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The effect of Fe doping on superconductivity in ZrRuP
Solid State Communications 151 (2011) 1504–1506
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
The effect of Fe doping on superconductivity in ZrRuP Leslie M. Schoop a , Vadim Ksenofontov a , Teuta Gasi a , R.J. Cava b , Claudia Felser a,∗ a
Institut für Anorganische und Analytische Chemie, Johannes Gutenberg - Universität, 55099 Mainz, Germany
b
Department of Chemistry, Princeton University, Princeton NJ 08544, USA
article
info
Article history: Received 4 April 2011 Accepted 8 June 2011 by E.Y. Andrei Available online 16 June 2011 Keywords: A. Superconductors A. Magnetic doping C. Mößbauer spectroscopy
abstract This work reports the structure and superconducting properties of the superconductor ZrRuP doped with Fe; the ZrRu1−x Fex P solid solution was investigated by means of X-ray powder diffraction, SQUID magnetometry and Mößbauer spectroscopy. It is shown that the modification of the superconducting properties by doping with Fe is similar to the effect of chemical pressure and that the Fe doped compounds do not show any magnetic ordering. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ternary transition metal phosphides have been known to be superconductors since the discovery of superconductivity in ZrRuP with a transition temperature (Tc ) of 13 K by Barz et al. [1]. Of the whole family MM′ X, where M = Ti, Zr or Hf, M′ = Ru, Os, and X = P or As, ZrRuP has the highest Tc [2]. Doping ZrRuP with B at the P position or with Rh at the Ru position lowers Tc [3,4]. ZrRuP exists in two different crystal structures, with hexagonal (h-ZrRuP) and orthorhombic (o-ZrRuP) symmetry. o-ZrRuP crystallizes in the Co2 P structure with the space group Pnma [5]. h-ZrRuP crystallizes ¯ in the Fe2 P structure with the space group P 62m. Both structures are shown in the inset of Fig. 1. The hexagonal phase with Tc = 13 K is higher as Tc of the orthorhombic phase where Tc = 4 K. Applying pressure on ZrRuP lowers Tc in both phases [6]. h-ZrRuP also has a remarkably high critical field Hc2 of 17 T [7]. Both structures are layered and have similar electronic densities of states at the Fermi level [5]. ZrFeP is a ferromagnet [1]. It crystallizes only in the orthorhombic structure. In this study, the primary goal was to test the idea of ‘chemical’ pressure in ZrRuP when doped with Fe at the Ru position. Therefore, we used the advantage of the difference of the chemical radii of Ru and Fe and their isovalency. Due to the smaller radius of Fe compared to Ru the lattice constant of the compound should decrease which would lead to the same result as applied pressure. The second goal was to study the effects of magnetic doping on the superconductivity. Before the discovery of the new
family of Fe based superconductors in 2008 [8], magnetic elements have been avoided in intermetallic superconducting materials due to the antagonistic interrelation between superconductivity and magnetism. The discovery of the Fe based superconductors has opened new aspects regarding the understanding of superconductivity. Therefore, one can expect that doping with a magnetic element may shed light on the mechanism of superconductivity and the possible coexistence of superconductivity and magnetism. 2. Experimental setup ZrRu1 –xFex P samples with varying compositions of x = 0, 0.05, 0.2, 0.4, 0.5, 0.6, 0.8, 1 were prepared by arc melting of stoichiometric amounts of prereacted powders of FeP and RuP with Zr in an argon atmosphere. The resulting polycrystalline ingots were annealed at ≈1000 °C in evacuated quartz tubes for 3 days. The crystal structure was determined by X-ray powder diffraction (XRD) using Cu Kα radiation. Powder diffraction patterns for x = 0.05, 0.4, 0.5 are shown in Fig. 1. They show a structural change from the hexagonal structure to the orthorhombic structure with an increasing fraction of Fe. The orthorhombic structure appears at an Fe concentration of x ≥ 0.2. At a concentration of x ≥ 0.5 only the orthorhombic phase persists. 57 Fe Mößbauer measurements were performed using a helium cryostat in a standard transmission geometry. The magnetic properties were investigated by a superconducting quantum interference device (SQUID) (MPMS-XL-5, Quantum Design). 3. Results and discussion
