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NbOsSi and TaOsSi – Two new superconducting ternary osmium silicides
Accepted Manuscript NbOsSi and TaOsSi – Two new superconducting ternary osmium silicides Christopher Benndorf, Lukas Heletta, Gunter Heymann, Hubert Huppertz, Hellmut Eckert, Rainer Pöttgen PII:
S1293-2558(17)30146-2
DOI:
10.1016/j.solidstatesciences.2017.04.002
Reference:
SSSCIE 5488
To appear in:
Solid State Sciences
Received Date: 10 February 2017 Accepted Date: 7 April 2017
Please cite this article as: C. Benndorf, L. Heletta, G. Heymann, H. Huppertz, H. Eckert, R. Pöttgen, NbOsSi and TaOsSi – Two new superconducting ternary osmium silicides, Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.04.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT To be submitted to Solid State Sci.
08.04.17 15:04
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NbOsSi and TaOsSi – two new superconducting ternary osmium silicides Christopher Benndorfa,b, Lukas Helettaa, Gunter Heymannc, Hubert Huppertzc, Hellmut Eckertb *, Rainer Pöttgena * Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany
b
Institut für Physikalische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany and Institute of Physics in Sao Carlos, University of Sao Paulo, Sao Carlos, SP 13560-590, Brazil
c
Institut für Allgemeine, Anorganische und Theoretische Chemie, Leopold-FranzensUniversität Innsbruck, Innrain 80–82, A-6020 Innsbruck, Austria
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a
E-mail addresses:
Article history: Received ........
[email protected] (H. Eckert), [email protected] (R. Pöttgen) INFO
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* Corresponding authors: Institut für Anorganische und Analytische Chemie and Institut für Physikalische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany, Fax: +49–251–36002
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Accepted ........
Available online …… Keywords:
Ternary silicides Crystal structure
Superconductivity 29
Si solid state NMR spectroscopy
ABSTRACT
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ACCEPTED MANUSCRIPT The new equiatomic silicides NbOsSi and TaOsSi as well as ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) were prepared from the elements by arc-melting. These silicides crystallize with the orthorhombic TiNiSi type structure, space group Pnma. Irregularly shaped crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were
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separated from the annealed samples and investigated by single-crystal X-ray diffraction (a = 640.46(7), b = 404.07(5), c = 743.66(8) pm, wR2 = 0.0285, 390 F2 values, 20 variables for ZrOsSi; a = 629.78(6), b = 388.72(4), c = 727.48(7) pm, wR2 = 0.0350, 397 F2 values, 20 variables for NbOsSi, a = 626.80(6), b = 389.36(4), c =
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726.22(7) pm, wR2 = 0.0501, 385 F2 values, 20 variables for TaOsSi, a = 653.48(8), b = 395.35(4), c = 739.19(8) pm, wR2 = 0.0427, 413 F2 values, 20 variables for ZrIrSi and a = 646.34(12), b = 393.57(7), c = 736.8(14) pm, wR2 = 0.0582, 371 F2 values, 20
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variables for HfIrSi). The striking structural motifs in the new osmium compounds are three-dimensional [OsSi] networks (Os–Si: 240-251 pm) in which the osmium atoms have strongly distorted tetrahedral silicon coordination. High-pressure/high-temperature experiments (9.5 GPa / 1520 K) on TaOsSi gave no hint for a structural phase transition. Temperature dependent measurements of the magnetic susceptibility and the
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electrical conductivity of NbOsSi and TaOsSi showed superconductivity below TC = 3.5 and 5.5 K, respectively.
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Si solid state MAS NMR investigations of the prepared
silicides approved the structural models and showed a correlation between the observed 29
2017 Elsevier. All rights reserved.
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Si resonance shifts and the electronegativity of the involved refractory metal.
1. Introduction
Phase analytical studies in the binary system Os-Si by X-ray powder diffraction,
metallographic techniques, thermal and microprobe analysis revealed the existence of three semiconducting silicides [1]: OsSi (FeSi type), Os2Si3 (Ru2Ge3 type) and OsSi2 (FeSi2 type) [2-4]. The experimentally determined properties were fully underlined by electronic structure calculations on DFT level [5-7], additionally leading to the elastic constants and the enthalpies of formation.
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ACCEPTED MANUSCRIPT Combination with an electron-poor transition metal (T), a rare earth or an actinoid metal leads to a variety of ternary phases with interesting properties. Representative examples in the field of rare earth compounds are CeOs2Si2 with almost tetravalent cerium and the 42 K antiferromagnet TbOs2Si2 [8]. Of the actinoid compounds U2OsSi3 [9] shows spin fluctuation behavior while PuOs2Si2 [10] remains paramagnetic down to
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4 K.
Phase analytical studies with the electron-poor transition metals led to the equiatomic silicides TiOsSi, ZrOsSi and HfOsSi [11, 12]. The titanium and hafnium
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compound crystallize with the orthorhombic TiFeSi type structure [13], a superstructure of ZrNiAl. ZrOsSi is isotypic with ZrOsP [14] and adopts the TiNiSi type. The striking property of ZrOsSi is its superconducting transition at a critical temperature of 1.72 K
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[11]. The so far highest transition temperature for the equiatomic TT’Si silicides (for a literature overview see [15]) is 10.3 K for ZrRhSi [16]. Screening the family of TT’Si silicides for possible element combinations it becomes readily clear that several osmium-containing phases are missing. During our systematic phase analytical work on TT’Si silicides [15, 17, 18] we have now obtained the new superconducting silicides NbOsSi and TaOsSi. Their synthesis, structural characterization and a detailed 29Si solid
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state NMR spectroscopic study along with the isotypic phases ZrOsSi [11], ZrIrSi [19], HfIrSi [19], NbIrSi [15], TaIrSi [15], NbPtSi [20] and TaPtSi [21] is reported herein.
2.1. Synthesis
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2. Experimental
NbOsSi, TaOsSi and the isotypic compounds ZrOsSi, TIrSi and TPtSi (T = Zr, Hf,
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Nb, Ta) were prepared by arc melting [22] from the elements in the ideal 1 : 1 : 1 atomic ratio and a total mass of about 500 mg under argon atmosphere with ca. 800 mbar pressure. Starting materials were niobium and tantalum sheets (both WHS Sondermetalle, 99.6 %), zirconium sponge (Johnson Matthey 99.5 %), hafnium shavings (Chempur, 99.8 %), osmium (Merck, 99.9 %) and iridium (Agosi 99.9 %) powder, platinum sheets (Agosi 99.9 %) and silicon pieces (smart elements, ultrapure). Prior to use the powdered noble metals were cold-pressed into pellets of 6 mm diameter under a pressure of 100 bar and melted to small buttons. The argon was purified over titanium sponge (900 K), silica gel and molecular sieves. To increase the homogeneity
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ACCEPTED MANUSCRIPT of the samples, the prepared ingots were turned over and re-melted several times. The mass losses after synthesis were negligible. To increase crystallinity, the arc-melted samples were sealed in evacuated silica ampoules and annealed in a muffle furnace at 1223 K for 20 d, then slowly cooled down to room temperature with a cooling rate of 5
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K h–1. The polycrystalline silicides are highly brittle and show bright metallic lustre, reminding elemental silicon though less dark and greyish. The samples are resistant to moisture, air and concentrated mineral acids (HCl, HNO3 and mixtures), even in their
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powdered forms.
2.2. EDX data
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The ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi crystals studied on the diffractometers were semiquantitatively investigated by EDX analyses by use of a Zeiss EVO MA10 scanning electron microscope in variable pressure mode with Zr, Hf, Nb, Ta, Os, Ir, Pt and SiO2 as standards. The experimentally observed compositions (37±3 at.-% Zr : 29±3 at.-% Os : 34±3 at.-% Si; 35±3 at.-% Nb : 32±3 at.-% Os : 33±3 at.-% Si; 35±3 at.-% Ta : 31±3 at.-% Os : 34±3 at.-% Si; 25±3 at.-% Hf : 27±3 at.-% Ir : 48±3
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at.-% Si; 38±3 at.-% Zr : 30±3 at.-% Ir : 32±3 at.-% Si) were all close to the ideal ones. No impurity elements with an atomic number larger than sodium (detection limit of the instrument) were observed. The crystal fragments all showed extreme conchoidal
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fracture, hampering precise analyses.
2.3. X-Ray diffraction
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The purity of the prepared silicides was checked by Guinier powder X-ray diffraction using Cu-Kα1 radiation and α-quartz (a = 491.30 and c = 540.46 pm) as an internal reference substance. The Guinier camera (Enraf-Nonius FR552) was equipped with an imaging plate (Fuji film, BAS-READER 1800). Lattice parameters (Table 1) were derived from least-squares refinements. Correct indexing was ensured through intensity calculations [23]. Irregularly shaped single crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were isolated from the annealed and crushed ingots. The crystals were selected with the help of a light microscope and fixed to silica fibers with beeswax. The first investigation
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ACCEPTED MANUSCRIPT of the crystals was carried out on a Buerger precession camera (white Mo radiation, Fuji-film imaging plate) to check their quality for further intensity data collection. All data sets were collected at ambient temperature by using a Stoe IPDS-II image plate system (graphite monochromatized Mo radiation (λ = 71.073 pm); oscillation mode) or
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a Stoe StadiVari four-circle single crystal 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 corrections along with scaling were applied to the data sets. Details on the crystallographic data and structure refinements
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are given in Table 2.
