Chromium carbonyls – vapor pressure
Assessment of vapor pressure data of solid metal carbonyls
Raja Chellappa , Dhanesh Chandra
The Journal of Chemical Thermodynamics, Volume 37, Issue 4, April 2005, Pages 377–387
http://dx.doi.org/10.1016/j.jct.2004.10.002,
FIGURE 1. (a) Clausius–Clapeyron (C–C) plot of available experimental vapor pressure data of Cr(CO)6 (265 K < T < 427 K), (b) MOAR representation of the same data, (c) individual MOAR representation of the data. ×, Garner et al.[10]; +, Boni [17]; □, Cordes and Schreiner [16]; – –, Windsor and Blanchard [7]; ◊, Hieber and Romberg [15]; ∘, Baev et al.[13]; – - – - , Rezukhina and Shvyrev [14]; —, arc drawn by ln fMOAR of the regression equation determined by CG analysis
Chromium (0) hexacarbonyl (hexacarbonylchromium) Cr(CO)6
Chromium hexacarbonyl Cr(CO)6, is colorless or white sublimable solid, stable to air and moisture, melting at 164°C and decomposing over 180 °C (other data >150°C?) and dissolving in polar organic solvents.
It is synthesized by reduction of CrCl3 by Al at 300°C under high pressure of carbon monooxide (200-300 atm): CrCl3 + Al + 6 CO → Cr(CO)6 + AlCl3
Cr(CO)6 vapour upon thermal decomposition at low temperature (<300°C) produces a metallic phase with a face-centred cubic structure, distinct from the body-centred cubic structure of the pure metal. Solid pyrolysis products were identified as insterstitial oxycarbide (equimolar amounts of carbon and oxygen were found by microanalysis); the oxygen derives from the CO ligand of the precursors. [901]
Thermodynamic properties of Cr(CO)6 in the gaseous state from 100 to 600 K: C0p, S0, -(G0-H0Q)/T, H0-H0f, ΔH0f, ΔG0f, logKp were reported in [902]
The infrared absorption spectra of the hexacarbonyls of Cr(CO)6, werr studied in the gaseous and solid states; most of fundamental frequencies were assigned. Force constants were calculated using a resonance interaction potential function. Cr-C bond force constant is lower than in W(CO)6, but higher than in Mo(CO)6; a higher metal-carbon force constant is accompanied by a lower CO force constant, indicating a decrease in metal-ligand π bonding in the same order as the metal-carbon force constants. [903]
Cr(CO)6 has been applied as CVD source: Cr(CO)6 provides low deposition temperatures of 400°C, but the chromium films obtained can be contaminated with carbon, chromium carbide and chromium oxide. This can be explained by catalytic dissociation of the carbonyl CO taking place on the metal:
Cr(CO)6(g) → Cr(s) + Cr2O3(s) + Cr2C3(s) + CO(g)
This shows that metal carbonyls are only good precursors for those transition metal elements which do not allow surface dissociation of CO. It may be possible to minimize the C and O impurities upon addition of, for example, H2O.
