Chromium Telluride Research Articles

Soft x-ray magnetic circular dichroism study of Cr tellurides

Ferromagnetic chromium tellurides Cr5Te6(δ=0.17) and Cr2Te3(δ=0.33) have been investigated by Cr 2p x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). The observed XMCD spectra have been analyzed by means of a configuration-interaction cluster model calculation. From calculated results, we suggest that the doped holes created by the Cr deficiency exist mainly in the Te 5p orbital of Cr1−δTe.

Cr 2p XMCD spectra of ferromagnetic Cr 1− δTe: A configuration interaction picture

We have performed Cr 2p X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) study of ferromagnetic chromium tellurides Cr 1− δ Te ( δ = 0.11–0.33). A configuration interaction cluster model calculation has been applied to the observed XAS and XMCD spectra. It is found that the lineshape of the XMCD spectrum is quite sensitive to the charge transfer energy and the crystal field. It is clarified that the ground state is dominated by d 4 L _ states, suggesting that the doped holes by the Cr deficiency are likely to be in the Te 5p orbitals.

Chromium Tellurides: The Effect of Cation and Anion Substitution onto the Crystal Structures and Magnetic Properties

Chromium Tellurides: The Effect of Cation and Anion Substitution onto the Crystal Structures and Magnetic Properties

Magnetic properties and low temperature X-ray studies of the weak ferromagnetic monoclinic and trigonal chromium tellurides Cr 5Te 8

Monoclinic and trigonal Cr 5Te 8 show a transition into the ferromagnetic state with a Curie temperature T c, which sensitively depends on the actual composition. Monoclinic samples exhibit a lower T c despite their higher Cr content. This observation is explained on the basis of less effective ferromagnetic superexchange in the monoclinic compounds and the larger number of Cr atoms being antiferromagnetically coupled. Magnetization experiments performed at 5 K demonstrate that the compounds saturate already at rather low magnetic fields. In addition, small values are estimated for the coercitive field H c as well as for the remanence magnetization both being typical for weak ferromagnetic materials. The values for the saturation magnetization amount to about 72% and 65% for monoclinic and trigonal Cr 5Te 8, respectively. These low values can partially be explained on the basis of antiferromagnetically coupled Cr(III) d 3 centers. An additional spin canting is assumed to fully account for the reduced saturation moments. Above 300 K the susceptibilities follow a Curie–Weiss law with large positive values for the Weiss constant and magnetic moments in accordance with a Cr 3+ 3 d 3 spin configuration. Low temperature X-ray investigations reveal unusual thermal expansion of the a as well as b lattice parameters of the monoclinic and of the a lattice parameter of the trigonal sample, respectively. Near T c the slope of the thermal expansions changes significantly.

Reaction of modified SUS316 with tellurium under low oxygen potentials

Corrosion behavior of modified SUS316 has been studied under different oxygen potentials with tellurium vapor. Under the condition of oxygen potential ⩽−500 kJ/mol and tellurium vapor pressure of 8.9 × 10 −4 atm, duplex corrosion layers formed on the surface of specimens, which mainly consisted of chromium telluride, iron telluride and chromium oxide. The thickness of corrosion layer was increased with decreasing the oxygen potential. Furthermore, the amount of oxygen uptake in sample corroded with tellurium vapor was much greater than that in sample oxidized without tellurium vapor.

Low Temperature Synthesis of Chromium Tellurides Using Superlattice Reactants: Crystallization of Layered CrTe3 at 100 °C and the Decomposition into Cr2Te3.

