Table of contents

Initial oxidation processes on Si(001) have been studied by means of surface differential reflectance (SDR). The time courses of the SDR spectra measured during thermal oxidation at 820 and 920 K allowed two different growth modes, Langmuir-type adsorption and two-dimensional island growth, to be distinguished. No photon energy dependence was observed in the time course of the SDR intensity at either temperature. On the other hand, different uptake curves were observed at different photon energies for oxidation at 300 K. The difference between the oxidation mechanisms at 300 K and at high temperatures was qualitatively apparent from SDR results, because significant photon energy dependence was observed only at 300 K. Possible assignments of the spectral components in the SDR spectra are discussed.

Performing atomic-scale simulations, we study the interaction of the scanning tunneling microscopy (STM) tip with mesoscopic islands at zero bias voltage. Our calculations reveal tip-induced shape transitions in Co islands on Cu(100) as the tip approaches the surface. The structure of the islands and of the tip are found to depend strongly on the tip–substrate distance. A significant influence of the tip on atomic diffusion on the top and at the edges of the islands is demonstrated. The size-dependent strain relief in the islands caused by the tip and by the substrate is found to play a key role in atomistic processes on islands. Our results show that, for certain tip–surface separations, the hopping diffusion of Co adatoms on the top of Co islands and the upward mass transport at the edge of the islands can be strongly enhanced. Our findings point out the possibility of manipulating atomic motion on mesoscopic islands using the STM tip.

This article reviews the microscopic origin of properties due to transition-metal (TM) impurities, M, in insulator materials. Particular attention is paid to the influence of pressure upon impurity properties. Basic concepts such as the electronic localization in an MX_N complex, the electrostatic potential, V_R, arising from the rest of the lattice ions or the elastic coupling of the complex to the host lattice are initially exposed. The dependence of optical and magnetic parameters on the impurity–ligand distance, R, in cubic lattices is discussed in a first step. Emphasis is put on the actual origin of the R dependence of 10Dq. Examples revealing that laws for strict cubic symmetry cannot in general be transferred to lower symmetries are later given. This relevant fact is shown to come from allowed hybridizations like nd–(n+1)s as well as the influence of V_R. As a salient feature the different colour in ruby and emerald is stressed to arise from distinct V_R potentials in Al₂O₃ and Be₃Si₆Al₂O₁₈. The last part of this review deals with electronic instabilities. The phenomena associated with the Jahn–Teller (JT) effect in cubic lattices, the origin of the energy barrier among equivalent minima and the existence of coherent tunnelling in systems like MgO:Cu²⁺ are discussed. An increase of elastic coupling is pointed out to favour a transition from an elongated to a compressed equilibrium conformation. Interestingly the equilibrium geometry of JT ions in non-cubic lattices is shown to be controlled by mechanisms different to those in cubic systems, V_R playing again a key role. The relevance of first principles calculations for clarifying the subtle mechanisms behind off-centre instabilities is also pointed out. Examples concern monovalent and divalent TM impurities in lattices with the CaF₂ structure. The instability due to the transition from the ground to an excited state is finally considered. For complexes with significant elastic coupling vibrational frequencies and the Stokes shift are expected to undergo bigger changes through a chemical rather than a hydrostatic pressure. The reduction of Huang–Rhys factors upon increasing the pressure is discussed as well.

A topical review is given of the physics of submicron ferroelectrics, describing the application considerations for memory devices (both as switching memory elements for ferroelectric nonvolatile random access memories, FRAMs, and as passive capacitors for volatile dynamic random access memories, DRAMs) as well as the fundamental physics questions regarding both the thickness and lateral size of present interest.

The paper proposes and describes a physical model of thermally induced processes in binary lamellar systems. The model has been developed for the theoretical explanation of an experimentally revealed fact of thermal stabilization of intermetallic phases on the surface of a lamellar sample. Based on the model we developed an algorithm for calculations and a computer code that operates with three one-phase and two two-phase regions in the binary alloy state diagram. The computational model includes as inputs changes in concentration boundaries for existing phases with change in temperature, as well as arbitrary temperature–time regimes for the thermal treatment of the lamellar system under investigation. Good agreement between the theoretical calculations and Mössbauer investigations of binary lamellar Fe–Be systems has been achieved.

