Chromium (III) acetylacetonate Cr(acac)3 is violet crystalline solid with melting point 210-214°C and boiling point 340°C, vapor pressure is 3.328 Torr at 150°C; it can be sublimed without residue.
Semyannikov et al. measured equilibrium sublimation pressure of Cr(acac)3 (ln(P(Pa))= 39.197 − 15308.5/T(K)) at 320-476 K by combination of Knudsen's effusion procedure with gas phase composition mass spectrometric analysis (better for measuring low pressure) and the mass spectrometry-based “calibrated volume method” (CVM) (more efficient for high pressures). [963] Similar vapor pressure values for Cr(acac)3 were measured at 345-410K by Knudsen effusion method (Fig.) [856, 964] Pankajvalli et al. [[i], ref 132 in Fig] reported much lower vapor pressure, while Malkerova [[ii], ref126 in Fig] obtained much higher values by a non-recommended isoteniscope procedure.
The sublimation enthalpy of [Cr(acac)3] based on vapor pressure measurements is 128.20 kJ/mol at 345-410K [856], similar values were reported by Malkerova [[iii]], Semyannikov (127.28 ± 0.22 kJ/mol) [963] and (except lower temperature data) - by Lazarev et al. [[iv]] Sligthly different values were reported by Melia and Merrifield [[v], ref 133 in Fig], Quite low value of heat of sublimation 111.6±3 kJ/mol was estimated by the transpiration method. [844], what can be related to undersaturation of the carrier gas with the sample vapor.
The dependences of mass transfer rate during sublimation of film samples of Cr(acac)3 versus carrier gas flow rate (0.5–40 l/h), temperature (162–242∘C), aggregation state of precursor (powder, crystal, melt) and size of evaporation surface were studied. The evaporation enthalpy 79.4±4.2 kJ/mol and the sublimation enthalpy 133.8. ±-4.2 kJ/mol were reported. [[vi]] An empirical correlation betwen Cr(acac)3 mass transfer rate (sublimation rate) and crystallite dimensions was proposed: J=Const×T2.18×V0.43×exp(-ΔH/RT)×W1.62×L0.386 (W and L are width and length of a sample) [[vii]].
The sublimation entropy ΔS°sub (T) = 230.1 ± 0.5 J/molK was reported [963]
The diffusion coefficients of Cr(acac)3 in N2 and He were studied; the unexpectedly low values (DAB (cm2s-1)=4.45×10-4 T0.82 (Cr(acac) 3 in nitrogen). DAB (cm2s-1 )=2.93×10-4 T1.03 (for Cr(acac)3 in helium) (Fig) might be explained by some compound dimerization; however, no experimental evidence for dimerization is known from the literature. [[856,964]
The heat capacity of Cr(acac)3 was measured by the adiabatic method at 5–320 K, an anomaly at ∼60 K indicating phase transformation was observed. The Raman spectra were measured at 60–400 cm−1 frequency and 5–220 K temperature, a new line at 109 cm−1 appears at ∼60 K. Experimental heat capacity data were used for calculation of the thermodynamic functions (entropy, enthalpy and reduced Gibbs energy) at 298.15 K. [965]
[i] R. Pankajavalli, C. Mallika, O. M.Shreedharan, P. AntonyPremkumar, K. S. Nagaraja, V. S. Rangunath, Chem. Engg. Sci., 57, (2002) 3603.
[ii] I. P. Malkerova, A. S. Alikhanyan, V. B Lazarev, V. A. Bogdanov, V. I. Gorgoraki, Ya. Kh. Grinberg, Russ. J. Phys. Chem., 292, (1987) 376 (Engl.Transl.).
[iii] I. P. Malkerova, A. S. Alikhanyan, V. G. Sevastyanov, Ya. Kh. Grinberg, V. I. Gorgoraki, J. Inorg. Chem.USSR,, 35 (1990) 413 (Engl.Transl.).
[iv] V. B Lazarev, J. H. Greenberg, Z. P. Ozerova, G. A.Sharpataya, J. Therm. Anal.,33, (1988) 797.
[v] T. P. Melia, R. Merrifield, J. Inorg. Nucl. Chem., 32, (1970) 1489.