∗
Corresponding author. Tel.: +49 6131 39 26266; fax: +49 6131 39 26267. E-mail address: [email protected] (C. Felser). URL: http://www.superconductivity.de (C. Felser).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.06.009
Refinements of the X-ray diffraction patterns showed that the lattice constants of ZrRu1−x Fex P decrease monotonically with
L.M. Schoop et al. / Solid State Communications 151 (2011) 1504–1506
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Fig. 1. X-ray powder diffraction of ZrRu1−x Fex P and the corresponding hexagonal and orthorhombic crystal structures. Shown are the powder patterns measured with Cu Kα radiation at room temperature for x = 0.05, 0.4, and 0.5. ‘h’ indicates reflections belonging to the hexagonal phase, ‘o’ indicates the ones belonging to the orthorhombic phase. ZrRu0.95 Fe0.05 P crystallizes in the hexagonal structure, ZrRu0.5 Fe0.5 P in the orthorhombic one, and ZrRu0.6 Fe0.4 P is a mixture of both phases (online in color). Fig. 3. Mößbauer spectra of ZrRu0.5 Fe0.5 P at 295 K and 4.2 K (Online in color). Table 1 Critical temperatures of the superconducting samples. x in ZrRu1−x Fex P
Tc [K]
Structure
0.0 0.05 0.2
13.8 10.3 2.6
Hexagonal Hexagonal Mixed hex. and orth.
increasing amount of iron, which is in conformity with Vegard’s law [9]. Superconductivity was observed above 1.8 K in two doped samples with low Fe concentrations, for ZrRu0.95 Fe0.05 P and in ZrRu0.8 Fe0.2 P. Table 1 shows the corresponding Tc . Clearly, doping with Fe in h-ZrRuP results in a decrease of Tc . ZrRu0.6 Fe0.4 P does not show superconductivity above 1.8 K. The two samples ZrRu0.8 Fe0.2 P and ZrRu0.6 Fe0.4 P showed an impurity of ZrRu in their X-ray diffraction patterns, but because this impurity is non-magnetic it should not effect Tc [10]. Low magnetic moments were observed at 5 K for all samples except ZrRu0.95 Fe0.05 P. The magnetic moments per Fe atoms were determined by measuring M–H curves at 5 K. The magnetic moments of the samples with x = 0.2, 0.4, 0.5, 0.6, 0.8 are between 0.002 µB and 0.060 µB . For example ZrRu0.5 Fe0.5 P has a moment of 0.002 µB per Fe atom. This very small moment indicates that ZrRu0.5 Fe0.5 P
is not intrinsically magnetic. The observed magnetic signal most probably arises from the presence of a magnetic impurity. Fig. 2 shows the measured superconducting properties of ZrRu0.95 Fe0.05 P. The critical field Hc2 was not reached at 2 K in an applied field of 5 T. Mößbauer measurements of ZrRu1−x Fex P samples with x = 0.2, 0.4, 0.5, 0.6, 0.8, 1 were performed at 295 and 4.2 K. All samples except of ZrFeP show a paramagnetic doublet, which indicates the absence of magnetic order at room temperature. The Mößbauer spectrum of ZrFeP reveals a magnetic sextet, which indicates the presence of ferromagnetic order. Fig. 3 displays the Mößbauer spectra of ZrRu0.5 Fe0.5 P at 295 and 4.2 K. At room temperature the Mößbauer spectrum shows a non-magnetic doublet. The measurement of ZrRu0.5 Fe0.5 P at 4.2 K reveals a weak magnetic sextet. This sextet is attributed to the presence of a small amount of γ -FeOOH. This fraction of γ -FeOOH was found to be 7(1)%. This observation explains the magnetic moment determined in the SQUID measurements. Without this impurity, we can assume that the sample would display no magnetic moment, even though 50% of the Ru was substituted with Fe. Therefore Fe does not behave as a magnetic dopant in ZrFeP, even at high concentrations. The isomer shift δ between 0.34 and 0.39 mm/s is equivalent to the isomer shift of divalent Fe in a strongly covalent state.
