A change from second- to first-order transition in (La(1-x)Eu(x))0.67Ca0.33MnO3 (0 ≤ x ≤ 0.2)

A detailed investigation of the paramagnetic to ferromagnetic transition in (La(1-x)Eu(x))(0.67)Ca(0.33)MnO(3) having small Eu(3+)-content (0 ≤ x ≤ 0.2) has been carried out through resistivity and magnetization measurements. X-ray diffraction patterns of the compounds reveal a single phase (La(1-x)Eu(x))(0.67)Ca(0.33)MnO(3) (0 ≤ x ≤ 0.2) of an orthorhombic crystal structure after annealing the precursor at 800 °C for 2 h in air. With increasing Eu(3+)-content, the second-order transition (at x = 0 and 0.1) changes to first-order at x = 0.2. The experimental results demonstrate thermomagnetic irreversibility of the transition for x = 0.2 composition. This arises between the supercooling and superheating regimes where both the ferromagnetic and paramagnetic phases coexist.     

On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La_{1? x} Sr _x MnO₃ perovskite manganite materials with different compositions (x?=?0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T _c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.     

Transport effects in La0.67−x R x Sr0.33MnO3 (R=Eu, Gd)

An experimental study is reported on the temperature dependences of the magnetic susceptibility, electrical resistivity, magnetoresistance, thermo-and magnetothermopower, and on the Hall effect of the polycrystalline lanthanum manganites La_0.67?x R _x Sr_0.33MnO₃ (x=0 and 0.07, R=Eu, Gd) within the 77-to 430-K temperature range. Replacement of a small amount of lanthanum by europium or gadolinium was found to result in a considerable decrease in the resistivity and in a change in the pattern of its temperature dependence. The temperature dependence of both the ordinary and anomalous Hall coefficients near the Curie temperature is shown to be determined by the change in the number of carriers in delocalized states. A method is proposed for separation of the hole and electronic contributions to thermopower in the ferromagnetic region. The conduction mechanisms are discussed in terms of the concept of mobility edge motion.     

Magnetic and piezoelectric properties of the Bi1 − x La x FeO3 system near the transition from the polar to antipolar phase

The crystal structure, piezoelectric and magnetic properties of the Bi_{1 ? x} La _x FeO₃ solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.     

Electron-phonon coupling through the orthorhombic to rhombohedral phase transition in La2/3(Ca1−xSrx)1/3MnO3 manganites

We present a systematic study of the Raman modes of a ferromagnetic series of manganites, La_2/3(Ca_1?xSr_x)_1/3MnO₃ with 0?x?1, that presents two different regimes for the variation of the ferromagnetic order temperature, T_C, with Sr doping. This change occurs for x～0.5 when the temperature of the orthorhombic, Pbnm, to rhombohedral, R-3c, structural phase transition, T_t, coincides with T_C. We have analyzed the evolution of the frequencies and widths of the observed Raman modes as a function of Sr doping and temperature. At room temperature, features of the Pbnm structure are detected for compounds up to x=0.6, over the orthorhombic phase limit x=0.45 at 300 K. Octahedra bending modes behave as expected with doping while the tilt mode, which is related to Mn–O–Mn angles and therefore to electronic conduction mechanisms, presents different behaviors in both structures. The tilt frequency is much less sensitive to Mn–O–Mn angle in R-3c than in Pbnm structure indicating a reduction of electron-phonon coupling. Its width presents an anomalous behavior both as a function of doping and temperature with unexpectedly large width in the rhombohedral phase.     

A study of the microstructure of La1 − x Ca x MnO3 (x = 0.5, 0.8) solid solutions

HREM and XPD methods are used to study the microstructure of La_{1 − x} Ca _x MnO₃ perovskite-like oxides synthesized by pyrolysis of polymer-salt compositions. The XPD data show that the samples studied are single-phase solid solutions. Morphological transitions are observed in series of samples depending on the substitution parameter x. At 0 ≤ x ≤ 0.4 the samples can be characterized by rhombic symmetry (space group Pnmb); at 0.5 ≤ x ≤ 0.8 the symmetry increases to the tetragonal space group I4/mmm; at 0.9 ≤ x ≤ 1 the symmetry lowers to the monoclinic space group P1121. Heating of samples with x = 0.5 and 0.8 up to 1200°C in air does not lead to noticeable changes in the sample structure, which indicates their high thermal stability. When the La_0.2Ca_0.8MnO₃ sample is heated to 1100°C in vacuum nanocrystalline states with particle disintegration into microphases of different structures form because of a partial decomposition of the solid solution. As a result, a system of Mn₃O₄ nanoparticles appears, which is coherently bound with the perovskite phase of a defective structure.     

