Raman spectra of As4S4 polymorphs: Structural implications for amorphous As2S3 films

Raman-scattering spectra of α-As₄S₄ and β-As₄S₄ have been determined at 300 and 10 K. Although similar in overall aspect, the spectral signatures of the two polymorphs are clearly distinct. We have made a careful comparison of these first-order crystal line spectra to the sharp features reported in the Raman spectra of freshly-deposited films of amorphous As₂S₃. Prior proposals for the presence of As₄S₄ molecules in the unannealed films are supported by these comparisons, but recent contentions that actual microcrystals of β-As₄S₄ are present in the as-deposited material are clearly contradicted by the absence of any of the lattice-phonon lines which are prominent in the crystals at frequencies below 70 cm^-1.     

High pressure effect on the electrical properties of NiS2

A semiconductor-metal transition in the electrical resistance of NiS₂, which has been suggested to be a Mott transition, is observed with decreasing temperature under pressure up to 44 kbar. The transition temperature increases with pressure with a slope of $\text{dT}\text{dP}\text{= 6 ± 1 K/kbar}$. The activation energy in a semiconducting region is found to decrease with increasing pressure and to vanish at about 46 kbar. The critical pressure and temperature are predicted to be 46 ± 2 kbar and 350 ± 20 K.     

Pressure induced phase transition in the highly anisotropic compound: Y2[Pt(CN)4]3·21H2O

For the linear chain system Y₂[Pt(CN)₄]₃·21H₂O a pressure induced phase transition is observed by emission spectroscopy. At p_trans=(5±0.5) kbar and T=295 K the compound undergoes a first order phase transition, in the course of which the intra-chain Pt-Pt distance R shrinks by $\text{ΔR≈}\text{-0.03}\text{A}\text{?}$. An approximate value had already been found at standard pressure for a temperature induced phase transition (T_trans=218 K).     

Pressure dependence of the superconducting transition temperature of Th4H15

Measurements of the superconducting transition temperature of Th₄H₁₅ under hydrostatic pressure up to 28 kbar are reported. The initial linear increase is found to be $\text{+42}\text{mK}\text{kbar}$.     

Thermoelectric power and phase transition of polycrystalline As2Te3 under pressure

The pressure dependence of the thermoelectric power of monoclinic As₂Te₃ is measured up to 10 GPa using a Mao-Bell diamond anvil cell. The thermoelectric power never reaches an absolute value greater than the ambient pressure value of 242 μV/K. Evidence of a phase transition is present between 6 and 8 GPa where the thermoelectric power reaches an absolute value of 225 μV/K after passing through a minimum of S≈75 μV/K. X-ray diffraction experiments confirm that the resulting structure is β-As₂Te₃, which is isostructural with Bi₂Te₃ and Sb₂Te₃.     

Electron spin resonance investigation of the structural phase transitions in Alurea6(ClO4)3 and Ga urea6(ClO4)3

Second order structural phase transitions in Alur₆(ClO₄)₃ and Gaur₆(ClO₄)₃ with T_c ～ 300 K are studied by means of ESR on single crystals doped with the analogous Cr(III) compound. The transitions are antiferrodistortive and of the displacive type, the displacements resulting from the condensation of a X₂ mode $\text{(k = (0}\text{1}\text{2}\text{1}\text{2}\text{))}$ of the ClO₄ ions. The ESR parameters have the same temperature dependence as the order parameters and can be described by D and E～φ～. The space group describing the structure changes from S₆² to S₂¹, and the number of domains is multiplied by three. Above 300 K the crystals already consist of two domains, resulting from a ferrodistortive phase transition D_3d⁶ → S₆². The actual transition temperature of the latter phase transition lies at some temperature above the decomposition temperature of the crystals.     