The propagation of Nd long range magnetic order in the hexagonal and cubic sublattices has been investigated in double hexagonal compact Nd/Sm(001) superlattices by resonant x-ray magnetic scattering at the Nd L₂ absorption edge. For a superlattice with 3.7 nm thick Sm layers, the magnetic structure of the hexagonal sublattice propagates coherently through several bilayers, whereas the order in the cubic sublattice remains confined to single Nd blocks. For a superlattice with 1.4 nm thick Sm layers, the magnetic structures of both sublattices appear to propagate coherently through the superlattice. This is the first observation (i) of the long range coherent propagation of Nd order on the cubic sites between Nd blocks and (ii) of a different thickness dependence of the propagation of the Nd magnetic phases associated with the hexagonal and cubic sublattices. The propagation of the Nd magnetic order through Sm is interpreted in terms of generalized susceptibility of the Nd conduction electrons.

The effect of swift heavy ion (SHI) irradiation on InGaAs/GaAs heterostructures is studied using Raman spectroscopy and atomic force microscopy (AFM). The structures consist of molecular beam epitaxy (MBE) grown InGaAs layers on GaAs(001), having layer thicknesses of 12, 36, 60 and 96 nm. After irradiation, the GaAs type longitudinal optical (LO) mode blue shifted to higher frequency in thin samples and red shifted towards lower frequency in thick samples. These results are discussed invoking the penetration depth of the probe radiation (λ = 514.5 nm) in InGaAs. Deconvoluting the Raman spectra of thin samples indicates a compressive strain developed in the substrate, close to the interface upon irradiation. This modification and diffusion of indium across the interface results in an increase of strain and reduction of the defect densities in the InGaAs layer. The variations in FWHM of the Raman modes are discussed in detail. The surface morphology of these heterostructures has been studied by AFM before and after SHI irradiation. These studies, combined with Raman results, help to identify different relaxation regimes.

We study the sliding of elastic solids in adhesive contact with flat and rough interfaces. We consider the dependence of the sliding friction on the elastic modulus of the solids. For elastically hard solids with planar surfaces with incommensurate surface structures we observe extremely low friction (superlubricity), which very abruptly increases as the elastic modulus decreases. We show that even a relatively small surface roughness may completely kill the superlubricity state.

We have studied the magnetic properties of ⁵⁷Fe-doped NiO nanoparticles using Mössbauer spectroscopy and magnetization measurements. Two samples with different degrees of interparticle interaction were studied. In both samples the particles were characterized by high-resolution transmission electron microscopy and x-ray diffraction and found to be plate-shaped. Computer simulations showed that high-field Mössbauer data are very sensitive to the size of the uncompensated magnetic moment. From analyses of the Mössbauer spectra we have estimated that the size of the uncompensated magnetic moment is in accordance with a model based on random occupation of surface sites. The analyses of the magnetization data gave larger magnetic moments, but the difference can be explained by the different sensitivity of the two methods to a particle size distribution and by interactions between the particles, which may have a strong influence on the moments estimated from magnetization data.

The magnetism of surfaces and interfaces can differ strongly from the magnetism of corresponding bulk materials. In the case of actinide systems the study of the surfaces and interfaces is still at the very beginning. In this work, we investigated the electronic and magnetic properties of U films and U(001)₁/Fe(110)₃ multilayers within the framework of the density functional theory. We report both scalar-relativistic and fully relativistic calculations. The exchange correlation potential was treated in the generalized gradient approximation (GGA). In agreement with previous calculation by Stojić et al (2003 Phys. Rev. B 68 094407) we obtained the surface layer of the U films to be magnetic for the bulk lattice parameter. The dependence of the magnetic properties of the U films on the lattice parameter was studied. It is shown that decreasing distances between U atoms lead to decreasing magnetic moment and finally to the nonmagnetic ground state. The variation of the magnetic moment as a function of the lattice parameter is discontinuous. Using the frozen-magnon approach we evaluated the parameters of the inter-atomic exchange interactions and estimated the Curie temperature. The calculation for U(001)₁/Fe(110)₃ multilayers showed that the U layer is magnetic with the direction of the U moments opposite to the Fe moments. The importance of the U–Fe hybridization is revealed. Both the intra-layer (U–U, Fe–Fe) and inter-layer (U–Fe) exchange interactions were evaluated. The temperature dependence of the layer magnetizations was studied within the random-phase approximation for the Heisenberg Hamiltonian for classical spins.

