High H ionic conductivity in barium hydride

Transcription

1 High H ionic conductivity in barium hydride Maarten C. Verbraeken a, Chaksum Cheung a, Emmanuelle Suard b and John T.S. Irvine a a School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, Fife, UK, jtsi@st-and.ac.uk b Institut Laue-Langevin, BP 156, 6, rue Jules Horowit, Grenoble Cede 9, France NATURE MATERIALS 1

2 Thermal analysis Figure S1, the TGA of BaD 2 in argon atmosphere, shows a weight loss at 664 C due to the release of D 2 gas (as evidenced from the corresponding mass spectrometry peak with mass 4). This means that BaD 2 has a similar thermal stability to its Sr and Ca analogues, which have decomposition temperatures of 686 C and 668 C, respectively. The weight loss of BaD 2 is only 73% of the theoretical weight loss, which is in agreement with Peterson s phase diagram, showing an equilibrium of a hydrogen rich barium metal α-phase and the barium hydride β- phase at the temperature of the isotherm (750 C) in the TGA eperiment 1. No higher temperatures were used, as to avoid contamination of the equipment with barium vapour. Figure S1: TGA of BaD 2 in argon atmosphere coupled with mass spectrometry Structural information The cotunnite structure of the alkaline earth hydrides is in fact a distorted heagonal array of alkaline earth ions, with the ratio of the c/b aes slightly larger than the ideal value of 3. This makes the P n m a spacegroup a non-isomorphic subgroup of the P 6 3 / m m c spacegroup with a displacement of metal and hydride ions off their ideal high symmetry sites. This means that the following transposition equations apply: aorth = che and borth = ahe ; for the ideal heagonal symmetry c = 3 b applies. Table 1 in the supplementary ort orth information lists the actual and ideal c/b ratios and atomic coordinates for BaD 2 at room temperature whereas Figure S2 also gives a graphical representation of the relationship 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION between the cotunnite and heagonal structure. It may be argued that this phase transition from P n m a P 6 3 / m m c could be of second order nature, as the symmetry change only involves small changes in the unit cell parameters along with small shifts in the atomic coordinates. A gradual change in the c/b ais ratio and movement of atoms towards their high symmetry sites at low temperatures would be in agreement with a second order phase change. However, careful refinement of the two dominant phases at 500 C shows that there is a 0.9% volume contraction upon going to the heagonal symmetry. This gives a discontinuity in the evolution of lattice volume vs. temperature, suggesting that the phase transition is in fact first order in nature. This discontinuity is also apparent in the evolution of the common aes, i.e. a orth -ais (= c he -ais), b orth -ais (= a he -ais) and c/b ratio as shown in Figure S3. The phase change is furthermore visible using differential thermal analysis (DTA, Figure S4). Despite the slightly smaller lattice parameters, the variable temperature diffraction data in Figure 2a is in agreement and complimentary to the low temperature data by Ting et al. 2. Table S2 summarises the refinement information for both the room temperature as well as the high temperature (610 C) diffraction patterns. Table S1: Actual values of c/b and fractional coordinates in distorted heagonal array of Ba ions (T < 500 C) and values for ideal heagonal packing of Ba and D ions (T > 500 C). Actual values (< 500 C); RT data Ideal values (> 500 C) c/b Ba D (2d) D Figure S2: Relation between orthorhombic cotunnite and heagonal unit cells. NATURE MATERIALS 3

4 6.85 (a) a-ais (Å) a orth ais c he ais b orth, a he ais (Å) b orth ais a he ais c orth /b orth (b) c/b T ( C) T ( C) Ba orth Ba orth D2 (c) 0.72 D T ( C) Figure S3: First order phase change is apparent from the above three graphs. Evolution of the common ais between the two space groups (a orth and c he ) with temperature showing discontinuity at phase change (a). Temperature dependence for c/b ais ratio and b orth (= a he ) in (b) and atomic fractional coordinates in (c) show similar behaviour. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Figure S4: DTA showing phase transition in different atmospheres: BaH 2 in 5% H 2-95% argon (green), BaD 2 in 5% D 2-95% argon (red and blue) NATURE MATERIALS 5

