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.     

Reversible low-temperature transition in LaMnO<Subscript>3+δ</Subscript>

The evolution with a decrease in temperature of the basic rhombohedral structure of the LaMnO_3+δ compound having a specified constant oxygen concentration is considered. The X-ray diffraction analysis shows that a transition from rhombohedral to monoclinic phase begins at a temperature close to 200 K. The important feature of the revealed low-temperature phase transition is its reversibility with heating to room temperature. A deformation mechanism of structural phase transitions based on uniform distortions of a perovskite block with a doubled lattice parameter is proposed. The results of the analysis of the symmetry of atomic positions are in favor of two-step routes of phase transitions through dilatation and twinning.     

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.     

DC conductivity studies in K3Na(SeO4)2 single crystals

DC electrical conductivity studies were carried out along the three crystallographic axes for Tripotassium sodium diselenate (K₃Na(SeO₄)₂ or KNSe). Earlier studies of phase transition in this crystal show successive phase transitions at 334 K, 346 K, 730 K, and 758 K. In this paper we report the dc electrical conductivity measurements in the temperature region 303 K – 430 K along a, b and c – axes. An anomaly in conductivity was obtained around 341 K and another one around 333 K. These can be attributed as due to phase transitions in this crystal. A strong anomaly also has been observed along the c‐axis and comparatively week one along a and b axes around 395 K for the first time. This can be due to newly observed phase transition in the crystal. DSC taken for the sample also shows endothermic peak supporting the occurrence of newly observed phase transition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

EPR and pulsed NMR study of (NH4)2ZnBr4:Mn2+

The EPR and pulsed NMR measurements of (NH₄)₂ZnBr₄:Mn²⁺ have been carried out in the temperature range 110–473 K and 83–284 K, respectively. EPR spectra from three different paramagnetic entities have been observed. Intensity changes of certain lines in the EPR spectrum observed at ≈ 430 K indicate the occurrence of a phase transition. Another phase transition at ≈ 235 K is indicated as a change of slope in the T₁ vs 1/T curve.     

Studies on a new Rb3H9(PO4)4 crystal

A new type of trirubidium nonahydrogen tetraphosphate, Rb₃H₉(PO₄)₄, crystal has been crystallized. Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on the title compound. The crystal does not undergo any structural phase transition in the temperature range from 120 to 440 K except a melting transition at 399 K. The compound belongs to a monoclinic system with space group C2/c at room temperature. The structure is built on a framework of PO₄ tetrahedra linked by five types of OH‐O hydrogen bonds in the length range from 2.472(4) to 2.679(4) Å. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure and phase transition of single crystalline CsNH2SO3

Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on cesium sulfamate CsNH₂SO₃ single crystal. Two distinct endothermic peaks in the DSC curves are observed at 330 and 436 K. It is pointed out that the peak at 330 K is attributed to the structural phase transition, and the other peak at 436 K is associated with the thermal decomposition of the crystal. The structures in room‐ and high‐temperasture phases are determined, and the space group of the sample crystal is found to change from monoclinic P 2₁/c to orthorhombic Pnma. The structure of the room‐temperature phase consists of two different types of N‐H···O hydrogen bond, but in the high‐temperature phase there is no specific hydrogen bond between the NSO₃ pseudo‐tetrahera. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time evolution of a sol–gel process monitored by Mn2+ EPR spectroscopy

The time evolution of the tetraethoxysilane (TEOS)-based sol–gel process was investigated by EPR spectroscopy of a Mn²⁺ spin probe. Analysis of the EPR spectra recorded at room temperature (RT) and liquid nitrogen temperature (77 K) showed that the values of the g-factor (for the central Mn²⁺ sextet) were found to be, within experimental error, identical during the sol–gel process evolution. However, the peak-to-peak half width, ΔB_pp, of the spectral lines recorded at RT, the splitting of the forbidden hyperfine lines (and thus the zero-field splitting parameter, D) recorded at 77 K, and the averaged hyperfine splitting constant, A_av, at both temperatures, were found to change significantly during the various stages of the sol–gel process. These EPR spectral parameters allow us to monitor changes in the TEOS-based sol–gel reaction mixture during the liquid-to-solid-state transition. It was clearly demonstrated that the sol-to-gel transition as well as the hydration and dehydration process of gels could be successfully studied by Mn²⁺ EPR spectroscopy.     

Magnetic phase transition in cubic lanthanum manganite

Studies of magnetic phase transition (ferromagnetic-paramagnetic) in cubic LaMnO₃ have been carried out. Using neutron diffraction and calorimetric investigations the temperature of this transition was determined to be equal to 148 K. The studied phase transition is close to the second order transition by nature. Transition enthalpy is estimated from the given dependence of C_p(T) and is 17 cal/mole. Linear dependence of temperature of the given magnetic transition in weak magnetic fields is established.     