Photomodulation Raman scattering spectroscopy has been employed to study free charge trapping mechanisms at ZnSe–GaAs(001) heterostructure interfaces. This technique reveals that the interfacial region contains predominantly hole traps. Time dependent measurements of the photomodulated Raman scattering intensity show that interfacial charge-trap lifetime is ≈30 s for both electrons and holes.

The TiB₂(0001) surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. It is found that there are large relaxations within the top three layers for both termination surfaces, and the outermost and second interlayer relaxations for B-terminated surfaces are much larger than those for Ti-terminated surfaces. The charge depletion in the vacuum and the charge accumulations in the interlayer region between the first and second layers reinforces the interlayer Ti–B chemical bonds and reduces the outermost interlayer distance. Simultaneously, the charge accumulation for B-terminated surface is more than that for Ti-terminated surface, and the interlayer Ti–B bonds between the second and third layers are weakened more for the B-terminated surface. The Ti-terminated surface is thermodynamically more favourable in most of the range of μ_B^slab.

Density functional theory and a pseudopotential plane wave method are applied to study electronic and structural properties of the defect-free TiO₂(110) surface. The variations of the surface energy, work function, and atomic displacements are examined for partially and fully relaxed slabs modelling the rutile (110) surface, and consisting of up to 33 atomic layers. Relatively small relaxations of atomic positions in the outermost layers have a strong influence on the calculated surface energies and work functions. The effect of nonequivalence of the odd–even layer terminations is explored. A simple method is proposed which allows one to estimate accurate surface energies for relaxed systems from calculations for partially relaxed slabs.

A mean field approach is employed for determining the kinetics of two main quantities for film morphology, namely the number of islands and the film perimeter. These two quantities have been computed for both coalescence and impingement growth regimes. For each case the growth of two- and three-dimensional islands has been considered. In addition, the effect of the spatial correlation among nuclei has been taken into account on the grounds of the hard core interaction. Analytic formulae have been derived for each case.

Germanium carbide (Ge_1−xC_x) films have been prepared by RF reactive sputtering a pure Ge(111) target at different flow rate ratios of CH₄/(CH₄+Ar) in a CH₄/Ar mixture discharge, and it has been found that the composition, chemical bonding, optical and mechanical properties of Ge_1−xC_x films are remarkably influenced by the flow rate ratio of CH₄/(CH₄+Ar). The effects of the chemical bonding on the optical and mechanical properties of the Ge_1−xC_x films have been explored. In addition, an antireflection Ge_1−xC_x double-layer coating deposited on both sides of the ZnS substrate wafer has been developed for application as an infrared window. It is shown that the transmittance in the wavelength region between 8 and 12 µm and the hardness of the ZnS substrate have been significantly improved by the double-layer coating.

The electronic structure of Co-doped anatase TiO₂ epitaxial thin films grown at different partial oxygen pressures is investigated using soft x-ray emission spectroscopy. The resonantly excited Co L_2,3 x-ray emission spectra of ferromagnetic Ti_0.96Co_0.04O₂ samples for the oxygen-deficient regime show that the ratio of integral intensities for Co L₂ and L₃ emission lines significantly decreases with respect to nonmagnetic samples in the oxygen-rich regime. This is due to L₂L₃M_4,5 Coster–Kronig transitions and suggests that ferromagnetic Ti_0.96Co_0.04O₂ samples have n-type charge carriers and Co–Co bonds between substitutional and interstitial Co atoms are present while Co–O bonds are dominant in nonmagnetic Ti_0.96Co_0.04O₂ samples in the oxygen-rich regime. Electronic structure calculations show that the presence of free charge carriers and Co segregation play a crucial role in strong ferromagnetism at room temperature in Co-doped TiO₂.

Surface excitation spectra are calculated, including both collective and single-particle modes, and examined in detail. This is achieved by calculating the non-local dielectric function ε_p(Q,z,z^',ω) of the thin jellium film within the random phase approximation (RPA) (using local density approximation wavefunctions which actually takes us beyond the RPA), from which we then derive the spectral function. The high precision of the calculations enables us to analyse not only the collective (surface plasmon) modes and their dependence on the film thickness, but also the intra-band electron–hole excitations, and for the first time oscillatory structures due to inter-band transitions. The spectra are then analysed with special attention to their dependence on the slab thickness, and the periodic peaks observed due to single-particle excitations in the spectra.

Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time T_M, were carried out, in the 20–200 K temperature range, on an AsO₄⁴⁻ paramagnetic probe stabilized in RbH₂PO₄ (RDP), NH₄H₂PO₄ (ADP), and dipolar glass Rb_0.5(NH₄)_0.5H₂PO₄ (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the ¹⁴N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the bond both in ADP and RDP; and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number m_I of ⁷⁵As. The temperature dependence of T_M exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb_0.5(NH₄)_0.5H₂PO₄. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20–200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample.

The magnetic properties of Na₂CuP₂O₇ were investigated by means of ³¹P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the ³¹P NMR shift, the spin–lattice , and spin–spin (1/T₂) relaxation rate data as a function of temperature T. The temperature dependence of the NMR shift K(T) is well described by the S = 1/2 square lattice Heisenberg antiferromagnetic model with an intraplanar exchange of  K and a hyperfine coupling A = 3533 ± 185 Oe /μ_B. The ³¹P NMR spectrum was found to broaden abruptly below T∼10 K, signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data down to 1.8 K. The heat capacity appears to have a weak maximum around 10 K. With decrease in temperature, the spin–lattice relaxation rate 1/T₁ decreases monotonically and appears to agree well with the high temperature series expansion expression for a S = 1/2 2D square lattice.

The optimized effective potential (OEP) method provides an additional level of exactness in the computation of electronic structures, e.g. the exact exchange energy can be used. This extra freedom is likely to be important in moving density functional methods beyond traditional approximations such as the local density approximation. We provide a new density-matrix-based derivation of the gradient of the Kohn–Sham energy with respect to the effective potential. This gradient can be used to iteratively minimize the energy in order to find the OEP. Previous work has indicated how this can be done in the zero temperature limit. This paper generalizes the previous results to the finite temperature regime. Equating our gradient to zero gives a finite temperature version of the OEP equation.

Cerium for strontium substitution allows an oxygen deficient perovskite Sr_1−xCe_xCoO_3−δ to be stabilized with a cerium solubility limited to x≤0.15 (Trofimenko et al 1997 Solid State Ion. 100 183). For these samples, the magnetic properties depend clearly upon the oxygen content: Sr_0.9Ce_0.1CoO_2.74 and Sr_0.9Ce_0.1CoO_2.83 are weak and strong ferromagnets (T_C = 160 K), respectively, the maximum ac-magnetic susceptibility of the latter being larger by two orders of magnitude than that of the former. In contrast to other Sr_1−xL_xCoO_3−δ series (L =  lanthanide, Y³ or Th⁴⁺), the electrical resistivity (ρ) behaviour does not simply reflect the magnetic behaviour. For x = 0.10 the re-entrant ρ feature becomes more pronounced whereas the ferromagnetic fraction and cobalt oxidation state increase. This unexpected behaviour could be related to the Ce³⁺/Ce⁴⁺ mixed valency, the 4f localized moment of the Ce³⁺ cations interacting with the conduction electrons through a Kondo-like mechanism. It is also found that the increase of oxygen vacancies favours the appearance of magnetoresistance at low T, reaching −50% at 5 K in 7 T for the Sr_0.95Ce_0.05CoO_2.61 sample prepared in a sealed tube.

Al_1−xGa_xPO₄ solid solutions (x = 0.2, 0.3, 0.38, 0.7) and the pure AlPO₄ (x = 0) and GaPO₄ (x = 1) end members with the α-quartz-type structure were studied by Raman scattering. An investigation as a function of composition enabled the various modes to be assigned, in particular coupled and decoupled vibrations. The tetrahedral tilting modes, which have been linked to high-temperature phase transitions to β-quartz-type forms, were found to be decoupled. In addition, it is shown that Raman spectroscopy is a powerful technique for determining the gallium content of these solid solutions. Single crystals with x = 0.2, 0.38, and 1.0 (GaPO₄) were investigated at high temperature. The composition Al_0.8Ga_0.2PO₄ was found to exhibit sequential transitions upon heating to the β-quartz and β-cristobalite forms at close to 993 K and 1073 K, respectively. Direct α-quartz–β-cristobalite transitions were observed for the two other compositions at close to 1083 K and 1253 K, respectively, upon heating. The spectra of the β-quartz and β-cristobalite forms indicate the presence of significant disorder. Back transformation to the α-quartz-type form occurred readily with a hysteresis of less than 100 K for the composition x = 0.38 and for pure GaPO₄. Rapid cooling was necessary to obtain the metastable α-cristobalite form. In contrast, for Al_0.80Ga_0.20PO₄, the α-cristobalite form was obtained even upon slow cooling.

