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The only known compound of sodium and hydrogen is archetypal ionic NaH. Application of high pressure is known to promote states with higher atomic coordination, but extensive searches for polyhydrides with unusual stoichiometry have had only limited success in spite of several theoretical predictions. Here we report the first observation of the formation of polyhydrides of Na (NaH3 and NaH7) above 40 GPa and 2,000 K. We combine synchrotron X-ray diffraction and Raman spectroscopy in a laser-heated diamond anvil cell and theoretical random structure searching, which both agree on the stable structures and compositions. Our results support the formation of multicenter bonding in a material with unusual stoichiometry. These results are applicable to the design of new energetic solids and high-temperature superconductors based on hydrogen-rich materials.

Three different sodium-silicon clathrate compounds–Na8Si46 (sI), Na24Si136 (sII), and a new structure, NaSi6–were obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900–1100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures, we observed the formation of a new intercalated compound, metallic NaSi6, which crystallizes in the orthorhombic Eu4Ga8Ge16 structure. High-pressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi6), compared to the typical characteristics achieved for single crystals obtained by conventional routes (RRR < 6). Since the Na-Si clathrates are stable only above 2 GPa, previous reports of their synthesis may be viewed as nonequilibrium, precursor-based routes to high-pressure phases at low-pressure conditions.

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Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca 2 H 5 composition, which is characterized by a distinctive vibration at 3789 cm −1 at 25 GPa. The observed Raman spectrum is in close agreement with first-principles calculations of a Ca 2 H 5 structure characterized by a lattice containing a central layer of H 2 molecules oriented along the (100) direction. At higher pressures (e.g., 116 GPa and 1600 K), we synthesize another phase, which has the composition of CaH 4 and a denser body-centered tetragonal structure. This weakly metallic phase also contains molecular-like H 2 units, and its spectroscopic as well as diffraction signatures match closely with those predicted from first-principles calculations. This phase is observed to persist on decompression to 60 GPa at room temperature. The elongation of the H−H bond in these hydrides is a result of the Ca−H 2 interaction, analogous to what occurs in molecular compounds, where H 2 binds side-on to a d-element, such as in Kubas complex.

High pressure plays an increasingly important role in both understanding superconductivity and the development of new superconducting materials. New superconductors were found in metallic and metal oxide systems at high pressure. However, because of the filled close-shell configuration, the superconductivity in molecular systems has been limited to charge-transferred salts and metal-doped carbon species with relatively low superconducting transition temperatures. Here, we report the low-temperature superconducting phase observed in diamagnetic carbon disulfide under high pressure. The superconductivity arises from a highly disordered extended state (CS4 phase or phase III[CS4]) at ∼6.2 K over a broad pressure range from 50 to 172 GPa. Based on the X-ray scattering data, we suggest that the local structural change from a tetrahedral to an octahedral configuration is responsible for the observed superconductivity.extended solids | magnetic ordering | metallization | nonconventional superconductors | non-Fermi liquids H ighly compressed low-Z molecular solids become extended solids in 3D network structures of polymeric and/or metallic states, as found in their periodic high-Z counterparts (1, 2). A relevant question is then, if these extended forms of simple molecular solids can give rise to novel properties such as superconductivity and magnetism, as often found in sp/spd-elemental metals and metallic alloys at low temperatures (3, 4). The theoretical prediction of high-temperature (possibly 300 K) superconductivity in metallic hydrogen at high pressure is stimulating in this regard (5), yet the superconductivity in simple molecular solids has only been observed in paramagnetic oxygen at T C = ∼0.6 K above 100 GPa (6).Recently, we have reported that carbon disulfide undergoes a series of pressure-induced transformations from a transparent molecular solid (Cmca, depicted as phase I) at 2 GPa, to a black polymer of (-S-(C=S)-) p with three-folded carbon atoms bonded to sulfur atoms (CS3 phase or phase II[CS3], signifying the threefold carbon coordination in the bracket) at 10 GPa and then to a highly reflective polymer with four-folded carbons (CS4 phase or III[CS4]) above 40-50 GPa (2). Although highly disordered, phase III[CS4] exhibits a remarkable electrical conductivity of ∼5 μΩ m at ambient temperatures similar to that of an elemental metal (rather than an organic polymer or a polymeric metal) (7). The resistivity ∼5 μΩ m of phase III[CS4] is close to that of elemental metals, such as titanium (0.42 μΩ m), europium (0.94 μΩ m), and intermetallic alloys, such as Nichrome (1.1 μΩ m), Pt/Pd (0.4 μΩ m), rather than organic metals. In the present study, we further show that the phase III[CS4] undergoes a magnetic ordering transition below ∼42 K and enters a superconducting state at ∼6.2 K, both observed over a large pressure range from 50 to 172 GPa (the maximum pressure studied) and exhibits the characteristics of a correlated intermetallic "molecular" alloy. The present results are summarized in the phase diagram...

