P. Berastegui

Synthesis and structural studies of Sr2Co2−xGaxO5, 0.3⩽x⩽0.8

The present thesis deals with the investigation of some perovskite related complex cobaltates. The phases Sr2Co2-xGaxO5 (0.3 ≤ x ≤ 0.7), Sr2Co2-xAlxO5 (0.3 ≤ x ≤ 0.5), Sr1-xBixCoO3-y (0.1 ≤ x ≤ 0.2), Sr0.75Y0.25Co1-xGaxO2.625 (0.125 ≤ x ≤ 0.375) and Sr0.75Y0.25Co1-xFexO2.625+δ (0.125 ≤ x ≤ 0.625) were synthesised and characterised. All these compounds crystallises with similar structures, they are all composed by altering layers of octahedra and tetrahedra although in the two former, the tetrahedra are organised in chains (the Brownmillerite structure), while in the latter three the tetrahedra arranges as segregated Co4O12 units (the 314 type structure). The techniques X-ray and neutron diffraction, transmission electron microscopy, thermal analysis and magnetic measurements were used to track structural and important physical properties.

Structure and conductivity of some fluoride ion conductors

Abstract The structure and conductivity of a number of doped β-PbF 2 and BiF 3 compounds have been investigated using neutron diffraction and impedance spectroscopy. Anion-deficient compounds in the system PbF 2 –MF with M=K, Rb display a high-temperature superionic phase with a body-centred cubic sublattice while an ordered cubic perovskite structure is observed for the composition PbCsF 3 . In the system PbF 2 –SnF 2 , the high-temperature structure of γ-PbSnF 4 with cubic symmetry has been characterised. The solid solutions in the system PbF 2 –BiF 3 , where vacancy clustering is observed, and in the systems BiF 3 –KF and BiF 3 –RbF have also been investigated at high temperature.

TlF and PbO under High Pressure: Unexpected Persistence of the Stereochemically Active Electron Pair

Even under a pressure of 46 GPa, the low-symmetry lone-pair structures of isoelectronic TIF and PbO (see picture for β-PbO), classic examples of systems with a stereochemically active lone pair, resist transformation into the corresponding high-symmetry NaCl and CsCl structures. Ab initio calculations allowed a simple bonding picture for lone-pair structures involving inert-pair elements to be developed.

Ag+ diffusion within the rock-salt structured superionic conductor Ag4Sn3S8

The temperature dependence of the crystal structure of the compound Ag4Sn3S8 and its Ag+ ionic conduction mechanisms have been investigated using impedance spectroscopy measurements, powder neutron diffraction studies and computer simulations. Under ambient conditions, Ag4Sn3S8 adopts a cubic structure in space group P4132 with a = 10.808 98(7) A and Z = 4. The S2− form a slightly distorted fcc structured anion sublattice with the 16 × Ag+ and 12 × Sn4+ arranged in an ordered manner over the 32 available octahedrally coordinated cavities. As a result, the crystal structure of Ag4Sn3S8 can be described as a slightly deformed, cation deficient rock-salt arrangement. Highly anisotropic thermal vibrations of the Ag+ are observed, especially at elevated temperatures. Above K, analysis of the powder neutron diffraction data provides evidence for limited thermally induced cation disorder, associated with an increasing occupancy of the empty octahedrally coordinated positions. This behaviour is successfully reproduced within molecular dynamics simulations, which identify two different Ag+ diffusion mechanisms which occur via a subset of the tetrahedrally coordinated interstices. A brief discussion of the wider issue of superionic behaviour within rock-salt structured compounds is also given.

