Phase Diagram Analysis of High-Pressure/High-Temperature Polymorphs of Ammonia Borane.

Ammonia borane (NH3BH3) is a promising hydrogen-storage material because of its high hydrogen density. It is employed as a hydrogen source when synthesizing superconducting polyhydrides under high pressure. Additionally, NH3BH3 is a crystallographically interesting compound that features protonic hydrogen (Hδ+) and hydridic hydrogen (Hδ-), and it forms a dihydrogen bond, which explains its stable existence as a solid. Herein, X-ray diffraction experiments were performed at high pressures (HPs) and high temperatures (HTs) of up to 30 GPa and 300 °C, respectively, to investigate the HP/HT phase diagram of NH3BH3. A new HP/HT phase (HPHT2) was identified above 9 GPa and 150 °C. Crystal-structure analysis using the Rietveld method and stability verification using density functional theory calculations revealed that HPHT2 has a P21/n (Z = 4) structure, similar to that of a previously reported HP/HT phase (HPHT) that appears at a lower pressure. HPHT2 is denser than the HP phases that appear at room temperature (HP1 and HP2) at the same pressure (up to ∼17 GPa). In the phase diagram, the phase-boundary line between HPHT and HP1 is a downward convex curve. These unconventional phenomena in the density and phase boundary can be attributed to the influence of dihydrogen bonding on the crystal structure and phase diagram.

Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran-CO2 Binary Hydrate.

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO2 below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO2 guests in small pentagonal dodecahedral 512 host water cages, not previously observed in the simple CO2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I41/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.

Dense Structure of Yttrium Nitride as a Post-Rock-Salt Phase: High-Pressure Experiments and Computations.

The post-rock-salt (B1) phase in yttrium nitride was investigated under pressures above 100 GPa. The starting B1 phase was prepared by a direct elemental reaction between yttrium metal and supercritical nitrogen in a laser-heated diamond anvil cell. Compression of the resulting compound under nitrogen pressure revealed that the experimental bulk modulus was significantly lower than that reported for previous compression experiments but consistent with values predicted by theoretical calculations. The post-B1 phase transition started at 116 GPa and was almost completed at 170 GPa, with slow kinetics of conversion. In contrast to the likely transformation to B2 from B1, the phase was determined to be B10, which is identical to a high-pressure BaO structure. Theoretical enthalpy calculation demonstrated that a transition from B1 to B10 takes place at 108 GPa prior to the B2 transition. The present YN is the first non-4f-nitride compound reported to crystallize with a B10 structure.

Influence of pressure-induced formation of dihydrogen bonds on lattice parameters, volume, and vibrational modes of ammonia borane.

The high-pressure phase of ammonia borane (NH3BH3) observed at ∼1.2 GPa has been reported to result in pressure-induced formation of dihydrogen bonds at ∼4 GPa. In this study, we performed high-pressure x-ray diffraction measurements on the high-pressure phase (up to ∼10.2 GPa) using a He hydrostatic pressure medium to examine the influence of the formation of dihydrogen bonds on the lattice parameters and unit cell volume of NH3BH3. We observed a unique behavior in the pressure dependence of lattice parameters close to the pressure at which the dihydrogen bond was formed. The lattice parameters demonstrated hysteresis curves under compression and decompression conditions but the unit cell volume did not. Moreover, the pressure dependence of the unit cell volume could not be expressed using a single Birch-Murnaghan equation within an acceptable margin of error, thus suggesting a change in bulk modulus under compression. These results are considered to have originated from the pressure-induced formation of dihydrogen bonds. Moreover, high-pressure Raman scattering measurements and a simulation using density functional theory calculations revealed the vibrational modes of the high-pressure phase of NH3BH3. The results demonstrated that librational modes were enhanced by forming dihydrogen bonds. Moreover, the intramolecular stretching modes of BN, BH, and NH monotonically shifted with pressure, while the symmetrical in-plane bending modes of BH3 and NH3 split irrespective of the formation of dihydrogen bonds.

Hidden Structural and Superconducting Phase Induced in Antiperovskite Arsenide SrPd3As.