Pulsed EPR spectroscopy of Gd3+ centers in Ca1−x R x F2+x (R=La, Y) mixed crystals

A study of Gd³⁺ centers in Ca_1?x R _x F_2+x (R=La, Y) crystals using pulsed EPR spectroscopy is presented. The echo-induced EPR (ESE-EPR) spectrum shows, besides the signal of slightly perturbed cubic Gd³⁺ centers, a broad signal at g≈2 due to Gd³⁺ centers at low symmetry sites. To describe the effects of R³⁺ ions on the EPR Gd³⁺, a model, including cubic and linear R³⁺?2F _i ^? centers, is developed. Its predictions are compared with the experimental results. The composition dependence of the EPR signal due to slightly perturbed cubic Gd³⁺ centers in mixed Ca_1?x R _x F_2+x crystals is explained taking into account the different clustering tendency in La and Y crystals. Moreover, the formation of mixed clusters involving R³⁺ and Gd³⁺ ions is proposed for both series of samples. A greater clustering trend is found in the Y crystals than in the La ones. Gd³⁺ ions are found to be a “non innocent” paramagnetic probe for structural studies in these mixed crystals.     

Features of high-frequency ultrasound propagation in the temperature range of structural and magnetic phase transitions in La1−x SrxMnO3 (x=0.175) manganite

The propagation of ultrasonic waves at a frequency of 770 MHz in a La_0.825Sr_0.175MnO₃ single crystal is investigated in the temperature range 350–150 K. It is found that the velocity, attenuation, and mode composition of ultrasonic waves change at temperatures of 315–280 and 220 K. These changes correlate with the structural and magnetic phase transitions and can be explained in terms of the Jahn-Teller distortions of the crystal lattice.     

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

Magnetocaloric effect and order of transition in (La_1?x Nd _x )_2/3(Ca_1?y Sr _y )_1/3MnO₃, prepared by conventional solid-state reaction, have been investigated. Using Banerjee criterion, we demonstrate first-order transition for (J1) and (J2 ) as well as second-order transition for (J3 ), (J4 ), and (J5 ) samples. The ΔS _M ^max is ranging between 9.18 Jkg^?1 K^?1 and 4.87 when Nd and Sr content changes leading to relative cooling power (RCP) varying between 330 and 229.35 J/kg. Both ΔS _M ^max and the RCP are found sensitive to the disorder σ ². The universal behavior obtained from ΔS variation curves confirmed the first-order transition for (J1) and (J2 ) samples and second-order transition for (J3), (J4), and (J5 ) samples obtained by Banerjee criterion. All samples with second-order phase transition exhibit inhomogeneous character estimated from local exponent n.     

Elastic and inelastic neutron scattering studies on the tetragonal to orthorhombic phase transition of La2−x Sr x CuO4±δ

The phase transition from the tetragonal to the orthorhombic structure in La_2–x Sr _x CuO₄₊ has been studied combining elastic and inelastic neutron scattering and specific heat measurements on the same well characterized crystals. This analysis focusses on the comparison between undoped and doped compounds. The proportionality between the orderparameter and the orthorhombic strain, predicted by the Landau theory, can be confirmed in La_2–x Sr _x CuO₄₊ in a wide concentration and temperature range. We observe different values for the critical exponent for doped superconducting and undoped insulating crystals. The temperature dependent study of the soft modes on a metallic superconducting crystal reveals an extremely large temperature range where this mode is strongly anharmonic. The comparison with previously published data shows that the frequencies of the soft modes at low temperatures decrease with doping, i.e. with increasing charge carrier concentration.     

On the microstructure of the charge density wave observed in La1− x Ca x MnO3

We have used low temperature (90?K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La_{1? x} Ca _x MnO₃. It has been stated that Mn³⁺ and Mn⁴⁺ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5?µm in La_0.48Ca_0.52MnO₃ had a wavevector which varied on a scale of tens of nanometres with an average of ?q??=?0.450a * and a standard deviation Δq?=?0.004?a* in its magnitude and Δθ?=?0.56° in its direction at 90?K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90?K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30?K above the ‘charge ordering temperature’.     

