A study of valency changes of vanadium ions in Al2O3 by γ irradiation

The effect of γ irradiation at 300 K on the concentrations of vanadium ions V³⁺, V⁴⁺ and V²⁺ in Al₂O₃ has been studied quantitatively, using three techniques: optical absorption (V³⁺), low temperature thermal conductivity measurements (V⁴⁺) and EPR (V²⁺). Several single crystals of Al₂O₃ doped with vanadium in a large range of concentration $\text{(2.8 × 10}{}^{18}\text{? 1.3 × 10}{}^{20}\text{at.}\text{cm}{}^3\text{)}$ have been measured. The evolution of the respective concentrations by γ irradiation as a function of the total vanadium content C is quite different in the two regions $\text{C< 1.2 × 10}{}^{19}\text{at.}\text{cm}{}^3$ and C larger than this value. A consistent analysis of the results has nevertheless been achieved, leading to the determination of the absolute concentrations of the three ions in the as-received and γ irradiated states for all samples with $\text{C<4.2 × 10}{}^{19}\text{at.}\text{cm}{}^3$ (room temperature annealing is observed above this value). The concentrations of V⁴⁺ and V²⁺ ions are always small, but V⁴⁺ ions are more stable: they are present in the as-received state at a level of 1% of the total concentration and a maximum value of $\text{/}\text{?}\text{2.3 × 10}{}^{18}\text{at.}\text{cm}{}^3$ is observed in the γ irradiated state; on the other hand there are less than 4.7 × 10¹⁵V²⁺ ions per cm³ in the as-received state and the maximum value is only $\text{4.2 × 10}{}^{17}\text{at.}\text{cm}{}^3$. Charge transfer between V ions only is not sufficient to explain the experimental results and other defects must be involved in the γ irradiation effect.     

Spectroscopy of 41K by thermal neutron capture in 40K

The γ-ray spectrum emitted after thermal neutron capture in ⁴⁰K has been studied at the ILL high flux reactor with curved crystal Bragg, pair and Ge(Li) spectometers. 585 transitions were assigned to the reaction ⁴⁰K(n, γ)⁴¹K and 490 of them were placed into a ⁴¹K level scheme; 68 new states are proposed. On the basis of γ-ray branches to states with established spin and parity, many new spin-parity assignments were made. The level energies up to 4 MeV were measured with a precision of 8–50 eV relative to the 411.8 keV ¹⁹⁸Au standard, those above 4 MeV with a precision of 50–100 eV. The spin of the capture state was found to be $\text{I =}\text{7}\text{2}$; the neutron binding energy was determined to E_B = 10095.25(10) keV. The level density of $\text{I}{}^{\mathrm{\pi }}\text{=}\text{5}\text{2}{}^{\mathrm{\pm }}\text{,}\text{7}\text{2}{}^{\mathrm{\pm }}\text{,}\text{9}\text{2}{}^{\mathrm{\pm }}$ states was analyzed in terms of the constant-temperature Fermi gas model. It was shown that in this spin window the level scheme is almost complete up to an excitation energy of 5 MeV.     

Diffusion and ionic conductivity in sodium beta alumina

Measurements of sodium tracer diffusion (D_t) and ionic conductivity (σ) have been made on the same single crystals of sodium beta-alumina of composition 1.23 Na₂0.11 Al₂O₃. The σ- measurements were made over the temperature 390–660 K using reversible (liquid sodium) electrodes. A fit to the conductivity data gives $\text{σT = 2470}\text{exp}\text{(?0.142}\text{eV}\text{kT}\text{)Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}\text{K}$. The D_t, measurements employed two techniques, i.e. nondestructive profiling over the temperature range 210–750 K and cation exchange over the temperature range 505–970 K. The results of the two techniques were in close agreement and can be expressed as $\text{D = 2.12 ×10}{}^{\mathrm{?4}}\text{exp}\text{(?0.169}\text{eV}\text{kT}\text{)}\text{cm}{}^2\text{sec}{}^{\mathrm{?1}}$ for T>520K and $\text{D = 2.45 × 10}{}^{\mathrm{?4}}\text{exp}\text{(?0.164}\text{eV}\text{kT}\text{)}\text{cm}{}^2\text{sec}\text{470}\text{K}$. The “transition” region between 470 and 520 K is not observed in the conductivity measurements. Silver cation exchange was used to determine the number of mobile sodium ions. A comparison of D_t and σ data yielded a Haven ratio that is temperature dependent, ranging in value from 0.45 at 870 K to 0.35 at 370 K.     

