Institute of Solid State Physics and Semiconductors

Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials. Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals. Here we report low-threshold amplified spontaneous emission and lasing from ∼10 nm monodisperse colloidal nanocrystals of caesium lead halide perovskites CsPbX3 (X=Cl, Br or I, or mixed Cl/Br and Br/I systems). We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440-700 nm) with low pump thresholds down to 5±1 μJ cm(-2) and high values of modal net gain of at least 450±30 cm(-1). Two kinds of lasing modes are successfully observed: whispering-gallery-mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX3 nanocrystals and random lasing in films of CsPbX3 nanocrystals.

A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 1018 and 4 × 1019 cm—3. It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich InxGa1—xN alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ∼ 2.5 eV allows one to reconcile our results and the literature data for the band gap of InxGa1—xN alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN–In2O3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.

Influence of cation order-disorder phenomena on the crystal structure, magnetic, and electrical transport properties of new CMR perovskites for $L{\mathrm{BaMn}}_{2}{\mathrm{O}}_{6\ensuremath{-}\ensuremath{\gamma}}$ $(L=\mathrm{Pr},$ Nd, Sm, Eu, Gd, Tb) series has been investigated. For each rare-earth ion three compounds have been synthesized by the topotactic reduction-oxidation method. Structural investigations have shown the oxygen-stoichiometry $L{\mathrm{BaMn}}_{2}{\mathrm{O}}_{6}$ compound obtained in air to be cubic with disordered ${L}^{3+}$ and ${\mathrm{Ba}}^{2+}$ cations whereas the oxygen-deficient $L{\mathrm{BaMn}}_{2}{\mathrm{O}}_{5}$ is tetragonal with ordered ${L}^{3+}$ and ${\mathrm{Ba}}^{2+}$ and alternate stacking of rare earth and barium containing layers along c. This crystal structure is similar to the ${\mathrm{YBaCuFeO}}_{5}$ related one. Another form of oxygen-stoichiometry $L{\mathrm{BaMn}}_{2}{\mathrm{O}}_{6}$ compound obtained by reoxidation of oxygen-deficient $L{\mathrm{BaMn}}_{2}{\mathrm{O}}_{5}$ is also tetragonal and retains the ordering of ${L}^{3+}$ and ${\mathrm{Ba}}^{2+}$ cations. It is notable that the reoxidized ${\mathrm{EuBaMn}}_{2}{\mathrm{O}}_{6}$ compound has an orthorhombic unit cell. It is observed that this type of cation ordering leads to considerable increase of transition temperature to paramagnetic state. For example, disordered ${\mathrm{EuBaMn}}_{2}{\mathrm{O}}_{6}$ compound has magnetic properties similar to spin glass and shows freezing temperature of magnetic moments ${T}_{f}\ensuremath{\approx}40\mathrm{K}$ while ordered ${\mathrm{EuBaMn}}_{2}{\mathrm{O}}_{6}$ is an inhomogeneous ferromagnet with Curie point ${T}_{C}\ensuremath{\approx}260\mathrm{K}.$ Electrical resistivity behavior correlates with magnetization. Below the ${T}_{C}$ the Pr, Nd, Sm based compounds undergo a transition to metallic state and demonstrate a peak of magnetoresistance. It is supposed that the remarkable changing of the magnetic and electrical properties of the reoxidized compounds is a consequence of the $L/\mathrm{B}\mathrm{a}$ ordering and can be explained on the base of the Goodenough-Kanamori rules for $180\ifmmode^\circ\else\textdegree\fi{}$ indirect superexchange interactions taking into account an ion size effect in A sublattice of perovskite.

Magnetization measurements of a ${\mathrm{Gd}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoO}}_{3}$ perovskitelike compound have revealed an anomalous behavior at ${T}_{i}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}240\mathrm{K}$ and ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}277\mathrm{K}$ that corresponds to appearing and disappearing spontaneous magnetization. The transition at ${T}_{i}$ is accompanied by the jump of conductivity and a giant magnetoresistance. Below ${T}_{i}$, ${\mathrm{Gd}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoO}}_{3}$ exhibits metamagnetic behavior. At about ${T}_{M}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}370\mathrm{K}$, a transition from a semiconductive to a quasimetallic state has been observed. It is supposed that a ferromagnetic low-spin cobalt state occurs in the temperature range between ${T}_{i}$ and ${T}_{C}$.

