Article pubs.acs.org/IC
Ba3Pt4Al4Structure, Properties, and Theoretical and NMR Spectroscopic Investigations of a Complex Platinide Featuring Heterocubane [Pt4Al4] Units Frank Stegemann,† Christopher Benndorf,†,‡ Timo Bartsch,† Rachid St. Touzani,§ Manfred Bartsch,∥ Helmut Zacharias,∥ Boniface P. T. Fokwa,§,# Hellmut Eckert,‡,⊥ and Oliver Janka*,† †
Institut für Anorganische und Analytische Chemie and ‡Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 28/30, 48149 Münster, Germany § Institut für Anorganische Chemie, RWTH Aachen, Landoltweg 1, D-52074 Aachen, Germany # Department of Chemistry, University of California Riverside, California 92521, United States ∥ Physikalisches Institut, Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany ⊥ Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos−SP 13566-590, Brazil S Supporting Information *
ABSTRACT: Ba3Pt4Al4 was prepared from the elements in niobium ampules and crystallizes in an orthorhombic structure, space group Cmcm (oP44, a = 1073.07(3), b = 812.30(3), c = 1182.69(3) pm) isopointal to the Zintl phase A2Zn5As4 (A = K, Rb). The structure features strands of distorted [Pt4Al4] heterocubane-like units connected by condensation over Pt/Al edges. These are arranged in a hexagonal rod packing by further condensation over Pt and Al atoms with the barium atoms located inside cavities of the [Pt4Al4]δ− framework. Structural relaxation confirmed the electronic stability of the new phase, while band structure calculations indicate metallic behavior. Crystal orbital Hamilton bonding analysis coupled with Bader effective charge analysis suggest a polar intermetallic phase in which strong Al−Pt covalent bonds are present, while a significant electron transfer from Ba to the [Pt4Al4]δ− network is found. By X-ray photoelectron spectroscopy measurements the Pt 4f5/2 and 4f7/2 energies for Ba3Pt4Al4 were found in the range of those of elemental Pt due to the electron transfer of Ba, while PtAl and PtAl2 show a pronounced shift toward a more cationic platinum state. 27Al magic-angle spinning NMR investigations verified the two independent crystallographic Al sites with differently distorted tetrahedrally coordinated [AlPt4] units. Peak assignments could be made based on both geometrical considerations and in relation to electric field gradient calculations.
1. INTRODUCTION In the ternary system of rare-earth platinum aluminum (RE− Pt−Al) a number of different compounds are known. Besides the equiatomic TiNiSi-type REPtAl compounds (RE = Y, La− Nd, Sm, Gd−Lu; Pnma)1−5 a lot of aluminum-rich compounds are reported, which have been synthesized from the elements or from Al flux reactions. RE4Pt9Al24 (RE = Y, Gd−Lu; P1;̅ Er4Pt9Al24-type)6 and RE2Pt6Al15 (RE = Ce, Sm, Gd−Tm; P63/ mmc; Sc1.2Fe4Si9.8-type or R3̅m; Gd1.33Pt3Al8-type)7−10 are two series of representatives that should be mentioned here. While the rare-earth-rich areas of the respective ternary systems are scarcely investigated, a series for the platinum-rich corner for the small lanthanides with composition REPt5Al2 (RE = Y, Gd−Tm) has been published recently.11 When going from the rare-earth elements to the more electropositive alkaline (A) and alkaline-earth (AE) metals the number of compounds is significantly smaller. The Pearson database12 lists only five © XXXX American Chemical Society
compounds for A/AE−Pt−Al: the equiatomic LiPtAl (P63/ mmc; BeZrSi-type)13 and CaPtAl (Pnma, TiNiSi-type)1 as well as the Heusler phases Li2PtAl (F4̅3m; Li2AgSb-type)13 and LiPtAl2 (Fm3̅m; MnCu2Al-type)13 and finally Mg3Pt2Al (Fd3̅m; Nb3Ni2Si-type).14 Recently Ca2Pt2Al, an intermetallic compound with pairwise distorted Pt2 dumbbells, was reported.15 All of these compounds exhibit a more or less pronounced framework of Pt and Al, with a certain degree of bonding between the framework building atoms, while the electropositive metals are located in cavities within the structure. During attempts to synthesize new intermetallic compounds in the so far empty system Ba−Pt−Al we obtained the new phase Ba3Pt4Al4, which crystallizes isopointal to the Zintl phase K2Zn5As4 in the orthorhombic crystal system with space group Received: August 12, 2015
A
DOI: 10.1021/acs.inorgchem.5b01842 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Cmcm (a = 1073.07(3), b = 812.30(3), c = 1182.69(3) pm). Herein we report on the synthesis and characterization of the title compound, its physical properties, as well as 27Al magicangle spinning (MAS) NMR spectroscopic investigations, band structure calculations, and X-ray photoelectron spectroscopy measurements.
