Crystal Structure Phase Transitions in the Perovskites A(Cu1−xMnx)Mn4O12(A = Na+, Ca2+, Y3+, La3+)

X-Ray diffraction and neutronographical studies on the crystal structure of the perovskitelike compounds A(Cu_1−xMn_x)Mn₄O₁₂ have been undertaken. It has been shown that the samples with 50% of Mn³⁺ in the octahedral sites have a cubic unit cell, while the samples with higher Mn³⁺ composition may have a monoclinic or rhombohedral unit cell. The temperature increase raises the symmetry to the cubic one. The concentration ranges with different symmetry type have been determined. The lowering of the symmetry of the unit cell is due to the cooperative Jahn-Teller effect.     

Crystal structure and magnetic properties of the LaCo0.5Fe0.5O3 perovskite

The magnetic and crystal structures of the LaCo_0.5Fe_0.5O₃ perovskite are investigated. It is established that the unit cell of this compound at room temperature is characterized by rhombohedral distortions. As the temperature decreases, the compound undergoes a structural phase transition from the rhombohedral phase to the orthorhombic phase in the temperature range 200–300 K. The LaCo_0.5Fe_0.5O₃ perovskite has an antiferromagnetic structure with the G _z spatial orientation of the antiferromagnetic vector. The magnetic properties of the LaCo_0.5Fe_0.5O₃ perovskite are interpreted within a model according to which the ground state of Co³⁺ ions is a low-spin state and the existence of the weak ferromagnetic component is associated with the exchange interactions between the Fe³⁺ ions.     

Ferromagnetic insulating and reentrant spin glass behavior in Mg doped La0.75Sr0.25MnO3 manganites

In this study, the effect of Mg substitution on structural, magnetic and electrical properties of La_0.75Sr_0.25Mn_1?xMg_xO₃ and La_0.75Sr_0.25?xMg_xMnO₃ (nominal compositions) samples are investigated by XRD, Ac susceptibility and electrical resistivity measurements. It is found that Mg does not replace La in the perovskite lattice. Also the results show that by increasing Mg doping levels, the paramagnetic–ferromagnetic and metal–insulator transition temperatures decrease. The reason for decreasing transition temperatures with increasing Mg concentration is, that the long-range FM order has been destroyed by the Mg, which is randomly occupying Mn site. This leads to the suppression of double-exchange interaction in the Mn³⁺–O–Mn⁴⁺ networks. Also the reentrant spin glass (RSG) state accompanied by FM transition, exists in high doped samples. The RSG state could be understood on the basis of double exchange ferromagnetic interaction in Mn³⁺–O–Mn⁴⁺ and super-exchange antiferromagnetic interaction in the Mn⁴⁺–O–Mn⁴⁺ networks.     

Structural phase transitions in the LaMnO3+λ system

Structural phase transitions in the LaMnO_3+λ system are studied at various temperatures and with different Mn⁴⁺ ion concentrations. Two structural phase transitions are established. The O′-orthorhombic – O-orthorhombic transition (O′ O transition) is due to removing the cooperative Jahn-Teller distortions and occurs in the Mn⁴⁺ concentration range from 10 to 14% at room temperature. The temperature of this transition varies from 710 K for the stoichiometric LaMnO₃ to room temperature for LaMnO_3.07. The O-orthorhombic – rhombohedral transition (O R transition) is due to the change of the rotation axis of the undistorted MnO₆ octahedra from [100] and [110] to [11 1]. The temperature of this transition varies from 1010 K for LaMnO₃ to 230 K for LaMnO_3.135. The structural phase diagram of temperature vs composition is plotted, indicating the existence of regions of the phase with different structures.     

Magnetic phase transitions in LaMnO3+λ

Magnetic properties of nonstoichinometric lanthanum manganite LaMnO_3+λ as a function of the Mn⁴⁺ concentration (from 0 to 27%) and temperature (from 77 to 300 K) are investigated. The Mn⁴⁺ ions concentration depends on the degree of oxidation λ. It is shown that at 0–10% Mn⁴⁺ there exists an antiferromagnetic ordering and at 10–14% — mixed ferro-antiferromagnetic ordering, while at Mn⁴⁺ concentration exceeding 14% the ferromagnetic ordering takes place. The concentrational antiferromagnetic-ferromagnetic transition at 14% Mn⁴⁺ is due to the crystal structural O' — O orthorhombic transition. In ferromagnetic O-orthorhombic lanthanum manganite the phase ferromagnetic-paramagnetic transition occurs via intermediate magnetic state where in the paramagnetic matrix there are noninteracting ferromagnetic impurity clusters. The presence of such magnetic state gives rise to anomalous behaviour of magnetic and galvanomagnetic properties of LaMnO_3+λ near Curie temperature.     

