High pressure optical study of KTbP2Se6

We have observed a pressure induced phase transition in KTbP₂Se₆ at about 9.2 GPa which was accompanied by a discontinuous jump of the absorption edge of about 0.5 eV into the red. We have proposed that the high pressure phase is due to charge transfer from the selenium to the phosphorous accompanied by the formation of a Se-Se bond. In order to exclude a model which involves a 4f-5d charge transfer in Tb, leading to the intervalence charge transfer from Tb³⁺P⁴⁺-Tb⁴⁺P³⁺,we have measured the pressure dependence of the luminescence of the crystal field split levels of the Tb-ion. A comparison of the pressure dependence of the luminescence spectra of KTb(MoO₄)₂ and KTbP₂Se₆ shows that such a charge transfer does not occur in KTbP₂Se₆.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

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.     

Reentrant high-conduction state in CuIr2S4 under pressure

The results of electrical resistance and angle dispersive X-ray diffraction measurements at high pressures and ambient temperature on the chalcogenide spinel, CuIr₂S₄ are reported. The resistance increases gradually and reaches around 12 GPa a value that is approximately forty times the initial value. Above 15 GPa, the resistance decreases up to 30 GPa and on further pressure increase tends to saturate at a value slightly above the ambient pressure value. Thus, the material exhibits a reentrant high conducting phase under pressure. The behaviour of the electrical resistance exhibits a close correlation with the structural evolution with pressure.     

Energetics and structural stability of Cs3C60

Using the ab initio pseudopotential total-energy method and the density-functional theory, we study the energetics of face-centered-cubic Cs₃C₆₀ which is a material of great interest as a possible high transition-temperature superconductor. At the optimized lattice constant the volume per C₆₀ is found to be smaller than the the most-stable hexagon-coordination A15 phase, while the total energy of the fcc phase is about 0.9 eV higher than the A15 phase. These results indicate that a low-temperature and high-pressure synthesis method might be a possible way to produce the fcc Cs₃C₆₀ phase. In addition, it is also found that the A15 Cs₃C₆₀ should show a phase transformation from a hexagon-coordination phase to a pentagon-coordination phase under hydrostatic pressure.     

Pressure-induced phase transition in defect Chalcopyrites HgAl2Se4 and CdAl2S4

Results of angle dispersive X-ray diffraction (ADXRD) measurements on the defect chalcopyrites (DCP), HgAl₂Se₄ and CdAl₂S₄ up to 22.2 and 34 GPa, respectively, are reported. The ambient tetragonal phase is retained in HgAl₂Se₄ and CdAl₂S₄ up to 13 and 9 GPa respectively. The values of the bulk modulus estimated from the Equation of State is 66(1.5) and 44.6(1) GPa for HgAl₂Se₄ and CdAl₂S₄ in the chalcopyrite phase. At higher pressure a disordered rock-salt structure and on pressure release a disordered zinc blende structure with broad X-ray diffraction lines are observed as is the case for several defect chalcopyrites.     

Pressure cycle of superconducting Cs0.8Fe2Se2 : A transport study

We report measurements of the temperature and pressure dependence of the electrical resistivity (ρ) of single-crystalline iron-based chalcogenide Cs_0.8Fe₂Se₂. In this material, superconductivity with a transition temperature develops from a normal state with extremely large resistivity. At ambient pressure, a large “hump” in the resistivity is observed around 200 K. Under pressure, the resistivity decreases by two orders of magnitude, concomitant with a sudden T_c suppression around . Even at 9 GPa a metallic resistivity state is not recovered, and the ρ(T) “hump” is still detected. A comparison of the data measured upon increasing and decreasing the external pressure leads us to suggest that the superconductivity is not related to this hump.     

Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb₂C₆₀ and Cs₂RbC₆₀ superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb₂C₆₀ the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs₂RbC₆₀ the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

The interaction of Cs and O2 on the basal plane of MoS2

The interaction of Cs and O₂ on MoS₂(0001) has been studied both in the alternate adsorption and the codeposition mode by LEED, AES, TDS and WF measurements at 170 and 300 K. Oxygen does not interact with Cs when θ_Cs?0.04 at 300 K or θ_Cs?0.08 at 170 K, where Cs is known to adsorb as strongly ionized, individual adatoms. The interaction at higher θ_Cs, where Cs is known to form clusters on MoS₂(0001), leads to clusters of a Cs/O complex characterized by a Cs(563 eV)/O(512 eV) Auger peak ratio of 1.1–1.3. The minimum WF is 2.1 eV at 300 and 170 K upon alternate adsorption, and 1.7 eV at both T upon codeposition. Upon heating, oxygen and Cs desorb independently, as no oxide desorption is observed. The Cs TDS spectrum is shifted to lower T in the presence of oxygen and a new desorption peak appears at ～ 880 K. The differences in the Cs/O interaction between MoS₂(0001) and other semiconductors and metals are attributed to the Cs clustering and the inertness of MoS₂(0001) to O₂ adsorption.     