2.4. High-pressure/high-temperature studies on TaOsSi
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Investigations concerning a possible high-pressure/high-temperature phase transition of TaOsSi were carried out via a multianvil press, equipped with a Walker-type module. Details about the technique and the construction of the different assemblies can be found in numerous references [24-27]. Carefully milled powder of TaOsSi was loaded into 14/8-assembly crucibles made of hexagonal boron nitride and compressed within 250 min to a pressure of 9.5 GPa. Subsequent heating to 1520 K within 15 min was
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followed from a period of 10 min of constant temperature. Afterwards the sample was gently cooled down to 920 K for the next 120 min. This annealing process under pressure at elevated temperatures is important to enhance the crystallinity of the sample and was followed by quenching the sample to room temperature. After decompression
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of the pressure cell, the sample was carefully separated from the surrounding assembly parts by mechanical fragmentation. The recovered polycrystalline sample was silvery
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with metallic lustre and stable in air.
2.5. Magnetic susceptibility and electrical resistivity measurements The measurements of the magnetic susceptibility and the electrical resistivity were
executed on a Quantum Design Physical Property Measurement System (PPMS) using the Vibrating Sample Magnetometer (VSM) and the ac-Transport option, respectively. For the susceptibility measurements 25-55 mg of polycrystalline pieces of the samples were attached to the sample holder rod of the VSM option using Kapton tape. The compounds were examined in the temperature range of 2.1-300 K with magnetic
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ACCEPTED MANUSCRIPT field strength of up to 10 kOe. Magnetization isotherms in the magnetic field range of ±7500 Oe of NbOsSi and TaOsSi were recorded at 2.5 K. The electrical resistivity measurements were executed in the temperature range of 2.1-300 K with a sweep rate of 0.1 K in zero-field using the van-der-Pauw technique
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[28]. Therefore, annealed ingots of both samples were imbedded in a poly(methyl methacrylate) matrix and abraded to expose a cross sectional area with a diameter of 3 mm. Subsequently, the polymer matrix was dissolved in acetone and the ingots were embedded again using a self-build mold allowing a parallel polishing of the opposing
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site. About 1 mm thick discs were obtained by abrading and polishing the other site and dissolving the polymer matrix again. The disc shaped samples were attached to a modified ac-transport puck of the PPMS. A van-der-Pauw press contact assembly with
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spring probes made of gold plated nickel made by Wimbush Science & Technology was used to measure the electrical properties. A measurement time for each metered value of 1 s was used for TaOsSi and NbOsSi with a maximum current of 10 and 25 mA and an ac-frequency of 29 and 79 Hz, respectively. The recorded data of the two channels was converted according to the van-der-Pauw equation
=
,
,
[28].
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2.6. 29Si solid state MAS NMR spectroscopy Si solid state NMR spectra were recorded with a Bruker DSX 400 (B0 = 9.4 T)
spectrometer and with a DSX 500 spectrometer interfaced with a 4.7 T magnet at
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resonance frequencies of 79.440 MHz and 39.720 MHz, respectively. The powdered samples were mixed with dry potassium chloride powder in an approximate ratio of 1–3 volume portions to reduce their electrical conductivity and their volumetric mass
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density and filled in conventional ZrO2 MAS rotors with a diameter of 4 mm. The 29Si spectra were recorded using magic angle spinning (MAS) conditions with spinning frequencies of 10 kHz. Solid tris(trimethylsilyl)silyl silane (TTMSS, δ = –9.75 ppm) was used as an external reference substance at ambient temperature. All spectra were recorded using a rotor-synchronized π/2 - τ - π - τ spin-echo sequence with typical π/2 pulse lengths of p1 = 4.75-5.0 µs and relaxation delays of d1 = 0.5-1 s. The data were recorded with the help of the Bruker Topspin software [29], experimental parameters were analyzed with the Dmfit software [30] package and are listed in Table 5.
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3. Results and discussion 3.1. Structure refinements The data sets of the investigated single crystals showed primitive orthorhombic lattices and systematic extinctions compatible with the space group Pnma. All atoms
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occupy the Wyckoff sites 4a (x, ¼, z). The structure solutions and refinements were achieved and carried out with the help of the JANA2006 software package (full-matrix least-squares on Fo2) [31] including the Superflip-Algorithm [32] for assignment of the initial atomic positions. The structures were refined with anisotropic displacement
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parameters for all atoms. Deviations from the ideal composition of the single crystals were excluded by refining the occupancy parameters in separate series of least-square
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cycles leading to full occupation within three standard deviations for all atomic sites. Final difference Fourier synthesis revealed no significant residual electron densities. All relevant data are listed in Tables 2-4.
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Further details of the structure refinements may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No’s. CSD–432594 (ZrOsSi), CSD–432597 (NbOsSi), CSD– 432596 (TaOsSi), CSD–432595 (ZrIrSi) and CSD–432598 (HfIrSi).
3.2. Crystal Chemistry
The ternary silicides NbOsSi, TaOsSi, ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) crystallize with the well-known TiNiSi type structure (space group Pnma),
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a ternary ordered variant of the binary KHg2 type [33]. Since the crystal chemistry of TiNiSi type compounds has intensively been reviewed [33-37] we only briefly
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summarize the structural characteristics for the present osmium compounds NbOsSi and TaOsSi.
Silicon and osmium atoms are forming a three-dimensional polyanionic [OsSi]
network. This network can be deduced by condensation of strongly puckered hexagonal Os3Si3 units via edge sharing parallel to the bc plane of the unit cell. Due to strong interlayer bonding within the crystallographic a direction the three-dimensional network is formed leading to a distorted tetrahedral coordination of the Os by Si atoms et vice versa. The short Os–Si-distances in the range of 240-244 pm, which are close to or even smaller than the sum of the atomic covalent radii (243 pm [38]), indicate strong bonding
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ACCEPTED MANUSCRIPT interactions between the network forming atoms comparable with binary osmium silicides (e.g. FeSi type OsSi: 246 pm [2]). The large cavities left are occupied by the atoms of the refractory metal component Nb and Ta, respectively, with interatomic distances in the range of 271-274 pm (T–Si), 287-296 pm (T–Os) and 326-331 pm (T– T). Figures 1 and 2 show the crystal structure and the first coordination sphere of the Ta
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atoms in TaOsSi as representatives for both compounds. Additionally, the distorted T’4Si4 units of ZrOsSi, NbOsSi, TaOsSi, NbIrSi and TaIrSi are shown as an overview. The interatomic Os–Os (288-289 pm) and Ir–Ir (281-282 pm) distances for the Nb and Ta compounds indicate moderate bonding interactions between the noble metal atoms
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and are increased in contrast to the pure elements (hcp Os: 6 × 268 pm and 6 × 273 pm; fcc Ir: 12 × 272 pm [39]). Due to the lowered VEC in ZrOsSi, decreased Os-Os
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interactions are observable manifesting in increased Os–Os distances of 302 pm equitable to a decreased distortion of the T’4Si4 tetragons [36].
Since several of the TT’Tr tetrelides and TT’Pn pnictides show temperature or pressure driven phase transitions, we studied the samples under such conditions. Neither annealing experiments nor the HP-HT treatment of the TaOsSi sample in the Walker
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module gave any hint for a structural transformation in the investigated pressure/temperature area.
3.3. Magnetic properties and electrical conductivity The temperature dependence of the magnetic susceptibility χ of NbOsSi and
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TaOsSi was measured in zero field cooled (ZFC) mode applying a magnetic field of 10 kOe in the temperature range of 3-300 K. In both compounds the Pauli paramagnetic
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contribution of the conduction electrons over compensates their intrinsic diamagnetism. The molar susceptibility at 300 K of the niobium sample is χ(300
K)
= 6.0(2)×10–5
emu×mol–1 and χ(300 K) = 8.9(2)×10–5 emu×mol–1 was detected for the tantalum sample. Additionally, the compounds were examined in zero-field cooled / field-cooled
(ZFC/FC) mode with a magnetic field strength of 20 Oe in the temperature range of 2.115 K. Both samples reveal a sharp drop in their susceptibilities indicating transition to a superconducting state as shown in the top panel of Figure 3. The critical temperature of NbOsSi (TCχ = 3.3(1) K) was determined by the onset of diamagnetism of the ZFC/FC measurement. The measured critical temperature of TaOsSi (TCχ = 5.5(1) K) is even
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ACCEPTED MANUSCRIPT higher. Magnetization isotherms of both samples measured at 2.5 K are depicted in Figure 4 and show the typical hysteresis of type II superconductors. The lower critical fields Hc1(TaOsSi) = 67(8) Oe and Hc1(NbOsSi) = 35(10) Oe are defined as a magnetic field at which the magnetization deviates from a linear relation. These low critical fields
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are similar to TaPtSi [21] with Hc1 = 44 Oe. The superconducting state of NbOsSi and TaOsSi was confirmed by electrical resistivity measurements given in the bottom panel of Figure 3. The electrical resistivity of NbOsSi rises with increasing temperature to ρ(300 K) = 6.17(1)×10–7 Ωcm, indicating the absence of a bandgap. The resistivity of both samples shows a transition to zero at
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TCρ=0 = 3.2(1) K for NbOsSi and at TCρ=0 = 5.4(1) K for TaOsSi which are in good agreement with the critical temperatures determined from the magnetic susceptibility
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measurements. The observed transition temperatures of NbOsSi and TaOsSi are in the typical low-temperature range of TT’Si silicides, e. g. ZrOsSi (TC = 1.72 K) [11], ZrIrSi (TC = 2.04 K) and HfIrSi (TC = 3.50 K) [19].
