Results are presented on our latest research, aimed at the development and study of oxide scintil... more Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO4, ZnMoO4, PbWO4, PbMoO4, and MgWO4) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in
Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (M... more Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (ME) interactions. However, the ability to design on-demand ME coupling remains challenging despite a variety of host-guest configurations and ME phases coexistence possibilities. Here, we report the effect of metal-ion substitution on the magnetic and electric properties in the paramagnetic ferroelectric NH 2 (CH 3) 2 Al 1−x Cr x (SO 4) 2 × 6H 2 O. Doing so we are able to induce and even tune a sign of the ME interactions, in the paramagnetic ferroelectric (FE) state. Both studied samples with x = 0.065 and x = 0.2 become paramagnetic, contrary to the initial diamagnetic compound. Due to the isomorphous substitution with Cr the ferroelectric phase transition temperature (T c) increases nonlinearly, with the shift being larger for the 6.5% of Cr. A magnetic field applied along the polar c axis increases ferroelectricity for the x = 0.065 sample and shifts T c to higher values, while inverse effects are observed for x = 0.2. The ME coupling coefficient α ME = 1.7 ns/m found for a crystal with Cr content of x = 0.2 is among the highest reported up to now. The observed sign change of α ME with a small change in Cr content paves the way for ME coupling engineering.
We have studied the crystalline structure and absorption spectra for zinc oxide films with differ... more We have studied the crystalline structure and absorption spectra for zinc oxide films with different levels of doping with cobalt, deposited by high-frequency magnetron sputtering on the glass substrates. We estimated the concentration of the free charge carriers and showed that the shift of the fundamental absorption edge in ZnO:Co films with dopant level increasing is explained by the Burstein-Moss effect.
The comparative study of the dielectric dispersion in the vicinity of the ferroelectric-ferroelas... more The comparative study of the dielectric dispersion in the vicinity of the ferroelectric-ferroelastic phase transition in NH2(CH3)2Ga1-xCrx(SO4)2 • 6H2O (x = 0; 0.065) crystals was performed. As it follows from the analysis of the temperature-frequency dependences of the dielectric permittivity and the Cole-Cole diagrams, the dielectric dispersion is connected with a critical slowing down of the dipole relaxation (fundamental dispersion) in the high frequency range and with the domain wall dynamics at lower frequencies of the measuring electric field. It has been shown that the metal ion substitution practically does not affect the temperatures of phase transitions but manifests itself in the parameters of the low frequency dielectric dispersion. The substitution of gallium with chromium is followed by an increase in the relaxation time, the activation energy and the half-width of the Gaussian, which describes the distribution of the relaxation times. This was explained by the fact that chromium ions, due to their larger sizes in comparison with gallium ions, cause local lattice deformations. The latter promote the formation of more massive dipole clusters in the vicinity of the ferroelectric phase transition.
Abstract Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by ... more Abstract Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400 K and pressures up to 9 GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration ( N O =(2–4)×1019 cm −3 ) than that in the bulk matrix ( N O −3 ). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients D O ( T , P ). The obtained values of D O ( T , P ) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.
j ) ( c I Influence of Electric Field and Mechanical Stresses on the Properties of Incommensurate... more j ) ( c I Influence of Electric Field and Mechanical Stresses on the Properties of Incommensurate Phases in (N(CH,),),ZnCl, and (N(CH,),),CoCl, Crystals BY S. SVELEBA (a), V. ZHMURKO (a), V. KAPUSTIANIK (a), I. POLOVINKO (a, c), and Z. TRYBULA (b) The temperature dependences of birefringence increment (6(An)) and dielectric permittivity ( E l ) of (N(CH,),),ZnCl, and (N(CH,)&CoCl, are measured. The long-periodic commensurate phases appear within the incommensurate phase under the influence of external electric fields (E) and mechanical stresses (G). The (E, T ) and ( 0 , T ) phase diagrams are obtained. Ans pa3nliYHblX Cpe30B KpllCTannOB (N(CH,),),ZnCl, H (N(CH,),),CoCl, H3MepeHbI TeMIIepaTypHbIe 3aBHCHMOCTM A3MeHeHHR ABYJIyYeIIpeJIOMJIeHHR (F(An)) H fill3JIeKTpHYeCKOfi ITpOHHUaeMOCTH (&'). npH ~0 3 n e i i c ~s ~~ B H ~W H H X ~J I ~K T ~H Y ~C K H X noneii ( E ) H MexaHarecKxx Hanpaxemii ( 0 ) B TeMnepaTypHoti o6nacm HeCOpa3MepHOfi aa3bI Ha3BaHHbIX KpACTannOB BbIRBneHbI fiJIHHHOIIepHOnHYeCKHe COpa3Mep-HbIe 4a3bI. nOCTPOeHbI Qa30BbIe ( E , T ) ll (G, T ) AllarpaMMbI.
