Low temperature phase transition in cubic elpasolithe crystal Cs2NaBiCl6

The structural properties of the cubic compound Cs₂NaBiCl₆ have been investigated. X-ray diffraction patterns were observed from 300 to 7 K. On lowering the temperature, this compound undergoes a structural transformation starting at 90 K. This structural evolution was monitored by the thermal behavior of the 400 and 440 reflections. The crystallographic data was completed by Raman scattering measurements on an oriented sample. The phase transformation involves only the external mode corresponding to the Cs⁺ translation. The lattice instability of Cs₂NaBiCl₆ appears to be related to the size of the cesium ion site. Comparison with results obtained for isomorphous compounds strongly supports our interpretations.     

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.     

Scintillation and anomalous emission in elpasolite Cs2LiLuCl6:Ce

The luminescence and scintillation properties of Cs₂LiLuCl₆:0.5%Ce³⁺ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d_e state of Ce. Contrary to Cs₃LuCl₆ and Cs₂LiYCl₆, where the same type of fast emission was observed, the emission in Cs₂LiLuCl₆ is still observed at room temperature. Assuming that the 5d_e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce³⁺. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d_t excited state of Ce³⁺ explains most of the results. Besides the 275 nm emission, the material shows 5d_t-4f Ce³⁺ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16-22 eV photons. The scintillation properties of Cs₂LiLuCl₆:Ce are briefly discussed.     

Raman and magnetic susceptibility studies of hexagonal elpasolite Cs2NaAlF6:Cr

Polarized Raman scattering spectra of distorted elpasolites Cs₂NaAlF₆ with a Cr³⁺ content of 0.1, 0.5 and 3.0 at% have been studied at both room temperature and 16 K. A shoulder located near the very intense band assigned to the AlF₆³⁻ A_1g mode indicates that the guest ion causes only small perturbations to the host lattice. Magnetic susceptibility measurements performed on the 0.1, 0.5, 3, 10 and 50 at% samples show that for particular concentrations the Cr³⁺ ions are not isolated, but participate to inter and/or intra-cluster magnetic exchange.     

Ferroelastic phase transition in elpasolite Tl2KInF6

A new elpasolite Tl₂KInF₆ has been synthesized. This compound undergoes at 228 K a ferroelastic phase transition from a room-temperature Fm3m variety to a monoclinic variety. X-ray, optical, calorimetric and DTA studies under hydrostatic pressure have been performed and compared with other elpasolite-type fluorides.     

Successive jahn-teller phase transitions in Cs2PbCu(NO2)6

The sequential Jahn-Teller phase transitions in Cs₂PbCu(NO₂)₆ were studied by neutron and X-ray diffraction technique. In each phase we observed satellite reflections. The position of satellites in the reciprocal space is commonly given by $\text{k}{}_{\mathrm{<mtext>0</mtext>}}\text{= (}\text{1}\text{2}\text{1}\text{2}\text{ζ)}$ with $\text{0?ζ?0.5.}$ In Phase II and Phase IV, the values of ζ are definitely determined as ζ = 0 and ζ = 0.5 respectively. In Phase III, ζ is not definite within $\text{0.20?ζ?0.25}$ depending on samples, thermal cycling,etc. Phase III may be an incommensurate phase although there is the tendency that the value of ζ is locked in either at 0.20 ($\text{1}\text{5}$) or at 0.25 ($\text{1}\text{4}$).     

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.     

Structures and phase transition in Rb2NaHoF6

The phase transition at T_c=172(2) K and high as well as the low temperature structures at 205 K and 17 K were studied with a low temperature Guinier diffractometer and camera. A second order phase transition (Γ-condensation of the T_1g mode) lowers the symmetry from Fm3m at T>;T_c to I4/m at T<T_c. In the low temperature phase the octahedra are tilted (5.2 degrees at 17 K) and slightly stretched. The increase of the tilt angle and the linear increase of the tetragonal distortion suggests a mean field power law. This classical behaviour would agree with the assumption of a Jahn Teller splitting of the Ho³⁺ electronic state.     

Soft mode at an incommensurate structural phase transition

We show within the RPA approximation that a soft mode is associated with an incommensurate structural phase transition in a simple microscopic model of two level ions interacting with the phonons of a crystal.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

Optical properties of Mn-activated Na2SnF6 and Cs2SnF6 red phosphors

Alkaline hexafluorostantanate red phosphors Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺ are synthesized by chemical reaction in HF/NaMnO₄ (CsMnO₄)/H₂O₂/H₂O mixed solutions immersed with tin metal. X-ray diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space group D_4h¹⁴ (Na₂SnF₆:Mn⁴⁺) and a trigonal symmetry with the space group D_3d³ (Cs₂SnF₆:Mn⁴⁺). Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck-Condon analysis of the PLE data yields the Mn⁴⁺-related optical transitions to occur at ∼2.39 and ∼2.38 eV (⁴A_2g→⁴T_2g) and at ∼2.83 and ∼2.76 eV (⁴A_2g→⁴T_1g) for Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺, respectively. The crystal field parameters (Dq) of the Mn⁴⁺ ions in the Na₂SnF₆ and Cs₂SnF₆ hosts are determined to be ∼1930 and ∼1920 cm⁻¹, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose-Einstein occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of Mn⁴⁺-activated alkaline hexafluoride salts.     

