Light-yellow europium(II) diiodide, prepared by the low-temperature reaction of europium and ammonium iodide in liquid ammonia at 200 K and characterized by high-resolution X-ray powder diffraction, represents a new phase for EuI2 that adopts an orthorhombic Pnma structure with all three atoms lying on 4c positions (.m.). It is isotypic with SrI2(IV). Temperature-dependent X-ray measurements performed to investigate the thermal stability of the new phase show that it decomposes irreversibly to amorphous material around 673 K. Total-energy density-functional calculations using the generalized gradient approximation suggest this to be the ground-state structure of EuI2.
Supporting information
All manipulations were performed under a clean argon atmosphere. Europium(II)
iodide was prepared according to the literature procedure of Howell &
Pytlewski (1969) by dissolving two equivalents of oven-dried ammonium
iodide
in liquid ammonia in a Schlenk flask cooled by an ethanol–dry-ice mixture.
The ammonia was condensed through a cooling finger placed on the Schlenk
flask. One equivalent of elemental europium was added and the reaction started
immediately. After the reaction had completed and no more hydrogen gas was
produced, the solvent was allowed to evaporate by removing the cooling bath.
The bright-yellow product was heated to 473 K for 12 h to remove any leftover
ammonia and to improve crystallization.
Density-functional total-energy calculations were performed using plane
waves/pseudopotentials and the computer program VASP (Vienna ab
initio package; Kresse & Furthmüller,
1996a,b; Kresse &
Hafner, 1993) employing the generalized gradient approximation (GGA) of
PBE
type (Perdew et al., 1996) and the projected–augmented wave
(PAW)
method (Blöchl, 1994). The cut-off energy was set to 500 eV. All four
cells
were allowed to change in volume and shape, and all atomic positions were
allowed to relax. The convergence criterion of the electronic-structure
calculation was chosen as 10 -5 eV because of the expected small energy
differences in the calculated structures.
Data collection: WinXPow (Stoe & Cie, 2005); cell refinement: WinXPow (Stoe & Cie, 2005); data reduction: WinXPow (Stoe & Cie, 2005); program(s) used to solve structure: FULLPROF (Rodriguez-Carvajal 1993); program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal 1993); molecular graphics: ATOMS (Dowty, 2005); software used to prepare material for publication: enCIFer (Allen et al., 2004).
europium(II) diiodide
top
Crystal data top
EuI2 | Dx = 5.371 Mg m−3 |
Mr = 405.76 | Cu Kα1 radiation, λ = 1.540590 Å |
Orthorhombic, Pnma | µ = 184.46 mm−1 |
a = 12.2665 (11) Å | T = 293 K |
b = 4.8875 (4) Å | Particle morphology: powder |
c = 8.3697 (8) Å | light yellow |
V = 501.78 (8) Å3 | cylinder, 0.3 × 0.3 mm |
Z = 4 | Specimen preparation: Prepared at 200 K and 101.3 kPa |
F(000) = 676 | |
Data collection top
Stoe STADI MP diffractometer | Data collection mode: transmission |
Ge monochromator | Scan method: step |
Specimen mounting: capillary tube | 2θmin = 5.031°, 2θmax = 109.011°, 2θstep = 0.020° |
Refinement top
Least-squares matrix: full | Profile function: pseudo-voigt |
