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The crystal structure of the high-pressure phase of hexahydro-1,3,5-trinitro-1,3,5-triazine ([gamma]-RDX) is investigated by using infrared spectroscopy and powder X-ray diffraction measurements followed by Rietveld refinements using a diamond anvil cell (DAC). The molecular structure of the [gamma] phase has displayed the same conformation as that of the alpha phase, but the torsion angles of N-N[O.sub.2] changed marginally.

A new intermediate phase (phase II)was found between phases Iand III in LiNH2in the pressure range of 10 to 13 GPathrough the analysis of infrared and powder X-ray diffraction measurementsto 25 GPa at room temperature. This result agreed with the predictionof a stable phase between phases I and III through theoretical calculations.Powder X-ray diffraction measurement and DFT calculation showed thatthis phase has a monoclinic structure with space group C2/c(Z= 8), which is the samestructure as that of a slightly tilted crystal lattice of the Fdddstructural model. The enthalpy of the C2/cstructure was also found to be almost the sameas that of the Fdddstructure. [ABSTRACT FROM AUTHOR]

(La05 -- xNa05+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 l30 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. [ABSTRACT FROM AUTHOR]

A new iron-based superconductor, (Ca,Pr)- FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P21/ m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a Tc of ∼20 K in both magnetization and resistivity measurements. [ABSTRACT FROM AUTHOR]

Experimental observations indicate that pressure treatments transform the monoclinic and orthorhombic low-temperature phases of Mg 2 NiH 4 into an unresolved pseudocubic phase even at room temperatures, resembling the temperature-induced phase transformation of the material. This pressure-induced phase transformation is anticipated to involve deformations of the tetrahedrally bonded NiH 4 molecular ion and the metal lattice. However, the precise mechanism underlying this transformation remains unclear. To elucidate this behavior, this study adopted first-principles density functional theory calculations to investigate the effect of applied pressure on the observed phase transition. The results revealed that the phase transition from the low-temperature phases to the pseudocubic phase occurs at approximately 8-9 GPa. Furthermore, at this pressure, the metal lattice deforms into an antifluorite-like structure, and the NiH 4 molecular ion undergoes substantial distortion, resulting in the alignment of hydrogen atoms along the crystal axes of the metal lattice. Furthermore, through a comprehensive structural exploration, we successfully identified several tetragonal and orthorhombic models that are energetically more stable than the low-temperature phases at low pressures. These models are potential candidates for elucidating the pseudocubic phase observed in experiments. Overall, our findings are anticipated to enhance the current understanding of the structural changes occurring during pressure and temperature-induced phase transitions.

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO 2 below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO 2 guests in small pentagonal dodecahedral 5 12 host water cages, not previously observed in the simple CO 2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd -3 m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I 4 1 / 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.

Enriching the material variation often contributes to the progress of materials science. We have discovered for the first time antiperovskite arsenide SrPd 3 As and revealed a hidden structural and superconducting phase in Sr(Pd 1- x Pt x ) 3 As. The Pd-rich samples (0 ≤ x ≤ 0.2) had the same noncentrosymmetric (NCS) tetragonal structure (a space group of I 4 1 md ) as SrPd 3 P. For the samples with 0.3 ≤ x ≤ 0.7, a centrosymmetric (CS) tetragonal structure ( P 4/ nmm ) identical to that of SrPt 3 P was found to appear, accompanied by superconductivity at a transition temperature ( T c ) up to 3.7 K. In the samples synthesized with Pt-rich nominal compositions (0.8 ≤ x ≤ 1.0), Sr 2 (Pd,Pt) 8- y As 1+ y with an intergrowth structure (CS-orthorhombic with Cmcm ) was crystallized. The phase diagram obtained for Sr(Pd,Pt) 3 As was analogous to that of (Ca,Sr)Pd 3 P in which superconductivity ( T c ≥ 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.

