The (Zn1-xCox)(4)B6O13 (x = 0, 0.375, 0.5, 0.75) solid solutions were prepared using solid-state synthesis. The borates crystallize in the cubic crystal system (I 4 3m). According to the X-ray powder diffraction data, the unit cell parameter a slightly decreases with the increase in the x(Co2+) content. Crystal structure can be described as a sodalite cage composed of the [B24O48] clusters built from the vertex-sharing [BO4](5-) tetrahedra with the [MO4](6-) (M = Zn, Co) tetrahedra located inside the cage to form the [M4O13](18-) supertetrahedron. The O(1) atom is coordinated by four M2+ cations forming the isolated oxo-centered [OM4](6+) tetrahedra. A thermal behavior of the solid solutions was investigated by in-situ powder XRD (103-573 K) and thermal analysis (303-1343 K). Melting temperature decreases from 1217 (x(Co2+) = 0) to 1176 K (x(Co2+) = 0.75). Low thermal expansion was observed in the range 103-573 K. The increase in the x(Co2+) content leads to an increase in values of average linear thermal expansion coefficient alpha from 2.4 (x(Co2+) = 0) to 6.7 x 10-6 K-1 (x(Co2+) = 0.75). Thermal expansion coefficients were also calculated for the bond lengths, polyhedral volume and interpolyhedral angles. The increase in thermal expansion with both cationic substitution and temperature is attributed to a widening of angles between the cage-forming [BO4](5-) tetrahedra and an elongation of the M2+-O bonds. With the increasing x(Co2+) content, a decrease in intensity of photoluminescence bands is observed, which is associated with concentration quenching. Increasing x(Co2+) content reduces the bandgap from 5.82 eV to 4.05 eV as evidenced by absorption spectroscopy positioning these solid solutions as promising wide-band semiconductors.

Single and polycrystalline samples of Mg2 – xMn1 + xBO5 (x = 0.0, 0.2, 0.4) oxyborates have been obtained for the first time by spontaneous crystallization from the solution–melt and through a solid-state reaction. X-ray diffraction studies have shown that compounds are crystallized with increasing manganese content in the ludwigite (space group Pbam)–hulsite (space group P2/m)–orthopinakiolite (space group Pbam) series and belong to the “3 Å wallpaper” borate family. A common property of materials is the presence of octahedral complexes (walls) consisting of manganese ions with a mixed valence at even crystallographic sites. The dc magnetization and specific heat of the Mg2 – xMn1 + xBO5 (x = 0.0, 0.2, 0.4) compounds have been studied for the first time. These studies have shown that cooling is accompanied by magnetic transitions, which are due to the ordering of several magnetic subsystems.

Ni2CrBO5 has been synthesized and investigated by X-ray diffraction, dc magnetization, and specific heat measurements. The unusual cation distribution has been established: the M1 and M3 sites are occupied by Ni2+ ions, the M2 site is by Cr3+ ions, and the M4 site is mixed, featuring both Ni2+ and Cr3+ ions. The magnetic order onsets at TN = 140 K, which is confirmed by a 7-type peak in specific heat, followed by the dome-shaped anomaly of the magnetization at about 30 K. In the magnetically ordered state, a remarkable sequence of temperature-induced magnetization reversal sensitive to the measurement's regime is observed. Dominant antiferromagnetic interactions are characterized by the Weiss temperature 0 = -73 K. The effective magnetic moment of 5.37 i B per formula unit is close to the spin-only one. The effect of the cation distribution on the magnetic properties is discussed.

Single crystals of Co2AlBO5 were synthesized using flux. The structural, magnetic, and electrical properties have been studied, with emphasis on cationic disorder effects. The Al3+ and Co2+ ions share four symmetry inequivalent sites. Large amplitudes of the displacement parameters for the M2 and M4 metal sites and the O4 oxygen site were found. The compound exhibits two magnetic transitions at T-1 = 41 K and T-2 = 20 K and shows a high crystallographic anisotropy. The random cationic distribution induces magnetic softness and an increase in electrical resistivity. The sources of the cationic disorder and the approach for controlling it are discussed.

