Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Influence of atmospheric humidity on the symmetry and phase transitions of layered potassium oxyfluorides K2NbOF5 · H2O

Crystals of K₂NbOF₅ · H₂O have been grown, polarization optical investigations have been performed, and the birefringence and rotation angle of the optical indicatrix have been measured in the temperature range 100–400 K. It has been found that, depending on the degree of atmospheric humidity, the layered K₂NbOF₅ · H₂O crystal at room temperature can be in three states, namely, A, B, and C, which differ in symmetry and properties of the crystal. The K₂NbOF₅ · H₂O crystal in the A state exists at a relative humidity RH = 90–100% and undergoes a first-order improper ferroelastic phase transition P $\bar 1$ ? C2/m, which is accompanied by strong anomalies of the optical characteristics, twinning, and shear strain x ₆ at temperatures T ₂ ^↓ = 308 K and T ₂ ^↑ = 313 K. The most stable state of the K₂NbOF₅ sdH₂O crystal is the B state (RH = 20–90%), which retains the monoclinic symmetry C2/m in the temperature range 100–370 K. In a dry atmosphere (RH = 0–20%) or at T ₁ ≈ 370 K, the crystal becomes anhydrous (K₂NbOF₅) with the symmetry P4/nmm (the C state). The difference between the crystals in the states A and B is explained by the presence or absence of water molecules in interlayer spaces.     

27Al Magic Angle Spinning Nuclear Magnetic Resonance Study of Al1−x Cr x K(SO4)2·12H2O (x = 0, 0.07, and 0.2)

The physical properties of Al_1?x Cr _x K(SO₄)₂·12H₂O (x = 0, 0.07, and 0.2) were studied as a function of temperature using magic angle spinning nuclear magnetic resonance for ²⁷Al. On the basis of the physical properties of pure AlK(SO₄)₂·12H₂O, the effects of partially replacing Al³⁺ with Cr³⁺ ions were examined. Molecular motion changed with the concentration of Cr³⁺ ions. The relaxation process near 320 K was found to undergo molecular motion as described by the Bloembergen–Purcell–Pound theory. The activation energies, phase transition temperatures, and spin–lattice relaxation times in the rotating frame T _1ρ changed with the concentration of paramagnetic ions.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Absorption measurements of H2O at high temperatures using a CO laser

Absorption of CO laser radiation (v = 8→7, J = 14→15 transition at 1901.762 cm^-1) by H₂O has been studied in shock-heated H₂/O₂/Ar mixtures over the temperature range 1300–2300 K. This laser transition is nearly coincident with the v₂-band 12_3,10←11_2,9 transition of H₂O at 1901.760 cm^-1, thereby providing a convenient and sensitive absorption-based H₂O diagnostic useful for studies of combustion. The collision-broadening parameter for this H₂O line, due to broadening by Ar, was determined to be 2γ (cm^-1atm^-1) = 0.027 (T/1300)^-0.9 in the temperature range 1300–2300 K. Calculations of the H₂O absorption coefficient (at 1901.762 cm^-1) based on this expression for 2γ are presented for the temperature range 300–2500 K and pressure range 0.3–1 atm.     

Thermal evidences for successive CDW phase transitions in 1T-TaS2

The heat capacity of 1T-TaS₂ has been measured over the temperature range including the successive phase transitions (140 K–370 K) by an adiabatic calorimeter. There are three transitions in the measured temperature range, two first-order transitions (at about 226 K (T₁) and about 353.5 K (T₃)) and one small anomaly at about 283 K (T₂) with a broad peak. The transition enthalpies are as follows; ΔH₁=52±5 cal·mol^-1, ΔH₂=7.5±2 cal· mol^-1 and ΔH₃=122±8cal·mol^-1.     

Thermal expansion of the one-dimensional conductor K2Pt(CN)4Br0.3·xH2O and K2Pt(CN)4·xH2O

Measurements of the thermal expansion coefficients of K₂Pt(CN)₄Br_0.3· xH₂O and K₂Pt(CN)₄·xH₂O show a large anisotropy of the a-and c- directions. Their temperature dependence could be described by a simple Grüneisen theory. In the range from 80–330°K no anomaly indicating a Kohn-Peierls transition could be found.     

