We report on crystal structure data and hydrogenation properties of 24 palladium-rich intermetallic compounds with elements from groups 12–16 of the Periodic Table. Refined crystal structures based on X-ray powder diffraction data are presented for Pd3As (Fe3P type structure) and several members of the Pd5TlAs type structure family. Hydrogenation was studied in situ by differential scanning calorimetry (DSC) under 5.0 MPa hydrogen pressure up to 430 °C. Pd0.75Zn0.25, PdCd, PdHg, Pd2Sn, Pd5Pb3, Pd13Pb9, Pd3As, Pd20Sb7, Pd8Sb3, Pd5Sb2, PdSb, Pd5Bi2, Pd17Se15, Pd4Se, Pd5TlAs, Pd5CdSe, Pd5CdAs, Pd5HgSe, Pd5InAs, Pd8In2Se and Pd3Bi2Se2 do not show any sign of hydrogen uptake according to DSC and X-ray diffraction. For Pd3Sn and Pd3Pb a significant hydrogen uptake with unit cell volume increases of 0.4 and 0.6 %, respectively, with a retained structure type of the parent intermetallic was observed. Hydrogenation of Pd5InSe yields Pd3InH≈0.9 and a mixture of palladium selenides. Thermal analysis experiments in helium and in hydrogen atmosphere show that this is a multistep reaction with a decomposition of Pd5InSe to Pd3In and a liquid phase and subsequent hydrogenation of Pd3In.
Palladium can take up large amounts of hydrogen to form solid solutions PdH1–x [1, 2]. Palladium-rich phases, where some of the palladium atoms have been substituted by other metals in an ordered fashion, either show reduced hydrogen capacity as compared to pure palladium, e.g. MPd3 phases (M = Mg , Ca , In , Tl , Y , Mn , Ce ), or no reactivity towards hydrogen like for Pd2Al, Pd3Al, Pd2Ga, Pd13Ga5, Pd5In3, Pd2In and Pd5As [10, 11]. In some cases the hydrogenation reactions in MPd3 phases are accompanied by transformation of one superstructure of the cubic close packing (ccp, Cu type) to another ccp superstructure, e.g. from ZrAl3 type MgPd3 to an AuCu3 type arrangement. This interesting rearrangement can formally be seen as a gliding of layers within the intermetallic structures, and through a hydrogenation-rearrangement-dehydrogenation cycle it produces metastable compounds not accessible via other synthesis routes [3, 12]. In order to shed more light on this phenomenon we aim at a systematic investigation of the hydrogenation behavior of palladium-rich compounds. Especially, we would like to understand the prerequisites for hydrogen uptake and the hydrogen induced rearrangement as a function of the substitute M. Choosing a variety of elements M from groups 12–16 of the Periodic Table may help to distinguish between size and electronic effects, which both may play a role for hydrogenation and atomic rearrangement in palladium-rich intermetallic compounds. To the best of our knowledge, hydrogenation reactions were not reported for any of these compounds as yet.
2 Results and discussion
Twenty-four binary and ternary palladium-rich compounds with elements from groups 12–16 of the Periodic Table were synthesized from the elements. In order to monitor their reactivity towards hydrogen in situ thermal analysis experiments (differential scanning calorimetry, DSC) were performed on powder samples under 5.0 MPa hydrogen pressure up to 430 °C (see Experimental Section). In such experiments hydrogen uptake is often easily detected by exothermic signals . In addition X-Ray powder diffraction (XRPD) data were taken before and after the hydrogenation DSC experiments. Rietveld analysis was used to accurately determine unit cell volumes (Table 1), because an increase often indicates a possible hydrogen uptake. A change of more than 0.2 % was considered significant in the following.
