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This work reports an X-ray diffraction, X-ray absorption, and Raman spectroscopy study of [(CH3)4N]2MnX4 (X = Cl, Br) under pressure. We show that both compounds share a similar phase diagram with pressure. A P21/c monoclinic structure describes precisely the [(CH3)4N]2MnCl4 crystal in the 0.1-6 GPa range, prior to crystal decomposition and amorphization, while [(CH3)4N]2MnBr4 can be described by a Pmcn orthorhombic structure in its stability pressure range of 0-3 GPa. These materials are attractive systems for pressure studies since they are readily compressible through the weak interaction between organic/inorganic [(CH3)4N+/MnX42-] tetrahedra through hydrogen bonds and contrast with the small compressibility of both tetrahedra. Here we determine the equation-of-state (EOS) of each crystal and compare it with the corresponding local EOS of the MnX42- and (CH3)4N+ tetrahedra, the compressibility of which is an order and 2 orders of magnitude smaller than the crystal compressibility, respectively, in both chloride and bromide. The variations of the Mn-Cl bond distance obtained by extended X-ray absorption fine structure and the frequency of the totally symmetric v1(A1) Raman mode of MnCl42- with pressure in [(CH3)4N]2MnCl4 allowed us to determine the associated Grüneisen parameter (γloc = 1.15) and hence an accurate local EOS. On the basis of a local compressibility model, we obtained the Grüneisen parameters and corresponding variations of the intramolecular Mn-Br and C-N bond distances of MnBr42- (γloc = 1.45) and (CH3)4N+ (γloc = 3.0) in [(CH3)4N]2MnBr4. [ABSTRACT FROM AUTHOR]

This work investigates the Mn2+ electronic structure and exciton dynamics in one-dimensional (1D) N(CH3)4MnCl3 (TMMC) through time-resolved excitation/emission spectroscopy and absorption measurements in the 0-10 GPa pressure range for different Cu2+ doping concentrations. The local and crystal structures have been analyzed by Raman spectroscopy and x-ray absorption measurements at the Mn K edge showing that the 1D chain structure is maintained in the whole explored pressure range. We show that both the first Mn2+ absorption band, 4T1(G), and its associated emission band experience very large pressure redshifts, which are associated with the crystal anisotropy providing large axial ligand fields at the Mn2+ site that increase with pressure. The red emission at 633 nm shows a large pressure variation of 22 nm/GPa (50 meV/GPa) making TMMC a suitable probe for using as a photoluminescence (PL) pressure gauge in the low-pressure regime. The energy-transfer exciton dynamics and trapping at non-PL centers have been explained through changes of the intrachain Mn-Mn exchange interaction and Cu2+-trap concentration carried out by applying pressure and doping, respectively. The model demonstrates that an increase of exchange interaction favors both the pumping capability and energy transfer yielding exciton migration. Under these conditions, we show that pressure enhances the PL efficiency of TMMC provided that the Cu2+ concentration responsible for the PL quenching is below 0.001 mol %. However, between 0.001% and 0.1%, the PL intensity reduces with pressure, and above 0.1%, the PL is practically quenched even at ambient conditions. [ABSTRACT FROM AUTHOR]

This work investigates the electronic structure and photoluminescence (PL) of Co2+-doped MgAl2O4 and their pressure dependence by time-resolved spectroscopy. The variations of the visible absorption band and its associated emission at 663 nm (τ = 130 ns at ambient conditions) with pressure/temperature can be explained on the basis of a configurational energy model. It provides an interpretation for both the electronic structure and the excited-state phenomena yielding photoluminescence emission and die subsequent quenching. We show that there is ah excited-state crossover (ESCO) [4T1(P) ↔2E(G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 K and 100 K to a narrow-line emission at lower temperatures. Contrary to expectations from the Tanabe-Sugano diagram, instead of enhancing ESCO phenomena, pressure reduces PL and even suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced nonradiative relaxation to lower excited states: 2E(G) → 4T1(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation. [ABSTRACT FROM AUTHOR]

This work investigates the Co2+(Td) electronic structure and photoluminescence (PL) in MgAl2O4:Co2+ and their variation with pressure by time-resolved spectroscopy. The absorption spectra and the pressure and temperature dependences of the emission band at 663 nm (τ=120 ns, at ambient conditions) can be explained on the basis of a configurational energy model, which provides an interpretation of the electronic structure and the excited-state phenomena. We show that there is an excited-state crossover [4T 1(P)↔2E (G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 and 100 K to a narrow-peak emission at 6 K. The pressure, instead of enhancing excited-state crossover phenomena, reduces PL and suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced non-radiative crossover relaxation to lower excited states: 2E (G)→4T 1(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation. [ABSTRACT FROM AUTHOR]