We report the novel pressure(P ) -temperature(T ) phase diagram of antiferromagnetism and superconductivity in CeRhIn5 and CeIn3 revealed by the 115 In nuclear-spin-lattice-relaxation (T1) measurement. In the itinerant magnet CeRhIn5, we found that the Néel temperature TN is reduced at P ≥ 1.23 GPa with an emergent pseudogap behavior. In CeIn3, the localized magnetic character is robust against the application of pressure up to P ∼ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.PACS numbers: PACS: 74.25. Ha, 74.62.Fj, 74.70.Tx, 75.30.Kz, 76.60.Gv It has been reported that a superconducting (SC) order in cerium (Ce)-based heavy-fermion (HF) compounds takes place nearby the border at which an antiferromagnetic (AF) order is suppressed by applying pressure (P ) to the HF-AF compounds CeCu 2 Ge 2 ,[1] CePd 2 Si 2 [2] and CeIn 3 [3]. The superconductivity in these compounds, however, occurs only in extreme conditions where the pressure exceeds ∼ 2 GPa and temperature (T ) is cooled down below ∼ 1 K. Indeed the experiments were restricted mainly to transport measurements. The discovery of P -induced HF superconductors in Ce-based HF-AF compounds has stimulated further experimental works under P [4,5,6,7]. In order to gain profound insight into a relationship between magnetism and superconductivity in HF systems, systematic NMR/NQR experiments under P are important, since they can probe the evolution of the magnetic properties toward the onset of SC phase.Recently, Hegger et al. found that a new HF material CeRhIn 5 consisting of alternating layers of CeIn 3 and RhIn 2 reveals an AF-to-SC transition at a relatively lower critical pressure P c = 1.63 GPa than in all previous examples [1,2,3]. The SC transition temperature T c = 2.2 K is the highest one to date among P -induced superconductors [4]. This finding has opened a way to investigate the P -induced evolution of both magnetic and SC properties over a wide P range. In the previous paper [7], the 115 In NQR study of CeRhIn 5 has clarified the P -induced anomalous magnetism and unconventional superconductivity. In the AF region, the Néel temperature T N exhibits a moderate variation, while the internal field H int at 115 In(1) site in the CeIn 3 plane due to the magnetic ordering is linearly reduced in P = 0 -1.23 GPa, extrapolated to zero at P * = 1.6 ± 0.1 GPa. This P * is comparable to P c = 1.63 GPa at which the SC signature appears [4], which was indicative of a second-order like AF-to-SC transition rather than the first-order one suggested previously [4]. At P = 2.1 GPa, it was found that the nuclear spin-lattice relaxation rate 1/T 1 reveals a T 3 dependence below the SC transition temperature T c , which shows the existence of line-nodes in the gap function [7]. It is, however, not yet clear how the electronic states change with P when the AF phase evolves into the SC phase.On the other hand, CeIn 3 c...

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