|Compound||Unit cell volume increase (%)||Secondary phases (XRD)||Space group (structure type)||a (Å)||b (Å)||c (Å)||β (°)||Unit cell volume V (Å3)|
|Pd0.75Zn0.25||–||Fm3̅m (Cu type)||3.8680(6)||57.87(1)|
|0.2||–||Fm3̅m (Cu type)||3.8708(1)||57.996(4)|
|PdCd||–||P4/mmm (AuCu type)||3.0318(2)||3.6234(3)||33.309(4)|
|–0.2||–||P4/mmm (AuCu type)||3.0263(2)||3.6310(3)||33.254(5)|
|PdHg||–||P4/mmm (AuCu type)||3.02697(5)||3.6967(1)||33.867(1)|
|0.0||–||P4/mmm (AuCu type)||3.02812(7)||3.6943(1)||33.875(2)|
|Pd2Sn||18 % Pd3Sn||Pnma (Co2Si type)||5.6424(2)||4.3072(1)||8.0899(3)||196.61(1)|
|0.0||20 % Pd3Sn||Pnma (Co2Si type)||5.64352(6)||4.30708(4)||8.08942(8)||196.630(3)|
|Pd3Sn||5 % Pd2Sn||Pm3̅m (AuCu3 type)||3.9779(1)||62.947(4)|
|0.4||5 % Pd2Sn||Pm3̅m (AuCu3 type)||3.9808(6)||63.084(2)|
|Pd5Pb3||2 % Pd3Pb||C2 (Ni5Ge3 type)||13.3202(6)||7.6611(3)||7.2602(6)||52.231(6)||585.66(8)|
|–0.3||7 % Pd3Pb||C2 (Ni5Ge3 type)||13.2978(4)||7.6582(2)||7.2398(2)||52.384(2)||584.02(3)|
|Pd13Pb9||31 % Pd5Pb3||C2/c (Pd13Pb9 type)||15.6057(3)||9.0577(2)||13.9238(4)||55.797(3)||1627.77(9)|
|–0.1||32 % Pd5Pb3||C2/c (Pd13Pb9 type)||15.6048(4)||9.0491(3)||13.9237(5)||55.806(3)||1626.3(1)|
|Pd3Pb||–||Pm3̅m (AuCu3 type)||4.03068(9)||65.484(3)|
|0.6||–||Pm3̅m (AuCu3 type)||4.03809(6)||65.846(2)|
|Pd3As||–||I4̅ (Ni3P type)||9.9762(2)||4.82209(8)||479.92(1)|
|Pd20Sb7||28 % Pd8Sb3||R3̅ (Pd20Sb7 type)||11.7259(2)||11.0173(2)||1311.89(5)|
|Pd5Sb2||–||P63cm (Pd5Sb2 type)||7.6154(2)||13.8835(4)||697.29(3)|
|Pd8Sb3||R3̅c (Yb8In3 type)||7.6059(2)||42.999(2)||2154.1(2)|
|PdSb||–||P63mc (NiAs type)||4.07457(6)||5.5873(1)||80.333(2)|
|Pd5Bi2||–||C2/m (Pd5Bi2 type)||14.3914(2)||5.76875(7)||6.73909(9)||489.13(1)|
|0.2||–||C2/m (Pd5Bi2 type)||14.3994(3)||5.7719(1)||6.74366(2)||490.03(2)|
|Pd17Se15||–||Pm3̅m (Pd17Se15 type)||10.6051(2)||1192.73(6)|
|–0.2||–||Pm3̅m (Pd17Se15 type)||10.5983(2)||1190.45(5)|
|Pd4Se||10 % Pd7Se2||P4̅21c (Pd4Se type)||5.23037(6)||5.6439(1)||154.398(5)|
|–0.3||–||P4̅21c (Pd4Se type)||5.2246(6)||5.63807(6)||153.900(3)|
|Pd5CdSe||20 % Pd34Se11||P4/mmm (Pd5TlAs type)||4.00638(4)||7.0052(1)||112.440(3)|
|–0.1||15 % Pd34Se11||P4/mmm (Pd5TlAs type)||4.00496(8)||7.0036(2)||112.335(5)|
|Pd5HgSe||–||P4/mmm (Pd5TlAs type)||4.01305(3)||7.03851(7)||113.352(2)|
|0.0||–||P4/mmm (Pd5TlAs type)||4.01365(2)||7.03873(5)||113.390(1)|
|Pd5InSe||–||P4/mmm (Pd5TlAs type)||4.0269(4)||6.9829(8)||113.23(2)|
|–||Reaction to mixture of 63 % Pd3InHx + 23 % Pd8In2Se + 14 % Pd34Se11|
|Pd8In2Se||27 % Pd5InSe||P4/mmm (Pd8In2Se type)||4.0067(1)||10.9451(6)||175.71(1)|
|–||Reaction to mixture of 68 % Pd3InHx + 11 % Pd8In2Se + 9 % Pd34Se11 + 9 % Pd7Se4 + 3 % Pd17Se15|
|Pd3Bi2Se2||–||C2/m (Ni3Bi2S2 type)||11.7081(5)||8.4083(4)||8.3927(5)||113.845(3)||595.89(5)|
|0.0||–||C2/m (Ni3Bi2S2 type)||11.712(1)||8.4094(9)||8.397(1)||113.893(6)||596.0(1)|
|Pd5CdAs||9 % CdPd + 7 % Pd2As||P4/mmm (Pd5TlAs type)||3.97566(5)||6.9867(2)||110.431(4)|
|0.1||10 % Pd2As||P4/mmm (Pd5TlAs type)||3.97709(3)||6.9887(1)||110.542(3)|
|Pd5InAs||11 % Pd1–xInx||P4/mmm (Pd5TlAs type)||3.9861(1)||6.9814(2)||110.925(5)|
|0.0||11 % Pd1–xInx||P4/mmm (Pd5TlAs type)||3.98639(8)||6.9816(2)||110.946(5)|
|Pd5TlAs||12 % Pd13Tl9||P4/mmm (Pd5TlAs type)||4.00146(9)||7.0427(2)||112.765(5)|
2.1 Binary palladium-rich compounds with elements of group 12
