M. Quintero

Low temperature magnetic behaviour of CuFeS2 from neutron diffraction data

Abstract Measurements of neutron diffraction spectra were made at temperatures 4.2, 25, 45, 65, 85, 150 and 300 K on powdered polycrystalline synthetic CuFeS2. Standard Rietveld profile analysis using GSAS showed that at all temperatures, a good fit to the data could be obtained with both the chemical and magnetic space groups taken as I 4 2d, and an antiferromagnetic configuration of the Fe spins, as reported previously by Donnay et al. However, graphs of lattice parameters a and c against temperature showed a distinct change in slope close to 50 K, indicating the presence of a transition. Graphs of integrated intensity of diffraction lines against temperature showed corresponding discontinuities in the case of magnetic and mixed nuclear-magnetic lines but not for nuclear lines, indicating that the transition was magnetic. Detailed calculations of predicted magnetic intensities showed that the intensity variations could be explained by Cu spins, having a paramagnetic arrangement down to 50 K and then ordering to an antiferromagnetic form at lower temperatures. The analysis gave a value of ∼ 0.05 μB for the magnetic moment of the Cu ions.

Analysis of direct exciton transitions in CuGa(SxSe1−x)2 alloys

Abstract The absorption coefficient spectra of the chalcopyrite alloys system CuOa(S x Se 1− x ) 2 were measured near the fundamental band edge at 77 K. From the analysis of the data, taking into account both the discrete and continuum exciton contributions, the exciton binding energy and the broadening were determined. It is shown that the exciton-phonon coupling is weak and that the absorption line shape is given by a symmetrical Lorentzian function. Also, it was found that the dependence of the oscillator strength on the energy gap can be explained by the dipole approximation.

Temperature variation of energy gaps and deformation potentials in CuGa(SzSe1−z)2 semiconductor alloys

Measurements of wavelength modulated reflectance in the temperature range 10–300 K have been made on single crystals of the system CuGa(SzSe1−z)2. For each sample, values of EA,EB, and EC corresponding to the three valence to conduction bands transitions have been determined. These results are analyzed using an equation proposed by Manoogian and Woolley [Can. J. Phys. 62, 285 (1984)]. From the analysis of the variation of the EA energy gap, with temperature and pressure the valence and conduction band, deformation potentials have been estimated for the present compounds and alloys.

Optical energy‐gap variation and deformation potentials in CuInTe2

Values of optical energy gap E0 as a function of pressure P up to 3 GPa at room temperature, and as a function of temperature T in the range 10–300 K at atmospheric pressure were obtained by optical‐absorption measurements on samples of CuInTe2. It was shown that in this pressure range, E0 varies linearly with P, dE0/dP having a value of 2.2×10−2 eV/GPa. The variation of E0 with T was fitted well by a simple Manoogian–Leclerc equation of the form E0(0) − E0(T) = UT + Vφ (coth φ/2T−1). Both dE0/dP and U can be related to (dE0/dT)2, the lattice dilation contribution to the variation of the energy gap with temperature, and the values obtained in the two cases showed good agreement. The Vφ term can be related to (dE0/dT)1, the energy‐gap variation due to electron‐phonon interaction. From the values of (dE0/dT)1 and (dE0/dT)2, values were determined for the acoustic deformation potentials of the conduction band Ce and of the valence band Ch.

Effects of crystallographic ordering on the magnetic behaviour of (AgIn)1-zMn2zTe2 and (CuIn)1-zMn2zTe2 alloys

Abstract Measurements of magnetic susceptibility in the temperature range 4.2–300 K were made on polycrystalline samples of the (AgIn) 1 - z Mn 2 z Te 2 and (CuIn) 1 - z Mn 2 z Te 2 alloys, and the data used to give values of spin-glass transition te mperature T g and Curie-Weiss paramagnetic temperature θ. For any sample for which the X-ray powder photograph indicated an apparently single phase condition, either zinc-blende or chalcopyrite, the susceptibility data could show up to three separate T g values. These different magnetic conditions are attributed to crystallographic ordering of the Mn ions on the chalcopyrite and zinc-blende lattices, the three observed T g values corresponding to disordered zinc-blende, ordered zinc-blende and ordered chalcopyrite. The value of θ obtained from the 1/χ vs. T plot is shown to be a weighted mean of the separate values of θ for the phases present. The relative sizes of the T g peaks and the values of θ for any given sample gives an indication of the amount of each phase present. These amounts were varied by using different methods of heat treatment and it was shown that the magnetic behaviour was consistent with the T ( z ) phase diagram for the two alloy systems.

