Fernando Aguado

High-pressure behaviour of KMF3 perovskites

The structural stability of cubic KMF3 (M: Mg, Zn, Co, Ni) perovskites has been studied by powder X-ray diffraction under pressure. Neither superlattice reflections nor peak splitting associated to a phase transition were detected in the 0–10 GPa pressure range. Furthermore, KMgF3 showed no structural changes up to 50 GPa. The results were compared with previous reports on isostructural KMnF3. Tolerance factor and site parameters ratio have been analysed as high-pressure stability indexes for this family of perovskites.

Correlations between structure and optical properties in Jahn–Teller Mn3+ fluorides: A study of TlMnF4 and NaMnF4 under pressure

This work investigates the Jahn–Teller (JT) distortion in different Mn3+ fluoride series by optical absorption (OA) spectroscopy. The aim is to establish correlations between the local structure of the formed MnF63− derived from x-ray diffraction and the JT splitting associated with the parent octahedral 5Eg(3z2−r2,x2−y2) and 5T2g(xy,xz,yz) states, Δe and Δt, obtained from the OA spectrum. A salient feature is the linear relation exhibited by both Δe and Δt with the tetragonal coordinate Qθ along the whole series. From these relations we derive suitable electron-ion coupling coefficients related to the 5Eg and 5T2g states whose values play a key role in the e⊗E and e⊗T JT theory, respectively. The results of these correlations are applied to investigate the structural variations undergone by the two-dimensional compounds NaMnF4 and TlMnF4 under pressure using OA spectroscopy. Interestingly, the analysis carried out is relevant since it provides useful information on the Mn3+ local structure, a task that i...

Bulk and Molecular Compressibilities of Organic–Inorganic Hybrids [(CH3)4N]2MnX4 (X = Cl, Br); Role of Intermolecular Interactions

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 ν₁(A₁) Raman mode of MnCl₄²⁻ with pressure in [(CH₃)₄N]₂MnCl₄ allowed us to determine the associated Grüneisen parameter (γ(loc) = 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⁻ (γ(loc) = 1.45) and (CH₃)₄N⁺ (γ(loc) = 3.0) in [(CH₃)₄N]₂MnBr₄.

Pressure Effects on the Cooperative Jahn-Teller Distortion in AMnF 4 (A=Na, Tl)

This work investigates the Optical Absorption spectrum of the AMnF 4 layer perovskites of Mn 3+ , and its variation with the pressure. We show that the crystal-field transition energies and their pressure shifts provide a very useful information about the local structural changes in the \hbox{MnF}_6^{3-} complex and how it changes with the pressure, once the correlations between crystal-field electronic structure and coordination geometry around Mn 3+ has been established. Along this work we demonstrate that the equatorial and axial distances decrease from 1.844 to 1.813 + , and from 2.167 to 2.090 + , respectively, in the 0-100 kbar range, leading to a partial reduction of the Jahn-Teller distortion.

Unraveling the coordination geometry of copper(II) ions in aqueous solution through absorption intensity.

