A. Molak

Electronic structure of NaNbO3–Mn single crystals

Abstract The XPS spectra of the NaNbO 3 single crystals doped with manganese ( x =0.1, 0.5, 1.0, 3.0, 5.0 and 10.0%) were measured using the PHI 5700/660 physical electronics photoelectron spectrometer at room temperature within the antiferroelectric phase. From the spectra the atomic concentration was calculated and compared to the nominal composition. The approximate solubility limit of the Mn ions, built into the sodium niobate crystal lattice, was determined as 1.0% Mn. The dependence of the core level peak position and valence band on manganese dopant was determined. The influence of sputtering with the Ar + ions on the Nb core line and valence band shape was observed. This effect was ascribed to the change in niobium valence due to the oxygen defects produced during the sputtering process.

Resistance Switching Induced in BiMnO3 Ceramics

In our study BiMnO3 ceramics has been sintered. The X-ray diffraction test has shown that the crystal lattice symmetry is monoclinic and centrosymmetric. Grain structure and stoichiometry has been determined by scanning electron microscopy whereas local disorder by X-ray photoemission spectroscopy. Leakage currents dominated the D–E hysteresis loop. Electric conductivity has been measured in the 85–770 K range. The studied fresh ceramics shows semiconductor properties. Charge-order transition occurrence has been deduced. The dc resistance switching occurs following the application of electroformation at a high temperature range. This effect is reversible, i.e. the Rdc(T) recovers semiconductor properties after thermal and electric field treatment.

Nano-scale chemical and structural segregation induced in surface layer of NaNbO3 crystals with thermal treatment at oxidising conditions studied by XPS, AFM, XRD, and electric properties tests

The process of ionic segregation in the surface layer induced by the thermal treatment of the NaNbO3 crystal was studied. The study of the as grown crystal and the crystal heated at 970 K in ambient air was carried out for comparison. The changes in the concentration of the elements within the surface layer were determined using XPS measurements. The AFM technique was applied to determine the surface morphology related to nano-scale transformation. The possibility of appearance of the Ruddlesden–Popper phases was detected by the XRD test. The change in the features of the electric permittivity, the loss tangent, and the electric conductivity was ascribed to the marked participation of the chemically modified surface layer in the measured effective electrical properties.

Insulator–metal transition in Mn-doped NaNbO3 induced by chemical and thermal treatment

The transition between insulator and metal conductor states, induced by oxygen non-stoichiometry, was studied for NaNbO3 : Mn crystals. Conditions for an optimal reduction were determined on the basis of TGA tests. The temperature dependencies of the resistance measured on the macroscale showed that the transition from thermally activated to metallic features depends on the level of oxygen deficiency. The LC-AFM measurement exhibited non-homogeneous electric resistance on the nanoscale. We ascribed the local insulator–metal transition to changeover in the electronic state of the Nb ions occurring in filaments. The Mn dopant stabilised the induced oxygen non-stoichiometry and the metallic conduction down to room temperature.

Optic properties and EPR spectra of PbTiO3:Mn crystals

Abstract The crystals PbTiO3:Mn have been obtained. The influence of Mn dopant on light absorption, electric conductivity, dielectric permittivity was observed. The defect centers of Mn24, Mn4+, Vo, Fe3+-Vo were determined with EPR test.

Properties of (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 analysed within dielectric permittivity, conductivity, electric modulus and derivative techniques approach

The (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 ceramics with perovskite structure were sintered. The XRD test proved that the samples are cubic (a = 3.920 ± 0.001 Å). Microstructure and atomic composition were determined with a SEM (JSM-5410) equipped with energy dispersion X-ray analyser (ISIS-300). The fluctuation in the chemical composition was found indicating on local disorder. Broadband dielectric spectroscopy in the range 10−1–3 · 107 Hz was applied within the range of 100–650 K. The real, ϵ′(f, T), and imaginary, ϵ″(f, T), parts of complex dielectric permitivity characteristics, both in the temperature and frequency domain, show relaxation processes partially covered by electric conductivity. At high temperatures the electric conductivity exhibits a thermally activated behaviour σ(f, T) ∝ exp(−E a/kT) but the variable range hopping (VRH) dependence σ ∝ exp[−(T 0/T)1/4] is manifested at low temperatures. The derivatives technique in the frequency (∂ log ϵ/∂ log ω) and temperature (∂ log ϵ/∂T) domain enabled various relaxation processes to be distinguished. The data converted to electric modulus representation, M*(f, T) = 1/ϵ*, exhibited clearly resolved relaxation peaks. The relaxation times obtained from the peaks position show a slightly non-Arrhenius temperature behaviour with the activation energy varying in 0.4–0.6 eV range and characteristic time of the electric conductivity relaxation of the order of 10−12 s. The relaxation times can be fitted at better accuracy with the VRH dependence where T 0 are of the order of 108 K. It is shown that the low frequency ac-conductivity converges to dc-conductivity and the relation σ(0) ∼ ωm ∼ τm −1 typical for the disordered solids applies. The conduction current relaxation relationship behaves in accord with the VRH system: σdc ∝ (T/T 0)q (e 2/kT) ωc, where ωc = νph exp[−(T 0/T)1/4] is valid for the locally disordered (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 compound.

