Mid-Infrared, Optically Active Black Phosphorus Thin Films on Centimeter Scale.

Black phosphorus (BP) is a narrow bandgap (∼0.3 eV) semiconductor with a great potential for optoelectronic devices in the mid-infrared wavelength. However, it has been challenging to achieve a high-quality scalable BP thin film. Here we present the successful synthesis of optically active BP films on a centimeter scale. We utilize the pulsed laser deposition of amorphous red phosphorus, another allotrope of phosphorus, followed by a high-pressure treatment at ∼8 GPa to induce a phase conversion into BP crystals. The crystalline quality was improved through thermal annealing, resulting in the observation of photoluminescence emission at mid-infrared wavelengths. We demonstrate high-pressure conversion on a centimeter scale with a continuous film with a thickness of ∼18 nm using a flat-belt-type high-pressure apparatus. This synthesis procedure presents a promising route to obtain optical-quality BP films, enabling the exploration of integrated optoelectronic device applications such as light-emitting devices and mid-infrared cameras on a chip scale.

High-Pressure Synthesis of Corundum-Type Ga2O3:Cr3+ and Application of Its Fluorescence to the Pressure Scale.

Cr3+-doped Ga2O3 crystals with a corundum structure were synthesized at high temperature and high pressure, and their excitation as well as fluorescence properties were evaluated. The crystals were green under white light illumination but deep red when exposed to ultraviolet light. This can mainly be attributed to R1 and R2 fluorescence spectra caused by the Cr3+ transition. The pressure-dependence of their fluorescence spectra is comparable with ruby (Al2O3:Cr3+), which is currently often used as a pressure scale. The excitation spectrum was shifted to the long-wavelength side compared with ruby, which enabled excitation with long-wavelength lasers, even if the pressure effect is considered. In addition, the R1 and R2 peaks were well-separated with increasing pressure, which might have advantages over the ruby scale.

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.

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.

An Exceptionally Narrow Band-Gap (∼4 eV) Silicate Predicted in the Cubic Perovskite Structure: BaSiO3.

The electronic structures of 35 A2+B4+O3 ternary cubic perovskite oxides, including their hypothetical chemical compositions, were calculated by a hybrid functional method with the expectation that peculiar electronic structures and unique carrier transport properties suitable for semiconductor applications would be hidden in high-symmetry cubic perovskite oxides. We found unique electronic structures of Si-based oxides (A = Mg, Ca, Sr, and Ba, and B = Si). In particular, the unreported cubic BaSiO3 has a very narrow band gap (4.1 eV) compared with conventional nontransition-metal silicates (e.g., ∼9 eV for SiO2 and the calculated value of 7.3 eV for orthorhombic BaSiO3) and a small electron effective mass (0.3m0, where m0 is the free electron rest mass). The narrow band gap is ascribed to the nonbonding state of Si 3s and the weakened Madelung potential. The existence of the predicted cubic perovskite structure of BaSiO3 was experimentally verified by applying a high pressure of 141 GPa. The present finding indicates that it could be possible to develop a new transparent oxide semiconductor of earth abundant silicates if the symmetry of its crystal structure is appropriately chosen. Cubic BaSiO3 is a candidate for high-performance oxide semiconductors if this phase can be stabilized at room temperature and ambient pressure.

Postperovskite phase transition of ZnGeO3: comparative crystal chemistry of postperovskite phase transition from germanate perovskites.

The postperovskite phase of ZnGeO3 was confirmed by laser heating experiments of the perovskite phase under 110-130 GPa at high temperature. Ab initio calculations indicated that the phase transition occurs at 133 GPa at 0 K. This postperovskite transition pressure is significantly higher than those reported for other germanates, such as MnGeO3 and MgGeO3. The comparative crystal chemistry of the perovskite-to-postperovskite transition suggests that a relatively elongated b-axis in the low-pressure range resulted in the delay in the transition to the postperovskite phase. Similar to most GdFeO3-type perovskites that transform to the CaIrO3-type postperovskite phase, ZnGeO3 perovskite eventually transformed to the CaIrO3-type postperovskite phase at a critical rotational angle of the GeO6 octahedron. The formation of the postperovskite structure at a very low critical rotational angle for MnGeO3 suggests that relatively large divalent cations likely break down the corner-sharing GeO6 frameworks without a large rotation of GeO6 to form the postperovskite phase.

Distinct responses to mechanical grinding and hydrostatic pressure in luminescent chromism of tetrathiazolylthiophene.

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.

Perovskite-to-postperovskite transitions in NaNiF3 and NaCoF3 and disproportionation of NaCoF3 postperovskite under high pressure and high temperature.

High-pressure structural phase transitions in NaNiF(3) and NaCoF(3) were investigated by conducting in situ synchrotron powder X-ray diffraction experiments using a diamond anvil cell. The perovskite phases (GdFeO(3) type) started to transform into postperovskite phases (CaIrO(3) type) at about 11-14 GPa, even at room temperature. The transition pressure is much lower than those of oxide perovskites. The anisotropic compression behavior led to heavily tilted octahedra that triggered the transition. Unlike oxide postperovskites, fluoropostperovskites remained after decompression to 1 atm. The postperovskite phase in NaCoF(3) broke down into a mixture of unknown phases after laser heating above 26 GPa, and the phases changed into amorphous ones when the pressure was released. High-pressure and high-temperature experiments using a multianvil apparatus were also conducted to elucidate the phase relations in NaCoF(3). Elemental analysis of the recovered amorphous samples indicated that the NaCoF(3) postperovskite disproportionated into two phases. This kind of disproportionation was not evident in NaNiF(3) even after laser heating at 54 GPa. In contrast to the single postpostperovskite phase reported in NaMgF(3), such a postpostperovskite phase was not found in the present compounds.

