Pressure effects on NaMnF4: Structural correlations and Jahn-Teller effect from crystal-field spectroscopy

This work investigates the optical absorption spectrum of the NaMnF4-layered perovskite and its variation with pressure. The spectrum basically consists of three broadbands located at 1.916, 2.263, and 2.817 eV, which correspond to the crystal-field (CF) transitions 5B1g⃗5Γi (Γi=A1g, B2g, and Eg) with the Jahn-Teller- (JT-) distorted MnF63- complex (Mn3+ d4 configuration). In addition, there are two spin-flip 5B1g⃗3B1g peaks at 2.397 and 2.890 eV, which are activated by the exchange mechanism. Their variation with pressure reveals that the JT energy does not change significantly with pressure: ∂EJT/∂P=0.8 meV/GPa. Furthermore, the variation of the JT tetragonal splitting of the parent octahedral eg and t2g, termed Δe and Δt, respectively, clearly indicate that ∂Δe/∂P≪∂Δt/∂P, although Δe≈4Δt. The CF energies and their pressure shift are explained in terms of local structural changes within the MnF63- complex induced by pressure. The structural correlation analysis reveals that the reduction of the MnF63- JT distortion is smaller than the expected one on the basis of the crystal volume reduction, thus indicating tilt phenomena. This interpretation is supported by the decrease of in-layer Mn-F-Mn superexchange, such as is derived from the optical spectra. We demonstrate that the equatorial and axial distances decrease from 1.839 to 1.808 Å and from 2.167 to 2.107 Å, respectively, in the 0–10 GPa range

Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskite CsMnF4

The interplay between the orbital ordering and the spin state in Jahn-Teller Mn3+ governing the optical, magnetic, and transport properties in the layered CsMnF4 perovskite is investigated. Such electronic effects are strongly coupled to the lattice and thus can be modified by external pressure. However, there is very little understanding of the structural conditions which are required to attain spin crossover in connection with the electronic structure of Mn3+. The distortion, spin state, and tilting of MnF6 octahedra in the insulating ferromagnet CsMnF4 are jointly studied by high-pressure optical spectroscopy. The insulating character of CsMnF4 allowed us to explore the electronic structure associated with the 3d levels of Mn3+ in the 0–46 GPa pressure range, an information which is obscured in most oxides due to metallization at high pressure. We show that the spin-crossover transition, related to the spin change, S=2→1, in Mn3+, takes place at 37 GPa with the simultaneous suppression of the axially elongated distortion associated with the Jahn-Teller effect. The wide stability pressure range of the Jahn-Teller distortion and high-spin state is explained in terms of crystalfield models including the Jahn-Teller stabilization energy. On this basis, we discuss the interplay between spin transition and Jahn-Teller effect comparing present findings with other results attained in Mn3+, Ni3+, and Co3

Pressure effects on Jahn-Teller distortion in perovskites: The roles of local and bulk compressibilities

The interplay between the Jahn-Teller (JT) effect and octahedron tilting in transition-metal perovskites is investigated as a function of pressure. Our focus is on its effects on the exchange and electron-phonon interactions, both having a strong influence on materials properties. We demonstrate that the JT distortion in Cu2+ and Mn3+ is reduced upon compression and is eventually suppressed at pressures above 20 GPa. X-ray diffraction and x-ray absorption measurements in A2CuCl4 layer perovskites (A: Rb, CnH2n+1NH3; n = 1–3) show that, although pressure slightly reduces the long Cu-Cl distance in comparison to the Cu-Cu distance in the layer, the JT distortion is stable in the 0–20 GPa range. The difference between lattice (βC 0 = 0.14 GPa−1) and local CuCl6 (β0 = 0.016 GPa−1) compressibilities, together with the high stability of the JT distortion, lead to CuCl6 tilts upon compression. The evolution of the elongated CuCl6 octahedron in A2CuCl4, as well as MnF6 in CsMnF4 and MnO6 in LaMnO3 and DyMnO3, toward a nearly regular octahedron takes place above 20 GPa, in agreement with experimental results and a model analysis based on the JT energy derived from optical absorption spectroscopy: EJT = 0.25–0.45 eV/Cu2+, EJT = 0.45 eV/Mn3+ (CsMnF4), and EJT = 0.25 eV/Mn3+ (LaMnO3). The proposed model clarifies controversial results about pressure-induced JT quenching in Cu2+ and Mn3+ systems, providing an efficient complementary means to predict pressure behavior in perovskites containing JT transition-metal ions.Financial support from the Spanish Ministerio de Ciencia e Innovacion (Grant No. MAT2008-06873-C02-01/MAT) ´ is acknowledged. X-ray absorption and diffraction experiments were done at D11/LURE (Project PS203-01), and ID09A/ESRF (HS2159), respectively. The MALTA Consolider Ingenio 2010 program (CSD2007-00045) is also acknowledged