Enriching the material variation often contributes to the progress of materials science. We have discovered for the first time antiperovskite arsenide SrPd3As and revealed a hidden structural and superconducting phase in Sr(Pd1-xPtx)3As. The Pd-rich samples (0 ≤ x ≤ 0.2) had the same noncentrosymmetric (NCS) tetragonal structure (a space group of I41md) as SrPd3P. For the samples with 0.3 ≤ x ≤ 0.7, a centrosymmetric (CS) tetragonal structure (P4/nmm) identical to that of SrPt3P was found to appear, accompanied by superconductivity at a transition temperature (Tc) up to 3.7 K. In the samples synthesized with Pt-rich nominal compositions (0.8 ≤ x ≤ 1.0), Sr2(Pd,Pt)8-yAs1+y with an intergrowth structure (CS-orthorhombic with Cmcm) was crystallized. The phase diagram obtained for Sr(Pd,Pt)3As was analogous to that of (Ca,Sr)Pd3P in which superconductivity (Tc ≥ 2 K) occurred in the CS phases induced by substitutions to the NCS phases. This study indicates the potential for further material variation expansion and the importance of elemental substitutions to reveal hidden phases in related antiperovskites.

High-Pressure Synthesis of Light Lanthanide Dodecaborides (PrB12 and CeB12): Effects of Valence Fluctuation on Volume and Formation Pressure.

Light lanthanide dodecaborides, RB12 (R = Pr and Ce), were synthesized from a stoichiometric mixture of hexaborides and boron using a laser-heated diamond anvil cell under high-pressure and high-temperature conditions. Contrary to the expectation that lighter lanthanide elements require higher pressure to crystallize RB12, in situ X-ray diffraction experiments reveal that cerium dodecaboride crystallizes at 26 GPa, which is significantly lower than that required to form the heavier praseodymium dodecaboride (35 GPa). In addition to the lower formation pressure, an anomalous volume reduction is also observed in CeB12, which can be explained by a valence fluctuation between Ce3+ and Ce4+ indicated by X-ray absorption near-edge structure measurements. A polyhedral coordination change from a truncated cube in RB6 to a truncated octahedron in RB12 and associated shortening of the R-B bond length result in an increase in bulk modulus and hardness.

Infrared spectra of the ß and γ phases of oleic acid under high pressure.

Infrared spectra for the ß and γ phases of oleic acid were obtained at room temperature (~25 °C) under pressures of up to 5 and 10 GPa, respectively. Density functional theory (DFT) calculations were performed to examine the changes in the infrared spectra and crystal lattices of the ß and γ phases that occur under pressure. Mode assignments for peaks that were observed to have a low-wavenumber shift with pressure were made based on the DFT-calculated infrared spectra. The CO stretching and OH stretching vibration peaks shifted to a lower wavenumber with pressure in both the ß and γ phases because the hydrogen bonds became stronger with pressure and the CO and OH bonds weakened. The difference in the shift amount between the two phases was related to anisotropic compression in the crystal lattice.

Antiperovskite Superconductor LaPd3P with Noncentrosymmetric Cubic Structure.

Antiperovskites are a promising candidate structure for the exploration of new materials. We discovered an antiperovskite phosphide, LaPd3P, following our recent synthesis of APd3P (A = Ca, Sr, Ba). While APd3P and (Ca,Sr)Pd3P were found to be tetragonal or orthorhombic systems, LaPd3P is a new prototype cubic system (a = 9.0317(1) Å) with a noncentrosymmetric space group (I4̅3m). LaPd3P exhibited superconductivity with a transition temperature (Tc) of 0.28 K. The upper critical field, Debye temperature, and Sommerfeld constant (γ) were determined to be 0.305(8) kOe, 267(1) K, and 6.06(4) mJ mol-1 K-2 f.u.-1, respectively. We performed first-principles electronic band structure calculations for LaPd3P and compared the theoretical and experimental results. The calculated Sommerfeld constant (2.24 mJ mol-1 K-2 f.u.-1) was much smaller than the experimental value of γ because the Fermi energy (EF) was located slightly below the density of states (DOS) pseudogap. This difference was explained by the increase in the DOS at EF due to the approximately 5 atom % La deficiency (hole doping) in the sample. The observed Tc value was much lower than that estimated using the Bardeen-Cooper-Schrieffer equation. To explain the discrepancy, we examined the possibility of an unconventional superconductivity in LaPd3P arising from the lack of space inversion symmetry.

Observation of Dihydrogen Bonds in High-Pressure Phases of Ammonia Borane by X-ray and Neutron Diffraction Measurements.

