Ca2IrD5: an order–disorder transition

Journal of Alloys and Compounds 363 (2004) 99–103

Ca2 IrD5: an order–disorder transition Ralph O. Moyer Jr. a , Brian H. Toby b,∗ b

a Department of Chemistry, Trinity College, Hartford, CT 06106-3100, USA NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-8562, USA

Received 16 January 2003; received in revised form 7 April 2003; accepted 11 April 2003

Abstract Neutron powder diffraction data were collected on a sample of Ca2 IrD5 for five temperatures from 20 to 295 K. At room temperature, the symmetry is face-centered cubic, but between 295 and 275 K a phase transition to body-centered tetragonal symmetry is seen. In the cubic phase, there is a single octahedral D site with 5/6 occupancy and an Ir–D distance of 1.702(3) Å. In the tetragonal phase, there are two D sites. The apical D site is 50% occupied and has an Ir–D bond distance of 1.805(6) Å, while the equatorial site is fully occupied and has an Ir–D bond distances of 1.681(3) Å. Weighted profile R-factors for the cubic and tetragonal structures are 0.0716, and 0.0733, respectively. In contrast to Sr2 IrD5 and Eu2 IrD5 , where minor amounts of the cubic phase persisted well below the cubic to tetragonal phase transition temperature, all Ca2 IrD5 completely transformed. Published by Elsevier B.V. Keywords: Metal hydrides; Chemical synthesis; Neutron diffraction; Crystal structure and symmetry; Phase transitions

1. Introduction The first homoleptic ternary metal hydrides (TMHs) possessing the well-known K2 PtCl6 structure appeared in the literature thirty years ago and currently more than a dozen have been reported [1,2]. Each of these compounds is composed of a d-block transition metal (TM) of the groups 8, 9 or 10, most commonly from the rarer platinum group: Ru, Os, Rh, or Ir, where the TM atom takes the Pt site of K2 PtCl6 . The eight-fold K+ site is most commonly occupied by a relatively strong electropositive element (M) from groups 1 or 2. It is well known that the crystal chemistry of divalent Eu and Yb, for binary oxides and hydrides, is similar to that of the heavier alkaline earth ions. These divalent lanthanide ions also occupy the K sites in several TMH compounds [3–6]. Structural phase transitions can occur for this class of TMHs. For the six-coordinate TMHs having the general formula M2 (TM)H6 , where TM is a group 8 element, there are no examples of compounds undergoing a temperature-dependent phase transition at atmospheric pres∗ Corresponding author. Tel.: +1-301-975-4297; fax: +1-301-921-9847. E-mail address: [email protected] (B.H. Toby).

0925-8388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/S0925-8388(03)00457-2

sure. Not so with TMHs with the formula M2 (TM)H5 ,where the TM is a member of the group 9 series. For those compounds that undergo a temperature-dependent phase change, the disordered high temperature cubic structure transforms to a more ordered tetragonal structure at low temperatures. The first example was reported by Zhuang et al. [7] for Sr2 IrD5 , but it was estimated that approximately 13% of the cubic phase remained untransformed even at 4.2 K. Close on the heels of this paper was another report by Zhuang et al. [8] with a low temperature structural study of Eu2 IrD5 that curiously gave no evidence of a structural phase transition. This work is now under question, as a very recent study by Kohlmann et al. [9], with a new preparation of Eu2 IrD5 , shows that the structural behavior of this compound does indeed behave similarly to Sr2 IrD5 . The cubic phase of Eu2 IrD5 also persists well below the phase transition temperature [9]. Strangely, when Rh is substituted for Ir in these M2 (TM)H5 compositions, there is no structural phase transition. Bronger et al. [10,11] have reported this to be the case when M=Ca or Sr; this was confirmed for Ca2 RhD5 in the course of this study. Interestingly, Zolliker et al. [12] reported that Mg2 CoD5 transforms from a disordered high temperature face centered cubic structure to a completely ordered primitive tetragonal form between 620 and 770 K.

