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.     

Anomalous behaviour of LaMnO3 with small structure distortions in magnetic phase transitions range

The magnetic field effect on conductivity and intensity of the magnetic neutron diffraction reflections as well as magnetization of lanthanum manganite with different crystal lattice distortions in the ferromagnetic-paramagnetic transitions range are investigated. It is shown that in the investigated samples near the paramagnetic-ferromagnetic transition temperature at H = 0 there exists a relatively wide transition range where the magnetic ordering occurs with a relatively small external magnetic field applied.     

Magnetic phase transitions in the Mn1-xCrxSe (x ≦ 0.4) system

Neutron diffraction and magnetic investigations have been made on the Mn_1-xCr_xSe (0 ≦ x ≦ 0.4) system solid solutions. Evidence is found of the appearance of magnetization in the samples with composition 0.05 ≦ x ≦ 0.4, exceeding in value that of antiferromagnet. The temperatures of magnetic transition, obtained from the magnetic measurements, do not fit the temperatures of disappearance of antiferromagnetic ordering obtained from the neutron diffraction studies. The antiferromagnetic structure with compositions 0.03 ≦ x ≦ 0.3 is analogous to the α-MnSe structure. The results are discussed under assumption of simultaneous coexistence of phases with different type of magnetic ordering.     

Structural phase transitions in heteroepitaxial ferroelectric films

The changes in the structural—deformational characteristics occurring during the phase transitions in the crystal lattice of epitaxially grown thin (Ba,Sr)TiO₃ films on the (001) cleavage of MgO crystals have been studied. It is shown that microdeformations along the surface normal of the wall of a c-domain film increase with the temperature and attain maximum values in the vicinity of the phase transition. No decrease in the dimensions of the coherent-scattering regions was observed. The comparison of the deformation of the “average” lattice and microdeformations led to the assumption that the transition from the paraelectric to the ferroelectric phase is accompanied by the dislocation-induced formation of a dipole-glass-type intermediate phase at the temperatures exceeding that of the phase transition.     

Sequence of phase transitions in PbHfO<Subscript>3</Subscript>

The temperature changes in the PbHfO₃ structure in the temperature range of 20 < t < 400°C have been studied by X-ray powder diffraction. The sequence of phase changes with an increase in temperature was found to be as follows: orthorhombic phase Pbam (O1), orthorhombic phase C2mm (O2), tetragonal phase P4mm (T), and cubic phase Pm3m (C). The C2mm and P4mm phases are ferroelectric, which is confirmed by measuring the dependences ɛ(t). The similarity of the transition pattern obtained with the known transition sequences for ferroelectric (barium titanate, potassium niobate, and lead titanate) and antiferroelectric (lead zirconate) oxides is analyzed.     

Orthorhombic — tetragonal phase transition and oxygen index of YBa2Cu3O6+δ

The orthorhombic — tetragonal phase transition of YBa₂Cu₃O_6+δ has been investigated by X-ray powder diffraction methods. The transition is of the second order or very close to second order and no coexistence of tetragonal and orthorhombic phases has been observed. The transition strongly depends on the oxygen index δ which changes with temperature. Typical patterns of the triplet 006, 020, and 002 obtained by precise X-ray powder diffraction measurements have been presented and shown to be useful for identification of YBa₂Cu₃O_6+δ and estimation of oxygen index δ.     

V-ΔT and V-λ relation for the naphthalene—adipic acid system

The mechanism of crystal growth from the undercooled melt is studied experimentally for the naphthalene-adipic ac d system by means of a simple capillary technique. The microscopic observations for the 0.11 molefraction of naphthalene-adipic acid mixture indicated lamellar structure. The relationship between lamellar spacing, λ, and crystallization velocity V was found to be λ²V = constant.     

On the existence of a pyrochlore-type phase in the system Bi2O3–TiO2

The existence of a bismuth titanate in the range between Bi₂Ti₄O₁₁ (Bi₂O₃ · 4 TiO₂) and Bi₄Ti₃O₁₂ (2 Bi₂O₃ 3TiO₂) has been investigated by X-ray diffraction of samples prepared by solid state reactions. For reaction temperatures above 1100 °C and a starting composition Bi₂O₃ · 2 TiO₂ there appeared additional lines which could be attributed to a cubic face-centred cell with a = 10.354 Å. A multiphase Rietveld analysis based on X-ray powder diffraction data confirmed the structural model of a pyrochlore for this compound. There is evidence that this phase belongs to the group of defect pyrochlores with a Bi³⁺-deficiency resulting in a composition of Bi_1.833Ti₂O_6.75.     

Interpolation in der Funktionstafel zum Meßkompensator 0…130 λ mit Kombinationsplatte aus Kalkspat (Drehkompensator nach Ehringhaus)

A formula for the calculation of the path difference of birefringent objects for any wave lengths between the spectral lines F and C is derived from the interpolation formula given by A. Ehringhaus formerly. According to this new relation the calculation of the interpolation values is possible more quickly than according to the old one.     

Ferroelectric phase transitions in KNO3 embedded into porous glasses

This paper presents the results of dielectric and dilatometric measurements of porous glasses filled with KNO₃ from water solution and from the melt. It was shown that ferroelectric phase transition is observed during the cooling process only in KNO₃ nanocrystals introduced into the porous matrix. The phase diagram of transition temperatures versus average values of pore dimensions was obtained. The phenomenological model describing the phase transition and size effects in KNO₃ crystals is proposed.     

