New observations on the pressure dependence of luminescence from Eu2+-doped MF2 (M = Ca, Sr, Ba) fluorides

The luminescence from Eu(2+) ions in MF2 (M = Ca, Sr, Ba) fluorides has been investigated under the pressure range of 0-8 GPa. The emission band originating from the 4f(6)5d(1) --> 4f(7) transition of Eu(2+) ions in CaF2 and SrF2 shows the red-shift as increasing pressure with pressure coefficients of -17 meV/GPa for CaF2 and -18 meV/GPa for SrF2. At atmospheric pressure, the emission spectrum of BaF2:Eu(2+) comprises two peaks at 2.20 and 2.75 eV from the impurity trapped exciton (ITE) and the self-trapped exciton (STE), respectively. As the pressure is increased, both emission peaks shift to higher energies, and the shifting rate is slowed by the phase transition from the cubic to orthorhombic phase at 4 GPa. Due to the phase transition at 4-5 GPa pressure, the ITE emission disappears gradually, and the STE emission is gradually replaced by the 4f(6)5d(1) --> 4f(7) transition of Eu(2+). Above 5 GPa, the pressure behavior of the 4f(6)5d(1) --> 4f(7) transition of Eu(2+) in BaF2:Eu(2+) is the same as the normal emission of Eu(2+) in CaF2 and SrF2 phosphors.     

X‐Ray Diffraction and Mössbauer Spectroscopy Studies of Pressure‐Induced Phase Transitions in a Mixed‐Valence Trinuclear Iron Complex

The mixed‐valence complex Fe₃O(cyanoacetate)₆(H₂O)₃ ( 1 ) has been studied by single‐crystal X‐ray diffraction analysis at pressures up to 5.3(1) GPa and by (synchrotron) Mössbauer spectroscopy at pressures up to 8(1) GPa. Crystal structure refinements were possible up to 4.0(1) GPa. In this pressure range, 1 undergoes two pressure‐induced phase transitions. The first phase transition at around 3 GPa is isosymmetric and involves a 60° rotation of 50 % of the cyanoacetate ligands. The second phase transition at around 4 GPa reduces the symmetry from rhombohedral to triclinic. Mössbauer spectra show that the complex becomes partially valence‐trapped after the second phase transition. This sluggish pressure‐induced valence‐trapping is in contrast to the very abrupt valence‐trapping observed when compound 1 is cooled from 130 to 120 K at ambient pressure.     

Crystal structure of the new FeSe(1-x) superconductor

The newly discovered superconductor FeSe(1-x) (x approximately 0.08, T(c)(onset) approximately 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure-the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO(1-x)F(x) (RE = rare earth) superconductors.     

High-pressure- and low-temperature-induced changes in [(CH3)2NH(CH2)2NH3][SbCl5

The structure of N,N-dimethylethylenediammonium pentachloroantimonate(III), [(CH3)2NH(CH2)2NH3][SbCl5], NNDP, was investigated at 100 and 15 K at ambient pressure, as well as at pressures up to 4.00 GPa at room temperature in the diamond-anvil cell. The stable structure at low temperatures and low pressures consists of isolated [SbCl5]2- anions and [(CH3)2NH(CH2)2NH3]2+ cations. The inorganic anions have a distorted square pyramidal geometry. They are arranged in linear chains parallel to the c axis. In contrast to the low-temperature studies, where no phase transition was detected, pressure induces a P2(1)/c --> P2(1)/n phase transition between 0.55 and 1.00 GPa, accompanied by a doubling of the a unit-cell parameter. This solid-solid transition results from changes in the electron configuration of the Sb(III) atom and formation of the Sb-Cl bridging bonds between inorganic polyhedra to form, at approximately 1.0 GPa, isolated [Sb2Cl10]4- units consisting of [SbCl6]3- octahedra and [SbCl5]2- square pyramids connected by a common corner. The intermolecular distances continuously decrease with further increase in pressure, and at approximately 3.1 GPa, zigzag [{SbCl5}n]2n- chains containing corner-sharing [SbCl6]3- octahedra are formed. The unit-cell volume of NNDP decreases by 18.15% between room pressure and 4.00 GPa. The linear distortions of the [SbCl5]2- and [SbCl6]3- polyhedra decrease with increasing pressure and decreasing temperature and indicate a reduction in the stereochemical activity of the lone electron pair on the Sb(III) atom.     