X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3+ -doped di... more X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3+ -doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from ferroelastic to ferroelectric phase. The spin Hamiltonian parameters have been determined for both phases. The spin Hamiltonian parameters in the ferroelectric phase are: g
Infrared absorption spectra of [(CH3)2CHNH3]4Cd2Cl10:Cu crystals with copper concentration of 0.6... more Infrared absorption spectra of [(CH3)2CHNH3]4Cd2Cl10:Cu crystals with copper concentration of 0.6 ± 0.1% and 1.8 ± 0.1% were investigated in the spectral range of 30-12000 cm −1 and Raman spectra of the same crystals were investigated within the range of 30-4000 cm −1. These crystals belong to a large family of compounds with the metal-halogen polyhedra of different types manifesting various phase transitions including those into the ferroelectric, ferroelastic and even ferroelastoelectric phases. The related crystals with an alkylammonium cation and transition metal ions in their structure were found to be multiferroics since they are characterized by the coexistence of ferroelectric and magnetic ordering. The obtained data for [(CH3)2CHNH3]4Cd2Cl10:Cu crystals were compared with those for the initial compound without copper. Increasing the Cu 2+ concentration in the crystals leads to merging of bands in the infrared spectra corresponding to the skeletal vibrations of IPA cations because of their interaction with the complex anions consisting of two types of octahedra: CdCl6 and CuCl6. Moreover, for all investigated crystals and geometries of experiment a considerable splitting of the Raman modes corresponding to skeletal vibrations of IPA cation and asymmetric stretching vibrations of the methyl groups were observed. This case corresponds to Davydov's splitting connected with the presence of different numbers of distinct IPA cations. A very similar character and level of the mentioned splitting testifies to the fact that the crystals doped with copper are characterized by the same structure of phase II as the initial IPACC crystal. The doped crystals with a higher concentration of Cu 2+ manifest very broad bands in the region above 9000 cm −1 which would be related to the intra-ion transitions in Cu 2+ ion. The performed analysis of the IR and Raman spectra both of the initial crystals and those doped with copper allowed to identify more precisely the real phonons which are most of all involved in the formation of the absorption edge. topics: multiferroics, infrared spectra, Raman spectra, ferroelastoelectric
Nanostructured CuO thin film was deposited on glass substrate using a radio-frequency magnetron s... more Nanostructured CuO thin film was deposited on glass substrate using a radio-frequency magnetron sputtering technique. The topological measurements performed by AFM showed the polycrystalline structure of deposited film and the size of CuO nanocrystallites was analyzed. The linear optical absorption of the CuO nanostructured film was studied by UV-Vis spectroscopy and the band-gap energies of indirect and direct transitions were obtained. By performing Z-scan measurements using a picosecond laser the nonlinear absorption coefficient and imaginary parts of the third-order nonlinear susceptibility of the CuO nanostructured film were determined. Nanostructured CuO film exhibited large optical nonlinearity due to saturable absorption caused by electrons filling of excited bands and/or depletion of ground band.
