3969
SOLIDSOLUTIONS OF ERBIUM AND YTTERBIUM OXIDES
Spectroscopic Properties of Solid Solutions of Erbium and Ytterbium Oxides by Leonard Grussla,b Pitman-Dunn Research Laboratories, Frankford Arsenal, Philadelphia, Pennsylvania
and Robert E. Salomon” Department of Chemistry, Temple Uninersity of the Commonwealth System of Higher Education, Philadelphia, Pennsylsania 19182 (Receiaed April 9, 1970)
Solid solutions of Erz03and YbzO3 were prepared over the entire composition range. The visible and ultraviolet diffuse reflectance spectra, the magnetic susceptibility, and the lattice parameters of these solutions were determined. An unusual competition in the intensity stealing of an ultraviolet charge-transfer band was observed and analyzed.
Introduction Paramagnetic ions in ionic lattices are subject to perturbations which may for convenience be classified into two categories. The first type are the crystal field perturbations together with possible covalent contributions from the neighboring ions. This is the full perturbation when the paramagnetic ions are dilute. The second type of perturbation, which comes into play a t higher concentrations, involves the resonance interactions between pairs and higher groups of identical ions. These interactions, unlike the former, are difficult to observe directly because changes in concentration not only change the resonance interactions but, in general, also change the crystal field produced by the second coordination sphere. The rare earth ions are unique in that they all produce nearly the same electrostatic field, since they differ only in their number of f electrons, and these are shielded by a closed shell of electrons with fairly similar radii. This similarity in outer electron configuration is of course the factor that makes it difficult to separate the rare earths from one another and conversely leads to isomorphous substitution. For many pairs this substitution occurs over the entire concentration range without change in structure. I n this work, the Erz03-Ybz03system was chosen for study. The sesquioxides of Erz03 and Ybz03are both cubic and belong to the Ia3 space group with 16 molecules per unit ce1L2 The crystal radii of trivalent Er and Yb are 0.87 and 0.85 8, respectively. The oxygens are arranged as a slightly asymmetric tetrahedron about the rare earth ion with metal-oxygen distances of about 2.0 The electronic configurations of Er3+ and Yb3+are 4fl15s25pG and 4f135s25pG and the ground states ~ / 2F,/z, ~ respectively. The energy levels of are 4 1 ~and Er3+and Yb3+in a number of different hosts are given by Diekee4 Er3+ exhibits a number of crystal field
transitions in the visible and ultraviolet, while for Yb3+the 2F7l2-+ 2F5/2manifold occurs in the region of 9700 8.5 I n the ultraviolet region below 2500 8 the optical transitions are described as either charge transfer from the oxygen 2p levels to empty 4f levels associated with the rare earth or transitions from oxygen 2p orbitals to oxygen 3s orbitals.6 The former is characteristic of Ybz03while the latter is characteristic of Erz03. Changes in the positions and intensities of electronic absorption bands in solid solutions of Er203with Yb203 can be attributed primarily to resonance interactions or possibly to small variations in the lattice parameters as the composition is varied.
Experimental Section Erbium and ytterbium oxide powders, 99.9% pure, were purchased from the llichigan Chemical Corp., St. Louis, Mich. Solid solutions were prepared by first mechanically mixing appropriate proportions of the two oxides. The mechanical mix was then dissolved in hot 10% HC1, and the solution was filtered. Coprecipitation of the hydrous oxides was effected by addition of sufficient NH40H to ensure complete precipitation. The filtered precipitates were thoroughly
* To whom correspondence should be addressed. (1) (a) This research is part of a dissertation submitted by L. Gruss to the Temple University Graduate Board in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (b) This work was supported by U. S. Army Material Command under Project No. 1T061102B32A, Research in Materials. (2) R. S.Roth and S. J. Schneider, J. Res. Nat. Bur. Stand., Sect. A , 64, 309 (1960). (3) R. W. G. Wyckoff, “Crystal Structures,” Vol. I, Interscience, New York, N. Y . , 1948, pp 2-4. (4) G. H. Dieke, “Spectra and Energy Levels of the Rare Earth Ions in Crystals,” Wiley, New York, N. Y . , 1968, Chapter 13. ( 5 ) J. Hoagschagen, Physica, 11, 513 (1944). (6) C. K. Jorgensen, R. Pappalardo, and E. Rittershaus, 2. Naturforsch., 20A, 54 (1964).