The magnetic and electronic structures of Fe4O5 have been investigated at ambient and high pressures via a combination of representation analysis, density functional theory (DFT+U) calculations, and Mössbauer spectroscopy. A few spin configurations corresponding to the different irreducible representations have been considered. The total-energy calculations reveal that the magnetic ground state of Fe4O5 corresponds to an orthogonal spin order. Depending on the magnetic propagation vector k, two spin-ordered phases with minimal energy differences are realized. The lowest energy magnetic phase is related to k = (0, 0, 0) and is characterized by ferromagnetic ordering of iron magnetic moments at prismatic sites along the b-axis and antiferromagnetic ordering of iron moments at octahedral sites along the c-axis. For the k = (1/2, 0, 0) phase, the moments in the prisms are antiferromagnetically ordered along the b-axis and the moments in the octahedra are still antiferromagnetically ordered along the c-axis. Under high pressure, Fe4O5 exhibits magnetic transitions with the corresponding electronic transitions of the metal-insulator type. At a critical pressure PC ∼ 60 GPa, the Fe ions at the octahedral sites undergo a high-spin to low-spin state crossover with a decrease in the unit-cell volume of ∼4%, while the Fe ions at the prismatic sites remain in the high-spin state up to 130 GPa. This site-dependent magnetic collapse is experimentally observed in the transformation of Mössbauer spectra measured at room temperature and high pressures.

Cu 2 FeBO 5 ludwigite single crystals have been grown from a solution–melt by spontaneous crystallization. Using the X-ray diffraction method, the crystal structure has been resolved in detail. Cations in sites M2, M3, and M4 have turned out to be structurally disordered. It has been found that oxygen atoms are disordered in one of five nonequivalent sites (O4). As can be seen from Mössbauer spectroscopy data, Fe 3+ ions occupy four nonequivalent sites with different distortions of coordination octahedra. In the temperature range 40 K ≤ T ≤ 300 K, the spectra represent a superposition of quadrupole doublets. Static susceptibility measurements have revealed two magnetic features at T 1 = 35 K and T 2 = 20 K and spin-glass effects. Specific heat measurements in the interval 4–300 K have not discovered magnetic-transition-related anomalies.

A tetravalent-substituted cobalt ludwigite Co2.5Ge0.5BO5 has been synthesized using the flux method. The compound undergoes two magnetic transitions: a long-range antiferromagnetic transition at TN1 = 84 K and a metamagnetic one at TN2 = 36 K. The sample-oriented magnetization measurements revealed a fully compensated magnetic moment along the a- and c-axes and an uncompensated one along the b-axis leading to high uniaxial anisotropy. A field-induced enhancement of the ferromagnetic correlations at TN2 is observed in specific heat measurements. The DFT+GGA calculation predicts the spin configuration of (↑↓↓↑) as a ground state with a magnetic moment of 1.37 μB/f.u. The strong hybridization of Ge(4s, 4p) with O (2p) orbitals resulting from the high electronegativity of Ge4+ is assumed to cause an increase in the interlayer interaction, contributing to the long-range magnetic order. The effect of two super-superexchange pathways Co2+-O-B-O-Co2+ and Co2+-O-M4-O-Co2+ on the magnetic state is discussed.

Changes in the magnetic properties of FeBO3 single crystals in the course of spin crossover occurring with increasing pressure up to 63 GPa are studied both experimentally and theoretically. Simultaneous measurements of the nuclear diffraction and nuclear forward scattering spectra make it possible to detect the antiferromagnetic high-spin state at low pressures up to 48 GPa and to reveal the antiferromagnetic state within the range characterized by the coexistence of Fe3+ ions in the high-spin and low-spin states, which occurs at pressures from 48 to 54 GPa, where the hysteresis takes place. Above 58 GPa, only the low-spin state is observed, whereas the magnetic order is absent down to 9 K. An analysis of changes in the exchange interactions manifesting themselves at spin crossover suggests that there exist competing ferromagnetic and antiferromagnetic contributions nearly compensating each other. A possible Néel temperature in the low-spin state does not exceed 7 K; thus, an ordered state cannot be observed in our experiments.

The investigations of the crystal structure, magnetic and electronic properties of Co3BO5 at high temperatures were carried out using powder X-ray diffraction, magnetic susceptibility, electrical resistivity, and thermopower measurements. The orthorhombic symmetry (Sp.gr. Pbam) was observed at 300 K and no evidence of structural phase transitions was found up to 1000 K. The compound shows a strong anisotropy of the thermal expansion. A large negative thermal expansion along the a-axis is observed over a wide temperature range (T = 300-600 K) with αa = -35 M K-1 at T = 500 K with simultaneous expansion along the b- and c-axes with αb = 70 M K-1 and αc = 110 M K-1, respectively. The mechanisms of thermal expansion are explored by structural analysis. The activation energy of the conductivity decreases significantly above 700 K. Electronic transport was found to be a dominant conduction mechanism in the entire temperature range. The correlations between the thermal expansion, electrical resistivity, and effective magnetic moment were revealed and attributed to the evolution of the spin state of Co3+ ions towards the spin crossover and gradual charge-ordering transition.

Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1 = 27 K and a ferrimagnetic one at TN2 = 14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected.

ludwigite N.V. Kazak, A. Arauzo, J. Bartolomé, M.S. Molokeev, V.A. Dudnikov, L.A. Solovyov, А.A.Borus, S.G. Ovchinnikov 1Kirensky Institute of Physics, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia 2Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza and Departamento de Física de la Materia Condensada, 50009 Zaragoza, Spain 3Servicio de Medidas Físicas, Universidad de Zaragoza, 50009 Zaragoza, Spain 4Research and Development Department, Kemerovo State University, Kemerovo, 650000, Russia 5Institute of Chemistry and Chemical Technology, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia

Mössbauer spectroscopy is used to study the characteristic features of the crystal lattice dynamics in powdered single crystals of Fe 1.75 V 0.25 BO 4 warwickite in the temperature range of 4.2–505 K. The Debye temperature (Θ D = 260 K) is determined from the temperature dependence of the probability of the Mössbauer effect in the thin absorber approximation. It is found that the electron delocalization related to the fast electronic transfer between neighboring Fe 3+ and Fe 2+ cations takes place in the temperature range of 260−505 K. As a result, iron cations exhibiting the mixed valence (Fe 2.5+ ) arise. This process correlates with a change in the elastic properties of the lattice. Such correlation leads to a sharp decrease in the recoil-free absorption of γ‑ray photons by the crystal lattice in the range of 260–400 K.

The magnetic contribution of the Co3+ ions in Co3BO5 has been investigated using the Co (K-edge) XMCD, dc magnetic susceptibility, and heat capacity measurements. The crystal structure of Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The results have been supplemented by the GGA+U calculations.

Cobalt pyroborate Co2B2O5 single crystals have been obtained by spontaneous crystallization from solution–melt. Powder X-ray diffraction measurements have revealed the triclinic symmetry $$P\overline 1 $$ with the lattice parameters $$a = 3.1666(7)$$ Å, $$b = 6.1543(6)$$ Å, $$c = 9.2785(12)$$ Å, $$\alpha = 104.240(5)^\circ $$ , $$\beta = 90.841(14)^\circ $$ , $$\gamma = 92.064(16)^\circ $$ , and $$V = 175.10(5)$$ Å3. Magnetic properties have been studied in the temperature range of 4.2–300 K and in magnetic fields up to 90 kOe by measuring the static magnetization and molar heat capacity. A transition to an antiferromagnetic state has been detected at TN = 45 K. A spin-flop transition occurs in the sample in strong magnetic fields.

The ludwigite Co2FeBO5 has been studied experimentally using 57Fe Mössbauer spectroscopy and theoretically using DFT + GGA calculations. The room-temperature Mössbauer spectra are composed of four quadrupole doublets corresponding to the high-spin Fe3+ ions in octahedral oxygen coordination. All components undergo splitting below 117 K due to the magnetic hyperfine fields. The DFT + GGA calculations performed for three models of Fe ion distributions have revealed that the ground state corresponds to the "Fe4(HS)" model with the high-spin Fe3+ ions located at the M4 site and the high-spin Co2+ ions located at the M1, M2, and M3 sites. A ferrimagnetic ground state, with the Co and Fe magnetic moments being nearly parallel to the b-axis and a total magnetic moment of circa 1.1μB f.u.-1, was found. The other Fe distributions cause an increase in the local octahedral distortions and transformation of the spin state. The calculated quadrupole splitting values are in good agreement with the experimental values obtained by Mössbauer spectroscopy.

The sign reversal of both magnetization and exchange bias field was studied in the polycrystalline Ni5.33Ta0.67B2O10. The crystal structure of Ni5.33Ta0.67B2O10 is quasi-low dimensional due to NiO6/TaO6 octahedra forming two dimensional infinite layers. The antiferromagnetic/ferrimagnetic phase transition was observed at T-N = 165 K. The negative exchange bias effect was found in the 30-90 K temperature range, whereas the positive exchange bias effect was observed at temperatures below 30 K. We assume that the exchange bias effect is due to different types of magnetic ordering of Ni2+ magnetic moments in two layers. (C) 2020 Elsevier B.V. All rights reserved.