Table of contents

The influence of pressure on the crystal structure of LaGaO₃ has been studied using synchrotron x-ray diffraction. At atmospheric pressure LaGaO₃ has an orthorhombic structure in Pnma and undergoes a first-order phase transition to a rhombohedral structure near 2.5 GPa. The transition is reversible and the orthorhombic structure is recovered upon releasing the pressure below 2 GPa.

The structure of (1-x)[Pb(Mg_1/3Nb_2/3)O₃]-xPbTiO₃ is tetragonal and rhombohedral for x⩾0.35 and x⩽0.30, respectively. The intrinsic width of the morphotropic phase boundary region (0.30<x<0.35) is an order of magnitude smaller than hitherto believed. The structure of the morphotropic phase for x = 0.34 is shown to be monoclinic with space group Pm and not a mixture of rhombohedral and tetragonal phases.

Through a simple gas reaction, single-crystalline β-Ga₂O₃ nanoribbons, nanosheets and nanowires were synthesized on a large scale at 700 °C. They were characterized by means of x-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and Raman spectroscopy. FE-SEM and TEM images showed that the product was composed of nanoribbons, nanosheets and nanowires. XRD, EDS, HRTEM, SAED and the Raman spectrum indicated that all of the nanoribbons, nanosheets and nanowires were of single-crystalline β-Ga₂O₃.

A new mechanism of simultaneous appearance of ferromagnetism and superconductivity based on interaction of electrons mediated by localized spins was recently proposed by Suhl. Here the superconducting critical temperature is calculated for this model and conditions for appearance of superconductivity are analysed. It is shown that this mechanism can lead only to an s-wave order parameter. Superconductivity appears together with ferromagnetism but persists only until the ferromagnetism is weak. In order for it not to be destroyed by the paramagnetic effect, the metal has to be of a heavy-fermion type. This all fits recent experimental data obtained for UGe₂.

⁵⁷Fe Mössbauer spectra of Fe₃O₂BO₃ reveal a combined effect of charge ordering and electron delocalization between 112 and 450 K. On the basis of the temperature dependence of the isomer shifts and quadrupole interactions, together with the information from previously obtained transport data, we are able to discuss the arrangement of Fe²⁺ and Fe³⁺ in the structure and the dynamics of the electronic configurations. We found a charge-delocalization transition around 300 K. Below this temperature, formation of pairs of Fe ions with mixed valence takes place in part of the crystalline structure.

Mixed-valence perovskite manganites (Re_1-xA_xMnO₃ where Re = rare earth, A = alkaline earth) provide a unique opportunity to study the relationships between the structure and the magnetotransport properties due to an interplay among charge carriers, magnetic coupling, orbital ordering and structural distortion. This makes these compounds very exciting from both the basic research and from the technological viewpoint. As the technology pursued with these materials requires film growth, extensive studies have been made on materials synthesis, structural and physical characterization and device fabrication. In this article, the results from the different experimental techniques and the effects of the deposition procedure of the manganite thin films are first reviewed. Second, the relation between the structural and the physical properties is mentioned, and the influence of strains discussed. Finally, possible applications of manganite thin films in spin electronics are presented.

Short-range structural and chemical ordering in Si-rich chemically deposited a-Si_1-xC_x thin films have been investigated via Raman scattering and the numerical modelling technique. Raman spectra have been presented over a wide frequency range including both Stokes and anti-Stokes scattering. The interpretation of the spectra is performed in terms of the whole density of vibrational states. In order to determine the latter, the structure of the a-Si_1-xC_x (x<0.5) system has been modelled and the corresponding dynamical properties have been computed in the harmonic approximation using the valence-force-field model. By integrating the Stokes and anti-Stokes, first-order, and multiple-order processes, a fit of the experimental Raman spectra has been achieved. As expected, our analysis shows a tendency to chemical ordering into a tetrahedrally coordinated network for Si-rich alloys. Nevertheless, a total chemical ordering is not achieved since homonuclear C-C bonds coexist with Si-Si and Si-C ones in these alloy compounds.