Nanocrystalline manganite La0.50Ba0.50MnO3 was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ∼30 nm to ∼7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ∼3.787 to ∼3.904 Å with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ∼30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T C from ∼264 to ∼331 K and in the spontaneous magnetic moment σ S from ∼1.52 to ∼3.31 μB/f.u. The anomalous behavior of the magnetic properties of the manganite La0.50Ba0.50MnO3 obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+-O-Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Cation-ordered manganites of the PrBaMn2O6 system have been obtained using a two-stage synthesis and characterized with respect to the chemical composition, crystal structure, magnetic and magnetotransport properties, and the stability of the ordered state on heating. The physical properties of the cation ordered PrBaMn2O6 manganites obtained using this method significantly differ from the properties of cation disordered Pr0.50Ba0.50MnO3 synthesized by means of the conventional ceramic technology and depend on the degree of ordering of the Pr3+ and Ba2+ cations. In particular, the cation-disordered Pr0.50Ba0.50MnO3 has a cubic perovskitelike unit cell $$(SG = Pm\bar 3m,Z = 1)$$ , while cation-ordered PrBaMn2O6 has a tetragonal unit cell (SG = P4/mmm, Z = 2). Cation states in the system under study are reversible. The cation-ordered PrBaMn2O6 state remains stable upon heating in an oxidizing medium (P[O2] = 1 bar) up to 1300°C. The ordering of the Pr3+ and Ba2+ cations leads to a significant increase in the critical temperatures of phase transitions. In particular, PrBaMn2O6 with the maximum degree of ordering is a metallic ferromagnet with the Curie temperature T C∼320 K, whereas T C of a fully disordered sample is on the order of 140 K. The samples with intermediate degrees of ordering contain two magnetic phases. Slightly below T C, all such samples exhibit a metal-insulator transition and a peak of the magnetoresistance, which amounts to approximately 10 and 65% in a magnetic field of 9 kOe for the fully ordered PrBaMn2O6 and disordered Pr0.50Ba0.50MnO3, respectively. The results are interpreted in terms of the Goodenough-Kanamori empirical rules for indirect exchange interactions with allowance for the degree of ordering of the Pr3+ and Ba2+ cations.

The results of experimental investigation of the chemical phase composition, crystal structure, and magnetic properties of the manganite La0.70Sr0.30MnO3−γ (0 ≤ γ ≤ 0.25) with perovskite structure depending on the concentration of oxygen vacancies are presented. It is found that the mean grain size of the stoichiometric solid solution of La0.70Sr0.30MnO3 amounts approximately to 10 μm, while the grain size for anion-deficient solid solutions of La0.70Sr0.30MnO3−γ is approximately 5 μm. It is found that samples with 0 ≤ γ ≤ 0.13 have a rhombohedral unit cell (with space group $$R\bar 3c$$ , Z = 2), while samples with γ ≥ 0.20 have a tetragonal unit cell (space group I4/mcm, Z = 2). It is proved experimentally that the magnetic phase state of the manganite La0.70Sr0.30MnO3−γ changes upon a decrease in the oxygen content. It is shown that anion-deficient solid solutions of La0.70Sr0.30MnO3−γ experience a number of successive magnetic phase transformations in the ground state from a ferromagnet (0 ≤ γ ≤ 0.05) to a charge-disordered antiferromagnet (γ = 0.25) via an inhomogeneous magnetic state similar to a cluster spin glass (0.13 ≤ γ ≤ 0.20). The mean size of ferromagnetic clusters (r ≈ 50 nm) in the spin glass state is estimated. It is shown that oxygen vacancies make a substantial contribution to the formation of magnetic properties of manganites. The generalized magnetic characteristics are presented in the form of concentration dependences of the spontaneous magnetic moment, coercive force, and the critical temperature of the magnetic transition. The most probable mechanism of formation of the magnetic phase state in Sr-substituted anion-deficient manganites is considered. It is assumed that in the absence of orbital ordering, a decrease in the magnetic ion coordination number leads to sign reversal in indirect superexchange interactions Mn3+-O-Mn3+.