H2O(g) + C(s) → H2(g) + CO(g)
Cr(CO)6 for Cr metal CVD
Hexacarbonylchromium Cr(CO)6 was the first organochromium compound used for low temperature (250°C-710‘C) chromium deposition. Without additive gas, coatings were usually described as a mixture of Cr metal, carbide (Cr3C2) and oxide (Cr2O3), with proportions dependent on the experimental conditions. In order to minimize homogeneous decomposition of carbonyl, cold-wall reactors were used. [904]
Thus, chromium coatings were grown from Cr(CO)6 at 250 and 650°C ; analyses revealed large amount of oxygen coming froncarbonyl groups. [904b]
Black chrome selective absorber coatings for high temperature photothermal energy conversion were grown on steel substrates from Cr(CO)6 by atmospheric pressure CVD in the presence of oxygen. The coatings demonstrated a significant solar absorptance combined with a high infrared reflectance necessary for efficient conversion of solar energy into heat; coatings remained intact after lifetime testing at 600°C in air for 1000 h. The coatings were characterised by the measurement of thermal optical properties (α, ϵ), XRD and by surface analysis using SEM and AES. [904e]
Cr(CO)6 for CrO2 CVD
Cr(CO)6 has been applied for CrO2 film deposition by laser- or plasma-assisted CVD at very low temperatures (15-21°C only; at 42°C film surface was rough). Chromium hexacarbonyl vapor was introduced into a vacuum deposition chamber at 10 mTorr, oxygen is introduced at 15-100 milliTorr. Chromium oxide layers was produced photolytically (by focussing UV laser beam onto a substrate) or using RF plasma; CrO2 were shown to be ferromagnetic according to measurements of magneto-opt. Kerr effect. [905, 905]
CrO2 thin films were grown at RT on sapphire substrates by laser-assisted CVD using KrF excimer laser photodissociation of Cr(CO)6, in dynamic O2 and Ar atmospheres at 10-5-10-1 mbar working pressures. CrO2 with significant amount of Cr2O3 was deposited at RT at 0.5 mbar. [869]
Cr(CO)6 for Cr2O3 CVD
Cr(CO)6 was applied for the frowth of thin chromia (Cr2O3) films as anti-wear protection layers on AISI 304 stainless steel, soda-lime glass and Si(001) substrates by hot-wall MOCVD at growth temperature 500 °C, process pressure 3 torr, using O2 mixed with H2O vapor as the reactant gas and N2 as the carrier gas. The highest growth rate obtained was 20 nm/min. Cr2O3 were grown according to measurement results: XPS (oxidation state +3 of chromium), FTIR (Cr2O3 characteristic absorption bands), and XRD (hexagonal Cr2O3 eskalonite structure). Films grown on stainless steel were analyzed by nanoindentation measurements and scratch test to determine the hardness and the film adhesion. [978]
Cr(CO)6 leads to oxide films by PECVD even in the presence of excess of H2[[i]]
Cr2O3 films were prepared by by atmospheric-pressure CVD method from chromium hexacarbonyl. The lower limit of the deposition temperature was 150°C, which could be reduced to 60°C by irradiation with UV light. The films exhibited preferred overgrowth with a vertically developed three-dimensional coarse structure. [968]
Cr(CO)6 for Cr oxycarbide and oxycarbonitride by CVD
Cr(CO)6 solutions in toluene or THF were used for the growth of nanocrystalline chromium oxycarbide (without NH3 addition) and oxy-carbonitride (with NH3) on stainless steel substrates by atmospheric pressure cold wall direct liquid injection (DLI) MOCVD at temperatures below 450 °C. The influence of injection parameters and general CVD conditions on the chemical, physical and structural coating characteristics was studied by XRD, SEM and EPMA. [907, 914]
Cr(CO)6 for CrP CVD
Chromium hexacarbonyl Cr(CO)6, in combination with cyclohexylphospine PH2Cy has been used for the CVD growth of crystalline chromium phosphide CrP thin films. [577]
Cr(CO)6 for Cr-doped InP MOVPE
Hexacarbonylchromium has been applied as Cr source for the MOVPE growth of chromium-doped semi-insulating InP . A compensating deep donor concentration of up to 3 × 1016 cm−3 with resistivity as high as 3×108 Ω cm was achieved. Cr-doped InP can be employed as a current blocking layer in buried heterostructure lasers with improved performance compared to Fe containing structures. [908]
Cr(CO)6 for Cr-doped GaAs MOVPE