The Journal of Physical Chemistry, Vol. 74, No. $8, 1970
3970
LEONARD GRUSSAND ROBERT E. SALOMON
washed with large volumes of hot distilled water until the washings were neutral to litmus. The precipitates were then dried, ground with a mortar and pestle, and passed through a 74-p sieve. The fine powder was then heated in air in a quartz tube to 1000° and held a t this temperature for over 100 hr. The long heating period ensured that the final samples were truly solid solutions and also eliminated any water, carbon dioxide, or nitrogen compounds. All samples, including pure Erz03and Ybz03, were subject t o the exact procedure described in the above to facilitate comparison of data. X-Ray diffraction patterns were obtained on all samples using a Debye-Scherrer 114.59-mm camera (Norelco) and a Norelco Basic X-ray diffraction unit employing filtered copper Ka! radiation. Lattice parameters were calculated from the Straumanis positioned film by averaging over the last ten hkl reflections. Diffuse reflectance spectra between 220-700 mp were obtained using a Cary Model 14 spectrophotometer equipped with a Model 1411 diffuse reflectance (ring collector) accessory. Most of the spectra mere taken a t approximately 297°K. For low-temperature measurements a specially designed cell7 which permitted measurements down to 78°K was used. Magnetic susceptibility measurements were made using the Guoy method a t a temperature of 295°K.
I n the visible region of the spectrum the sharp bands characteristic of Er3+ were observed. Table I1 lists the bands a t several compositions. The observed shifts were less than the experimental accuracy of 10 cm-I a t 297 and 78°K. I n the region between 39,600 and 42,500 cm-' the strong absorption characteristic of Olp + Yb4f and Ozp+ Oas transitions were observed. The spectra of pure Er203,pure Ybz03 and 25 Yb203-75 Erz03 are shown in Figure 1. The shoulder in pure \'b203 was resolved into a band by assuming both a gaussian shape and that the absorbance in the region from 240 to 250 mp is free from contributions from the 02,+ band. The latter is clearly suggested by the observed spectrum. When this mas done, the entire spectrum in the region 250 to 220 mp for all 11 compositions fit the following formula rather closely A(X,k)
=
+
A0E,203(A)
A*YbzO,(k)[I - (1 -
xYb)4]
(1)
where is the apparent absorbance of pure Er203, A*YI,,o, is the resolved 02, + Yb4r apparent absorbance of pure Ybz03,and X Y is~ the mole fraction of Ybz03. I
I
I
1
I
I
210
220
230
240
250
260
Results The X-ray patterns were indicative of solid solutions. The lattice parameters of pure Erz03and Yb& agreed with literature values.2s8 The lattice parameters, which varied linearly with composition, are given in Table I. The magnetic susceptibility varied in a completely linear manner with composition within the experimental error of =k2%. The susceptibilities of pure Erz03 and pure YbzOawere 9.2 and 4.2 BM, respectively, in agreement with reported value^.^-'^
Table I : Lattice Parameters and Concentrations of Erz03-Ybz03Solid Solutions
1.4 1.2
W A V E L E N G T H (my) Composition, mol %
Lattice parameter,
K
10.5481 1 0 . 0 0 1 1 10.5475 f 0 . 0 0 0 8 10.5415 i:0.0013 10,5380 2C 0.0014 10.5194=kO0.O011 10.5096 f 0.0011 10.4914=kO0.O011 10.4740 f 0 . 0 0 1 6 10.4621 + 0.0008 10.4451 & 0.0013 10.4389 f O . 0 0 0 9 10.4349 zt0.0008 10.43362C0.0008 ~~~~~~~
~
The Journal of Physical Chemistry, Vol. 74, No. 88, 1970
Figure 1. Comparison of experimental (-) and calculated (- - -) absorbance curves of the 75 Erz03-25 Yb208 solid solution in the ultraviolet region a t room temperature. The postulated A o ~ r z and ~ 3 A o ~ ~ curves z ~ 3are also shown. (7) J. G. Bendoraitis, Ph.D. Thesis, Temple University, Philadelphia, Pa., 1968. (8). National Bureau of Standards Circular 8, U. S. Government Printing Office, Washington, D. C., 1958, p 539. (9) F. Hund, Z. Phys., 33, 855 (1925). (10) J. H. Van Vleck and A. Frank, Phys. Rev., 34, 1494, 1625 (1929). (11) W. Klem and A. Kocsy, 2.Anorg. Allg. Chem., 233, 84 (1937). (12) M . M.Pinaeva and E. I. Krylov, Zh. Neorg. Khim., 11(4), 728 (1966).