Отримано композитний фотокаталiзатор, що складається з шару зародкiв ZnO з n-типом електропровiдн... more Отримано композитний фотокаталiзатор, що складається з шару зародкiв ZnO з n-типом електропровiдностi, "наноквiтiв" ZnO з n-типом електропровiдностi та поруватого кремнiю з p-типом електропровiдностi. Дослiджено фотодеґрадацiю барвника з використанням композитних фотокаталiзаторiв на основi тонких плiвок i "наноквiтiв". Використання "наноквiтiв" ZnO дало змогу полiпшити фотокаталiтичнi властивостi композиту. Ключовi слова: композит, фотокаталiзатор, фотодеґрадацiя, оксид цинку, поруватий кремнiй.
Gallium oxide has recently been identified as a promising scintillator. To assess its potential a... more Gallium oxide has recently been identified as a promising scintillator. To assess its potential as a detector material for ionizing radiation at low temperatures, we measured the luminescence and scintillation properties of an undoped Ga 2 O 3 crystal over the 7-295 K temperature range. The emission of the crystal is due to the radiative decay of self-trapped excitons and donor-acceptor pairs and peaks at a wavelength of 380 nm. The scintillation light output of the undoped Ga 2 O 3 increases with a decrease in temperature, reaching a maximum value of 19 300 6 2200 ph/MeV at 50 K. The measured luminescence kinetics has a recombination character with specific decay time (s 0.1) increasing from 1 to 1.8 ls at cooling. Since radiative decay in the crystal competes with nonradiative processes, material optimization could lead to the scintillator achieving a yield of 40800 ph/MeV, a figure considered to be an upper limit.
The recently reported ability to induce and tune a sign of the magnetoelectric (ME) interactions ... more The recently reported ability to induce and tune a sign of the magnetoelectric (ME) interactions in NH 2 (CH 3) 2 Al 1−x Cr x (SO 4) 2 • 6H 2 O crystals as a function of Cr content is further investigated here. The ME coupling and its larger absolute value for the sample with larger Cr content agrees qualitatively with thermodynamic Landau analysis. However, quantitative estimation implies other contributions as well. The observed nontrivial magnetic field dependence of ferroelectric transition can be explained within the model that takes into account influence of the arising local deformation of the lattice on the two types of DMA group responsible respectively for the ferroelectric and antiferroelectric ordering.
Abstract On the basis of absorption spectroscopy data the temperature evolution of the Cu2+ ion e... more Abstract On the basis of absorption spectroscopy data the temperature evolution of the Cu2+ ion environment in the crystal of [(CH3)2CHNH3]4Cd3Cl10 doped with Cu2+ (IPACC:Cu) was studied. As it follows from the temperature dependences of heat capacity and absorption coefficient at a constant wavelength IPACC:Cu crystals are characterised by I order phase transitions at T1 = 358 K and T3 = 253 K whereas the continuous changes observed at T2 = 293 K would be related to the first order phase transition close to the second order. For the detailed analysis of the crystal field spectra in the temperature range from T3 till T1 the special program package Crys Tool 2.0 based on the quantum mechanical models first of all on the model of normalized spherical harmonics (NSH) has been employed. It has been found that similarly to the crystal of ((CH3)2CHNH3)4Cd3Cl10 (IPACC) the investigated doped crystal contains the distorted octahedral metal-halogen complexes of two types and the degree of their distortion is considerable changed in vicinity of the phase transition at T2 = 293 K. The phase transitions at T2 and T3 were found to be shifted toward the low temperatures in comparison with initial IPACC crystal whereas the high temperature transition at T1 is shifted to the higher temperatures.
A simple two-step approach to produce ZnO nanorods and nanotubes by electron sputtering of the me... more A simple two-step approach to produce ZnO nanorods and nanotubes by electron sputtering of the metallic zinc in vacuum and then oxidizing it to ZnO in air is reported. It has been found that the samples synthesised by this method exhibited much more intense exciton photoluminescence around ∼370 nm in comparison with the usual flat zinc oxide thin films. The clearly distinguished bands corresponding to localised exciton states and phonon replicas testify to quite high crystallinity of the obtained nanorods (nanotubes). The slight but noticeable blueshift (4 meV) in the near band edge luminescence for the sample with nanotubes in comparison with the samples with nanorods and flat films could reflect the quantum confinement effect.