The compounds PdCd, PdHg (both AuCu type) and Pd0.75Zn0.25 (solid solution of Cu type) were synthesized from the elements. The latter was formed in an attempt to prepare an ordered phase Pd3Zn via mineralization by iodine within 3 months. Crystal structures refined on the basis of XRPD data (Table 1) are in good agreement with the literature [14–16]. No reaction with hydrogen is observed for any of the three compounds under given terms (Table 1).
2.2 Binary palladium-rich compounds with elements of group 14
The compounds Pd2Sn (Co2Si type), Pd5Pb3 (Ni5Ge3 type), and Pd13Pb9 (Pd13Pb9 type) do not show any reaction with hydrogen according to DSC experiments and by comparing XRPD data before and after the hydrogenation experiment. Refined crystal structures (Table 1) agree well with those from the literature [17–19]. The structures of Pd3Sn and Pd3Pb are also in accordance with literature . However, an increase of the unit cell volume by up to 0.4 and 0.6 % upon hydrogenation, respectively, indicates a possible hydrogen uptake. [Pd6] octahedral sites, which are preferred by hydrogen , are present in their structures (AuCu3 type). Several hydrides MPd3Hx with occupation of these interstices, resulting in a cubic anti-perovskite type structure, are known already (M = Mg , In , Tl , Y , Mn , Ce ). However, no significant thermal signal during the in situ DSC experiment could be observed. This may be assigned to low hydrogen content and small bonding energy of hydrogen in these compounds.
2.3 Binary palladium-rich compounds with elements of group 15
From the wide range of Pd–As compounds known [11, 21–33] only for PdAs2 and Pd5As high quality crystal structure data are available [33, 11]. For Pd3As  only a structure type was assigned, but no crystal structure was refined. This prompted us to reinvestigate the crystal structure of Pd3As and perform a Rietveld refinement based on XRPD data. The results (Table 2, Fig. 1) confirm the assignment of the Fe3P structure type (D0e) and provide the first refined structure data for Pd3As.
|Atom||Wyckoff site||Symmetry||x||y||z||Biso (Å2)|
Space group I4̅ , a = 9.9761(1) Å, c = 4.82191(8) Å.
Rp = 0.029; Rwp = 0.037; χ2 = 2.45; RBragg = 0.138.
Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: firstname.lastname@example.org, http://www.fizkarlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-430924.
XRPD data confirm the NiAs structure type for PdSb and the refined lattice parameters (Table 1) are in good agreement with literature [34, 35]. The correspondence of refined lattice parameters for Pd5Sb2, Pd8Sb3, Pd20Sb7, and Pd5Bi2 with those from literature [36–38] is also reasonable. According to the absence of thermal effects in DSC experiments and the absence of significant unit cell volume changes (no XRD for Pd20Sb7 and Pd8Sb3 after hydrogenation), none of the Pd–As, Pd–Sb and Pd–Bi phases investigated shows any signs for hydrogen uptake.
2.4 Binary palladium-rich compounds with elements of group 16
Pd4Se and Pd17Se15 were synthesized and their structure are in good accord with literature data [39, 40]. The unit cell volumes of both compounds decrease slightly during the hydrogenation experiment, which might be connected to the reaction of the side product in case of Pd4Se (Table 1). DSC and XRPD experiments suggest the absence of significant hydrogen incorporation.
