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Narrow band optical absorption of CrF2
Spectrochimics
Acta, 1966, Vol. 22, pp. 1381 to 1382. Pergamon Press Ltd. Printed in Northern Ireland
RESEARCH NOTES
Narrow
band
optical
absorption
of CrF,*
(Received 11 3March 1965) LITTLE information is available on the optical absorption of the [A]( 3~2)~electronic configuration, especially the Cra’ ion, and only tentative a&gnments have been given to the reported transitions [l]. Recently C-K [2] has observed broad absorption bands in the reflectance spectra of powder samples of CrF, and CrCl,. In this note we report observations of the optical absorption
‘1: 4.500 5,000
6,000
7,000
6,000
9,000
WAVELENGTH
l0,000
II,000
12,000
13,000
(A,
Fig. 1. The absorption spectrum of CrF, in the spectral region from 0.45 p to 1.3 p. The sharp lines, not completely resolved in this spectrum, are indicated by arrows. No additional structure is observed to 3.3 ,u. of crystals of CrF, from 0.45 ,u to 3.3 ~1. Two unresolved bands, and a third band can be distinguished in the absorption spectrum, in agreement with results of the reflectance measurements. In addition, four very narrow absorption lines are observed in the visible region. Platelets of CrF, were prepared by the temperature gradient method from CrF, powder in an inert g&s atmosphere. The X-ray spectra showed no evidence of other phases present in these crystals. The room temperature absorption spectrum of CrF, crystals from 0.45 ,u to 1.3 p is shown in Fig. 1. This spectral proflle was measured with a Beckman DK-2A spectrophotometer equipped with a special oondensing system for investigation of small samples. The broad * This work, supported by the U.S. Office of Naval Research, Contract No. Nonr-4127(00), is part of Project DEFENDER under the joint sponsorship of the Advanced Research Projects Agency, the Office of Naval Research and the Department of Defense. [l] C. J. BALLHAUSEN, Ligand Field Theory, McGraw-Hill, [2] R. J. H. &BRIE, J. Chem. Sot. 1964, 417 (1964). 1381
New York (1962).
1382
Research notes
absorption band with the maximum at roughly 7000 A appears to be composed of two unresolved absorption bands. Another band beginning (with decreasingwavelength) at 4750 A is present. In addition, four sharp lines are observed, which are indicated by arrowsin Fig. 1. No additional transitionswere observed in further experimentswhich examined the spectral region to 3.3 p. The resolution of the Beckman spectrophotometerwas insufllcientto study the four sharp lines. Further measurementswere performed on these lines using photographictechniqueswith a Jarrell-Ash f/0.3 grating spectrograph at 77’K. These transitions were determined to be centered at 5894 A, 5364 A, 5218 A and 4950 A. The 6900 A transition appears to have a linewidth of less than 30 A (~100 cm-l), while the three remainingtransitionsappear to be less than 20 A ( ~80 cm-l) in width. These measurementswereperformedwell above the Nobeltemperature (TN = 53°K) [3] to eliminate effects due to magnetic ordering. Although the Cr2+ site exhibits tetragonal distortion [4] presumably due to the Jahn-Teller effect of this configuration[I], consistent assignmentsmay be given to these transitions on the basis of the calculationby TANABEand SUQANO[5] for the splitting of the (d)4 configurationin a weak octahedralfield. CLARKhas attributed the two unresolvedbands with peaks at 7100 A and 9000 A to a splitting of the 6E + 6T2 transitionby the tetragonal distortion. For the undistorted ionic site, the separationE(5E -+ ST,) = 10 Dq N 14,000 cm-‘, or Dq N 1,400 cm-l. Although the experimental result agrees with the previous investigations for metal aqua-ions, this Dq value may be 50 percent too large because of the site distortion [6]. The four narrow bands may