Piezochromism in nickel salicylaldoximato complexes: tuning crystal-field splitting with high pressure

The crystal structures of bis(3-fluoro-salicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6?GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni???Ni distance decreases from 3.19?? at ambient pressure to 2.82?? at 5.4?GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011?? for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620?nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x(2)-y(2)) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5?GPa.     

Synthesis, structural transformation, thermal stability, valence state, and magnetic and electronic properties of PbNiO3 with perovskite- and LiNbO3-type structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.     

Formation of antiferromagnetically coupled C(60)(*)(-) and diamagnetic (C(70)(-))(2) dimers in ionic complexes of fullerenes with (MDABCO(+))(2).M(II)TPP (M = Zn, Co, Mn, and Fe) assemblies

A series of ionic multicomponent complexes comprising C60 and C70 anions and coordinating assemblies of methyldiazabicyclooctane cations with metal tetraphenylporphyrins, (MDABCO+)2.MIITPP.(C60(70)-)2.Sol. (C60, M = Zn (1); C60, M = Co (2); C60, M = Mn (3); C60, M = Fe (4); C70, M = Mn (5); and C70, M = Fe (6)) has been obtained. IR- and UV-vis-NIR spectra of 1-6 justified the formation of C60*- in 1-4 and single-bonded (C70-)2 dimers in 5 and 6. Co and Mn atoms are six-coordinated in the (MDABCO+)2.MIITPP units with relatively long M-N bonds of 2.475(2), 2.553(2), and 2.511(3) A for 2, 3, and 5, respectively. Isostructural complexes 2 and 3 contain C60*- zigzag chains separated by the (MDABCO+)2.MIITPP units, whereas in 5 the layers formed by the (C70-)2 dimers alternate with those composed of the (MDABCO+)2.MnIITPP units and noncoordinating MDABCO+ cations. Negative Weiss constants of -13 (1), -2 (3), and -2 (4) K indicate the antiferromagnetic interaction of spins, which decreases the magnetic moment of the complexes below 70-120 K. The EPR signals of 1 and 4 attributed to C60*- are split into two components at the same temperatures, which broaden and shift to higher and lower magnetic fields with the temperature decrease. Complexes 2 and 3 show single EPR signals with g-factors equal to 2.1082 and approximately 2.4 at 293 K, respectively. These values are mean between those characteristic of MIITPP and C60*-, and, consequently, the signals appear due to exchange coupling between these paramagnetic species. The antiferromagnetic ordering of C60*- spins below 70-100 K shifts g-factor values closer to those characteristic of individual MIITPP (g = 2.1907 (2) and approximately 4.9 (3) at 4 K). In contrast to 1-4, complex 5 shows paramagnetic behavior with Weiss constant close to 0.     

Pressure-induced cubic to monoclinic phase transformation in erbium sesquioxide Er(2)O(3)

Cubic Er(2)O(3) was compressed in a symmetric diamond anvil cell at room temperature and studied in situ using energy-dispersive X-ray diffraction. A transition to a monoclinic phase began at 9.9 GPa and was complete at 16.3 GPa and was accompanied by a approximately 9% volume decrease. The monoclinic phase was stable up to at least 30 GPa and could be quenched to ambient conditions. The normalized lattice parameter compression data for both phases were fit to linear equations, and the volume compression data were fit to third-order Birch-Murnaghan equations of state. The zero-pressure isothermal bulk moduli (B(0)) and the first-pressure derivatives (B(0)') for the cubic and monoclinic phases were 200(6) GPa and 8.4 and also 202(2) GPa and 1.0, respectively. Ab initio density functional theory calculations were performed to determine optimized lattice parameters and atom positions for the cubic, monoclinic, and hexagonal phases of Er(2)O(3). The calculated X-ray spectra and predicted transition pressure are in good qualitative agreement with the experimental results.     

Piezochromism in Nickel Salicylaldoximato Complexes: Tuning Crystal‐Field Splitting with High Pressure

The crystal structures of bis(3‐fluoro‐salicylaldoximato)nickel(II) and bis(3‐methoxy‐salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni???Ni distance decreases from 3.19 Å at ambient pressure to 2.82 Å at 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni–ligand distances also shorten, on average by 0.017 and 0.011 Å for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x²?y²) orbital. Time‐dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni–ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV‐visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa.     

