EPR spectra of colour centres in x-irradiated BaCl2:K+ and BaCl2:Ag+

X-irradiation at 80°K leads to the formation of V_K centres and, in addition, perturbed F centres in the case of BaCl₂:K⁺. The V_K centres spectra exhibit a superhyperfine structure. A warming to 130°K of crystals X-irradiated at 80°K causes the V_K centres to be perturbed, and besides leads to the formation of (AgCl₄)^2? complexes in BaCl₂:Ag⁺.     

ESR and endor studies of the VK center in SrF2

The results of electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) studies of a self-trapped hole (V_K center) in SrF₂ are reported. The g-factor and hyperfine interaction constants were determined for the flourine nuclei forming the center and for those in the nearest three different sites. The values of hyperfine interaction constants are intermediate to the known values for CaF₂ and BaF₂.     

ESR and photoacoustic spectra of V2O5−MO2 (M = Se, Te, Ge) glasses

ESR spectra of V₂O₅?MO₂ (M = Ge, Se, Te) glasses are investigated in the range 298–498 K. The spectra at 298 K are characteristic of V⁴⁺ with the 3d¹ electron localized on a single ${}^{51}\text{V}\text{(I =}\text{7}\text{2}$) in the glass network. At higher temperature, the hyperfine structure progressively broadens, leading eventually to a broad, single ESR peak. These results are consistent with thermally-induced electron hopping from V⁴⁺ to V⁵⁺. Photoacoustic spectra of the glass at 298 K are characteristic of V⁴⁺ in a distorted octa environment. A correlation of ESR and PAS data suggests that covalency increased as M is charged from Ge through Te to Se.     

Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8

A method to grow single crystals of ammonium vanadate (IV, V) (NH₄)₂V₃O₈ has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are $\text{a = 8.891 ± 0.004}\text{A}\text{?}$ and $\text{c = 5.582 + 0.002}\text{A}\text{?}$. The V⁵⁺ atom lies at the center of a triangular pyramide (VO₄ tetrahedron) while the V⁴⁺ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO₅ whose symmetry point group is almost C_4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g_∥ = 1.9263 et g_τ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T₂ = 0.4 × 10^?7s and for spin-lattice relaxation T₁ = 1.6 × 10^?7s. The magnetic structure is characterized by an exchange narrowing ω_e = 3 × 10¹⁰rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P_2c4bm as the interaction between V⁴⁺ ions from consecutive planes parallel to (001).     

Electrical and optical properties of single crystal In2Te3 and Ga2Te3

Electrical conductivity and thermopower measurements are reported for the defect semiconductors p-In₂Te₃ and n-Ga₂Te₃. The hole mobility μ_p in the former varied as Tⁿwheren=+5.98 forT<350 K and n=-4.13 forT>350 K showing a maximum of 210 cm²V^-1 sec^-1 at 350 K. Electron mobility in n-Ga₂Te₃ also went through a maximum at 320 K. The optical band-gaps for both were found to be direct, with values of 1.01 and 1.08 eV for In₂Te₃ and Ga₂Te₃ respectively at 300 K. Pronounced effects of annealing on TEP and optical absorption gave evidence of defect ordering at low temperatures followed by defect creation at T>350 K.     

Correlation between the high temperature anomalies in V2O3 and the lattice parameters

A single phase corundum type structure was observed in V₂O₃ from 165 to 1100K. An anomalous behaviour of the lattice parameters of the rhombohedral unit cell was found in the high temperature region where V₂O₃ undergoes a broad electrical transition. The unit cell edge a_R contracts above 165 K, reaching a minimum at 533 K, after which there is a sharp increase. Above 595 Ka_R expands continuously and then almost linearly above 775 K. The temperature dependence of the rhombohedral angle shows two distinct regions, and the volume expansion follows the variation of angle; the change of the rate of expansion coincides with the minimum of a_R. The contraction of a_R corresponds to a reduction in certain second-neighbor VV distances.Our results show that a gradual transition is taking place in V₂O₃ from an α type structure to a β type characteristic of the Cr-doped V₂O₃ compounds. In addition, the sharp dip of the a_R curve occurs at nearly the same temperature as the peak previously observed in small angle scattering of neutrons. It is presumed that the two effects are related, and that the critical-like behaviour of a_R is due to some dynamic magnetoelastic effect involving second neighbor VV interactions.     