Crystal structure as well as magnetization and electrical transport vs. temperature and field for the La3+1−xBa2+xMn3+O2−3−x/2 (0 ≤ x ≤ 0.50) anion-deficient manganites have been studied. It is established that the samples in the region 0 ≤ x ≤ 0.05 are O/-orthorhombic perovskites and are rhombohedric with 0.10 ≤ x ≤ 0.25, whereas with 0.27 ≤ x ≤ 0.50 they are cubic. As the doping level increases the samples undergo a transition from a weak ferromagnetic (x = 0) to an inhomogeneous ferromagnetic (x ≥ 0.03) state. At x ≥ 0.12 samples show cluster spin glass properties with the temperature of magnetic moment freezing ∼45 K. All the reduced samples are semiconductors and show considerable magnetoresistance over a wide temperature range. The largest magnetoresistance ratio is observed for the x = 0.30 sample. Concentration dependences of spontaneous magnetization and the magnetic ordering temperature for the reduced La3+1−xBa2+xMn3+O2−3−x/2 manganites have been established by magnetic measurements and compared with those for stoichiometric ones. Magnetic data for the reduced La3+1−xBa2+xMn3+O2−3−x/2 manganites have been compared with those for the stoichiometric La3+1−xBa2+xMn3+1−xMn4+xO2−3. The magnetic state of the anion-deficient samples is interpreted on the basis of the superexchange interaction model.

For La 0.825 3+ Sr 0.175 2 +Mn3+O 2.912 2− anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point T C ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (T C ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, V fr ≈ 22%, of the indirect Mn3+-O-Mn3+ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La 0.825 3+ Sr 0.175 2+ Mn3+O 2.912 2− anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.

The structure and magnetic properties of the Ba-ordered state in solid solutions of manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd) with a cation ratio Ln3+/Ba2+ ≫ 1 are studied experimentally. The samples are obtained by two-stage synthesis. The initial stoichiometric Ba-disordered solid solutions Ln0.70Ba0.30MnO3 synthesized in air according to traditional ceramic technology are characterized by the orthorhombic (Imma, Z = 4) perovskite-like unit cell and are ferromagnets with Curie temperatures T C ≈ 173 and ≈ 143 K for Pr and Nd, respectively. The average size <D> of a crystalline in the initial samples is 5 μm. It is found that annealing of the initial samples in a vacuum of P[O2] = 10−4 Pa leads to their separation into three phases: (1) the anion-deficient ordered LnBaMn2O5 phase described by a tetragonal (P4/mmm, Z = 2) perovskite-like unit cell, as well as the phases (2) Ln2O3 (P $$\bar 3$$ m1, Z = 1) and (3) MnO (Fm $$\bar 3$$ m, Z = 2). Reduction leads to the formation of a nanocomposite with an average crystallite size <D> = 100 nm. Anion-deficient Ba-ordered phases of LnBaMn2O5 exhibit ferrimagnetic properties with Néel temperatures T N ≈ 113 and ≈123 K for Pr and Nd, respectively. Annealing of anion-deficient samples in air at a moderate temperature of T = 800°C does not change the average size of the nanocrystallite, but noticeably alters their phase composition. Stoichiometric nanocomposites consist of two perovskite-like phases: (1) the Ba-deficient ordered stoichiometric phase LnBaMn2O6, which is described by a tetragonal (P4/mmm, Z = 2) unit cell and has the Curie temperatures T C ≈ 313 (Pr) and ≈303 K (Nd), and (2) the Ba-disordered superstoichiometric phase Ln0.90Ba0.10MnO3+δ, which is described by an orthorhombic (Imma, Z = 4) unit cell and has Curie temperatures T C ≈ 138 (Pr) and ≈123 K (Nd). The two magnetic phases of the Ba-ordered nanocomposite are exchange-coupled. For the low-temperature magnetic phase, a temperature hysteresis is observed at ΔT ≈ 22 K in a field of 10 Oe and at ΔT ≈ 5 K in a field of 1 kOe. It is shown that states with different degrees of ordering of cations in the A sublattice can be obtained employing different technological conditions of treatment. The significant changes in the magnetic properties of Ba-ordered nanocomposites are explained on the basis of chemical phase separation taking into account the effect of compression, which is a consequence of the action of chemical (cation ordering) and external (surface tension) pressures.