Solvent effects on the S0(1(1)Ag-) --> S2(1(1)Bu+) transition of beta-carotene, echinenone, canthaxanthin, and astaxanthin in supercritical CO2 and CF3H

Solvent-induced spectral shifts of the four C40 carotenoids, beta-carotene, echinenone, canthaxantin, and astaxanthin, have been studied in supercritical CO2 and CF3H. In situ absorption spectroscopic analysis was used to determine the maximum peak position of the electronic transitions from the ground state (1(1)Ag-) to the S2 state (1(1)Bu+) of the carotenoids. The medium polarizability function, R(n) = (n2 - 1)/(n2 + 2) of the refractive index of the solvent was varied over the range R(n) = 0.08-0.14, by changing the pressure of CO2 or CF3H between 90 and 300 bar at the temperature 308 K. For all the carotenoids studied here, a significant hypsochromic shift of ca. 20-30 nm was observed in supercritical fluids as compared to that in nonpolar liquids. The spectral shifts in supercritical fluids were compared with those in liquids and showed a clear linear dependence on the medium polarizability. The temperature-dependent shift of the absorption maxima was less significant. Interestingly, there was almost no difference in the energetic position of the absorption maxima in supercritical CO2 and CF3H at a given R(n) value. This is in contrast to previous extrapolations from studies in liquids at larger R(n) values, which yielded different slopes of the R(n)-dependent spectral shifts for polar and nonpolar solvents toward the gas-phase limit of R(n) = 0. The current experimental results in the gas-to-liquid range show that the polarity of the solvent has only a minor influence on the 1(1)Ag- --> 1(1)Bu+ transition energy in the region of low R(n). We also obtain more reliable extrapolations of this 0-0 transition energy to the gas-phase limit nu(0-0)(gas-phase) approximately (23,000 +/- 120) cm(-1) for beta-carotene.     

Struture stability and compressibility of iron-based superconductor Nd(O0.88F0.12)FeAs under high pressure

The high-pressure angle-dispersive X-ray diffraction experiments on the iron-based superconductor Nd(O0.88F0.12)FeAs were performed up to 32.7 GPa at room temperature. An isostructural phase transition starts at approximately 10 GPa. When pressure is higher than 13.5 GPa, Nd(O0.88F0.12)FeAs completely transforms to a high-pressure phase, which remains the same tetragonal structure with a larger a-axis and smaller c-axis than those of the low-pressure phase. The ambient conditions isothermal bulk moduli B0 are derived as 102(2) and 245(9) GPa for the low-pressure phase and high-pressure phase, respectively. The structure analysis based on the Rietveld refinement methods shows the difference of pressure dependence of the Fe-As and Nd-(O, F) bonding distances, as well as As-Fe-As and Nd-(O, F)-Nd angles between the low-pressure phase and high-pressure phase.     

Pressure-induced first-order phase transition in (NH4)2V3O8 fresnoite: a double coordination change for V4+ and V5+

The high-pressure behaviour of (NH(4))(2)V(3)O(8) with the fresnoite structure (P4bm, Z = 2) has been studied at room temperature with single-crystal X-ray diffraction in diamond anvil cells using laboratory and synchrotron facilities. At ambient conditions, the crystal structure is composed of layers of corner-sharing V(5+)O(4) tetrahedra and V(4+)O(5) square pyramids separated by layers of the NH(4)(+) cations. At about 3 GPa, there occurs a reversible first-order phase transition to a three-dimensional structure (P4/mbm, Z = 2) built of corner-sharing V(5+)O(5) trigonal bipyramids and V(4+)O(6) octahedra. The NH(4)(+) cations fill up the interstitial sites in the tunnels formed by the vanadate framework. Up to the phase transition, the a lattice parameter of the low-pressure polymorph does not change while the contraction perpendicular to the stacking of the V(3)O(8) slabs accounts entirely for the bulk compressibility. Above the phase transition, the a lattice parameter slightly expands. The structural features of the high-pressure phase of (NH(4))(2)V(3)O(8) are compared to those of other vanadium oxides.     

Photomagnetic effect in a cyanide-bridged mixed-valence {Fe(II)2Fe(III)2} molecular square

The self-assembly of [Fe(III)(Tp)(CN)(3)](-) and [Fe(II)(bik)(2)(S)(2)](2+) affords the cyanide-bridged mixed valence {Fe(III)(2)Fe(II)(2)}(2+) molecular square, which exhibits a photomagnetic effect under laser light irradiation at low temperature and also shows thermal spin-state conversion near ambient temperature.     

