Molecular distortion and ferroelasticity in K2Hg(CN)4 investigated by Raman scattering

The results of the temperature dependence of the Raman spectrum in K₂Hg(CN)₄ indicate that a trigonal distortion of the Hg(CN)⁼₄ ions exists in the room-temperature phase and thus its crystal structure is not cubic as previously reported. The ferroelasticity of the low-temperature phase and the phase transition at 110.5 K are apparently triggered by the molecular distortion.     

First-order magnetic phase transition in the tetragonal K2NpO4

Mössbauer effect and susceptibility studies of the tetragonal K₂NpO₄ compound were performed. K₂NpO₄ ordered magnetically at 19.5(5) K by a first-order phase transition. The ordered moment that was derived from H_eff is ≈0.6μ_B. From the isomer shift of the Mössbauer spectra (-57(1) mm/s rel. NpAl₂) it was concluded that the ground state of the Np ion in the specified compound was a ²F_{$\text{5}\text{2}$} Kramer's doublet.     

ESR study of phase transitions in single crystals of Cd(BF4)2, 6H2O

From an ESR study of Mn²⁺ doped single crystals of Cd(BF₄)₂, 6H₂O, two first order structural phase transitions at 324 ± 2 K (monoclinic to trigonal) and at 177 ± 2 K (trigonal to monoclinic or triclinic) are detected and studied for the first time.     

Stress dependence of Tc for the incommensurate phase transition in K2SeO4

We have measured the uniaxial stress dependence of the frequency of the zone center A₁ soft mode in the low temperature phase of K₂SeO₄. The frequency shows a strong decrease with stress. We infer from this a similar decrease of the incommensurate phase transition temperature T_c. This behavior is explained by detailed lattice dynamical calculations for K₂SeO₄ which reveal the unusual balance between Coulomb and short-range forces in this structure. Our calculations show that this balance is changed by stress in such a way that the quasi-harmonic normal mode frequencies harden drastically. T_c is reduced by this purely volume dependent effect. This unusual balance between Coulumb and short-range forces also provides a natural explanation for the occurence of an incommensurate phase transition in K₂SeO₄. We infer that similar transitions are likely in isomorphous structures.     

Heusler合金Pd2 CrAl四方变形、磁性及弹性常数的第一性原理计算

采用基于密度泛函理论的投影缀加波方法研究了Heusler合金Pd₂CrAl的四方变形、磁性和弹性常数. 四方变形中,Pd₂CrAl在c/a≈1.20处出现总能的局域最小值,对应一个稳定的马氏体. Pd₂CrAl的L2₁结构和四方结构的单胞总磁矩分别为3.825μ_B和3.512μ_B. 在这两种结构中Cr原子均是Pd₂CrAl总磁矩的主要贡献者,Pd和Cr原子间存在很强的杂化作用,Cr的3d电子的t_2g和e_g两个亚能带是Pd₂CrAl磁性的主要来源. 弹性常数的计算结果显示,Pd₂CrAl的L2₁结构和四方结构的弹性常数均满足相应结构的稳定性判据. 关键词： Heusler合金 四方变形 磁性 弹性常数     

Texture in a ternary Nd16.2Fe78.2B5.6 powder using a modified hydrogenation–disproportionation–desorption–recombination process

A modified hydrogenation–disproportionation desorption-recombination (HDDR) process consisting of (i) solid disproportionation and (ii) slow recombination under partial hydrogen pressure has been applied to a Nd_16.2Fe_78.2B_5.6 alloy. Scanning electron microscopy shows that an initially fine rod-like structure of NdH_2±x and Fe observed after 15 min of hydrogenation at 900°C is transformed into a granular morphology with prolonged annealing. Simultaneously, finely dispersed tetragonal Fe₃B particles of 10–50 nm diameter exist. XRD studies show that this metastable Fe₃B phase is transformed to Fe₂B and Fe on further annealing. Short solid-disproportionation times result in a higher degree of anisotropy after recombination, whereas long annealing times and conventional processing lead to isotropic material. It is concluded that the formation of the intermediate tetragonal Fe₃B phase after solid disproportionation is pivotal for the inducement of texture in HDDR processed ternary NdFeB-type alloys.     

Brillouin scattering in Hg2Cl2

Hg₂Cl₂ undergoes a second order phase transition around 185 K. All the elastic constants have been measured above (tetragonal phase) and below (orthorhombic phase) the transition point. The results support the hypothesis that the transition is induced by the condensation of a transversal acoustical mode at an X point of the first Brillouin zone in the tetragonal phase. The coefficients of the coupling terms in the free energy are evaluated and the relative values of the spontaneous deformations are evaluated.     