A metastable perovskite phase of Bi(Mg 1/2 Ti 1/2 )O 3 composition (BMT) has been obtained under high-pressure (6 GPa) and high-temperature (1270 K) conditions. X-ray and electron diffraction studies have revealed a complex superstructure originating from antiferroelectric-like displacements of Bi 3+ cations and octahedral tilting. A group theoretical analysis was used to enumerate the possible crystal structures compatible with both types of the distortions. On the basis of the symmetry arguments in combination with the diffraction methods, it has been concluded that the perovskite BMT is characterized by the orthorhombic Pnnm space group with unit cell parameters a = 11.3207(10) Å, b = 5.6433(10) Å, and c = 7.8314(10) Å. A relation of the BMT crystal structure to that commonly accepted for antiferroelectric perovskite PbZrO 3 (PZ) is discussed.

The microstructure, crystal structure, and magnetotransport properties of microsized and nanosized Badoped manganites have been investigated. A “two-step” reduction-reoxidation procedure has been used to obtain nanosized ceramic manganite Nd0.70Ba0.30MnO3 (II). The parent microsized manganite Nd0.70Ba0.30MnO3 (I) was prepared by usual ceramic technology in air. Then the sample was annealed in vacuum. The grain size of the reduced sample, determined by scanning electron microscopy, decreased from ∼5 μm down to ∼100 nm. To obtain the oxygen stoichiometry nanosized sample, the Nd0.70Ba0.30MnO2.60 was again annealed in air. It is established that the (I) sample is a pseudocubic perovskite, whereas (II) is tetrahedral as a consequence of Nd3+ and Ba2+ ions as well as the ordering of oxygen vacancies. The (I) sample is a ferromagnet with T C ≈ 140 K. It has metal-insulator transition at T MI ≈ 135 K and a peak of magnetoresistance ∼50% in a field of 9 kOe. For the (II) sample, the critical points of phase transitions move to higher temperatures, T C ≈ 320 K and T MI ≈ 310 K. The magnetoresistance of the (II) sample at room temperature (T ≈ 293 K) is about 7% in a field of 9 kOe. The magnetotransport properties are interpreted in the framework of the nanosized effect.

Nanocrystalline manganite La0.50Ba0.50MnO3 was synthesized by the soft-chemical method. For this purpose, the sol-gel method was modified and improved. A trihydric saturated alcohol, e.g., glycerol, was suggested as a new organic matrix. The crystal structure of the composite was studied by X-ray powder diffraction at room temperature. A La0.50Ba0.50MnO3 powder annealed in air at T = 500°C is characterized by a perovskite-like cubic structure with the unit-cell parameter a = 3.869 Å. The chemical composition of the sample was studied by electron-probe X-ray microanalysis. The La: Ba: Mn cation ratio in the material was 1: 1: 2. The surface topography was examined with a scanning electron microscope. The crystallite size was ∼30 nm. The dependence of the crystal structure and the surface topology on the annealing temperature was studied. The high-temperature treatment in air resulted in the growth of larger, micrometer-size, crystallites.

Magnetization and electrical resistivity studies have established that ${\mathrm{La}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoO}}_{3\ensuremath{-}\ensuremath{\gamma}}$ (LaBaC) is a metallic ferromagnet whereas ${\mathrm{Pr}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoO}}_{3\ensuremath{-}\ensuremath{\gamma}}$ (PrBaC) is a magnetic semiconductor. The magnitudes of spontaneous magnetizations are in agreement with intermediate (LaBaC) and low-spin (PrBaC) cobalt states with itinerant $3d$ electrons. ${R}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoO}}_{3\ensuremath{-}\ensuremath{\gamma}} (R\mathrm{BaC},$ $R=\mathrm{Sm},$ Eu, Gd, Tb, Dy) exhibit a sequence of a phase transitions at temperatures ${T}_{i}, {T}_{C},$ and ${T}_{M}$ on heating. Below ${T}_{i}$ these materials are antiferromagnets, whereas in the temperature interval between ${T}_{i}$ and ${T}_{C},$ a ferromagnetic behavior has been revealed. For TbBaC the temperatures ${T}_{i}$ and ${T}_{C}$ are 245 and 285 K, respectively. The phase transition at ${T}_{i}$ is accompanied by a jump of electrical resistivity and peak of magnetoresistance. For TbBaC an external magnetic field of 14 T leads to decreasing ${T}_{i}$ from 245 to 85 K. At around ${T}_{M}$ (337 K for TbBaC) the first-order phase transition from semiconductive to quasimetallic state (high-temperature phase) has been observed. The transition occurs without changes of orthorhombic symmetry, and the unit-cell volume of the high-temperature phase was shown to be larger as compared to the low-temperature one (approximately 0.2%). It is supposed that the transition at ${T}_{M}$ results from oxygen vacancy ordering.