The crystal structure of α-PbSnF4 and its anion diffusion mechanism

The crystal structure of PbSnF4 and the nature of the anion diffusion mechanism which characterizes its high ionic conductivity have been investigated by impedance spectroscopy, powder neutron diffraction and computer simulation methods. The ionic conductivity of PbSnF4 undergoes small, but abrupt, increases at 608(4) and 672(3)?K characteristic of the and phase transitions. The ambient temperature ?-PbSnF4 phase possesses a tetragonal crystal structure (space group P4/nmm), derived from the cubic fluorite arrangement by ordering of the cations in the scheme PbPbSnSnPbPb along the [001] direction. However, the Sn2+?Sn2+ layers contain essentially no F?, with the displaced anions residing in the Pb2+?Sn2+ layers and showing significant disorder, particularly at temperatures close to the upper limit of stability of the ? phase. Computer simulations, using interionic potentials derived from first-principles calculations and containing realistic representations of polarization effects, are in good agreement with the measured ionic conductivity and successfully reproduce the experimentally determined ionic distribution. Analysis of the simulated ionic motions demonstrate that the impressive ionic conductivity of ?-PbSnF4 at temperatures close to ambient is a consequence of anion diffusion within the Pb2+?Sn2+ layers, whilst those F? within the Pb2+?Pb2+ layers are immobile. At temperatures close to the melting point of the simulated system, increased transfer of anions between the various Pb2+?Pb2+, Pb2+?Sn2+, Sn2+?Sn2+ layers is observed, as the system tends towards a more isotropic anion diffusion process.

Structural and superionic properties of Ag+-rich ternary phases within the AgI?MI2 systems

The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag2CdI4, Ag2ZnI4 and Ag3SnI5 have been investigated by powder diffraction and impedance spectroscopy techniques. e-Ag2CdI4 adopts a tetragonal crystal structure under ambient conditions and abrupt increases in the ionic conductivity are observed at 407(2), 447(3) and 532(4) K, consistent with the sequence of transitions e-Ag2CdI 4 → β-Ag2CdI 4 + β-AgI + CdI2 → α-AgI + CdI2 → α-Ag2CdI4. Hexagonal β-Ag2CdI4 is metastable at ambient temperature. The ambient-temperature β phase of Ag2ZnI4 is orthorhombic and the structures of β-Ag2CdI4 and β-Ag2ZnI4 can, respectively, be considered as ordered derivatives of the wurtzite (β) and zincblende (γ) phases of AgI. On heating Ag2ZnI4, there is a 12-fold increase in ionic conductivity at 481(1) K and a further eightfold increase at 542(3) K. These changes result from decomposition of β-Ag2ZnI4 into α-AgI + ZnI2, followed by the appearance of superionic α-Ag2ZnI4 at the higher temperature. The hexagonal crystal structure of α-Ag2ZnI4 is a dynamically disordered counterpart to the β modification. Ag3SnI5 is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (σ ≈ 0.19Ω−1 cm−1 at 420 K) due to dynamic disorder of the Ag+ and Sn2+ within a cubic close packed I− sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI–MI2 compounds is discussed, with reference to recently published studies of Ag4PbI6 and Ag2HgI4 and new data for the temperature dependence of the ionic conductivity of the latter compound.

From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: A Remarkable Onset of Zn‐Cluster Formation in an Intermetallic Framework