originate from one or more of the four-field independent transitions (6E -+ sE), (5E -, sT2), (5E ‘L sA1), and (SE -+ SA2). For example, both the sA, and 2E electronicconfigurationscontain unpairede-orbital electrons,where the Jahn-Teller distortionis expected to be large. Assuming, therefore, that the Jahn-Teller effect is approximately the same in both con&urations, then the separation calculated by TANABE and STJ~ANO [5], i.e. E(5E + 8A2) = 9B + 4C, should be approximately equal to this separation in the distorted site. Using calculated values for the chromous ion [7], B - 810 cm-1 and C - 3565, E(sE + sA2) - 21,300 cm-l. The value for this transition, determined from the measured absorption spectrum of the sharp lines, should be E(5E + sA2) - 20,000 cm-l. (In the isoelectronicMn3+ ion absorption spectrum in the strong field region, sharp bands are observed in potassium manganioyanide [6] and manganese trifluoride [2].) Narrow bands have also been observed by us in the absorption of CrC&. However, the complexity of the spectra of several samples suggests contamination by the hydrates of this salt, which we have been unable to eliminate. Acknewbdgententa-The assistance of G. F. Sanrvm with these experiments is gratefully acknowledged. We are pleased to acknowledgeseveral fruitful discussionswith D. L. LYONS. Spew Rand Research Center Sudbwy,
[3] [4] [6] [6]
Massachusett-s
W. W. HOLLOWAY,JR. M. KESTICXAN
J. W. CBBLE,M. K. WILKINSONand E. 0. WOLLAN, Phye. Rev. 118,950 (1900). K. H. JACKand R. MAITLAND,Proc. Chnc. Sot. (London) 1957, 232 (1967). Y. TANABEand S. STJ~ANO, J. Phy8. Sot. (Japan) 9,753, 766 (1964). J. S. GRIFFITH,The Theory of Tramdim-Me&d Iona Seo 11.3 Cambridge University Press (1961). [7] D. S. MCCLURE,Solid State Phys. 9, 399 (1959). [8] G. D. JONESand W. A. RUNCI~~AN, Proc. Phy8. Sot. (London) 76, 996 (1960).
Acta, 1966, Vol. 22, pp. 1381 to 1382. Pergamon Press Ltd. Printed in Northern Ireland
RESEARCH NOTES
Narrow
band
optical
absorption
of CrF,*
(Received 11 3March 1965) LITTLE information is available on the optical absorption of the [A]( 3~2)~electronic configuration, especially the Cra’ ion, and only tentative a&gnments have been given to the reported transitions [l]. Recently C-K [2] has observed broad absorption bands in the reflectance spectra of powder samples of CrF, and CrCl,. In this note we report observations of the optical absorption
‘1: 4.500 5,000
6,000
7,000
6,000
9,000
WAVELENGTH
l0,000
II,000
12,000
13,000
(A,
Fig. 1. The absorption spectrum of CrF, in the spectral region from 0.45 p to 1.3 p. The sharp lines, not completely resolved in this spectrum, are indicated by arrows. No additional structure is observed to 3.3 ,u. of crystals of CrF, from 0.45 ,u to 3.3 ~1. Two unresolved bands, and a third band can be distinguished in the absorption spectrum, in agreement with results of the reflectance measurements. In addition, four very narrow absorption lines are observed in the visible region. Platelets of CrF, were prepared by the temperature gradient method from CrF, powder in an inert g&s atmosphere. The X-ray spectra showed no evidence of other phases present in these crystals. The room temperature absorption spectrum of CrF, crystals from 0.45 ,u to 1.3 p is shown in Fig. 1. This spectral proflle was measured with a Beckman DK-2A spectrophotometer equipped with a special oondensing system for investigation of small samples. The broad * This work, supported by the U.S. Office of Naval Research, Contract No. Nonr-4127(00), is part of Project DEFENDER under the joint sponsorship of the Advanced Research Projects Agency, the Office of Naval Research and the Department of Defense. [l] C. J. BALLHAUSEN, Ligand Field Theory, McGraw-Hill, [2] R. J. H. &BRIE, J. Chem. Sot. 1964, 417 (1964). 1381
New York (1962).
