The effect of high pressure on the electric field gradients at two inequivalent lattice sites of tantalum in ω-zirconium

The pressure variation of the electric field gradients at substitutional tantalum atoms on the two lattice sites (A and B) of ω-zirconium have been determined to be (? lnq ^A/?P)_T = ?1.05(26)10^?3 kbar^?1 and (? lnq ^B/?P)_T = +6.03(40)10^?3 kbar^?1. In addition the thermal expansion coefficients of the unit cell of ω-Zr were found to be α_‖ = +3.5(3)10^?6 K^?6 and α_⊥ = +7.9(4)10^?6 K^?1. A combined analysis of the pressure dependence and the previously determined temperature dependence of the electric field gradients supports the assumption of two different bonding types at the A and B sites.     

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 pressure dependence of the electric field gradient at tantalum in group IIIb metals: Sc,Y, and Lu

The electric field gradientq acting on¹⁸¹Ta nuclei embedded into Sc, Y, and Lu was found to increase with hydrostatic pressure with relative slopes of (dlnq/dP) _T=300K=+8.0(3)10^?3 kbar^?1, +9.2(6)10^?3 kbar^?1, +10.2(1.3)10^?3 kbar^?1 for Sc, Y, and Lu, respectively.     

Pressure dependence of superconductivity in the pseudo-one-dimensional compound Tl2Mo6Se6

Measurements of the temperature and pressure dependences of the resistivity of the pseudo-one-dimensional ternary compound Tl₂Mo₆Se₆ are presented. We find that the conductivity parallel to the highly conducting c-axis is enhanced by pressure and the superconducting transition temperature T_c is suppressed by pressure at a rate $\text{?T}{}_{\mathrm{c}}\text{?P}\text{=?7.6×10}{}^{\mathrm{?5}}$ kbar^?1. These results are discussed in relation to the current models of transport in one-dimensional conductors.     

Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

The results of the Mössbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > T _N demonstrate that the main part (～90%) of iron atoms are in the low-spin state Fe²⁺. The other atoms can be attributed to the impurity NaFe₂As₂ phase or to the extended defects in NaFeAs. The structural phase transition (at T _S ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < T _N, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (H _Fe) at ⁵⁷Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(H _Fe) allows us to determine the temperature of the magnetic phase transition (T _N = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the H _Fe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (V _ZZ ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at ⁵⁷Fe nuclei.     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.     

Diffusion of titanium in slightly reduced rutile

The self-diffusion of ⁴⁴Ti in slightly reduced rutile. TiO_2?δ, was measured along the c axis over the temperature range of 1000–1100°C between 0.2 and 1 × 10^?18atm. oxygen pressure. These measurements enabled the determination of the defect structure of TiO_2-δ for 0.02 ?gd ? 0.001. For oxygen pressures between 1 × 10^?13 and 1 × 10^?16atm. at 1058.4°C random tetravalent titanium atoms are the predominant defects evident from self-diffusion. The enthalpy of motion was determined as ΔH_m = 57.03 ± 4.9% kcal/mole. From the activation energy at 1.69 × 10^?16atm., the enthalpy of formation for tetravalent titanium interstitials was determined as ΔH_f = 276 ± 15.6% kcal/mole.For oxygen pressures less than 1 × 10^?16atm. at 1058.4°C, the tracer diffusion coefficient shows a continuous decline as the oxygen pressure is lowered. Comparisons with thermogravimetric studies and consideration of the similarity in structure between nonstoichiometric point defect phases and the first homologous series phase indicate that the order-disorder transition retains a considerable degree of short range order below the critical concentration in the form of Wadsley defects.     

Pressure dependence of the superconducting transition temperature of a (Mo0.6Ru0.4)86B14 metallic glass

We have studied the pressure dependence of the superconducting transition temperature of amorphous (Mo_0.6Ru_0.4)₈₆B₁₄ for hydrostatic pressures up to P ～ 9 kbar. The transition temperature T_c decreases with pressure at a rate dT_c/dP=-(9±1) mK kbar^-1. We estimate the Grüneisen parameter and the volume dependence of the electron-phonon coupling constant.     

Charge transport and pressure dependence of Tc of single crystal,ferromagnetic EuB6

We present Hall Effect and resistivity data which demonstrate that EuB₆ is a degenerate semiconductor transforming into a metal or semimetal below the ferromagnetic ordering temperature, T_c = 13.7K. We also report an anomalously large, positive pressure dependence of T_c, (1/T_c)(ΔT_c/ΔP) ? 4 × 10^?2 kbar^?1.     

The temperature and pressure dependence of the electric field gradient at tantalum in rhenium

The quadrupole interaction (QI) at substitutional Ta atoms in a Re matrix has been investigated by TDPAC between 1.2 K and 450 K. It was found to depend only weakly on temperature with a minium around 200 K. No change in the QI was observed when the sample was cooled into the superconducting state. The QI was found to increase with pressure with the coefficient (? lnq/?P)_T=+0.93(23)10^?3 kbar^?1 at 300 K. A discussion of the temperature and pressure variations in connection with published Mössbauer data reveals the role of the conduction electron contribution to the electric field gradient.     

