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1.
The specific heat of [NH2(CH3)2]2ZnCl4 was measured calorimetrically in the temperature region 80–300 K. As the temperature T decreases, the C p (T) dependence indicates a phase transition sequence, with the phase transition at T6=151 K observed for the first time. The thermodynamic characteristics of the crystal were refined. The transformation occurring at T2=298.3 K is shown to be an incommensurate-commensurate phase transition. 相似文献
2.
The heat capacity of the [[N(C2H5)4]2CdBr4 crystal is measured by the calorimetric method in the temperature range from 80 to 300 K. It is revealed for the first time that the temperature dependence of the heat capacity C p (T) exhibits an anomaly associated with the first-order phase transition occurring at the temperature T 1 = 226.5 K. A long relaxation of the temperature of the crystal is observed in the temperature range 150–165 K. 相似文献
3.
The polarized Raman spectra of SrB4O7 (SBO) single crystals are studied in detail in the temperature range of 300–1273 K. The TO, LO, and IO phonon lines of A1, A2, B1, and B2 symmetries of rhombic SBO at 300 K are identified. The behavior of the Raman spectra of SBO crystals is studied upon heating up to their melting. The relation of Raman spectra with the structure of boron–oxygen fragments, as well as the transformation of spectra in the process of melting of SBO crystals, is discussed. 相似文献
4.
E. P. Smirnova A. Sotnikov S. Ktitorov N. Zaitseva H. Schmidt M. Weihnacht 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,83(1):39-45
The longitudinal acoustic wave velocity and attenuation in
BiFeO3 ceramics have been measured by ultrasonic
pulse-echo technique at a frequency of 10 MHz in the temperature range from
4.2 K to 830 K. The anomalies observed in the sound velocity and attenuation
behavior versus temperature are attributed to the assumed relaxation in the
temperature range 200–500 K and antiferromagnetic phase transition at
higher temperatures. Order parameter fluctuations along with
magnetostriction are discussed as the factors determining the acoustic wave
velocity anomaly in the vicinity of the antiferromagnetic phase transition
point. 相似文献
5.
Solid solutions in the Csx(NH4)1?xLiSO4 (0≤x≤0.35) system are grown and investigated. The birefringence (na?nb) and the heat capacity are measured in the temperature range 100–530 K. The (x-T) phase diagram is constructed. It is demonstrated that the substitution of cesium for ammonium in the NH4LiSO4 crystal affects the transition temperatures in such a way that the region of the ferroelectric phase increases and the ferroelastic phase disappears at x>0.22. The character of the high-temperature transition remains unchanged (2β=0.24±0.01 for all compositions), but the birefringence anomaly and enthalpy decrease. As the concentration x increases, the low-temperature transition becomes more similar to a first-order transition: the birefringence jump δn and the temperature hysteresis ΔT increase. 相似文献
6.
Yu. V. Shaldin S. Matyjasik M. Kh. Rabadanov N. Angert M. Roth M. Tseitlin 《Physics of the Solid State》2006,48(5):912-918
The results of measuring the pyroelectric coefficient γ s σ of nominally perfect KTiOPO4 (KTP) crystals grown from solution in a melt with a potassium to phosphorus ratio of ~2 are presented. The γ s σ (T) dependence is monotonic in the range from 4.2 to 250 K. Deviations from a linear dependence are observed beginning from 250 K, which is considered to be due to interstitial-potassium transport in the KTP crystal field. The spontaneous polarization of unclamped KTP samples is estimated from the results of the measurements. In terms of the crystal-physics approach, it is shown that the main contribution to a polar state of KTP is made by the dipole moments of two nonequivalent mesoscopic tetrahedra forming two sublattices that are polarized in opposite directions and bound by Ti(1) ions. 相似文献
7.
The temperature dependence of the permittivity ε of PbWO4 crystals is studied in the range T = 290–550 K at a frequency of 1 kHz. The ε(T) dependences measured on heating and cooling are different. On heating, groups of narrow maxima at 290–330 K and 330–400 K are observed in the ε(T) curves. The first group of peaks is dominant. High-temperature polarization produces an additional broad peak in the ε (T) curve at 400 K. A linear ε(T) dependence is observed in the range 400–470 K. Above 470 K, the variation in ε(T) closely follows an exponential law. Restoring relaxation of ε in the range 25–30 at 290 K after high-temperature sample heating proceeds exponentially in a few stages. The features of ε(T) curves are determined by the dipole polarization and the hopping mechanism of charge exchange between complex dipole associates. Such structural defects may be pairs of doubly charged lead and oxygen vacancies (diplons). These defects also form a basis for more complicated defect complexes with localized holes (or electrons) at the corresponding vacancies. 相似文献
8.