CaCu3Ti4O12: Pressure dependence of electronic and vibrational structures

The effects of pressure in electronic and vibrational properties of the double perovskite CaCu3Ti4O12 have been investigated in the 0-25 GPa range by optical absorption and Raman spectroscopy. Besides a full structural characterization, we aim at unveiling whether the ambient Im3 crystal structure is stable under high pressure conditions and how its giant dielectric permitivity and electronic gap varies with pressure. Results show that there is evidence of neither structural phase transition nor metallization in CaCu3Ti4O12 in the explored pressure range. We have observed the eight Raman active modes associated with its Im3 crystal phase and obtained their corresponding frequency and pressure shift. Moreover, the direct electronic band gap (2.20 eV), which is mainly associated with the oxygen-to-copper charge transfer states, increases slightly with pressure at a rate of 13 meV GPa?1 from 0 to 10 GPa. Above this pressure is almost constant (Eg = 2.3 eV). The results highlight the high stability of the compound in its Im3 phase against compression.This work has been supported by the Spanish Ministerio de Ciencia, Innovaci´on y Universidades (Projects MAT2015-69508-P, PGC2018-101464-B-I00 and MALTA-Consolider Team RED2018- 102612-T). E.J. thanks for an FPI research grant (Ref. BES-2016-077449)

Tunable interlayer hydrophobicity in a nanostructured high charge organo-mica

A tunable hydrophobicity, from a fully hydrophobic medium to an amphiphilic quasi-solution, has been obtained in the interlayer space of a synthetic high charged mica by ion exchange reaction with amine cations. The structural and intercalation properties of the hybrids after the exchange with the n-alkylammonium cations: [RNH3]+, [RNH(CH3)2]+ and [RN(CH3]3+ with C16 alkyl chain length have been determined by termogravimetric/differential scanning calorimetry analysis (TGA-DSC) and mass spectrometry (MS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Transmission electron microscopy (TEM) has been used as a complementary technique to provide new insights into the morphology of the exchanged products. Coverage and cation distribution have been correlated with layer charge and steric effects. Thus, a full organo-clay is obtained when the primary amine cations are adsorbed between the layers. However, a homogenous single phase of mixed organic/inorganic cations is formed in the same interlayer with the tertiary amine cations. Mixed ion clays combining both exchangeable inorganic and adsorbent organic ions in their interlayer space can be potential materials to be used as adsorbents for water decontamination, independently of the hydrophilic/hydrophobic nature of the pollutants. For the quaternary amine cations steric effects preclude the coexistence of both organic and inorganic species in the same interlayer of the clay so phase segregation together with a heterogeneous phase of organic and inorganic galleries in the same particle can be observed.Funding from the Ministerio de Economía y Competitividad, under project MAT2015-63929-R is also acknowledged. Fernando Aguado received funding from the Ministerio de Economía y Competitividad, project MAT2015-69508-P

Reversibility of the zinc-blende to rock-salt phase transition in cadmium sulfide nanocrystals

CdS nanoparticles prepared by a mechanochemical reaction in a planetary ball mill have been investigated by x-ray diffraction, optical absorption, and Raman scattering under high pressure conditions up to 11 GPa. The zinc-blende (ZB) to rock-salt phase transition is observed around 6 GPa in all experiments, the transition pressure being similar to the one measured in CdS colloidal nanocrystals, and much higher than in bulk (around 3 GPa). The direct optical energy gap in ZB-CdS increases with pressure, and suddenly drops when the pressure is raised above 6 GPa, according to the high-pressure indirect-gap behavior. A linear blue-shift of the CdS Raman spectra is observed upon increasing pressure. Both Raman and x-ray diffraction studies indicate that the phase transition has a large hysteresis, making the ZB phase barely recoverable at ambient conditions. Cell parameters and bulk modulus measured in CdS nanoparticles clearly show that the nanoparticles at ambient conditions are subject to an initial pressure in comparison to CdS bulk.This work was financially supported by the Spanish Ministerio de Ciencia e Innovación (Projects MAT2008-06873-C02-01/MAT, MAT2011-28868-C02-01 and CTQ2009-14596-C02-01), the Comunidad de Madrid and European Social Fund (Project S2009/PPQ-1551 4161893 (QUIMAPRES)), and the MALTA-Consolider Ingenio 2010 (Reference CSD2007-00045). R.M.-R. thanks the Spanish MEC for a FPI research grant (Reference BES-2006-13359). The expert assistance of L. Rodríguez at the SERTEM is gratefully acknowledged

Adsorptive capture of ionic and non-ionic pollutants using a versatile hybrid amphiphilic-nanomica

A versatile, functional nanomaterial for the removal of ionic and non-ionic pollutants is presented in this work. For that purpose, the high charge mica Na-4-Mica was exchanged with the cationic surfactant (C16H33NH(CH3)2)+. The intercalation of the tertiary amine in the swellable nano-clay provides the optimal hydrophilic/hydrophobic nature in the bidimensional galleries of the nanomaterial responsible for the dual functionality. The organo-mica, made by functionalization with C16H33NH3+, was also synthesized for comparison purposes. Both samples were characterized by X-ray diffraction techniques and transmission electron microscopy. Then, the samples were exposed to a saturated atmosphere of cyclohexylamine for two days, and the adsorption capacity was evaluated by thermogravimetric measurements. Eu3+ cations served as a proof of concept for the adsorption of ionic pollutants in an aqueous solution. Optical measurements were used to identify the adsorption mechanism of Eu3+ cations, since Eu3+ emissions, including the relative intensity of different f–f transitions and the luminescence lifetime, can be used as an ideal spectroscopic probe to characterize the local environment. Finally, the stability of the amphiphilic hybrid nanomaterial after the adsorption was also tested.We would like to thank IDIVAL for financial support, project number INNVAL19/1