NMR study of prssure-induced magnetic transition in EuSe

The zero-field ¹⁵³Eu NMR in EuSe has been observed as a function of the pressure up to 14 kbar in the temperature range between 1.7 and 4.2 K. A sequent magnetic phase transition of NSNS-NNS-NNSS-NNN occurs with pressure at 1.7 K. In the NNSS state (2.0 ? P ? 4.8 kbar) the NMR frequency v is insensitive to the pressure. In the ferromagnetic state (P ? 4.8 kbar) v depends remarkably on the pressure, and the exchange interaction between the nearest Eu neighbours J₁ is shown to increase rapidly with decreasing the Eu-Eu distance. The pressure dependences of the Eu hyperfine field suggest that the magnetic moment of a Eu²⁺ ion in the ferromagnetic state under pressure is smaller than the ideal saturation value at 0 K.     

Superconductivity in α- and ω-Zirconium under high pressure

The pressure dependence of the superconducting transition temperatureT _c (p) of α-Zr has been investigated in both solid and liquid pressure transmitting media. Up to about 45 kbardT _c /dp was measured to be + 3.5 × 10^?6 K/bar. Cold working at 4.2 K produced a strong irreversible effect onT _c . The superconductivity of the high pressure phase, ω-Zr, has been studied in its region of stability, i.e. above 60 kbar. For ω-Zr,dT _c /dp=+7.7 × 10^?6K/bar, andT _c (0)=0.72 K (by extrapolation).     

Magnetic hyperfine interactions of 119Sn probe atoms in the binary perovskite CaCu3Mn4O12

The manganite CaCu₃Mn₄O₁₂ doped by ¹¹⁹Sn atoms (about 1 at % with respect to Mn atoms) is studied by Mössbauer spectroscopy. The introduction of diamagnetic tin atoms is found not to affect the structure of the manganite. Tetravalent tin atoms are shown to substitute for the isovalent manganese atoms that are located in an octahedral oxygen surrounding. The cluster method of molecular orbitals is used to calculate the contributions of Mn⁴⁺ and Cu²⁺ cations that belong to different structural sublattices to the hyperfine magnetic field at ¹¹⁹Sn nuclei (H _Sn = 105 kOe at T = 77 K). These partial contributions are analyzed, and the intrasublattice Mn⁴⁺-O-Mn⁴⁺ exchange interactions are found to play a significant role in the formation of the magnetic structure of the manganite.     

Pressure dependence of the absorption edge for Cd1-xMnxTe

The effect of pressure on the optical absorption edge of mixed crystals Cd_1-xMn_xTe with different manganese concentrations is reported. The observed absorption edge shifts to higher energy with increasing pressure at a rate of α=7?8×10^?3 eV/kbar and a second order coefficient of β=-4×10^?5 eV/kbar² for x<0.5, to lower energy with increasing pressure at a rate of α=-5.0 ×10^?3 eV/kbar for x?0.5. A phase transition occurs for all the samples studied. The absorption edge of the new phase is outside the wavenumber range of the instrument. The physical origins of different pressure coefficients are discussed in the light of the deformation potentials of energy band states and the hybridization of the Mn²⁺ 3d levels with the p-like states in the valence band.     

Investigation of the effect of resonant environment (Mössbauer screen) on the observed half-life of the 119m1Sn isomer (E = 23.8 keV, T 1/2 ∼ 18 ns)

In order to analyze the effect of resonant environment on the γ decay of excited nuclei, the half-life of the ^119m1Sn isomer (E = 23.8 keV, T _1/2 ～ 18 ns), which appears from the decay of the ^119m2Sn state (E = 89 keV, T _1/2 ～ 293 d), has been measured by the e-γ coincidence method on samples consisting of the mixture of tin oxides containing the ¹¹⁹Sn stable nucleus and ^119m2Sn radioactive nuclei with various values of the ¹¹⁹SnO₂/^119m2SnO₂ ratio. For the samples, where this ratio is equal to 6.2 × 10⁴, 1.2 × 10⁶, 6.5 × 10⁶, and 1.7 × 10⁷, the half-life T _1/2is equal to 18.69(2), 18.71(2), 18.91(5), and 19.43(4) ns, respectively. For the reference sample (a metal with a ¹¹⁹Sn/^119m2Sn ratio of 1.5 × 10⁵), the value T _1/2 = 18.68(6) ns is obtained.     