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Mixed crystal TMHs of the K2 PtCl6 structural family have been reported, i.e. [M2−x Eux ]RuH6 where M=Ca or Sr, [Sr2−x Eux ](TM)H5 where TM=Ir or Rh, and [Ca2−x Eux ]IrH5 where 0 ≤ x ≤ 2 [14–16]. All of the materials obey Vegard’s rule, where complete miscibility occurs at the 8-fold sites of the platinumate structure. The rule requires that the substituting pair of atoms must be in the same group and have atomic radii within 15%. To extend Vegard’s rule with a substitution at the 4-fold sites, we attempted to form solid solutions of Ca2 [Ir1−x Rhx ]D5 , since both Ir and Rh are in group 9, have the same oxidation state and have almost indistinguishable atomic radii. However, repeated synthesis attempts resulted in mixtures of Ca2 IrD5 and Ca2 RhD5 , implying that Ca2 [Ir1−x Rhx ]D5 does not form below 700 ◦ C. We report here the complete structural elucidation for Ca2 IrD5 . The synthesis and structural study by X-ray powder diffraction of Ca2 IrH5 first appeared in 1971 [13].

2. Experimental

Table 1 Summary of Rietveld refinement parameters for Ca2 IrD5 Cell Temperature (K) a (Å) c (Å) Unit cell volume (Å3 ) Space group Refined composition Weighted profile R-factor (Rwp ) Unweighted profile R-factor (Rp ) Reduced χ2 Integrated intensity R-factor (RF2 ) Reflection d-space range (Å−1 ) Number of reflections Ir impurity mass fraction (%)

Cubic 295 7.2479(2) 380.74(1) Fm3m Ca2 IrD5.04(7) 0.0862 0.0716 0.880 0.0606 0.8–4.2 38 5.8 (5)

Tetragonal 20 5.0314(2) 7.4373(4) 188.28(1) I4/mmm Ca2 IrD5.12(6) 0.0923 0.0733 0.911 0.0593 0.8–4.2 78 4.4 (3)

closed-cycle He refrigerator was used for temperature control. Crystallographic analyses were performed using Rietveld analysis, as implemented in the GSAS software using the EXPGUI interface [17–19]. For the principal phase, the low-angle peak asymmetry model of Finger et al. [20] was used.

2.1. Synthesis Ca2 IrD5 was formed by a high temperature solid/gas phase reaction between calcium deuteride (CaD2 ) and iridium at approximately 700 ◦ C and ≈100 kPa (1 atmosphere) of deuterium. The general procedure was reported earlier [13] and is epitomized with the following equation: 2CaD2 (s) + Ir(s) + 1/2D2 → Ca2 IrD5 (s) An additional step, where the CaD2 is sieved through an 80-mesh screen was added to this procedure, as will be discussed further in Section 3. The same conditions were used to form Ca2 RhD5 . These conditions were also used in our unsuccessful attempts to form the mixed crystal system Ca2 [Ir1−x Rhx ]D5 . In this case, CaD2 was reacted with mixtures of Ir and Rh, with Ir:Rh ratios of 3:1, 1:1 and 1:3. 2.2. Neutron powder diffraction Neutron powder diffraction data were collected using the 32 detector BT-1 neutron powder diffractometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research reactor, NBSR. A Cu(311) monochromator with a 90◦ take-off angle, λ=1.5402(1) Å, as well as a Ge(311) monochromator with a 75◦ take-off angle, λ=2.0783(2) Å. For both an in-pile collimation of 15 min of arc were used. Data were collected over the range of 3–168◦ 2θ with a step size of 0.05◦ . The instrument is further described in the NCNR website (http://www. ncnr.nist.gov/xtal). Samples of ≈1 g were sealed in a 6-mm diameter vanadium container inside a He filled glove box where H2 O and O2 levels are monitored and are typically <10 ppm. A