High-pressure X-ray and neutron diffraction analyses of an ambient-pressure phase (AP) and two high-pressure phases (HP1 and HP2) of ammonia borane (i.e., NH3BH3 and ND3BD3) were conducted to investigate the relationship between their crystal structures and dihydrogen bonds. It was confirmed that the hydrogen atoms in AP formed dihydrogen bonds between adjacent molecules, and the H-H distance between the hydrogen atoms forming this interaction was shorter than 2.4 Å, which was nearly 2 times larger than the van der Waals radius of hydrogen. In the case of half of the hydrogen bonds, a phase transition from AP to the first high-pressure phase (HP1) at ∼1.2 GPa resulted in an increase in the H-H distances, which suggested that the dihydrogen bonds were broken. However, when HP1 was further pressurized to ∼4 GPa, all of the H-H distances became shorter than 2.4 Å again, which implied the occurrence of pressure-induced re-formation of the dihydrogen bonds. It was speculated that the re-formation was consistent with a second-order phase transition suggested in previous studies by Raman spectroscopy and X-ray diffraction measurement. Furthermore, at ∼11 GPa, HP1 transformed to the second high-pressure phase (HP2), and its structure was determined to be P21 (Z = 2). In this phase transition, the inclination of the molecule axis became larger, and the number of types of dihydrogen bonds increased from 6 to 11. At 18.9 GPa, which was close to the upper pressure limit of HP2, the shortest dihydrogen bond decreased to ∼1.65 Å. Additionally, the X-ray diffraction results suggested another phase transition to the third high-pressure phase (HP3) at ∼20 GPa. The outcomes of this study confirmed experimentally for the first time that the structural change under pressure causes the breakage and re-formation of the dihydrogen bonds of NH3BH3.

Experimental and Computational Determination of Optimal Boron Content in Layered Superconductor Sc20C8-xBxC20.

It is generally difficult to quantify the amount of light elements in materials because of their low X-ray-scattering power, as this means that they cannot be easily estimated via X-ray analyses. Meanwhile, the recently reported layered superconductor, Sc20C8-xBxC20, requires a small amount of boron, which is a light element, for its structural stability. In this context, here, we quantitatively evaluate the optimal x value using both experimental and computational approaches. Using the high-pressure synthesis approach, which can maintain the starting composition even after sintering, we obtain the Sc20(C,B)8C20 phase by the reaction of the previously reported Sc15C19 and B (Sc15ByC19). Our experiments demonstrate that an increase in y values promotes the phase formation of the Sc20(C,B)8C20 structure; however, there appears to be an upper limit to the nominal y value to form this phase. The maximum critical temperature (Tc = 7.6 K) is found to correspond with the actual x value of x ≈ 5 under the assumption that the sample with the same Tc as the reported value (7.7 K) possesses the optimal x amount. Moreover, we construct the energy convex hull diagram by calculating the formation enthalpy based on first principles. Our computational results indicate that the composition of Sc20C4B4C20 (x = 4) is the most thermodynamically stable, which is reasonably consistent with the experimentally obtained value.

Structural Phase Transitions and Superconductivity Induced in Antiperovskite Phosphide CaPd3P.

In this study, we succeeded in synthesizing new antiperovskite phosphides MPd3P (M = Ca, Sr, Ba) and discovered the appearance of a superconducting phase (0.17 ≤ x ≤ 0.55) in a solid solution (Ca1-xSrx)Pd3P. Three perovskite-related crystal structures were identified in (Ca1-xSrx)Pd3P, and a phase diagram was built on the basis of experimental results. The first phase transition from centrosymmetric (Pnma) to noncentrosymmetric orthorhombic (Aba2) occurred in CaPd3P near room temperature. The phase transition temperature decreased as Ca2+ was replaced with a larger-sized isovalent Sr2+. Bulk superconductivity at a critical temperature (Tc) of approximately 3.5 K was observed in a range of x = 0.17-0.55; this was associated with the centrosymmetric orthorhombic phase. Thereafter, a noncentrosymmetric tetragonal phase (I41md) remained stable for 0.6 ≤ x ≤ 1.0, and superconductivity was significantly suppressed as samples with x = 0.75 and 1.0 showed Tc values as low as 0.32 K and 57 mK, respectively. For further substitution with a larger-sized isovalent Ba2+, namely, (Sr1-yBay)Pd3P, the tetragonal phase continued throughout the composition range. BaPd3P no longer showed superconductivity down to 20 mK. Since the inversion symmetry of structure and superconductivity can be precisely controlled in (Ca1-xSrx)Pd3P, this material may offer a unique opportunity to study the relationship between inversion symmetry and superconductivity.

Superconductivity in a Scandium Borocarbide with a Layered Crystal Structure.