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Low temperature synthesis of chromium tellurides using superlattice reactants: crystallisation of layered CrTe 3 at 100°C and the decomposition into Cr 2Te 3

Thin chromium and tellurium superlattice reactants with individual layer thickness ranging from a few Angstrom to thousands of Angstrom were deposited on (100)-Si substrate and the reactions were investigated with ex-situ and in-situ X-ray powder diffraction and X-ray reflectometry. The number of layers was adjusted between 3 and 653 and the compositions of the films ranged from 71 to 86 at% Te. Samples with thick Cr/Te double-layers (300–3000 Å) react at about 300°C to form crystalline and highly textured (h00)-CrTe 3. The temperature of formation is dramatically reduced to 100°C using Cr–Te multi-layers exhibiting Cr/Te double-layers with a thickness below 25 Å. At about 275°C (h00)-CrTe 3 is decomposed followed by the crystallisation of Cr 2Te 3 which also crystallises as a highly oriented material with a (00l)-texture. In a narrow temperature range both compounds coexist. The decomposition of the monochromium tritelluride followed by the crystallisation of Cr 2Te 3 could only be observed using the thin film samples whereas for the thick superlattices no such decomposition could be observed. We also show the very detailed analysis of a sample containing 6 Cr/Te double-layers. The results obtained with different analytical techniques like energy dispersive X-ray spectroscopy, atomic force microscopy, transmission electron microscopy, and X-ray reflectometry lead to a very comprehensive picture of the multi-layer structure of this sample.

Heat capacities of Cr 5Te 8 and Ni 5Te 8 from 325 to 930 K

Heat capacities of monoclinic Cr 5Te 8(m) and hexagonal Ni 5Te 8(hex) have been measured from 325 to 930 K using an adiabatic scanning calorimeter. For monoclinic Cr 5Te 8(m) the λ-type heat capacity anomaly was observed at 815 K, presumably due to the order—disorder of chromium atoms and vacancies in chromium tellurides. The transition entropy change of the Cr 5Te 8(m) sample obtained in this experiment compared reasonably well with that calculated on the basis of the order—disorder transition from vacancy-ordered Cr 5Te 8-type to vacancy-ordered Cr 3Te 4-type crystal structures. For the hexagonal Ni 5Te 8(hex) sample two excess heat capacities were observed around 400 K and above 700 K. The excess heat capacity at low temperature may be connected to the order—disorder of nickel atoms and vacancies in this structure and that at high temperature may be related to the activation processes such as defect formation, electron excitation and electron—hole pair formation.

Hexakis(triethylphosphine)octatelluridohexachromium and a molecule-based synthesis of chromium telluride, Cr3Te4

The molecular cluster compound Cr6Te8(PEt3)6 (1) is formed by the reaction of TePEt3 with either (Et3P)2Cr(allyl)2 or Cr(2,4-dimethylpentadienyl)2. This compound can be converted to the extended inorganic solid state compound Cr3Te4 by simple thermolysis. We have determined the structure of the title compound crystallographically (monoclinic; a = 13.076(5) A, b = 21.194(7) A, c = 23.694(7) A, β = 105.21(5)°, V = 6276(10) A3, Z = 4). The molecule is formed by a Cr6 octahedron, a Te8 cube, and a (PEt3)6 octahedron, all of which are concentric. We compare and contrast the structure and properties of the cluster with those of related solid-state compounds.

Heat capacity of chromium-tellurium alloy at high temperatures

Heat capacities of monoclinic Cr 5Te 8(m) (CrTe 1.528, CrTe 1.576 and CrTe 1.590) and trigonal Cr 5Te 8(tr) (CrTe 1.609) were measured from 220 to 900 K by using an adiabatic scanning calorimeter. Heat capacity anomalies were observed at 841, 842 and 815 K for monoclinic Cr 5Te 8(m) (CrTe 1.528, CrTe 1.576 and CrTe 1.590, respectively) and at 776 K for trigonal Cr 5Te 8(tr) (CrTe 1.609), presumably due to order-disorder of chromium vacancies in chromium tellurides. The transition entropy changes of Cr 5Te 8(m) and Cr 5Te 8(tr) phases obtained in this experiment were smaller than those calculated from the entropy changes due to the partial disordering of chromium vacancies originally limited to every second chromium layer. This fact suggests that the completely-ordered phase is not formed in spite of slowly cooling the sample or that the partially-ordered phase still remains even at the high temperatures at which measurments were made in this study.

High Temperature Corrosion of Iron Chromium Alloys by Tellurium.