Structure and magnetism of low-doped monoclinic crystals of (Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>Se<Subscript>3</Subscript>

The influence that doping with Mn³⁺ ions has on the fine crystal structure of the monoclinic compound Ga₂Se₃ was studied by single-crystal neutron diffraction and the measurements of magnetic properties. It was found that the structural state of the crystals changes even at relatively low doping levels (x = 0.04). Local Jahn-Teller distortions were shown to be responsible for the formation of the fine structure and magnetism in this type of compounds.     

Paramagnetic behavior of Zn1−xMnxO at room temperature

Mn‐doped ZnO were synthesized by solid state reaction and sol‐gel method respectively. It was found that samples synthesized by solid state reaction containing Mn₂O₃ and MnO₂ are a mixture of ferromagnetic and paramagnetic phases. Contrary, samples without second phases were found to be paramagnetic at room temperature. According to previous report, interface effects between Zn‐rich Mn₂O₃ and MnO₂ interfaces may be the origin of the ferromagnetic behavior observed in our samples prepared by solid reaction, so the alloy of Zn_1−xMn_xO may be paramagnetic at room temperature. Prepared by sol‐gel technique, the samples without second phases in the XRD patterns are also room‐temperature paramagnetic. Therefore we believe that the magnetism of Zn_1−xMn_xO is paramagnetic at room temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic properties of the Mg2FeV3O11−x site-disordered vanadate

The magnetic properties of the Mg₂FeV₃O_11−x ternary vanadate, characterized by disorder between diamagnetic Mg²⁺ and high-spin Fe³⁺ ions, are studied using dc magnetization and electron paramagnetic resonance (EPR). The dc susceptibility shows antiferromagnetic interactions between Fe³⁺ spins with a Curie–Weiss temperature of Θ = −50(1) K, followed by spin-glass-like freezing at T_f ≈ 2.8 K, suggesting significant spin frustration. Temperature-dependent EPR measurements confirm the antiferromagnetic coupling of Fe³⁺ spins at high temperatures, while a distinct divergence is observed at T ≈ 50 K. This behavior is associated with the formation of spin clusters providing two different energy scales for the magnetic interactions. The magnetic response of Mg₂FeV₃O_11−x is similar to that of the Zn-analogue compound, though the observed differences of the implicated energy scales indicate that magnetic inhomogeneity depends on the extent of cation disorder.     

Linear trinuclear mixed valance MnIII-MnII-MnIII complex: Synthesis,crystal structure and characterization

A new linear trinuclear Mn^III-Mn^II-Mn^III complex 1 has been synthesized and characterized by elemental, spectral, X-ray and magnetic analysis. X-ray diffraction studies show that the central Mn^II ion is located at a crystallographic inversion center and is triply bridged to the terminal Mn^III ions through one methoxide, one syn-syn carboxylate and one hydroxyl oxygen bridges with the short Mn^III…Mn^II distance that is 3.047 Å. The intermolecular C-H…O, C-H…π and ring-metal interactions are observed in the hydrogen-bonded assembly of 1. Magnetic studies reveal that the mixed-valence complex 1 has S = 3/2 ground state with antiferromagnetic exchange interactions between Mn^II and Mn^III ions.     

Magnetic behavior and microstructure of x%V2O5·(100−x)%As2O3 glasses

The magnetic properties and microstructure of x%V₂O₅·(100−x)%As₂O₃ glasses with x varying in the range 40 to 90 mol% were investigated in order to elucidate their magnetic ordering. Weak antiferromagnetic interactions between V⁴⁺ ions were observed. Glasses with x ⩾ 60 separated into two glassy phases. The effect of microstructure on magnetic properties of these glasses was investigated. Phase separation increases with increasing V₂O₅ content and produces a broadening of the EPR line width of glasses with high vanadium content (x > 70). The c = V⁴⁺/V_total ratio of x%V₂O₅·(100−x)%As₂O₃ glasses, determined from EPR and chemical analysis, are considerably greater than those usually reported for x%V₂O₅·(100−x)%P₂O₅ glasses.     