Crystal Structure of the High Pressure Phase(II) in CuGeO3

Crystal structures of the ambient pressure and temperature phase (phase I) and high pressure phase (phase II) in CuGeO₃ were studied by means of the high pressure single‐crystal X‐ray diffraction method in a diamond anvil cell using high power X‐ray generator and imaging plate detector. The pressure dependence of the atomic displacements in the phase I was investigated under the hydrostatic pressure of 0.1 MPa and 2.9 and 3.9 GPa. The lattice is particularly compressive in the b direction. In phase I the rippled layers are formed by the corner‐shared chains of GeO₄ tetrahedra and edge‐linked planar CuO₄. Major effects of pressure, directly related to the shortening of the b‐axis, consist of an enhanced folding of the rippled layers towards the b‐direction and of a shortening of the weak Cu–O bond. The crystal structure of phase II is monoclinic, a = 4.935(57), b = 6.754(14), c = 6.208(11) Å, β = 92.67(3)°, space group; P2₁/c. The transition from phase I to II involves a corrugated arrangement of the both cation with some oxygens around the c‐axis. Ge ion at the transition point of 6.4 GPa changes its coordination number from four‐fold to five‐fold, and Cu ion occupies a position of seven‐fold site. The structure of phase II is explained as a slab structure having unique edge‐ and corner‐sharing arrangements of GeO₅ and CuO₇ polyhedra. The average Ge–O and Cu–O distances in phase II is 1.92 and 2.17 Å, respectively, at 6.5 GPa.     

Thermal, pressure and light induced spin transition in the two-dimensional coordination polymer [Fe(pmd)2[Cu(CN)2]2

A complete structural, calorimetric, and magnetic characterisation of the 2D coordination spin crossover polymer [Fe(pmd)(2)[Cu(CN)(2)](2)] is reported. The crystal structure has been investigated below room temperature at 180 K and 90 K, and at 30 K after irradiating the sample at low temperature with green light (lambda = 532 nm). The volume cell contraction through the thermal spin transition is only 18 A(3) which is lower than the usually observed value of around 25-30 A(3) while the average Fe-N bond distances decrease by the typical value of about 0.19 A. The structural data of the irradiated state indicate that the high spin state is well induced since the cell parameters are consistent with the data at 180 K. Calorimetric and photo-calorimetric experiments have also been performed. The entropy content for the thermal spin transition, DeltaS = 35-37 J mol(-1) K(-1) lies in the lowest range of the typical values and correlates with the low volume cell contraction. The combination of the crystallographic and calorimetric data predicts, in accordance with a mean-field approach, a linear pressure dependence of the critical temperature with a slope of 302 K GPa(-1). Magnetic measurements under pressure reveal an anomalous behaviour since the critical temperature and hysteresis do not change up to 0.22 GPa but an apparent linear dependence is obtained for higher pressures (up to 0.8 GPa) with a slope two times higher than the mean-field estimation.     

Raman scattering spectroscopic study of n-tetradecane under high pressure and ambient temperature

The Raman spectroscopy of n-tetradecane was investigated in a Moissanite anvil cell at pressure from 0.1 MPa to 1.4 GPa and ambient temperature. The result shows that the liquid-solid phase transition of n-tetradecane takes place at around 302.8 MPa and the corresponding DeltaV(m) obtained is about -9.6 cm(-3)/mol. Above 302.8 MPa, the frequencies of CH(2) and CH(3) symmetric stretching and asymmetric stretching vibration shift to higher wave numbers in a linear manner with increasing pressure, which can be expressed as: nu(s)(CH(3))=0.013P+2882.0; nu(as)(CH(3))=0.014P+2961.6; nu(s)(CH(2))=0.013P+2850.8; nu(as)(CH(2))=0.009P+2923.2. This relationship indicates that n-tetradecane can be a reliable pressure gauge for the experimental study within the pressure range of 0.3-1.4 GPa.     

A novel antiferromagnetic organic superconductor kappa-(BETS)(2)FeBr(4) [where BETS = bis(ethylenedithio)tetraselenafulvalene].