Solvation of transition-metal (M) salts in aqueous solution is a seminal issue in coordination chemistry and has implications for life, the environment, and industry. A fundamental aspect like the formation of hexaaqua transition-metal complexes M(H2O)6 2+ is unclear for Cu ions and still needs clarification. Since the pioneering studies of Cu aqueous solutions by optical spectroscopy, and later by neutron diffraction, X-ray absorption, nuclear magnetic resonance, quantum mechanical calculations, and molecular dynamics simulations, the real Cu coordination is controversial and lacks of appropriate structural models. In particular, efforts to detect formation of Cu(H2O)5 2+ dynamically exchanging single water molecules among equivalent pyramidal configurations were ineffective through local probes like X-ray absorption or optical absorption. The similitude of the d-d electronic spectra of Cu ions in aqueous solution and Tutton salts like Cs2Cu(SO4)2·6H2O, where copper forms an axially elongated Cu(H2O)6 2+ octahedron of D4h symmetry, [17] suggested that the same complex is present in solution. In fact, chemistry lab courses often use optical absorption spectroscopy to underline the suitability of the Cu(H2O)6 2+ complex as a basic unit for explaining the band structure at 1.5 eV and its associated blue color in both the hydrate salt and aqueous solution in terms of a Jahn– Teller (JT) distorted complex. However, in light of recent dynamical studies, instead of a stable hexaaqua complex in water solution, Cu(H2O)6 2+ ought to be regarded as a timeaveraged system rather than the instantaneous coordination of the actual complex, which is assumed to be mainly Cu(H2O)5 . 10,15] Nevertheless, X-ray and optical spectroscopy with the support of first-principle calculations do not clarify whether the actual coordination corresponds to Cu(H2O)5 , Cu(H2O)6 , or an intermediate geometry. Instead several Cu(H2O)n 2+ species with n ranging from 4 to 6 were proposed on the basis of X-ray absorption analysis. The closely related free-energy calculated for these species indicates the delicate balance of the complex stability in aqueous solution. Furthermore, the calculated electronic density of states related to d orbitals provides similar spectral shapes for those coordination geometries, making them barely distinguishable through optical spectroscopy. This peculiarity, together with dilution in water, makes the Cu structure in aqueous solution subtle and difficult to unmask, but it still constitutes a fundamental issue in coordination chemistry. Experimentally, transition energy and oscillator strength of crystal-field d-d transitions can be explained on the basis of the transition-metal complex irrespective of the host (solid or liquid). This is due to the local character of d orbitals involved in the crystal-field transitions. The main role of the host is to exert an effective (chemical) pressure on the complex that can eventually modify both, bond distances and symmetry. Such structural changes can be detected by spectroscopic techniques. In this way the correlations between optical absorption spectroscopy and crystal structure are commonly used to elucidate the actual symmetry of transition-metal ions in proteins. In particular, the oscillator strength is a sensitive parameter to distinguish between centrosymmetric and noncentrosymmetric systems. Bearing all this in mind, we have undertaken a comparative spectroscopic study between hexaaquaD4h Cu(H2O)6 2+ in Cs2Cu(SO4)2·6H2O salt, average CuO5 in glass and Cu 2+ ions in aqueous solution. Searching for simple ideas to intricate problems, we demonstrate that, unlike the d band structure, the absorption intensity provides key information to unveil the local symmetry of Cu ions in aqueous solution. For instance, the oscillator strength of d-d transitions (fd-d) is known to decrease by an order of magnitude upon transformation of CuO5 (C4v) into CuO6 (D4h) in CuMoO4. [19] This result makes it an efficient parameter to explore the coordination of the Cu ion. Figure 1 compares similar bluish Cu systems containing well-defined coordination geometries: the Tutton salt Cs2Cu(SO4)2·6H2O with elongated Cu(H2O)6 2+ of D4h symmetry, [20] CuSO4·5H2O with elongated Cu[(H2O)4O2] 2 with two oxygen ligands from SO4 2 anions, and Cu-doped glass with average CuO5 coordination. [22] Although all these systems exhibit quite a similar color (absorption spectrum), the strength of blue portion is markedly different in each system as a consequence of the different coordination polyhedron and the Cu concentration. The pale-blue of the Tutton salt contrasts with the stronger blue of CuSO4·5H2O in spite of the fact that the d-d transition oscillator strength of the band at 1.5 eVat room temperature (RT) is similar in both systems (fd-d= 5.8 10 5 and 6.8 10 , [*] S. G mez-Salces, Dr. F. Aguado, Prof. F. Rodr guez MALTA Consolider Team, DCITIMAC, Facultad de Ciencias Universidad de Cantabria 39005 Santander (Spain) E-mail: fernando.aguado@unican.es