A Comparative Study of (Cu–III–Se2)x–(FeSe)1—x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy

Polycrystalline samples of (Cu-III-Se 2 )-(FeSe) 1-x alloys (III: Al, Ga and In) were prepared by the usual melting and annealing method in the entire composition range 1≤ x < 0. X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and Scanning Electron Microscopy (SEM) techniques were used to characterize the products. From SEM technique, the stoichiometric relation for each sample was checked to be very close to the nominal values, with deviations less than 10%. XRD measurements showed that, for the three alloy families, there is a wide single-phase field, with a chalcopyrite-like structure, for 1 < x < 0.5. The lattice parameters vary linearly with composition and the variation can be predicted using Jaffe and Zungers model for chalcopyrites, Paulings set of covalent radii, and the covalent radius of 1.35 A for Fe. From DTA measurements, thermal transitions in the temperature range 300 K < T < 1400 K were obtained. The melting point decreases from that of extreme compounds (x = 0 and x x = 1) towards an intersection point (local minimum) at around x = 0.3. These results suggest that under adequate thermal treatments, the single-phase field could be increasing up to around x 0.3. In a closer analysis, a pronounced local minimum has been observed at x = 2/3 for Ga and In-based alloys, suggesting that the composition x 2/3 is a new class of compounds, the (Cu-III) 2 FeSe 5 .

X-ray diffraction (XRD) studies on (CuAlSe2)x (FeSe)1−x alloys

X-ray diffraction (XRD) studies have been performed in (CuAlSe2)x—(FeSe) 12x alloys prepared by the usual melt and anneal technique. The results showed that in the interval 0.9 . x . 0.33, a tetragonal chalcopyrite-like structure is the predominant phase ( a) with traces of FeSe (b). For 0.33 . x . 0.1, a and b phases coexist, in approximately same relative amounts. Additional lines in the diffraction spectra of the new compounds CuFeAlSe3 (x 5 0.5) and CuFe2AlSe4 (x 5 0.33) suggest a crystallographic transition from tetragonal (I) to tetragonal (P). In our knowledge is the first time that alloys of the type (I-III-VI 2)-(FeVI) have been studied in the entire composition range.

The crystal structure of copper iron selenide, CuFeSe2

The crystal structure of CuFeSe2 has been found to be tetragonal, space group P42c [D212, N° 112], with a=5.530(1), c=11.049(2)A and Z=4. The structure is a superstructure based upon a cubic close-packed array of anions with the cations occupying a fraction of the available tetrahedral sites. It can be viewed as a sulvanite derivative structure instead of a chalcopyrite-type of structure as expected for a typical I-III-VI2 semiconductor compound. The structure was solved by analyzing the two possible arrangements permitted by the subgroup-supergroup relationships. The refinement, carried out by full-matrix least-squares techniques, led to R(F) = 0.082 and Rw(F)=0.069 using all 310 independent reflections measured with a four-circle single-crystal diffractometer.

T(z) diagram and optical energy gap values of Cd 1-z Mn z Ga 2 Se 4 alloys

The T(z) diagram of the system Cd1−zMnzGa2Se4 was obtained from x-ray diffraction and differential thermal analysis measurements. It was found that at lower temperatures, a single phase solid solution occurs across the whole compositional range and values of lattice parameters were determined as a function of z. At higher temperatures, an order-disorder transition occurs, in the range 0 < z < 0.6 to a partially ordered tetragonal structure and for 0.6 < z < 1. 0 to a disordered defect zinc-blende structure. In the T(z) diagram, both the ordering boundary and the solidus curve appear to show discontinuities at z = 0.6, corresponding to the change in the disordered phase. It is suggested that the symmetries of the terminal compounds are different one from the other. Optical absorption measurements were made at 300 K to show the variation of the direct optical energy gap Eo with z, and again the values appear to divide into two parts below the above z = 0.6.

Phase diagram, optical energy gap, and magnetic susceptibility of (CuGa)1−zMn2zTe2 alloys

Abstract Polycrystalline samples of (CuGa) 1− z Mn 2 z Te 2 alloys were prepared by the melt and anneal technique and were used in differential thermal analysis, lattice parameter, optical energy gap, and magnetic susceptibility measurements. It was found that in the equilibrium diagram the range of single-phase solid solution at room temperature is very small with the chalcopyrite phase existing for 0 ≤ z ≤ 0.04 and no zinc blende phase occurring. However, above 600°C, the chalcopyrite and zinc blende phases together show single-phase behavior out to z = 0.4. Samples water-quenched from 650°C were found to be predominantly metastable manganese-ordered zinc blende. Thus the optical and magnetic data for this phase can be obtained and is in good agreement with the values obtained previously for other chalcopyrite-derived semimagnetic semiconductors.