Identification of a mixed-valence triclinic product formed by the partial bromination of K2Pt(CN)4

X-ray diffraction measurements as well as the results of chemical analysis are presented as evidence that the partial bromination of K₂Pt(CN)₄ results in at least two products which differ in their structural features and stoichiometry. Triclinic crystals grow from acidic aqueous solution whereas the known tetragonal phase grows from basic solutions. The Pt-Pt spacing is 2.95 Å in the triclinic phase as compared to a Pt-Pt spacing of 2.88 Å in the tetragonal phase.     

Theoretical investigation of the local lattice structure of Mn 2+ ion doped in tetragonal K 2ZnF 4 crystal

The complete energy matrices for a d⁵configuration ion in a tetragonal ligand-field has been constructed on the basis of the complete set of basis of d⁵configuration (252 dimension), and the relationship between the low-symmetry EPR parameters b₂ ⁰ ,b₄ ⁰ and the local distortion parameters has been established based on the complete energy matrices. As an application, we have studied the EPR parameters and the local lattice structure of Mn²⁺ ion doped in tetragonal K₂ZnF₄ system. The calculation indicated that the local lattice structure around a tetragonal Mn²⁺ ion center has an expansion distortion. Simultaneously, the local lattice structure parameters R₁ =2.0727 ?, R₂ =2.0801 ? at room temperature (295 K) and R₁ ^′ = 2.0439 ?, R₂ ^′ =2.05478 ? at low temperature (4.2 K) are determined.     

Dielectric properties of PbSrWO4 and PbBaWO4 compounds

PbSrWO₄ and PbBaWO₄ have been synthesised by the solid state reaction technique XRD patterns show them to be tetragonal. Dielectric constant (K^/) and Dielectric loss (K ^//) of PbSrWO₄ and PbBaWO₄ have been measured at 1 kHz in the temperature range of 300 to 1050 K. The log K^/ vs T as well as log K ^// vs T plot of PbSrWO₄ and PbBaWO₄ shows rapid increase of dielectric constant above 590 K and 640 K, respectively.     

Pressure-induced transformations and optical properties of the two-dimensional tetragonal polymer of C60 at pressures up to 30 GPa

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C₆₀ have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C₆₀ and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C₆₀ were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C₆₀ monomer. The deformation potential and the Grüneisen parameters of the 2D(T) polymeric phase of C₆₀ have been determined and compared with those of the pristine material.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Raman and brillouin scattering in a single crystal of K2Hg(CN)4

The temperature dependence of the elastic constants of cubic K₂Hg(CN)₄ has been measured by optical Brillouin spectroscopy in the temperature range from 300-110 K. A decrease of all three elastic constants was found in agreement with the results of ultrasonic measurements. Raman measurements in the temperature range from 300-4 K revealed a splitting of the F_2g modes corresponding to a trigonal distortion of the Hg(CN)₄-tetrahedra in the low temperature phase.     

Heusler合金Mn2 NiAl的电子结构和磁性对四方畸变的响应及其压力响应

运用基于密度泛函理论的第一性原理的投影缀加波方法,对Hg₂CuTi型Mn₂NiAl在由立方结构至四方结构的畸变过程中电子结构和磁性的变化规律及其对压力响应的规律进行了研究.研究发现:在由奥氏体相到马氏体相的相变中,由于Ni-Mn(A)原子间距的减小而使得杂化程度增强,导致占据态的态密度向低能区域移动,体系的能量降低,致使在马氏体相中的稳定性增大;在从奥氏体相到马氏体相的相变中,能带变宽,成键作用加强,从而在马氏体相中的稳定性增大;在四方畸变过程中,总磁矩的变化主要来源于Ni原子磁矩的变化;计算得到Mn₂NiAl的零压体积弹性模量为125.69 GPa,其抗压缩性比其他常见的Heusler型合金弱. 关键词： 第一性原理 电子结构 磁性 四方畸变     

Electron energy structure of defect chalcopyrite CdGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>

The CdGa₂Se₄ compound, with a crystalline lattice of defect chalcopyrite, shows birefringence. However, in contrast to the similar CdGa₂S₄ compound, CdGa₂Se₄ has no experimentally observed isotropic points in the optical spectral range. In this study, the electron energy structure and optical spectra of CdGa₂Se₄ are calculated for three different experimentally obtained sets of crystalline parameters. Only one of these sets shows the absence of isotropic points.     