Hexacarbonyl chromium proved to be useful for controllable doping of GaAs grown epitaxially by MOCVD using metalalkyl - hydride vapour system. The concentration of incorporated Cr and the solubility saturation level were determined. Semi-insulating epitaxial GaAs:Cr layers were applied as buffer layers in MESFETs. [909]
[i] H. Suhr, J. Bald, L. Deutschmann, A. Etspüler, E. Feurer, H. Grünwald, C. Haag, H. Holzschuh, C. Oehr, S. Reich, R. Schmid, I. Traus, B. Waimer, A. Weber, H. Wendel
JOURNAL DE PHYSIQUE, Colloque C5, supplément au n05, Tome 50, mai 1989
Chromium (VI) oxide (chromium trioxide) CrO3
The crystal structure of CrO3 consists of infinite chains of corner-sharing CrO4 tetrahedra running parallel to the c axis. The bridging Cr-O bond length is 1.748 Ǻ, the terminal Cr-O length is 1.599 Ǻ. The angle at the brodging O atom is 143°. [858]
Chromium trioxide CrO3 with O2 as carrier gas (200sccm), has been applied as precursor for the selective atmospheric-pressre CVD growth of epitaxial CrO2 (110) thin films on MgO(001) substrates with TiO2(110) buffer layers obtained by oxidizing TiN(001) thin films upon annealing process in low-pressure oxygen. XRD and SEM revealed that CrO2(110) films are orthogonally twinned and formed with small grains of CrO2 of 3-5 µm size. Epitaxial relationship of the sample was CrO2(110)[001]∥TiO2(110)[001]∥MgO(001)[110]
Chromium trioxide CrO3 has been used for the deposition of CrO2 thin films on sapphire (0001) at growth temperature 390°C (just below decomposition temperature of CrO2 to Cr2O3), what allowed to obtain epitaxial CrO2 films. The precursor was evaporated at 260°C (melting point of CrO3) and trasported to the deposition area by the O2 carrier gas flow of 500sccm.
Highly oriented a-axis CrO2 films (containing highly oriented (0001)Cr2O3) were grown on Al2O3(0001) substrates by atmospheric pressure (AP) CVD at low deposition temperature (330 °C) using CrO3 as precursor (vaporised at 260°C), and oxygen as carrier gas. Deposition rate was significantly varying with the substrate temperature, whereas film surface microstructure depended mainly on film thickness. CrO2 growth kinetics was dominated by a surface reaction mechanism with activation energy of 121.0 ± 4.3 kJ/mol. Sharp magnetisation transition at 375K and saturation magnetisation of 1.92 µB/f.u., close to the bulk value of 2µB/f.u. were observed.The presence of a Cr2O3 layer at the CrO2 film/interface was explained by some structural effects during the study of the growth process as a function of the deposition time [861]
Epitaxial CrO2 films were grown by atmospheric pressure CVD using thermal decomposition of gaseous CrO3 onto rutile (TiO2) single crystals in air. Pure CrO2 epitaxial films were produced the optimum substrate temperature of 390°C; layers included Cr2O5 impurity at growth temperature of 380°C, and Cr2O3 appeared at 400°C. Magnetic domain patterns of these films were studied by longitudinal Kerr effect. [863]
Thin epitaxial CrO2 films (a thickness series 27-535 nm) were deposited on single-crystal rutile TiO2(100) substrates by CVD using CrO3 as a solid precursor; layer magnetic anisotropy was strongly thickness-dependent according to the ferromagnetic resonance (FMR) studies. In the thinnest films (27 nm), the strain-induced anisotropy was predominant and the easy magnetization axis switched from the [001] direction (characteristic of the bulk magnets) to the [010] direction. [864,865]
Solid CrO3 precursor was used for the CVD growth of epitaxial CrO2 (100) and CrO2 (110) films on TiO2 (100) and TiO2 (110) substrates, respectively. Layer-by-layer growth mode was observed on TiO2 (100) resulting in smooth surfaces but significant out-of-plane compressive stress. In contrast, films on TiO2 (110) follow an islandlike growth mode; even the thinnest films (∼35 nm) are practically strain free. CrO2 (100) films display strong change of magnetic anisotropy with increasing thickness due to the substrate-induced stress, in contrast to the CrO2 (110) films. [866]
CrO3 was apllied as the precursor for the atmospheric-pressure CVD growth assisted by AAO templatesof the high-density vertically aligned CrO2 nanowire arrays which presented significantly improved coercivity compared with CrO2 films or bulk. Nanowire lenght was strongly influenced by the AAO template pore diameter. [867]