[vi] N. E. Fedotova, N. V Gelfond, I. K Igumenov, A.N Mikheev, N.B Morozova, R.H Tuffias, Internat. J. Therm. Sci., Vol. 40, Iss. 5, May 2001, Pages 469-477
[vii] N.E. Fedotova, A.N. Mikheev, N.V. Gelfond, I.K. Igumenov, N.B. Morozova, R.H. Tuffias, J. Phys. IV France 09 (1999) Pr8-251-Pr8-258, DOI: 10.1051/jp4:1999831 Proceedings of the Twelfth European Conference on Chemical Vapour Deposition
Cr(acac)3 for Cr metal CVD
Cr(acac)3 was applied as the precursor for the Plasma Assisted (PA) MOCVD growth of Cr metal layers at a substrate temperature 550 °C and plasma power density 70 mW/cm2. Nanocrystalline, free from pores/ cracks Cr films with 1200 HV hardness were obtained. [966]
Cr(acac)3 for Cr2O3 CVD
Thin Cr2O3 films were prepared from chromium acetylacetonate by MOCVD on stainless steel as corrosion protection layers at high temperatures Deposited film growth kinetics depended on the substrate temperature, gas flow rate and substrate position in the reactor. The influence of the deposition parameters on the growth rate and film uniformity was studied. [967]
Shiny smooth chromium oxide Cr2O3 thin films were prepared by atmospheric-pressure CVD method from Cr(acac)3 at a process temperature above 400°C. The films exhibited a significant solar absorptance combined with a high infrared reflectance, necessary for efficient conversion of solar energy into heat. [968]
Cr2O3 coatings were grown on SiC and Al2O3−SiO2 based ceramic fibers by thermal MOCVD in air at 500°C. The coated fibers exhibited some reduction in tensile strength but still had considerable strength for use in composites. [969]
Cr(acac)3 for Cr2O3 by ALD
Chromia (Cr2O3) thin films as modification layers of porous oxides supported on metal substrates (monoliths) were prepared from Cr(acac)3 by ALE [970]
Cr(acac)3 for CuCrO2 CVD
CuCrO2 crystalline thin films were grown on glass substrates by LP-MOCVD using Cr(acac)3 combined with Cu(acac)2. They exhibit p-type conductivity by Hall (unlike other transparent conducting oxides having n-type conductivity) with the highest carrier Hall mobility reported for p-type transparent conducting oxides with Cu-delafossite structure. Growth rate of 13 nm/min was achieved at 550°C process temperature. (101), (012), (104), and (110) refelctions were found by XRD. The direct optical bandgap of 3.08 eV was calculated. [971]
Cr(acac)3 for CrN by CVD
Hard nanocrystalline chromium nitride (CrN) thin films were grown on stainless steel and YSZ substrates using plasma assisted (PA) MOCVD technique, by using a volatile mixture of Cr(acac)3 and either NH4I or ammonium bifluoride (NH4HF2) as precursor. Nitrogen and hydrogen were used as carrier gases; growth rate up to ~0.9 μm/h was achieved in optimum conditions. Quite smooth surface morphology was observed for CrN films deposited on polished YSZ substrates, whereas microstructure of the coatings grown on well-polished stainless steel contained globular particles. The deposited CrN films were characterised by XRD, microhardness studies and microscopy. [973]
Cr(acac)3 for Cr-doped Al2O3 (ruby) by CVD