Using molecular dynamics simulations based on an effective potential, in analogy to the tight binding theory in the second-moment approximation, we studied the vibrational behaviour of Cu islands of various sizes on the Cu(111) surface. We found that small clusters are contracted by as much 10% (for a dimer) from the relative positions of the relaxed interlayers of the island's atoms; the effect is less important with increasing cluster size, N, attaining the same value as for the surface atoms for islands with more than Nc = 33 atoms. In addition, from the calculated mean-square displacements, we found that in the direction normal to the surface the vibrational amplitudes of the island's atoms do not depend on the size of the cluster, while they are increased by a factor of five in the in-plane directions. These findings are compatible with the behaviour of the cluster phonon modes. Indeed, we found that small islands introduce new vibrational modes that diminish with increasing cluster size, regaining the surface vibrational behaviour. These results suggest that small 2D islands (containing up to 10 atoms) formed on the Cu(111) face are strained; for larger islands this compressive stress is released and the islands exhibit the vibrational behaviour of the substrate for N>Nc. This characteristic cluster size, Nc, coincides with the characteristic island size above which the migration energy of the island's atoms saturates to the value of the step-energy barrier.

Density functional theory and a plane-wave basis pseudopotential technique are applied to calculate the bulk and the (0001) surface properties of hexagonal-close-packed (hcp) beryllium and magnesium. The calculations were performed with two forms of the exchange-correlation functional. The bulk properties of the two metals calculated within the generalized gradient approximation (GGA) are closer to the measured ones than the local density approximation (LDA) results are. The GGA also provides results for lattice relaxations, work functions, and surface energies which are in a better agreement with experimental values than the LDA results are. The changes of surface energy and work function with slab thickness varying between 3 and 13 atomic layers were considered. Quantum-size effects did not influence the calculated work function and surface energies significantly.

Stopping powers of polycarbonate, Mylar and Havar for 1.0-3.25 MeV/nucleon ⁴⁰Ar-ions have been determined by the transmission method in two geometries. The stopping power values were obtained within uncertainty of 2.1-4.5% for the various materials. The present results are compared with the predictions obtained by the most commonly used procedures employed in obtaining stopping powers. These include the Northcliffe and Schilling model, semi-empirical parametrization of Ziegler et al (SRIM2000) with and without the cores and bonds model and the Hubert et al formulation. SRIM2000 values were in good agreement in case of Mylar and Havar, on average within 3% of present results. For polycarbonate the differences were less than 6% on average. The cores and bonds (CAB) model improved the parametrization values slightly. The Northcliffe and Schilling model and the Hubert et al formulation both yielded values within 5% or less for Mylar and polycarbonate. For the Havar the Hubert et al formulation and the present results disagreed by 10% on average.

2.5% and 8% tin- and 8% titanium-doped γ-Fe₂O₃ have been synthesized and examined by x-ray powder diffraction, EXAFS, electron microscopy and by ⁵⁷Fe- and ¹¹⁹Sn-Mössbauer spectroscopy. The Sn- and Ti-K-edge EXAFS show that both tin and titanium adopt octahedral sites in the spinel related γ-Fe₂O₃ structure. However, whereas tin substitutes for iron on one of the fully occupied sites, titanium adopts the octahedral site, which is only partially occupied. The ⁵⁷Fe-Mössbauer spectra recorded in the presence of a longitudinal magnetic field of 2-8 T confirm that the tetravalent ions adopt the octahedral sites. The canting angles for both sublattices in γ-Fe₂O₃ were determined from the in-field Mössbauer spectra. The ¹¹⁹Sn-Mössbauer spectra showed that the maximum hyperfine field sensed by the Sn⁴⁺ ions in γ-Fe₂O₃ is about 2/3 of that observed in tin-doped Fe₃O₄ (magnetite).

The electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia (YSZ) materials. The positions of the peaks in the near-edge structure are identical in both techniques. Differences observed in the intensities of the features are attributed to the effect of specimen charging in the XANES experiments. Analysis of near-edge structure reveals that both the crystallographic phase and the metal fraction of yttrium present can be determined directly from the oxygen K-edge data opening up opportunities for characterization of interfacial phenomena in YSZ materials with sub-nanometre resolution using ELNES.

The x-ray diffraction patterns for Ce³⁺-doped BaMgF₄ (BMF) crystals suggest the existence of superlattice structure. The superlattice model is consistent with the characterization of the 4f¹ ground state of Ce³⁺ as a probe ion using the electron spin-resonance (ESR) technique. The distinct Ce³⁺ luminescence spectra with different peak energies and lifetimes also support the superlattice model. Although the detailed superlattice structure could not be analysed using the diffraction spots, a model has been proposed, taking into account the eight Ce³⁺ polyhedra with different anion coordinations in the unit cell of the BMF crystal obtained from the ESR experiments.

Crystal structures and magnetic and thermal properties of neodymium ruthenate Nd₃RuO₇ have been investigated. The specific heat and neutron diffraction measurements showed a monoclinic–orthorhombic structural phase transition at 130 K. The monoclinic crystal structure at 100 K is well described with the space group P2₁/m (a = 10.8893(4) Å, b = 7.3864(2) Å, c = 7.4731(2) Å and β = 90.008(6)°). The results of the magnetic susceptibility measurements show the existence of the antiferromagnetic transition with a weak ferromagnetic component at 19 K. Specific heat and neutron diffraction measurements also indicated long-range magnetic ordering of both Nd³⁺ and Ru⁵⁺ ions below 19 K. The magnetic sublattice of Nd³⁺ ions was twice as large as the size of the crystal lattice along the b-axis. On the other hand, the size of the magnetic sublattice of Ru⁵⁺ was the same as that of the crystal lattice. Ru⁵⁺ ions were coupled anti-ferromagnetically along the c-axis.

We report single-crystalline growth of Tb-doped GaN grown by gas source molecular beam epitaxy using NH₃ as a nitrogen source, and the incorporation site of Tb in GaN. It was found that most of the Tb atoms are substitutionally incorporated into the Ga lattice site by Rutherford-backscattering ion channelling and extended x-ray absorption fine-structure analysis. Photoluminescence spectra, which show sharp peaks originating from 4f intra-atomic transitions of Tb³⁺ ions, with near-band-edge emission of GaN, were observed from the film whose Tb content was roughly estimated to be 2%, and their properties are discussed.

Small-polaron behaviour in a two-dimensional honeycomb net is studied by applying the strong-coupling perturbative method to the Holstein molecular crystal model. We find that small optical polarons can be mobile also if the electrons are strongly coupled to the lattice. Before the polarons localize and become very heavy, there is in fact a window of electron-phonon couplings in which the polarons are small and have masses of order ≃5-50 times the bare band mass according to the value of the adiabaticity parameter. The two-dimensional honeycomb net favours the mobility of small optical polarons, in comparison with the square lattice.

The angular and polarization dependences of acoustic phonon emission from a quasi-2D hole gas in quantum wells are theoretically investigated. The contribution of both deformation potential and piezoelectric coupling is considered in the hole–phonon interaction. We have also taken into account the effect of anisotropy of the valence band which gives rise to the hole–phonon interaction due to deformation potential coupling for both LA and TA phonons. Finally, the theory is applied to calculate the rates of acoustic phonon emission in GaAs quantum wells.

We have performed calculations on the fullerene cage structures and the binding energies of Si_n (n = 20, 24, 26, 28, 30, 32) clusters by the full-potential linear-muffin-tin-orbital molecular dynamics (FP-LMTO-MD) method. It is found that the fullerene cages are not stable, and relax into structures which are severely distorted. Except for Si₂₀, their atomic arrangement tends towards tetrahedral geometry. After the structural distortion, about two silicon atoms can still be filled into the inside spaces of the distorted cages for Si_n (n = 26-32).