3.4. 29Si solid state MAS NMR spectroscopy
Figure 5 shows the experimental 29Si solid state MAS NMR spectra along with line-
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shape simulations for the new compounds NbOsSi and TaOsSi as well as for the isotypic phases ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta). Consistent with the crystal structure, we observe single-peak spectra, whose resonance shifts vary between 48 and 434 ppm. These values are substantially lower than those observed in
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other silicides, e. g. RERh4Si2 (RE = Sc, Y, Lu; δ(Si1) [40]), RE3Rh9Si2Sn3 (RE = Y, Lu)[41]), RET2Si2 [42] and Sc3TSi3 [43]. In the case of ZrIrSi, HfIrSi, NbPtSi and
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TaPtSi weak spinning sidebands could be observed in addition to the central resonance reflecting the effect of the magnetic shielding anisotropy in the order of magnitude of 100 ppm. While the comparison of nuclear magnetic resonance frequencies between compounds having different crystal structures does not offer much physical insight, recent systematic NMR spectroscopic investigations resulted in the discovery of meaningful shift trends for some series of isostructural compounds. In this regard, the class of compounds with the 1:1:1 composition crystallizing with the TiNiSi structure type is an important model system [42].
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ACCEPTED MANUSCRIPT Quite generally, the
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Si resonance frequencies in these compounds arise from a
contribution due to diamagnetic shielding and orbital coupling (“chemical shift”) and a contribution due to unpaired spin density of s-electrons near the Fermi edge (“Knight shift”) [44-48]. As these different individual contributions are inseparable from each other, the physical reason for the shift variations in the present series of compounds
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cannot be elucidated experimentally. It is worth mentioning that for all, the osmiumcontaining compounds TOsSi (T = Zr, Nb, Ta), the Ir-containg compounds TIrSi (T = Zr, Hf, Nb, Ta) and the Pt-containing compounds TPtSi (T = Zr [18], Hf [18], Nb, Ta) the 29Si resonance shift decreases with increasing Pauling electronegativity (EN) of the
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refractory metal (EN(Hf) = 1.30; EN(Zr) = 1.33; EN(Ta) = 1.5; EN(Nb) = 1.6 [38]). Analogous electronegativity effects were previously observed for other nuclei such as Al, 89Y and
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Pt NMR shifts in various series of isotypic intermetallics [18, 49, 50],
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where they could be linked to Fermi edge s-electron densities via band structure calculations. Based on this analogy we argue that the trend shown in Figure 6 arises dominantly from Knight shift variations. On the other hand, no comparable correlation between the electronegativity of the involved noble metal T’ upon the resulting
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4. Conclusions
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resonance shift can be observed, indicating a more complex relationship.
Phase analytical studies revealed the new superconducting silicides NbOsSi and TaOsSi which are isotypic with TiNiSi, space group Pnma. The occurrence of
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superconductivity in NbOsSi and TaOsSi is in line with the two presumptions made in previous publications. The presence of 3d elements in ternary TT’Si silicides [11], TT’P
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phosphides [51] and TT’As arsenides [52] seems to hinder the appearance of superconductivity. The TiNiSi type structure rather than the ordered Fe2P type structures (ZrNiAl type) seems to favor the occurrence of superconductivity in ternary silicides [11]. A critical look at the possible element combinations reveals potential for other new compounds. Systematic studies are in progress.
Acknowledgments We thank Dipl.-Ing. U. Ch. Rodewald for the single-crystal intensity data collections and M. Sc. S. Seidel for EDX investigations of the single crystals.
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[41] D. Voßwinkel, O. Niehaus, B. Gerke, C. Benndorf, H. Eckert, R. Pöttgen, Z. Anorg. Allg. Chem. 641 (2015) 238. [42] C. Benndorf, doctoral thesis, WWU Münster, 2016. [43] T. Harmening, D. Mohr, H. Eckert, A. Al Alam, S. F. Matar, R. Pöttgen, Z. Anorg. Allg. Chem. 636 (2010) 1839.
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[44] F. Haarmann, in eMagRes (Eds.: R. K. Harris, R. E. Wasylishen), John Wiley & Sons, Ltd, Chichester, 2011. [45] A. Abragam, Principles of Nuclear Magnetism, Oxford University, Oxford, 1961.
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[46] C. P. Slichter, Principles of Magnetic Resonance, 3rd ed., Springer, Berlin, 1990. [47] M. d’Avezac, N. Marzari, F. Mauri, Phys. Rev. B 76 (2007) 165122. [48] R. Laskowski, P. Blaha, J. Phys. Chem. C 119 (2015) 731. [49] C. Benndorf, O. Niehaus, H. Eckert, O. Janka, Z. Anorg. Allg. Chem. 641 (2015) 168. [50] M. Johnscher, S. Stein, O. Niehaus, C. Benndorf, L. Heletta, M. Kersting, C. Höting, H. Eckert, R. Pöttgen, Solid State Sci. 52 (2016) 57. [51] H. Barz, H. C. Ku, G. P. Meisner, Z. Fisk, B. T. Matthias, Proc. Natl. Acad. Sci. 77 (1980) 3132. [52] G. P. Meisner, H. C. Ku, H. Barz, Mater. Res. Bull. 18 (1983) 983.
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ACCEPTED MANUSCRIPT Table 1 Refined lattice parameters from Guinier powder data of the ternary silicides TOsSi (T = Zr, Nb, Ta), TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta). Standard deviations are given in parentheses.
TaIrSi NbPtSi TaPtSi
Table 2
V (nm³) 0.1925 0.1931 0.1781 0.1772 0.1910 0.1912 0.1874 0.1880 0.1765 0.1771 0.1756 0.1756 0.1808 0.1800 0.1797 0.1789
reference this work [11] this work this work this work [19] this work [19] this work [15] this work [15] this work [20] this work [21]
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NbIrSi
c (pm) 743.66(8) 745.3 727.48(7) 726.23(7) 739.19(8) 737.62 736.8(1) 737.63 726.72(9) 727.70(3) 726.89(8) 726.78(3) 737.13(9) 735.6 736.7(2) 735.3
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HfIrSi
b (pm) 404.07(5) 404.1 388.72(4) 389.36(4) 395.35(4) 395.37 393.57(7) 393.8 379.11(4) 379.48(2) 378.58(6) 378.69(2) 381.97(5) 381.7 380.90(8) 380.8
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NbOsSi TaOsSi ZrIrSi
a (pm) 640.46(7) 641.1 629.78(6) 626.80(6) 653.48(8) 655.78 646.3(1) 647.1 640.56(7) 641.27(3) 637.99(8) 638.11(3) 642.02(8) 641.1 640.4(1) 638.8
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Compound ZrOsSi
Crystal data and structure refinement for the silicides NbOsSi, TaOsSi, ZrOsSi, ZrIrSi and HfIrSi; TiNiSi type structure; space group Pnma; Z = 4.