Hydrostatic pressure study of the structural phase transitions and superconductivity in single crystals of (Ba1−xKx)Fe2As2 (x=0 and 0.45) and CaFe2As2

We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe₂As₂, BaFe₂As₂, and (Ba_0.55K_0.45)Fe₂As₂, by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe₂As₂ and BaFe₂As₂ (lightly doped with Sn) display structural phase transitions near 170 and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba_0.55K_0.45)Fe₂As₂ is superconducting for T<30 K. The effect of pressure on BaFe₂As₂ is to shift the onset of the crystallographic transformation down in temperature at the rate of ~−1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba_0.55K_0.45)Fe₂As₂ is to shift the onset of superconductivity to lower temperatures at the rate of ~−0.21 K/kbar. The effect of pressure on CaFe₂As₂ is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P<5 kbar. However, higher pressures bring about another phase transformation characterized by reduced-resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (H_c2) data in (Ba_0.55K_0.45)Fe₂As₂ and CaFe₂As₂ are discussed.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Hole mobility in evaporated layers of As2S3.CdI2

The hole drift mobility in As₂S₃.CdI₂ has been determined using Spear's method. The room-temperature value is 10^?4$\text{cm}{}^2\text{Vs}$, the activation energy 0.2 eV. The hole yield appears to be controlled by geminate recombination.     

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.     

Vibronic properties of the F+ centre in strontium oxide

The shape and moments of the F⁺ absorption band in SrO have been measured between 4.2K and 300K. Analysis of the moments gives the effective frequency of the vibrational modes interacting with the centre as $\text{h}\text{?}\text{ω = 236 ± 20 cm}{}^{-1}$, and the Huang-Rhys factors for cubic and non-cubic modes as S_C = 3 ± 1 and S_NC = 4 ± 1 respectively.     

Neutron scattering study of the phase transition in s-triazine at high pressure

The elastic transition in s-triazine (C₃N₃H₃) from a trigonal ($\text{R}\text{3}\text{c}$) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.     

Synthesis and physical properties of new superconducting Chevrel phases HgxMo6S8

New ternary molybdenum chalcogenides Hg_xMo₆S₈ (0 < x < 1) have been synthesized and investigated for their structural, magnetic, and superconducting behavior. These new phases prepared at low temperature by reaction of mercury with Mo₆S₈ (obtained from oxidation of Cu₂Mo₆S₈ by iodine) have been characterized by x-ray diffraction, static Faraday susceptibility, and superconducting transition temperature studies. HgMo₆S₈ crystallizes in the rhombohedral space group $\text{R}\text{3 (}\text{a}{}_{\mathrm{r}}\text{= 6.51}\text{A}\text{?}\text{, α}{}_{\mathrm{<mtext>r</mtext>}}\text{= 92.53°)}$, has a paramagnetic, temperature dependent susceptibility, and superconducts at 8.1K. As the mercury content decreases, a continuous decrease in T_c from 8.1K to 1.7K for x=0 (Mo₆S₈) is observed.     

Commensurability and fluctuating conductivity in the organic conductor TSF-TCNQ

We report the pressure dependence of the phase transition in TSF-TCNQ as determined by resistivity measurements. We find a narrow pressure domain centered on 6.25 kbar where the transition temperature peaks above a monotonous pressure dependence. We suggest that there is a commensurate × 3 superlattice in this pressure regime resulting from an increase in charge transfer from 0.63 under ambient pressure to $\text{2}\text{3}$.A drop of longitudinal conductivity related to the × 3 commensurability is visible in the same temperature region where X-ray diffuse scattering data show 1-D features for the precursor fluctuations. We suggest that like TTF-TCNQ, the metallic conductivity of TSF-TCNQ is also influenced by the fluctuating Fröhlich mode.     

The antiferroelectric phase transition in TlH2PO4 and TlD2PO4

It is shown by X-ray diffraction measurements that the transitions at 230 K in TlH₂PO₄ and at ≈ 350 K in TlD₂PO₄ are structurally equivalent. In both cases, on cooling through the transition, new reflections appear at points ($\text{h +}\text{1}\text{2}$, $\text{k +}\text{1}\text{2}$, l) referred to the primitive monoclinic unit cell of the high-temperature phase. In TlH₂PO₄ some distinct but broadened scattering remains at the ($\text{h +}\text{1}\text{2}$, $\text{k +}\text{1}\text{2}$, l) points up to 10 K, or more, above the transition temperature.     