Optical characterization of the oxygen dimer in silicon has been performed for the first time. The vibrational IR absorption bands at 1012, 1060, and $1105{\mathrm{cm}}^{\ensuremath{-}1}$ are shown to arise from this complex. Using heat-treatment studies, the dimer binding energy is determined to be about 0.3 eV. Indications of the high migration ability of the dimer predicted earlier are found as well.

The crystal structure and the magnetic and electric properties of La0.70Ba0.30MnO3 − γ manganite (0≤γ≤0.30) with a perovskite structure were studied experimentally depending on the concentration of oxygen vacancies. The stoichiometric La0.70Ba0.30MnO3 compound (γ = 0) had cubic unit cell symmetry, which did not change as oxygen deficiency increased up to γ=0.30. A decrease in the content of oxygen in the compound under study caused the occurrence of several sequential magnetic phase transitions in the ground state, from the ferromagnetic state at γ=0 through the cluster spin glass state (γ=0.15) to the antiferromagnetic state (γ=0.30) with the presence of a small ferromagnetic component. The specific electric resistance grew to become activation in character at γ=0.11, and the metal-semiconductor transition disappeared as oxygen deficiency increased. The magnetoresistance of anion-deficient compositions included (1) magnetoresistance close to the temperature of the transition to the magnetically ordered state and (2) low-temperature magnetoresistance. The magnetoresistance peak at T C disappeared as γ increased (γ=0.11), whereas the low-temperature magnetoresistance component first increased to attain a maximum of about 34% at γ=0.15 and then decreased. The results of experimental studies were used to construct a magnetic phase diagram. These results could be interpreted within the framework of superexchange magnetic ordering theory. The suggestion was made that Mn3+-O-Mn3+ indirect exchange interactions were positive in the orbitally disordered phase only when manganese was in octahedral coordination, whereas these interactions became negative if at least one of the Mn3+ ions was five-coordinate or had a smaller coordination number.

The magnetic, resonance, and electric properties of LaxMnO3+δ (0.815≤x≤1.0) polycrystalline samples have been studied in the temperature range of 77–370 K and at high pressures of up to 11.5 kbar. It is shown that the increase in the La/Mn ratio gives rise to a change in the low temperature magnetic state from ferromagnetic to cluster spin glass, as well as to a drastic transformation of electric properties. A peculiar double-peaked shape is characteristic of the resistivity versus temperature curves for the intermediate range of x values. Within this range, the application of high pressures drastically changes both the value of resistivity and the character of its temperature dependence. It is shown that the approach, which regards the state of LaxMnO3+δ polycrystals as a mixture of interpenetrating paramagnetic insulating and ferromagnetic metallic phases, is able to successfully describe the peculiarities of the temperature dependence of total resistivity, as well as the features of its transformation under hydrostatic compression. It is demonstrated that the formation of a low temperature resistance peak is a result of a wide-temperature-region coexistence of the phases, which exhibits opposite trends in the temperature dependences of resistivity. The conclusion is made that not only does hydrostatic compression result in drastic changes in the relative volume fractions of the coexisting phases, but it also affects the intrinsic parameters of each of the phases.