3. Results and discussion The structure of Ca2 IrD5 was determined at 295 and 20 K from the neutron diffraction data shown in Fig. 1. A relatively minor amount of unreacted Ir metal was noted as an impurity phase in the samples. However, no CaD2 reactant was observed in these samples. It is instructive to note that in our initial experiments, unreacted CaD2 was always seen in the neutron diffraction patterns, despite the absence of peaks for this phase in the X-ray diffraction patterns. We interpreted this to mean that Ir reacts by diffusing into the CaD2 and that unreacted CaD2 remained at the core of the larger grains. Sieving the crushed CaD2 to remove the larger grains eliminated the unreacted CaD2 from the product. The room temperature powder neutron diffraction data were indexed using the Fm3m (#225) space group and the diffraction data collected at 20 K were indexed using the I4/mmm (#139) space group.1 . A summary of the Rietveld analyses at both temperatures is shown in Table 1. The atomic parameters for Ca2 IrD5 at 295 K are presented in Table 2. Fig. 2a portrays the cubic K2 PtCl6 structure, where the Ca is found in the Wyckoff 8c sites (larger spheres). The Ir atoms are found in the Wyckoff 4a sites (octahedra centers). The nearest neighbor Ca–Ir distance is 3.1384(1) Å. D atoms occur at Wyckoff 24e sites (small 1 A referee has pointed out that, according to Landau theory, a phase transformation from Fm3m to I4/mmm must be second order. Attempts were made to model the structure with symmetries lower than I4/mmm, but the data do not offer sufficient sensitivity to prove or disprove a lower symmetry space group. Likewise, with the relatively coarse temperature steps used, no conclusions may be made concerning coexistence of the tetragonal and cubic phases at the phase transition.

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Fig. 1. Neutron diffraction of Ca2 IrD5 at (a) 295 K and (b) 20 K. The upper box shows the observed diffraction data (crosses) as well as the computed pattern and fitted background function (line and dashed line, respectively) from the Rietveld results. Vertical lines in the middle indicate reflection positions for cubic Ca2 IrD5 (top) and Ir (below). The middle curve shows the difference between the observed and computed data. The lower curve shows the same differences, but divided by the standard uncertainties. Sections of the plots are enlarged to show details better, where the magnification factor is shown at the top of the graph.

Table 2 Structural parameters for cubic Ca2 IrD5 at 295 K Atom

Position

Displacement, 100×Uiso (Å)

Site multiplicity

Fractional occupancy

Ir Ca D

x=y=z=0 x = y = z = 1/4 x = y = 0; z = 0.2348(4)

0.98(7) 1.31(8) 2.67(6)

4 8 24

1 1 0.864(10)

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Fig. 2. Plots of Ca2 IrD5 structures at 295 and 20 K. (a) The 295 K cubic form and (b) 20 K tetragonal form.

spheres at octahedral apices), with six D sites in an octahedral arrangement around each Ir atom. The Ir–D bond distance is 1.702(3) Å and the shortest Ca–D distance is 2.5649(1) Å. The D occupancy refines to 0.854(10), which closely agrees with the expected occupancy of 5/6 for a six-fold disorder. The atomic parameters for Ca2 IrD5 at 20 K are presented in Table 3. In contrast to the 295 K cubic structure, at 20 K the lattice contracts in the a and b directions and elongates along c. Fig. 2b shows this low temperature structure, a tetragonally distorted K2 PtCl6 structure. The Ca atoms are found in the Wyckoff 4d sites (larger spheres). The Ir atoms are found in Wyckoff 2a sites (octahedra centers). The D

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Table 3 Structural parameters for cubic Ca2 IrD5 at 20 K Atom

Position

Displacement, 100×Uiso (Å)

Site multiplicity

Fractional occupancy

Ir Ca D1 D2

x=y=z=0 x = 1/2, y = 0, z = 1/4 x = y = 0; z = 0.2427(9) x = y = 0.2362(4), z = 0

0.21(8) 0.42(9) 1.7(2) 1.54(9)