The discovery of nearly room-temperature superconductivity in superhydrides has motivated further materials research for conventional superconductors. To realize the moderately high critical temperature (Tc) in materials containing light elements, we explored new superconducting phases in a scandium borocarbide system. Here, we report the observation of superconductivity in a new ternary Sc-B-C compound. The crystal structure, which was determined through a Rietveld analysis, belongs to tetragonal space group P4/ncc. By complementarily using the density functional theory calculations, a chemical formula of the compound was found to be expressed as Sc20C8-xBxC20 (x = 1 or 2). Interestingly, a small amount of B is essential to stabilize the present structure. Our experiments revealed the typical type-II superconductivity at Tc = 7.7 K. Additionally, we calculated the density of states within a first-principles approach and found that the contribution of the Sc-3d orbital was mainly responsible for the superconductivity.

Superconductivity in Uncollapsed Tetragonal LaFe2As2.

We report synthesis, crystal structure, and superconductivity in ThCr2Si2-type LaFe2As2 (La122). La122 was synthesized at 960 °C for 1.5 h under a pressure of 3.4 GPa. An as-synthesized La122 (nonsuperconductor) had a collapsed tetragonal structure with a short c-axis length of 11.0144(4) Å as observed in CaFe2As2 under pressure. The collapsed tetragonal structure transformed into an uncollapsed tetragonal structure by annealing the as-synthesized La122 at 500 °C. The c-axis length remarkably extended to 11.7317(4) Å, and superconductivity emerged at 12.1 K in the uncollapsed tetragonal La122. A cylindrical hole-like Fermi surface around the Γ point that plays an important role for an s± wave pairing in iron-based superconductors was missing in the uncollapsed tetragonal La122 because of heavy electron doping. Superconductivity in La122 may be closely related to that induced in CaFe2As2 under pressure.

Superconductivity in a 122-type Fe-based compound (La,Na,K)Fe2As2.

We synthesized a Fe-based superconductor (FeSC), (La,Na,K)Fe2As2, and characterized its superconducting properties. It was found that (La,Na,K)Fe2As2 has a 122-type (ThCr2Si2-type) structure with a space group I4/mmm (No. 139), identical to (Ba,K)Fe2As2 and (La,Na)Fe2As2 but distinct from so-called 1144-type FeSCs such as CaKFe4As4 and (La,Na)CsFe4As4. The results demonstrate that the formation of the 1144-type phase necessitates the large ionic radius mismatch among the so-called A-site constituent elements of the AFe2As2 formula. The lattice constants are a = 3.850(1) Å and c = 13.21(1) Å. The La, Na, and K ions occupy the same atomic site of Wyckoff position 1a. Electrical resistivity and magnetic susceptibility show the superconducting transition at 22.5 K. The transition temperature (Tc) of (La,Na,K)Fe2As2 is comparable with that of 122-type (La,Na)Fe2As2 and 1144-type (La,Na)AFe4As4 (A = Rb, Cs), while being more than 10 K lower than those of typical 122- and 1144-type FeSCs. The results suggest that the random distribution of La3+ and Na+ ions is the main reason for lower Tc in the AE = (La,Na) 122-type and 1144-type FeSCs.

Fe-Based Superconductors of ( Ln0.5- xNa0.5+ x)Fe2As2 ( Ln = Ce, Pr).

Recently, we succeeded in synthesizing (La0.5- xNa0.5+ x)Fe2As2 ((La,Na)122) with a solid solution range of 0 ≤ x ≤ 0.35. Superconductivity was induced for 0.15 ≤ x ≤ 0.35, with the highest transition temperature Tc = 27.0 K for x = 0.3. Here, we report the synthesis and physical properties of analogous compounds ( Ln0.5- xNa0.5+ x)Fe2As2 (( Ln,Na)122) ( Ln = Ce, Pr). Samples were synthesized by precisely tuning the reaction temperature according to Ln and x. The solid solution ranges, 0.1 ≤ x ≤ 0.3 ( Ln = Ce) and 0.15 ≤ x ≤ 0.25 ( Ln = Pr), become narrower with increasing atomic number of Ln (which decreases the ionic radius of Ln3+). Bulk superconductivity emerged for 0.2 ≤ x ≤ 0.3 and 0.15 ≤ x ≤ 0.25 with the highest Tc of 25.6 K ( x = 0.3) and 24.7 K ( x = 0.25) for Ln = Ce and Pr, respectively. Crystal structures refined via the Rietveld analysis method showed that the ( Ln,Na)122 compounds ( Ln = La, Ce, Pr) with the highest Tc have almost the same As-Fe-As bond angles (∼107°) and As heights from Fe planes (∼1.43 Å). In addition to the solid solution ranges, the phases in the samples changed depending on the ionic radius of Ln3+. The ( Ln,Na)122 phase competes with the non-superconducting CaFe4As3(143)-type phase of ( Ln,Na)Fe4As3 for Ln = Ce and Pr, whereas only the ( Ln,Na)122 phase was stable for Ln = La. The 143-type phase alone was observed for Ln = Nd, and neither 122- nor 143-type phases were observed for Ln = Sm and Gd.