NbOsSi
TaOsSi
ZrOsSi
ZrIrSi
HfIrSi
Formula weight (g mol–1) Unit cell dimensions (pm)
311.2 a = 629.78(6) b = 388.72(4) c = 727.48(7) 0.1781 STADIVARI 11.61 15 × 15 × 20 40 0–180 / 0.3 7.0, -6.0, 0.03 77.8 524 4–33 ±9, ±5, ±11 5219 397 / 0.0317 357 / 0.0099 397 / 20 1.25 0.0155 / 0.0342 0.0181 / 0.0350 153(8) 1.60 / -1.12
399.2 a = 626.80(6) b = 389.36(4) c = 729.22(7) 0.1772 IPDS-II 14.96 10 × 10 × 30 840 0–180 / 1.0 13.0, 2.7, 0.015 133.4 652 4–33 ±9, ±6, ±11 4354 385 / 0.0914 339 / 0.0129 385 / 20 1.67 0.0239 / 0.0492 0.0286 / 0.0501 1380(90) 3.83 / –2.54
309.5 a = 640.46(7) b = 404.07(5) c = 743.66(8) 0.1925 STADIVARI 10.68 10 × 10 × 20 40 0–180 / 0.3 7.0,-6.0, 0.030 71.5 520 4–33 ±9, ±6, ±11 5377 390 / 0.0353 351 / 0.0098 390 / 20 1.02 0.0121 / 0.0279 0.0153 / 0.0285 103(6) 0.69 / –0.79
311.5 a = 653.48(8) b = 395.35(4) c = 739.19(8) 0.1910 IPDS-II 10.84 15 × 20 × 25 70 0–180 / 1.0 12.0, 3.0, 0.020 75.2 524 4-33 ±10, ±6, ±11 5386 413 / 0.0753 369 / 0.0119 413 / 20 1.51 0.0191 / 0.0419 0.0249 / 0.0427 2860(130) 2.51 / -2.78
398.8 a = 646.34(12) b = 393.57(7) c = 736.8(14) 0.1874 STADIVARI 14.13 10 × 10 × 15 40 0–180 / 0.3 6.0,-4.0, 0.030 126.4 652 4-33 ±9, ±5, ±11 1576 371 / 0.0463 318 / 0.0200 371 / 20 1.66 0.0260 / 0.0576 0.0314 / 0.0582 275(14) 2.48 / -2.38
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Empirical formula
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Unit cell volume (nm³) Diffractometer type Calculated density (g cm−3) Crystal size (µm3) Exposure time (s) ω range; increment (°) Integr. param. (A, B, EMS) Absorption coefficient (mm−1) F(000) θ range for data collection (°) Range in hkl Total no. reflections Independent reflections / Rint Reflections with I ≥ 3σ(I) Data / parameters Goodness-of-fit on F2 R1 / wR2 for I > 3σ(I)) R1 / wR2 (all data) Extinction coefficient Largest diff. peak / hole (e Å-3)
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Table 3 Atomic coordinates and anisotropic displacement parameters (/pm2) for the silicides ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi. All atoms lie on Wyckoff sites 4c (x, ¼, z). The anisotropic displacement factor exponent takes the form – 2π2[(ha*)2U11+…+2hka*b*U12]. Ueq is defined as a third of the trace of the orthogonalized Uij tensor. U12 = U23 = 0. z
U11
U22
U13
Ueq
126(2) 109(1) 113(6)
115(2) 108(1) 131(6)
–10(2) –1(1) –11(5)
120(1) 109(1) 120(3)
155(2) 115(1) 119(7)
118(2) 115(1) 129(7)
–5(2) –3(1) –11(6)
131(1) 116(1) 127(4)
87(2) 53(2) 91(15)
54(2) 49(2) 52(12)
–4(2) –7(1) –18(11)
63(1) 49(1) 61(8)
59(2) 41(3) 41(8)
36(1) 36(3) 51(9)
–1(1) –1(2) –7(7)
45(1) 39(2) 43(5)
127(3) 105(3) 123(15)
120(3) 115(3) 105(16)
–2(2) –3(2) 10(15)
123(2) 113(2) 122(10)
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ZrOsSi Zr 0.01335(7) 0.68396(6) 120(2) Os 0.16141(3) 0.05823(3) 109(1) Si 0.2866(2) 0.3777(2) 115(6) NbOsSi Nb 0.02004(8) 0.67927(8) 121(2) Os 0.15409(3) 0.05964(3) 116(1) Si 0.2743(3) 0.3754(3) 133(7) TaOsSi Ta 0.01996(8) 0.67920(6) 48(2) Os 0.15594(8) 0.05987(6) 45(2) Si 0.2744(6) 0.3750(5) 39(13) ZrIrSi Zr 0.01768(4) 0.68174(4) 39(1) Ir 0.15156(11) 0.06453(11) 39(3) Si 0.2780(3) 0.3816(3) 37(9) HfIrSi Hf 0.01925(10) 0.68110(9) 122(3) Ir 0.15367(9) 0.06436(9) 119(3) Si 0.2766(7) 0.3793(7) 136(18)
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Table 4 Interatomic distances (/pm) in the structures of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi calculated from the powder lattice parameters. All distances of the first coordination spheres are listed. Standard deviations are less or equal 0.2 pm. Si Si Si Os Os Os Os Nb Nb Si Si Si Si Os Nb Nb Nb Nb Os Os Os Nb Nb Nb Nb
271.5 273.0 273.7 288.7 289.3 293.0 295.7 326.3 331.3 359.0 240.4 241.9 243.8 288.1 288.7 289.3 293.0 295.7 240.4 241.9 243.8 271.5 273.0 273.7 359.0
TaOsSi Ta: 2 1 2 1 1 2 2 2 2 1 Os: 2 1 1 1 2 1 2 2 Si: 2 1 1 2 1 2 1
Si Si Si Os Os Os Os Ta Ta Si Si Si Si Ta Os Ta Ta Ta Os Os Os Ta Ta Ta Ta
271.1 272.5 273.4 286.7 289.3 293.2 294.4 326.0 329.8 358.5 240.5 240.6 243.8 286.7 289.3 289.3 293.2 294.4 240.5 240.6 243.8 271.1 272.5 273.4 358.5
ZrIrSi Zr: 1 2 2 2 1 1 2 2 2 1 Ir: 2 1 1 2 2 1 1 2 Si: 2 1 1 1 2 2 1
Si Si Si Ir Ir Ir Ir Zr Zr Si Si Si Si Zr Ir Zr Zr Zr Ir Ir Ir Zr Zr Zr Zr
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Table 5 29 Si NMR spectroscopic parameters of the silicides TOsSi, TIrSi and TPtSi: resonance shifts δ (± 1 /ppm); full width at half maximum ∆ (±0.01 /kHz), degree of Gaussian (G) vs. Lorentzian (L) character of the central signal, 90° pulse length p1 (/μs), relaxation delay d1 (/s) and flux density of external magnetic field B0 (/T).
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compound δ ∆ ZrOsSi 388 0.44 NbOsSi 48 1.66 TaOsSi 147 0.95 ZrIrSi 378 0.36 HfIrSi 434 0.73 NbIrSi 244 2.22 TaIrSi 255 1.15 a ZrPtSi 340 0.69 HfPtSia 352 0.68 NbPtSi 260 2.86 TaPtSi 277 1.92 a data taken from [18]
G/L 0.49 0.74 0.38 0.28 0.14 0.59 0.46 0.15 0.44 0.39 0.59
p1 4.75 4.75 4.75 4.75 4.75 5.0 4.75 4.75 4.75 4.75 4.75
d1 0.5 0.5 0.5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5
B0 9.4 9.4 9.4 9.4 9.4 4.7 9.4 9.4 9.4 9.4 9.4
279.6 280.4 280.6 294.1 296.2 300.6 305.5 334.4 342.0 358.8 243.9 247.3 248.5 294.1 295.7 296.2 300.6 305.5 243.9 247.3 248.5 279.6 280.4 280.6 358.8
HfIrSi Hf: 1 2 2 2 1 1 2 2 2 1 Ir: 2 1 1 2 2 1 1 2 Si: 2 1 1 1 2 2 1
νrot 10 10 10 10 10 10 10 10 10 10 10
Si Si Si Ir Ir Ir Ir Hf Hf Si Si Si Si Hf Ir Hf Hf Hf Ir Ir Ir Hf Hf Hf Hf
277.7 278.0 278.3 293.9 295.4 297.6 301.4 332.5 338.7 359.9 243.6 245.3 247.2 293.9 295.3 295.4 297.6 301.4 243.6 245.3 247.2 277.7 278.0 278.3 359.9
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NbOsSi Nb: 2 1 2 1 1 2 2 2 2 1 Os: 2 1 1 2 1 1 2 2 Si: 2 1 1 2 1 2 1
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279.3 282.5 287.2 288.5 294.0 300.2 304.9 335.0 340.6 356.9 244.7 244.8 250.7 288.5 294.0 300.2 301.8 304.9 244.7 244.8 250.7 279.3 282.5 287.2 356.9
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Si Si Si Os Os Os Os Zr Zr Si Si Si Si Zr Zr Zr Os Zr Os Os Os Zr Zr Zr Zr
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ZrOsSi Zr: 2 2 1 1 1 2 2 2 2 1 Os: 1 2 1 1 1 2 2 2 Si: 1 2 1 2 2 1 1
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Fig. 1. Crystal structure of TaOsSi. The tantalum, osmium and silicon atoms are drawn as dark grey, blue and open cycles, respectively. The polyanionic [OsSi] network is emphasized.
Fig. 2. (Right) Coordination of the tantalum atoms in TaOsSi. The tantalum, osmium and silicon atoms are drawn as dark grey, blue and open cycles, respectively. (Left) The characteristic T2Si2 units (with angles /° and interatomic distances /pm) in ZrOsSi, NbOsSi, NbIrSi, TaOsSi and TaIrSi.
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Fig. 3. (top) Temperature dependence of the magnetic susceptibility of NbOsSi and TaOsSi from 2.1 to 15 K measured in ZFC/FC mode at 20 Oe. (bottom) Electrical resistivity as a function of temperature in a range of 2.1-15 K measured with no external magnetic field.
Fig. 4. Magnetization isotherms of NbOsSi and TaOsSi measured at 2.5 K in a magnetic field range of ±7500 Oe.
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Fig. 5. 29Si solid state MAS NMR spectra and simulations (colored lines) of the silicides TOsSi (T = Zr, Nb, Ta), TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) recorded at B0 = 9.4 T and 4.7 T and νrot = 10 kHz.
Fig. 6. Experimental 29Si NMR resonance shifts of the investigated silicides TOsSi, TIrSi and TPtSi in dependence on the involved refractory metal T. The compounds of the titanium group members are depicted with red, those of the vanadium group members with blue cycles, respectively. The data of ZrPtSi and HfPtSi are taken from [18].
S1293-2558(17)30146-2
DOI:
10.1016/j.solidstatesciences.2017.04.002
Reference:
SSSCIE 5488
To appear in:
Solid State Sciences
Received Date: 10 February 2017 Accepted Date: 7 April 2017
Please cite this article as: C. Benndorf, L. Heletta, G. Heymann, H. Huppertz, H. Eckert, R. Pöttgen, NbOsSi and TaOsSi – Two new superconducting ternary osmium silicides, Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.04.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT To be submitted to Solid State Sci.