A low-temperature tilter system and its application to the measurement of the anisotropy of optical rotation in K2Zncl4 in the vicinity of the phase transition at 145 k

The ferroelectric/ferroelastic phase transition of K₂ZnCl₄ at 145 K has been investigated employing the tilter method (Kaminsky and Glazer (1996) Ferroelectrics 183, 133) which was adapted to low-temperature experiments. We were able to observe the anisotropy of optical rotation in K₂ZnCl₄ which shows a distinct discontinuity at the transition temperature. A precursor-optical rotation in the ferroelectric phase is discussed in connection with a cluster ordering scheme suggested in an earlier X-ray study. The rotation of the optical indicatrix which accompanies the transition from the orthorhombic to the monoclinic system shows a Landau-type temperature dependence. Model calculations based on the dipole-dipole interaction between the atoms of the structures at different temperatures are in qualitative agreement with the experimental results.     

Spectroscopic properties of Fe ions at tetragonal sites—Crystal field effects and microscopic modeling of spin Hamiltonian parameters for Fe (S=2) ions in K2FeF4 and K2ZnF4

Magnetic and spectroscopic properties of the planar antiferromagnet K₂FeF₄ are determined by the Fe²⁺ ions at tetragonal sites. The two-dimensional easy-plane anisotropy exhibited by K₂FeF₄ is due to the zero field splitting (ZFS) terms arising from the orbital singlet ground state of Fe²⁺ ions with the spin S=2. To provide insight into the single-ion magnetic anisotropy of K₂FeF₄, the crystal field theory and the microscopic spin Hamiltonian (MSH) approach based on the tensor method is adopted. Survey of available experimental data on the crystal field energy levels and free-ion parameters for Fe²⁺ ions in K₂FeF₄ and related compounds is carried out to provide input for microscopic modeling of the ZFS parameters and the Zeeman electronic ones. The ZFS parameters are expressed in the extended Stevens notation and include contributions up to the fourth-order using as perturbation the spin-orbit and electronic spin-spin couplings within the tetragonal crystal field states of the ground ⁵D multiplet. Modeling of the ZFS parameters and the Zeeman electronic ones is carried out. Variation of these parameters is studied taking into account reasonable ranges of the microscopic ones, i.e. the spin-orbit and spin-spin coupling constants, and the energy level splittings, suitable for Fe²⁺ ions in K₂FeF₄ and Fe²⁺:K₂ZnF₄. Conversions between the ZFS parameters in the extended Stevens notation and the conventional ones are considered to enable comparison with the data of others. Comparative analysis of the MSH formulas derived earlier and our more complete ones indicates the importance of terms omitted earlier as well as the fourth-order ZFS parameters and the spin-spin coupling related contributions. The results may be useful also for Fe²⁺ ions at axial symmetry sites in related systems, i.e. Fe:K₂MnF₄, Rb₂Co_1−xFe_xF₄, Fe²⁺:Rb₂CrCl₄, and Fe²⁺:Rb₂ZnCl₄.     

Low-frequency internal friction of polycrystalline ZrO2-4 mol % Y2O3 in the temperature range 273–373 K

The specific features in the low-frequency internal friction, structure, and phase composition of polycrystalline ZrO₂-4 mol % Y₂O₃ are investigated in the temperature range 273–373 K. It is demonstrated that the low-frequency internal friction exhibits two peaks upon heating and cooling. The former peak is observed at a temperature of approximately 293 K, and the latter peak is revealed at 313 K. It is assumed that the peak observed in the low-frequency internal friction at a temperature of 293 K is attributed to relaxation dissipation of energy during motion of twin boundaries in the monoclinic phase, whereas the peak at 313 K is associated with relaxation processes due to displacement of the boundaries of tetragonal T′ domains.     

Specific heat behaviour of K2SeO4 in the 48K–800K temperature range a new phase transition

A calorimetric study performed on the ferroelectric K₂SeO₄ in the 48K–800K temperature range confirmed the existence of three phase transition and revealed the occurrence of a new one at 56.03±0.05K (T^IV_c). The critical measurements for the commensurate-incommensurate phase transition seem to show a cross-over behaviour.     

Local structure of disordered PbSc<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> in the region of the diffuse tetragonal phase-rhombohedral phase transition

The local structure of the ferroelectric-relaxor PbSc_1/2Nb_1/2O₃ in the temperature range from 550 to 220 K has been investigated using ⁴⁵Sc nuclear magnetic resonance. It has been found that, in the paraelectric phase at temperatures below 550 K, the crystal consists of regions of an ordered elpasolite structure and inclusions of the disordered tetragonal perovskite phase with displacements along directions of the [001] type. The relative weight of the tetragonal structure in the region of the paraelectric phase is approximately equal to 0.28. Below the temperature of the phase transition from the disordered modification to the polar phase, the relative weight of the tetragonal phase decreases with decreasing temperature. The tetragonal structure is replaced by the trigonal polar structure. In a wide temperature range (∼50 K), there exists a heterophase structure that is characteristic of relaxors. Note that the correlation length of displacements in the tetragonal phase should be very small to explain the absence of indications of the existence of this phase in the diffraction data.