Superconductivity in a New 1144-Type Family of (La,Na)AFe4As4 (A = Rb or Cs).

We discovered novel Fe-based superconductors (FeSCs) (La,Na)AFe4As4, where A = Rb or Cs, and characterized their superconducting properties. (La,Na)AFe4As4 is a so-called 1144-type compound with a tetragonal unit cell classified into space group P4/mmm (no. 123). The lattice constants are a = 3.861(1) Å and c = 13.26(1) Å for (La,Na)RbFe4As4 and a = 3.880(1) Å and c = 13.60(1) Å for (La,Na)CsFe4As4. The Rietveld refinement results on the powder X-ray diffraction suggest that the La/Na ratio is rather fixed as La:Na = 0.44(5):0.56(5). The electrical resistivity and magnetic susceptibility show superconducting transition at 25.5 K for (La,Na)RbFe4As4 and 24.0 K for (La,Na)CsFe4As4. The superconducting transition temperature (Tc) of (La,Na)AFe4As4 is comparable with that of 122-type (La,Na)Fe2As2 and lower than that of typical 122-type or 1144-type FeSCs by more than 10 K. The possible reasons for lower Tc are discussed in terms of the structural modification, carrier concentration, and chemical disorder.

Superconductivity on Hole-Doping Side of (La0.5-xNa0.5+x)Fe2As2.

(La0.5-xNa0.5+x)Fe2As2 ((La,Na)122) is an interesting system in the sense that either electrons (x < 0) or holes (x > 0) can be doped into the Fe2As2 layers, simply by changing the composition value x. However, only nonbulk superconducting samples (single crystals) with x = 0.1 have been synthesized to date. Here, we successfully synthesize polycrystalline samples with a wide hole-doping composition range of 0 ≤ x ≤ 0.35 via a conventional solid-state reaction, by tuning the reaction temperature according to x. The parent compound, (La0.5Na0.5)Fe2As2 (x = 0), is a nonsuperconductor with a resistivity anomaly at 130 K due to structural and antiferromagnetic transitions. We find that the temperature of the resistivity anomaly decreases with increasing x and that bulk superconductivity emerges for 0.15 ≤ x ≤ 0.35. The maximum transition temperature is 27.0 K, for x = 0.3. An electronic phase diagram for the hole-doping side is constructed. However, electron-doped samples (x < 0) cannot be synthesized; thus, the other half of the electronic phase diagram of (La,Na)122 requires resolution to study the electron-hole symmetry in Fe-based superconductors.

Synthesis and Superconductivity of a Strontium Digermanide SrGe2-δ with ThSi2 Structure.

We have succeeded in crystallizing a new strontium digermanide (SrGe2-δ) with the ThSi2-type structure (tetragonal SrGe2), which is theoretically predicted to compete with the EuGe2-type one (trigonal SrGe2) under pressure. The tetragonal SrGe2 appeared as a metastable phase in samples at approximately 900 °C under a pressure of 2 GPa. X-ray diffraction studies show that the tetragonal SrGe2 is formed by the reaction between trigonal SrGe2 and excess Sr. The composition of the tetragonal SrGe2 was analyzed to be SrGe1.66(4). Lattice parameters for the tetragonal SrGe2 are determined to be a = 4.559(4) Å and c = 14.42(1) Å. The tetragonal SrGe2 shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (Tc) of 7.3 K, which is the highest among compounds with the ThSi2-type structure. Superconducting properties of the tetragonal SrGe2, such as the upper critical field, and the effect of pressure on Tc, are presented and superconductivity is discussed on the basis of electronic band structure calculations.

Phase Transition of a Structure†II Cubic Clathrate Hydrate to a Tetragonal Form.

The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH4 ) and propanol are reported from powder X-ray diffraction measurements. The deformation of host water cages at the cubic-tetragonal phase transition of 2-propanol+CH4 hydrate, but not 1-propanol+CH4 hydrate, was observed below about 110 K. It is shown that the deformation of the host water cages of 2-propanol+CH4 hydrate can be explained by the restriction of the motion of 2-propanol within the 5(12) 6(4) host water cages. This result provides a low-temperature structure due to a temperature-induced symmetry-lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.

New-Structure-Type Fe-Based Superconductors: CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs).

Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.