Utilizing the harmonic approximation, we introduce a simple technique for the generation of ensembles of thermally disordered atomic structures, and calculate for the MgO crystal electron densities of states (DOS) and the on-site potential probability distributions for the ensembles generated within the classical and quasi-classical harmonic approximations, molecular dynamics and uncorrelated Gaussian atomic disorder models. An account of the zero-energy vibrations even at room temperature results in a significant increase in the mean square atomic displacements and, thus, in the probability distributions of the electrostatic potential and ultimately in the extent of the band tails in the electron DOS. We also demonstrate that the correlations in atomic positions affect the electronic structure. We have evaluated directly the on-site potential autocorrelation function (PAF) for all disorder models, as well as the temperature dependence of the PAF. The correlation length, L, is shown to be less than the second nearest neighbour at low temperature and decreasing to below the nearest neighbour distance at T = 500 K. The short correlation length obtained in the direct modelling is in agreement with the Urbach–Martienssen rule for the optical absorption edge observed experimentally.

We present a theoretical study on the density of states (DoS) of a two-dimensional electron gas subjected simultaneously to an intense laser field and a strong perpendicular magnetic field. We find that in the Faraday geometry, due to the coupling of the magnetic field to the laser field, the DoS of a Landau level (LL) is split into a series of peaks centred at E = E_N + (eF₀)²/[4m*(ω²-ω_c²)] + mℏω. Here, ω and F₀ are respectively the frequency and the electric field strength of the laser field, ω_c is the cyclotron frequency, E_N is the Nth LL energy, and m = 0,±1,±2,... corresponding to different optical processes. This is electrically analogous to the Franz-Keldysh effect for an electron gas driven by a strong external field and we therefore name it the magneto-optical Franz-Keldysh effect.

The magnetic form factor of the heavy fermion compound CeRu₂Si₂ was measured by polarized neutron diffraction in the Pauli paramagnetic phase and above the metamagnetic transition. The magnetization density is characteristic of 4f electrons in both phases with an almost pure |5/2,5/2⟩ ground-state wavefunction. The only field effect observed in this experiment corresponds to a nonlinear variation of the magnetization of the 4f electrons.

We have studied the structural, magnetic and electrical properties of hole-doped cobaltites with compositions, La_0.7(Ca_1-xBa_x)_0.3CoO₃ (x = 0.0, 0.5 and 1.0). Due to the large difference in size between the Ca and Ba ions (size mismatch), these compounds show interesting changes in the magnetic and transport behaviours with an increase in the concentration of Ba. The distortion in the perovskite structure decreases with the substitution of Ba for Ca. Magnetic studies indicate that the anisotropy in their ferromagnetic characters, which contribute to the observations of large thermomagnetic irreversibilities and large coercive fields in these compounds, change significantly with the substitution of Ba for Ca. The unusual magnetic behaviour of the La_0.7Ba_0.3CoO₃ compound, as reflected in its M-H behaviour, has been explained on the basis of a possible coexistence of different magnetic phases. Observations of the low-temperature resistivity minimum, and negative magnetoresistance up to 26% in the low-temperature insulating regions of these polycrystalline compounds, have been ascribed to grain boundary effects.

We study the electron-phonon scattering rate 1/τ_ep in impure metals in the dirty limit. We show that, if all impurities are substitutional, the previous Reizer-Sergeyev result, 1/τ_ep~T⁴, holds even when discreteness of the lattice structure is taken into account, where T = temperature. However, the result is modified when we also allow for random positional shift of impurities, in which case the result 1/τ_ep~T², is obtained.