Dielectric and Optical Studies of the Low-Temperature Phase in NaNbO3: Effect of Manganese Dopant

Pure and Mn-doped NaNbO3 crystals have been investigated dielectrically between 380 and 20 K. Below room temperature, no phase transition is observed in pure sodium niobate. With increasing Mn content, a structural phase transition is induced around 140 K. The dielectric constant shows a maximum for the sample with 0.7 wt.%, and shows large thermal hysteresis. In the low-temperature region, ferroelectric activities could not be detected in either pure or lightly doped NaNbO3. The phase diagram of Tc vs the concentration of Mn is obtained. Domain motion associated with this phase transition is observed around the transition temperature. This evidence proved that the low-temperature phase is induced by light doping with Mn ions. The uniaxial stress decreases Tc; dTc/dσ=-80 K/kbar on heating and -600 K/kbar on cooling.

Electrical Conduction Relaxation in the Bi(Mn1/3Nb2/3)O3 and (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 Ceramics

New Bi(Mn 1/3 Nb 2/3 )O 3 ceramics were obtained. Their pseudo-perovskite symmetry with monoclinic distortion was determined by XRD test. Dispersion in permittivity ε′ (T,f) was observed. Small polaron mechanism of electric conductivity was deduced. Several techniques were applied to evaluate electric conduction relaxation times τ(T,f) from ε′(T,f), ε”(T,f), tan δ(T,f), and M” (T,f) dependences. The relaxation times τ (T,f) temperature dependence was fitted with high accuracy by Fermi glass related term τ (T,f)∼ exp[(T0/T)1/4] corresponding to variable range hopping of small polaron model. Perovskite cubic structure was determined for (Bi 1/9 Na 2/3 )(Mn 1/3 Nb 2/3 )O 3 ceramics. The E a , τ0, T 0, and electric conductivity σ values were changed by Na ions addition.

Electric Current Relaxation and Resistance Switching in Non-Homogeneous Bismuth Manganite

Bismuth manganite ceramics has been obtained by high temperature sintering in air. It was found from the X-ray diffraction pattern analysis that the ceramics consists of two centrosymmetric phases: the cubic I23 and the orthorhombic Pbam. Impedance has been measured at radiofrequencies in 85–500 K range. Two relaxation processes have been discerned. One process, which shows characteristic time value τ0 ≈ 10−12 s, occurs in 120–230 K range and it has been assigned to polaronic conductivity. The other process occurs in 270–310 K range and it exhibits longer characteristic time value τ0 ≈ 10−8 s. The resistance Rdc(T) temperature dependence shows semiconductor features with activation energy Ea = 0.28–0.40 eV for the as-sintered samples. The semiconductor to metallic-type transition has been induced after electroformation procedure carried out at 790 K in ambient air. We propose that extended defects, related to occurrence of the two phases, facilitate resistive switching.

The Estimation of the Mn Atoms Chemical Bonding in (Na1-xBix)(Nb1-yMny)O3 Ceramics and Changeover in the Electrical Properties

The chemical bonding of Mn atoms in (Na1-xBix)(Nb1-yMny)O3 ceramics were analysed using the X-ray Absorption Near Edge Structure (XANES) measurement at the Mn L3,2–edge. In all the compounds coexistence of the covalent bonding MnCV state and ionic Mn+k states was found. The increase in the Bi and Mn content caused the marked dispersion and increased the values of electric conductivity and dielectric permittivity of the ceramics.