Volume 61, Issue 12

Among the thermodynamic parameters governing the preparation of novel materials, temperature (T) and pressure (p) play an important role. In Materials Chemistry, the synthesis of materials needs energy in order to enhance the diffusion of atoms to the equilibrium positions required by the specific structure and to induce the formation of chemical bonds. The comparison of the energy conveyed by both parameters (p and T) underlines that high pressures can be associated - in liquid or solid media - with soft processes. Consequently this paper describes the main factors induced by the parameter pressure that are able to support new structural forms or generate novel materials. Two different approaches are presented: (i) for a given composition with characteristic chemical bonds, high pressures can induce structural transformations, (ii) high pressures lead to the formation of novel materials from different precursors through the formation of new chemical bonds.

The new filled skutterudite TbFe 4 P 12 has been prepared at around 4 GPa and 1050 °C. Powder X-ray diffraction of TbT 4 P 12 (T = Fe and Ru) has been studied with synchrotron radiation at ambient pressure. The crystal structures of both compounds were refined by Rietveld methods at ambient pressure. The positional parameters, bond lengths and bond angles have been obtained for TbT 4 P 12 (T = Fe and Ru). The electrical and magnetic properties of TbFe 4 P 12 have been investigated at low temperatures. The susceptibility of this phosphide follows a Curie-Weiss behavior at higher temperatures. The linear slope of the χ−1 vs. T curve from 15 to 300 K yields an effective magnetic moment of 9.48 μB. This value is close to the magnetic moment of the Tb 3+ ion calculated from Hund’s rule, 9.72 μB. The ferromagnetic transition of TbFe 4 P 12 was observed at around 10 K, and an electrical anomaly based on the magnetic ordering was detected. The relationship between the crystal structure and the physical properties of TbT 4 P 12 (T = Fe and Ru) is discussed.

The hexagonal high-pressure (HP) modifications of the ternary compounds REPtSn (RE = La, Pr, Sm) were prepared under multianvil high-pressure (9 - 14 GPa) high-temperature (1050 - 1400 °C) conditions from the orthorhombic normal-pressure (NP) modifications. The HP-REPtSn stannides were investigated by X-ray diffraction on powders and single crystals: ZrNiAl type, space group P6̅2m, a = 762.6(2), c = 418.55(7) pm, wR2 = 0.1147, 256 F 2 values and 14 variables for HP-LaPtSn, a = 754.97(7), c = 412.64(3) pm, wR2 = 0.0782, 252 F 2 values and 14 variables for HP-PrPtSn, and a = 750.1(2), c = 407.6(1) pm, wR2 = 0.1060, 229 F 2 values and 14 variables for HP-SmPtSn. The high-pressure modifications have two crystallographically independent platinum positions in trigonal prismatic coordination, Pt1Sn 6 RE 3 and Pt2Sn 3 RE 6 . The shortest interatomic distances occur between the platinum and tin atoms with in the three-dimensional [PtSn] networks. The rare earth atoms fill distorted hexagonal channels within these networks and they are bound through short RE-Pt contacts. Susceptibility measurements of HP-PrPtSn reveal paramagnetic behaviour with an experimental magnetic moment of 3.31(2) μB/Pr atom. Low-temperature susceptibility and specific heat data point to inhomogeneous magnetism in HP-PrPtSn.

The binary phase SrSi 6 was prepared at a pressure of 10(1) GPa and a temperature of 1520(150) K. Single crystal refinements and powder diffraction data reveal that the compound crystallizes in the orthorhombic crystal system (Cmcm, a=4.4692(4), b=10.256(1), c=11.698(1)Å ) and is isopointal to EuGa 2 Ge 4 and EuSi 6 . Exothermic decomposition of the compound into SrSi 2 and Si at 797 K and ambient pressure indicates that the compound is a metastable high-pressure phase. Analysis of the chemical bonding using the electron localization function and calculated charge densities reveals covalent bonding within the polyanion of four-bonded silicon atoms. Strontium cations are enclosed in the 3D net resulting in an electron balance Sr 2+ [Si 0 ] 2− 6 . In the electronic density of states the excess electrons of the framework are assigned to a filling of antibonding bands. SrSi 6 is a metallic conductor with an electrical resistivity of ρ ≈ 250 μΩ·cm at 300 K. Magnetization measurements indicate diamagnetic behaviour (χ 0 = −50 · 10 −6 emu·mol −1 ).