The influence of phase transitions in the Rb<Subscript>2</Subscript>CdI<Subscript>4</Subscript> ferroelastic on the exciton absorption spectrum

The parameters of the long-wavelength exciton band for Rb₂CdI₄ films are investigated in the temperature range 90–410 K. It is found that the Rb₂CdI₄ films undergo a sequence of phase transitions at temperatures T_c1=380 K (paraphase → incommensurate phase), T_c2=290 K (incommensurate phase → ferroelastic phase I), and T_c3 = 210 K (ferroelastic phase I → ferroelastic phase II). The parameters of the exciton band (such as the spectral position and the half-width) measured during heating and cooling of the Rb₂CdI₄ film differ significantly. This is especially true for the incommensurate phase. Upon heating of the incommensurate phase, the domain boundaries become frozen, whereas the cooling of this phase is accompanied by the generation of solitons and their pinning, which, in turn, results in a first-order phase transition at the temperature T_c2. It is revealed that the oscillator strength of the exciton band anomalously increases in the range of existence of commensurate phase I (T_c3<-T<-T_c2) due to ordering of the Rb₂CdI₄ crystal lattice.     

Absence of magnetic ordering in low dimensional mixed magnetic Mn1−xNixCl2·H2O

A new mixed magnet, Mn_1−xNi_xCl₂·H₂O, is examined by dc magnetization and susceptibility measurements across the entire composition range. The pure components are quasi-one-dimensional Heisenberg antiferromagnets ordering at 2.17 K (Mn) and 5.6₅ K (Ni) due to weaker interchain exchange supplementing the dominant exchange along MCl₂MCl₂M… chemical and structural chains. High temperature magnetic susceptibilities yield Curie and Weiss constants in χ_M=C/(T−θ). C(x) is linear but θ(x) displays curvature, which is analyzed to show that unlike-ion exchange is ferromagnetic and similar in size to like-ion. Most notable is the absence of antiferromagnetic susceptibility maxima down to 1.6 K from x=0.10 to 0.95. For x=0.05 a susceptibility maximum appears, with T_max almost 20% lower than in the pure Mn component but T_c reduced by 2%. The size of the susceptibility is enhanced by admixture, the effect of disrupted antiferromagnetic tendencies. Magnetization isotherms evolve with composition. Larger values of magnetization, under the same measuring conditions, occur for mixtures than for pure components, consistent with frustration, which weakens antiferromagnetic alignment tendencies. The competing ferromagnetic (Ni) and antiferromagnetic (Mn) intrachain interactions, along with disorder and low dimensional characteristics, presumably lead to the absence of magnetic order over a remarkably broad composition range.     

Incommensurate Phases of the MgSiF6·6H2O Crystals: EPR and Group-Theoretical Studies

The group-theoretical study of the structural phase transition to incommensurate state of MgSiF₆·6H₂O crystals, revealed by the electron paramagnetic resonance (EPR) method, as well as analysis of the EPR results, are presented. The consideration of temperature dependences of Mn²⁺ admixture ion EPR spectrum symmetry and parameters leads to the conclusion that at T _i1 = 370 ± 0.3 K they undergo second-order structural phase transition to incommensurately modulated state, the order parameter of this transition may be the angle of [Mg(H₂O)₆]²⁺ octahedra rotation around crystal C ₃ axis. At temperature decreasing below T _i1 the gradual transformation of plane-wave modulation of lattice displacements into soliton mode occurs, which is interrupted by the first-order phase transition at T _i2 = 343 ± 0.3 K accompanied by abrupt decrease in modulation amplitude. At T _c = 298.5 ± 0.3 K the first-order improper ferroelastic phase transition into monoclinic phase occurs. The group-theoretical analysis of the phase transition at T _i1 in the investigated crystals, carried out for the first time, has shown that the existence of the incommensurately modulated phase is conditioned by the fundamental reasons (presence of Lifshitz invariant). The conclusions of this analysis on the nature of order parameter, the structural motifs of incommensurate phase and the possible character of temperature evolution of the structure are in agreement with the EPR investigation data.     

Spin correlations in YBa2(Cu1−x Fx)3O7+y ceramic

To detect scattering by magnetic correlations and to estimate their characteristic space scale, YBa₂(Cu_1?x Fe_x)₃O_7+y ceramic with x=0.13 and y=0.4 is investigated by the small-angle scattering of polarized neutrons. The measurements are carried out in the range of temperatures 15 K?T?315 K and magnetic fields 0<H?4500 Oe. Anomalies in the temperature curves of the intensity I(T,q) (where q is the momentum transfer) and the polarization P(T,q) are observed in the temperature range T<40 K. Interference between nuclear and magnetic scattering is also observed in this temperature range. The observed phenomena are interpreted as scattering by magnetic correlations having a scale 70 Å<R<370 Å. Irreversible effects and the type of magnetic ordering are discussed.     