Photoconductivity and the structural phase transition in SrTiO3

Detailed photoconductivity measurements have been performed in nominally pure SrTiO₃ in order to elucidate the effect of the antiferrodistorsive cubic-tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region.     

Helimagnetism in the cubic Laves phase YMn2

Neutron diffraction experiments on YMn₂ using a wave length of λ_N = 2.483 Å show a splitting of the magnetic peaks. The magnetic structure is helimagnetic consistent with an angle modulation of the previously reported antiferromagnetic structure. The NMR spectrum can be explained as arising from a perturbation of the helix by the magnetocrystalline anisotropy. Below T_N the observed frustration of the negative Mn interactions is inherent from the topology of the crystallographic structure. Above T_N, it creates short range ordering whose thermal decrease may explain the increase in the paramagnetic neutron scattering as the temperature is increased.     

High pressure electrical transport measurements of Cs2MoS4 and KTbP2Se6 and X-ray diffraction study of Cs2MoS4

We report the results of electrical resistance measurements at high pressures on Cs₂MoS₄ and KTbP₂Se₆. The results of high pressure X-ray diffraction study of Cs₂MoS₄ are also presented. Interestingly, in the case of Cs₂MoS₄ the resistance vs. pressure follows the behavior of the absorption edge vs. pressure obtained from our optical measurements lending further support to a direct-indirect band crossing. In the case of KTbP₂Se₆,the phase transition at about 9.2 GPa is reflected in a sharp drop of the resistance. In addition we report the pressure dependence of the lattice constants as well as the equation of state of Cs₂MoS₄.     

Zone-center soft mode behavior in the cubic phase of NaNbO3

The temperature dependence of the zone-center polar lattice vibrations, including a soft mode, in the cubic phase of NaNbO₃ as observed by infrared reflectivity spectroscopy, is reported. Data are analysed and discussed in the light of results previously obtained in other oxidic perovskites.     

Enhanced ferromagnetism in Mn-doped TiO2 films during the structural phase transition

The crystal structures and magnetic properties of Ti_0.9747Mn_0.0253O₂ films prepared by sol–gel dip coating have been investigated. Room temperature ferromagnetism was observed both in the films of pure anatase phase and of mixed anatase and rutile phases. For the first time, enhancement of the ferromagnetism was revealed as the phase transition from anatase to rutile occurs: from 0.7±0.01μ_B/Mn to 1.1±0.05μ_B/Mn. The possible mechanism for the observed magnetism enhancement is discussed.     

The magnetic structure of Cs0.4 Mn0.4 Fe1.6 F6 and of Cs0.4 Ni0.4 Fe1.5 F6

The magnetic structure of the two compounds as determined by neutron diffraction is reported. Both compounds are antiferromagnetic with Néel-temperatures of 115 and 100 K respectively. Both compounds show a very pronounced diffuse scattering.     

Optical spectroscopy of Yb in the Cs2NaYF6 single crystal

Results of the optical spectroscopy investigation of the cubic paramagnetic center Yb³⁺ ion in the Cs₂NaYF₆ single crystal are presented. The Stark level energies of the Yb³⁺ multiplets are established from absorption, luminescence and excitation luminescence spectra and the crystal field parameters are calculated. Information about the phonon spectra of Cs₂NaYF₆ crystals is obtained from the electron-vibrational structure of the optical absorption and luminescence spectra.     

Giant thermodynamical optical effect near ferroelastic phase transition point in Cs3Bi2I9 layered crystal

For the first time we have found a new giant thermodynamical optical effect near the ferroelastic phase transition point in Cs₃Bi₂I₉ layered crystal. The effect is appeared as periodical oscillations in time of the reflection coefficient. This phenomenon is caused by the small temperature deviations in thermodynamical system the appearance of which in the reflection spectra is strongly amplified in the ferroelastic phase transition point. The optical oscillations are explained on the base of a model that takes into account the temperature dependence of the refractive index through the order parameter (spontaneous strain) of the crystal.     

The phase transition in VO2

The electronic d-states in VO₂ are investigated by a semi-empirical MO LCAO method. The calculations of clusters for the crystal structures above and below the phase transition point show that the energy gap is mainly formed by the crystal distortions. The energies calculated agree well with the ESCA experimental estimations and explain the large gap values obtained from the optical data.