EuI₂, a low-temperature europium(II) iodide phase

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.

S1. Comment

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 I_Bragg = a(T_c - T)^b was performed using the integrated intensities of the (112) reflections at different temperatures; the critical temperature T_c 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 T_c = 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.

S2. Experimental

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.

S3. Refinement

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.

Computing details

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).

Figures

	Fig. 1. The unit cell of the title compound, with all atoms drawn as spheres of arbitrary radii. [Symmetry codes: (i) -x + 1, y + 1/2, -z + 1; (ii) -x + 1/2, -y + 1, z + 0.5; (iii) -x + 1/2, -y + 1, z - 1/2; (iv) x + 1/2, y, -z + 1/2; (v) x + 1/2, -y + 1/2, -z + 1/2; (vi) -x, y + 1/2, -z.]
	Fig. 2. Schematic view of the EuI7 polyhedron. Atoms Eu1, I1^i, I1^ii and I2^iii form a mirror plane passing through the polyhedron. [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; (iv) x, y + 1, z.]
	Fig. 3. The temperature-dependent X-ray powder diffraction of EuI2(IV). In the electronic version of the journal, high and low absolute intensities are displayed in yellow and blue, respectively. [AUTHOR: Please examine in greyscale (for printed journal) to see if a revised presentation is required]
	Fig. 4. Plot of the integrated intensity (triangles) and the position of the (112) reflection (circles) versus temperature. At temperatures above 680 K, the linear decrease of the position of the (112) reflection and the trend of the integrated intensity are interrupted.
	Fig. 5. Energy–volume plot of the four known EuI2 phases on the basis of density-functional (GGA) calculations.

europium(II) diiodide

Crystal data

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) Å^3	cylinder, 0.3 × 0.3 mm
Z = 4	Specimen preparation: Prepared at 200 K and 101.3 kPa
F(000) = 676

Data collection

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

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
χ^2 = 2.220
5200 data points	Background function: fourier-background
Excluded region(s): No excluded regions	Preferred orientation correction: none

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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)

Eu1—I1	3.379 (12)	Eu1—I2	3.281 (17)
Eu1—I1^i	3.379 (12)	Eu1—I2^i	3.281 (17)
Eu1—I1^ii	3.472 (6)	Eu1—I2^iv	3.365 (17)
Eu1—I1^iii	3.311 (12)
I1—Eu1—I1^i	92.6 (3)	I1^iii—Eu1—I2	78.1 (4)
I1—Eu1—I1^ii	77.8 (4)	I1^iii—Eu1—I2^iv	75.3 (7)
I1—Eu1—I1^iii	124.3 (6)	I2—Eu1—I2^i	96.3 (4)
I1—Eu1—I2	80.3 (3)	I2—Eu1—I2^iv	124.1 (9)
I1—Eu1—I2^i	155.2 (8)	Eu1^v—I1—Eu1^vi	102.2 (3)
I1—Eu1—I2^iv	76.0 (4)	Eu1^v—I1—Eu1^iv	102.5 (4)
I1^ii—Eu1—I1^iii	143.1 (4)	Eu1^vi—I1—Eu1^iv	144.0 (3)
I1^ii—Eu1—I2	77.5 (5)	Eu1^v—I2—Eu1^iii	103.4 (5)
I1^ii—Eu1—I2^iv	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.