The second harmonic generation of antiferromagnetic and dielectric multilayers is analysed by using a conventional nonlinear optics approach and transfer matrix formalism. The theoretical modelling of the multilayers is configured in Voigt geometry in order to observe second harmonic transmission and reflection through the film system, with the assumption of weak nonlinearity and no depletion of incident waves. With these, some of the linear and second harmonic transmissions and reflections are calculated numerically and shown graphically.

To reveal the energetic sequence of the alloy phases in the Co–Au system, the lattice constants, cohesive energies, and bulk modulus of the fcc Au, hcp Co, the B1, B2, and L1₀ structured CoAu phases, and the D0₃, L1₂, and D0₁₉ structured Co₃Au and CoAu₃ phases, respectively, are acquired by first-principles calculations within the generalized-gradient approximation (GGA) as well as within the local density approximation (LDA). In addition, the magnetic moment of the Co atom in the studied phases are also calculated. To further examine the structural stability, the elastic constants of the studied phases are calculated and the results suggest that the fcc-type structures could be elastically stable at Co/Au = 1:3, 1:1, and 3:1, whereas the hcp-type structures could be stable at Co/Au = 1:3 and 3:1. Moreover, the spatial valence charge density (SVCD) and spin density of the studied phases are also calculated to clarify the physical origin of the structural stability. It turns out that, in the relatively stable phases, the high SVCDs mostly distribute between the similar atoms, thus forming the attractive covalent bonding to stabilize the respective structures, and that the spin density may also play an important role in influencing the stability of the ferromagnetic metastable phases.

The electronic structure of the tetragonal U₂T₂In (T = Ni, Rh, Pt) compounds in the paramagnetic phase were studied by x-ray photoelectron spectroscopy (XPS). Both valence band and core level spectra were analysed. The experimental data are compared with the calculations of the density of states using the tight-binding linear muffin-tin orbital method (TB-LMTO) and full-potential local-orbital full-relativistic method (FPLO). The calculated data reveal a dominant U 5f electron character for the states near the Fermi level E_F with a small contribution from U 5d, Ni 3d, Rh 4d, Pt 5d and In 5p states. The XPS valence bands of these compounds are characterized by a sharp peak of the U 5f states near the Fermi level (E_F) and broad peaks of the Ni 3d, Rh 4d and Pt 5d states at about 2.6, 3.2 and 4.0 eV below E_F, respectively. The small change in the position of the U 5f peak with respect to E_F is −0.35 eV for T = Ni and −0.15 eV for T = Rh and Pt. A satellite between the Ni 2p_1/2 and Ni 2p_3/2 peaks is visible, suggesting that the Ni 3d band is not completely filled, and the existence of a small induced magnetic moment on the Ni atoms cannot be ruled out.

Single-phased polycrystalline BiMnO₃ (hereinafter abbreviated as BMO) ceramic was fabricated via high-pressure solid-state reaction. Microstructure modification of selective grains, signalled by emergence of superlattice diffraction, was scrutinized by means of electron diffraction (ED) combined with high-resolution transmission electron microscopy (HRTEM). It was clearly evidenced that the well established C 2 monoclinic substructure (a = 9.53 Å, b = 5.61 Å, c = 9.85 Å and β = 110.67°) of BMO (Atou et al 1999 J. Solid State Chem.145 639) is metastable and prone to be transformed to a new pseudocubic superstructure (a≈b≈c≈15.8 Å and α≈β≈γ≈90°) (Yang et al 2006 Phys. Rev. B 73 024114) when irradiated continuously by an electron beam. Magnetization measurement unveiled a unique ferromagnetic phase transition at 103 K, which corroborated our speculation that as-prepared BMO ceramic is free of polymorphism at ambient conditions.

The effects of doping with magnetic Mn ions or nonmagnetic Ga ions on the structural, electrical transport and magnetic properties of Na_0.75CoO₂ have been investigated. It has been found that the lattice parameter c of the samples increases with Ga or Mn ion doping. Ga doping raises the electrical resistivity of Na_0.75CoO₂, but the metallic conducting behaviour of the compound has not been influenced. In contrast, 5% Mn doping leads to a metal–insulator transition at low temperatures in Na_0.75Co_1−yMn_yO₂. The susceptibility of the Ga doped sample shows strong magnetic field dependence, while the susceptibility of the Mn doped samples is not very sensitive to the magnetic field. This work implies that magnetic interaction plays an important role in Na_xCoO₂.

