Exotic Magnetism in Perovskite KOsO_{3}.

A new perovskite KOsO_{3} has been stabilized under high-pressure and high-temperature conditions. It is cubic at 500 K (Pm-3m) and undergoes subsequent phase transitions to tetragonal at 320 K (P4/mmm) and rhombohedral (R-3m) at 230 K as shown from refining synchrotron x-ray powder diffraction (SXRD) data. The larger orbital overlap integral and the extended wave function of 5d electrons in the perovskite KOsO_{3} allow to explore physics from the regime where Mott and Hund's rule couplings dominate to the state where the multiple interactions are on equal footing. We demonstrate an exotic magnetic ordering phase found by neutron powder diffraction along with physical properties via a suite of measurements including magnetic and transport properties, differential scanning calorimetry, and specific heat, which provide comprehensive information for a system at the crossover from localized to itinerant electronic behavior.

Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn7O12 Using 57Fe Probe Mössbauer Spectroscopy.

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with 57Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn6.9657Fe0.04O12 undergoes a cascade of structural (T1 ≈ 590 K, T2 ≈ 442 K, and T3 ≈ 240 K) and magnetic (TN1 ≈ 57 K, TN2 ≈ 50 K, and TN3 ≈ 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi3+ ions, confirmed the presence of the local dipole moments pBi, which are randomly oriented in the paraelectric cubic phase (T > T1). The unusual behavior of the parameters of hyperfine interactions between T1 and T2 was attributed to the dynamic Jahn-Teller effect that leads to the softening of the orbital mode of Mn3+ ions. The parameters of the hyperfine interactions of 57Fe in the phases with non-zero spontaneous electrical polarization (Ps), including the P1 ↔ Im transition at T3, were analyzed. On the basis of the structural data and the quadrupole splitting Δ(T) derived from the 57Fe Mössbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe Ps(T) dependence. The Ps(T) dependence around the Im ↔ I2/m phase transition at T2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < TN1) are also discussed.

Anisotropic Thermal Expansion and a Second-Order Charge Order Transition in the Ferrimagnetic Dy2CuZnMn4O12 Perovskite with Triple A-Site Cation Ordering.

Dy2CuZnMn4O12 perovskite, belonging to the A-site columnar-ordered quadruple perovskite family with the general composition of A2A'A″B4O12, was prepared by a high-pressure, high-temperature method at 6 GPa and 1500 K. Its crystal structure was studied by synchrotron powder X-ray diffraction between 100 and 800 K. The ideal cation distribution (without antisite disorder) was found to be realized within the sensitivity of the synchrotron X-ray diffraction method. Between 100 and 400 K, it crystallizes in space group Pmmn (no. 59) and has layered charge ordering of Mn3+ and Mn4+ at the B sites. Above 425 K, it crystallizes in space group P42/nmc (no. 137) with one crystallographic B site and an average Mn3.5+ oxidation state. The charge ordering transition (at TCO = 425 K) appears to be of the second order as no anomalies were found on differential scanning calorimetry curves and temperature dependence of the unit cell volume, and the orthorhombic a and b lattice parameters merge gradually. The compound demonstrates anisotropic thermal expansion with the c lattice parameter decreasing with increasing temperature above 280 K. A ferrimagnetic transition occurs at TC = 116 K with an additional, gradual rise of magnetic susceptibilities below 45 K, probably due to increases of the ordered moments of the Dy sublattices.

Cd2FeReO6: A High-TC Double Perovskite Oxide with Remarkable Tunneling Magnetoresistance.

In this study, we successfully synthesized the double perovskite oxide Cd2FeReO6 by using a high-temperature and high-pressure method. The crystal structure was confirmed to belong to the P21/n space group, exhibiting approximately 68% ordering of Fe3+ and Re5+ ions at the perovskite B-site with the remaining regions showing antisite disorder. The measured Curie temperature of Cd2FeReO6 was 460 K, slightly lower than expected but still significantly above room temperature. Remarkably, Cd2FeReO6 displayed a remarkable low-field butterfly type tunneling magnetoresistance of -23% (-37% between the lowest and the largest values) at 5 K and 90 kOe, the highest among the A2FeReO6 (A = Ca, Sr, Pb, Ba) family. First-principles calculations provided insight into the origin of this observed magnetoresistance behavior, revealing Cd2FeReO6's half-metallic ferrimagnetic nature. This research extends our understanding of the double perovskite family and emphasizes its potential significance in the domains of spintronics and materials science. The exploration of differing magnetoresistance behaviors between Cd2FeReO6 and Ca2FeReO6, along with the influence of antisite disorder in Cd2FeReO6, opens intriguing avenues for further research.