Quantitative analysis of blistering upon annealing of hydrogen-ion-implanted diamond single crystals

Local blistering observed upon high-temperature annealing of natural diamond single crystals implanted by 350-keV hydrogen ions with a dose of 12 × 10¹⁶ cm^?2 is studied. Based on room-temperature measurements, Griffith cracking criterion, and gas law, model quantitative calculations of blister size (R _j = (0.361?5.568) × 10^?3 m, V _j = (307?9695) × 10^?18 m³) and the amount of molecules in a blister (n _j = (0.448?10.95) × 10¹³) are carried out for the first time. At room temperature, T ₁ = 293 K, the amount of local elastic stresses σ_ij in the upper layer of the diamond is counterbalanced by (inner) hydrogen pressure P _ij of the (σ _j1 = P _j1 = (2.968?6.439) × 10⁷ Pa). At annealing temperature T ₂ = 1693 K, the hydrogen pressure rises to P _j2 = (0.1717?0.8750) GPa. Under subsequent annealing at a still higher temperature, T ₃ = 1743 K, the pressure in the blisters might be expected to grow to P _j3 = (0.1747?0.9010) GPa; however, some of blisters collapse and thin diamond slices flake away.     

The infrared spectrum of OH-compensated defect sites in C-doped MgO and CaO single crystals

The IR spectra of OH-compensated point defects in MgO (and CaO) single crystals of various purity grades were reinvestigated. Three distinct groups of IR bands appear in the O-H stretching region: A, B and C around 3550 cm^?1 (3650 cm^?1), 3300 cm^?1 (3450 cm^?1) and 3700cm^?1 (3750cm^?1). They are assigned as follows: band A to the fully compensated, band B to the half compensated and band C to the overcompensated cation vacancies, [O?V”_catH?]^×, [O?V”_cat], and [O?O?V”_catH?$\text{]}\text{?}$, respectively.Upon cooling to 80 K the band A shows a complex behavior partly due to the formation of Ha molecules by charge transfer and concommittant O^? formation: [? (H₂)”_cat?]^×. The O^? represent defect electrons or positive holes in the O^2? matrix.Bands A and B show a characteristic multiplet splitting which is caused by local lattice strains coming from carbon atoms on near-by interstitial position. The intensity ratios between the multiplet components remain constant regardless of temperature pretreatments up to 1470 K, but strong variations of the integral intensities are observed. These are caused by the highly mobile C atoms entering and leaving reversibly the cation vacancy sites as a function of temperature and of the quenching speed. When the C atoms push the H₂ molecules onto interstitial sites, an H-H stretching signal appears around 4150cm^?1.     

Hyperfine interactions of tin atoms in samarium

The hyperfine interactions of ¹¹⁹Sn impurity atoms in samarium at temperatures from 5 to 70 K are investigated by Mössbauer spectroscopy. The distributions P of magnetic hyperfine fields B _hf for tin atoms at sites of the hexagonal [P _h(B _hf)] and cubic [P _c(B _hf)] samarium sublattices are determined from the experimental absorption spectra. Ion ordering in pairs of magnetic centers located in layers of the cubic sublattice is observed by Mössbauer spectroscopy for the first time. Each magnetic center involves ordered ions at the nearest neighbor sites of the tin atom replacing the samarium ion at the hexagonal lattice site. The quadrupole coupling constant e ² q _hQ=0.59±0.12 mm/s is determined for tin atoms at the hexagonal sublattice sites of samarium. The quadrupole interaction of tin atoms in heavy rare-earth metals (from Tb to Er) with a hexagonal close-packed structure is discussed.     

Energy band structure of TiO2 and it's dependence on uniaxial stress

The electronic band structure of titanium dioxyde is calculated. The evolution of the levels and of the optical transitions under an uniaxial stress along the c axis has also been calculated.We confirm the existence, of a first order forbidden direct optical gap, and of a maximum along the Δ direction; the theoretical pressure coefficients ?E/?P for the Γ⁺₇ → Γ⁺₆ and Δ₅ (max) → Γ⁺₆ transitions are respectively 0.600 and 0.950 meV kbar^-1 while the corresponding experimental values are 0.625 and 0.985 meV kbar^-1.     

Volume properties and compressibility of the graphite-potassium-mercury compounds

Abstract

The volume properties of graphite intercalation compounds (GIGS) C4KHg and of the initial intermetallic compounds KHg and KHg₂ have been investigated in the piston-cylinder apparatus, using the direct volumetric technique, under pressures up to 25 kbar. The compounds, average compressibility K₊, was determined to be 3.8×10^?3 kbar^?1 for C₄KHg, 3.0×10^?3 kbar-^?1 for C₈KHg, 4.8×10^?1 kbar^?1 for KHg, and 4.0 ×10^?1 kbar^?1 for KHg₂ at pressures of 0-20 kbar. The compressibility of the “two-dimensional” KHg layer in the GIC under various pressure conditions has been estimated. These estimates permit comparison of KHg properties in the “three dimensional” and “quasi-two dimensional” states. It was concluded that the influence of the graphite matrix on the intercalant is insignificant for this type ternary GIC.