Results are presented on our latest research, aimed at the development and study of oxide scintil... more Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO4, ZnMoO4, PbWO4, PbMoO4, and MgWO4) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in
Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (M... more Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (ME) interactions. However, the ability to design on-demand ME coupling remains challenging despite a variety of host-guest configurations and ME phases coexistence possibilities. Here, we report the effect of metal-ion substitution on the magnetic and electric properties in the paramagnetic ferroelectric NH 2 (CH 3) 2 Al 1−x Cr x (SO 4) 2 × 6H 2 O. Doing so we are able to induce and even tune a sign of the ME interactions, in the paramagnetic ferroelectric (FE) state. Both studied samples with x = 0.065 and x = 0.2 become paramagnetic, contrary to the initial diamagnetic compound. Due to the isomorphous substitution with Cr the ferroelectric phase transition temperature (T c) increases nonlinearly, with the shift being larger for the 6.5% of Cr. A magnetic field applied along the polar c axis increases ferroelectricity for the x = 0.065 sample and shifts T c to higher values, while inverse effects are observed for x = 0.2. The ME coupling coefficient α ME = 1.7 ns/m found for a crystal with Cr content of x = 0.2 is among the highest reported up to now. The observed sign change of α ME with a small change in Cr content paves the way for ME coupling engineering.
We have studied the crystalline structure and absorption spectra for zinc oxide films with differ... more We have studied the crystalline structure and absorption spectra for zinc oxide films with different levels of doping with cobalt, deposited by high-frequency magnetron sputtering on the glass substrates. We estimated the concentration of the free charge carriers and showed that the shift of the fundamental absorption edge in ZnO:Co films with dopant level increasing is explained by the Burstein-Moss effect.
The comparative study of the dielectric dispersion in the vicinity of the ferroelectric-ferroelas... more The comparative study of the dielectric dispersion in the vicinity of the ferroelectric-ferroelastic phase transition in NH2(CH3)2Ga1-xCrx(SO4)2 • 6H2O (x = 0; 0.065) crystals was performed. As it follows from the analysis of the temperature-frequency dependences of the dielectric permittivity and the Cole-Cole diagrams, the dielectric dispersion is connected with a critical slowing down of the dipole relaxation (fundamental dispersion) in the high frequency range and with the domain wall dynamics at lower frequencies of the measuring electric field. It has been shown that the metal ion substitution practically does not affect the temperatures of phase transitions but manifests itself in the parameters of the low frequency dielectric dispersion. The substitution of gallium with chromium is followed by an increase in the relaxation time, the activation energy and the half-width of the Gaussian, which describes the distribution of the relaxation times. This was explained by the fact that chromium ions, due to their larger sizes in comparison with gallium ions, cause local lattice deformations. The latter promote the formation of more massive dipole clusters in the vicinity of the ferroelectric phase transition.
Abstract Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by ... more Abstract Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400 K and pressures up to 9 GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration ( N O =(2–4)×1019 cm −3 ) than that in the bulk matrix ( N O −3 ). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients D O ( T , P ). The obtained values of D O ( T , P ) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.
j ) ( c I Influence of Electric Field and Mechanical Stresses on the Properties of Incommensurate... more j ) ( c I Influence of Electric Field and Mechanical Stresses on the Properties of Incommensurate Phases in (N(CH,),),ZnCl, and (N(CH,),),CoCl, Crystals BY S. SVELEBA (a), V. ZHMURKO (a), V. KAPUSTIANIK (a), I. POLOVINKO (a, c), and Z. TRYBULA (b) The temperature dependences of birefringence increment (6(An)) and dielectric permittivity ( E l ) of (N(CH,),),ZnCl, and (N(CH,)&CoCl, are measured. The long-periodic commensurate phases appear within the incommensurate phase under the influence of external electric fields (E) and mechanical stresses (G). The (E, T ) and ( 0 , T ) phase diagrams are obtained. Ans pa3nliYHblX Cpe30B KpllCTannOB (N(CH,),),ZnCl, H (N(CH,),),CoCl, H3MepeHbI TeMIIepaTypHbIe 3aBHCHMOCTM A3MeHeHHR ABYJIyYeIIpeJIOMJIeHHR (F(An)) H fill3JIeKTpHYeCKOfi ITpOHHUaeMOCTH (&'). npH ~0 3 n e i i c ~s ~~ B H ~W H H X ~J I ~K T ~H Y ~C K H X noneii ( E ) H MexaHarecKxx Hanpaxemii ( 0 ) B TeMnepaTypHoti o6nacm HeCOpa3MepHOfi aa3bI Ha3BaHHbIX KpACTannOB BbIRBneHbI fiJIHHHOIIepHOnHYeCKHe COpa3Mep-HbIe 4a3bI. nOCTPOeHbI Qa30BbIe ( E , T ) ll (G, T ) AllarpaMMbI.