2.5 Ternary palladium-rich compounds with Pd5TlAs type and related structures
The structure type of Pd5TlAs may be described as an intergrowth structure with AuCu3 like and CsCl like slabs . It may also be derived from a cubic close packing by doubling of one lattice parameter, i.e. a cubic-tetragonal transition, and ordering the eight atomic positions in a 5:1:1:1 fashion with five palladium, one thallium, one arsenic atom and one vacancy. It thus represents one of many ordered intermetallic compounds related to the cubic close packing by crystallographic group–subgroup relationships [42–44]. This structure type exhibits many octahedral sites, which seem to be attractive for incorporation of hydrogen. Compounds with this type of structure thus seem to be good candidates for hydrogenation. The refined structures of Pd5InAs, Pd5HgSe and Pd5InSe are in good agreement with literature [43, 45–47]. For all other compounds listed in Tables 1 and 3 these are the first refined crystal structure data. The only free positional parameter, z(Pd2) is higher for the compounds with arsenic than for those with selenium resulting in shorter Pd–As as compared to Pd–Se distances as expected from atomic sizes (Table 3). The c/a ratio of the Pd5TlAs type structure is always considerably smaller than two. That means that the smaller selenium or arsenic atoms are packed closer along the crystallographic c direction than the larger cadmium, mercury, indium or thallium atoms.
From all compounds listed above only Pd5InSe exhibits reactivity towards hydrogen (Table 1). The distorted [Pd4M2] (M = Cd, Hg, In) and [Pd5X] (X = As, Se) octahedral sites in Pd5MX are two possible hydrogen positons in this structure. Occupation by hydrogen would lead to distances between 1.96 and 2.06 Å, which are comparable with palladium-hydrogen distances in Pd3InH0.89 (2.01 Å ). However, unreasonably short distances As–H, Se–H, Cd–H, Hg–H, In–H, Tl–H would result, which probably prevents hydrogen from entering the structures.
Pd5InSe and Pd8In2Se  form Pd3InHx during the hydrogenation experiment. Differential thermal analysis (DTA) of Pd5InSe in helium atmosphere was executed and the results have shown two reversible thermal signals with a hysteresis (Fig. 2, top). X-ray powder diffraction after the DTA confirmed that the reaction is fully reversible. To investigate the intermediates, a temperature-resolved XRPD experiment was carried out. The false color plot (Fig. 2, bottom) shows the formation of Pd8In2Se at about 525 °C. Upon further heating Pd8In2Se decomposed at about 625 °C and Pd3In (ZrAl3 type) was formed. On cooling (not shown here) Pd5InSe returned.
We suggest the following reactions to take place:
For the proposed liquid phase we have indirect evidence from annealing experiments. Pd5InSe samples were annealed at 600 and 800 °C, i.e. after the first and second thermal signal in the DTA, respectively, and the silica glass ampoules were quenched in water. The main phases were Pd8In2Se at 600 °C and Pd3In at 800 °C with secondary phases Pd34Se11, Pd7Se4 and Pd17Se15. The latter forms from the liquid (2 Pd + Se) by quenching according to the phase diagram  of the system Pd–Se. Thus, hydrogen plays a role in the reactions (Table 1) only insofar that it reacts with Pd3In formed by thermal decomposition of Pd5InSe.
2.6 The half-antiperovskite Pd3Bi2Se2
The half-antiperovskite Pd3Bi2Se2 (Ni3Bi2S2 type)  was investigated for its hydrogenation properties. [Pd6] octahedral sites in its crystal structure, albeit strongly distorted, suggested this structure family to be a good hydrogenation candidate from geometric reasons. However, neither in situ DSC nor comparison of unit cell volumes before and after the hydrogenation experiment hint towards any hydrogen uptake.
Twenty-four palladium-rich intermetallic compounds were synthesized, their crystal structures refined by the Rietveld method and their hydrogenation behavior investigated. For Pd3As, Pd5CdSe, Pd5CdAs, Pd5TlAs, the refined crystal structure data are presented for the first time. From all investigated compounds, only Pd3Sn and Pd3Pb show signs of significant hydrogen uptake while retaining the structure type of the parent intermetallic compound. No hydrogen uptake could be observed for any compound containing an element of groups 15 or 16. This seems to confirm the assumption that electronic effects are more important than geometric effects in palladium-rich intermetallic compounds . Pd5InSe decomposes to Pd3In and palladium selenides, the former reacting with hydrogen to yield Pd3InH≈0.9.