Hybrid Improper Ferroelectricity in Columnar (NaY)MnMnTi4 O12.

We show that cation ordering on A site columns, oppositely displaced via coupling to B site octahedral tilts, results in a polar phase of the columnar perovskite (NaY)MnMnTi4 O12 . This scheme is similar to hybrid improper ferroelectricity found in layered perovskites, and can be considered a realisation of hybrid improper ferroelectricity in columnar perovskites. The cation ordering is controlled by annealing temperature and when present it also polarises the local dipoles associated with pseudo-Jahn-Teller active Mn2+ ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass. Below TN ≈12 K, Mn2+ spins order, making the columnar perovskites rare systems in which ordered electric and magnetic dipoles may reside on the same transition metal sublattice.

Abnormal Eu3+ → Eu2+ Reduction in Ca9-xMnxEu(PO4)7 Phosphors: Structure and Luminescent Properties.

ß-Ca3(PO4)2-type phosphors Ca9-xMnxEu(PO4)7 have been synthesized by high-temperature solid-phase reactions. The crystal structure of Ca8MnEu(PO4)7 was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu3+ → Eu2+ in air was observed in Ca9-xMnxEu(PO4)7 according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu3+ shows partial reduction and coexistence of Eu2+/3+ states. It reflects in combination of a broad band from the Eu2+ 4f65d1 → 4f7 transition and a series of sharp lines attributed to 5D0 → 7FJ transitions of Eu3+. Eu2+/Eu3+ ions are redistributed among two crystal sites, M1 and M3, while Mn2+ fully occupies octahedral site M5 in Ca8MnEu(PO4)7. The main emission band was attributed to the 5D0 → 7F2 electric dipole transition of Eu3+ at 395 nm excitation. The abnormal quenching of Eu3+ emission was observed in Ca9-xMnxEu(PO4)7 phosphors with doping of the host by Mn2+ ions. The phenomena of abnormal reduction and quenching were discussed in detail.

Ferrimagnetic Ordering and Spin-Glass State in Diluted GdFeO3-Type Perovskites (Lu0.5Mn0.5)(Mn1-xTix)O3 with x = 0.25, 0.50, and 0.75.

ABO3 perovskite materials with small cations at the A site, especially those with ordered cation arrangements, have attracted a great deal of interest because they show unusual physical properties and deviations from the general characteristics of perovskites. In this work, perovskite solid solutions (Lu0.5Mn0.5)(Mn1-xTix)O3 with x = 0.25, 0.50, and 0.75 were synthesized by means of a high-pressure, high-temperature method at approximately 6 GPa and approximately 1550 K. All the samples crystallize in the GdFeO3-type perovskite structure (space group Pnma) and have random distributions of the small Lu3+ and Mn2+ cations at the A site and Mn4+/3+/2+ and Ti4+ cations at the B site, as determined by Rietveld analysis of high-quality synchrotron X-ray powder diffraction data. Lattice parameters are a = 5.4431 Å, b = 7.4358 Å, c = 5.1872 Å (for x = 0.25); a = 5.4872 Å, b = 7.4863 Å, c = 5.2027 Å (for x = 0.50); and a = 5.4772 Å, b = 7.6027 Å, c = 5.2340 Å (for x = 0.75). Despite a significant dilution of the A and B sublattices by non-magnetic Ti4+ cations, the x = 0.25 and 0.50 samples show long-range ferrimagnetic order below TC = 89 K and 36 K, respectively. Mn cations at both A and B sublattices are involved in the long-range magnetic order. The x = 0.75 sample shows a spin-glass transition at TSG = 6 K and a large frustration index of approximately 22. A temperature-independent dielectric constant was observed for x = 0.50 (approximately 32 between 5 and 150 K) and for x = 0.75 (approximately 50 between 5 and 250 K).

Decay Kinetics of Gd3Al2Ga3O12:Ce3+ Luminescence under Dense Laser Irradiation.