Using ab initio calculations, we have studied shearing in Nb₂AlC, where NbC and Al layers are interleaved. The stress–strain analysis of this deformation mode reveals Nb–Al bond breaking, while the Nb–C bond length decreases by 4.1%. Furthermore, there is no evidence for phase transformation during deformation. This is consistent with basal slip and may be understood on the basis of the electronic structure: bands below the Fermi level are responsible for the dd bonding between NbC basal planes and only a single band with a weak dd interaction is not resistant to shearing, while all other bands are unaffected. The Al–Nb bonding character can be described as mainly metallic with weak covalent–ionic contributions. Our study demonstrates that Al layers move with relative ease under shear strain. Phase conservation upon shearing is unusual for carbides and may be due to the layered nature of the phase studied. Here, we describe the electronic origin of basal slip in Nb₂AlC, the atomic mechanism which enables reversible plasticity in this class of materials.

The temperature dependence of the magnetization of a light emitting diode having a ferromagnetic contact (spin-LED) is measured from 2 to 300 K in magnetic fields from 30 to 70 kOe and it is found that it originates from the GaAs substrate. The magnetization of GaAs comprises a van Vleck-type paramagnetic contribution to the susceptibility which scales inversely with the band gap of the semiconductor. Thus, the temperature dependence of the band gap of GaAs accounts for the non-linear temperature dependent magnetic susceptibility of GaAs and thus, at large magnetic fields, for the spin-LED.

The heat capacity of two relaxors BaTi_1−xZr_xO₃ (x = 0.35,0.40) was measured using adiabatic calorimetry in the temperature range 100–360 K. The C_p(T) dependence of both compositions is characterized by the presence of two diffuse anomalies near the Burns temperature T_d and the temperature of the maximum in permittivity T_m in the temperature ranges 250–350 K and 120–200 K. The anomalous heat capacity near T_d was analysed taking into account the distribution of Zr concentration in nanoregions leading to the distribution of their transition temperatures into the polar phase. Excess heat capacity near T_m was discussed in the framework of the spherical random bond–random field model. The results are compared with the data obtained by the same procedures for PbMg_1/3Nb_2/3O₃ studied experimentally earlier.

The physical properties of parkerite and its related compound, Bi₂Ni₃X₂ (X =  S, Se), were studied. The electrical resistivity of both compounds shows typical metallic behaviour up to 400 K. The resistivity and specific heat measurements at low temperatures reveal that these compounds are superconducting with a transition temperature of ∼0.7 K. The upper critical fields at 0 K, μ₀H_c2(0), are 95 mT (Bi₂Ni₃S₂) and 75 mT (Bi₂Ni₃Se₂). In the normal state, the electronic specific heat coefficient, γ, and the Debye temperature, Θ_D, are found to be 11.4 mJ mol⁻¹ K⁻² and 189 K, respectively, for Bi₂Ni₃S₂, and 12.2 mJ mol⁻¹ K⁻² and 173 K, respectively, for Bi₂Ni₃Se₂. From the electronic specific heat in the superconducting temperature range, it was found these compounds belong to the weak-coupling BCS superconductors.

In this paper, we examine the possible influence of extrinsic factors on the electrical and magnetotransport of La_0.67Ca_0.33Mn_1−xRu_xO₃ (x≤0.10). Ru substitution results in double metal–insulator transitions (MITs) at T_MI1 and T_MI2, both exhibiting magnetoresistance (MR). No additional magnetic signal corresponding to a second low-temperature maximum (LTM) at T_MI2 could be observed, either in ac susceptibility (χ^') or in specific heat (C_p). Typical grain sizes of ∼18 000–20 000 nm, as estimated from the scanning electron microscope (SEM) micrographs, are not so small as to warrant an LTM. The absence of additional peaks in the high statistics powder x-ray diffraction (XRD), a linear systematic increase of the unit cell parameters, close matching of the transition temperatures in resistivity, χ^' and C_p and their linear systematic decrease with x, and an homogeneous distribution of Mn, Ru and O at arbitrarily selected regions within and across the grains exclude chemical inhomogeneity in the samples. The insensitivity of grain boundary MR at 5 K to Ru composition indicates that the grain boundary is not altered to result in an LTM. Oxygen stoichiometry of all the compounds is close to the nominal value of 3. These results not only exclude the extrinsic factors, but also establish that double MITs, both exhibiting MR, are intrinsic to Ru substituted La_0.67Ca_0.33MnO₃.