Mössbauer study of (NH4)2FeCl5·H2O

Mössbauer spectra of the compound (NH₄)₂FeCl₅·H₂O have been studied as a function of temperature. Two phase transitions are observed in the temperature range between 7 K and 9 K. The transition at 9 K is structural and presents an unusually high thermal hysteresis. AroundT=8 K the substance orders magnetically and different Fe³⁺ contributions are present.     

Optical investigations of the effect of gradual substitution NH<Subscript>4</Subscript> → Cs on the ferroelastic phase transition in a CsLiSO<Subscript>4</Subscript> crystal

Crystals of Cs_x(NH₄)_1?xLiSO₄ (0.39≤x≤1.0) solid solutions are grown and investigated using polarized light microscopy and measurements of the birefringence in the temperature range 100–530 K. The (x-T) phase diagram of the Cs_x(NH₄)_1?xLiSO₄ solid solutions is constructed. It is demonstrated that, upon substitution of ammonium for cesium in the CsLiSO₄ crystal, the phase transition temperature gradually increases to such a degree that the ferroelastic phase can exist at room temperature. The triple point of intersection of the Pmcn, P2₁cn, and P112₁/n phase boundaries is determined. It is established that the introduction of ammonium in small amounts has an unusually strong effect on the refractive properties and character of the ferroelastic phase transition in the CsLiSO₄ crystal.     

Transverse-and zero-field μSR investigation of magnetism and superconductivity in (Y1−xPr x )Ba2Cu3O7

Zero-field muon-spin-rotation (μSR) measurements on (Y₁? _x Pr _x )Ba₂Cu₃O₇ [x=1.0, 0.8, 0.6, and 0.54] show evidence for antiferromagnetic ordering of the Cu moments within the Cu?O planes, with Néel temperatures 285, 220, 35. 30 and 20 K, respectively. Forx=1.0 the local muon magnetic field is ≈16 mT, but decreases to ≈12 mT at 17K, due to additional magnetic ordering. The zero-field data, in conjunction with transport data, allow construction of a complete phase diagram for this system. Transverse-field (1 kOe) μSR data forx=0.2 (T _c =75 K) show that the muon depolarization is determined primarily by the Cu nuclear moments forT>T _c , and by the vortex state forT _c . Fitting the superconducting-state data to a BCS model yields an extrapolated zero-temperature magnetic penetration depth of 2170 Å.     

An ESR study of phase transitions in crystals of M(BF4)2·6H2O (M = Fe,Co, Ni,Zn)

From a temperature dependent ESR study of Mn²⁺-doped crystals of M(BF₄)₂·6H₂O, M Zn, Co and Ni, new structural phase transitions have been detected and studied. First order structural phase transitions occur in Co(BF₄)₂·6H₂O at T₁ ~ 281K, T₂~189 K and T₃~172K (during cooling), in Zn(BF₄)₂·6H₂O at T₁ ~ 286 K and in Ni(BF₄)₂·6H₂O at T₁ ~ 301 K. A continuous phase transition occurs in Co(BF₄)₂·6H₂O at T_p ~ 257 K, in Zn(BF₄)₂·6H₂O at T_p ~ 277 K and in Ni(BF₄)₂·6H₂O at T_p ~ 294 K. The ESR spectral characteristics suggest similarities in the structures of these fluoroborate compounds in the phase above T₁ with the room temperature structure of Mg(ClO₄)₂·6H₂O. All these compounds are found to have a tendency to crystallise in a triply-twinned pseudo-hexagonal form, although the unit cell above T₁ is found to be orthorhombic. The structural changes related to the water octahedron around the metal at T₁ were found to be very small and basically the same for these three compounds. Although the unit cell structure of Fe(BF₄)₂·6H₂O above the first order phase transition temperature T₁ was found to be similar to that of the other fluoroborate compounds, the structural changes occurring at T₁ appeared to be quite different. The low temperature thermal behaviour differs considerably in the Co, Fe and Zn compounds.     