08.04.17 15:04
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NbOsSi and TaOsSi – two new superconducting ternary osmium silicides Christopher Benndorfa,b, Lukas Helettaa, Gunter Heymannc, Hubert Huppertzc, Hellmut Eckertb *, Rainer Pöttgena * Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany
b
Institut für Physikalische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany and Institute of Physics in Sao Carlos, University of Sao Paulo, Sao Carlos, SP 13560-590, Brazil
c
Institut für Allgemeine, Anorganische und Theoretische Chemie, Leopold-FranzensUniversität Innsbruck, Innrain 80–82, A-6020 Innsbruck, Austria
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E-mail addresses:
Article history: Received ........
[email protected] (H. Eckert), [email protected] (R. Pöttgen) INFO
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* Corresponding authors: Institut für Anorganische und Analytische Chemie and Institut für Physikalische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany, Fax: +49–251–36002
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Accepted ........
Available online …… Keywords:
Ternary silicides Crystal structure
Superconductivity 29
Si solid state NMR spectroscopy
ABSTRACT
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ACCEPTED MANUSCRIPT The new equiatomic silicides NbOsSi and TaOsSi as well as ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) were prepared from the elements by arc-melting. These silicides crystallize with the orthorhombic TiNiSi type structure, space group Pnma. Irregularly shaped crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were
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separated from the annealed samples and investigated by single-crystal X-ray diffraction (a = 640.46(7), b = 404.07(5), c = 743.66(8) pm, wR2 = 0.0285, 390 F2 values, 20 variables for ZrOsSi; a = 629.78(6), b = 388.72(4), c = 727.48(7) pm, wR2 = 0.0350, 397 F2 values, 20 variables for NbOsSi, a = 626.80(6), b = 389.36(4), c =
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726.22(7) pm, wR2 = 0.0501, 385 F2 values, 20 variables for TaOsSi, a = 653.48(8), b = 395.35(4), c = 739.19(8) pm, wR2 = 0.0427, 413 F2 values, 20 variables for ZrIrSi and a = 646.34(12), b = 393.57(7), c = 736.8(14) pm, wR2 = 0.0582, 371 F2 values, 20
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variables for HfIrSi). The striking structural motifs in the new osmium compounds are three-dimensional [OsSi] networks (Os–Si: 240-251 pm) in which the osmium atoms have strongly distorted tetrahedral silicon coordination. High-pressure/high-temperature experiments (9.5 GPa / 1520 K) on TaOsSi gave no hint for a structural phase transition. Temperature dependent measurements of the magnetic susceptibility and the
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electrical conductivity of NbOsSi and TaOsSi showed superconductivity below TC = 3.5 and 5.5 K, respectively.
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Si solid state MAS NMR investigations of the prepared
silicides approved the structural models and showed a correlation between the observed 29
2017 Elsevier. All rights reserved.
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Si resonance shifts and the electronegativity of the involved refractory metal.
1. Introduction
Phase analytical studies in the binary system Os-Si by X-ray powder diffraction,
metallographic techniques, thermal and microprobe analysis revealed the existence of three semiconducting silicides [1]: OsSi (FeSi type), Os2Si3 (Ru2Ge3 type) and OsSi2 (FeSi2 type) [2-4]. The experimentally determined properties were fully underlined by electronic structure calculations on DFT level [5-7], additionally leading to the elastic constants and the enthalpies of formation.
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ACCEPTED MANUSCRIPT Combination with an electron-poor transition metal (T), a rare earth or an actinoid metal leads to a variety of ternary phases with interesting properties. Representative examples in the field of rare earth compounds are CeOs2Si2 with almost tetravalent cerium and the 42 K antiferromagnet TbOs2Si2 [8]. Of the actinoid compounds U2OsSi3 [9] shows spin fluctuation behavior while PuOs2Si2 [10] remains paramagnetic down to
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4 K.
Phase analytical studies with the electron-poor transition metals led to the equiatomic silicides TiOsSi, ZrOsSi and HfOsSi [11, 12]. The titanium and hafnium
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compound crystallize with the orthorhombic TiFeSi type structure [13], a superstructure of ZrNiAl. ZrOsSi is isotypic with ZrOsP [14] and adopts the TiNiSi type. The striking property of ZrOsSi is its superconducting transition at a critical temperature of 1.72 K
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[11]. The so far highest transition temperature for the equiatomic TT’Si silicides (for a literature overview see [15]) is 10.3 K for ZrRhSi [16]. Screening the family of TT’Si silicides for possible element combinations it becomes readily clear that several osmium-containing phases are missing. During our systematic phase analytical work on TT’Si silicides [15, 17, 18] we have now obtained the new superconducting silicides NbOsSi and TaOsSi. Their synthesis, structural characterization and a detailed 29Si solid
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state NMR spectroscopic study along with the isotypic phases ZrOsSi [11], ZrIrSi [19], HfIrSi [19], NbIrSi [15], TaIrSi [15], NbPtSi [20] and TaPtSi [21] is reported herein.
2.1. Synthesis
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2. Experimental
NbOsSi, TaOsSi and the isotypic compounds ZrOsSi, TIrSi and TPtSi (T = Zr, Hf,
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Nb, Ta) were prepared by arc melting [22] from the elements in the ideal 1 : 1 : 1 atomic ratio and a total mass of about 500 mg under argon atmosphere with ca. 800 mbar pressure. Starting materials were niobium and tantalum sheets (both WHS Sondermetalle, 99.6 %), zirconium sponge (Johnson Matthey 99.5 %), hafnium shavings (Chempur, 99.8 %), osmium (Merck, 99.9 %) and iridium (Agosi 99.9 %) powder, platinum sheets (Agosi 99.9 %) and silicon pieces (smart elements, ultrapure). Prior to use the powdered noble metals were cold-pressed into pellets of 6 mm diameter under a pressure of 100 bar and melted to small buttons. The argon was purified over titanium sponge (900 K), silica gel and molecular sieves. To increase the homogeneity
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K h–1. The polycrystalline silicides are highly brittle and show bright metallic lustre, reminding elemental silicon though less dark and greyish. The samples are resistant to moisture, air and concentrated mineral acids (HCl, HNO3 and mixtures), even in their
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2.2. EDX data
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The ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi crystals studied on the diffractometers were semiquantitatively investigated by EDX analyses by use of a Zeiss EVO MA10 scanning electron microscope in variable pressure mode with Zr, Hf, Nb, Ta, Os, Ir, Pt and SiO2 as standards. The experimentally observed compositions (37±3 at.-% Zr : 29±3 at.-% Os : 34±3 at.-% Si; 35±3 at.-% Nb : 32±3 at.-% Os : 33±3 at.-% Si; 35±3 at.-% Ta : 31±3 at.-% Os : 34±3 at.-% Si; 25±3 at.-% Hf : 27±3 at.-% Ir : 48±3
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at.-% Si; 38±3 at.-% Zr : 30±3 at.-% Ir : 32±3 at.-% Si) were all close to the ideal ones. No impurity elements with an atomic number larger than sodium (detection limit of the instrument) were observed. The crystal fragments all showed extreme conchoidal
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fracture, hampering precise analyses.
2.3. X-Ray diffraction
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The purity of the prepared silicides was checked by Guinier powder X-ray diffraction using Cu-Kα1 radiation and α-quartz (a = 491.30 and c = 540.46 pm) as an internal reference substance. The Guinier camera (Enraf-Nonius FR552) was equipped with an imaging plate (Fuji film, BAS-READER 1800). Lattice parameters (Table 1) were derived from least-squares refinements. Correct indexing was ensured through intensity calculations [23]. Irregularly shaped single crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were isolated from the annealed and crushed ingots. The crystals were selected with the help of a light microscope and fixed to silica fibers with beeswax. The first investigation
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a Stoe StadiVari four-circle single crystal 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 corrections along with scaling were applied to the data sets. Details on the crystallographic data and structure refinements
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2.4. High-pressure/high-temperature studies on TaOsSi
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Investigations concerning a possible high-pressure/high-temperature phase transition of TaOsSi were carried out via a multianvil press, equipped with a Walker-type module. Details about the technique and the construction of the different assemblies can be found in numerous references [24-27]. Carefully milled powder of TaOsSi was loaded into 14/8-assembly crucibles made of hexagonal boron nitride and compressed within 250 min to a pressure of 9.5 GPa. Subsequent heating to 1520 K within 15 min was
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followed from a period of 10 min of constant temperature. Afterwards the sample was gently cooled down to 920 K for the next 120 min. This annealing process under pressure at elevated temperatures is important to enhance the crystallinity of the sample and was followed by quenching the sample to room temperature. After decompression
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of the pressure cell, the sample was carefully separated from the surrounding assembly parts by mechanical fragmentation. The recovered polycrystalline sample was silvery
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with metallic lustre and stable in air.
