 2.745(4) A ˚ 3.526(2) and 3.713(3) A (Fig. 3). 3.3. Comparison of the structures at room and low-temperatures At low-temperature (120 K), the title compound crystallizes in the P21 monoclinic space group. It follows that the decrease
of temperature does not affect the structural arrangement, the main feature of the structure remains based on (HgBr3 )n− n chains running along a-axis (Fig. 2). 3.3.1. Temperature effect on the lattice parameters The temperature dependence of all lattice parameters and the cell volume was determined from single crystal data collected in the temperature range between 120 and 299 K.
Fig. 3. Environment of mercury and nitrogen atoms at 120 and 299 K.
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Table 4 ˚ and angles (◦ ) at different temperatures Comparison of bond distances (A) T = 120 K
T = 299 K
Bonds in HgBr4 tetrahedra ˚ Hg Br(1) (A) ˚ Hg Br(2) (A)
2.749(12)–2.834(13) 2.504(12)–2.520(12)
2.854(3) 2.481(3)–2.524(3)
Angles in HgBr4 tetrahedra Bri Hg Brj (min) (◦ ) Bri Hg Brj (max) (◦ )
100.23(14)–100.76(16) 143.34(8)–143.68(8)
99.82(7) 146.83(15)
(c) However, symmetry decrease lead to large changes in the environment of the nitrogen atoms: at 120 K the nitrogen site divides into two crystallographic independent sites (Fig. 4). Decreasing temperature from 299 to 120 K leads to a relative displacement of nitrogen atoms accompanied by a large
Nitrogen Bromine bonds ˚ N Br (A)
3.416(3)–3.614(8) for N(1) 3.37(5)–3.57(2) for N(2)
3.526(4)–3.713(9)
The net behaviour is an increase of the unit cell volume as well as the “a” and “c” parameters especially with increasing temperature (Fig. 4). This corresponds to the increase of the Hg Br(1) distances, directed along [1 0 1] direction (Table 4). On the other hand, “b” and “β” parameters remain practically constant with temperature. However, “b”, “c” and “V” parameters especially exhibit one anomaly at 200 K which coincides with the exothermic peak already observed in the DSC curve. Albeit weak evidence, these results strongly suggest that there are changes in the crystal structure at this temperature. Furthermore, from Fig. 4, one notices that the monoclinic “β” angle decreases and approaches 90◦ with increasing temperature. This behaviour indicates a trend of NH4 HgBr3 ·H2 O to higher symmetry in the room-temperature phase. 3.3.2. Temperature effect on the crystal structure Decreasing temperature gives the following results: (a) NH4 HgBr3 ·H2 O crystals present two different phases on cooling from room temperature. The structural phase transformation can be attributed to an orientational disorder of ammonium groups as also observed in the majority of ammonium mercury compounds such as ␣-NH4 HgCl3 [3–4], Cs0.7 (NH4 )0.3 HgCl3 [5], (NH4 )2 Hg3 Cl8 ·2H2 O [14] and Cs0.92 (NH4 )0.08 HgBr4 [6]. According to the single crystal X-ray diffraction and DSC results already presented here the phase transition sequence for NH4 HgBr3 ·H2 O would be summarized as follows:
In general, no significant modifications in the internal structure of a tetrahedron could be noted in the two phases (120 and 299 K). (b) The symmetry decreases when the temperature decreases and the agreement factor decreased from 0.055 to 0.048 and the thermal motion of Hg and Br is reduced. Thus, the mercury atom at low temperature tends to have more ordered and stable tetrahedral coordination.
Fig. 4. Temperature dependence of volume “V” and the lattice parameters “a”, “b”, “c”, and “β” for NH4 HgBr3 ·H2 O.
M. Loukil et al. / Journal of Alloys and Compounds 463 (2008) 100–106
105
Table 5 Infrared frequencies (cm−1 ) and band assignments at room and high temperature T = 300 K IR frequencies 450 1408 1614 3012 3120 3588
T = 540 K (cm−1 )
Assignment
Intensity
IR frequencies
m vs m sh s mb
– 1392 – 2998 3158 –
(cm−1 )
Intensity – vs – sh s –
H2 O libration νNH4 + H O H deformation νNH4 + νNH4 + O H stretching
Relative intensities: sh, shoulder; m, medium; mb, medium broad; s, strong; vs, very strong.