3971
SOLIDSOLUTIONS OF ERBIUM AND YTTERBIUM OXIDES
Table I1 : Comparison of the Absorption Peak Positions of Solid Solutions of ErzOa-YbzOt with Pure ErzOs a t Room Temperature ErzOa (104om-1)
95 Erzoa-5 YblOa (104 om-1)
90 ErzOa-10 YbzOa (IO4 om-])
75 Erz03-25 YbzOa (104 om-1)
65 Erzos-35 YbzOa (104 om-1)
50 ErzOa-50 YbzOa (104 cm-1)
Y
3
8
8
8
1,5119 1.5280 1.9154 1.9252 2,0447 2,6331 2.6380 2.7222 3.8778
I
1.5120 1.5282 1.9148 1.9252 2,0443 2.6332 2.6398 2.7235 3.8769
1.5118 1.5284 1.9154 1,9252 2,0438 2.6332 2.6398 2.7235 3.8796
1.5119 1.5282 1 ,9146 1.9248 2.0448 2.6337 2.63S6 2.7244 3.8796
1.5121 1.5279 1,9150 1,9252 2,0440 2 6344 2.6398 2.7229 3.8796
1.5121 1.5272 1.9164 1.9247 2.0445 2.6328 2,6393 2.7243 3.8782
ErzOa (104 om-1)
35 ErzOa-65 YbaO3 (104 om-1)
25 ErzOa-75 YbzOa (104 om-1)
10 ErzOa-90 YbzOa (104 om-1)
5 EraOa-95 YbzOs (104 om-1)
3
3
3
3
8
1,5119 1.5280 1.9184 1.9252 2,0447 2.6331 2.6380 2.7222 3.8778
1.5122 1.5277 1.9154 1.9252 2.0453 2.6337 2 6394 2,7247 3.8776
1.5121 1.5280 1.9152 1.9252 2.0446 2.6332 2.6398 2.7225 3.8786
1.5117 1.5275 1.9159 1 $9257 2.0445 2.6332 2.6376 2.7230
1.5118 1.5280 1.9156 1.9258 2.0445 2.6328 2.6398 2.7249
The apparent absorbance is the logarithm of the ratio of light diffusely reflected from a MgC03 white blank to the intensity of light diffusely reflected from the sample. Although the relation bethveen the true absorbance and the ahparent absorbance (Le., diffuse reflectance) is a complicated function of particle size and absorption coefficient, it is clear that if the particle size distribution of two samples is the same, and the apparent absorbances are the same, then the true absorbances are the same. The most important aspect of eq 1 and the observed data is the relative insensitivity of apparent absorbance to composition. It is recognized that although the diffuse reflectance may distort the spectrum the correlations at a given wavelength are quite good as long as the absorbance does not become too large. A further justification for the use of apparent absorbance data in this work was provided by a check on the diffuse reflectance on mechanical mixtures of Erz03 and Yb&. The total apparent absorbances in this check were indeed sums of the separate absorbances.