Chaleur spécifique et transformation martensitique dans le système Nb1−xSnx

Experimental data on the superconducting temperature, and low temperature specific heat, together with X-ray and neutron investigations are reported for two sets of Nb_xSn_1?x, samples with the A15 type structure. The first one, intended for a study of the effect of stoichiometry, consists of cast alloys with lattice constants ranging from 5.273 to 5.289 Å, γ's from 4.9 to $\text{9}\text{mJ}\text{K}{}^2$ g-at and T_c's from 6 to 17.9 K. The second one consists of nearly stoichiometric sintered alloys with lattice constants ranging from 5.288 to 5.291 Å, γ's from 5.6 to 13.5 $\text{mJ}\text{K}{}^2$ g-at and T_c's close to 18 K. The martensitic transformation ratio has been determined by neutron diffraction in the latter set and the values 32, 77 and 95% were obtained. The analysis of the specific heat jumps at T_c shows that the superconducting T_c of the tetragonal phase is 17.9 K and both T_c and γ are reduced in the cubic phase.Phonon information inferred from specific heat data are indicative of a structure with a typical energy of 9 meV in the density of states. The agreement with spectra obtained by inelastic neutron scattering is good.     

Determination of the fermi surface of monoclinic SrAs3 by Shubnikov dehaas effect

The electronic properties of single crystals of monoclinic SrAs₃ have been studied by investigating the temperature dependence of electrical conductivity, Hall effect and Shubnikov de Haas (SdH) oscillations. At 4.2 K, SrAs₃ is predominantly p-conducting with a typical hole concentration of 6 × 10¹⁷cm^-3. The Hall coefficient changes its sign near 80 K. The angular dependence of the SdH oscillations was used to map out the shape of the Fermi-surface of holes. Two asymmetric, quasi-ellipsoidal Fermi-bodies are located in the first Brillouin zone. The cyclotron effective masses $\text{m}{}^1$ for two crystallographic directions were calculated from the temperature dependence of the amplitude of the oscillations: $\text{m}{}^1\text{(B6a)=(0.70± 0.002)m}{}_0$and $\text{m}{}^1\text{(B6c}{}^1\text{)=(0.095±0.003)m}{}_0$, respectively. There are indications for a third Fermi-surface which is attributed to electrons.     

Plasma dynamics in GaAs under strong picosecond surface excitation

We have investigated the time-resolved luminescence of GaAs in air at room temperature, under strong picosecond surface excitation. The energy density was in the range 4–40 $\text{mJ}\text{cm}{}^2$ and the excitation wavelength λ_ex ? 0.53 μ. The peak temperature reached by the generated electron-hole plasma at the end of the excitation pulse amounts to T_M ? 720 K at an energy density of about 10 $\text{mJ}\text{cm}{}^2$. Further increase of the power induces surface damage on the sample. The plasma relaxes its kinetic energy at a slow rate of the order of 10¹⁰$\text{eV}\text{s}$, supporting our previous calculations which concluded that electron-phonon interactions are strongly screened by intravalley free-carrier collisions at high plasma density, so that the plasma cooling slows down.     

Charge density wave commensurability in 2H-TaS2 and AgxTaS2

The charge density wave transition in 2H-TaS₂near 75 K has been observed to be incommensurate, using electron diffraction, with $\text{q}{}^1\text{= (}\text{0.338}\text{±}\text{0.002}\text{)a}{}^1{}_0$ along the 〈10.0〉 directions which, within the experimental uncertainty, remains temperature independent to about 14 K. Incommensurate charge density formation is also observed in Ag_xTaS₂ samples for $\text{x?}\text{0.26}$ with an increase in q¹ to $\text{(}\text{0.347}\text{±}\text{0.002}\text{)a}{}^1{}_0$ when x?0.26. Within the experimental error q¹ appears to be temperature independent to 25 K.     

A new organic low temperature conductor: HMTSF-TNAP

Conductivity and optical data on a new organic, conducting charge transfer salt Δ^{2, 2′}-Bi-(4,5-trimethylene-1,3-diselenole) 11,11′,12,12′-tetracyano-2, 6-napthoquinodimethane (HMTSF-TNAP) are given. $\text{σ(300}\text{K}\text{)= 2400 ± 600 Ω}{}^{-1}\text{cm}{}^{-1}$. A maximum in σ(T) is found at T_M = 47 K with σ(T_M)/σ(300 K) = 6.0 ± 10%, and $\text{σ(1.5}\text{K}\text{) > 250 Ω}{}^{-1}$. σ(T) is well defined in the high temperature region, but is sample dependent at low temperatures. The optical data indicate a bandwidth and carrier density comparable to that of HMTSF-TCNQ.     