The electronic properties and thermal stability of centers incorporating a vacancy and a group-V-impurity atom (P, As, Sb, or Bi) in Ge crystals have been investigated. The vacancy-group-V-impurity atom pairs ($E$ centers) have been induced by irradiation with $^{60}\mathrm{Co}$ $\ensuremath{\gamma}$ rays and studied by means of capacitance transient techniques with the use of Au-Ge Schottky barriers. It is argued that the $E$ centers in Ge have three charge states: double negative, single negative and neutral, and introduce two energy levels into the gap. There are pronounced changes in the activation energies of charge carrier emission for the particular states with the changes in the type of impurity atoms. The emission of an electron from the doubly negatively charged state of the centers is accompanied by a large change in entropy $(\ensuremath{\Delta}S)$, so, the free energy of the electron ionization, $\ensuremath{\Delta}G(\ensuremath{-}\ensuremath{-}∕\ensuremath{-})=\ensuremath{\Delta}H(\ensuremath{-}\ensuremath{-}∕\ensuremath{-})\ensuremath{-}T\ensuremath{\Delta}S(\ensuremath{-}\ensuremath{-}∕\ensuremath{-})$, changes significantly with temperature. Consequently, the position of the second acceptor level of the $E$ centers ${E(\ensuremath{-}\ensuremath{-}∕\ensuremath{-})={E}_{c}\ensuremath{-}\ensuremath{\Delta}G(\ensuremath{-}\ensuremath{-}∕\ensuremath{-})}$ is temperature dependent. In Ge crystals having shallow donor concentrations in the range ${10}^{13}--{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at equilibrium conditions half-occupancy of the doubly negatively charged state of the vacancy-group-V-impurity atom pairs occurs when the Fermi level is at ${E}_{c}\ensuremath{-}(0.18--0.22)\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Changes in the entropy of ionization and the energy of electron emission for the double negative state of the $E$ centers follow the Meyer-Neldel rule. It has been shown that the directly measured capture cross sections of electrons at the singly negatively charged $E$ centers are temperature dependent and can be described by the multiphonon-assisted capture model. The first acceptor level of the $E$ centers is in the lower part of the band gap. The formation of one vacancy--group-V-impurity atom complex results in the removal of at least two electrons from the conduction band in $n$-type Ge. It is thought that the $E$ centers are responsible for the fast free carrier removal and $n\ensuremath{\rightarrow}p$ conversion of the conductivity type in oxygen-lean Ge crystals upon electron- or $\ensuremath{\gamma}$-irradiation at room temperature. The thermal stability of the $E$ centers in Ge has been found to increase with an increase in the size of donor atoms.

Nd0.70Ba0.30MnO3+δ manganites with an ordered arrangement of Nd3+ and Ba2+ cations are fabricated. The initial stoichiometric Ba-disordered Nd0.70Ba0.30MnO3 solid solution is synthesized in air using a traditional ceramic technology, is characterized by an orthorhombic unit cell (space group Imma, Z = 4), and is ferromagnetic with Curie temperature T C ≈ 151 K. The average crystallite size in the initial sample is 〈D〉 ≈ 2.169 μm. Annealing of the initial sample in a reducing atmosphere at a pressure P[O2] ≈ 10−4 Pa and then in air at T = 800°C leads to the formation of a material with a crystallite size 〈D〉 ≈ 440 nm. This material consists of the following two perovskite phases: a Ba-ordered stoichiometric NdBaMn2O6 phase with a tetragonal unit cell (P4/mmm, Z = 2) and a Curie temperature of ∼301 K and a Ba-disordered hyperstoichiometric Nd0.90Ba0.10MnO3+δ phase with an orthorhombic unit cell (Imma, Z = 4) and T C ∼ 121 K. Significant changes in the magnetic properties are explained in terms of chemical phase separation with allowance for cation ordering.

An experimental investigation is performed of the crystal structure and magnetic and electrical properties of anion-deficient compositions of La 1− 3+ Ba x 2+ Mn3+O 3− 2/2− (0≤x≤0.30) which do not contain manganese ions of different vacancies. It is found that all reduced samples are single-phase perovskites with O-orthorhombic (x=0, 0.05), rhombohedral (x= 0.10, 0.15, 0.20, 0.25), and cubic (x=0.30) symmetry of the unit cell. It is observed that systems of the compounds being treated experience a transition from a weakly ferromagnetic (x=0) to a nonuniform ferromagnetic (0≤x≤0.10) state. An increase in the degree of nonstoichiometry with respect to oxygen leads to the emergence of the antiferromagnetic orbitally disordered phase. For compounds with x>0.20, clearly defined properties are observed that are characteristic of cluster spin glass with the freezing temperature of magnetic moments T F ∼45 K. The maximal amount of the ferromagnetic component is registered for x=0.15. All of the reduced samples are semiconductors. As the substitution level increases, the electrical resistivity (at room temperature) first decreases in magnitude (0≤x≤0.15) and then increases (x>0.15). The magnetoresistance of all reduced samples increases gradually upon transition to a magnetically ordered state and reaches its maximal value at the liquid nitrogen temperature. A hypothetical magnetic phase diagram is constructed for the system of anion-deficient compositions of La 1− 3+ Ba x 2+ Mn3+O 3− 2/2− (0≤x≤0.30) being treated. The investigation results contribute to understanding the nature of 180-degree indirect superexchange interactions between ions of trivalent manganese. It is assumed that the Mn3+-O-Mn3+ superexchange interactions are negative in the orbitally disordered phase in the case of pentahedral coordination of Mn3+ ions.