Rp = 1.482 | 20 parameters |
Rwp = 1.912 | 0 restraints |
Rexp = 1.288 | 0 constraints |
RBragg = 8.093 | Weighting scheme based on measured s.u.'s Standard least squares refinement |
χ2 = 2.220 | |
5200 data points | Background function: fourier-background |
Excluded region(s): No excluded regions | Preferred orientation correction: none |
Crystal data top
EuI2 | V = 501.78 (8) Å3 |
Mr = 405.76 | Z = 4 |
Orthorhombic, Pnma | Cu Kα1 radiation, λ = 1.540590 Å |
a = 12.2665 (11) Å | µ = 184.46 mm−1 |
b = 4.8875 (4) Å | T = 293 K |
c = 8.3697 (8) Å | cylinder, 0.3 × 0.3 mm |
Data collection top
Stoe STADI MP diffractometer | Scan method: step |
Specimen mounting: capillary tube | 2θmin = 5.031°, 2θmax = 109.011°, 2θstep = 0.020° |
Data collection mode: transmission | |
Refinement top
Rp = 1.482 | χ2 = 2.220 |
Rwp = 1.912 | 5200 data points |
Rexp = 1.288 | 20 parameters |
RBragg = 8.093 | 0 restraints |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Eu1 | 0.3287 (3) | 0.75000 | 0.631 (1) | 0.0309 (18) | |
I1 | 0.3883 (4) | 0.25000 | 0.366 (2) | 0.000 (2) | |
I2 | 0.3852 (4) | 0.25000 | 0.879 (2) | 0.007 (2) | |
Geometric parameters (Å, º) top
Eu1—I1 | 3.379 (12) | Eu1—I2 | 3.281 (17) |
Eu1—I1i | 3.379 (12) | Eu1—I2i | 3.281 (17) |
Eu1—I1ii | 3.472 (6) | Eu1—I2iv | 3.365 (17) |
Eu1—I1iii | 3.311 (12) | | |
| | | |
I1—Eu1—I1i | 92.6 (3) | I1iii—Eu1—I2 | 78.1 (4) |
I1—Eu1—I1ii | 77.8 (4) | I1iii—Eu1—I2iv | 75.3 (7) |
I1—Eu1—I1iii | 124.3 (6) | I2—Eu1—I2i | 96.3 (4) |
I1—Eu1—I2 | 80.3 (3) | I2—Eu1—I2iv | 124.1 (9) |
I1—Eu1—I2i | 155.2 (8) | Eu1v—I1—Eu1vi | 102.2 (3) |
I1—Eu1—I2iv | 76.0 (4) | Eu1v—I1—Eu1iv | 102.5 (4) |
I1ii—Eu1—I1iii | 143.1 (4) | Eu1vi—I1—Eu1iv | 144.0 (3) |
I1ii—Eu1—I2 | 77.5 (5) | Eu1v—I2—Eu1iii | 103.4 (5) |
I1ii—Eu1—I2iv | 141.7 (5) | | |
Symmetry codes: (i) x, y+1, z; (ii) −x+1, y+1/2, −z+1; (iii) −x+1/2, −y+1, z+1/2; (iv) −x+1/2, −y+1, z−1/2; (v) x, y−1, z; (vi) −x+1, y−1/2, −z+1. |
Experimental details
Crystal data |
Chemical formula | EuI2 |
Mr | 405.76 |
Crystal system, space group | Orthorhombic, Pnma |
Temperature (K) | 293 |
a, b, c (Å) | 12.2665 (11), 4.8875 (4), 8.3697 (8) |
V (Å3) | 501.78 (8) |
Z | 4 |
Radiation type | Cu Kα1, λ = 1.540590 Å |
µ (mm−1) | 184.46 |
Specimen shape, size (mm) | Cylinder, 0.3 × 0.3 |
|
Data collection |
Diffractometer | Stoe STADI MP diffractometer |
Specimen mounting | Capillary tube |
Data collection mode | Transmission |
Scan method | Step |
2θ values (°) | 2θmin = 5.031 2θmax = 109.011 2θstep = 0.020 |
|
Refinement |
R factors and goodness of fit | Rp = 1.482, Rwp = 1.912, Rexp = 1.288, RBragg = 8.093, χ2 = 2.220 |
No. of data points | 5200 |
No. of parameters | 20 |
Selected bond lengths (Å) topEu1—I1 | 3.379 (12) | Eu1—I2 | 3.281 (17) |
Eu1—I1i | 3.472 (6) | Eu1—I2iii | 3.365 (17) |
Eu1—I1ii | 3.311 (12) | | |
Symmetry codes: (i) −x+1, y+1/2, −z+1; (ii) −x+1/2, −y+1, z+1/2; (iii) −x+1/2, −y+1, z−1/2. |
Because of the exceptional luminescence properties of Eu2+ ions, there is an ongoing interest in synthetic techniques to generate europium(II) compounds easily at high purities. During a low-temperature synthesis of europium(II) iodide we found a new phase of this compound.