Chromium-doped aluminum oxide (ruby) thin films were deposited from Cr(acac)3 and oxygen on Si(100) and stainless steel substrates by cold-wall CVD at temperatures 1000-1200°C and pressures 200-240 hPa (200-240 mbar). Al2O3 films containing 1 % Cr were grown according to XRD, SEM and EDX spectroscopy. Mainly γ-Al2O3 and θ-Al2O3 films were grown at 1000°C, whereas at higher temperatures α-Al2O3 were obtained. The temperature-dependent luminescence in the films was studied by laser excitation. α-Al2O3:Cr films are promising as temperature sensors, the typical phosphorescence of ruby with phosphorescence lifetimes between 2.3 ms at 298 K and 6 μs at 813 K was observed. [972]
Cr(acac)3 for Cr-doped ZnO by CVD
Chromium acetylacetonate Cr(acac)3 was applied as chromium precursor for the preparation of Cr-doped ZnO powders (nanocrystalline particles) by chemical vapor synthesis (CVS) which is a modified CVD process. Increasing dopant concentration resulted in the decrease of grain size. Doping level also has slight influence on the lattice constants (extracted by the Rietveld method from XRD data). Cr dopant atoms are incorporated into the wurtzite host lattice according to XRD and EXAFS. [974]
Chromium tris(trifluoroacetylacetonate) Cr(tfac)3
Chromium tris(trifluoroacetylacetonate) Cr(tfac)3 is solid with melting point 185°C (other data 140°C). Its calculated vapor pressure 9.910 Torr at 150°C, the estimated sublimation enthalpy and entropy are: ΔHsubl 71 kJ/mol, ΔSsubl 186 J/molK. [980]
Cr(tfac)3 has been studied by the quasi-equilibrium thermogravimetry (with vapour pressure-calibrated sample holders); ‘p-T’ relationships (in the range 0.0006 to 0.11 atm) were obtained; evaporation enthalpy and entropy were measured.[[i]]
Cr(tfac)3 (Cr(tfa)3) has been considered as potential precursor for CVD of Cr-containing layers. Its average decomposition temperature in vapor phase with Cr2O3 formation is 315°C (in Ar, O2 atmosphere). [975]
[i] V. A. Logvinenko, G. V. Gavrilova, N. B. Morozova, J. Therm.Anal. Calorim., vol. 52, Number 2, 341-344, DOI: 10.1023/A:1010101724203
Chromium tris(hexafluoroacetylacetonate) Cr(hfac)3
Chromium (III) hexafluoroacetylacetonate Cr(hfac)3 is a volatile solid with melting point 84°C and calculated vapor pressure 29.511 Torr/ 150°C. Its sublimation enthalpy and entropy were estimated: ΔHsubl 46 kJ/mol, ΔSsubl 134 J/molK[980] Crystal structure of Cr(hfac)3 was determined: it is octahedral compound with a Cr–O distance 1.943(5) Å. [977]
Cr(hfac)3was applied as a precursor for the growth of chromia (Cr2O3) thin 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. All films were crystalline with hexagonal Cr2O3 eskalonite structure; XRD results were confirmed by FTIR (absorption bands characteristic of Cr2O3) and XPS (oxidation state +3 of chromium). Films were analyzed by XRD, AFM, RBS, nuclear reaction analysis (NRA), elastic recoil detection (ERD), XPS, and FTIR spectroscopy [978]
Cr(hfac)3 (Cr(hfa)3) has been reported to be useful for the deposition of Cr2O3 films on Si and SiO2 substrates at 100°C deposition temperature by Supercritical Fluid Transport Chemical Deposition, using N2O as supercritical fluid solvent.[[i]].
Cr metal layers were reported to be photochemically derived from Cr(hfac)3[[ii]]
[i] B. N. Hansen, B. M. Hybertson, R. M. Barkley, R. E. Sievers, Chem. Mater. 1992, 4, 749
[ii] J. Cheon, H.-K. Kang, Jeffrey I Zink, Coord. Chem. Rev., Vol. 200-202, May 2000, Pages 1009-1032
Chromium tris(2,2,6,6-tetramethyl-3,5-heptanedionate) Cr(thd)3 (Cr(tmhd)3)
Chromium tris(2,2,6,6-tetramethyl-3,5-heptanedionate) Cr(thd)3 is one of the most useful precursors for the MOCVD of chromium-contaning layers.