6 Table S2: Refinement results for neutron powder diffraction on BaD 2, obtained on instrument D1A (ILL), neutron wavelength λ = Å. Refinement parameter T = 298 K T = 883 K D1 on 4f T = 883 K D1 on 2d Spacegroup P n m a P 6 3 / m m c P 6 3 / m m c a (Å) (1) (2) (2) b (Å) (1) - - c (Å) (1) (4) (5) V (Å 3 ) (6) (10) (11) Ba, 4c (,¼,) Ba, 2b (⅓,⅔,¼) Ba, 2b (⅓,⅔,¼) β D1, 4c (,¼,) β D2, 4c (,¼,) β (3) (2) 65(5) 158(10) 52(3) -6(2) 1.24(4) (2) (2) 143(5) 320(12) 77(3) -3(3) 2.18(4) 0.928(7) (2) (2) 140(4) 378(11) 118(3) -8(3) 2.71(4) 0.911(6) D1, 4f (⅓,⅔,) D2, 2a (0,0,0) 407(17) 407(17) 372(14) -204(8) 4.3(1) (5) 801(36) 801(36) 486(24) -400(18) 7.6(2) 0.473(5) 1808(47) 1808(47) 378(21) -904(23) 11.6(2) 0.829(9) D1, 2d (⅓,⅔,¾) D2, 2a (0,0,0) 545(24) 545(24) 249(13) -272(12) 4.2(1) R p R wp R ep χ β ij = B 4 ij a and B 523(43) 523(43) 2050(48) -262(21) 11.3(4) 0.39(2) 2129(66) 2129(66) 1014(48) -1064(33) 16.2(5) 1.00(2) 2 ij = 8π Uij, where a* are the reciprocal lattice parameters and U ij is the mean square atomic displacement in the ij direction. B equivalent are the equivalent isotropic thermal displacement parameters. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION Table S3: Selected bond distances for BaD 2 at 298K and 883K. T = 298 K T = 883 K D1 on 4f T = 883 K D1 on 2d Ba D (2) (7) (1) Ba D (2) 2.832(3) (2) Ba D (2) Ba D (2) (1) (1) Ba D (2) Ba D (2) D1 D (1) D1 D (1) 2.819(1) (1) D1 D (2) D1 D (2) D1 D (2) D2 D (2) (2) (2) D2 D (2) Refinement of high temperature phase Refinement of the high temperature phase is possible with D1 either positioned on the high symmetry 2d site (,, ) or the lower symmetry 4f ( ) 3 3 4,, site (with being close to ¾). When D1 is positioned on the 2d however, this results in poorer reliability factors (R p = 3.78, R wp = 4.39) and chi squared (χ 2 = 1.69) values. The reason for this is most probably the tight space of the D1 site, which is now planar trigonally coordinated to Ba, with Ba-D1 distance of 2.57 Å, smaller than the smallest Ba-D distance at room temperature, i.e Å. This model also does not converge completely, unless the separate D1 and D2 occupancies are fied. Table S2 shows that due to constraints, the D2 occupancy is refined as 1.00(2), however no convergence is obtained. Although the 2d site may seem preferable from a symmetry point of view and also since this atomic position has been previously reported for the related compound SrF 2 under high pressure 3, it fails to eplain the observed physical properties. The D D hopping distances are practically identical to those in the cotunnite structure and can therefore not account for the sudden jump in conductivity. The presence of a half occupied NATURE MATERIALS

8 1 2 site when D1 resides on 4f ( ),, however, does eplain this jump of an order magnitude, 3 3 due to the creation of charge carriers and seems therefore more likely to be the more correct atomic position. Figure S5 gives a graphical representation of the 2d site splitting into two 4f sites. This causes an increase in the Ba D1 distance from 2.57 Å to 2.63 Å and concomitant halving of the D1 occupancy. Figure S5: Representation of the site splitting of D1. By moving from a 2d to a 4f site, the Ba D1 distance increases from 2.57 Å to 2.63 Å, whilst concomitantly creating a half vacant site for ionic conduction. Co-eistence of cotunnite and heagonal BaD 2 at 500 C The neutron diffraction pattern at 500 C reveals co-eistence of both low and high temperature phases of BaD 2. The pattern and its refinement are shown in Figure S6. The refinement gives a phase fraction of Cotunnite Heagonal = 59.8(4) 40.2(7), with the following reliability factors for the overall fit: R p = 2.73, R wp = 3.29, R ep = 3.22 and χ 2 = NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Figure S6: Neutron diffraction pattern and refinement for BaD 2 at 500 C (a). Top vertical green marks show cotunnite reflections, bottom marks show reflections for heagonal phase. Zoomed section of same pattern and fit (b). Electrical properties Figure S7 (Figure 3 in the manuscript) shows a typical impedance spectrum for BaH 2 recorded at 320 C. It shows the grain boundary response at high frequency and the electrode behaviour at intermediate low frequency. The arc for the bulk response is only visible at temperatures up to 250 C, due to the small time constant for the coupled geometric capacitance (in the order of F/cm). The grain boundary response was however visible up to 420 C, enabling deconvolution of the bulk and grain boundary conductivities up to this temperature. The equivalent circuit depicted in Figure S8 was used to model the impedance data and isolate the contributions of the bulk and grain boundary to the conductivity. In this equivalent circuit, the subscript gb stands for grain boundary and Q is a constant phase element (CPE), which is defined by Y Y ( jω ) CPE 0 n =. At T > 270 C the (RQ) bulk element is replaced by a series resistance preceded by an inductor L (due to the leads of the jig). At T > 420 C, (RQ) gb disappears as well and the series resistance now represents the total conductivity (including contributions from the measuring jig). Figure S9 shows the impedance spectrum for BaH 2 at 560 C, showing a clear electrode response indicative of hydride electrochemistry. NATURE MATERIALS 9

10 -800 BaH 2 T=320 C Z" (Ω) H H Electrode Z' (Ω) Figure S7: Impedance spectrum of BaH 2 at 320 C, showing grain boundary and electrode response L R bulk R gb R electrode Q bulk Q gb Q electrode Figure S8: Equivalent circuit used to model impedance data obtained for BaH 2. The subscript gb stands for grain boundary and Q is a constant phase element (CPE), defined by Y Y ( jω ) CPE 0 n = BaH 2 T=560 C H 6.5 H Z" (Ω) R bulk + R gb Z' (Ω) Figure S9: High temperature impedance spectrum for BaH 2, intercept on Z ais giving sum of bulk and grain boundary elements (dominated by bulk at this temperature) showing a clear electrode response owing to hydride electrochemistry ( H2 + 2e 2H ) 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION References 1 Peterson, D. T. & Indig, M. The barium-barium hydride system. J. Am. Chem. Soc. 82, (1960). 2 Ting, V. P., Henry, P. F., Kohlmann, H., Wilson, C. C. & Weller, M. T. Structural isotope effects in metal hydrides and deuterides. Physical Chemistry Chemical Physics 12, (2010). 3 Wang, J. S. et al. Structural phase transitions of SrF2 at high pressure. J. Solid State Chem. 186, (2012). NATURE MATERIALS 11