Fig. 2. Superconducting properties of ZrRu0.95 Fe0.05 P. Shown is the temperature dependence of the susceptibility at a magnetic field of 10 Oe (a), measured in ZFC and FC modes, and the dependence on the magnetic moment of the magnetic field at 2 K (b). The hysteresis shows the typical butterfly shape for type II superconductors (Online in color).
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L.M. Schoop et al. / Solid State Communications 151 (2011) 1504–1506
orthorhombic phase is higher, resulting in a higher quadrupole splitting. The Mößbauer data therefore support the structural transition seen in the X-ray data. Fig. 4 shows the isomer shift δ as well as the quadrupole splitting 1EQ of the studied samples at room temperature as a function of Fe doping. 4. Summary In summary, we have found that doping ZrRuP with the magnetic element Fe does not destroy the superconductivity in an abrupt way. Fe is not magnetic in the doped samples; rather we showed that Fe doped into h-ZrRuP acts like chemical pressure, which decreases Tc in conformity with the results for physical pressure [6]. However the decrease of Tc with Fe doping is much faster than with physical pressure. Additional to the volume decrease, Tc is also influenced by higher disorder in the doped samples which leads to more scattering centers. The intrinsic magnetic moments are very low or absent in Fe doped ZrRuP, although ZrFeP is known to be a ferromagnetically ordered compound. Further investigations including the influence of pressure on the superconducting properties would be of interest. Acknowledgment Fig. 4. Dependence of the isomer shift and the quadrupole splitting for ZrRu1−x Fex P as a function of the Fe concentration x (Online in color).
The resonance line is shifted to slower velocities with increasing amount of doped Fe, which corresponds to an increase of the selectron density at the Fe nuclei due to the doping. In accordance with Mößbauer studies, pressure application also increases the electron density, which corresponds to a shift of the resonance line to the left. This behavior was also observed in FeSe [11]. However, while in FeSe Tc increases with increasing pressure, in ZrRuP Tc decreases with Fe doping. The values of the isomer shift indicate that the s-electron density is higher in the hexagonal phase than in the orthorhombic one. The quadrupole splitting 1EQ in the orthorhombic phase (ca. 0.5 mm/s) is notably higher than the quadrupole splitting in the hexagonal phase (ca. 0.36 mm/s). In the hexagonal phase Fe is trigonally coordinated with P atoms, while the Zr atoms are more distant. In the orthorhombic structure, the nearest neighbors of an Fe atom are two Zr atoms together with two P atoms. Therefore the electrical field gradient in the
The work at Princeton was supported by the Air Force MURI on superconductivity. References [1] H. Barz, H. Ku, G. Meisner, Z. Fisk, B. Matthias, Proceedings of the National Academy of Sciences of the United States of America 77 (6) (1980) 3132. [2] G. Meisner, H. Ku, Applied Physics A 31 (1983) 201–212. [3] R. Schaak, R. Cava, Materials research bulletin 39 (9) (2004) 1231–1235. [4] I. Shirotani, D. Kato, A. Nishimoto, T. Yagi, Journal of Physics: Condensed Matter, 13 (41) 9393. [5] I. Shirotani, K. Tachi, N. Ichihashi, T. Adachi, T. Kikegawa, O. Shimomura, Physics Letters A 205 (1) (1995) 77–80. [6] H. Salamati, F. Razavi, G. Quirion, Physica C: Superconductivity 292 (1–2) (1997) 79–82. [7] R. Müller, R. Shelton, J. Richardson Jr., R. Jacobson, Journal of the Less Common Metals 92 (1) (1983) 177–183. doi:10.1016/0022-5088(83)90240-0. [8] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, Journal of the American Chemical Society 130 (11) (2008) 3296–3297. doi:10.1021/ja800073m. [9] L. Vegard, Zeitschrift für Physik A: Hadrons and Nuclei 5 (1) (1921) 17–26. [10] P. Anderson, Journal of Physics and Chemistry of Solids 11 (1–2) (1959) 26–30. [11] S. Medvedev, T. McQueen, I. Troyan, T. Palasyuk, M. Eremets, R. Cava, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, et al., Nature Materials 8 (8) (2009) 630–633.