The crystal structure of CaIrO3 post-perovskite revisited

Abstract The structure of CaIrO3 post-perovskite has been determined using CAD4 and Bruker SMART diffractometers for two different crystals. In both cases the structure was solved by Direct Methods which revealed all four atoms in the asymmetric unit. Anisotropic refinement of all atoms gave (CAD4/SMART): R1 = 0.016/0.008, wR2 = 0.032/0.009. There is excellent agreement between the structure models of the two studies. However, some notable differences were found between the structures determined here and that reported by Sugahara et al., (2008): most obviously, we observe no significant anisotropic displacements for any of the atoms. Anisotropic displacement parameters for Ca, Ir and O atoms are very similar to those of their counterparts in MgSiO3 perovskite. The close similarity of the structures of the two crystals determined here using two very different diffractometers suggests that the derived model structure reported here is better-constrained and more representative of the characteristic features of CaIrO3 post-perovskite than that obtained by Sugahara et al., (2008).

Three-dimensional magnetic ordering in the Rb2CuCl4 layer perovskite—structural correlations

This work investigates the magnetic structure of Rb2CuCl4 as a function of pressure and temperature using neutron diffraction. As in most A2CuCl4 layered perovskites, there is a 2D ferromagnetic order within the layers. This behaviour is due to the Jahn–Teller (JT) antiferrodistortive structure of the CuCl6 units. Rb2CuCl4 undergoes a 3D magnetic transition at TN = 16 K, which mainly depends on the weak antiferromagnetic interlayer interaction. The pressure slightly increases TN ,a s∂TN/∂P = 0.13 K kbar −1 .T his behaviour is interpreted in terms of pressure-induced tilts and reduction of interlayer distance, both effects increasing the antiferromagnetic exchange coupling between layers. The results are compared with previous magnetic studies under chemical and hydrostatic pressure along layered perovskites series of [CnH2n+1NH3]2CuCl4 (n = 1–3) and BMnF4 (B = Li, Na, K, Rb, Tl, Cs and NH4 )i nvol ving JT ions of Cu 2+ and Mn 3+ ,r esp ectively. We show that the ratio of the interlayer to intralayer coupling, and thus the nature of the magnetic order, can be tuned by chemical or hydrostatic pressure along the A2CuCl4 series. The present findings stress the relevance of octahedral tilts on the magnetic behaviour of layered perovskites.

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 pressu...

Discontinuous temperature-dependent macroscopic strain due to ferroelastic domain switching and structural phase transitions in barium strontium titanate

Remnant strain has been measured as a function of temperature in (Ba0.8Sr0.2)TiO3 (BST) ceramic by mechanical poling in three point bending configuration. BST ceramic exhibits recoverable macroscopic strain with shape memory effect and three jumps in the temperature-dependent strain during thermal cycling under applied force. The jumps are associated with the three structural phase transitions of BST, as confirmed by the simultaneous measurements of dynamic modulus and internal friction. In addition, the orthorhombic phase of BST exhibits a significantly higher strain comparing to that in the tetragonal and rhombohedral phases. X-ray diffraction confirms that the macroscopic strain is due to ferroelastic domain switching and particularly the dominant contribution to the higher macroscopic strain at orthorhombic phase is the higher probability of non-180° domain switching rather than the variation of domain switching strain at different phases.

Pressure-induced closure of the jahn-teller distortion in Rb2CuCl4(H2O)2

This work investigates the pressure-induced variation of the local structure around Cu2+ as well as the crystal structure in Rb2CuCl4(H2O)2 through XAS and XRD techniques. The application of pressure induces a structural change in the Jahn-Teller (JT) [Formula: See Text] complex from axially elongated to compressed. This change leads to the closing of the 2D JT distortion related to the four in-plane Cl− ligands, which are responsible for the antiferrodistortive structure displayed by the crystal. It is shown that the presence of water ligands enhances a JT release. Their associated axial ligand-field favours the occurrence of such a local structural transition below the metallization pressure. The results are compared with recent pressure experiments on A2CuCl4 systems.