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1.
I. N. Flerov M. V. Gorev V. D. Fokina M. S. Molokeev A. D. Vasil’ev A. F. Bovina N. M. Laptash 《Physics of the Solid State》2006,48(8):1559-1567
The heat capacity, T-p phase diagrams, and unit cell parameters of cryolite (NH4)3Ti(O2)F5 were studied over a wide temperature range. A phase transition was found near 226 K, and its thermodynamic characteristics and their dependence on the crystallization conditions were determined. The coordinates and thermal parameters of atoms in the Fm3m phase were refined. An analysis of the electron density distribution and the transition entropy showed that the mechanism of the structural transition involves, above all, rotation of the Ti(O2)F5 octahedra. Possible models of disordering of tetrahedral ammonium groups are considered. 相似文献
2.
E. V. Bogdanov E. I. Pogoreltsev S. V. Mel’nikova M. V. Gorev I. N. Flerov M. S. Molokeev A. V. Kartashev A. G. Kocharova N. M. Laptash 《Physics of the Solid State》2013,55(2):409-418
Thermal, physical, structural, optical, and dielectric investigations have been performed for oxyfluoride solid solutions (NH4)2W1 — x Mo x O2F4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1). The character of the influence of the chemical and hydrostatic pressures on the stability of the parent (space group Cmcm) and distorted ferroelastic and antiferroelectric phases has been determined by analyzing the temperature-pressure, unit cell volume-composition, and temperature-composition phase diagrams. The specific features of the nature and mechanism of the phase transitions have been discussed using the available data on the structural, entropy, and dielectric parameters. 相似文献
3.
I. N. Flerov M. V. Gorev V. D. Fokina A. F. Bovina M. S. Molokeev Yu. V. Boĭko V. N. Voronov A. G. Kocharova 《Physics of the Solid State》2006,48(1):106-112
A new chemical compound, (NH4)2KWO3F3, was synthesized. The Rietveld-refined crystal structure was found to be cubic at room temperature and to belong to the elpasolite
family (space group
). The heat capacity and unit cell parameters were studied within a broad temperature range. A second-order phase transition
was found to occur at 235.4 K and to be well described in terms of phenomenological theory. Hydrostatic pressure broadens
the temperature interval of stability of the cubic phase (dT0/dp = −10.8 K GPa−1). A possible model of structural ordering based on a comparison of the entropy parameters and electron density distribution
in oxygen and fluorine atoms is discussed. 相似文献
4.
For the first time, the specific heat of single phase, stoichiometric, high transition temperature (21.8 K) A-15 Nb3Ge has been measured. From the data between 4 and 29 K, the linear term coefficient, γ, of the specific heat is found to be 30.3±1. mJ/mole-K2 and the Debye temperature, ?D, is 302±2 K. The bulk energy gap parameter, 2Δ/kTc, is found to be 4.2±0.2, in agreement with tunneling measurements. 相似文献
5.
M. V. Gorev E. V. Bogdanov I. N. Flerov A. G. Kocharova N. M. Laptash 《Physics of the Solid State》2010,52(1):167-175
The thermal expansion along the principal crystallographic axes of the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides has been studied. The anomalous behavior of α
i
(T) due to the phase transitions has been revealed at T
1 = 271.4 K and T
2 ≈ 180 K for the molybdate and at T
1 = 201.5 K and T
2 ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσ
i
, which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been
determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations.
The p-T and α
i
-T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted
phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of
the structural phase transitions are as follows: ΔS
BCE varies from approximately −10 to −17 J/mol K and ΔT
AD ≈ 8−17 K for the molybdate and ΔS
BCE varies from approximately −10 to −17 J/mol K and ΔT
AD ≈ 8−13 K for the tungstate. 相似文献
6.
《Current Applied Physics》2020,20(7):866-870
(Li + Nb) co-doped (Li + Nb)xFe2-xO3 (with x = 0.0005, 0.005, 0.05, and 0.1) ceramics were prepared by sol-gel method. Their structural, dielectric, humidity, and magnetic properties were investigated. Colossal permittivity (~104) was approached or achieved in all doped samples even with a very small doping level of x = 0.0005. The colossal permittivity behavior is composed of two dielectric relaxations with the low-temperature one being a polaron relaxation due to electrons hopping between Fe3+ and Fe4+ ions and the high-temperature one being a Maxwell-Wagner relaxation caused by humidity-sensing properties. 相似文献
7.
S. V. Mel’nikova S. V. Misyul’ A. F. Bovina M. L. Afanas’ev 《Physics of the Solid State》2001,43(8):1594-1597
Optical polarization and x-ray diffraction studies of powders and single-crystal plates of different cuts of an (NH4)3GaF6 crystal are carried out in a broad temperature range, including the phase transition point. The following sequence of symmetry changes is found to occur: $O_h^5 - Fm3m (Z = 4) \leftrightarrow C_i^1 - I\bar 1(Z = 16)$ . 相似文献
8.
The structures of two phases of the (NH4)2KWO3F3 crystal have been determined from X-ray diffraction data obtained for a powder sample. The profile and structural parameters
have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with
invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical
and noncritical atomic orderings and allows one to interpret the experimental data. It has been found that the symmetry transformation
in the crystal can be schematically represented in the form
Fm[`3]m\xrightarrow[(h,0,0,0,0,0)]10 - 10(X5- )PmmnFm\bar 3m\xrightarrow[{(\eta ,0,0,0,0,0)}]{{10 - 10(X_5^ - )}}Pmmn. This transformation is accompanied by the ordering of WO3F3 polyhedra and the displacement of NH4 and K ions. 相似文献
9.
