Thermal vibration in the cubic phase of K2SnCl6

The rectilinear and angular mean-square displacements of the atoms in K₂SnCl₆ have been measured from 293 to 450 K using single-crystal X-ray diffraction. The motion of the tin and chlorine atoms has been found to consist, essentially, of rigid-body translation and libration of the SnCl₆ octahedra. The root-mean-square angle of libration of the SnCl₆ octahedra, $\text{φ}{}^2$^{$\text{1}\text{2}$}, ranged from 5.7 to 6.1°. The slow variation of $\text{φ}{}^2$ with temperature, together with the measured non-zero value of one of the fourth cumulants of the chlorine atom displacement, are indicative of highly non-linear coupled motion associated with the soft-mode phase transition at 261.5 K.     

Inelastic neutron scattering from transverse acoustic modes in K2SnCl6

In cubic K₂SnCl₆ the dispersion curve für q q; [110] of [1$\text{1}$0] polarized acoustic phonons has been measured at three temperatures near the phase transition temperature T_c1. The acoustic branch shows no temperature dependence in the low wave vector region and stiffens slightly near the X-point of the fcc-Brillouin zone as the temperature approaches the phase transition T_c1= 262 K from above. The results support a previously developed model on acoustic anomalies in this compound.     

Pressure dependence of phase transitions in K2SnCl6 from NQR frequency measurements

The influence of hydrostatic pressure 0 ? p ? 4 kbar on the ³⁵Cl NQR in K₂SnCl₆ was studied in the temperature range 238 K ? T ? 300 K. The phase transition temperatures T_C1 and T_C2 were determined from changes in the NQR line pattern.The phase boundaries in the p-T diagram are straight lines in the region studied. The pressure coefficients are given by dT_C1/dP = 1.35 (10) K kbar^?1 and dT_C2/dP=?1.25 (20) K kbar^?1.     

The lattice parameters of K2SnCl6 at low temperatures determined by high resolution single crystal X-ray diffraction

The temperature dependence of lattice parameters of K₂SnCl₆ was measured in the temperature range 300 K>T>240 K by means of high resolution single crystal X-ray diffraction experiments. The cubic phase ($\text{a=9.986(2)}\text{A}\text{?}\text{,}\text{Z}\text{=4,}\text{T}\text{=280 K}$) was confirmed, whereas in disagreement to earlier authors the intermediate phase was proved to be orthorhombic ($\text{a}\text{=9.966(2)}\text{A}\text{?}\text{, (}\text{b}\text{=9.948(2)}\text{A}\text{?}\text{,}\text{c}\text{=10.002(2)}\text{A}\text{?}\text{,}\text{Z}\text{=4,}\text{T}\text{=258 K}$), and the monoclinic phase to have another unit cell ($\text{a}\text{=9.951(2)}\text{A}\text{?}\text{,}\text{b}\text{=9.948(2)}\text{A}\text{?}\text{,}\text{c}\text{=10.005(2)}\text{A}\text{?}\text{, β=90.099°,}\text{Z}\text{=4,}\text{T}\text{=250 K}$). In a small temperature range between the cubic and the orthorhombic phase reflexion splittings and the occurence of superstructure reflexions indicate a further intermediate phase.     

Evidence of phase transitions in K2PbCu(NO2)6

The heat capacity of K₂PbCu(NO₂)₆ shows anomalies at 273K and 280K, which are accompanied by length changes. At 280 K a change in the E.S.R. spectrum is observed.     

Soft rotary mode and structural phase transitions in K2ReCl6

The triple-axis neutron scattering technique has been used to observe the soft-phonon modes associated with the second-order structural phase transitions in the cubic antifluorite K₂ReCl₆. The transverse (ReCl₆)^?2 rotary mode has been found to be soft all along the [ζ00] direction, with the structural distortions being triggered by a condensation first at the zone center and then at the [100] zone boundary. In addition to the soft phonon modes there is a temperature dependent central component to the spectrum along the [ζ00] direction.     

X-ray investigations of modulations in Cs2ZnI4

The X-ray diffractometer patterns of a reciprocal space have been analyzed in the modulated phases of Cs₂ZnI₄ crystals. The character of the atomic displacements in the modulation wave has been established. The temperature behaviour of both a modulation wave vector and amplitude has been determined from the position and intensity of satellite reflections.     

Muonic X-ray intensities in SF6 and in H2+SF6

With a high pressure gas target muonic F and S X-ray intensities in SF₆ and H₂+SF₆ have been measured. It was found that the distribution of muons between sulphur and fluorine is not at all equal to the ratio of the atomic numbers Z.     

Pion-pion phase shift and K2π decay

We remark that, using the Cabibbo transformation properties for the weak Hamiltonian we can obtain Δ₀ = δ₀⁽⁰⁾ (s = M_K⁽²⁾ (s = M_K²) from the experimental decay rates for K_s → 2π, K⁺ → π⁰π⁺, obtaining δ₀ = 58.0° ± 4.6°. This result implies in particular a value of ≈ 0.6 for the S-wave isospin zero scattering length, in accordance with K_e4 results but in violent disagreement with Weinberg's calculation.     

Uniaxial stress measurements on Rb2PbCu(NO2)6 and K2PbCu(NO2)6

EPR measurements on crystals of compressed tetragonal Rb₂PbCu(NO₂)₆ and K₂PbCu(NO₂)₆ subjected to uniaxial stress have been carried out at various temperatures. The results indicate that uniaxial stress can reorient the crystal axes in both compounds and that smaller stresses are required for Rb₂PbCu(NO₂)₆ than for K₂PbCu(NO₂)₆ at comparable temperatures. Larger stresses are required at lower temperatures.     