Cr(thd)3 – crystal structure
The crystal structures and thermal stabilities of six polymorphs of Cr(thd)3 have been studied using single crystal X-ray diffraction and differential scanning calorimetry. [979] Polymorph 1 (plate-shaped crystals) is thermodynamically stable below ca. 110 °C and forms [a = 9.927(5), b = 18.010(5), c = 21.427(5) Å, β = 95.461(5)° at 295 K; space group C2/c]. Its crystal structure is isotypic with Mn(thd)3. Polymorph 4 (needle shaped) is metastable to 1 and was prepared by precipitation from solution. [a = 28.54(3), b = 19.14(2), c = 21.92(2) Å, β = 97.31(2)° at 295 K; space group C2/c] This structure is isotypic with monoclinic Co(thd)3. Modifications 2, 2*, 5 (needle-shaped crystals): orthorhombic for 2 [: a = 18.97(7), b = 18.69(7), and c = 10.62(5) Å], tetragonal for 5 [a = b = 18.93(5) and c = 10.57(4) Å], both at 110 K]. Crystals of 2 and 5 have limited lifetime, which depends on storage temperature and exposure to X-rays. The conversion sequence is: 5 → 2 → 1; the first step takes 3-4 h, the second ~20 h. The structural distinction between 2 and 2* is associated with rotational disorder among the Cr(thd)3 molecules. 2* and 3 are high-temperature modifications with stability range ca. 155–200 and ca. 200–235 °C, respectively [Cr(thd)3 melts at ca. 235 °C]. Quenched 2* can be retained as metastable at room temp. for some weeks, whereas 3 is not quenchable [3 is cubic [a = 13.357(4) Å at 225 °C; with strong rotational disorder of the molecules] according to High-temperature PXD data. There are only minor variations in bond lengths and angles between 1 and 4 or from 100 to 295 K, disorder is limited.
Cr(thd)3 – thermal properties
Thermal properties of Cr(thd)3 (sublimation temperature, decomposition temperatures), as a promising precursor for ALCVD of chromium-containing layers, were studied by thermogravimetry under vacuum. [[i]]
Thernal stability (by TGA), vapor pressure, gaseous diffusion coefficients) of Cr(thd)3 were in detail investigated by Siddiqi et al. [[ii]]
The heat of sublimation of Cr(thd)3 derived from the vapor pressure equation was 127.45 kJ/mol (340-405 K). The gaseous diffusion coefficient of [Cr(tmhd)3] was measured in helium atmosphere only (because the mass loss in nitrogen was very small at low temperatures). [[iii]]
.
The gaseous diffusion coefficient of [Cr(tmhd)3] in nitrogen was estimated using hard sphere model; they are expected to be lower than in helium.
[i] O. Nilsen, H. Fjellvåg, A. Kjekshus, Thermochimica Acta, Volume 404, Issues 1-2, 4 September 2003, Pages 187-192
[ii] M.A. Siddiqi, R. A. Siddiqui, B. Atakan. J. Chem. Eng. Data 55 (2010), 2149-2154.
[iii] R.A. Siddiqui, PhD Thesis, Univerity Duisburg-Essen, 2009, http://duepublico.uni-duisburg-essen.de/servlets/DerivateServlet/Derivate-22988/Rehan_Dissertation.pdf
Cr(thd)3 for Cr2O3 by MOCVD
Chromium tris(2,2,6,6-tetramethyl-3,5-heptanedionate) Cr(thd)3 (dissolved in the 9:1 THF:tetraglyme as solvent) has been considered as potentially applicable as precursor for the MOCVD growth of Cr2O3 thin films [981]
Indeed, Cr(thd)3was successfully utilised as MOCVD precursor for the growth of Cr2O3 thin films as anti-wear protection layers on AISI 304 stainless steel, soda-lime glass and Si(001) in the hot-wall reactor at growth temperature 500 °C, process pressure 3 torr, using O2 mixture with H2O vapor as the reactant gas and N2 carrier gas. According to XRD measurements, all layers had hexagonal Cr2O3 eskalonite crystalline structure, confirmed by XPS (Cr in oxidation state +3) and FTIR (Cr2O3 characteristic absorption bands). [978]
Cr(thd)3 for (Cr,Zn)O by MOCVD
(Cr,Zn)O films have been grown by MOCVD on Si(100) substrates, by using simultaneous deposition of 0.15 M Cr(thd)3 and 0.025 M Zn(thd)2 in THF solvent (with flow ratio of 1:10) in the oxidizing environment. Nanocrystalline ZnCr2O4 and disordered Cr2O3 formed as the secondary Cr-containing phases, rather than uniform Cr doping. ZnCr2O4 crystallites dispersed throughout the film, disordered Cr2O3 layer may form at the interface. The results are consistent with Cr exhibiting octahedral, rather than tetrahedral coordination with O. [982]