Perovskite, LiNbO3, corundum, and hexagonal polymorphs of (In(1-x)M(x))MO3.

LiNbO(3) (LN), corundum (cor), and hexagonal (hex) phases of (In(1-x)M(x))MO(3) (x = 0.143; M = Fe(0.5)Mn(0.5)) were prepared. Their crystal structures were investigated with synchrotron X-ray powder diffraction, and their properties were studied by differential thermal analysis, magnetic measurements, and Mössbauer spectroscopy. The LN-phase was prepared at high pressure of 6 GPa and 1770 K; it crystallizes in space group R3c with a = 5.25054(7) Å, c = 13.96084(17) Å, and has a long-range antiferromagnetic ordering near T(N) = 270 K. The cor- and hex-phases were obtained at ambient pressure by heating the LN-phase in air up to 870 and 1220 K, respectively. The cor-phase crystallizes in space group R-3c with a = 5.25047(10) Å, c = 14.0750(2) Å, and the hex-phase in space group P6(3)/mmc with a = 3.34340(18) Å, c = 11.8734(5) Å. T(N) of the cor-phase is about 200 K, and T(N) of the hex-phase is about 140 K. During irreversible transformations of LN-(In(1-x)M(x))MO(3) with the (partial) cation ordering, the In(3+), Mn(3+), and Fe(3+) cations become completely disordered in one crystallographic site of the corundum structure, and then they are (partially) ordered again in the hex-phase. LN-(In(1-x)M(x))MO(3) exhibits a reversible transformation to a perovskite GdFeO(3)-type structure (space group Pnma; a = 5.2946(3) Å, b = 7.5339(4) Å, c = 5.0739(2) Å at 10.3 GPa) at room temperature and pressure of about 5 GPa.

Pressure-induced spin-state transition in BiCoO3.

The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.

Dense yttria phase eclipsing the A-type sesquioxide structure: high-pressure experiments and ab initio calculations.

In situ X-ray diffraction experiments and ab initio calculations elucidated the high-pressure phase transition properties of yttrium sesquioxides. The C-, B-, and A-type sesquioxides structure sequence observed in the room-temperature compression does not coincide with the high-pressure phase sequence of yttrium sesquioxides at high temperature. A reconstructive-type transformation taking place at high temperature yields the Gd(2)S(3) structure around 8 GPa with a drastic change in cation-oxygen coordinations. Ab initio structural optimization suggests that a displacive-type transformation from B- to A-type sesquioxides structure metastably occurs under pressure at room temperature. The calculated density of states indicates that the transition to the Gd(2)S(3) structure causes a significant decrease in the band gap. The Gd(2)S(3) phase was also found to be partially recovered at ambient pressure. We briefly discuss the quenchability of the Gd(2)S(3) structure in sesquioxides on the basis of the enthalpy differences between the ambient phase and the recovered products.

High-pressure phase transition to the Gd(2)S(3) structure in Sc(2)O(3): a new trend in dense structures in sesquioxides.

In situ X-ray diffraction experiments using a laser-heated diamond anvil cell revealed a novel dense phase with the Gd(2)S(3) structure stabilizing in Sc(2)O(3) at pressures over 19 GPa. Although no phase transformation was induced during room-temperature compression up to 31 GPa, the C rare earth sesquioxide structure transformed into the B rare earth sesquioxide structure at 10 GPa after laser annealing and subsequently into the Gd(2)S(3) structure at 19 GPa. Neither the A rare earth sesquioxide structure nor the U(2)S(3) structure was found in Sc(2)O(3). Static density functional lattice energy calculations demonstrated that the C structure prefers Gd(2)S(3) over U(2)S(3) as the post phase. Sc(2)O(3) is the second sesquioxide, after In(2)O(3), to crystallize into a Gd(2)S(3) structure at high pressures and high temperatures.

Peculiar high-pressure behavior of BiMnO3.

High-pressure structural properties of perovskite-type BiMnO(3) have been investigated by synchrotron X-ray powder diffraction at room temperature. A new monoclinic phase having P2(1)/c symmetry was found between about 1.5 and 5.5 GPa. Above 8 GPa, the orthorhombic GdFeO(3)-type phase (space group Pnma) is stable. The crystal structure of BiMnO(3) at 8.6 GPa and room temperature was investigated (a = 5.5132(3) A, b = 7.5752(3) A, c = 5.4535(3) A). The orthorhombic phase of BiMnO(3) has an orbital order similar to LaMnO(3) but with a different arrangement of orbitals in the ac plane. High-pressure room-temperature behavior of BiMnO(3) differs from high-temperature behavior at ambient pressure in comparison with BiCrO(3) and BiScO(3). These findings may open new directions in investigation of BiMnO(3).