Optical properties of positive hole centres obtained in X-irradiated SrCl2 doped with an alkaline cation

The optical absorption spectra and the thermoluminescence curves can be accounted for with the assumption that, at L.He.T., there exist pure V_k centres which, by warming, are converted gradually to perturbed V_k centres. The transformation is complete at 130 K.     

Luminescence centers in phosphorus-doped Cd1−xMgxTe mixed crystals

The emission and excitation spectra of P-doped Cd_1?xMg_xTe (x ? 0.45) mixed crystals have been measured over the temperature range of 300 ～ 4.2 K to reinvestigate the photoluminescence process of two broad emission bands; the 690 nm band of the as- grown crystals and the 610 nm band of the annealed crystal. The excitation spectrum of each band has a characteristic peak at 550 nm near the absorption edge of this crystal. Together with the data of the decay characteristic and the halfwidth of each band, we discuss the difference in the recombination process when the crystal is excited under two different radiation at 365 and 570 nm (close to the 550 nm radiation). It is concluded that both emission bands arise from donor-acceptor pairs and a localized center or self- activated center; the associated pair is V_Cd-interstitial P in the as-grown and V_Te ? P_Te in the annealed crystal.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Kβ2 X-ray emission bands of Ga and As and their compounds

Ga and As Kβ₂ emission bands of the compounds under study consist of a more or less pronounced Kβ₂ main band, a short-wavelength side band which according to calculations by the ‘sudden approximation’ method may be essentially assigned to a KM_IV,V → M_IV,VN_II,III satellite, and long-wavelength 3d → 1s bands. Compounds such as Ga₂(SO₄)₃, Ga(NO₃)₃, Ga₂O₃, NaH₂AsO₄, K₃AsO₄, and As₂O₃ also show long-wavelength side-maxima at a distance of about 12 and 15 eV, respectively, from the main band, which are due to electron transitions from bands or levels with a preponderant 0 2s character.The Kβ₂ main band of A^IIIB^V compounds is less pronounced owing to the widths of the K levels and to instrumental distortions. The distance between the maxima of the state density of the upper valence bands can be recognized only by a shoulder or asymmetry of the band at the long-wavelength side. By calculation of the Ga and As Kβ₂ bands in GaAs with a pseudopotential kp band structure method, and allowing for the influence of both the transition probability and instrumental distortion excellent agreement with experiment is obtained.     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

Effect of V K centers on positron annihilation in KCl and Ag-doped KCl

Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ₂) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV _K centers. Work support in part by the Research Corporation. Paper C1 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

X-ray photoelectron spectroscopy characterization of the layered intercalated compound K2xMn1 − xPS3

Polycrystalline powders of the layered MnPS₃ compound have been intercalated with K⁺ ions by ion-exchange to yield the K_2xMn_1 − xPS₃ intercalate. X-ray photoelectron spectroscopy has been applied to learn about the electronic structure of this compound. In particular, we have studied the XPS spectra of the Mn 2p and 3p, P and S 2p, K 2p and 3p core levels and of the valence band region. The binding energies for various core levels of the elements present in this compound and their observed chemical shifts are analyzed. The data give evidence for the lack of non-equivalent atoms of K, Mn, P and S. Shake-up satellites are present at the Mn 2p and 3p core levels. The occurrence of such lines allows us to hypothesize that K_2xMn_1 − xPS₃ is a large-gap insulating Mn compound. Confirmation that only an ion transfer accompanies the intercalation process is given from both the strong observed similarity with the corresponding XPS spectra in MnPS₃ and the observed binding energy positions of the K 2p and 3p levels. As regards the valence band XPS spectrum, the observed analogies with the corresponding XPS spectra of the pure compound and of other K compounds have allowed us to single out two regions and their probable contributors.     