A new silicon-rich binary compound BaSi 6 has been prepared by the treatment of the Ba 8 Si 46 clathrate compound under a pressure of 15 GPa at 1000 °C, or from a stoichiometric mixture of BaSi 2 and Si by treatment under similar high-pressure and high-temperature conditions. The Rietveld refinements revealed that BaSi 6 is isomorphous with EuGa 2 Ge 4 , and crystallizes with space group Cmcm and the lattice parameters a = 4.485(1), b = 10.375(2), and c = 11.969(3) Å . Each Ba atom is surrounded by 18 Si atoms in an irregularly shaped polyhedron @Si 18 . The polyhedra are connected by sharing faces to form Ba containing tunnels along the a axis. All of the Si-rich compounds so far with atomic ratios Si/Ba > 2 in the binary system have been prepared only under high-pressure and high-temperature conditions. There is a general tendency that the Si/Ba ratio of the compounds increases with an increase of the pressure in the preparation.

A new anhydrous high-pressure synthesis route was developed for rutile type RhO 2 . Rhodium(III) chloride was reacted with Na 2 O 2 at 600 °C under an oxygen pressure of 200 MPa yielding RhO 2 and NaCl. X-ray powder diffractometer data and TEM observations confirmed the rutile structure: P4 2 /mnm, a = 448.7(1), c = 308.9(1) pm, x(O) = 0.3125(8), leading to Rh-O distances of 195.0 (4x) and 198.3 pm (2x) in the slightly elongated octahedra. Structural transformations associated with the production of oxygen vacancies can be initiated by electron-beam heating.

We have synthesized two new series of metastable oxides, namely REMn 2 O 5 and RECu 3 Mn 4 O 12 (RE = rare earths) under moderate pressure conditions. A novel series of ferrimagnetic oxides has been obtained by replacing Mn 3+ by Fe 3+ in the parent REMn 2 O 5 compounds (RE = Y, Dy, Ho, Er, Tm, Yb). The crystal structure has been studied by neutron powder diffraction (NPD); it contains chains of edge-linked Mn 4+ O 6 octahedra connected via dimeric groups of Fe 3+ O 5 square pyramids. The magnetic susceptibility and the thermal evolution of the NPD patterns reveal the onset of a ferrimagnetic structure below T C ≈ 165 K, characterized by the propagation vector k = 0. Immediately below T C , the Fe 3+ and Mn 4+ moments lie in an antiparallel arrangement along the c-axis direction. At lower temperatures, the magnetic moment of the rare-earth cations also participates in the magnetic structure, adopting a parallel arrangement with the Fe 3+ spins. Some new derivatives of CaCu 3 Mn 4 O 12 have been prepared at moderate pressures of 2 GPa by replacing Ca 2+ by RE 3+ cations in the series RECu 3 Mn 4 O 12 (RE = Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb); the concomitant electronic injection leads to a substantial contribution to T C . The crystal structures of the new materials were refined in the space group Im3̅ from NPD data for the non-absorbing RE cations. The unit cell parameters are considerably expanded with respect to CaCu 3 Mn 4 O 12 , as a result of the electronic injection. The r. t. magnetic structure displays a ferrimagnetic coupling between Mn 3+/4+ and Cu 2+ spins; at low temperatures there is an antiferromagnetic coupling of the RE 3+ moments with the Mn substructure, which substantially reduces the susceptibility and the saturation magnetization.

Perovskite and related Ruddlesden-Popper type transition metal oxides synthesised at high pressures and temperatures during the last decade are reviewed. More than 60 such new materials have been reported since 1995. Important developments have included perovskites with complex cation orderings on A and B sites, multiferroic bismuth-based perovskites, and new manganites showing colossal magnetoresistance (CMR) and charge ordering properties.

High oxygen pressures are a fruitful tool for the stabilization of the highest formal oxidation states of transition metals (M n+ ) leading to the strongest chemical bonds; the improvement of the M n+ -O bond covalency induces different electronic phenomena. Among the physical characterizations applied to investigate such phenomena, 57 Fe and 119 Sn Mössbauer spectra are evaluated for studying unusual electronic configurations, orbital ordering, charge disproportionation and insulator-metal transitions in the perovskites series of 57 Fe doped RENiO 3 nickelates (RE = rare earths, Y and Tl) and 119 Sn doped AEFeO 3 ferrates (AE = Ca, Sr).