Features of magnetic properties of La x MnO3 + δ (0.815 ≤ x ≤ 1.0)

Magnetic and resonance studies of the system of polycrystalline samples of self-doped manganites La _x MnO_{3 + δ} (x = 0.815, 0.90, 0.94, 0.97, and 1.0) have been performed in a temperature range of 77–300 K. According to ⁵⁵Mn NMR data, all samples contain a ferromagnetic phase at 77 K. As the defect density increases (x changes from 1.0 to 0.815), samples become more magnetically ordered. In this case, the ferromagnetic state of the system gradually changes from a mixed state in which both ferromagnetic insulating (basic) and ferromagnetic metal (for x = 0.97 and 1.0) phases coexist to only the ferromagnetic metal state (for x = 0.815 and 0.90). It has been shown that both ferromagnetic metal and ferromagnetic insulating phases are inhomogeneous, and either phase consists of two phases with different dynamics of nuclear spins and different Curie temperatures. The diagram of the magnetic phase state of the La _x MnO_{3 + δ} system (x = 0.815, 0.90, 0.94, 0.97, 1.0) has been constructed for a temperature range of 120–240 K and Mn⁴⁺ contents of 12–30%.     

Structural phase transitions in the Bi1−x La x FeO3 system

The crystal structure and phase composition of the Bi_1?x La _x FeO₃ system have been studied by the methods of neutron and x-ray diffractions and electron microscopy. It has been revealed that a unit cell of the solid solutions at room temperature is described by the R3c, Imma, and Pnma space groups in the concentration ranges 0 < x < 0.19, 0.4 < x < 0.5, and x > 0.5, respectively. For 0.2 < x < 0.4, the solid solutions are not formed.     

Mössbauer and X-Ray studies of the dynamics of phase transformations and suppression of polymorphism in the LaMn1 − x Fe x O3 + δ compound (x = 0.015–0.500)

Physics of the Solid State - The effect of iron impurity on the structural transformations and the suppression of polymorphism of the LaMn1 ? x Fe x O3 + δ compound (x =...     

Calorimetric investigations of phase transitions in the cryolites (NH4)3Ga1−x ScxF6(x=1.0,0.1,0)

The specific heat of the cryolites (NH₄)₃Ga_1−x Sc_xF₆ with x=1.0, 0.1, and 0 was measured. The thermodynamic parameters of the phase transitions were determined. A previously unknown phase transition was found in the scandium compound at T=243 K. Fiz. Tverd. Tela (St. Petersburg) 41, 523–528 (March 1998)     

Percolation conductivity of single-crystal manganite Pr1 − x Sr x MnO3 (x = 0.22; 0.24) in the phase-separation state

This paper presents an experimental study of the temperature dependences of the electrical resistivity, thermopower, and magnetoresistance of single-crystal manganite Pr_{1 ? x} Sr _x MnO₃ (x = 0.22, 0.24). The results obtained have been analyzed in terms of the theory of hopping electrical conductivity of manganites in the phase-separation state and the main postulates of percolation theory. In the 2–400-K interval, hopping conductivity with temperature-dependent activation energy has been observed. Near the Curie temperature, the monotonic exponential growth of electrical resistivity with decreasing temperature becomes distorted by formation of finite metallic clusters. For T ≤ 36 K, the conductivity of Pr_0.8Sr_0.22MnO₃ follows the Mott law.     

Effect of oxygen non-stoichiometry on the antiferromagnet-ferromagnet transition in Nd0.5Ca0.5MnO3−δ(δ≤0.12)

A decrease in the oxygen content in Nd_0.5Ca_0.5MnO_3?δ down to γ≤0.12 is shown to bring about a strong decrease in the magnetic field inducing a transition from the antiferromagnetic charge-ordered to the ferromagnetic charge-disordered state. The ferromagnetic phase in a Nd_0.5Ca_0.5MnO_2.92 sample is stable in the absence of an external magnetic field. A further increase in the content of oxygen vacancies stabilizes the antiferromagnetic charge-disordered state.