Yu. N. Ivanov I. P. Aleksandrova A. A. Sukhovsky A. I. Baranov 《Physics of the Solid State》2007,49(6):1142-1148
A crystal of the Cs5H3(SO4)4 · xH2O (x ≈ 0.5) (PCHS) compound, which belongs to the family of proton conductors with a complex system of hydrogen bonds, is investigated by 2H NMR spectroscopy. The temperature and orientation dependences of the 2H NMR spectra are measured and analyzed. It is established that, upon transition to the glassy phase at the temperature T g = 260 K, the parameters characterizing the proton exchange between positions in hydrogen bonds remain unchanged to within the limits of experimental error. The protons in the two-dimensional network of hydrogen bonds in the (001) plane are dynamically disordered over possible positions down to temperatures considerably lower than the glass transition point T g . However, water molecules are fixed at particular structural positions in the phase transition range. In PCHS crystals with a nonstoichiometric water content, this circumstance can be responsible for the frustration that leads to the formation of the glassy state. 相似文献
9.
The heat capacity of [(C2H5)2NH2]2CuCl4 crystals, both nonirradiated and γ-irradiated to a dose of 107R, was studied in the temperature interval 90–330 K by adiabatic calorimetry. The temperature dependence of Cp(T) was found to have a peak-shaped anomaly in the region of the thermochromic phase transition (PT) at T = 322.7 K. Smoothened experimental heat capacity data were used to calculate the changes in the thermodynamic functions. The changes in the entropy and enthalpy of the thermochromic PT were determined to be ΔS = 42 J K?1 mol?1 and ΔH = 13653 J mol?1 for the nonirradiated crystals and ΔS = 39 J K?1 mol?1 and ΔH = 12120 J mol?1 for the irradiated crystals, respectively. Irradiation of a [(C2H5)2NH2]2CuCl4 crystal by γ rays to a dose of 107 R was shown to shift the PT point toward lower temperatures by ΔT ≈ 1.7 K. 相似文献
10.
The electrical conductivity σ and dielectric properties (?, tanδ) of β-BaB2O4 were studied in the temperature range 90–300 K. The quantities σ, ?, and tanδ were measured at frequencies of 0.1, 1, and 10 kHz and 1 MHz. The dielectric permittivity and electrical conductivity were found to grow with increasing temperature at all frequencies. The permittivity decreases and the electrical conductivity increases (by several orders of magnitude) with increasing frequency. Maxima were observed in the σ=f(T) and tanδ=f(T) curves for all frequencies; the maxima shift toward higher temperatures with increasing frequency. 相似文献
11.
The reflection R(?ω), transmission t(?ω), absorption α(?ω), and refraction n(?ω) spectra of polycrystalline In2O3–SrO samples with low optical transparency, which contain In2O3 and In2SrO4 crystallites with In4SrO6 + δ interlayers, are examined. In the region of small ?ω values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences n(?ω) and α(?ω) are calculated using the classical electrodynamics relations. The results are compared to the data based on the t(?ω) spectra. The calculated absorption spectra are interpreted within the model with an overlap of tails of the density of states in the valence band and in the conduction band. A “negative” gap E gn in the density of states with a width from–0.12 to–0.47 eV is formed in highly disordered samples in this model. It is demonstrated that the high density of defects and the band of deep acceptor states of strontium in the major matrix In2O3 phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap E gd = 1.3 eV corresponding to the In2SrO4 phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed. 相似文献
12.
Organic–inorganic hybrid sample [N(C4H9)4]2Cu2Cl6 was prepared via the reaction between copper chloride and tetrabutylammonium chloride. The compound was characterized by
X-ray powder diffraction, IR, Raman, differential scanning calorimetry (DSC), DTA-TGA analysis and electrical impedance spectroscopy.