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
The effect of Fe doping on superconductivity in ZrRuP Leslie M. Schoop a , Vadim Ksenofontov a , Teuta Gasi a , R.J. Cava b , Claudia Felser a,∗ a
Institut für Anorganische und Analytische Chemie, Johannes Gutenberg - Universität, 55099 Mainz, Germany
b
Department of Chemistry, Princeton University, Princeton NJ 08544, USA
article
info
Article history: Received 4 April 2011 Accepted 8 June 2011 by E.Y. Andrei Available online 16 June 2011 Keywords: A. Superconductors A. Magnetic doping C. Mößbauer spectroscopy
abstract This work reports the structure and superconducting properties of the superconductor ZrRuP doped with Fe; the ZrRu1−x Fex P solid solution was investigated by means of X-ray powder diffraction, SQUID magnetometry and Mößbauer spectroscopy. It is shown that the modification of the superconducting properties by doping with Fe is similar to the effect of chemical pressure and that the Fe doped compounds do not show any magnetic ordering. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ternary transition metal phosphides have been known to be superconductors since the discovery of superconductivity in ZrRuP with a transition temperature (Tc ) of 13 K by Barz et al. [1]. Of the whole family MM′ X, where M = Ti, Zr or Hf, M′ = Ru, Os, and X = P or As, ZrRuP has the highest Tc [2]. Doping ZrRuP with B at the P position or with Rh at the Ru position lowers Tc [3,4]. ZrRuP exists in two different crystal structures, with hexagonal (h-ZrRuP) and orthorhombic (o-ZrRuP) symmetry. o-ZrRuP crystallizes in the Co2 P structure with the space group Pnma [5]. h-ZrRuP crystallizes ¯ in the Fe2 P structure with the space group P 62m. Both structures are shown in the inset of Fig. 1. The hexagonal phase with Tc = 13 K is higher as Tc of the orthorhombic phase where Tc = 4 K. Applying pressure on ZrRuP lowers Tc in both phases [6]. h-ZrRuP also has a remarkably high critical field Hc2 of 17 T [7]. Both structures are layered and have similar electronic densities of states at the Fermi level [5]. ZrFeP is a ferromagnet [1]. It crystallizes only in the orthorhombic structure. In this study, the primary goal was to test the idea of ‘chemical’ pressure in ZrRuP when doped with Fe at the Ru position. Therefore, we used the advantage of the difference of the chemical radii of Ru and Fe and their isovalency. Due to the smaller radius of Fe compared to Ru the lattice constant of the compound should decrease which would lead to the same result as applied pressure. The second goal was to study the effects of magnetic doping on the superconductivity. Before the discovery of the new
family of Fe based superconductors in 2008 [8], magnetic elements have been avoided in intermetallic superconducting materials due to the antagonistic interrelation between superconductivity and magnetism. The discovery of the Fe based superconductors has opened new aspects regarding the understanding of superconductivity. Therefore, one can expect that doping with a magnetic element may shed light on the mechanism of superconductivity and the possible coexistence of superconductivity and magnetism. 2. Experimental setup ZrRu1 –xFex P samples with varying compositions of x = 0, 0.05, 0.2, 0.4, 0.5, 0.6, 0.8, 1 were prepared by arc melting of stoichiometric amounts of prereacted powders of FeP and RuP with Zr in an argon atmosphere. The resulting polycrystalline ingots were annealed at ≈1000 °C in evacuated quartz tubes for 3 days. The crystal structure was determined by X-ray powder diffraction (XRD) using Cu Kα radiation. Powder diffraction patterns for x = 0.05, 0.4, 0.5 are shown in Fig. 1. They show a structural change from the hexagonal structure to the orthorhombic structure with an increasing fraction of Fe. The orthorhombic structure appears at an Fe concentration of x ≥ 0.2. At a concentration of x ≥ 0.5 only the orthorhombic phase persists. 57 Fe Mößbauer measurements were performed using a helium cryostat in a standard transmission geometry. The magnetic properties were investigated by a superconducting quantum interference device (SQUID) (MPMS-XL-5, Quantum Design). 3. Results and discussion