E.S.R. and optical study of the interstitial h centres in heavily neutron irradiated Lif

The paramagnetic spectra of the hole-centres created in LiF by a heavy neutron irradiation at 78°K have been studied by E.S.R. and optical techniques. Samples which have been irradiated at a dose at least equal to $\text{10}{}^{19}\text{solneutrons}\text{cm}{}^2\text{exhibit}$, after they have been warmed at 200°K, several spectra related to 〈111〉-oriented asymmetric centres. These centres are stable up to 500°K. One of them, designated the H_N1 centre, has been studied in detail. The analysis of the paramagnetic spectrum shows that this defect is an interstitial fluorine atom which forms an asymmetric ($\text{F}{}^{\mathrm{<mtext>-</mtext><mtext>?1</mtext><mtext>3</mtext>}}\text{- F}{}^{\mathrm{<mtext>-</mtext><mtext>?2</mtext><mtext>3</mtext>}}$) molecule ion with a lattice fluorine ion. Optical experiments have shown that the excited Σ level is at 4.1 eV from the ground Σ level. Two excited π levels have been found at 2.5 and 2.1 eV. This asymmetric H centre is formed from a primary symmetric H centre, designated the H_N2 centre. The H_N2 centre is 〈110〉-oriented. The transformation of the symmetric H_N2 centre into the asymmetric H_N1 one is associated with important changes in the values of the isotropic hyperfine interaction and of the g_⊥ shift.     

Oпpeдeлehиe othocиteлъhoгo pacпoлoжehия heкomБиhиpyющиx элeкtpohhъix coctoяhий дbyxatomhъix moлeкyл. Ochobhoe элeкtpohhoe coctoяhиe ZrO

A method is proposed for determining the energy difference of noncombining electronic states and the type of ground electronic state of molecules, based on studying the temperature dependence of integral absorption coefficients of molecular bands in a reflected shock wave plasma over a wide range of temperatures. The method is used to determine the relative position of singlet and triplet ZrO electronic states. As the result of measurements of integral absorption coefficients of 0, 0 bands of ${}^1\text{Σ}{}^{\mathrm{+}}\text{-}{}^1\text{Σ}{}^{\mathrm{+}}$ and ${}^3\text{Δ -}{}^3\text{Δ}$ systems in the temperature interval 3370–5200°K, it is shown that ¹Σ⁺ state is the ZrO ground state, whilst T₀(a³Δ) = 1700 ± 250 cm^-1.     

Pressure spectroscopy of impurity states in GaSb(Se)

Galvanomagnetic effects and Shubnikov-de Haas oscillations are studied in monocrystalline GaSb(Se) samples ($\text{n}{}_{\mathrm{<mtext>se</mtext>}}\text{= 10}{}^{17}\text{?10}{}^{18}\text{cm}{}^{\mathrm{?3}}$) in magnetic fields up to 50 kOe at temperatures 0,07 K ?T?300 K under pressures p?12 kbar. Pressure spectroscopy has been used to determine the density of states in impurity bands and to investigate the anomalies of the transport properties of Γ-electrons near mobility edge ε_c on both metal and insulator sides.     

Anisotropic magnetic susceptibility and phase transitions in intercalated graphite: KC24

We measured the magnetic susceptibility of KC₂₄ from 4.2 to 300 K and found no anomalies near the phase transitions at 95 and 123 K as observed in the resistivity. We conclude that the transitions must be due to order- disorder transitions of the K atoms and not charge density wave formation. The susceptibility is anisotropic; at room temperature $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{6}\text{c}\text{)= + 1.50 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 2\%}$ and $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{⊥}\text{c}\text{)= + 0.045 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 50\%}$. This anisotropy is not understood in terms of simple rigid band extensions of the band structure of graphite.     