Light lanthanide dodecaborides, RB 12 (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 RB 12 , 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 CeB 12 , which can be explained by a valence fluctuation between Ce 3+ and Ce 4+ indicated by X-ray absorption near-edge structure measurements. A polyhedral coordination change from a truncated cube in RB 6 to a truncated octahedron in RB 12 and associated shortening of the R-B bond length result in an increase in bulk modulus and hardness.

Antiperovskites are a promising candidate structure for the exploration of new materials. We discovered an antiperovskite phosphide, LaPd 3 P, following our recent synthesis of A Pd 3 P ( A = Ca, Sr, Ba). While A Pd 3 P and (Ca,Sr)Pd 3 P were found to be tetragonal or orthorhombic systems, LaPd 3 P is a new prototype cubic system ( a = 9.0317(1) Å) with a noncentrosymmetric space group ( I 4̅3 m ). LaPd 3 P exhibited superconductivity with a transition temperature ( T c ) 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 LaPd 3 P 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 ( E F ) was located slightly below the density of states (DOS) pseudogap. This difference was explained by the increase in the DOS at E F due to the approximately 5 atom % La deficiency (hole doping) in the sample. The observed T c 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 LaPd 3 P arising from the lack of space inversion symmetry.

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., NH 3 BH 3 and ND 3 BD 3 ) 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 P 2 1 ( 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 NH 3 BH 3 .

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, Sc 20 C 8- x B x C 20 , 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 Sc 20 (C,B) 8 C 20 phase by the reaction of the previously reported Sc 15 C 19 and B (Sc 15 B y C 19 ). Our experiments demonstrate that an increase in y values promotes the phase formation of the Sc 20 (C,B) 8 C 20 structure; however, there appears to be an upper limit to the nominal y value to form this phase. The maximum critical temperature ( T c = 7.6 K) is found to correspond with the actual x value of x ≈ 5 under the assumption that the sample with the same T c 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 Sc 20 C 4 B 4 C 20 ( x = 4) is the most thermodynamically stable, which is reasonably consistent with the experimentally obtained value.

In this study, we succeeded in synthesizing new antiperovskite phosphides M Pd 3 P (M = Ca, Sr, Ba) and discovered the appearance of a superconducting phase (0.17 ≤ x ≤ 0.55) in a solid solution (Ca 1- x Sr x )Pd 3 P. Three perovskite-related crystal structures were identified in (Ca 1- x Sr x )Pd 3 P, and a phase diagram was built on the basis of experimental results. The first phase transition from centrosymmetric ( Pnma ) to noncentrosymmetric orthorhombic ( Aba 2) occurred in CaPd 3 P near room temperature. The phase transition temperature decreased as Ca 2+ was replaced with a larger-sized isovalent Sr 2+ . Bulk superconductivity at a critical temperature ( T c ) 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 ( I 4 1 md ) remained stable for 0.6 ≤ x ≤ 1.0, and superconductivity was significantly suppressed as samples with x = 0.75 and 1.0 showed T c values as low as 0.32 K and 57 mK, respectively. For further substitution with a larger-sized isovalent Ba 2+ , namely, (Sr 1- y Ba y )Pd 3 P, the tetragonal phase continued throughout the composition range. BaPd 3 P no longer showed superconductivity down to 20 mK. Since the inversion symmetry of structure and superconductivity can be precisely controlled in (Ca 1- x Sr x )Pd 3 P, this material may offer a unique opportunity to study the relationship between inversion symmetry and superconductivity.

We report synthesis, crystal structure, and superconductivity in ThCr 2 Si 2 -type LaFe 2 As 2 (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 CaFe 2 As 2 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 CaFe 2 As 2 under pressure.

