R. Tovar

Crystallographic properties of I2–Fe–iv–vi4 magnetic semiconductor compounds

X-ray powder diffraction measurements were made at room temperature on sixteen polycrystalline samples of I{sub 2}-Fe-IV-VI{sub 4} (I: Cu, Ag; IV: Si, Ge, Sn, Pb; VI: Se, Te) magnetic semiconductor compounds. The diffraction patterns were analyzed to determine values of lattice parameter for each compound. The results showed that Cu{sub 2}FeSiSe{sub 4}, Cu{sub 2}FeGeSe{sub 4} and Cu{sub 2}FeSnSe{sub 4} have the tetragonal stannite structure (I{bar 4}2m), while the rest of the materials have an orthorhombic superstructure of quartzite which is known as wurtz-stannite (Pmn2{sub 1}). It was found that, when the values of the effective parameter a{sub e} = (V/N){sup 1/3} are plotted against the molecular weight W of the materials, the tetragonal and orthorhombic compounds lie on different straight lines. In addition, differential thermal analysis (DTA) measurements were made, and the peaks on the DTA cooling curves were used to determine values corresponding to the melting temperature for the compounds.

Crystal growth and characterization of the cubic semiconductor Cu2SnSe4

X-ray powder diffraction study of the p-type semiconductor Cu2SnSe4 shows that this material crystallizes in the cubic structure, space group F4¯3m, with unit cell parameter a=5.6846(3) A. The temperature variation of the hole concentration between 120 and 300 K, obtained from the Hall effect and electrical resistivity measurements, is due to the thermal activation of an acceptor level with ionization energy of about 0.02 eV. The temperature variation of the hole mobility is explained by considering the scattering of charge carriers by ionized impurities and acoustic phonons. From this analysis, the density-of-states effective mass of the holes is estimated to be about 0.8 me, me being the free electron effective mass. From the optical absorption spectra, the fundamental absorption edge is found to be direct. The value of the lowest energy gap and the spin-orbit splitting were estimated to be about 0.35 and 0.20 eV, respectively. The temperature dependence of the magnetization measurements shows that Cu2S...

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.

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.

Temperature variation of lattice parameters and thermal expansion coefficients of the compound ZnGa2Se4

Abstract X-ray powder diffraction measurements on ZnGa2Se4, a II-III2-VI4 semiconductor compound, were made in the temperature range between 300 K and 700 K, a region in which this material has a defect tetragonal structure. From the analysis of the X-ray diffraction lines, accurate lattice parameter values were determined as a function of temperature. These results allowed the evaluation of the thermal expansion coefficients of the corresponding parameters. The observed increase, with the temperature, of the tetragonal deformation δ(= 2 − c a ) as well as of the crystal anisotropy Aa(= αa − αc, with αa > αc) was then explained by using the relations proposed by Abrahams and Bernstein.

The (AgGa)1−zMn2zTe2 alloys: Phase relations and the effects of ordering

Abstract Polycrystalline samples of (AgGa) 1− z Mn 2 z Te 2 alloys were prepared by the melt and anneal technique and were used in lattice parameter, optical energy gap E o , and differential thermal analysis measurements. Both chalcopyrite α and zinc blende β single-phase fields are found and the results indicate that in both of these cases ordering of the manganese can occur at lower temperatures to give ordered α′ and β′ structures. The composition and temperature ranges of these fields are shown on a T ( z ) diagram. It is found that the managese ordering has an appreciable effect on E o . This is demonstrated by the different aiming points at z = 1.0 for the E o vs z lines in the different fields, the values being ∼2.2 eV for α, ∼1.9 eV for β′, and 1.35 eV for α′.

Crystallographic and magnetic properties of Cu2FeGeSe4 and Cu2FeGeTe4 compounds

X-ray powder diffraction measurements, at room temperature, and magnetic susceptibility measurements, in the temperature range from 2 to 300 K, were made on polycrystalline samples of Cu2FeGeSe4 and Cu2FeGeTe4 magnetic semiconductor compounds. Magnetization measurements at 2, 4.2 and 77 K in magnetic fields up to 35 T were carried out on Cu2FeGeSe4 compounds. From the analysis of the X-ray diffraction lines, it was found that Cu2FeGeSe4 and Cu2FeGeTe4 have, respectively, stannite and monoclinic structures. The resulting 1/X versus T curves showed that Cu2FeGeSe4 is antiferromagnetic with a Neel temperature T N = 20 K while Cu2FeGeTe4 is ferrimagnetic with T N = 160.1 K. The magnetization and susceptibility results obtained on Cu2FeGeSe4 showed the presence of bound magnetic polarons (BMPs) in agreement with earlier studies made on this type of materials [1, 2].

T(z) diagram of the Mn3zIn2(1-z)Te3 system in the range 0 < z < 0.7

The T(z) phase diagram of the system Mn3zIn2(1-z)Te3 was determined in the composition range 0 < z < 0.75 from differential thermal analysis and X-ray diffraction measurements. It was shown that a single-phase solid solution with the adamantine structure occurs in the composition range 0 < z < 0.26, but that for the rest of the composition range investigated two phase conditions apply. In the single-phase range, at higher temperature the structure is zinc-blende, with Mn atoms, In atoms and lattice vacancies arranged at random on the cation sublattice. Below ∼620°C, two separate ordered structures occur, one with 0 < z < ∼ 0.03, corresponding to the orthorhombic ordered structure of In2Te3, and the other with ∼0.09 < z < 0.26, corresponding to the tetragonal ordered structure of MnIn2Te4.

Temperature variation of lattice parameter and optical energy gap values of the compounds CdIn2Te4 and MnIn2Te4

Abstract X-Ray powder diffraction measurements, in the range from 300 K to 700 K, and optical absorption measurements, in the range 20–300 K, were made on polycrystalline samples of CdIn2Te4 and MnIn2Te4 compounds. In each case, from the analysis of the X-ray diffraction lines, accurate lattice parameter values were determined as a function of temperature, thus the thermal expansion coefficients of the materials are obtained. The absorption measurements were used to determine values of the optical energy gap Eg as a function of temperature. The resulting curves of Eg vs T were fitted to a simplified Manoogian-Leclerc equation and the fitted coefficients used to give values of ( dE g dT ) 1 and ( dE g dT ) 2 due to lattice dilation and electron-phonon contributions respectively. Hence, the deformation potentials of the valence and conduction bands were estimated for both compounds.