Discussion The linear dependence of magnetic susceptibility on concentration and the invariance of the crystal field peaks with composition are in accord with the view that the f orbitals on different atoms do not mix to any significant degree and that the crystal field about each metal ion is invariant with composition. The small but significant shift in lattice parameter (0.1 A) would be expected to cause some shift in those peaks. The
.
absence of any such shift is surprising if all the metal ion-oxygen distances vary as does the lattice parameters. It is conceivable that the individual metaloxygen distances are preserved and that the lattice parameters reflect any average change in bond length. Transitions involving f --t d, f --t g, f --t p, etc., for Er3+ and Yb3+ occur in the vacuum ~ l t r a v i o l e t ~ ~ - ’ ~ and, hence, do not overlap the charge-transfer band observed in this work. Lohlahas shown that the lowest 4f + 5d band of Er3+ and Yb3+ in a CaFz crystal at room temperature occurs at 156 and 141 mp, respectively. Hence, transitions involving 4f + 5g, 4f --t 6p, etc., would absorb at a much higher energy so that these intraatomic transitions are well enough removed from the charge-transfer bands described here. The most striking result of this work is eq 1. The term [l - (1 - X Y I , ) ~is] the fraction of oxygens adjacent to at least one ytterbium. Hence, the intensity of the charge-transfer band originating on a given oxygen would appear to be independent of the number of terminal states or independent of the degeneracy of the final level. From another point of view, eq 1 appears to violate Beer’s law. We propose the following explanation of this result. The transition 0 2 , --+ Ybdf may be nominally forbidden (13) G. H. Dieke and H. M. Crosswhite, A p p l . Opt., 2 , 675 (1963). (14) S. R. Sinha, “Complexes of the Rare Earths,” Pergamon Press, New York, N. Y., 1966, p 91. (15) B. W. Bryant, J . Opt. Soc. Amer., 55, 771 (1965). (16) E. Loh, P h y s . Rev.,147, No. 1, 332 (1966). The Journal of Physical Chemistry, Vol. 74, No. 22, 1070
3972
LEONARD GRUSSAND ROBERT E. SALOMON
or extremely weak since the overlap between these orbitals is negligible because of the shielding of the 4f orbitals by the 5s25p6orbitals. The oxygen 3s orbitals which have a larger radius may, on the other hand, overlap the 4f orbitals and mix with them. They are fairly close in energy. The 02, -+ 0 3 5 transition is strongly allowed. The mixing coefficients can be readily calculated using the method of linear variations. I n this analysis, the approximations that f orbitals on different ytterbiums are orthogonal, that f orbitals are orthogonal to the Oss orbitals, and that the matrix elements of the Hamiltonian connecting the ossorbital with each 4f orbital are identical, were made. These are essentially the Hiickel 1 4 0 approximations. The result which emerges from this analysis is that no matter how many Yb ligands surround an oxygen only one f level is perturbed and of course this is the only level which contains any 3s character. The square of the coefficient of the 3s function when there are jYb ligands, L,, is found in eq 2 to be
.5
-
2
-
PLD
.3-
2
(3)
L,(O) approaches 0.5 as 0 approaches 0 and becomes proportional to j as 0 goes to infinity. L,(O) is proportional to the intensity of the transition. Hence for small 0 (0 < 1) the intensities as can be seen from eq 2 are nearly equal (Ll(1) = L2(l) = Lr(1) = L(1)). Although the intensities for one, two, three, and four Yb ligand environments approach each other as 0 -+ 0, the energy of the perturbed 4f level is expected to shift. The difference in energy between the perturbed f levels when there are n and n' ligands is found to be En - En'
=
(3sIH13~)- (4flH14f) X 2
[4L-$-4-]
values calculated from eq 2 at 0 = 1 and the statistical equation which follows, an almost perfect fit of the
+
A 'Yb2oa (4xYb(l - XYb)aL1f A = A E ~ ~ o____ ~ L4
4
6X2yb(1 - XYb)'LZ. .1
L
+
4x3Yb(l - XYb)L3
0
(4)
For 0 = 1, the calculated shift is less than 25 8 which is within experimental observation. Using the L
YI
U
where
I
2
4
6
.8
IO
MOLE FRACTION YbpOl
Figure 2. Comparison of the experimental (-) and calculated (- -) absorbance as a function of mole fraction at a wavelength of 245 mp.
-
The Journal of Physical Chemistry, VoZ. 74,No. $8, 1070
+ x4YbL4)
(5)
absorbance data was obtained. I n this equation we have multiplied the fraction of oxygens surrounded by j ligands by L3.and summed the results. The remarkable fit of this equation to the experimerltal data is illustrated a t a selected wavelength (246 mp) by Figure 2.