Enhanced magnetocaloric effect in Eu-doped La0.7Ca0.3MnO3 compounds

Orthorhombic La_0.7-xEu_xCa_0.3MnO₃ samples (x = 0.04–0.12) with apparent density of ρ = 3.9–4.1 g/cm³ prepared by solid-state reactions have been studied. The analysis of temperature-dependent magnetization for an applied field H = 500 Oe indicated a decrease of the Curie temperature (T_C) from about 225 K for x = 0.04 through 189 K for x = 0.08–146 K for x = 0.12. The magnetocaloric (MC) study upon analyzing M(H, T) data has revealed that the magnetic entropy change around T_C reaches the maximum (|ΔS_max|), which is dependent on both x and H. For an applied field interval of ΔH = 60 kOe, |ΔS_max| values are about 5.88, 4.93, and 4.71 J/kg⋅K for x = 0.04, 0.08, and 0.12, respectively. Though |ΔS_max| decreases with increasing x, relative cooling power (RCP) increases remarkably from 383 J/kg for x = 0.04 to about 428 J/kg for x = 0.08 and 0.12. This is related to the widening of the ferromagnetic-paramagnetic transition region when x increases. Particularly, if combining two compounds with x = 0.04 and 0.08 (or 0.12) as refrigerant blocks for MC applications, a cooling device can work in a large temperature range of 145–270 K, corresponding to RCP ≈ 640 J/kg for H = 60 kOe. M(H) analyses around T_C have proved x = 0.04 exhibiting the mixture of first- and second-order phase transitions while x = 0.08 and 0.12 exhibit a second-order nature. The obtained results show potential applications of Eu-doped La_0.7Ca_0.3MnO₃ materials for magnetic refrigeration below room temperature.     

Antiferromagnetic ordering in the double perovskites La2−xSrxCoRuO6

The electric transport and magnetic susceptibility of double perovskites La_2?xSr_xCoRuO₆ have been studied over a temperature range up to 800–1000 K. The crystal and magnetic structure has been determined by neutron diffraction on two samples of the series, x=0.6 and 1.4, which represent the electron- and hole-doped systems with respect to “ideal” single-valent insulator x=1. The study shows that spins in both the Co and Ru f. c. c. like sublattices exhibit a long-range ordering of the antiferromagnetic type II (T_N=60 K for x=0.6 and T_N=60–80 K for x=1.4).     

The properties of the (H2O)6 water cluster that depend on its density during temperature transitions

Water clusters (H₂O)₆ are simulated by the Monte Carlo method with the Metropolis function at various temperatures (T ₁ = 273 K, T ₂ = 298 K, and T′₁= 373 K) and densities (ρ₁ = 0.9998 g/cm³, ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system. It is established that the number of retained most probable configuration types at ρ₁ = 0.9998 g/cm³ during temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K is smaller than at ρ₃ = 0.00059 g/cm³. This result was acquired on the background of the following invariable parameters of the system with the same temperature transitions for each of three values of density: (i) the average number of retained most probable configuration types, (ii) the average fraction of weight coefficients of the most probable configuration types, and (iii) the average potential energy. The configuration type that was retained among the most probable configuration types of the system for all values of density (ρ₁ = 0.9998 g/cm², ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system for temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K was also revealed.     

Structure and magnetic properties of the intermetallic La2Co17−xMox (x=0.5, 1, 1.5, 2) and La2Co16−yFeyMo (y=0, 1, 2, 3, 4, 6) compounds

The effect of Mo and Fe atoms on the crystal structure and magnetic properties of the intermetallic La₂Co_17−xMo_x (x=0.5, 1, 1.5, 2), and La₂Co_16−yFe_yMo (y=0, 1, 2, 3, 4, 6) compounds have been studied by X-ray diffractometry, magnetic measurements and Mössbauer spectroscopy. All samples belong to the rhombohedral Th₂Zn₁₇-type structure and their lattice parameters a and c increase both with Mo and Fe content. From the La–Co–Mo samples only the one with x=0.5 presents planar anisotropy, whereas from the La–Co–Fe–Mo samples only the y=1 has uniaxial anisotropy. The magnetization M_S and the Curie temperature T_C decrease upon Mo substitution, whereas the anisotropy field H_A does not change significantly. In the Fe-substituted compounds M_S increases, but the Curie temperature increases slightly for 0⩽y⩽4 but decreases in y=6. The low temperature M–T curve shows that the samples La₂Co_16.5Mo_0.5, and La₂Co₁₀Fe₆Mo present a spin reorientation transitions at 70 and 260 K, respectively. Mössbauer samples were obtained for all Fe-containing samples in the temperature range 20–300 K. Above 260 K a jump in the values of the hyperfine fields and quadrupole splitting parameters is observed which can be associated to the spin reorientation.