Superfluidity of neutron matter: (II). Triplet pairing

The possibility of neutron triplet pairing and superfluidity in neutron star matter is investigated, and the energy gap and corresponding critical temperature is calculated or estimated as a function of Fermi momentum or density. The calculations are performed for a “one-pion-exchange gaussian” potential, and compared with the results for neutron and proton singlet pairing and superfluidity calculated earlier.The results indicate that neutron superfluidity, corresponding specifically to ³P₂ state pairing, may exist in a high-density shell in the nuclear-matter region of a neutron star, i.e. for 1.6 × 10¹⁴g/cm³ < ρ < 1.4 × 10¹⁵g/cm³, and the maximum self-consistent energy gap is Δ⁰₁k_F ≈ 0.6 MeV and Δ⁰₃(k_F) ≈ 0.1 MeV for an effective mass $\text{m}{}^1\text{≈ 0.75}\text{and}\text{k}{}_{\mathrm{<mtext>F</mtext>}}\text{≈ 2.1}\text{fm}{}^{\mathrm{?1}}$, i.e. for a mas ? ≈ 5.2 × 10¹⁴g/cm³. For $\text{m}{}^1\text{= 1.0}$ we get correspondingly Δ⁰₁(k_F) ≈ 3.3 MeV and Δ⁰₃(k_F) ≈ 0.6 MeV for k_F ≈ 2.2 fm^?1.     

Magnetization,magnetocrystalline anisotropy,magnetostriction and elastic constants of the Heusler alloy: Cu2Mn Al

Three kg of stoichiometric single crystals of Cu₂Mn Al have been grown by the Bridgman method. The thermal variations of their magnetization, anisotropy, magnetostriction and elastic constants have been measured between 4.2 and 300 K. This alloy exhibits a small magnetocrystalline anisotropy, but a large elastic anisotropy $\text{(}\text{C}{}^{\mathrm{?}}\text{C}{}^{\mathrm{\gamma }}\text{= 4.9)}$ which explains the strong anisotropy of the magnetostriction $\text{(}\text{λ}{}^{\mathrm{\gamma ,2}}\text{λ}{}^{\mathrm{?,2}}\text{= 10)}$. The magnetic moment per Mn atom at 0 K is 3.6 μ_B.     

Resonance neutron capture gamma rays in 177Hf

Primary capture γ-rays have been studied for 38 ¹⁷⁷Hf neutron resonances with energies in the range 1–165 eV. Intensities were measured for 29 transitions ending at states with an excitation energy in ¹⁷⁸Hf up to 2050 keV. The analysis was facilitated by the previous knowledge of the spin and parity of all neutron resonances and of most low-lying states. For nine final levels, which had not previously been seen, information on J and π was deduced from the corresponding average intensities. The distribution of partial widths was fitted with a χ² function with ν = 1.38_?0.13^+0.18 degrees of freedom for E1 radiation and ν = 1.5_?0.40^+0.60 for M1 radiation. The average El reduced photon strength was found to be $\text{S}{}_{\mathrm{<mtext>El</mtext>}}\text{= 〈Γ}{}_{\mathrm{\gamma itij}}\text{/DE}{}_{\mathrm{\gamma }}{}^5\text{〉A}{}^{\mathrm{<mtext>?8</mtext><mtext>3</mtext>}}\text{= (4.8 ± 1.0) × 10}{}^{\mathrm{?15}}\text{MeV}{}^{\mathrm{?5}}$ and the ratio between El and Ml intensities equal to 5.5 ± 1.4. A comparison of this value for the El strength with those reported for other nuclei with A$\text{\$}\text{?}$= 100 showed that the intensities follow the A-dependence predicted by the Brink-Axel model. A non-statistical effect was observed, consisting of an enhancement of El transition probalilities to K = 2, 3 final states as compared to K = 0, 4 states.     

The examination of the Faraday effect and location of the Cr level in Cr doped PbTe

Faraday effect, absorption coefficient and Hall effect have been examined in Cr doped PbTe single crystals. The effective masses of carriers m_F and then values of effective masses at the bottom of conductivity band m_F(0) have been calculated. It is shown that m_F in Cr doped PbTe is comparable with m_F in n-type PbTe not doped with chromium, with the same free carrier concentration, and the relative temperature variation of m_F(0) corresponds to relative variation of Eg. In the absorption spectrum the additional absorption maximum is found at the energy 0.11–0.14 eV. The long-wave side of the peak is shifted towards longer waves as the temperature is increased. Calculation shows that chromium level is located in the conduction band at ΔE = 0.11 eV in the limit T → 0, and is shifted down towards the bottom of the conduction band with a constant rate of $\text{0.8 × 10}{}^{\mathrm{?4}}\text{eV}\text{K}$ within the temperature range of 4.4–300 K and $\text{3.3 × 10}{}^{\mathrm{?4}}\text{eV}\text{K}$ within the temperature range 300–800 K.     