2.5. Magnetic susceptibility and electrical resistivity measurements The measurements of the magnetic susceptibility and the electrical resistivity were
executed on a Quantum Design Physical Property Measurement System (PPMS) using the Vibrating Sample Magnetometer (VSM) and the ac-Transport option, respectively. For the susceptibility measurements 25-55 mg of polycrystalline pieces of the samples were attached to the sample holder rod of the VSM option using Kapton tape. The compounds were examined in the temperature range of 2.1-300 K with magnetic
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[28]. Therefore, annealed ingots of both samples were imbedded in a poly(methyl methacrylate) matrix and abraded to expose a cross sectional area with a diameter of 3 mm. Subsequently, the polymer matrix was dissolved in acetone and the ingots were embedded again using a self-build mold allowing a parallel polishing of the opposing
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site. About 1 mm thick discs were obtained by abrading and polishing the other site and dissolving the polymer matrix again. The disc shaped samples were attached to a modified ac-transport puck of the PPMS. A van-der-Pauw press contact assembly with
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spring probes made of gold plated nickel made by Wimbush Science & Technology was used to measure the electrical properties. A measurement time for each metered value of 1 s was used for TaOsSi and NbOsSi with a maximum current of 10 and 25 mA and an ac-frequency of 29 and 79 Hz, respectively. The recorded data of the two channels was converted according to the van-der-Pauw equation
=
,
,
[28].
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2.6. 29Si solid state MAS NMR spectroscopy Si solid state NMR spectra were recorded with a Bruker DSX 400 (B0 = 9.4 T)
spectrometer and with a DSX 500 spectrometer interfaced with a 4.7 T magnet at
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resonance frequencies of 79.440 MHz and 39.720 MHz, respectively. The powdered samples were mixed with dry potassium chloride powder in an approximate ratio of 1–3 volume portions to reduce their electrical conductivity and their volumetric mass
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density and filled in conventional ZrO2 MAS rotors with a diameter of 4 mm. The 29Si spectra were recorded using magic angle spinning (MAS) conditions with spinning frequencies of 10 kHz. Solid tris(trimethylsilyl)silyl silane (TTMSS, δ = –9.75 ppm) was used as an external reference substance at ambient temperature. All spectra were recorded using a rotor-synchronized π/2 - τ - π - τ spin-echo sequence with typical π/2 pulse lengths of p1 = 4.75-5.0 µs and relaxation delays of d1 = 0.5-1 s. The data were recorded with the help of the Bruker Topspin software [29], experimental parameters were analyzed with the Dmfit software [30] package and are listed in Table 5.
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3. Results and discussion 3.1. Structure refinements The data sets of the investigated single crystals showed primitive orthorhombic lattices and systematic extinctions compatible with the space group Pnma. All atoms
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occupy the Wyckoff sites 4a (x, ¼, z). The structure solutions and refinements were achieved and carried out with the help of the JANA2006 software package (full-matrix least-squares on Fo2) [31] including the Superflip-Algorithm [32] for assignment of the initial atomic positions. The structures were refined with anisotropic displacement
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parameters for all atoms. Deviations from the ideal composition of the single crystals were excluded by refining the occupancy parameters in separate series of least-square
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cycles leading to full occupation within three standard deviations for all atomic sites. Final difference Fourier synthesis revealed no significant residual electron densities. All relevant data are listed in Tables 2-4.
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Further details of the structure refinements may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No’s. CSD–432594 (ZrOsSi), CSD–432597 (NbOsSi), CSD– 432596 (TaOsSi), CSD–432595 (ZrIrSi) and CSD–432598 (HfIrSi).
3.2. Crystal Chemistry
The ternary silicides NbOsSi, TaOsSi, ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) crystallize with the well-known TiNiSi type structure (space group Pnma),
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a ternary ordered variant of the binary KHg2 type [33]. Since the crystal chemistry of TiNiSi type compounds has intensively been reviewed [33-37] we only briefly
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summarize the structural characteristics for the present osmium compounds NbOsSi and TaOsSi.
Silicon and osmium atoms are forming a three-dimensional polyanionic [OsSi]
network. This network can be deduced by condensation of strongly puckered hexagonal Os3Si3 units via edge sharing parallel to the bc plane of the unit cell. Due to strong interlayer bonding within the crystallographic a direction the three-dimensional network is formed leading to a distorted tetrahedral coordination of the Os by Si atoms et vice versa. The short Os–Si-distances in the range of 240-244 pm, which are close to or even smaller than the sum of the atomic covalent radii (243 pm [38]), indicate strong bonding
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ACCEPTED MANUSCRIPT interactions between the network forming atoms comparable with binary osmium silicides (e.g. FeSi type OsSi: 246 pm [2]). The large cavities left are occupied by the atoms of the refractory metal component Nb and Ta, respectively, with interatomic distances in the range of 271-274 pm (T–Si), 287-296 pm (T–Os) and 326-331 pm (T– T). Figures 1 and 2 show the crystal structure and the first coordination sphere of the Ta
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atoms in TaOsSi as representatives for both compounds. Additionally, the distorted T’4Si4 units of ZrOsSi, NbOsSi, TaOsSi, NbIrSi and TaIrSi are shown as an overview. The interatomic Os–Os (288-289 pm) and Ir–Ir (281-282 pm) distances for the Nb and Ta compounds indicate moderate bonding interactions between the noble metal atoms
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and are increased in contrast to the pure elements (hcp Os: 6 × 268 pm and 6 × 273 pm; fcc Ir: 12 × 272 pm [39]). Due to the lowered VEC in ZrOsSi, decreased Os-Os
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interactions are observable manifesting in increased Os–Os distances of 302 pm equitable to a decreased distortion of the T’4Si4 tetragons [36].
Since several of the TT’Tr tetrelides and TT’Pn pnictides show temperature or pressure driven phase transitions, we studied the samples under such conditions. Neither annealing experiments nor the HP-HT treatment of the TaOsSi sample in the Walker
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module gave any hint for a structural transformation in the investigated pressure/temperature area.
3.3. Magnetic properties and electrical conductivity The temperature dependence of the magnetic susceptibility χ of NbOsSi and
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TaOsSi was measured in zero field cooled (ZFC) mode applying a magnetic field of 10 kOe in the temperature range of 3-300 K. In both compounds the Pauli paramagnetic
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contribution of the conduction electrons over compensates their intrinsic diamagnetism. The molar susceptibility at 300 K of the niobium sample is χ(300
K)
= 6.0(2)×10–5
emu×mol–1 and χ(300 K) = 8.9(2)×10–5 emu×mol–1 was detected for the tantalum sample. Additionally, the compounds were examined in zero-field cooled / field-cooled
(ZFC/FC) mode with a magnetic field strength of 20 Oe in the temperature range of 2.115 K. Both samples reveal a sharp drop in their susceptibilities indicating transition to a superconducting state as shown in the top panel of Figure 3. The critical temperature of NbOsSi (TCχ = 3.3(1) K) was determined by the onset of diamagnetism of the ZFC/FC measurement. The measured critical temperature of TaOsSi (TCχ = 5.5(1) K) is even
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ACCEPTED MANUSCRIPT higher. Magnetization isotherms of both samples measured at 2.5 K are depicted in Figure 4 and show the typical hysteresis of type II superconductors. The lower critical fields Hc1(TaOsSi) = 67(8) Oe and Hc1(NbOsSi) = 35(10) Oe are defined as a magnetic field at which the magnetization deviates from a linear relation. These low critical fields
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are similar to TaPtSi [21] with Hc1 = 44 Oe. The superconducting state of NbOsSi and TaOsSi was confirmed by electrical resistivity measurements given in the bottom panel of Figure 3. The electrical resistivity of NbOsSi rises with increasing temperature to ρ(300 K) = 6.17(1)×10–7 Ωcm, indicating the absence of a bandgap. The resistivity of both samples shows a transition to zero at
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TCρ=0 = 3.2(1) K for NbOsSi and at TCρ=0 = 5.4(1) K for TaOsSi which are in good agreement with the critical temperatures determined from the magnetic susceptibility
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measurements. The observed transition temperatures of NbOsSi and TaOsSi are in the typical low-temperature range of TT’Si silicides, e. g. ZrOsSi (TC = 1.72 K) [11], ZrIrSi (TC = 2.04 K) and HfIrSi (TC = 3.50 K) [19].
3.4. 29Si solid state MAS NMR spectroscopy
Figure 5 shows the experimental 29Si solid state MAS NMR spectra along with line-
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shape simulations for the new compounds NbOsSi and TaOsSi as well as for the isotypic phases ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta). Consistent with the crystal structure, we observe single-peak spectra, whose resonance shifts vary between 48 and 434 ppm. These values are substantially lower than those observed in
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other silicides, e. g. RERh4Si2 (RE = Sc, Y, Lu; δ(Si1) [40]), RE3Rh9Si2Sn3 (RE = Y, Lu)[41]), RET2Si2 [42] and Sc3TSi3 [43]. In the case of ZrIrSi, HfIrSi, NbPtSi and
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TaPtSi weak spinning sidebands could be observed in addition to the central resonance reflecting the effect of the magnetic shielding anisotropy in the order of magnitude of 100 ppm. While the comparison of nuclear magnetic resonance frequencies between compounds having different crystal structures does not offer much physical insight, recent systematic NMR spectroscopic investigations resulted in the discovery of meaningful shift trends for some series of isostructural compounds. In this regard, the class of compounds with the 1:1:1 composition crystallizing with the TiNiSi structure type is an important model system [42].