We have studied the electrical resistivity of YbNi₂Ge₂ under hydrostatic pressure up to 100 kbar. With increasing pressure the system is continuously tuned from an intermediate valence state at low pressure to a magnetically ordered Kondo lattice at high pressure. The critical pressure for the appearance of a magnetic ground state is near 50 kbar. Below P_c the resistivity shows a T² dependence, characteristic of a Fermi liquid description at low temperatures. The coefficient of the T² term increases drastically on approaching P_c. Above P_c, the magnetic transition temperature T_m increases linearly with pressure. The magnetoresistance at high pressure suggests the importance of ferromagnetic coupling.

The effect of an external harmonic signal on the screw instability of the current in the electron-hole plasma has been studied experimentally in Ge at 77 K and 300 K. The influence exerted by external signals with various amplitudes and frequencies, applied to a sample both additively and multiplicatively, on the synchronization, amplification and stability of the system in absolute and convective modes of instability excitation has been investigated at points of bifurcation in a wide region of the parametric space.

Defects in Ce³⁺- and Eu²⁺-doped alkaline-earth fluorides, created by vacuum ultraviolet (UV) photons at energies lower than at the bandgap, were investigated by various methods: thermostimulated luminescence, photostimulated luminescence and optical absorption.

The CaF₂:Eu²⁺ thermoluminescence curves in the range of 60-330 K due to various types of trapped holes were the same after vacuum UV illumination as after x-irradiation. Thermoluminescence curves of Ce³⁺-doped alkaline-earth fluorides created by vacuum UV illumination or x-irradiation were generally similar. However, V_k thermoluminescence peaks were absent in vacuum UV illuminated CaF₂:Ce³⁺ and SrF₂:Ce³⁺ crystals. This fact is obviously associated with the presence of charge-compensating fluorine interstitials in Ce³⁺-doped crystals.

The creation of Ce²⁺ characteristic bands was observed in photostimulated luminescence spectra as well as in optical absorption spectra of vacuum UV illuminated or x-irradiated Ce³⁺-doped crystals. The suppression of hole thermoluminescence peaks in CaF₂:Eu²⁺ crystals by blue light is due to the photoionization of Eu⁺ ions.

The proposed mechanism for the creation of trapped hole and trapped-electron defects by vacuum UV illumination involves charge-transfer-type transitions, in which the electron transfers from the valence band to an impurity level lying in the bandgap. Comparison of all energies involved of transitions in the crystals investigated shows that the sum of all the transition energies is less than that of the bandgap by 1.5-3.5 eV. This energy difference can be considered to be the energy of lattice relaxation around the created Ce²⁺ or Eu⁺ ions.

The energy profile of the density of states (DOS) over the mobility gap is determined jointly by the defect pool model (DPM) calculation and the Fourier transform of the transient photoconductivity (TPC), for intrinsic and phosphorus (P)-doped a-Si:H. From the Fourier transform of the TPC, we measure, as a doping effect, an increase of the DOS around the donor energy level, at about 0.16 eV below the conduction mobility edge, and a decrease of the tail width below this level from 21 to 15 meV. This disorder effect on the conduction band tail caused by the P dopant is consistent with the induced doping changes in the dangling bond defect distribution calculated by the DPM. TPC decays are then generated by numerical simulation using this DOS distribution and compared to experimental TPC data. All observed features in the transient photoresponse are reproduced by the simulation, namely the short time rapid decrease followed by the long power law decay in the intrinsic a-Si:H, and the long non-dispersive flat region in the P-doped a-Si:H.

A theory for the absorption and scattering (sideband generation) of near-infrared light by undoped quantum wells subjected to intense terahertz radiation and a quantizing magnetic field is presented. It is based on rigorous stationary wave functions for Landau levels driven by an alternating-current field. Due to their characteristic energy structure, the response of Landau levels to the terahertz radiation diverges at cyclotron resonance. This singularity, absent in the conventional optical Stark effect of atoms, leads to the disappearance of both absorption and sideband intensities (terahertz-induced transparency) at resonance or in the limit of strong terahertz intensity.