∗
Corresponding author. Tel.: +49 6131 39 26266; fax: +49 6131 39 26267. E-mail address: [email protected] (C. Felser). URL: http://www.superconductivity.de (C. Felser).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.06.009
Refinements of the X-ray diffraction patterns showed that the lattice constants of ZrRu1−x Fex P decrease monotonically with
L.M. Schoop et al. / Solid State Communications 151 (2011) 1504–1506
1505
Fig. 1. X-ray powder diffraction of ZrRu1−x Fex P and the corresponding hexagonal and orthorhombic crystal structures. Shown are the powder patterns measured with Cu Kα radiation at room temperature for x = 0.05, 0.4, and 0.5. ‘h’ indicates reflections belonging to the hexagonal phase, ‘o’ indicates the ones belonging to the orthorhombic phase. ZrRu0.95 Fe0.05 P crystallizes in the hexagonal structure, ZrRu0.5 Fe0.5 P in the orthorhombic one, and ZrRu0.6 Fe0.4 P is a mixture of both phases (online in color). Fig. 3. Mößbauer spectra of ZrRu0.5 Fe0.5 P at 295 K and 4.2 K (Online in color). Table 1 Critical temperatures of the superconducting samples. x in ZrRu1−x Fex P
Tc [K]
Structure
0.0 0.05 0.2
13.8 10.3 2.6
Hexagonal Hexagonal Mixed hex. and orth.
increasing amount of iron, which is in conformity with Vegard’s law [9]. Superconductivity was observed above 1.8 K in two doped samples with low Fe concentrations, for ZrRu0.95 Fe0.05 P and in ZrRu0.8 Fe0.2 P. Table 1 shows the corresponding Tc . Clearly, doping with Fe in h-ZrRuP results in a decrease of Tc . ZrRu0.6 Fe0.4 P does not show superconductivity above 1.8 K. The two samples ZrRu0.8 Fe0.2 P and ZrRu0.6 Fe0.4 P showed an impurity of ZrRu in their X-ray diffraction patterns, but because this impurity is non-magnetic it should not effect Tc [10]. Low magnetic moments were observed at 5 K for all samples except ZrRu0.95 Fe0.05 P. The magnetic moments per Fe atoms were determined by measuring M–H curves at 5 K. The magnetic moments of the samples with x = 0.2, 0.4, 0.5, 0.6, 0.8 are between 0.002 µB and 0.060 µB . For example ZrRu0.5 Fe0.5 P has a moment of 0.002 µB per Fe atom. This very small moment indicates that ZrRu0.5 Fe0.5 P
is not intrinsically magnetic. The observed magnetic signal most probably arises from the presence of a magnetic impurity. Fig. 2 shows the measured superconducting properties of ZrRu0.95 Fe0.05 P. The critical field Hc2 was not reached at 2 K in an applied field of 5 T. Mößbauer measurements of ZrRu1−x Fex P samples with x = 0.2, 0.4, 0.5, 0.6, 0.8, 1 were performed at 295 and 4.2 K. All samples except of ZrFeP show a paramagnetic doublet, which indicates the absence of magnetic order at room temperature. The Mößbauer spectrum of ZrFeP reveals a magnetic sextet, which indicates the presence of ferromagnetic order. Fig. 3 displays the Mößbauer spectra of ZrRu0.5 Fe0.5 P at 295 and 4.2 K. At room temperature the Mößbauer spectrum shows a non-magnetic doublet. The measurement of ZrRu0.5 Fe0.5 P at 4.2 K reveals a weak magnetic sextet. This sextet is attributed to the presence of a small amount of γ -FeOOH. This fraction of γ -FeOOH was found to be 7(1)%. This observation explains the magnetic moment determined in the SQUID measurements. Without this impurity, we can assume that the sample would display no magnetic moment, even though 50% of the Ru was substituted with Fe. Therefore Fe does not behave as a magnetic dopant in ZrFeP, even at high concentrations. The isomer shift δ between 0.34 and 0.39 mm/s is equivalent to the isomer shift of divalent Fe in a strongly covalent state.