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ACCEPTED MANUSCRIPT Quite generally, the
29
Si resonance frequencies in these compounds arise from a
contribution due to diamagnetic shielding and orbital coupling (“chemical shift”) and a contribution due to unpaired spin density of s-electrons near the Fermi edge (“Knight shift”) [44-48]. As these different individual contributions are inseparable from each other, the physical reason for the shift variations in the present series of compounds
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cannot be elucidated experimentally. It is worth mentioning that for all, the osmiumcontaining compounds TOsSi (T = Zr, Nb, Ta), the Ir-containg compounds TIrSi (T = Zr, Hf, Nb, Ta) and the Pt-containing compounds TPtSi (T = Zr [18], Hf [18], Nb, Ta) the 29Si resonance shift decreases with increasing Pauling electronegativity (EN) of the
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refractory metal (EN(Hf) = 1.30; EN(Zr) = 1.33; EN(Ta) = 1.5; EN(Nb) = 1.6 [38]). Analogous electronegativity effects were previously observed for other nuclei such as Al, 89Y and
195
Pt NMR shifts in various series of isotypic intermetallics [18, 49, 50],
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27
where they could be linked to Fermi edge s-electron densities via band structure calculations. Based on this analogy we argue that the trend shown in Figure 6 arises dominantly from Knight shift variations. On the other hand, no comparable correlation between the electronegativity of the involved noble metal T’ upon the resulting
29
Si
4. Conclusions
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resonance shift can be observed, indicating a more complex relationship.
Phase analytical studies revealed the new superconducting silicides NbOsSi and TaOsSi which are isotypic with TiNiSi, space group Pnma. The occurrence of
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superconductivity in NbOsSi and TaOsSi is in line with the two presumptions made in previous publications. The presence of 3d elements in ternary TT’Si silicides [11], TT’P
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phosphides [51] and TT’As arsenides [52] seems to hinder the appearance of superconductivity. The TiNiSi type structure rather than the ordered Fe2P type structures (ZrNiAl type) seems to favor the occurrence of superconductivity in ternary silicides [11]. A critical look at the possible element combinations reveals potential for other new compounds. Systematic studies are in progress.
Acknowledgments We thank Dipl.-Ing. U. Ch. Rodewald for the single-crystal intensity data collections and M. Sc. S. Seidel for EDX investigations of the single crystals.
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[14] G. P. Meisner, H. C. Ku, Appl. Phys. A 31 (1983) 201.
[15] R. Mishra, R. Pöttgen, G. Kotzyba, Z. Naturforsch. 56b (2001) 463. [16] I. Shirotani, Y. Konno, Y. Okada, Ch. Sekine, S. Todo, T. Yagi, Solid State Commun. 108 (1998) 967. [17] M. Valldor, R. Pöttgen, Z. Naturforsch. 61b (2006) 339.
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[18] C. Benndorf, H. Eckert, R. Pöttgen, Dalton Trans. 45 (2016) 8215.
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[19] W. X. Zhong, B. Chevalier, J. Etourneau, P. Hagenmuller, Mater. Res. Bull. 20 (1985) 517. [20] S. M. Yashiro, A. Kasahi, R. Kasai, H. Samata, Y. Nagata, J. Alloys Compd. 309 (2000) 51. [21] S. Yashiro, R. Kasai, A. Kasahi, Y. Nagata, H. Samata, T. Uchida, M. D. Lan, J. Phys. Chem. Solids 61 (2000) 1591. [22] R. Pöttgen, T. Gulden, A. Simon, GIT Labor-Fachzeitschrift 43 (1999) 133. [23] K. Yvon, W. Jeitschko, E. Parthé, J. Appl. Crystallogr. 10 (1977) 73. [24] H. Huppertz, Z. Kristallogr. 219 (2004) 330. [25] D. C. Rubie, Phase Trans. 68 (1999) 431. [26] D. Walker, Am. Miner. 76 (1991) 1092. [27] D. Walker, M. A. Carpenter, C. M. Hitch, Am. Miner. 75 (1990) 1020.
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[32] L. Palatinus, G. Chapuis, J. Appl. Crystallogr. 40 (2007) 786. [33] R.-D. Hoffmann, R. Pöttgen, Z. Kristallogr. 216 (2001) 127.
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[34] E. Parthé, L. Gelato, B. Chabot, M. Penzo, K. Cenzual, R. Gladyshevskii, TYPIX– Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types. Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th edition, Springer, Berlin (Germany), 1993. [35] G. Nuspl, K. Polborn, J. Evers, G. A. Landrum, R. Hoffmann, Inorg. Chem. 35 (1996) 6922.
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[36] G. A. Landrum, R. Hoffmann, J. Evers, H. Boysen, Inorg. Chem. 37 (1998) 5754. [37] M. D. Bojin, R. Hoffmann, Helv. Chim. Acta 86 (2003) 1683. [38] J. Emsley, The Elements, Oxford University Press, 1999.
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[41] D. Voßwinkel, O. Niehaus, B. Gerke, C. Benndorf, H. Eckert, R. Pöttgen, Z. Anorg. Allg. Chem. 641 (2015) 238. [42] C. Benndorf, doctoral thesis, WWU Münster, 2016. [43] T. Harmening, D. Mohr, H. Eckert, A. Al Alam, S. F. Matar, R. Pöttgen, Z. Anorg. Allg. Chem. 636 (2010) 1839.
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[44] F. Haarmann, in eMagRes (Eds.: R. K. Harris, R. E. Wasylishen), John Wiley & Sons, Ltd, Chichester, 2011. [45] A. Abragam, Principles of Nuclear Magnetism, Oxford University, Oxford, 1961.
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[46] C. P. Slichter, Principles of Magnetic Resonance, 3rd ed., Springer, Berlin, 1990. [47] M. d’Avezac, N. Marzari, F. Mauri, Phys. Rev. B 76 (2007) 165122. [48] R. Laskowski, P. Blaha, J. Phys. Chem. C 119 (2015) 731. [49] C. Benndorf, O. Niehaus, H. Eckert, O. Janka, Z. Anorg. Allg. Chem. 641 (2015) 168. [50] M. Johnscher, S. Stein, O. Niehaus, C. Benndorf, L. Heletta, M. Kersting, C. Höting, H. Eckert, R. Pöttgen, Solid State Sci. 52 (2016) 57. [51] H. Barz, H. C. Ku, G. P. Meisner, Z. Fisk, B. T. Matthias, Proc. Natl. Acad. Sci. 77 (1980) 3132. [52] G. P. Meisner, H. C. Ku, H. Barz, Mater. Res. Bull. 18 (1983) 983.
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ACCEPTED MANUSCRIPT Table 1 Refined lattice parameters from Guinier powder data of the ternary silicides TOsSi (T = Zr, Nb, Ta), TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta). Standard deviations are given in parentheses.
TaIrSi NbPtSi TaPtSi
Table 2
V (nm³) 0.1925 0.1931 0.1781 0.1772 0.1910 0.1912 0.1874 0.1880 0.1765 0.1771 0.1756 0.1756 0.1808 0.1800 0.1797 0.1789
reference this work [11] this work this work this work [19] this work [19] this work [15] this work [15] this work [20] this work [21]
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NbIrSi
c (pm) 743.66(8) 745.3 727.48(7) 726.23(7) 739.19(8) 737.62 736.8(1) 737.63 726.72(9) 727.70(3) 726.89(8) 726.78(3) 737.13(9) 735.6 736.7(2) 735.3
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HfIrSi
b (pm) 404.07(5) 404.1 388.72(4) 389.36(4) 395.35(4) 395.37 393.57(7) 393.8 379.11(4) 379.48(2) 378.58(6) 378.69(2) 381.97(5) 381.7 380.90(8) 380.8
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NbOsSi TaOsSi ZrIrSi
a (pm) 640.46(7) 641.1 629.78(6) 626.80(6) 653.48(8) 655.78 646.3(1) 647.1 640.56(7) 641.27(3) 637.99(8) 638.11(3) 642.02(8) 641.1 640.4(1) 638.8
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Compound ZrOsSi
Crystal data and structure refinement for the silicides NbOsSi, TaOsSi, ZrOsSi, ZrIrSi and HfIrSi; TiNiSi type structure; space group Pnma; Z = 4.