A new series of Ti-substituted derivatives of Tl₂Mn₂O₇ pyrochlore have been prepared under moderate pressures (P = 2 GPa). Materials of nominal stoichiometry Tl₂Mn_2-xTi_xO₇, with 0⩽x⩽0.4, have been characterized by neutron powder diffraction (NPD), magnetic, magnetotransport, and Hall measurements. The Ti-substituted materials are ferromagnetic, with Curie temperatures slightly reduced with respect to that of Tl₂Mn₂O₇, as a result of the introduction of a non-magnetic cation into the Mn sublattice, without nominal change of valence upon doping. Unlike the undoped (x = 0) compound, which shows a significant thallium and oxygen deficiency in the O' sublattice of the pyrochlore structure, the Ti-doped materials are fully stoichiometric, as deduced from NPD data. This result is consistent with the net increase observed in the number of carriers (electrons), which explains the reduction in resistivity and magnetoresistance.

Evidence for an unusual double charge disproportionation, Mn⁴⁺ + Co²⁺ → Mn³⁺ + Co³⁺ → Mn⁴⁺ + Co²⁺, is observed in nanocrystalline LaMn_0.5Co_0.5O₃ synthesized by a low-temperature method and annealed in the temperature range 200–1300 °C. The Curie temperature of nanocrystalline LaMn_0.5Co_0.5O₃ is increased from 150 to 230 K when annealed in the temperature range 200–700 °C and is further decreased back to 150 K when annealed in the temperature range 700–1300 °C. This increase and decrease in T_c is found to be associated with the conversion of the spin states of Mn and Co from Mn⁴⁺ and Co²⁺ to Mn³⁺ and low-spin Co³⁺, and again back to Mn⁴⁺ and Co²⁺ as evidenced from x-ray photoelectron spectroscopic studies.

By means of specific heat, magnetic susceptibility, and neutron powder diffraction experiments, we reinvestigated new samples of DyPd₃S₄ synthesized by an improved method. We found a new phase transition at T_Q = 3.4 K above the two magnetic transitions around 1 K, which was characterized by a distinct peak in the specific heat and only a small anomaly in the magnetic susceptibility. The neutron powder diffraction experiment demonstrated neither new magnetic reflections nor evidence of a change of the crystal structure below T_Q. It was furthermore found that the quartet crystalline-electric-field ground state is consistent with the result for the magnetic entropy deduced from the specific heat and that T_Q increases with the external magnetic field. These results strongly imply that the new phase transition at T_Q is due to antiferroquadrupolar ordering.

Cu, Fe, Gd and Nd dopants were added to HNb₂O₅ in a range of concentrations to investigate the influence on the magnetic properties. The dopants fill the vacant sites in the channels along the b axis of the structure. Magnetic moments tend to remain localized in the dopants. AC susceptibility measurements display low temperature undulations associated with short range magnetic correlations within small clusters (of two or three ions) of dopants. Glauber spin-flip kinetics for Ising spin rings can be applied to these data. Although the magnetic behaviour is mostly Curie–Weiss above 20 K, the susceptibility data display a temperature-independent residual paramagnetic signal. We attribute this signal to Van Vleck orbital paramagnetism associated with bonding–antibonding transitions within Nb⁴⁺ or Cu²⁺ spin-paired dimers.

The role of nuclear spins in decoherence and dephasing of a solid-state phase qubit is investigated. Both effects of static spin environment and spin polarization fluctuations in time are considered on the basis of non-Markovian Langevin-Bloch equations. We find conditions when coupling of a phase qubit to a bath of nuclear spins does not impair coherent evolution of the qubit.