2 4 4 8

1 1 0.541(14) 0.988(15)

atoms (small spheres), form a distorted octahedron around the Ir site. The equatorial D atoms, labeled as D2, are in the Wyckoff 8h sites and have an Ir–D bond distance of 1.681(3) Å. The axial D atoms, labeled as D1, are in Wyckoff 4e sites and have an Ir–D bond distance of 1.805(6) Å. The refined occupancies agree well with half-occupancy for D1 and full occupancy for D2. There is a minor change in the nearest neighbor Ca–Ir distance to 3.1282(1) Å. The shortest Ca–D1 and Ca–D2 distances are 2.5163(2) and 2.5751(1) Å, respectively. Diffraction patterns were collected at temperatures of 20, 77, 295 K and then at 250 and 275 K. The phase transition was complete and reversible within the time-scale of our measurements (h). The phase transition from cubic to tetragonal is demonstrated by monitoring the splitting of the cubic (400) reflection into the (004) and (220) reflections of the tetragonal structure, as is shown in Fig. 3. Lattice parameters are plotted in Fig. 4. It can be clearly seen from these figures that Ca2 IrD5 transforms from a cubic to tetragonal structure between 275 and 250 K. Diffraction measurements were also made using samples of Ca2 RhD5 , which confirmed that this material remains cubic over the temperature range 20–295 K. We also attempted to form Ir/Rh solid solutions: Ca2 Irx Rh1−x D5 with x=0.25, 0.5 and 0.75. These materials, however, are physical mixtures of Ca2 IrD5 and Ca2 RhD5 . At room temperature, the lattice constants for Ca2 IrD5 [7.2479(2) Å] and Ca2 RhD5

Fig. 3. Temperature dependence of the intensity in the region of the cubic (400) reflection for Ca2 IrD5 .

[7.2470(1) Å] are sufficiently close that the material appears to be single phase. However, at temperatures below the Ca2 IrD5 phase transformation, two phases are seen. Phase abundance and site occupancies determined by Rietveld refinements also gave no evidence for a Ca2 [Irx Rh1−x ]D5 solid solution at any value of x. Given that the reaction apparently occurs with Ir and Rh diffusing into grains of CaD2 , the mixture appears to be the thermodynamic product under the synthesis conditions, rather than a metastable product, due to a kinetic barrier. It should be noted that an inadvertent Rh impurity in the starting materials would result in a sample that appears to be single phase Ca2 IrD5 at room temperature. Upon cooling, both cubic and tetragonal phase components would be seen, which would have the appearance of an incomplete phase transition. This is one possible explanation for the earlier results where both cubic and tetragonal M2 IrD5 (M=Sr or Eu) appeared to coexist [7,9]. The observation that Ca2 [Irx Rh1−x ]D5 does not form below 700 ◦ C, despite the similarities between Ir+ and Rh+ , is quite interesting and warrants computational investigation, as it seems to reflect subtle differences in bonding for Ir versus Rh. These systems could be useful test cases for comparing different levels of theory.

Fig. 4. Temperature dependence of the unit cell constants for Ca2 IrD5 . Symbols used are: (triangle) cubic a; (squares) tetragonal a; (circles) √ tetragonal c. The tetragonal a values are multiplied by 2. Standard uncertainties are shown with horizontal lines placed at s ± 1σ, but are too small to be discerned on this scale.

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4. Conclusions The temperature of the cubic to tetragonal phase transition for M2 (TM)H5 is influenced by the choice of M and TM. The transition temperature is 75 K higher for Ca2 IrH5 compared to Sr2 IrH5 . In the case of Mg2 CoH5 , the transition is even higher, appreciably above room temperature. We speculate that this transition temperature may be inversely related to the atomic mass of the M atoms, but this must be confirmed through synthesis and diffraction studies of additional iridium pentadeuteride TMHs. The result that solid solutions of type Ca2 [Irx Rh1−x ]D5 could not be formed, despite the close agreement in both size and chemical properties for Rh and Ir was not expected. This result warrants further study.

Acknowledgements Certain trade names and company products are identified in order to specify experimental procedures adequately. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose. A Trinity College Faculty Research Grant is gratefully acknowledged by ROM Jr. We thank both referees for their useful comments.

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