NbOsSi
TaOsSi
ZrOsSi
ZrIrSi
HfIrSi
Formula weight (g mol–1) Unit cell dimensions (pm)
311.2 a = 629.78(6) b = 388.72(4) c = 727.48(7) 0.1781 STADIVARI 11.61 15 × 15 × 20 40 0–180 / 0.3 7.0, -6.0, 0.03 77.8 524 4–33 ±9, ±5, ±11 5219 397 / 0.0317 357 / 0.0099 397 / 20 1.25 0.0155 / 0.0342 0.0181 / 0.0350 153(8) 1.60 / -1.12
399.2 a = 626.80(6) b = 389.36(4) c = 729.22(7) 0.1772 IPDS-II 14.96 10 × 10 × 30 840 0–180 / 1.0 13.0, 2.7, 0.015 133.4 652 4–33 ±9, ±6, ±11 4354 385 / 0.0914 339 / 0.0129 385 / 20 1.67 0.0239 / 0.0492 0.0286 / 0.0501 1380(90) 3.83 / –2.54
309.5 a = 640.46(7) b = 404.07(5) c = 743.66(8) 0.1925 STADIVARI 10.68 10 × 10 × 20 40 0–180 / 0.3 7.0,-6.0, 0.030 71.5 520 4–33 ±9, ±6, ±11 5377 390 / 0.0353 351 / 0.0098 390 / 20 1.02 0.0121 / 0.0279 0.0153 / 0.0285 103(6) 0.69 / –0.79
311.5 a = 653.48(8) b = 395.35(4) c = 739.19(8) 0.1910 IPDS-II 10.84 15 × 20 × 25 70 0–180 / 1.0 12.0, 3.0, 0.020 75.2 524 4-33 ±10, ±6, ±11 5386 413 / 0.0753 369 / 0.0119 413 / 20 1.51 0.0191 / 0.0419 0.0249 / 0.0427 2860(130) 2.51 / -2.78
398.8 a = 646.34(12) b = 393.57(7) c = 736.8(14) 0.1874 STADIVARI 14.13 10 × 10 × 15 40 0–180 / 0.3 6.0,-4.0, 0.030 126.4 652 4-33 ±9, ±5, ±11 1576 371 / 0.0463 318 / 0.0200 371 / 20 1.66 0.0260 / 0.0576 0.0314 / 0.0582 275(14) 2.48 / -2.38
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Empirical formula
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Unit cell volume (nm³) Diffractometer type Calculated density (g cm−3) Crystal size (µm3) Exposure time (s) ω range; increment (°) Integr. param. (A, B, EMS) Absorption coefficient (mm−1) F(000) θ range for data collection (°) Range in hkl Total no. reflections Independent reflections / Rint Reflections with I ≥ 3σ(I) Data / parameters Goodness-of-fit on F2 R1 / wR2 for I > 3σ(I)) R1 / wR2 (all data) Extinction coefficient Largest diff. peak / hole (e Å-3)
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Table 3 Atomic coordinates and anisotropic displacement parameters (/pm2) for the silicides ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi. All atoms lie on Wyckoff sites 4c (x, ¼, z). The anisotropic displacement factor exponent takes the form – 2π2[(ha*)2U11+…+2hka*b*U12]. Ueq is defined as a third of the trace of the orthogonalized Uij tensor. U12 = U23 = 0. z
U11
U22
U13
Ueq
126(2) 109(1) 113(6)
115(2) 108(1) 131(6)
–10(2) –1(1) –11(5)
120(1) 109(1) 120(3)
155(2) 115(1) 119(7)
118(2) 115(1) 129(7)
–5(2) –3(1) –11(6)
131(1) 116(1) 127(4)
87(2) 53(2) 91(15)
54(2) 49(2) 52(12)
–4(2) –7(1) –18(11)
63(1) 49(1) 61(8)
59(2) 41(3) 41(8)
36(1) 36(3) 51(9)
–1(1) –1(2) –7(7)
45(1) 39(2) 43(5)
127(3) 105(3) 123(15)
120(3) 115(3) 105(16)
–2(2) –3(2) 10(15)
123(2) 113(2) 122(10)
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ZrOsSi Zr 0.01335(7) 0.68396(6) 120(2) Os 0.16141(3) 0.05823(3) 109(1) Si 0.2866(2) 0.3777(2) 115(6) NbOsSi Nb 0.02004(8) 0.67927(8) 121(2) Os 0.15409(3) 0.05964(3) 116(1) Si 0.2743(3) 0.3754(3) 133(7) TaOsSi Ta 0.01996(8) 0.67920(6) 48(2) Os 0.15594(8) 0.05987(6) 45(2) Si 0.2744(6) 0.3750(5) 39(13) ZrIrSi Zr 0.01768(4) 0.68174(4) 39(1) Ir 0.15156(11) 0.06453(11) 39(3) Si 0.2780(3) 0.3816(3) 37(9) HfIrSi Hf 0.01925(10) 0.68110(9) 122(3) Ir 0.15367(9) 0.06436(9) 119(3) Si 0.2766(7) 0.3793(7) 136(18)
U33
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x
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Atom
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Table 4 Interatomic distances (/pm) in the structures of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi calculated from the powder lattice parameters. All distances of the first coordination spheres are listed. Standard deviations are less or equal 0.2 pm. Si Si Si Os Os Os Os Nb Nb Si Si Si Si Os Nb Nb Nb Nb Os Os Os Nb Nb Nb Nb
271.5 273.0 273.7 288.7 289.3 293.0 295.7 326.3 331.3 359.0 240.4 241.9 243.8 288.1 288.7 289.3 293.0 295.7 240.4 241.9 243.8 271.5 273.0 273.7 359.0
TaOsSi Ta: 2 1 2 1 1 2 2 2 2 1 Os: 2 1 1 1 2 1 2 2 Si: 2 1 1 2 1 2 1
Si Si Si Os Os Os Os Ta Ta Si Si Si Si Ta Os Ta Ta Ta Os Os Os Ta Ta Ta Ta
271.1 272.5 273.4 286.7 289.3 293.2 294.4 326.0 329.8 358.5 240.5 240.6 243.8 286.7 289.3 289.3 293.2 294.4 240.5 240.6 243.8 271.1 272.5 273.4 358.5
ZrIrSi Zr: 1 2 2 2 1 1 2 2 2 1 Ir: 2 1 1 2 2 1 1 2 Si: 2 1 1 1 2 2 1
Si Si Si Ir Ir Ir Ir Zr Zr Si Si Si Si Zr Ir Zr Zr Zr Ir Ir Ir Zr Zr Zr Zr
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Table 5 29 Si NMR spectroscopic parameters of the silicides TOsSi, TIrSi and TPtSi: resonance shifts δ (± 1 /ppm); full width at half maximum ∆ (±0.01 /kHz), degree of Gaussian (G) vs. Lorentzian (L) character of the central signal, 90° pulse length p1 (/μs), relaxation delay d1 (/s) and flux density of external magnetic field B0 (/T).
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compound δ ∆ ZrOsSi 388 0.44 NbOsSi 48 1.66 TaOsSi 147 0.95 ZrIrSi 378 0.36 HfIrSi 434 0.73 NbIrSi 244 2.22 TaIrSi 255 1.15 a ZrPtSi 340 0.69 HfPtSia 352 0.68 NbPtSi 260 2.86 TaPtSi 277 1.92 a data taken from [18]
G/L 0.49 0.74 0.38 0.28 0.14 0.59 0.46 0.15 0.44 0.39 0.59
p1 4.75 4.75 4.75 4.75 4.75 5.0 4.75 4.75 4.75 4.75 4.75
d1 0.5 0.5 0.5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5
B0 9.4 9.4 9.4 9.4 9.4 4.7 9.4 9.4 9.4 9.4 9.4
279.6 280.4 280.6 294.1 296.2 300.6 305.5 334.4 342.0 358.8 243.9 247.3 248.5 294.1 295.7 296.2 300.6 305.5 243.9 247.3 248.5 279.6 280.4 280.6 358.8
HfIrSi Hf: 1 2 2 2 1 1 2 2 2 1 Ir: 2 1 1 2 2 1 1 2 Si: 2 1 1 1 2 2 1
νrot 10 10 10 10 10 10 10 10 10 10 10
Si Si Si Ir Ir Ir Ir Hf Hf Si Si Si Si Hf Ir Hf Hf Hf Ir Ir Ir Hf Hf Hf Hf
277.7 278.0 278.3 293.9 295.4 297.6 301.4 332.5 338.7 359.9 243.6 245.3 247.2 293.9 295.3 295.4 297.6 301.4 243.6 245.3 247.2 277.7 278.0 278.3 359.9
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NbOsSi Nb: 2 1 2 1 1 2 2 2 2 1 Os: 2 1 1 2 1 1 2 2 Si: 2 1 1 2 1 2 1
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279.3 282.5 287.2 288.5 294.0 300.2 304.9 335.0 340.6 356.9 244.7 244.8 250.7 288.5 294.0 300.2 301.8 304.9 244.7 244.8 250.7 279.3 282.5 287.2 356.9
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Si Si Si Os Os Os Os Zr Zr Si Si Si Si Zr Zr Zr Os Zr Os Os Os Zr Zr Zr Zr
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ZrOsSi Zr: 2 2 1 1 1 2 2 2 2 1 Os: 1 2 1 1 1 2 2 2 Si: 1 2 1 2 2 1 1
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Fig. 1. Crystal structure of TaOsSi. The tantalum, osmium and silicon atoms are drawn as dark grey, blue and open cycles, respectively. The polyanionic [OsSi] network is emphasized.
Fig. 2. (Right) Coordination of the tantalum atoms in TaOsSi. The tantalum, osmium and silicon atoms are drawn as dark grey, blue and open cycles, respectively. (Left) The characteristic T2Si2 units (with angles /° and interatomic distances /pm) in ZrOsSi, NbOsSi, NbIrSi, TaOsSi and TaIrSi.
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Fig. 3. (top) Temperature dependence of the magnetic susceptibility of NbOsSi and TaOsSi from 2.1 to 15 K measured in ZFC/FC mode at 20 Oe. (bottom) Electrical resistivity as a function of temperature in a range of 2.1-15 K measured with no external magnetic field.
Fig. 4. Magnetization isotherms of NbOsSi and TaOsSi measured at 2.5 K in a magnetic field range of ±7500 Oe.
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Fig. 5. 29Si solid state MAS NMR spectra and simulations (colored lines) of the silicides TOsSi (T = Zr, Nb, Ta), TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) recorded at B0 = 9.4 T and 4.7 T and νrot = 10 kHz.
Fig. 6. Experimental 29Si NMR resonance shifts of the investigated silicides TOsSi, TIrSi and TPtSi in dependence on the involved refractory metal T. The compounds of the titanium group members are depicted with red, those of the vanadium group members with blue cycles, respectively. The data of ZrPtSi and HfPtSi are taken from [18].