Superhard superconducting samples with a critical temperature of T C = 10.5 - 12.6 K were obtained by high-pressure / high-temperature sintering of synthetic diamond powders coated with a niobium film and in 50% - 50% composition with superhard C 60 fullerene. Superhard superconductors with T C = 9.3 K were obtained when diamond and molybdenum powders were sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with T C = 36.1 - 37.5 K have been obtained in the systems diamond-MgB 2 and cubic boron nitride-MgB 2 .

Solid fullerenes C 60 and C 70 have been treated at high pressure of 15 GPa and high temperatures of 520 - 1820 K for a time of exposure of 60 s and a quenching rate of 300 K s −1 using a toroid-type apparatus. X-ray diffraction and Raman spectra confirm the realization of 3D-polymerized phases in these solids at 15 GPa. The pressure/temperature maps of synthesis of metastable carbon phases on the basis of C 60 and C 70 have thus been extended to 15 GPa. The longitudinal and shear sound wave velocities were measured by acoustic microscopy techniques. A maximum sound wave velocity of (21±1)×10 5 m s −1 was observed in the sample synthesized from C 60 at T = 1170 K. The elastic constants were calculated using experimental data. The acoustic microscopy images of experimental samples have been investigated.

This work reports a new route for preparing cubic BN (c-BN) nanocrystals under moderate conditions. Considering the instability of BN nanoparticles, we have investigated the structural transformation of BN nanoparticles in benzene. The results revealed that a great deal of hexagonal BN (h-BN) nanoparticles had been converted into c-BN or wurtzitic BN (w-BN) at 280 °C and 50 MPa. As the temperature was increased from 280 to 300 °C, h-BN nanoparticles gradually disappeared, and the content of both c-BN or w-BN increased. Similarly, increase of pressure or hot-pressing time showed the same effect. These results may open up an opportunity for synthesizing other superhard nanomaterials such as diamond and cubic carbon nitride under moderate conditions.

Bulk samples (with volumes up to ~ 7.5 mm 3 ) of boron-doped diamonds (BDD) were synthesized by means of direct reaction between boron carbide and graphite in a multianvil apparatus at high pressures and high temperatures (HPHT). X-ray diffraction data revealed the presence in BDD of a very small amount of a highly boron-enriched phase (B 50 C 2 ) and traces of the B 13 C 2 used as an initial material. The absence of B 50 C 2 in the product of treatment of pure B 13 C 2 under the same HPHT conditions suggests that boron-rich carbides exsolute from diamond on quenching leading to boron depletion of the diamond matrix. These observations imply that boron solubility in diamond increases at high pressure and high temperature. This result may have important implications for the understanding of the mechanism of boron incorporation into diamond at HPHT synthesis and for the interpretation of the data on superconductivity of polycrystalline BDD.

Platinum nanoparticles and nanorods were synthesized by microwave-assisted solvothermal techniques. Changing the reaction conditions controlled particle size and morphology. The effects of the reaction conditions, such as the molar ratio of the polyvinylpyrrolidone (PVP) repeating unit to the metal sources, the concentration of metal sources, the reaction temperature, and the presence of distilled water were investigated. Nanoparticles of Pt were approximately 3 nm in size. Produced nanoparticles and nanorods were characterized by transmission electron microscopy. Image J TM software was used to calculate the particle size and size distribution.

Melting phase relations in the methane-ammonia-water system up to 3 GPa have been obtained in a series of in situ experiments in externally heated diamond anvil cells. The melting temperature of methane clathrate hydrates increases rapidly above pressures of ~ 1.5 GPa, and does not appear to be significantly affected by the presence of ammonia. The reaction of the hydrate formation at pressures 2 - 3 GPa is kinetically impeded. Our data show that the high-pressure methane hydrate has the maximum melting temperature among the clathrate hydrates studied so far.

The compression behavior of Zr-doped nano anatase Ti 0.90 Zr 0.10 O 2 synthesized by the sol-gel method was studied using a diamond anvil cell (DAC) up to ~ 13 GPa. The compressibility parallel to the a axis decreases strongly at ~ 4 GPa and other structural parameters also change with pressure. The parameters of the third order Birch-Murnaghan equation of state were fitted to: V 0 = 139.6(0) Å3, K 0 = 213(9) GPa and K′ = 17.9(2). Ab initio electronic structure simulations indicate that the Zr atoms cluster in the crystal. The effects of chemical substitution as well as of microstructure, especially the crystallite size, on the mechanical properties are discussed.