The electrical and magnetic properties of kappa-(BETS)(2)FeBr(4) salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (T(N) = 2.5 K). The characteristic field dependence of the magnetization at 2.0 K indicates a clear metamagnetic behavior. The small resistivity drop observed at T(N) clearly shows the existence of the interaction between pi metal electrons and localized magnetic moments of Fe(3+) ions. In addition, this system underwent a superconducting transition at 1.1 K. That is, kappa-(BETS)(2)FeBr(4) is the first antiferromagnetic organic metal exhibiting a superconducting transition below Néel temperature. The magnetic field dependence of the superconducting critical temperature indicated that the superconductivity in this system is strongly anisotropic also in the conduction plane because of the existence of the metamagnetically induced internal field based on the antiferromagnetic ordering of the Fe(3+) 3d spins in contrast to the cases of the other conventional organic superconductors. Furthermore, the specific heat measurement exhibited a lambda-type large peak of zero-field specific heat corresponding to the three-dimensional antiferromagnetic ordering of high-spin Fe(3+) ions. The lack of distinct anomaly in the C(p) vs T curve at T(c) suggests the coexistence of the superconductivity and the antiferromagnetic order below T(c).     

The high-pressure electronic structure of the [Ni(ptdt)2] organic molecular conductor

The electronic structure of the single component molecular crystal [Ni(ptdt)(2)] (ptdt = propylenedithiotetrathiafulvalenedithiolate) is determined at ambient and high pressure using density functional theory. The electronic structure of this crystal is found to be of the "crossing bands" type with respect to the dispersion of the HOMO and LUMO, resulting in a small, non-zero density of states at the Fermi energy at ambient pressure, indicating that this crystal is a "poor quality" metal, and is consistent with the crystal's resistivity exhibiting a semiconductor-like temperature dependence. The ambient pressure band structure is found to be predominantly one-dimensional, reflecting enhanced intermolecular interactions along the [100] stacking direction. Our calculations indicate that the band structure becomes two-dimensional at high pressures and reveals the role of shortened intermolecular contacts in this phenomenon. The integrity of the molecular structure is found to be maintained up to at least 22 GPa. The electronic structure is found to exhibit a crossing bands nature up to 22 GPa, where enhanced intermolecular interactions increase the Brillouin zone centre HOMO-LUMO gap from 0.05 eV at ambient pressure to 0.15 eV at 22 GPa; this enhanced HOMO-LUMO interaction ensures that enhancement of a metallic state in this crystal cannot be simply achieved through the application of pressure, but rather requires some rearrangement of the molecular packing. Enhanced HOMO-LUMO interactions result in a small density of states at the Fermi energy for the high pressure window 19.8-22 GPa, and our calculations show that there is no change in the nature of the electronic structure at the Fermi energy for these pressures. We correspondingly find no evidence of an electronic semiconducting-metal insulator transition for these pressures, contrary to recent experimental evidence [Cui et al., J. Am. Chem. Soc. 131, 6358 (2009)].     

Disorder in BaThF6--refinement of anharmonic displacement parameters from high-pressure single-crystal X-ray diffraction data

The structure of BaThF(6) has been investigated as a function of temperature and pressure with single-crystal X-ray diffraction using synchrotron radiation. The compound crystallizes in the tysonite structure, space group P6(3)/mmc (a = 4.296(1) ? and c = 7.571(1) ? at ambient conditions). It is stable at least down to 150 K and up to 4 GPa. In the entire range of pressures and temperatures studied here, the compound is characterized by a high degree of disorder, both on the cationic and anionic positions. Despite the different valence states and sizes, both cations occupy the same crystallographic site in the ideal tysonite structure. The cationic disorder is described by two alternative approaches. The first model corresponds to a split-atom position model in which Ba(2+) is maintained on a special position with site symmetry 6m2, while Th(4+) is slightly displaced from the respective position. In the second model, both cations are maintained on the ideal position and anharmonic displacement parameters using a tensor of third order are introduced. Anharmonic displacement parameters have been refined from high-pressure single-crystal X-ray data measured in situ in a diamond anvil cell for the first time. The feasibility and general problems of anharmonic refinements of high-pressure X-ray data are further commented.     

High pressure synthesis of late rare earth RFeAs(O,F) superconductors; R = Tb and Dy

New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures T(c) = 46 and 45 K and very high critical fields, >/=100 T, have been prepared at 1100-1150 degrees C and 10-12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La-Nd, Sm, Gd) high temperature superconductors.