The decay kinetics of Gd3Al2Ga3O12:Ce3+ single crystal luminescence were studied under dense laser excitation. It was shown that the decay times as well as the intensity of Ce3+ luminescence depend on the excitation density. The observed effects were ascribed to the interaction between excitons as well as to the features of energy transfer from the excitons to Ce3+. The numerical simulation of the experimental results was performed for justification of the proposed model.

High-Pressure Synthesis and Magnetic and Electrical Properties of Fe-Doped Bi3Re3O11 and Bi3Os3O11.

Under high-pressure and high-temperature conditions, doped Bi3Re3O11 and Bi3Os3O11 with Fe up to 29 atomic % were synthesized. The crystal structures and chemical compositions of Bi3Os2.45Fe0.55O11 and Bi3Re2.13Fe0.87O11 were determined by synchrotron powder X-ray diffraction and electron probe microanalysis. Both crystal structures were explained by a KSbO3-type model with the space group Pn3̅. Magnetic and electronic transport property measurements showed that Bi3Os2.45Fe0.55O11 exhibited a ferrimagnetic transition at the highest magnetic ordering temperature of 490 K in the KSbO3-type, while Bi3Re2.13Fe0.87O11 exhibited a spin glassy behavior below 22 K. The magnetoresistance at 5 K and 90 kOe was almost zero for Bi3Os2.45Fe0.55O11, but -10% for Bi3Re2.13Fe0.87O11. These results suggest that KSbO3- type 5d oxides, which exhibit only weak temperature-dependent paramagnetism to date, are a group of compounds that can be converted into spintronic materials by doping with 3d elements, leading to the development of new KSbO3-type materials with both theoretical and practical significance.

Dielectric and Spin-Glass Magnetic Properties of the A-Site Columnar-Ordered Quadruple Perovskite Sm2CuMn(MnTi3)O12.

Perovskite-type ABO3 oxides show a number of cation-ordered structures, which have significant effects on their properties. The rock-salt-type order is dominant for B cations, and the layered order for A cations. In this work, we prepared a new perovskite-type oxide, Sm2CuMn(MnTi3)O12, with a rare columnar A-site order using a high-pressure, high-temperature method at about 6 GPa and about 1700 K. Its crystal structure was studied with synchrotron powder X-ray diffraction. The compound crystallizes in space group P42/nmc (No. 137) at room temperature with a = 7.53477 Å and c = 7.69788 Å. The magnetic properties of the compound were studied with dc and ac magnetic susceptibility measurements and specific heat. Spin-glass (SG) magnetic properties were found with TSG = 7 K, while specific heat, in the form of Cp/T, showed a strong, very broad anomaly developing below 20 K and peaking at 4 K. The dielectric constant of Sm2CuMn(MnTi3)O12 was nearly frequency and temperature independent between 8 K and 200 K, with a value of about 50. Cu2+ doping drastically modified the magnetic and dielectric properties of Sm2CuMn(MnTi3)O12 in comparison with the parent compound Sm2MnMn(MnTi3)O12, which showed a long-range ferrimagnetic order at 34-40 K. The antisite disorder of Cu2+ and Mn2+ cations between square-planar and octahedral sites was responsible for the SG magnetic properties of Sm2CuMn(MnTi3)O12.

Crystal structure of the cubic double-perovskite Sr2Cr0.84Ni0.09Os1.07O6.

The crystal structure of the cubic double-perovskite Sr2Cr0.84Ni0.09Os1.07O6, grown at high pressure, was solved using intensity data measured at 113 K. The Os site was modelled with a partial Ni occupancy, and the Cr site was modelled with both Os and Ni partial occupancy. The refined structure shows that this cubic form is stable at 113 K.

Triple A-Site Cation Ordering in the Ferrimagnetic Y2CuGaMn4O12 Perovskite.