1382
Research notes
absorption band with the maximum at roughly 7000 A appears to be composed of two unresolved absorption bands. Another band beginning (with decreasingwavelength) at 4750 A is present. In addition, four sharp lines are observed, which are indicated by arrowsin Fig. 1. No additional transitionswere observed in further experimentswhich examined the spectral region to 3.3 p. The resolution of the Beckman spectrophotometerwas insufllcientto study the four sharp lines. Further measurementswere performed on these lines using photographictechniqueswith a Jarrell-Ash f/0.3 grating spectrograph at 77’K. These transitions were determined to be centered at 5894 A, 5364 A, 5218 A and 4950 A. The 6900 A transition appears to have a linewidth of less than 30 A (~100 cm-l), while the three remainingtransitionsappear to be less than 20 A ( ~80 cm-l) in width. These measurementswereperformedwell above the Nobeltemperature (TN = 53°K) [3] to eliminate effects due to magnetic ordering. Although the Cr2+ site exhibits tetragonal distortion [4] presumably due to the Jahn-Teller effect of this configuration[I], consistent assignmentsmay be given to these transitions on the basis of the calculationby TANABEand SUQANO[5] for the splitting of the (d)4 configurationin a weak octahedralfield. CLARKhas attributed the two unresolvedbands with peaks at 7100 A and 9000 A to a splitting of the 6E + 6T2 transitionby the tetragonal distortion. For the undistorted ionic site, the separationE(5E -+ ST,) = 10 Dq N 14,000 cm-‘, or Dq N 1,400 cm-l. Although the experimental result agrees with the previous investigations for metal aqua-ions, this Dq value may be 50 percent too large because of the site distortion [6]. The four narrow bands may originate from one or more of the four-field independent transitions (6E -+ sE), (5E -, sT2), (5E ‘L sA1), and (SE -+ SA2). For example, both the sA, and 2E electronicconfigurationscontain unpairede-orbital electrons,where the Jahn-Teller distortionis expected to be large. Assuming, therefore, that the Jahn-Teller effect is approximately the same in both con&urations, then the separation calculated by TANABE and STJ~ANO [5], i.e. E(5E + 8A2) = 9B + 4C, should be approximately equal to this separation in the distorted site. Using calculated values for the chromous ion [7], B - 810 cm-1 and C - 3565, E(sE + sA2) - 21,300 cm-l. The value for this transition, determined from the measured absorption spectrum of the sharp lines, should be E(5E + sA2) - 20,000 cm-l. (In the isoelectronicMn3+ ion absorption spectrum in the strong field region, sharp bands are observed in potassium manganioyanide [6] and manganese trifluoride [2].) Narrow bands have also been observed by us in the absorption of CrC&. However, the complexity of the spectra of several samples suggests contamination by the hydrates of this salt, which we have been unable to eliminate. Acknewbdgententa-The assistance of G. F. Sanrvm with these experiments is gratefully acknowledged. We are pleased to acknowledgeseveral fruitful discussionswith D. L. LYONS. Spew Rand Research Center Sudbwy,
[3] [4] [6] [6]
Massachusett-s
W. W. HOLLOWAY,JR. M. KESTICXAN
J. W. CBBLE,M. K. WILKINSONand E. 0. WOLLAN, Phye. Rev. 118,950 (1900). K. H. JACKand R. MAITLAND,Proc. Chnc. Sot. (London) 1957, 232 (1967). Y. TANABEand S. STJ~ANO, J. Phy8. Sot. (Japan) 9,753, 766 (1964). J. S. GRIFFITH,The Theory of Tramdim-Me&d Iona Seo 11.3 Cambridge University Press (1961). [7] D. S. MCCLURE,Solid State Phys. 9, 399 (1959). [8] G. D. JONESand W. A. RUNCI~~AN, Proc. Phy8. Sot. (London) 76, 996 (1960).