Elastic anomaly in K2ZnCl4 around the normal-incommensurate-commensurate phase transition points

The ultrasonic velocities in K₂ZnCl₄ were measured as functions of temperature. At the incommensurate-commensurate phase transition point the velocity of the c₅₅ mode showed a clear anomaly, whereas anomalies were hardly observable in all other modes. The results are analyzed in terms of a phenomenological theory.     

Adiabatic potential energies for Na2 and K2

Adiabatic potential energies (Σ states) were calculated for the systems Na₂ and K₂. Wells followed by bumps were observed for the ³Σ_u excited states dissociating to Na(3s) + Na(3p) and K(4s) + K(4p). Double minima for the ¹Σ_u states dissociating to Na(3s) + Na(4s) and K(4s) + K(5s) are reported.     

Successive Jahn-Teller transitions and the canted pseudospin state in K2PbCu(NO2)6

The successive phase transitions in K₂PbCu(NO₂)₆ has been studied by precise X-ray measurements. The unit cell in Phase III is doubled along each of the cubic principal axes. The experimental results can be interpreted as sequencial cooperative Jahn-Teller transitions where the higher transition is due to the coupling with the uniform distortion, while the lower transition is caused by the coupling with the uniform distortion, while the lower transition is caused by the coupling with the zone boundary phonon mode with $\text{k}{}_0\text{= (}\text{1}\text{2}\text{,}\text{1}\text{2}\text{,}\text{1}\text{2}$. The electronic state in Phase III is characterized as the “canted pseudospin” state.     

EPR lineshape study of the incommensurate phase in γ-irradiated K2SeO4

The temperature dependence of the SeO^4-₄ EPR frequencies and the asymmetric broadening of the EPR lines in the incommensurate phase of K₂SeO₄ can be explained by an incommensurate spatial modulation of the g tensors which corresponds to the “broad” phase soliton limit. A comparison between the experimental and calculated lineshape shows a ≈1% volume fraction of commensurate regions in the middle of the incommensurate phase at 110 K.     

Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

Spectroscopic investigation of [(NH4)xK1−x]2SnCl6 mixed crystals

Polycrystalline samples of the solid solution [(NH₄)_xK_1?x]₂SnCl₆(0?x? 1) have been investigated by DSC, X-ray diffraction and Raman scattering experiments. Substitution of K⁺ by NH⁺₄ depresses the phase transition temperature T₁. For 0? x ?0.05 a linear temperature coefficient $\text{d}\text{T}{}_1\text{dx=?5.16}\text{K/mol}$ % is obtained. The cubic lattice constant roughly obeys Vegard's law, whereas the linewidth of the SnCl₆^2?F_2g internal vibration displays a nonlinear dependence on composition.     

Magnetic study 2D-antiferromagnets Ba2NiF6 and Ba2FeF6 by neutron diffraction and mossbauer spectroscopy

The 2D-antiferromagnetism of tetragonal Ba₂NiF₆ and Ba₂FeF6 — whose structure derives from that of K₂NiF₄ — has been studied by magnetization measurements and by powder neutron diffraction. The magnetic cell is 2a, 2$\text{a}$,$\text{c}$ and the magnetic moments (μ_Ni²⁺ = 1.9μB and μ_Fe²⁺ = 3.46μB) lie along c. This has been confirmed by Mossbauer spectroscopy for Ba₂FeF₆.     

Intralamellar structural modifications related to the proton exchanging in K4Nb6O17 layered phase

Basic structural aspects about the layered hexaniobate of K₄Nb₆O₁₇ composition and its proton-exchanged form were investigated mainly by spectroscopic techniques. Raman spectra of hydrous K₄Nb₆O₁₇ and H₂K₂Nb₆O₁₇·H₂O show significant modifications in the 950-800 cm⁻¹ region (Nb-O stretching mode of highly distorted NbO₆ octahedra). The band at 900 cm⁻¹ shifts to 940 cm⁻¹ after the replacement of K⁺ ion by proton. Raman spectra of the original materials and the related deuterated samples are similar suggesting that no isotopic effect occurs. Major modifications were observed when H₂K₂Nb₆O₁₇ was dehydrated: the relative intensity of the band at 940 cm⁻¹ decreases and new bands seems to be present at about 860-890 cm⁻¹. The H⁺ ions should be shielded by the hydration sphere what preclude the interaction with the layers. Removing the water molecules, H⁺ ions can establish a strong interaction with oxygen atoms, decreasing the bond order of Nb-O linkage. X-ray absorption near edge structure studies performed at Nb K-edge indicate that the niobium coordination number and oxidation state remain identical after the replacement of potassium by proton. From the refinement of the fine structure, it appears that the Nb-Nb coordination shell is divided into two main contributions of about 0.33 and 0.39 nm, and interestingly the population, i.e., the number of backscattering atoms is inversed between the two hexaniobate materials.     

Time behaviour of diffuse band emission in Na2 and K2

The C states of sodium and potassium dimers were excited with short laser pulses (about 500 ps) and time evolution of the diffuse bands in Na₂ (413–456 nm) and K₂ (550–583 nm) was observed in fluorescence. The measured decay times were interpreted as lifetimes of unidentified triplet states giving origin to the diffuse bands. Our results contradict the values reported before.