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.     

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.     

Photoelectrical memory effect in ZnIn2Se4

The compound ZnIn₂Se₄ with n-type conductivity is shown to exhibit electro-optical memory effect and negative resistance effect similar to those previously reported for ZnIn₂S₄. At low temperature ZnIn₂Se₄ material will present a high conductivity during and after illumination by light of energy greater than the band gap. This high conductivity state can be quenched in three different ways (i) by heating the sample until it reaches room temperature, (ii) by illumination with monochromatic light of appropriate energy, (iii) by an electric field. In the latter case the material exhibits a negative resistance effect in the transition between the low and high conductivity conditions. These charge storage phenomena have been explained by assuming the presence of a level, localized in the forbidden gap, which is twofold negative charged and presents a repulsive barrier to the recombination of electrons.     

Etude des transitions de phases des composes Rb2KMO3F3, Cs2KMO3F3 et Cs2RbMO3F3 (M = Mo,W)

Crystallographic, micro-D.T.A., dielectric and pyroelectric measurements have been performed for the Rb₂KMO₃F₃, Cs₂KMO₃F₃ and Cs₂RbMO₃F₃ (M = Mo, W) compounds. Two types of transitions have been detected : the first one, for Rb₂KMoO₃F₃ at low temperature, is due to the disappearance of the superstructure detected for Rb₃MoO₃F₃α, the second one is of ferroelectric - paraelectric type (Rb₂KMO₃F₃ and Cs₂RbMO₃F₃ ; M = Mo, W). The transition temperatures have been compared vs the size of the alcaline ions and the nature of the M-O bonds. The increase of the transition temperatures from the A₂A'MO₃F₃ to the A₃MO₃F₃ compounds is likely due to the simultaneous presence of the A⁺ cations in the former ones in 6 and 12 coordination sites.     

Estimation of T1 for Co2+ ions from the temperature variation of the ESR linewidths for VO2+ in Cs2Co(SeO4)2·6H2O single crystals

The electron spin resonance of VO²⁺ is studied in single crystals of Cs₂M″ (SeO₄)₂·6H₂O (M″ = Zn, Co) from 290 to 77 K at ～ 9.45 GHz. The line broadening of VO²⁺ spectra on cooling the Cs₂Co(SeO₄)₂·6H₂O crystal is explained on the basis of host spin-lattice relaxation narrowing. T₁ for Co²⁺ is estimated to be ≈ 1.7 × 10^?12 sec. at 290 K.     

Soft mode and structural phase transition in the cubic elpasolite Cs2NaNdCl6

Inelastic neutron scattering measurements have been performed on the elpasolite Cs₂NaNdCl₆ in the cubic phase. It is found that the cubic-to-tetragonal phase transition at 137 K is driven by a zone-center soft phonon mode. The mode is of symmetry Λ^′₁₅ and corresponds to a rotation of the Cl₆ octahedra. The characteristics of the phase transition are similar to those observed in some compounds of the perovskite and antifluorite structures.     

Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2MP2O7 (M=Ca, Sr)

The excitation and emission spectra of octahedrally coordinated europium ion (Eu²⁺) ions in Cs₂M²⁺P₂O₇ (M²⁺=Ca, Sr) are reported and discussed. The remarkable features of the Eu²⁺ luminescence in these phosphate materials include (a) very large Stokes shift of emission (∼1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu²⁺ luminescence in phosphate-based materials. The broad emission bands of Eu²⁺ in Cs₂CaP₂O₇ and Cs₂SrP₂O₇ peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is ∼1.2 μs. The nature of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f⁶5d¹→4f⁷[⁸S_7/2] emission.     

Low temperature photoconductivity of ZnIn2Se4 and CdIn2Se4

The photoconductivity of ZnIn₂Se₄ and CdIn₂Se₄ single crystals has been studied at 4.2 K. The spectra are compared to reflectivity spectra and relevant structures are interpreted in terms of interband transitions. The absence of excitonic transitions is discussed in connection with the crystalline perfection.     

Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS₂ has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS₂. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS₂ surface depend strongly on substrate temperature. Cesium is deposited on MoS₂ in two adsorption states. Although MoS₂ is extremely inert to O₂ adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS₂.     

Cation ordering and crystal structures in AGa2X4 compounds (CoGa2S4, CdGa2S4, CdGa2Se4, HgGa2Se4, HgGa2Te4)

Single crystals of some AGa₂X₄ compounds (CoGa₂S₄, CdGa₂S₄, CdGa₂Se₄, HgGa₂Se₄, HgGa₂Te₄) were prepared by chemical vapour deposition and flux method.The X-ray structural investigations indicated blende or defect chalcopyrite structures.A simple relationship is suggested between the c/a ratio and the cationic sublattice ordering.