The interplay between spin states and metallization in compressed CoCl2 is investigated by combining diffraction, resistivity and spectroscopy techniques under high-pressure conditions and ab-initio calculations. A pressure-induced metallization along with a Co2+ high-spin (S = 3/2) to low-spin (S = 1/2) crossover transition is observed at high pressure near 70 GPa. This metallization process, which is associated with the p-d charge-transfer band gap closure, maintains the localization of 3d electrons around Co2+, demonstrating that metallization and localized Co2+ -3d low-spin magnetism can coexist prior to the full 3d-electron delocalization (Mott-Hubbard d-d breakdown) at pressures greater than 180 GPa. [ABSTRACT FROM AUTHOR]

The interplay between spin states and metallization in compressed CoCl2 is investigated by combining diffraction, resistivity and spectroscopy techniques under high-pressure conditions and ab-initio calculations. A pressure-induced metallization along with a Co2+ high-spin (S = 3/2) to low-spin (S = 1/2) crossover transition is observed at high pressure near 70 GPa. This metallization process, which is associated with the p-d charge-transfer band gap closure, maintains the localization of 3d electrons around Co2+, demonstrating that metallization and localized Co2+ -3d low-spin magnetism can coexist prior to the full 3d-electron delocalization (Mott-Hubbard d-d breakdown) at pressures greater than 180 GPa. [ABSTRACT FROM AUTHOR]

X‐ray absorption spectroscopy is a powerful tool to probe in situ changes in the local structure of a material induced by pressure. In article no. 1800073 it was used by Kuzmin et al. to study the behavior of copper nitride (Cu3N) crystal lattice at high pressure (0–26.7 GPa). The analysis of the Cu K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) suggests that at all pressures the local atomic environment around copper atoms remains close to that in cubic Pm‐3m phase. At the same time, a reduction of the lattice parameter of Cu3N by about 2% was found upon increasing pressure up to 26.7 GPa. These results indicate that the transition to the metal state in Cu3N above 5 GPa, observed previously using pressure‐dependent electrical resistance and optical absorption measurements, is due to the band gap collapse with a decrease of the unit cell volume. [ABSTRACT FROM AUTHOR]

The magnetic properties of a series of trimetallic (Co,Ni)Fe Prussian blue analogues (PBAs) were investigated by SQUID magnetometry and X-ray magnetic circular dichroism (XMCD) at the three transition metal (TM) K-edges. In turn, the PBA trimetallic series was used as a tool in order to better understand the information contained in TM K-edge XMCD and particularly the chemical nature of the probed species (extended sub-lattice or localized entities). The results show that the magnetic behavior of the compounds is dictated by competing exchange interactions between the Co-Fe and Ni-Fe pairs, without spin frustration. They also show that XMCD at the TM K-edge is a local atomic probe of the element at the N side of the cyanide bridge and a local probe of the absorbing atom and its first magnetic neighbors on the C side of the bridge. At last, XMCD at the TM K-edge turns out to be highly sensitive to very small structural distortions.

Polycrystalline Sr2Fe1+xRe1-xO6 samples have been synthesized and structurally characterized by X-ray powder diffraction, transmission electron microscopy and X-ray absorption spectroscopy. Resistivity strongly increases with x, but a large and negative magnetoresistance persists up to x = 0.33. This is discussed considering the charge delocalization in iron and rhenium t2g orbitals.

A new oxide NaCu 3 Fe 2 Os 2 O 12 is synthesized using high pressure and temperature conditions. The Rietveld structural analysis shows that the compound possesses both A- and B-site ordered quadruple perovskite structure in Pn3̅ symmetry. The valence states of transition metals are confirmed to be Cu 2+ /Fe 3+ /Os 5.5+ . The three transition metals all take part in magnetic interactions and generate strong Cu 2+ (↑)Fe 3+ (↑)Os 5.5+ (↓) ferrimagnetic superexchange interactions with a high Curie temperature about 380 K. Electrical transport measurements suggest its half-metallic properties. The first-principles theoretical calculations demonstrate that the compound has a spin-down conducting band and a spin-up insulating band with a wide energy gap.

X-ray diffraction and ultrasonic measurements have been carried out simultaneously up to 7 GPa at ambient temperature on a polycrystalline sample of Fe64Ni36 Invar alloy. The bulk modulus is found to increase linearly with pressure with an unusual low value (1.4) of dB/dP up to about 3.1(2) GPa followed by a regular slope (3.6) at higher pressure. The observation of these two distinct regimes is in qualitative agreement with previous results on the variation of the iron magnetic moment, and can be interpreted using the 2gamma-state model in terms of gradual population of low spin-small volume state at the expense of the high spin-large volume state under pressure.