At the moment there are three well described phases of europium(II) iodide: EuI2 crystallizing in P21/c (Bärnighausen & Schulz, 1969) and in Pbca (Bärnighausen & Schulz, 1969), and EuI2 synthesized at high pressure crystallizing in Pnma (Beck, 1979). In 1989 (Liu & Eick, 1989) a new strontium(II) iodide structure in Pnma, called SrI2(IV), was suggested. It was also mentioned by these authors that this structure was known for SmI2 and EuI2, but the details were never published. In 1995 (Wang et al., 1995), this same europium(II) iodide was reported, too, but the phase was not stable at ambient temperature and could only be stabilized through `high-pressure treatment', despite the fact that its molar volume is larger than those of all other phases. We now report a simple low-temperature route to this EuI2 phase, which should indeed be called EuI2(IV) by analogy with SrI2(IV).
The compound crystallizes in the centrosymmetric orthorhombic space group Pnma with all three symmetry-inequivalent atoms situated on 4c positions (.m.) (Fig. 1). The structure is built up by edge-sharing polyhedra. The sevenfold coordination of Eu2+, resulting in monocapped trigonal prisms, is typical for the europium(II) iodides. In the coordination polyhedron, a mirror plane passes through atoms I1i, I1ii, I2iii and Eu1, such that there are four different Eu—I bond lengths averaging 3.35 Å (Fig. 2; symmetry codes given in Fig. 2?).
To compare the four different structures, the packing of the EuI7 polyhedra must be considered. In the EuI2(IV) phase, the structure is built up by edge-sharing polyhedra forming parallel two-dimensional layers within the bc plane. The layers are connected through additional edge-sharing of the monocapped site. EuI2 in P21/c and Pbca exhibit very similar packing to each other, in which the polyhedra are tilted such that the layer structure is broken up into a three-dimensional network of edge- and corner-sharing polyhedra. The high-pressure EuI2 structure in Pnma contains strongly distorted EuI7 polyhedra forming a dense three-dimensional network. There is no obvious relationship between the two structures crystallizing in Pnma.
For thermochemical analysis, temperature-dependent X-ray diffraction was performed using a Huber Guinier diffractometer G644 in the temperature range 300–773 K (Fig. 3). It is obvious that the phase starts decomposing at about 640 K where all four reflections diminish in intensity. After heating, the resulting compound was slowly cooled down to room temperature but the reflections of the original phase were not re-observed; instead, the unstructured diffraction data point towards amorphous EuI2. Thus, the phase transition is irreversible because EuI2(IV) seemingly does not recrystallize from a high temperature.
Fig. 4 displays the course of the integrated intensity of the (112) reflection versus temperature, which matches the steady increase in the lattice parameters due to higher thermal motion. To determine the decomposition temperature more reliably, an exponential fit according to IBragg = a(Tc - T)b was performed using the integrated intensities of the (112) reflections at different temperatures; the critical temperature Tc equals the decomposition temperature. On the basis of the data points up to 680 K, the parameters give a = 250 (15) K-1, a decomposition temperature of Tc = 673 (1) K and b = 0.034 (7). Data at higher temperatures were not taken into account due to complicated behaviour that may be due to inhomogeneous heating of the sample.
All four europium iodides in which Eu2+ experiences sevenfold coordination were theoretically compared by means of total-energy density-functional calculations. The results (Fig. 5) corroborate the experimental results. EuI2 in Pnma obtained at high pressure and high temperature lies 11–16 kJ mol-1 higher in energy than the other phases and has the lowest volume. EuI2 in Pbca and P21/c may be obtained using similar experimental conditions, and the theoretical calculations show that they are close to each other in energy and volume. The low-temperature EuI2(IV) phase, also crystallizing in Pnma, has the highest volume but lies lowest in energy.