A new member of A-site columnar-ordered A2A'A″B4O12 quadruple perovskites with the composition of Y2CuGaMn4O12 was prepared by a high-pressure, high-temperature method at 6 GPa and about 1500 K. Its crystal structure and cation distributions were studied by powder synchrotron X-ray and neutron diffraction. There is a triple A-site cation ordering with some degrees of anti-site disorder among sites occupied by 3d transition metals: [Y2]A[Cu0.8Mn0.2]A'[Ga0.8Mn0.2]A″[Mn3.6Cu0.2Ga0.2]BO12. It has the space group P42/nmc (no. 137) between 1.5 and 873 K with a = 7.33884 Å and c = 7.66251 Å at 297 K. Despite anti-site disorder, it exhibits a long-range ferrimagnetic order at TC = 115 K with the ordered moment of 2.19 µB at each B site and 0.89 µB at the A' or A″ site. Magnetic moments are aligned along the c axis; all moments are ordered ferromagnetically at the B sites, and the moments at the A' or A″ site are ordered in the opposite direction. Cu2+ doping drastically changes magnetic properties as "parent" Y2MnGaMn4O12 just shows spin-glass magnetic properties without long-range ordering. Anisotropic thermal expansion was observed in Y2CuGaMn4O12: the lattice parameter a almost linearly decreases from 1.5 K to TC and then monotonically increases up to 873 K (almost linearly from 300 K); the parameter c monotonically increases from 1.5 to 300 K and then decreases up to 600 K.

Temperature-Driven Transformation of the Crystal and Magnetic Structures of BiFe0.7Mn0.3O3 Ceramics.

The compound BiFe0.7Mn0.3O3 consisting at room temperature of coexistent anti-polar orthorhombic and polar rhombohedral phases has a metastable structural state, which has been studied by laboratory X-ray, synchrotron and neutron diffraction, magnetometry, differential thermal analysis, and differential scanning calorimetry. Thermal annealing of the sample at temperatures above the temperature-driven phase transition into the single phase rhombohedral structure (~700 K) causes an increase of the volume fraction of the rhombohedral phase at room temperature from ~10% up to ~30%, which is accompanied by the modification of the magnetic state, leading to strengthening of a ferromagnetic component. A strong external magnetic field (~5 T) applied to the sample notably changes its magnetic properties, as well as provides a reinforcement of the ferromagnetic component, thus leading to an interaction between two magnetic subsystems formed by the antiferromagnetic matrix with non-collinear alignment of magnetic moments and the nanoscale ferromagnetic clusters coexisting within it. The modification of the structural state and magnetic properties of the compounds and a correlation between different structural and magnetic phases are discussed focusing on the effect of thermal annealing and the impact of an external magnetic field.

Aurivillius Phase Bi4V3O12 with d1 Magnetic Cations, Anisotropic and Negative Thermal Expansion, Multiple Structural Transitions, and Low-Dimensional Magnetism.

Aurivillius phases are an important class of inorganic compounds as they often show ferroelectric properties, and some members of this family are used in nonvolatile ferroelectric memories. The majority of Aurivillius phases have nonmagnetic d0 cations in the perovskite block. Bi4Ti3O12 is the best-known and extensively studied compound within this family. Here, using a high-pressure, high-temperature synthesis method, we could successfully prepare a full magnetic analogue, Bi4V3O12, with d1 cations. Bi4V3O12 is unstable in air above about 520 K. However, in an inert atmosphere, Bi4V3O12 demonstrates two first-order reversible structural transitions near 525 and 760 K. The high-temperature prototypical phase is the same in both Bi4V3O12 and Bi4Ti3O12 with tetragonal (T) I4/mmm symmetry and aT = 3.85608(5) Å and cT = 32.6920(8) Å (at 850 K) for Bi4V3O12, while the low-temperature phases are different. Bi4V3O12 shows anisotropic thermal expansion above 300 K and negative volumetric thermal expansion above about 700 K. Magnetic measurements showed a broad maximum near 70 K on magnetic susceptibility, indicating the presence of low-dimensional magnetism with strong antiferromagnetic interactions between V4+ ions with the Curie-Weiss temperature of about -370 K. But no long-range magnetic ordering was found in Bi4V3O12 down to 2 K.

Structural and Magnetic Phase Transitions in BiFe1 - xMnxO3 Solid Solution Driven by Temperature.