The reaction rates between iron-chromium alloys (1.17, 5.65 and 11.96 a/0Cr) and tellurium were measured in the temperature range of 8731, 023 K at 350 Pa of tellurium vapor pressure, and in the tellurium vapor pressure range of 66.7600 Pa at 923 K. The electron probe microanalysis, marker experiment and X-ray diffractometry were employed to clarify the mechanism of the telluride scale growth. The reaction rates between iron-chromium alloys and tellurium obeyed the parabolic rate law. It was observed that the telluride scale consists of the inner, the intermediate and the outer layers ; chromium is concentrated in the inner layer which may grow by the inward diffusion of tellurium ; the intermediate layer consists of mainly β-iron telluride, and the outer layer consists of both δ- and δ'-iron tellurides below 980 K, and δ-iron telluride alone above 980 K. The intermediate and outer layers grow by the outward diffusion of iron in iron tellurides. The protection effect by chromium may be explained by the fact that chromium telluride covered over the surface of iron-chromium alloy interrupts the outward diffusion of iron. The reaction between iron-chromium alloy and tellurium can be classified into three regions from the view point of the activation energy. From the comparison of the activation energy of the reaction between iron-chromium alloy and tellurium with that for the diffusion of iron in iron tellurides, the rate-determining step for the scale formation for each region was discussed.

Chemical reactions between caesium, tellurium and oxygen with fast breeder reactor cladding alloys

Abstract The corrosion of the alloys PE16 and M316 by Cs : Te mixtures (1:1, 2:1 and 4:1) has been studied in sealed capsules under partial pressures of O2 set by metal/metal oxide couples at 948 K for 168 h. The alloys suffered severe intergranular corrosion by the 1:1 mixtures and corrosion seemed independent of O2 potential. The 2:1 and 4:1 mixtures produced a matrix (alternate layering) type of corrosion and the depth of corrosion increased with increasing O2 potential. At the highest potential the 4:1 mixture differentiated the two alloys; PE16 suffered a combination of intergranular and matrix attack whereas M316 was penetrated throughout (mainly by Cs/oxygen) intergranularly. Generally Cs, Te, Cr and oxygen were found associated (as a mixture of caesium chromate and chromium telluride), and PE16 was more resistant than M316 steel to corrosion.

Magnetic and electrical properties of a new chromium telluride phase: CrTe 2

A new material, CrTe 2, is prepared by using a rapid precipitation metathesis reaction between the Zintl material K 2Te 2 and CrCl 2 in solution. The d.c. conductivity of this material exhibits semiconductivity and is dominated by the variable range hopping mechanism over a wide temperature region. This material undergoes a transition to a spin glass state at 18 K, that is characterized by d.c. susceptibility measurement as well as the behavior of the remanent at magnetization fields in the range 0–5 Tesla.

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Electronic band-structure calculations are presented for the ferromagnetic compounds CrTe, Cr3Te4 and Cr2Te4 and Cr2Te3. In these compounds the Cr3d-Te5p covalency and the Cr3d(z2)-Cr3d(z2)-overlap along the c axis are the most important interactions. The magnetic polarisation of Te is parallel to the Cr local moment in CrTe, antiparallel to it in Cr2Te3 and about zero in Cr2Te4. Measurements of electronic transport properties (resistivity, Hall effect and thermo-electric power) and magnetic properties of Cr1- delta Te( delta =0.1) and Cr3+xTe4(x=0.2) indicate that these chromium tellurides are p-type metals, with strong interaction between the holes in the Te5p band and the Cr magnetic moments. In the literature the variation of the magnetic properties of Cr3.2Te4 near Ts approximately=100 K has been attributed to a change from a canted antiferromagnetic to a collinear ferromagnetic structure. However, our Hall-effect measurements indicate that the spin structure is not collinear ferromagnetic above Ts.