Infrared reflection spectra of selected modifications of SiO2 and Al2O3

Infrared reflection spectra were measured within the wave number range of 1400 – −400 cm⁻¹ for following SiO₂ and Al₂O₃ modifications: quartz, opal, quartz glass, cristobalite, tridymite, corundum, sapphire, ruby, gibbsite, boehmite, and diaspore. The spectral bands were attributed, according to literature quotations, to individual vibrations of bonds Si O, O Si O, Si O Si, Al O, O Al O, and Al O Al. The evaluation of infrared reflection spectra has shown that they are very sensitive to internal vibrations of Si O, Al O, which are affected by the symmetry of the surrounding crystal field. They are therefore selective for individual minerals and thus suitable for quick identifying purposes.     

Neutron diffraction studies of the orthorhombic lattice distortions and magnetic properties of lanthanum manganite obtained under different technological regimes

Neutron diffraction studies have been carried out on two series of lanthanum manganite compositions obtained under different technological regimes. Based on the plot of angle dependence β° characterizing the degree of monoclinic distortions on “tolerance factor” t Mn³⁺ concentration is determined in the compositions obtained by us. With partial oxygen pressure increasing in annealing atmosphere the crystalline transition (orthorhomb-cube) temperature lowers from 583 ± 10°K to 508 ± 10°K. The correlation between magnetic properties of the compositions obtained and the degree of orthorhombic distortions γ′ = \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm b}\sqrt {{\rm 2/c}} $\end{document}\end{document} is established. Neutron diffraction studies of magnetic properties of LaMnO₃ compositions show that with decrease of orthorhombic distortion degree the decrease of antiferromagnetic component and lowering of the Néel point occur, while the ferromagnetic contribution increases and the Curie point rises.     

Indications of the magnetic state in the charge distributions in MnO, CoO, and NiO. I: Para-and antiferromagnetism of MnO

X-ray diffraction intensities from MnO and CoO were measured above and below their Néel temperatures and from NiO, below the Néel temperature. To detect possible characteristics of the paramagnetic and antiferromagnetic states of the crystals, the data were subjected to direct multipole analysis of the atomic-charge densities. For MnO, both spherical and nonspherical accumulation-of-charge densities indicate the exchange of the roles played by manganese and oxygen in the magnetic phase transition. Both spherical and nonspherical features characteristic of the ionic nature are inherent in both states. The electron counts of the density peaks correspond to Mn²⁺ and O^1?, with the tenth electron of O2? being distributed in a wider region. In the paramagnetic state, there is an electronic Mn-Mn bond which seems to be formed due to coupling with the tenth electron of O^2? and builds up a three-dimensional net of the charge density with the “cages” surrounding oxygen atoms. In the antiferromagnetic state, some Mn-Mn bonds disappear, while the preserved nonspherical ionic features enhance the role of oxygen atoms in electronic coupling.     

Crystal growth of Ca3−xYxMn2Ge3O12 and phase relations in the PbO?B2O3?GeO2 based melts

The bulk crystals of Ca_3−xY_xMn₂Ge₃O₁₂ garnets were grown on a seed from the PbO B₂O₃ GeO₂ based melts, where the primary crystallization field of garnets was found. The saturation temperatures of the melts were determined before each growth run. The dependence of Y incorporation into the crystals versus the melt composition was also evaluated.     

Instability of the local environment of Mn<Superscript>2+</Superscript> in BaF<Subscript>2</Subscript>

Excitation and luminescence spectra and luminescence lifetime of Mn²⁺ ion in BaF₂ crystals at 77 K have been investigated for the first time. Mn²⁺ ions in the crystal are coordinated by six and eight fluorine ions, have a trigonal environment, and form exchange-coupled pairs. Several types of centers of the Mn²⁺ ion are formed mainly because of the large difference in the Mn²⁺ and Ba²⁺ ionic radii, which causes instability of the local structure around the activator and its strain, and exchange striction. An increase in the impurity concentration enhances these factors, thus changing the relative concentration of various centers.     