Preparation and semiconducting properties of Th3As4

The resistivity, thermoelectric power and Hall constant in the temperature range of 78–830 K were determined for polycrystalline Th₃As₄ samples obtained by annealing thin thorium slabs in arsenic vapour. The samples examined were n-type semiconductors with a carrier concentration ranging from 1.0 × 10¹⁸cm^?3 to 2.8 × 10¹⁸ cm^?3 for which the effective mass was found to be equal to 0.55–0.76m₀. The Hall mobility, about 450cm²V^?1s^?1 at room temperature, obeys a $\text{T}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$ law at high temperatures. On the basis of the electrical measurements the forbidden gap of Th₃As₄ was found to be equal to 0.43 eV.     

Electron diffraction and infrared study of the semimetallic layered compound Ti1−xVxSe2

The crystals of the system Ti_1?xV_xSe₂ (C ≤ x ≤ 0.05) undergo a second order structural phase transition. Electron diffraction studies show that the transition temperature decreases with progressive V-doping. The phase transition considerably affects the infrared reflectivity, measured at 300 and 77 K in the spectral range 40 cm^?1 to 4000 cm^?1.The presence of free carriers and the existence of optical infrared active E_u phonon modes $\text{(}\text{E}\text{⊥}\text{c}\text{)}$ confer their characteristic appearance to the spectra. At room temperature one phonon structure is measured at 143 cm^?1. At 77 K a new series of phonon peaks appears up to a V-concentration of 5 % as a direct consequence of superlattice formation.At room temperature the plasma-edge shifts towards higher frequencies as the vanadium concentration increases. This effect is caused by a large increase of $\text{N}\text{m}{}^1$, associated with the mixing of impurities. It is indicative of the small density of states at the Fermi level in semimetallic TiSe₂. Our results suggest a phase transition driven by lattice dynamical effects.     

Photoluminescence decay affected by variable range hopping in band tail in amorphous As2S3

Photoluminescence decay was observed with various excitation energies and at various temperatures in amorphous As₂S₃. The decay at high temperature or with the high excitation energy had the $\text{t}{}^{\mathrm{<mtext>-3</mtext><mtext>2</mtext>}}$ long time behavior. This decay is concluded to arise from the combination of the tunneling recombination and the variable range hopping of carriers in the band tail.     

Birefringence,X-ray and neutron diffraction measurements on the structural phase transitions of (CH3NH3)2 MnCl4 and (CH3NH3)2FeCl4

We used optical birefringence, X-ray and neutron diffraction methods with single crystals to study the structural phase transitions of the perowskite-type layer structures of (CH₃NH₃)₂MeCl₄ with Me=Mn, Fe. The Mn-compound shows the following structural transitions at 394 K — a continuous order-disorder phase transition from tetragonal symmetry $\text{I4}\text{mmm}$ to orthorhombic space group Abma (Cmca in reference 10); at 257 K — a discontinuous transition to a second tetragonal modification; at 95 K — a discontinuous transition to a monoclinic phase. For the Fe-compound the corresponding transition temperatures are 328 K and 231 K, respectively. A low temperature monoclinic phase could not be observed. The lattice parameters of the different modifications were determined as a function of temperature. The temperature dependent course of the order parameter has been investigated for the order—disorder transition. For both compounds, all the methods used gave the same value for the critical exponent of β = 0.315.     

Elastic moduli and mode gammas of GaP: Their relationship to those of other isomorphic crystals and the high pressure structural-electrical transition

The transit times of ultrasonic waves associated with piezoelectrically inactive modes have been measured in sulfur-dope, n-type GaP using hydrostatic pressures up to 5 kbar. The velocities and elastic constants of most modes increase linearly with pressure but $\text{C}{}_{\mathrm{S}}\text{=}\text{1}\text{2}\text{(}\text{C}{}_{11}\text{?}\text{C}{}_{12}\text{)}$ decreases slightly. The behavior of the elastic constants and their associated mode gammas are compared to those of other III–V compounds and of Si and Ge. We suggest that the behavior of C_S is related to the occurrence of the structural-electrical transition which occurs at high pressure. A modified Born criterion for lattice stability developed by Demarest $\text{et}$$\text{al}$. is used to explain this relationship. The contribution of thermal dilatation to the temperature dependence of each elastic constant is deduced. Using it and data from the literature we find that the explicit thermal contribution to each elastic constant is significant and in the case of C₁₂ has an anomalous sign.