Effect of pressure on the band gap and the local FeO6 environment in BiFeO3

BiFeO3 exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi3+ and Fe3+ cations. Here, we investigate the local structure of Fe3+ as a function of pressure through absorption crystal-field spectroscopy in the 0–18 GPa range. We focus on the influence of phase transitions on the Fe3+ off-center displacement through the energy (E) and oscillator strength (fd−d) of the 4T1 and 4T2 Fe3+ (3d5) bands observed below the band gap (Egap = 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both 4T1 and 4T2 bands, consistent with the compression of the FeO6 octahedron under pressure. On the other hand, the transition oscillator strength (fd−d = 3 × 10−5), enabled by both the exchange mechanism and the off-center Fe3+ distortion, slightly increases with pressure. The absence of notable anomalies in the variation of E(P) and fd−d (P) through the phase sequence from the ferroelectric rhombohedral R3c phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe3+. While this local polarity is correlated and expected in the ferroelectric R3c phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities.Financial support from the Spanish Ministerio de Educación y Ciencia (Project No MAT2008-06873-C02-01), and MALTA CONSOLIDER-INGENIO 2010 (Ref. CDS2007-0045) is acknowledged

Eu3+ luminescence in high charge mica: an in situ probe for the encapsulation of radioactive waste in geological repositories

Isolation of high-level radioactive waste (HLW) in deep geological repositories (DGR) through a multibarrier concept is the most accepted approach to ensure long-term safety. Clay minerals are one of the most promising materials to be used as engineered barriers. In particular, high charge micas, as components of the engineered barrier, show superselectivity for some radioactive isotopes and a large adsorption capacity, which is almost twice that of the other low charge aluminosilicates. In addition, high charge micas are optimum candidates for decontamination of nuclear waste through two different mechanisms; namely an ion exchange reaction and a nonreversible mechanism involving the formation of new stable crystalline phases under hydrothermal conditions. In this work, we report a new in situ optical sensor based on the incorporation of Eu3+ in these high charge micas for tracking the long-term physical-chemical behavior of HLW contaminants in DRG under mild hydrothermal conditions. The incorporation of Eu3+ into the interlayer space of the mica originates a well resolved green and red luminescence, from both the 5D1 and 5D0 excited states, respectively. The formation of new crystalline phases under hydrothermal conditions involves important changes in the Eu3+ emission spectra and lifetime. The most interesting features of Eu3+ luminescence to be used as an optical sensor are (1) the presence or absence of the Eu3+ green emission from the 5D1 excited state, (2) the energy shift of the 5D0 → 7F0 transition, (3) the crystal-field splitting of the 7F1 Eu3+ level, and (4) the observed luminescence lifetimes, which are directly related to the interaction mechanisms between the lanthanide ions and the silicate network.Funding from projects MAT2015-63929-R, MAT2015-69508-P, PI16/00496, and NVAL16/17-IDIVAL is gratefully acknowledged

Exploring the local environment of the engineered nanoclay Mica-4 under hydrothermal conditions using Eu3+ as a luminescent probe

High charge mica Na4Al4Si4Mg6O20F4, Mica-4, is a promising candidate as a filling material to immobilize high-level radioactive waste in deep geological repositories due to its extraordinary adsorption capacity. In contrast to traditional clay materials, the structural composition of this mica, with a high content of aluminum in the tetrahedral sheet, enhances its chemical reactivity, favoring the formation of new crystalline phases under mild hydrothermal conditions, and thus providing a definitive isolation of the radionuclides in the engineered barrier. Moreover, this synthetic clay has some features that allow its use as an optical sensor by doping with luminescent rare earth cations such as Eu3+. In this paper we discuss the local structure of the nanoclay Mica-4 using Eu3+ as a local probe to track the physical and chemical modifications under hydrothermal conditions. For that purpose, a set of hydrothermal experiments has been carried out heating Mica-4 and an aqueous Eu(NO3)3 solution in a stainless steel reactor at different temperatures and times. Optical properties of the as-treated samples were characterized by spectroscopic measurements. The fine peak structure of emission and the relative intensity of different Eu3+ transitions as well as the luminescence lifetime have been correlated with the structure and composition of this nanoclay, and the interaction mechanisms between the lanthanide ions and the clay mineral at different temperatures and times. Special attention has been paid to understanding the role of the aluminum content, which may act as either an aggregating or dispersing agent, in the optical features and reactivity of the system.We would like to thank Instituto de Investigación Marqués de Valdecilla (IDIVAL) (Projects NVAL16/17 and INNVAL19/18) and Ministerio de Ciencia, Innovación y Universidades (Project PGC2018–101464-B-100) for financial support