Table 1. Crystal Data and Structure Refinement for Ba3Pt4Al4, Space Group Cmcm, Z = 4 molar mass (g·mol−1) unit cell dimensions (pm)
1300.3 1073.07(3) 812.30(3) 1182.69(3) 1031 8.38 10 × 25 × 50 STOE StadiVari Mo Kα (71.073 pm) 0.191/0.748 65.6 2128 3.2 to 35.4° ±17, ±13, ±19 22 831 1254 (Rint = 0.0513) 974 (Rσ = 0.0199) 1254/35 0.78 R1 = 0.0136 wR2 = 0.0269 R1 = 0.0223 wR2 = 0.0283 31(5) 0.75/−0.63
a= b= c=
volume (Å3) calculated density (g·cm−3) crystal size (μm3) diffractometer wavelength transm. ratio (min/max) absorption coefficient (mm−1) F(000) θ range range in hkl total no. reflections independent reflections reflections with I ≥ 3σ(I) data/parameter goodness-of-fit on F2 final R indices [I ≥ 3σ(I)]
2. EXPERIMENTAL SECTION 2.1. Synthesis. Starting materials for the preparation of the Ba3Pt4Al4 intermetallics were pieces of barium (Alfa Aesar, 99%), platinum sheets (Agosi AG, 99.9%), and aluminum turnings (Koch Chemicals, 99.99%). The elements were weighed in first attempts in a 1−1−1 ratio, and later they were weighed in the ideal atomic ratio and arc welded16 in niobium tubes in a dried argon atmosphere of ∼800 mbar. Argon was purified with titanium sponge (900 K), silica gel, and molecular sieves. The niobium ampules were subsequently sealed in evacuated silica tubes and placed in a box furnace (Nabertherm). The containers were heated to 1473 K, kept at that temperature for 10 min, cooled to 1073 K over 24 h, and kept at this temperature for another 7 d, followed by furnace-cooling to room temperature. Single crystals were collected from the crushed samples. The samples with a 1−1−1 starting ratio showed rapid oxidation after exposing them to air due to the excess barium; the desired product Ba3Pt4Al4 shows metallic luster and is stable in air for several days. Fast decomposition is observed when it is exposed to moisture or water and rather slow decomposition occurs upon grinding the material to a powder. PtAl and PtAl2 for X-ray photoelectron spectroscopy (XPS) and NMR investigations were prepared by arc-melting of the elements in a water-cooled copper hearth.16 The samples were used as obtained; the purity was checked by powder X-ray diffraction. PtAl is silver-colored and shows metallic luster, while PtAl2 has a yellowish color with metallic luster. K2PtCl6 was prepared as described in the literature.17 2.2. Structure Determination. The polycrystalline samples of Ba3Pt4Al4 were analyzed by powder X-ray diffraction: Guinier technique, image plate system Fujifilm, BAS-1800, Cu Kα1 radiation and α-quartz (a = 491.30 pm, c = 540.46 pm) as an internal standard. The lattice parameters were deduced from least-squares fits. Correct indexing was ensured by intensity calculations.18 Small crystallites were selected from the crushed samples and glued to thin quartz fibers using beeswax to investigate them by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, Fujifilm, BAS1800) to check their quality. An intensity data set of Ba3Pt4Al4 was collected at room temperature by use of a Stoe StadiVari four-circle diffractometer (Mo Kα radiation (λ = 71.073 pm), μ-source; oscillation mode; hybrid-pixel-sensor, Dectris Pilatus 100 K) with an open Eulerian cradle setup. Numerical absorption correction along with scaling was applied to the data set. Details of the data collection and the structure refinement are listed in Tables 1−3 and Table S1. Additional crystallographic information is available in the Supporting Information. 2.3. Structure Refinement. Structure solution was achieved by using the Superflip package19 included in Jana2006,20,21 which was used for structure refinement. The centrosymmetric space group Cmcm was chosen based on the Laue symmetry and systematic absences. All initial atomic positions could be easily assigned by the charge-flipping algorithm of Superflip. The refinement of the structure was possible with anisotropic atomic displacement parameters for all atoms. As a check for the correct composition, the occupancy parameters were refined in a separate series of least-squares cycles, and all sites were found to be fully occupied within three standard deviations. Finally the atomic positions were standardized using the program Structure tidy.22 2.4. Energy-Dispersive X-ray Analyses. The single crystal measured on the diffractometer was analyzed semiquantitatively with a Zeiss EVO MA10 scanning electron microscope with BaSO4, Pt, and Al2O3 as standards. No impurity elements heavier than sodium (detection limit of the instrument) were observed. The experimentally determined element ratios Ba/Pt/Al were within 1 atom % of the stoichiometric values.
R indices (all data) extinction coefficient largest diff. peak and hole (e−·Å3)
Table 2. Atomic Positions of Ba3Pt4Al4, Space Group Cmcm, Z=4 atom
Wyckoff site
x
y
z
Ba1 Ba2 Pt1 Pt2 Al1 Al2
8e 4c 8g 8f 8g 8f
0.21975(3) 0 0.18695(2) 0 0.36477(14) 0
0 0.02351(6) 0.34583(2) 0.29878(3) 0.14893(20) 0.3967(2)
0 1/4 1/4 0.57330(1) 1/4 0.12403(13)
2.5. Physical Property Measurements. Magnetic measurements of Ba3Pt4Al4 were performed on a Quantum Design Physical Property Measurement System using the Vibrating Sample Magnetometer option. Pieces of the samples were ground to a powder, packed in a polyethylene capsule, and attached to the sample holder rod. The measurements were performed in the temperature range of 2−300 K with magnetic flux densities up to 10 kOe. 2.6. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectra were measured using an Axis-Ultra spectrometer (Kratos, Manchester, U.K.) in ultrahigh vacuum (pressure