CrO2 thin films were deposited using CrO3 precursor by laser-assisted CVD [868]
CrO3 powder as precursor was used for the preparation of CrO2 films by thermal CVD on sapphire at 320 - 410 ºC temperatures and 50 - 500 sccm O2 carrier gas flow. Highly oriented a-axis CrO2 films were prepared at temperatures as low as 330 ºC; they kept high quality magnetic and transport properties as those deposited at higher temperatures. [869]
High-quality epitaxial CrO2(100) films were prepared on TiO2(100) substrates by CVD using CrO3 as precursor material. The spin-resolved electronic structure of the films was investigated by means of x-ray absorption and spin-resolved photoemission spectroscopy. Near EF an energy gap was observed for spin-down electrons; a spin polarization of about +(90 ± 10)% was found at 293 K, in agreement with the half-metallic nature of CrO2. [870]
Epitaxial CrO2/RuO2 thin film heterostructures were grown on TiO2(100) substrates by atmospheric pressure CVD using CrO3 and Ru(thd)3 precursors. Current-in-plane and current-perpendicular-to-plane giant magnetoresistive stacks were fabricated with either Co or another epitaxial CrO2 layer as the top electrode, however, magnetoresistance was low, probably due to the appearance of a chemically and magnetically disordered layer at the CrO2 and RuO2 interfaces when Cr2O3 was transformed into rutile structures during its intermixing with RuO2. [871]
Decomposition of CrO3 on the substrate contained in a pressure vessel, which can be considered as a kind of CVD, was used for the epitaxial growth of oriented layers of CrO2 on on the {0001} planes of single crystal Al2O3 and Fe2O3{100}, and on the {110}, {210}, {001} surfaces of single crystal TiO2 (rutile). The CrO2 layer begins to form during the decomposition of Cr2O5 by nucleation of oriented CrO2 at many sites on the substrate surface, with the perfection of the oriented layer being limited mainly by the defects on the surface of the substrates. Film characterization by chemical analysis and XRD indicated that the epitaxial CrO2 is identical in all respects to the bulk material; the oriented CrO2 layers have been characterized by magnetic measurements [[i][PS1] ]
Octachromium unicosoxide Cr8O21
Octachromium unicosoxide Cr8O21 is the intermediate oxide between CrO3 and CrO2, it was possible to be prepare it ex situ, store and use directly for the CVD growth. Cr8O21 is formed during CrO3 decomposition (i.e. CrO3 does not decompose directly to CrO2 and oxygen, as it had been previously thought). Ivanov proposed the hypothesis that the role of Cr8O21 in the CVD process is to exude unstable molecules of CrO4, and that the reaction on the substrate is the decomposition CrO4→ CrO2 + O2.
Octachromium unicosoxide Cr8O21 has been applied for the CVD growth of highly ordered CrO2 films on TiO2(110), TiO2(100), and Al2O3(0001) substrates, as well as variety CrO2-containing heterostructures such as superconductor/insulator/CrO2 and Co(polycr.)/AlOx/CrOx/CrO2(100)/substrate-TiO2(100). [872]
[i] R.C. DeVries, Materials Research Bulletin, Volume 1, Issue 2, October 1966, Pages 83-93
[PS1]R.C. DeVries, Materials Research Bulletin, Volume 1, Issue 2, October 1966, Pages 83-93
Oriented layers of CrO2 have been grown epitaxially on {100}, {110}, {210} and {001} surfaces of single crystal TiO2 (rutile) and on the {0001} planes of single crystal Al2O3 and Fe2O3 by the decomposition of CrO3 on the substrate contained in a pressure vessel. The area of these layers is limited primarily by the size of the single crystal substrates. Beside meeting structural requirements the substrate must not form stable compounds with molten CrO3 or the other chromium oxides resulting from CrO3 decomposition. The CrO2 layer begins to form during the decomposition of Cr2O5 by nucleation of oriented CrO2 at many sites on the substrate surface. Defects on the surface of the substrates limit the perfection of the oriented layer. Characterization of the films by both chemical analysis and x-ray techniques indicate that the epitaxial CrO2 is identical in all respects to the bulk material. The oriented layers have been used for magnetic measurements.