decrease of the thermal motion, resulting in a contraction of the cavity around the ammonium groups (Table 4): this decrease is induced by the hydrogen bond network between N atoms of the ammonium groups and Br atoms. (d) By examination of Table 4, one observes especially slightly ˚ at 299 K instead of shorter distances Hg-Br (2.481(3) A ˚ 2.504(12) A at 120 K). At 120 K we also notice, amongst the HgBr4 groups, that the largest bond Hg-Br increases ˚ relatively from 2.834(13) to 2.854(3) A. An obvious explanation for the phase transitions is the assumption of an “order-disorder” reorientation of the NH4 + group. As a conclusion, based on the present results, we can propose that the ammonium group (NH4 + ) exhibits a dynamic orientational disorder at room temperature, which might be similar to that observed in many ammonium compounds such as NH4 HgI3 ·H2 O [13] and (NH4 )2 Hg3 Cl8 ·2H2 O [14]. 3.4. IR spectroscopy investigation We have undertaken an IR study between 300 and 550 K, in order to gain more information on the crystal dynamics and to determine the nature of the high temperature phase transi-
tion (Fig. 5). The frequencies of the observed two phases are listed in Table 5. The IR bands associated with the NH4 + group were assigned by comparison with the spectra of NH4 HgI3 ·H2 O [13]. From these curves, it could be seen that up to about 340 K, the intensities of the bands characteristic of water molecules at about 492, 1594 and 3550 cm−1 decrease gradually and vanish totally at about 550 K. These spectroscopic results show that the title compound loses water molecules of crystallization gradually from 340 to 550 K. 4. Summary The ammonium mercury compound NH4 HgBr3 ·H2 O crystallises in the orthorhombic space group Cmc21 , and in the monoclinic P21 space group at 299 and 120 K temperatures, respectively. The main building blocks of the title compound, are strongly distorted [HgBr4 ] tetrahedra connected via common bromine atoms, which result in a one dimensional arrangement of (HgBr3 )n− n channels propagating parallel to the a-axis. The crystalline building stability is ensured by hydrogen bonding contacts: N H· · ·Br, O H· · ·Br and N H· · ·O which provide a linkage between ammonium groups, water molecules, and (HgBr3 )n− n chains. It follows that the decrease of temperature does not affect the structural arrangement, but it leads to a decrease of lattice cell dimensions. The ammonium disorder at room temperature influences the distortion of the tetrahedral [HgBr4 ] resulting from the increase of the electrostatic interaction between N and Br atoms. The single crystal X-ray diffraction and DSC results show that at low-temperature NH4 HgBr3 ·H2 O plates present a phase transition from monoclinic through orthorhombic room temperature structure. IR-spectroscopy shows that this compound loses crystallization water gradually in the temperature range [340–550 K]. This phase transformation can be explained by an orientational “order-disorder” phase transition of NH4 tetrahedra at low-temperature. Acknowledgements
Fig. 5. IR spectra at different temperatures of NH4 HgBr3 ·H2 O.
M. Loukil would like to thank the Deutscher Akademischer Austausch-Dienst (DAAD) for support in enabling the visit to
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Darmstadt (Germany). This study has been supported by the Tunisian Ministry of Scientific Research and Technology. References [1] V.M. Padmanabham, V.S. Yadava, Acta Cryst. B25 (1969) 647. [2] G. Natta, L. Passerini, Gazz. Chim. Itali 58 (1928) 472. [3] K. Negita, N. Nakamura, H. Chihara, Chem. Phys. Lett. 63 (N1) (1979) 187. [4] T. Asaji, T. Ishizaka, Z. Naturforsch. 55 (a) (2000) 83. [5] A. Kabadou, R. Ben Hassan, A. Ben Salah, T. Jouini, Phys. Stat. Sol. 208 (b) (1998) 387. [6] S. Walha, A. Kabadou, R. Ben Hassen, A. Madani, J. Jaud, R. Abdelhedi, Ben A. Salah, Ann. Chim. Sci. Mat. 26 (5) (2001) 43. [7] M. Loukil, A. Kabadou, I. Svoboda, H. Ehrenberg, A. Ben Salah, H. Fuess, Z. Kristallogr, NCS 218 (2003) 29.
[8] M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah, H. Fuess, Z. Kristallogr, NCS 218 (2003) 269. [9] R.C. Clark, J.S. Reid, CrysAlis RED, Oxford Diffraction Ltd., Program for analytical numeric absorption correction, Version 170.17 (2003). [10] G.M. Sheldrick, SHELXS-97, program for the solution of crystal structures, University of G¨ottingen, Germany, 1990. [11] G.M. Sheldrick, SHELXL-97, program for crystal structure determination, University of G¨ottingen, Germany, 1997. [12] K. Brandenburg, DIAMOND, Crystal Impact GbR, Version 2.1 e, Bonn, Germany, 2001. [13] M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah, H. Fuess, J. Alloys Comp. 386 (2005) 107. [14] M. Loukil, A. Kabadou, A. Ben Salah, H. Fuess, J. Alloys Comp. 428 (2007) 65.