A. N. Vtyurin A. Bulou A. S. Krylov M. L. Afanas’ev A. P. Shebanin 《Physics of the Solid State》2001,43(12):2307-2310
This paper reports on a Raman study of the cubic-to-monoclinic phase transition in (NH4)3ScF6 cryolite. We observed sharp anomalies in the frequencies and half-widths of the Raman lines corresponding to internal vibrations of the ScF 6 3+ ions and to lattice vibrations; no soft lattice mode condensation was revealed. It is concluded that the phase transition studied is related primarily to the orientational ordering of these ions. 相似文献
10.
I. N. Flerov V. D. Fokina M. V. Gorev A. D. Vasiliev A. F. Bovina M. S. Molokeev A. G. Kocharova N. M. Laptash 《Physics of the Solid State》2006,48(4):759-764
Precision studies of the thermophysical properties and structure of an (NH4)2WO2F4 crystal have been performed. It was established reliably that there is a sequence of two phase transitions at T 1 = 201 K and T 2 = 160 K characterized by wedging out of an intermediate phase with an increase in pressure. The role of tetrahedral and octahedral ionic groups in the mechanism of the structural transitions was determined. 相似文献
11.
Optical observations of growth twins and ferroelastic domains and measurements of the rotation of the optical indicatrix were carried out for Rb3H(SeO4)2 and (NH4)3H(SO4)2 using an optical microscope. Taking into account the symmetry reduction from the rhombohedral (Rm) to the monoclinic phase (B2/a) the occurrence of domains and growth twins can be well described. The orientations of oblique ferroelastic walls are well determined by the spontaneous strains s e 11 and s e 23 at room temperature. 相似文献
12.
《Journal of Magnetic Resonance (1969)》1983,51(1):95-102
The temperature dependence of the 35Cl NQR frequencies of 4-chloroanilinium cations in (4-ClC6H4NH3)2MnCl4, (4-ClC6H4NH3)2CuCl4, and (4-ClC6H4NH3)2CuBr4 was studied over a wide range of temperature. For the two copper(II) complexes, magnetic susceptibilities were measured below approximately 20 K. The lattice constants of these isomorphous orthorhombic crystals were determined. Magnetic phase transitions to three-dimensionally long-range-ordered states were found at 40, 8, and 13 K for the respective complexes. Additional structural phase transitions were also found at higher temperatures for all of the complexes studied. Magnetic properties in the ordered state and the nature of both types of phase transitions are discussed. 相似文献
13.
14.
The generic magnetic phase diagram of multiferroic RMn2O5 (with R=Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R's and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magnetoelectric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed. 相似文献
15.
I. N. Flerov V. D. Fokina A. F. Bovina E. V. Bogdanov M. S. Molokeev A. G. Kocharova E. I. Pogorel’tsev N. M. Laptash 《Physics of the Solid State》2008,50(3):515-524
The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH4)3MoO3F3 cryolite (space group Fm $ \overline 3 The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH4)3MoO3F3 cryolite (space group Fm
m) are investigated. It is revealed that the compound undergoes ferroelectric and ferroelastic structural phase transitions
at temperatures of 297 and 205 K, respectively. The mechanism of structural distortions is discussed in terms of the entropy
parameters, pressure-temperature phase diagrams, and electron density maps for critical atoms. An analysis is made of the
influence of the cation size and shape on the phase transitions in oxyfluorides of the general formula A
2
A′MO3 (A,A′ = NH4, K; M = Mo, W).
Original Russian Text ? I.N. Flerov, V.D. Fokina, A.F. Bovina, E.V. Bogdanov, M.S. Molokeev, A.G. Kocharova, E.I. Pogorel’tsev,
N.M. Laptash, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 3, pp. 497–506. 相似文献
16.
M. Paasch M. Winterlich R. Böhmer R. Sonntag G. J. McIntyre A. Loidl 《Zeitschrift für Physik B Condensed Matter》1995,99(3):333-338
The χ,T phase diagram of (NH4I)x(KI)1?x has been determined using neutron diffraction experiments and dielectric spectroscopy. At low temperatures and with decreasing χ, the sequence γ, β, ε and glass phase has been detected. The critical concentration χc≈0.55 separates the glassy phase with frozen-in orientational disorder from the ε phase which reveals long-range orientational order. Close to χc our experiments reveal evidence for two subsequent glass transitions. 相似文献
17.
M. Paasch M. Winterlich R. Böhmer R. Sonntag G. J. McIntyre A. Loidl 《Zeitschrift für Physik B Condensed Matter》1995,99(1):333-338
The χ,T phase diagram of (NH4I)x(KI)1−x has been determined using neutron diffraction experiments and dielectric spectroscopy. At low temperatures and with decreasing
χ, the sequence γ, β, ε and glass phase has been detected. The critical concentration χc≈0.55 separates the glassy phase with frozen-in orientational disorder from the ε phase which reveals long-range orientational
order. Close to χc our experiments reveal evidence for two subsequent glass transitions. 相似文献
18.
19.
M. Mansour-Gabr 《Phase Transitions》2013,86(2):183-198
The AC dielectric permittivity as a function of temperature (100–400 K) at different frequencies (between 80 Hz and 20 kHz) for the layered alkylene diammonium insulator containing Cd, namely, [(NH3)2(CH2)7CdCl2Br2] has been measured. The formation of the compound was confirmed by microchemical analysis and IR absorption spectrometry. X-ray powder diffraction indicates an orthorhombic unit cell of dimensions: a = 10.219(2) Å, b = 9.168(2) Å and c = 38.694(4) Å. The AC conductivity ([sgrave]) is presented as a function of temperature and frequency. The conductivity and permittivity results indicate the presence of first-order phase transitions at 317 and 345 K. This has been confirmed by thermal analysis techniques. The activation energies were of values ranging between 0.15 and 0.62 eV depending upon the temperature range and the applied frequencies. 相似文献