X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3+ -doped di... more X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3+ -doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from ferroelastic to ferroelectric phase. The spin Hamiltonian parameters have been determined for both phases. The spin Hamiltonian parameters in the ferroelectric phase are: g
Infrared absorption spectra of [(CH3)2CHNH3]4Cd2Cl10:Cu crystals with copper concentration of 0.6... more Infrared absorption spectra of [(CH3)2CHNH3]4Cd2Cl10:Cu crystals with copper concentration of 0.6 ± 0.1% and 1.8 ± 0.1% were investigated in the spectral range of 30-12000 cm −1 and Raman spectra of the same crystals were investigated within the range of 30-4000 cm −1. These crystals belong to a large family of compounds with the metal-halogen polyhedra of different types manifesting various phase transitions including those into the ferroelectric, ferroelastic and even ferroelastoelectric phases. The related crystals with an alkylammonium cation and transition metal ions in their structure were found to be multiferroics since they are characterized by the coexistence of ferroelectric and magnetic ordering. The obtained data for [(CH3)2CHNH3]4Cd2Cl10:Cu crystals were compared with those for the initial compound without copper. Increasing the Cu 2+ concentration in the crystals leads to merging of bands in the infrared spectra corresponding to the skeletal vibrations of IPA cations because of their interaction with the complex anions consisting of two types of octahedra: CdCl6 and CuCl6. Moreover, for all investigated crystals and geometries of experiment a considerable splitting of the Raman modes corresponding to skeletal vibrations of IPA cation and asymmetric stretching vibrations of the methyl groups were observed. This case corresponds to Davydov's splitting connected with the presence of different numbers of distinct IPA cations. A very similar character and level of the mentioned splitting testifies to the fact that the crystals doped with copper are characterized by the same structure of phase II as the initial IPACC crystal. The doped crystals with a higher concentration of Cu 2+ manifest very broad bands in the region above 9000 cm −1 which would be related to the intra-ion transitions in Cu 2+ ion. The performed analysis of the IR and Raman spectra both of the initial crystals and those doped with copper allowed to identify more precisely the real phonons which are most of all involved in the formation of the absorption edge. topics: multiferroics, infrared spectra, Raman spectra, ferroelastoelectric
Nanostructured CuO thin film was deposited on glass substrate using a radio-frequency magnetron s... more Nanostructured CuO thin film was deposited on glass substrate using a radio-frequency magnetron sputtering technique. The topological measurements performed by AFM showed the polycrystalline structure of deposited film and the size of CuO nanocrystallites was analyzed. The linear optical absorption of the CuO nanostructured film was studied by UV-Vis spectroscopy and the band-gap energies of indirect and direct transitions were obtained. By performing Z-scan measurements using a picosecond laser the nonlinear absorption coefficient and imaginary parts of the third-order nonlinear susceptibility of the CuO nanostructured film were determined. Nanostructured CuO film exhibited large optical nonlinearity due to saturable absorption caused by electrons filling of excited bands and/or depletion of ground band.