The series of isotypic compounds V8Ga41 --> V8Ga36.9Zn4.1 --> Cr8Ga29.5Zn11.2 --> Mn8Ga27.4Zn13.6 with the V8Ga41 structure type (space group R3, Z = 3) was prepared and structurally characterised by X-ray diffraction experiments (V8Ga41: a 13.9351(5), 14.8828(12); V8Ga36.9Zn4.1: a = 13.9244(7), c = 14.8660(9): Cr8Ga29.8Zn11.2: 13.7153(5), c = 14.6872(9); Mn8Ga27.4Zn13.6: a = 13.6033(6), c = 14.6058(16)). The site occupancies of the ternary compounds were refined from neutron powder-diffraction data and exposed a startling segregation of Zn and Ga, which finally resulted in the formation of separated Zn13 cluster entities-corresponding to almost ideal centred cuboctahedra or small pieces of fcc metal-in the Mn compound, which has the highest Zn content in the series. The homogeneity ranges of the underlying phases T8Ga41 xZnx were determined to be 0 < x < 4.1(3), 8.7(3) < x < 11.2(3) and 13.6(4) < x < 16.5(3) for T = V, Cr and Mn, respectively. The different ranges of composition of the phases reflect the requirement of an optimum electron concentration for a stable V8Ga41-type structure, which is in the narrow range between 159 and 165 electrons per formula unit. First-principles electronic-structure calculations could explain this fact by the occurrence of a pseudo gap in the density of states at which the Fermi level is put for this particular electron concentration. Furthermore the nature of the Zn/Ga segregation was revealed: T-Zn interactions were found to be considerably weaker than those for T-Ga. This places the Zn atoms as far as possible from the T atoms, thus leading to the formation of cuboctahedral Zn13 entities.

Structural description of the superionic behaviour in the system (AgI)x–(PbI2)1−x, 2/3≤x≤4/5

Abstract The crystal structure and ionic conductivity of the Ag + -rich phases in (AgI) x –(PbI 2 ) 1− x have been investigated by powder neutron diffraction and impedance spectroscopy studies of samples with x =2/3 and x =4/5. The phase diagram in this region appears to be somewhat simpler than that presented previously with a single phase stable at T ≥420 K ( x =2/3) and T ≥400 K ( x =4/5). This is a superionic phase, with σ ~0.1 Ω −1 cm −1 at T =420 K, in which the anions form a face centred cubic ( f.c.c. ) sublattice of lattice parameter a ~6.34 A. We therefore label this phase as f.c.c. -Ag x Pb 1− x I 2− x . The structure adopts the centrosymmetric space group Fm 3 m , with the majority of cations (over 90%) located in the octahedral 4( b ) cavities and the remainder within the tetrahedral 8( c ) interstices. Whilst this sixfold co-ordination to I − is to be expected for Pb 2+ , it is relatively unusual for Ag + , especially within a superionic phase. To highlight this behaviour, comparison is made with the f.c.c. -structured superionic phase α-Ag 2 HgI 4 . On cooling, the f.c.c. -Ag x Pb 1− x I 2− x phase with x =4/5 dissociates at T ≈400 K to form AgI and PbI 2 . The former adopts its zinc blende-structured γ phase whilst, unusually, the latter is stabilised in its 6 R polytype rather than the normal 2 H modification. The γ-AgI+6 R -PbI 2 mixture appears to be stable at ambient temperature indefinitely. For the x =2/3 case, 6 R -PbI 2 appears separately on cooling at ≈420 K and, on further cooling, complete dissociation to form a two-phase mixture of γ-AgI and 6 R -PbI 2 occurs by ≈390 K.

A cubic calcium oxynitrido-silicate, Ca2.89Si2N1.76O4.24

The title compound, tricalcium oxynitride silicate, with composition Ca3-xSi2N2-2xO4+2x (x ≃ 0.12), is a perovskite-related calcium oxynitrido silicate containing isolated oxynitrido silicate 12-rings. The N atoms are statistically disordered with O atoms (occupancy ratio N:O = 0.88:0.12) and occupy the bridging positions in the 12 ring oxynitrido silicate anion, while the remaining O atoms are located at the terminal positions of the Si(O,N)4 tetrahedra. The majority of the Ca2+ cations fill the channels along [100] in the packing of the 12-ring anions. The rest of these cations are located at several positions, with partial occupancy, in channels along the body diagonals.

Raman active modes in Nd2BaCu3Oz compound

Abstract In the present paper, group theoretical analysis was carried out for the insulating phase Nd 2 BaCu 3 O z (Nd213). Raman spectra were collected for different polarization geometry using bulk sample and micro setup. The polarization analysis of Raman active lattice modes was performed and the origin of additional mode in oxygen-deficient samples was discussed.