DSC studies indicate a presence of one-phase transition at 343 K. The complex impedance of compound [N(C4H9)4]2Cu2Cl6 have been investigated in temperature and frequency ranges 300–380 K and 200 Hz–5 MHz, respectively. The Z′ and Z″ versus frequency plots are well fitted to an equivalent circuit model. The circuits consist of the parallel combination
of bulk resistance R
p and constant phase elements CPE. The frequency dependence of the conductivity is interpreted in term of Jonscher's law: s(w) = sdc + Awn \sigma (\omega ){ } = {\sigma_{\rm{dc}}} + { }A{\omega^n} . The conductivity follows the Arrhenius relation. The variation of the value of these elements with temperatures confirmed
the availability of the phase transition at 343 K detected by DSC and electrical measurements. 相似文献
13.
Composite cathode material LiFePO4–Li3V2(PO4)3 is synthesized through a chemical reduction and lithiation using FeVO4·xH2O as both iron and vanadium sources. The structural properties of LiFePO4–Li3V2(PO4)3 are investigated. X-ray diffraction results show the composite material containing olivine type LiFePO4 and monoclinic Li3V2(PO4)3 phases. High-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry results indicate that mutual
doping effects take place between the LiFePO4 and Li3V2(PO4)3 particles with V3+ doping the LiFePO4 while Fe2+ dopes the Li3V2(PO4)3. LiFePO4–Li3V2(PO4)3 nanocomposites are formed in the carbon webs. There is no structural compatibility between monoclinic (Li3V2(PO4)3) and olivine (LiFePO4) domains in composite material LiFePO4–Li3V2(PO4)3. 相似文献
14.
The NH2 radical absolute concentrations were measured using intracavity laser absorption spectroscopy during the process of ammonia pyrolysis at relatively low temperatures of 800–1000 K. The NH2 spectra have been recorded at pressures of 12–380 Torr. The observed absolute concentrations (in the order of 1011 molecules/cm3) were higher than was predicted by the gas-phase pyrolysis mechanisms. These mechanisms also overestimate the activation energy of the process. The experimental observations can be explained by the existence of an additional channel of NH2 formation – probably on the surface of the reaction vessel. The addition of oxygen to the reaction mixture causes a considerable ( four-fold) increase in the NH2 radical concentration. PACS 33.20.Kf; 42.62.Fi; 82.20.Pm 相似文献
15.
We have studied the magnetic structure of Fe[Fe(CN)6]·4H2O, prepared by precipitation method, using neutron diffraction technique. Temperature dependent DC magnetization study down
to 4.2 K shows that the compound undergoes from a high temperature disordered (paramagnetic) to an ordered magnetic phase
transition at 22.6 K. Rietveld analysis of neutron diffraction pattern at 60 K (in its paramagnetic phase) revealed a face
centred cubic structure with space group Fm3m. The structure contains three-dimensional network of straight Fe3+-C≡N-Fe3+ chains along the edges of the unit cell cube. Fe3+ ions occupy 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2) positions. Fe3+(0, 0, 0) is surrounded octahedrally by six nitrogen atoms and Fe3+ (1/2, 1/2, 1/2) is surrounded octahedrally by six carbon atoms. Magnetic Rietveld refinement of neutron diffraction data
at 11 K shows a ferromagnetic coupling between the two inequivalent Fe3+ sites. Refinement yielded an ordered moment of 4.4(6) and 0.8(6) μB per Fe ion located at (0, 0, 0) and (1/2, 1/2, 1/2), respectively. Ordered moments are found to align along the face diagonal.
The observed net moment from low temperature neutron diffraction study is consistent with DC magnetization results. 相似文献
16.
The effect of IIIA metal and transition metalT substitution for Fe on crystal structure, magnetostriction and spontaneous magnetostriction, anisotropy and spin reorientation of a series of polycrystalline Tb0.3 Dy0.7 (Fe0.9 T 0.1)1.95 (T=Mn, Fe, Co, B, Al, Ga) alloys at room temperature were investigated systematically. It was found that the primary phase of the Tb0.3 Dy0.7 (Fe0.9 T 0.1)1.95 alloys is the MgCu2-type cubic Laves phase structure for different substitution. The magnetostrictionλ s decrases greatly for the substitution of IIIA metal, B, Al and Ga, but is saturated more easily for Al and Ga substitution, showing that the Al and Ga substitution is beneficial to a decrease in the magnetocrystalline anisotropy of Tb0.3 Dy0.7 (Fe0.9 T 0.1)1.95 alloys. However, the substitution of transition metal Mn and Co decreases slightly the magnetostrictionλ s . It was also found that the effect of different substitutions on the spontaneous magnetostrictionλ 111 is distinct. The analysis of the Mössbauer spectra indicates that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry for Al and Ga substitution, namely spin reorientation, but it does not change evidently for B, Mn and Co substitution. 相似文献
17.