Fig. 2. Superconducting properties of ZrRu0.95 Fe0.05 P. Shown is the temperature dependence of the susceptibility at a magnetic field of 10 Oe (a), measured in ZFC and FC modes, and the dependence on the magnetic moment of the magnetic field at 2 K (b). The hysteresis shows the typical butterfly shape for type II superconductors (Online in color).
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L.M. Schoop et al. / Solid State Communications 151 (2011) 1504–1506
orthorhombic phase is higher, resulting in a higher quadrupole splitting. The Mößbauer data therefore support the structural transition seen in the X-ray data. Fig. 4 shows the isomer shift δ as well as the quadrupole splitting 1EQ of the studied samples at room temperature as a function of Fe doping. 4. Summary In summary, we have found that doping ZrRuP with the magnetic element Fe does not destroy the superconductivity in an abrupt way. Fe is not magnetic in the doped samples; rather we showed that Fe doped into h-ZrRuP acts like chemical pressure, which decreases Tc in conformity with the results for physical pressure [6]. However the decrease of Tc with Fe doping is much faster than with physical pressure. Additional to the volume decrease, Tc is also influenced by higher disorder in the doped samples which leads to more scattering centers. The intrinsic magnetic moments are very low or absent in Fe doped ZrRuP, although ZrFeP is known to be a ferromagnetically ordered compound. Further investigations including the influence of pressure on the superconducting properties would be of interest. Acknowledgment Fig. 4. Dependence of the isomer shift and the quadrupole splitting for ZrRu1−x Fex P as a function of the Fe concentration x (Online in color).
The resonance line is shifted to slower velocities with increasing amount of doped Fe, which corresponds to an increase of the selectron density at the Fe nuclei due to the doping. In accordance with Mößbauer studies, pressure application also increases the electron density, which corresponds to a shift of the resonance line to the left. This behavior was also observed in FeSe [11]. However, while in FeSe Tc increases with increasing pressure, in ZrRuP Tc decreases with Fe doping. The values of the isomer shift indicate that the s-electron density is higher in the hexagonal phase than in the orthorhombic one. The quadrupole splitting 1EQ in the orthorhombic phase (ca. 0.5 mm/s) is notably higher than the quadrupole splitting in the hexagonal phase (ca. 0.36 mm/s). In the hexagonal phase Fe is trigonally coordinated with P atoms, while the Zr atoms are more distant. In the orthorhombic structure, the nearest neighbors of an Fe atom are two Zr atoms together with two P atoms. Therefore the electrical field gradient in the
The work at Princeton was supported by the Air Force MURI on superconductivity. References [1] H. Barz, H. Ku, G. Meisner, Z. Fisk, B. Matthias, Proceedings of the National Academy of Sciences of the United States of America 77 (6) (1980) 3132. [2] G. Meisner, H. Ku, Applied Physics A 31 (1983) 201–212. [3] R. Schaak, R. Cava, Materials research bulletin 39 (9) (2004) 1231–1235. [4] I. Shirotani, D. Kato, A. Nishimoto, T. Yagi, Journal of Physics: Condensed Matter, 13 (41) 9393. [5] I. Shirotani, K. Tachi, N. Ichihashi, T. Adachi, T. Kikegawa, O. Shimomura, Physics Letters A 205 (1) (1995) 77–80. [6] H. Salamati, F. Razavi, G. Quirion, Physica C: Superconductivity 292 (1–2) (1997) 79–82. [7] R. Müller, R. Shelton, J. Richardson Jr., R. Jacobson, Journal of the Less Common Metals 92 (1) (1983) 177–183. doi:10.1016/0022-5088(83)90240-0. [8] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, Journal of the American Chemical Society 130 (11) (2008) 3296–3297. doi:10.1021/ja800073m. [9] L. Vegard, Zeitschrift für Physik A: Hadrons and Nuclei 5 (1) (1921) 17–26. [10] P. Anderson, Journal of Physics and Chemistry of Solids 11 (1–2) (1959) 26–30. [11] S. Medvedev, T. McQueen, I. Troyan, T. Palasyuk, M. Eremets, R. Cava, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, et al., Nature Materials 8 (8) (2009) 630–633.