The crystal structure and magnetic state of the (1 − x)BiFeO3-(x)BiMnO3 solid solution has been analyzed by X-ray diffraction using lab-based and synchrotron radiation facilities, magnetization measurements, differential thermal analysis, and differential scanning calorimetry. Dopant concentration increases lead to the room-temperature structural transitions from the polar-active rhombohedral phase to the antipolar orthorhombic phase, and then to the monoclinic phase accompanied by the formation of two-phase regions consisting of the adjacent structural phases in the concentration ranges 0.25 < x1 < 0.30 and 0.50 ≤ x2 < 0.65, respectively. The accompanied changes in the magnetic structure refer to the magnetic transitions from the modulated antiferromagnetic structure to the non-colinear antiferromagnetic structure, and then to the orbitally ordered ferromagnetic structure. The compounds with a two-phase structural state at room temperature are characterized by irreversible temperature-driven structural transitions, which favor the stabilization of high-temperature structural phases. The magnetic structure of the compounds also exhibits an irreversible temperature-induced transition, resulting in an increase of the contribution from the magnetic phase associated with the high-temperature structural phase. The relationship between the structural parameters and the magnetic state of the compounds with a metastable structure is studied and discussed depending on the chemical composition and heating prehistory.

Unexpected Phonon Behaviour in BiFexCr1-xO3, a Material System Different from Its BiFeO3 and BiCrO3 Parents.

The dielectric function and the bandgap of BiFe0.5Cr0.5O3 thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO3 and BiCrO3. The bandgap value of BiFe0.5Cr0.5O3 is lower than that of BiFeO3 and BiCrO3, due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm-1 with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe0.5Cr0.5O3 thin films and BiFexCr1-xO3 polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe0.3Cr0.7O3.

K5Eu1-xTbx(MoO4)4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb3+ → Eu3+ Energy Transfer.

This paper describes the influence of sintering conditions and Eu3+/Tb3+ content on the structure and luminescent properties of K5Eu1-xTbx(MoO4)4 (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K5Tb(MoO4)4 (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group R3̅m) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the ß â†” α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)LT-K5Eu0.6Tb0.4(MoO4)4 structure was refined in the C2/m space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)D superspace group C2/m(0ß0)00 and the modulation vector q = 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to 5D4 → 7FJ (J = 3-6) Tb3+ transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the 5D4 → 7F5 Tb3+ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2-0.9 at λex = 377 nm exhibited an intense red emission at ∼615 nm due to the 5D0 → 7F2 Eu3+ transition, thus indicating an efficient energy transfer from Tb3+ to Eu3+.

Crystal and Magnetic Structure Transitions in BiMnO3+δ Ceramics Driven by Cation Vacancies and Temperature.

The crystal structure of BiMnO3+δ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P21/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO3+δ (δ ≤ 0.14) at temperatures above 470 °C.

The rich physics of A-site-ordered quadruple perovskite manganites AMn7O12.

Perovskite-structure AMnO3 manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron-phonon coupling, and Jahn-Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn7O12 were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn7O12 compounds host rich physics beyond the canonical AMnO3 materials.

KTb(MoO4)2 Green Phosphor with K+-Ion Conductivity: Derived from Different Synthesis Routes.

The influence of different synthesis routes on the structure and luminescent properties of KTb(MoO4)2 (KTMO) was studied. KTMO samples were prepared by solid-state, hydrothermal, and Czochralski techniques. These methods lead to the following different crystal structures: a triclinic scheelite-type α-phase is the result for the solid-state method, and an orthorhombic KY(MoO4)2-type γ-phase is the result for the hydrothermal and Czochralski techniques. The triclinic α-KTMO phase transforms into the orthorhombic γ-phase when heated at 1273 K above the melting point, while KTMO prepared by the hydrothermal method does not show phase transitions. The influence of treatment conditions on the average crystallite size of orthorhombic KTMO was revealed by X-ray diffraction line broadening measurements. The electrical conductivity was measured on KTMO single crystals. The orthorhombic structure of KTMO that was prepared by the hydrothermal method was refined using synchrotron powder X-ray diffraction data. K+ cations are located in extensive two-dimensional channels along the c-axis and the a-axis. The possibility of K+ migration inside these channels was confirmed by electrical conductivity measurements, where strong anisotropy was observed in different crystallographic directions. The evolution of luminescent properties as a result of synthesis routes and heating and cooling conditions was studied and compared with data for the average crystallite size calculation and the grain size determination. All samples' emission spectra exhibit a strong green emission at 545 nm due to the 5D4 → 7F5 Tb3+ transition. The maximum of the integral intensity emission for the 5D4 → 7F5 emission under λex = 380 nm excitation was found for the KTMO crashed single crystal.