Electronic band-structure calculations of some magnetic chromium compounds

Band-structure calculations of CrS, CrSe, Cr3Se4 and CrSb are presented. Together with the accompanying results for the chromium tellurides, these calculations give a coherent picture of the changes in the electronic structure caused by anion substitution and by introduction of cation vacancies. The importance of the Cr-X covalency and the 3d(z2)-3d(z2) overlap of Cr neighbours along the c axis is stressed. Further, the bandstructure calculations shed some light on the formation, the variation in magnitude and the coupling of the Cr magnetic moments and the indirect magnetic polarisation of the anion bands.

Thermochemistry of metal-rich chromium telluride and its role in fuel-clad chemical interactions

Vaporisation of Cr-Te alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te 2(g) and Te(g) over the two-phase field, Cr + CrTe x , were determined in the temperature ranges 1015 to 1138 K and 1180 to 1285 K, respectively. The temperature dependencies of the partial pressures have indicated that nearly equimolar proportions of Te and Te 2 are present in the vapor phase and that there is a phase transformation in CrTe x at 1160 ± 20 K. The Cr-rich phase boundaries of the nonstoichiometric CrTe x were delineated at 1075 K (50.72 ± 0.7 at% Te) as well as at 1235 K (48.25 ± 0.9 at% Te) by a continuous monitoring of the intensities of Te + and Te + 2 as a function of time, starting with samples having 55.63 and 57.22 at% Te. Enthalpies and Gibbs energy changes were derived for the equilibria. CrTe x(s) ai Cr(s) + ( x i )Te i(g) [ x = 1.029 and 0.932; i = 1 and 2] and Te 2( g) ai 2 Te( g). Enthalpies and Gibbs energies of formation of CrTe 1.0.29 and CrTe 0.932 were arrived at. The tellurium potentials which would be required for the formation of MTe x ( M = Fe, Cr, and Ni) in Type 316 stainless steel and those likely to exist in the fuel-cladding gap of a mixed-oxide fuel pin were computed.

Chemical reactions and Schottky barrier formation at Cr/n-CdTe interfaces

The formation of Schottky barriers at Cr/n-CdTe interfaces has been investigated in detail using angle-resolved ultraviolet photo-emission, soft X-ray photo-emission and current-voltage techniques. The CdTe surfaces studied were prepared by cleaving in ultra-high vacuum, cleaving in air, and by chemical etching. The results indicate strong chemical reactions between both CdTe and its native oxide (TeO2) with evaporated Cr layers. A typical high-coverage Cr/clean-CdTe interface consists of a mixture of cadmium chromium telluride and metallically bonded cadmium. In the case of oxidised CdTe surfaces, oxides of chromium and chromium telluride also exist at the interface. The resulting barriers are less than about 0.35 eV, showing Ohmic behaviour, and are almost independent of the surface preparation. These chemical reactions at the interface are discussed on the basis of the published bulk thermodynamic data.

Optical and magneto-optical properties of thin film chromium telluride

Two new techniques for the preparation of thin films of chromium telluride on glass substrates are described. The ferromagnetic properties of the films are discussed briefly, and the results of an ellipsometric determination of the basic complex optical and magneto-optical constants, n and Q, in the visible region of the spectrum are presented. The values obtained for the constants are used to evaluate the merit of the material as a magneto-optical storage medium by calculating the signal-to-noise ratios associated with various simple readout systems. The results are then compared with those obtained by carrying out identical calculations for the intermetallic compound manganese-bismuth.

Thermodynamic properties of the chromium telluride Cr2Te3

In an investigation, based on measurements of the emf of concentration galvanic cells, determinations were made of the isobaric-isothermal potential, heat, and entropy of formation of the chromium telluride Cr2Te3 over the temperature range 350–450°C.

Chemical stability of tellurides of subgroup VIa transition metals in various corrosive media

Tellurides of Subgroup VIa transition metals are fully decomposed by oxidizing acids. The reactions of hydrochloric acid and dilute sulfuric acid with chromium telluride are accompanied by hydrogen telluride evolution, which is not observed in the reactions of these acids with molybdenum and tungsten tellurides. The hypothesis is advanced that the proportion of ionic bonding is higher in chromium telluride than in molybdenum and tungsten tellurides.