The effect of temperature on the raman spectra of strontium formate dihydrate single crystals

The temperature dependence of the polarized Raman Spectra (3000 cm⁻¹ – 10 cm⁻¹, between 293 K and 10 K) of Sr(HCOO)₂ · 2H₂O(2D₂O) and partially deuterated samples is reported. From the analysis of the internal and external modes associated with water molecules and formate ions a phase transition is deduced. This is interpreted on the basis of the positional disorder of the hydrogen atoms, situated in non-equivalent lattice positions at low temperature.     

Preparation and studies of new crystals in the K3H(SO4)2-(NH4)3H(SO4)2-H2O system

To elucidate the effect of isomorphic substitution on the kinetics of phase transitions, single crystals of (K _x (NH₄)_1?x ) _m H _n (SO₄)_{(m + n)/2} · yH₂O solid solutions are grown from the K₃H(SO₄)₂-(NH₄)₃H(SO₄)₂-H₂O system, whose end members are known to undergo superprotonic phase transitions of fundamentally different kinetics. The chemical composition of the single crystals grown is determined by energy dispersive X-ray microanalysis. The thermal and optical behavior of (K,NH₄)₉H₇(SO₄)₈ · H₂O single crystals is studied in the temperature range 295–420 K and the crystal structure at 295 K is determined. A comparison of the results of the studies with data for crystal K₉H₇(SO₄)₈ · H₂O published earlier shows that the substitution of ammonium for potassium atoms lowers the temperature of the structural phase transition by 8 K.     

Cr3+ luminescence quenching in stoichiometric lithium niobate crystals

Cr³⁺-doped stoichiometric LiNbO₃ crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 × 10⁸ s⁻¹ is about 10⁴ times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr³⁺ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr³⁺ radiative lifetime in LiNbO₃ decreases from 8 to 4 μs when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr³⁺ ions to be a potential active media for tunable lasers.     

Synthesis and characterization of dihydroxylammonium tetrachlorocuprate dihydrate crystals

A new hydroxylammonium compound, [(NH₃OH)₂CuCl₄], was synthesized and its crystals were grown at room temperature by slow evaporation of aqueous solutions. The crystals were characterized through powder XRD, thermogravimetry (TG), differential thermal analysis (DTA), low temperature differential scanning calorimetric (DSC) and FTIR spectra. The X‐ray powder diffraction confirms the crystallinity of the compound. A fitting decomposition pattern of the compound was formulated based on the TG and thereby confirming the formation of the compound in the stoichiometric ratio. The thermal anomalies occurring in the low temperature DSC indicate successive phase transitions. The low temperature phase transitions are attributed to the ordering of [NH₃OH]⁺ ions. While most of the phase transitions are of first order type, the one occurring at –126 °C is of second order. Two glass transitions occur when the compound was cooled between –157.9 and –136.9 °C. The characteristic vibration bands due to [NH₃OH]⁺ and CuClequation/tex2gif-stack-1.gif ions are observed in the IR spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and thermoluminescence properties of R2O–RF–B2O3 glass systems doped with MnO

Alkali fluoroborate glass systems containing manganese cations have been thoroughly investigated in order to obtain information about the structural role of manganese in such glass hosts. The amorphous phase of the prepared glass samples R₂O–RF–B₂O₃:MnO (with R = Li and Na) was confirmed from their X-ray diffraction. From the infrared spectra of these glass systems it was concluded that the glass structure contains two group of bands; one due to trigonal BO₃ units and the second due to the tetrahedral BO₄ units. As manganese was introduced, replacing lithium or sodium, it acts as a network modifier and the intensity of the second group of bands increases at the expense of the first group of bands. The optical absorption spectra of R₂O–RF–B₂O₃:MnO exhibited two conventional absorption bands; one due to Mn²⁺ ions and the other due to Mn³⁺ ions. The ESR spectra of these glasses showed a six-line hyper-fine structure centered at g = 2.01 (due to Mn²⁺ ions) and another signal at g = 4.3 (due to Mn³⁺ ions). The intensity of optical absorption bands and the ESR signal due to Mn²⁺ ions decreases with increasing MnO concentration indicating the conversion of Mn²⁺ ions into Mn³⁺ ions in the glass network. The thermoluminescence studies on these glass systems showed a quenching of TL output with increase in the concentration of MnO. All the obtained results were discussed on the basis of the glass structure and the conversion of Mn²⁺ into Mn³⁺ ions with increasing concentration of MnO in the glass systems.     