Recently, we succeeded in synthesizing (La 0.5- x Na 0.5+ x )Fe 2 As 2 ((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 T c = 27.0 K for x = 0.3. Here, we report the synthesis and physical properties of analogous compounds ( Ln 0.5- x Na 0.5+ x )Fe 2 As 2 (( 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 Ln 3+ ). Bulk superconductivity emerged for 0.2 ≤ x ≤ 0.3 and 0.15 ≤ x ≤ 0.25 with the highest T c 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 T c 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 Ln 3+ . The ( Ln,Na)122 phase competes with the non-superconducting CaFe 4 As 3 (143)-type phase of ( Ln,Na)Fe 4 As 3 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.

We discovered novel Fe-based superconductors (FeSCs) (La,Na)AFe 4 As 4 , where A = Rb or Cs, and characterized their superconducting properties. (La,Na)AFe 4 As 4 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)RbFe 4 As 4 and a = 3.880(1) Å and c = 13.60(1) Å for (La,Na)CsFe 4 As 4 . 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)RbFe 4 As 4 and 24.0 K for (La,Na)CsFe 4 As 4 . The superconducting transition temperature (T c ) of (La,Na)AFe 4 As 4 is comparable with that of 122-type (La,Na)Fe 2 As 2 and lower than that of typical 122-type or 1144-type FeSCs by more than 10 K. The possible reasons for lower T c are discussed in terms of the structural modification, carrier concentration, and chemical disorder.

(La 0.5-x Na 0.5+x )Fe 2 As 2 ((La,Na)122) is an interesting system in the sense that either electrons (x < 0) or holes (x > 0) can be doped into the Fe 2 As 2 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, (La 0.5 Na 0.5 )Fe 2 As 2 (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.

We have succeeded in crystallizing a new strontium digermanide (SrGe 2-δ ) with the ThSi 2 -type structure (tetragonal SrGe 2 ), which is theoretically predicted to compete with the EuGe 2 -type one (trigonal SrGe 2 ) under pressure. The tetragonal SrGe 2 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 SrGe 2 is formed by the reaction between trigonal SrGe 2 and excess Sr. The composition of the tetragonal SrGe 2 was analyzed to be SrGe 1.66(4) . Lattice parameters for the tetragonal SrGe 2 are determined to be a = 4.559(4) Å and c = 14.42(1) Å. The tetragonal SrGe 2 shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (T c ) of 7.3 K, which is the highest among compounds with the ThSi 2 -type structure. Superconducting properties of the tetragonal SrGe 2 , such as the upper critical field, and the effect of pressure on T c , are presented and superconductivity is discussed on the basis of electronic band structure calculations.

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.

The Ba-Ir-Ge ternary compounds BaIr2Ge7 and Ba3Ir4Ge16 exhibit superconductivity (SC) at 2.5 and 5.2 K, respectively. Detailed single-crystal structural analysis revealed that these compounds share unique quasi-two-dimensional networks composed of crown-shaped Ge rings that accommodate Ba atoms at the center, referred to as "edge-shared crown-shaped BaGe16 polyhedra". The layered Ba-Ge network yielded a modest anisotropy of 1.3-1.4 in the upper critical field, which is in good agreement with the band structure calculations. The Ba-Ge structural unit is similar to cage structures seen in various clathrates in which the anharmonic vibration of the central atoms, the so-called "rattling" behavior, brings about strong-coupling SC. However, each Ba-Ge unit is relatively small compared to these materials, which likely excludes the possibility of unconventional SC.

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

Luminescent mechanochromism has been intensively studied in the past few years. However, the difference in the anisotropic grinding and the isotropic compression is not clearly distinguished in many cases, in spite of the importance of this discrimination for the application of such mechanochromic materials. We now report the distinct luminescent responses of a new organic fluorophore, tetrathiazolylthiophene, to these stresses. The multichromism is achieved over the entire visible region using the single fluorophore. The different mechanisms of a blue shift by grinding crystals and of a red shift under hydrostatic pressure are fully investigated, which includes a high-pressure single-crystal X-ray diffraction analysis. The anisotropic and isotropic modes of mechanical loading suppress and enhance the excimer formation, respectively, in the 3D hydrogen-bond network.