A study of oxygen self-diffusion in the C-direction of rutile using a nuclear technique

Oxygen-18 has been used as a tracer to study diffusion in the c-direction of rutile single crystals in the temperature range 1173–1473K. The distribution of the tracer was determined by means of a nuclear technique. The diffusion coefficient can be represented in this temperature range by the equation $\text{D = 2.4 × 10}{}^{\mathrm{?6}}\text{exp}\text{?}\text{(2.826 ± 0.05) × 10}{}^5\text{RT}\text{m}{}^2\text{s}{}^{\mathrm{?1}}$. A diffusion coefficient obtained at 1673 K using a different technique agrees well with the value obtained by extrapolation from the other results. Consideration of the results from specimens with different impurity concentrations leads to the conclusion that the diffusion which has been studied is extrinsic.The results obtained in this investigation are in good agreement with those of Haul and Dümbgen[l] and (by extrapolation) Gruenwald and Gordon [2] but not with those of Doskocil and Pospisil[3] and Bagshaw and Hyde[4].     

Metal-nonmetal transition in solid argon-lanthanum mixtures

A continuous metal-nonmetal transition is observed in solid ArLa mixtures. The d.c. conductivity σ starts at a La atomic fraction c_La ? 0.15 with $\text{σ = 2 × 10}{}^{-2}\text{[Ω-cm]}{}^{-1}$ and increases exponentially with c_La until σ reaches the minimum metallic conductivity $\text{σ}{}_{\min }\text{= 300 ± 100[Ω-cm]}{}^{-1}$ at c_La ? 0.4. The temperature dependence of σ shows variable range hopping between localized states for c>_La ? 0.4 and metallic behaviour for $\text{c}{}_{\mathrm{<mtext>La</mtext>}}\text{? 0.4.}$     

Defect dominated conductivity in Zn3P2

The electrical resistivity and Hall coefficient of Zn₃P₂ have been measured for single crystal and thin polycrystalline film samples which were annealed over a range of equilibrium vapor compositions and temperatures. The room temperature electrical resistivity of single crystal samples annealed at 573 K varied from approximately 10⁵Ω-cm for single crystals heated in equilibrium with zinc to 10 Ω-cm for those annealed in a phosphorus rich ambient. Hall measurements indicate that a variation in carrier concentration is responsible for these changes. The experimentally observed dependence of carrier concentration [h° ], (cm^?3) on phosphorus pressure is given by [h°] = 1.32 · 10¹⁶ [p(P₄)]^0.13 for samples annealed at 573 K. The experimentally determined pressure dependence is in good agreement with a model based on phosphorus interstitials acting as acceptors. The pressure and temperature dependence of the carrier concentration yield the equilibrium constant K_I for the formation of interstitial phosphorus defects according to the reaction

$\text{1}\text{4}\text{P}{}_4\text{→ P′}{}_{\mathrm{i}}\text{+ h°}$

where

$\text{K}{}_{\mathrm{I}}\text{= 10}{}^{\mathrm{42.4\; \pm \; 2}}\text{cm}{}^{\mathrm{?6}}\text{torr}{}^{0.25}\text{[p(P}{}_4\text{)]}{}^{\mathrm{?0.25}}\text{exp(?1.18}\text{ev}\text{kT}\text{)}$

. The accommodation of phosphorus interstitials is discussed in light of the crystal structure of Zn₃P₂.     

Electrical transport and magnetic properties of Nd2(WO4)3

Measurements of the molar magnetic susceptibility (X_m) of a powdered sample of Nd₂(WO₄)₃ in the temperature range 300–900 K, and the electrical conductivity (σ) and dielectric constant (?$\text{)}\text{?}$ of pressed pellets of the compound in the temperature range 4.2–1180 K are reported. X_m obeys the Curie-Weiss law with a Curie constant C= 3.13 K/mole, a paramagnetic Curie temperature θ= ?60 K and a moment of Bohr magnetons, p= 3.49 for the Nd³⁺ ion. The electrical conductivity data can be explained in terms of the usual band model and impurity levels. Both the σ and ?$\text{\$}\text{?}$data indicate some sort of phase transition round 1025 K. The conductivity follows Mott's law $\text{σ = A exp (?B/T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{)}$ in the temperature range $\text{200 < T < 3000}\text{K with}\text{B = 45.00 (}\text{K}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{and}\text{A = 1.38 × 10}{}^{\mathrm{?5}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$. The dielectric constant increases slowly up to 600 K, as is usual for ionic solids. The increase becomes much faster above 600 K, which is attributed to space-charge polarization of thermally generated charge carriers.