4 Experimental section
4.1 Synthesis of intermetallic compounds
Intermetallic compounds were synthesized from stoichiometric mixtures of the elements in evacuated silica tubes. Temperature treatment is given in the following as final temperature, holding time and heating rate, e.g. 875 °C 24 h 1 °C min–1 describes a temperature treatment of heating with 1 °C min–1 to 875 °C, holding this temperature for 24 h and cooling with the natural cooling rate of the furnace. In some cases a few mg of iodine was added (chemical vapor transport reaction), denoted by “I2”.
Pd0.75Zn0.25: 1000 °C 4 d
PdCd: 770 °C 3 h, 280 °C 14 d
PdHg: I2, 400 °C 9 d 1 °C min–1
Pd2Sn: I2, 720 °C 5 d 1 °C min–1
Pd3Sn: 720 °C 72 h 1 °C min–1
Pd5Pb3: 1200 °C 2 h 1.5 °C min–1 quenched at 900 °C, ground in mortar, 370 °C 11 d 1 °C min–1
Pd13Pb9: 1200 °C 2 h 1.5 °C min–1 quenched at 900 °C, ground in mortar, 520 °C 9 d 1 °C min–1
Pd3Pb: I2, 875 °C 24 h 1 °C min–1, 430 °C 7 d
Pd3As: 650 °C 2 h 3.5 °C min–1, 1000 °C 72 h 2 °C min–1
Pd20Sb7: 900 °C 168 h 2.1 °C min–1, 25 °C 1.2 °C min–1
Pd8Sb3: 1000 °C 120 h 1.8 °C min–1
Pd5Sb2: 1000 °C 168 h 1.3 °C min–1
PdSb: 850 °C 6 h 2 °C min–1, 750 °C 48 h 1.7 °C min–1
Pd5Bi2: I2, 450 °C 4 d 0.5 °C min–1
Pd17Se15: 430 °C 7 d 1 °C min–1
Pd4Se: 700 °C 2 h 1 °C min–1, 375 °C 7 d
Pd5TlAs: 650 °C 2 h 3.4 °C min–1, 1000 °C 30 h 1.9 °C min–1
Pd5CdSe, Pd5CdAs: 750 °C 6 d 1 °C min–1
Pd5HgSe I2, 400 °C 10 d
Pd5InSe, Pd5InAs: I2, 950 °C 4 h; 700 °C 7 d
Pd8In2Se 950 °C 4 h; 750 °C 6 d
Pd3Bi2Se2: 1200 °C 1 h 3 °C min–1; 500 °C 500 h
Products were obtained as gray powders, some of them with a silvery luster.
4.2 X-ray powder diffraction (XRPD)
XRPD data were collected using flat reflection samples on a Panalytical X’Pert at T = 23(1) °C with CuKα radiation or using flat transmission samples on a Huber Guinier G670 camera with an image plate system using either CuKα1 or MoKα1 radiation. Rietveld refinements were carried out with the program FullProf  and pseudo-Voigt as profile function. Absorption effects were modelled with a fixed overall thermal displacement parameter of –1.8 Å2 in the refinement of the crystal structure of Pd3As.
4.3 Thermal analysis (in situ DSC and ex situ DTA)
Differential scanning calorimetry was performed in situ under hydrogen pressures on a Q1000 DSC (TA Instruments) equipped with a gas pressure chamber. Twenty to thirty milligrams of the powdered intermetallics was put in aluminum crucibles, which were closed with an aluminum lid. These were placed inside the pressure chamber, which was then purged several times with hydrogen gas before filling it to the final hydrogen gas pressure of 5.0 MPa. Samples were heated to 430 °C with 10 °C min–1, held at that temperature for a minimum of 1 h, and cooled to 27 °C with 10 °C min–1. Usually, two or three such runs were performed, before the hydrogen pressure was released, the sample taken out and structural characterization undertaken. After releasing the gas pressure, the products were characterized ex situ by XRPD. The difference thermal analysis (DTA) of Pd5InSe in helium atmosphere was carried out on a Netzsch F1 Jupiter device using sintered alumina crucibles and a heating rate of 10 K min–1.
Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.
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