Temperature dependence of pressure broadening of NH3 perturbed by H2 and N2

The temperature dependence of pressure broadening of 134 rovibrational transitions of several branches in the ν₄ and 2ν₂ bands of ammonia perturbed by H₂ and N₂ has been measured using a high-resolution Fourier transform spectrometer. The temperature range covered during the experiments was between 235 and 296 K. The pressure-broadening linewidths were obtained using the method of multipressure fitting to the measured shapes of the lines. These broadenings were also calculated using a semiclassical model leading to a reasonable agreement with the observations and reproduces well the strong systematic experimental J and K quantum number dependencies. The retrieved values of the linewidths, along with those previously determined from the spectra at room temperature, were used to derive the temperature dependence of both H₂ and N₂ broadening of NH₃ lines. The broadening coefficients were shown to fit closely the well-known exponential law. For both experimental and theoretical results, the temperature exponent n has been obtained. Careful inspection of the experimental values shows that, contrary to the linewidths, the coefficient n is nearly K independent within each J multiplet. Also for a given J it does not seem to exhibit any noticeable variation with the type of rotational transition. On the other hand, the calculated n values exhibit a strong J and K systematic dependencies. n increases with K for a given J, decreases with J for a given K and are independent of the type of rotational transition.     

SrTb2Fe2O7 studied by mössbauer spectra of 57Fe

The temperature dependence of the hyperfine parameters of ⁵⁷Fe in SrTb₂Fe₂O₇ gave a magnetic ordering temperature T_N=542 K, saturation effective magnetic fieldH_eff(0)=552 kGand Debye temperature θ_D=330 K. The princip al axis of the PFG tensorV_zzis angled at 9° to the crystallographic c axis. Mössbauer spectra at 4.2 K reveal reorientation of iron spin.     

Hydrogen indium vacancy complex VInH4 in n-type InP studied by positron-lifetime

Positron-lifetime experiments have been carried out on two undoped n-type liquid encapsulated Czochralski (LEC)-grown InP samples with different stoichiometric compositions in the temperature range 10-300 K. For temperatures below 120 K for P-rich InP and 100 K for In-rich InP, the positron average lifetime began to increase rapidly and then leveled off, which was associated with the charge state change of hydrogen indium vacancy complexes from (V_InH₄)⁺ to (V_InH₄)⁰. This phenomenon was more obvious in P-rich samples that have a higher concentration of V_InH₄. The transformation temperature of approximately 120 K suggests that the complex V_InH₄ is a donor defect and that the ionization energy is about 0.01 eV. The ionization of neutral V_InH₄ accounted for the decrease of the positron average lifetime when the sample was illuminated with a photon energy of 1.32 eV at 70 K. These results provide evidence for hydrogen complex defects in undoped LEC InP.     

K-shell excitation spectra of CO,N2 and O2

Electron energy loss spectra of CO, N₂ and O₂ have been recorded in the regions of carbon, nitrogen and oxygen K-shell excitation and ionisation. These results are compared to previous energy loss, photoabsorption and theoretical studies of the same spectral regions. Several inconsistencies in the published spectra are clarified in the present work. Comparisons with recent calculations of the K-shell continua of these molecules are presented. Vibrational structure in the K → π * transitions of CO (C 1s) and N₂ (N 1s) has been resolved in high-resolution studies (< 0.1 eV FWHM) of these species.     

Farbzentren in Ammoniumhalogeniden

Color centers produced by X-rays in ammonium halides at various temperatures between 20°K and room temperature have been investigated by means of paramagnetic resonance and by optical methods. Two kinds of paramagnetic defects were found to be predominant, the self-trapped hole (V _K-center) and another electron deficiency center involving a NH₃ ⁺-radical. The electronic structure of theV _K-center is the same as in the alkali halides, except that the orientation of the molecular axis is along [100] instead of [110]. The kinetics of the thermally activated motion of theV _K-centers and of their recombination with electrons has been studied. The electronic structure of the second center was derived from the hyperfine spectrum of the paramagnetic resonance. The rotation of the NH₃ ⁺ ion and its connection with the order-disorder transition in NH₄Cl has been studied.     

Thermally stimulated conductivity induced in SrF2:Tb by u.v. light

Thermally-stimulated-conductivity was excited in SrF₂:Tb crystals by non-ionizing u.v. light. The electrical glow curves were studied in the range 80°–300°K, and thermal activation energies were computed by various methods. A composed glow peak, with maxima at 157° and 169°K, is attributed to the two stages of thermal decay of V_K centers.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.