I. F. Voloshin A. V. Kalinov L. M. Fisher V. A. Yampol’skii 《Journal of Experimental and Theoretical Physics》2009,108(3):411-416
The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa2Cu3O7 ? δ samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δth = 0.37. For δ < δth (T c > 60 K, where T c is the superconducting transition temperature), function S(T) has the well-known peak at T/T c = 0.4. For δ > δth (at T c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T c = 0.1. The threshold value δth of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition. 相似文献
18.
The pyroelectric properties of samples cut from various growth sectors of RbTiOPO4 single crystals grown from solution in a melt were measured in the temperature range from 4.2 to 300 K. The experimental values of the pyroelectric coefficient range from ?1.3 × 10?5 to ?4.6 × 10?5 C/m2 K. For the samples cut from the (100) sector, pronounced anomalies were revealed at 85 and 275 K, which, in our opinion, can be due to the contribution of associates formed by the coordination tetrahedra PO4(1) and PO4(2) and interstitial rubidium Rb i . At T > 280 K, superionic conductivity begins to manifest itself in all of the samples studied, which indicates the decomposition of the dipole complexes with increasing temperature. From the measured pyroelectric coefficient and birefringence along the polar direction, the spontaneous polarization of rubidium titanyl is calculated to be 0.5 C/m2 at 250 K, which is comparable in magnitude to that of lithium tantalate. 相似文献
19.
S. V. Gudenko A. Yu. Yakubovskii O. Yu. Gorbenko A. R. Kaul 《Physics of the Solid State》2004,46(11):2094-2102
The EPR of Mn ions in the (La1?yPry)0.7Ca0.3MnO3 system has been studied within a broad range of temperatures (4<T<600 K) and Pr concentrations (0≤y≤1), as well as under isotope substitution of 18O for 16O. All compositions were shown to undergo transitions to a magnetically ordered state with decreasing temperature. Magnetic phase diagrams were constructed for systems with different oxygen isotopes. The diagrams include paramagnetic, ferromagnetic, and antiferromagnetic regions. In the paramagnetic region, at temperatures not too close to the phase transition points, the Mn ion linewidth ΔH pp (T) is related to the magnetic susceptibility χ(T) through the relation ΔH pp (T) = [χ0/χ(T)]ΔH pp (∞) + ΔH0, where ΔH pp (∞) is the width of the exchange-narrowed line in the high-temperature approximation, χ0 ∝ 1/T is the susceptibility of noninteracting ions, and ΔH0 is the residual width originating from the sample porosity and resonance-field scatter in unoriented grains of a powder sample. An analysis of the data on ΔH pp (∞), ΔH0, and χ(T) made it possible to estimate the symmetric and antisymmetric exchange interaction of Mn ions and of the noncubic crystal-field component of the oxygen ions. These parameters were found to be independent of the oxygen isotope species to within experimental error. 相似文献
20.
Jinguang Cheng Yu Sui Yang Wang Xianjie Wang Wenhui Su 《Applied Physics A: Materials Science & Processing》2009,94(4):911-916
Transportation and thermodynamic properties of misfit-layered polycrystalline [Ca2CoO3]0.62[CoO2] were measured in order to clarify the nature of metal– semiconductor transition (MST) at T
MS≈400 K, above which the simultaneous decrease of resistivity and increase of thermopower with temperature give rise to a great
enhancement of thermoelectric power factor up to 1000 K. A first-order phase transition characteristic around T
MS was revealed by anomalies of resistivity, differential scanning calorimetry, and thermal expansion. The first-order characteristic
of the MST can be rationalized from the Virial theorem at an itinerant to localized electron transition in the narrow e
T
band within the [CoO2] plane. Above T
MS, the reduction of the retained delocalized states within the matrix of localized states and the enhancement of charge carrier
effective mass with increasing temperature might account for the considerable enhancement of the thermopower. 相似文献