Flux growth of ZnO crystals doped by transition metals

ZnO crystals doped with Cr, Mn, Fe and Co were grown by the flux method. The prepared crystals revealed no phase separation detectable by X-ray diffraction. Structure properties were characterized by Raman and photoluminescence spectroscopy. For ZnO:Co, Mn and Cr, no spontaneous ferromagnetic moment was observed up to T=2 K whereas for the ZnO:Fe crystals the m(H) curves suggest the existence of 5 nm superparamagnetic iron clusters. At low temperatures the m(H) curves can be interpreted as a superposition of major paramagnetic and minor antiferromagnetic contribution. The paramagnetic part corresponds to the presence of Co²⁺, Fe³⁺, Mn²⁺ ions and small Cr atom clusters.     

Crystal and Molecular Structure of 1-Ethyl-3-[tris(trimethylsiloxanyl)silyl]pyrrolinium Hydrochloride

The crystal and molecular structure of 1-Ethyl-3[tris(trimethylsiloxyl)silyl]pyrrolinium hydro-chloride (C₁₅H₃₈N⁺O₃Si₄ · C¹⁻) has been determined by direct methods. The title compound crystallizes in the monoclinic space group C2/c with a = 20.640(3), b = 19.494(2), c = 27.34(3) Å, β = 90.60(4)°, V = 11000(13) ų, Z = 16, D_x = 1.034 Mg m⁻³. There are two molecules with different conformations in the crystal. The pyrroline rings are non-planar.-The Si O Si angles range from 149(1)° to 163(1)°. Two of the SiMe₃ groups are disordered. All molecules are connected by C¹⁻ – N⁺ contacts and C¹⁻ - HN⁺ hydrogen bonds to form double chains.     

High-pressure Synthesis and Properties of Manganates with Ilmenite Structure

A²⁺Mn⁴⁺0₃(A  Ni, Co, Mg, Cu) manganates with ilmenite structure have been obtained by solid phase synthesis at high pressure and temperature. The synthesis conditions for single phase samples have been determined, and investigations of the magnetic, electric, magnetooptic, and resonance properties were carried out.     

Single crystal preparation of MnxCr3–xO4 by flux method

Mn_xCr_{3 x}O₄ was prepared by the flux method. Melts of PbO PbF₂, Bi₂O₃ B₂O₃, B₂O₃, Na₂B₄O₇, and Na₂W₂O₇ Na₂WO₄ were used. The best results could be yielded with the PbO PbF₂ flux, from which crystals with 2–4 mm in thickness were grown. The Bi₂O₃ B₂O₃ flux produced crystals with 1–2 mm in thickness. The spinell structure of the chromite was proved by X-ray investigation.     

Structural aspects of the giant oxygen isotope effect in perovskite manganese oxides

The structure of manganites R _1?x A _x MnO₃ (R = La, Sm, or a mixture of rare earth cations; A = Ca, Sr; x = 0.3, 0.5) has been studied by neutron diffraction to find out the reasons for the giant oxygen isotope effect—the transition from a low-temperature metallic state to an insulating state as a result of replacement of ¹⁶O with ¹⁸O. It is shown that this effect is observed in compositions in which the two phases P1 and P2 coexist at low temperatures. They have the same crystal symmetry (sp. gr. Pnma) but different types of Jahn-Teller distortions of oxygen octahedra and different magnetic structure. Phase P1 has a conductivity of the metallic type, weakly distorted MnO₆ octahedra, and a ferromagnetic structure. Phase P2 is insulating, with MnO₆ octahedra extended or compressed in the apical direction and the moments of Mn ions forming an antiferromagnetic structure. The relative volume of phases P1 and P2 in samples depends on the average radius of the A cation and changes occurring upon replacement of ¹⁶O with ¹⁸O. The percolation transition from the metallic to insulating state upon substitution of ¹⁶O with ¹⁸O is caused by the sharp decrease in the volume of the ferromagnetic metallic phase in favor of the insulating antiferromagnetic phase. An effect of the sample microstructure on the formation of the two-phase state is found.