Studies on the growth and optical characterization of Tm3+‐doped BaY2F8 single crystals

The BaY₂F₈ crystals doped with different concentrations of Tm³⁺ ions were prepared by the temperature gradient technique (TGT). X‐ray powder diffraction was applied to analyze the phase. The cracking phenomenon along (010) and (100) planes of the crystals grown by temperature gradient technique was studied on the basis of the structure of BaY₂F₈ crystals. The absorption spectra were measured and investigated in the ultraviolet‐visible and near‐infrared ranges at room temperature. Several characteristic absorption bands of Tm³⁺‐doped BaY₂F₈ crystal were observed. The emission and excitation spectra were obtained and investigated at room temperature and 12 K, showing the characteristic emission peaks of Tm³⁺ ions. The temperature dependence of Photoluminescence curve was also investigated in the range of 12–296 K. The luminescence intensity of emission bands decreased with increasing temperature, while the effective bandwidth increased. The up‐conversion spectrum excited at 650 nm was recorded and up‐conversion mechanism was analyzed in detail. The result showed the purple, green and yellow emissions corresponding to ³P₁→³F₃, ¹D₂→³H₅ and ³P₀→¹G₄ transitions, respectively.     

Synthesis and thermal,phase transition and FTIR spectral characterization of tetraethylammonium tetrachlorocuprate (II) crystals

Single crystals of tetraethylammonium tetrachlorocuprate (II), [N(C₂H₅)₄]₂CuCl₄, were grown by slow evaporation method at room temperature. The crystals were characterized through powder XRD, thermogravimetric (TG‐DTA), low temperature differential scanning calorimetric (DSC) studies and FTIR spectroscopy. While the powder XRD pattern of the compound shows sharp Bragg peaks confirming the crystallinity of the compound, the TG‐DTA studies confirm formation of the compound in the stoichiometric ratio. The thermal anomalies observed in DSC curve at ‐120°C in the heating cycle and around ‐30°C in the cooling cycle indicate a first order phase transition. The phase transition was predicted to be associated with the ordering of CuCl₄^2‐ and successive long range orientation of [N(C₂H₅)₄]⁺ ions which are disordered at high temperatures. The cationic [N(C₂H₅)₄]⁺ plays a role in phase transitions at low temperatures. The sharp exothermic peak observed in high temperature DSC indicates a structural phase transition when [N(C₂H₅)₄]CuCl₃ is formed on heating the compound. The FTIR spectra of the compound characterize the various chemical bonding and water molecules adsorbed in the compound. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In‐situ investigation of the temperature dependent structural phase transition in CuInSe2 by synchrotron radiation

The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe₂ was investigated by in‐situ high temperature synchrotron radiation X‐ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, |1‐η|, and distortion |u‐¼| (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x‐coordinate). The thermal expansion coefficients α_a and α_c of the tetragonal lattice constant which are related to the linear thermal expansion coefficient α_L were obtained, as were α_a of the cubic lattice constant, also α_u and α_η. The transition temperature is clearly identified via a strong anomaly in α_L. The temperature dependence of the anion position parameter was found to be rather weak, nearly α_u∼0, whereas α_η increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and |1‐η| as well as |u‐¼| go to zero with |T‐T_trans|^0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu‐In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu⁺ were found to occupy the Me1 site with a corresponding 67% of In³⁺ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with |T‐T_trans|^β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order‐disorder transitions according to the Ising model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theory of the formation of the ordered LiZn0.5Mn1.5O4 phase

A theory of the structural phase transition in LiZn_0.5Mn_1.5O₄ cathode material is proposed. The symmetry of the order parameter, thermodynamics, and mechanisms of formation of the atomic structure of low-symmetry ordered cubic lithium-zinc-manganese oxide spinel have been studied. The critical order parameter inducing the phase transition has been found. It is shown that the calculated LiZn_0.5Mn_1.5O₄ structure is formed due to displacements and orderings of lithium, zinc, manganese, and oxygen atoms. Within the Landau theory of phase transitions, it is shown that the phase states may change from high-symmetry cubic disordered Fd3m phase to the low-symmetry ordered cubic P2₁3 phase as a result of first-order phase transitions.     

Temperature Dependence of the Absorption Spectra and Optical Parameters in TGS and Cu2+-Doped TGS Crystals

The effect of temperature on the optical absorption spectra and optical parameters is investigated for pure TGS and TGS doped with Cu²⁺ ions. Absorption measurements cover the range from room temperature to about 355 K in the energy range 3-5.5 eV. The temperature dependence of the band gap E_g(T) reveals an anomaly at the phase transition temperature for both pure and Cu²⁺-doped TGS crystals. In the region of the absorption edge the absorption coefficient is found to display Urbach-rule behaviour. The characteristic Urbach parameters are determined and their temperature dependence is investigated.