Отримано композитний фотокаталiзатор, що складається з шару зародкiв ZnO з n-типом електропровiдн... more Отримано композитний фотокаталiзатор, що складається з шару зародкiв ZnO з n-типом електропровiдностi, "наноквiтiв" ZnO з n-типом електропровiдностi та поруватого кремнiю з p-типом електропровiдностi. Дослiджено фотодеґрадацiю барвника з використанням композитних фотокаталiзаторiв на основi тонких плiвок i "наноквiтiв". Використання "наноквiтiв" ZnO дало змогу полiпшити фотокаталiтичнi властивостi композиту. Ключовi слова: композит, фотокаталiзатор, фотодеґрадацiя, оксид цинку, поруватий кремнiй.
Gallium oxide has recently been identified as a promising scintillator. To assess its potential a... more Gallium oxide has recently been identified as a promising scintillator. To assess its potential as a detector material for ionizing radiation at low temperatures, we measured the luminescence and scintillation properties of an undoped Ga 2 O 3 crystal over the 7-295 K temperature range. The emission of the crystal is due to the radiative decay of self-trapped excitons and donor-acceptor pairs and peaks at a wavelength of 380 nm. The scintillation light output of the undoped Ga 2 O 3 increases with a decrease in temperature, reaching a maximum value of 19 300 6 2200 ph/MeV at 50 K. The measured luminescence kinetics has a recombination character with specific decay time (s 0.1) increasing from 1 to 1.8 ls at cooling. Since radiative decay in the crystal competes with nonradiative processes, material optimization could lead to the scintillator achieving a yield of 40800 ph/MeV, a figure considered to be an upper limit.
The recently reported ability to induce and tune a sign of the magnetoelectric (ME) interactions ... more The recently reported ability to induce and tune a sign of the magnetoelectric (ME) interactions in NH 2 (CH 3) 2 Al 1−x Cr x (SO 4) 2 • 6H 2 O crystals as a function of Cr content is further investigated here. The ME coupling and its larger absolute value for the sample with larger Cr content agrees qualitatively with thermodynamic Landau analysis. However, quantitative estimation implies other contributions as well. The observed nontrivial magnetic field dependence of ferroelectric transition can be explained within the model that takes into account influence of the arising local deformation of the lattice on the two types of DMA group responsible respectively for the ferroelectric and antiferroelectric ordering.
Abstract On the basis of absorption spectroscopy data the temperature evolution of the Cu2+ ion e... more Abstract On the basis of absorption spectroscopy data the temperature evolution of the Cu2+ ion environment in the crystal of [(CH3)2CHNH3]4Cd3Cl10 doped with Cu2+ (IPACC:Cu) was studied. As it follows from the temperature dependences of heat capacity and absorption coefficient at a constant wavelength IPACC:Cu crystals are characterised by I order phase transitions at T1 = 358 K and T3 = 253 K whereas the continuous changes observed at T2 = 293 K would be related to the first order phase transition close to the second order. For the detailed analysis of the crystal field spectra in the temperature range from T3 till T1 the special program package Crys Tool 2.0 based on the quantum mechanical models first of all on the model of normalized spherical harmonics (NSH) has been employed. It has been found that similarly to the crystal of ((CH3)2CHNH3)4Cd3Cl10 (IPACC) the investigated doped crystal contains the distorted octahedral metal-halogen complexes of two types and the degree of their distortion is considerable changed in vicinity of the phase transition at T2 = 293 K. The phase transitions at T2 and T3 were found to be shifted toward the low temperatures in comparison with initial IPACC crystal whereas the high temperature transition at T1 is shifted to the higher temperatures.
A simple two-step approach to produce ZnO nanorods and nanotubes by electron sputtering of the me... more A simple two-step approach to produce ZnO nanorods and nanotubes by electron sputtering of the metallic zinc in vacuum and then oxidizing it to ZnO in air is reported. It has been found that the samples synthesised by this method exhibited much more intense exciton photoluminescence around ∼370 nm in comparison with the usual flat zinc oxide thin films. The clearly distinguished bands corresponding to localised exciton states and phonon replicas testify to quite high crystallinity of the obtained nanorods (nanotubes). The slight but noticeable blueshift (4 meV) in the near band edge luminescence for the sample with nanotubes in comparison with the samples with nanorods and flat films could reflect the quantum confinement effect.
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Papers by V. Kapustianyk