Neutron diffraction study of ordering in the γ-phase of the aluminium-magnesium system

In the present work neutron diffraction study is made of the ordering effekt within the whole region of the existence of the γ-phase of the Al-Mg system. Depending on thermal treatment conditions, the order coefficient S varies from 0.15 to 0.7 for compounds containing 59 and 60 at.% Mg at temperatures ∼330 and below 150°C, respectively. – At decreased annealing temperatures the ordinate of the highest ordering is shown to approach the composition which may be approximately characterized as Al₂Mg₃. The temperature increase results in the decrease of the ordering degree, and the ordinate of the highest ordering displaces to the composition which may be expressed as Al₁₂Mg₁₇. – Kurnakov's point is determined which is lying at ∼330°C for the composition containing 59 at.% Mg.     

Improper ferromagnetic phase transitions in crystals

A complete set of possible improper phase transitions leading to polarization and magnetization of crystals is found from tensor bases of representations of the black-and-white point group of crystal symmetry by using the multiplication table of irreducible representations. The dependences of the secondary order parameter on the primary order parameter are presented for different classes of the symmetry group.     

Crystal growth of β-Phthalocyanines from the vapour phase in closed ampoules

An influence of temperature gradient, inert gas pressure and pulling rate on the growth parameters of β-phthalocyanine single crystals grown in the closed ampoules is investigated. The morphology and structure — sensitive properties of β-CuPc crystals grown under different conditions are compared.     

Laser-induced phase transitions in PbI2

Pure crystals of PbI₂ have been grown using zone-refining system. The structural phase transition 12 R → 2 H has been observed when exposed to ruby laser light. The results have been explained on the basis of structural considerations of PbI₂ and compared with the earlier existing findings in literature.     

Polymorphism and phase transitions in TlS

The conditions for synthesis and the range of existence of the monoclinic TlS modification in the Tl-S system have been determined by means of differential thermal, X-ray diffraction, and microstructural analyses. It was shown that the monoclinic phase is a structural analogue of TlGaSe₂. Possible variants of formation of structural sheets and the nature of phase transitions in the monoclinic TlS modification are discussed.     

A study of the influence of short-range order,clustering and crystal lattice defects on positron annihilation in α and α + β brass

A theoretical model is proposed allowing the consideration of the influence of local neighbourhood of an atom on the positron density enhancement at one or another type of atom in a binary alloy. With the help of this model the direction of the change of angular correlation peak height caused by appearance of short range order or clustering can be predicted. Cu–Zn-alloys containing 5.4, 15, and 37 weight % Zn are investigated. The peak height of samples quenched from 300°C. show a weak minimum at 90°C. This seems not to be caused by the increase of short-range order but rather by a process of clustering. probably by segregation of Zn atoms at stacking faults and dislocations. The observed recovery of plastically deformed brass can be well explained by annealing out of vacancies and following recrystallization, where the α + β brass shows characteristic deformation induced properties. Peak height measurements of recovery of crystal lattice defects in binary alloys seems to be only weakly influenced by effects such as short-range order or tendency of clustering. The formation energy of monovacancies in Cu-37 wt% Zn is determined to be (0.81 ± 0.02) eV.     

Phase equilibria and prospects of crystal growth in the system LiF–GdF3–LuF3

Scheelite type LiGdF₄, LiLuF₄, and mixtures of both end members were prepared by a hydrofluorination route from the rare earth oxides and commercial LiF. The samples were purified by melting in HF/Ar mixtures, and were investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and energy dispersive X‐ray spectroscopy (EDX) techniques. Both end members show unlimited miscibility in the solid phase. Mixed crystals containing at least 65 mol‐% LiLuF₄ melt under direct formation of the liquid phase. The gap between solidus and liquidus is narrow. LiGdF₄ and mixed crystals with less then 65 mol‐% LiLuF₄ decompose peritectically under formation of (Gd,Lu)F₃. Crystal growth is expected to be possible either from Lu‐rich melts with the appropriate scheelite composition or from Gd‐rich melts containing an excess of LiF. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the coexistence of phases at the α–β transition in quartz

The character of the α–β transition in crystals of natural and synthetic quartz has been studied by measuring the refractive index. The coexistence of two different crystallographic modifications over some temperature range has been established. The α–β phase transition is supposed to be attributed to the near-critical phase transitions. In synthetic quartz with Al and Fe structural impurity the α–β transition consists of two successive transitions. First the transition from the α-phase to an intermediate one takes place, then the intermediate phase is transformed into the β-Phase. Within a certain temperature range there exists only the intermediate phase.     

Structure and phase transitions in trigonal Cs3Sb2I9 crystals

The atomic structure of Cs₃Sb₂I₉ single crystals was refined using X-ray diffraction data (sp. gr. P3m1; wR = 1.58% and R = 3.07%). The phase transitions revealed earlier were found to be accompanied by the appearance of superstructural reflections. At T _c1 = 86 K, the reflections indicating doubling of the c parameter. At T _c2 = 73 K, a first-order phase transition takes place accompanied by “freezing” of the satellites h, k + 1/2, l + 1/2 reflections. In the temperature range from 73 to 78 K, an incommensurate phase providing the satellites h, k + 1/2 + δ, l + 1/2 is formed.