Spin-lattice relaxation time (T₁), isotropic Knight shift (K_iso) and quadrupole coupling constant (C_Q) have been measured in technetium metal by pulsed ⁹⁹Tc NMR in a magnetic field of 7.04 T in the temperature range 120-400 K. It was found that (T₁×T)^-1 = 3.21±0.35 (s K)^-1, K_iso(T) = 7305-1.52T ppm; the quadrupole coupling constant C_Q = 5.74±0.05 MHz is temperature independent. Experimental data on T₁ and K_iso are analysed in terms of the contact, d-polarization and orbital hyperfine interactions. It is shown that the main contribution to the relaxation rate comes from the contact interaction and the Knight shift is governed by the orbital interaction. The electric field gradient at the ⁹⁹Tc nucleus site is considered to be a sum of the electron q^el and lattice q^lat contributions with different signs. The calculated lattice contribution is positive and constitutes about 30% of the electronic contribution. The obtained values of q^el and q^lat are compared with data for other metals with hexagonal close-packed lattices.

The crystal and molecular structure of a new ferroelectric from the pyridinium salt group [C₅H₅NH]⁺IO₄^- was determined by the x-ray diffraction method at 350, 300 and 100 K. The high temperature and intermediate phases are orthorhombic, while the low temperature phase is monoclinic, with the following sequences of space groups and continuous phase transitions: . The two orthorhombic phases are isostructural with PyReO₄, where the pyridinium cation is disordered and the periodate anion is ordered. The low temperature phase is well ordered. The intermediate and low temperature phases are ferroelectric. Measurements of spin-lattice relaxation time and complex permittivity have been performed for polycrystalline and monocrystalline samples as a function of pressure and temperature. The potential shape and heights of the energy barriers for cation reorientations in ferroelectric phases have been proposed. The complex permittivity measurements indicate the order-disorder character of the ferroelectric phase transition and continuous slowing of the cation dynamics with decreasing temperature.

Site-selective spectroscopy fluorescence experiments (emission and excitation) in the near-infrared region associated with the ⁴I_15/2↔⁴I_11/2 transitions of Er³⁺ ions have been successfully used to show the presence of two different Er³⁺ centres in the Bi₁₂SiO₂₀ piezoelectric crystal. Green (545-570 nm), red (650-690 nm) and near-infrared (850-890 nm) up-converted emissions have been observed and resolved for each type of centre under excitation up to the ⁴I_11/2 state. The Stark energy level schemes of the three lower energy states of Er³⁺ ions have been determined and compared for both centres, showing two quite different crystalline field environments. The mechanisms responsible for the up-conversion process (excited state absorption and/or energy transfer up-conversion) are also analysed.

Behaviours of Pb²⁺ ions during thermal treatments of PbCl₂-deposited CsCl crystals have been observed in situ by optical absorption and reflection spectroscopy. In the early stages of the treatments, the Pb²⁺ ions take part in the formation of Cs₄PbCl₆ crystallites near the surface of the CsCl crystals. The crystallites exhibit a novel absorption spectrum suggesting that the 6s and 6p states of the Pb²⁺ ions are strongly localized. On heavy annealing at a high temperature (673 K) followed by a rapid cooling to room temperature, the Pb²⁺ ions are uniformly dispersed throughout the CsCl crystals, exhibiting the absorption spectrum attributable to isolated Pb²⁺-ion centres.

We report O and Ca K-edges x-ray absorption near edge structure (XANES) spectra of Ba_1-xCa_xTiO₃ (x = 0.01 and 0.08), BaTiO₃ and CaTiO₃ and the electronic structure of Ba_0.875Ca_0.125TiO₃ obtained by first-principles calculation. The characteristic features in the O K-edge XANES spectra of these ferroelectric perovskites are influenced by the Ca concentration. They differ substantially from those of the reference TiO₂. The O K-edge spectra suggest that the combination of the alkaline-earth-metal oxides, CaO and/or BaO, with TiO₂ enhance the effective charge of the O ions. Thus, a large dipole moment may result from the displacement of the Ti ion from the centre of the TiO₆ octahedron leading to collective displacement of Ti ions through attractive dipole-dipole couplings and may give rise to ferroelectricity. In the Ca K-edge XANES spectra there is a pre-edge feature similar to those found in other 3d transition-metal perovskites, which may provide information about hole doping.