- Email: [email protected]
161Dy isomer shifts in dysprosium compounds
J. Phys. Chem.
Solids
Pergamon Press 1969.Vol. 30, pp. 2159-2167.
lslDy ISOMER
SHIFTS
Printed inGreat Britain.
IN DYSPROSIUM
COMPOUNDS*
TOM P. ABELESt and WILLIAM G. BOS Department of Chemistry and P. J. OUSEPH Engineering Physics Department, University of Louisville, Louisville, Ky 40208, U.S.A. (Received
6 January
1969; in revisedform
17 April 1969)
Abstract-MMiissbauer absorption spectra were obtained for the 25.7 keV transitions of 161Dy in dysprosium metal and several compounds at room temperature. Isomer shifts relative to DyF, are: Dy (metal), 3.05 mm set-I; DyN, 0.85 mm set-‘; Dy,O,, 0.56 mm set-‘; DYH,,~, 0.55 mm set-‘; DyH2.,, 0.50 mm set-‘; DyF, . *HH,O,-0.04; and Dy.JSO& . 8H,O, -046 mm set-I. The relative isomer shifts of these compounds are attributed to varying degrees of covalency. The isomer shifts for the hydrides are compared to the predictions of the protonic and hydridic models for these compounds id appear to favor the hydrid; model. INTRODUCTION MEASUREMENTS
absorber of different chemical composition, the isomer shift, 6, is given by [l]
*Based in part, on a Ph.D. dissertation submitted by T. P. Abeles to the Graduate School, University of Louisville ( 1968). tPresent address: Department of Chemistry; Virginia Polytechnic Institute; Blacksburg, Va 2406 1, U.S.A.
where 2 is the atomic number of the nucleus, e is the charge of the electron, R is the radius of the nucleus in the ground state AR = R,, RYd is the difference in the radii of the excited and ground states, S’(Z) is a relativistic correction factor and [$J(O)/,~and lt,!~(O)l,~ are the electron densities at the nucleus in the absorber and source, respectively. For a given isomeric transition the isomer shift is directly proportional to the difference in electron density at the nucleus in the absorber and in the source with a proportionality constant determined by the characteristics of the nucleus. If AR/R is positive, a positive 6 indicates a greater electron density at the nucleus in the absorber than in the source. Since the isomer shift is a linear function of IW)la2comparisons of electron densities in different absorbers can be made without explicit reference to the source. The dominant direct contribution to 1$(0)12 is from s electrons, though relativistic
of the isomer shifts of Miissbauer absorption lines afford a means for the study of the electronic structure of solids. The spacing of nuclear energy levels is sensitive to the chemical environment of the nucleus because of hyperfine interactions between the nucleus and the electrons surrounding it. Thus the energy of the y-radiation emitted or absorbed in a transition between the nuclear ground state and an excited state will in general differ from one compound to another. The y-radiation emitted by the nuclei in a source will not be of the energy required for absorption by nuclei in a sample of different chemical composition. If, however, the source is moved at an appropriate velocity relative to the absorber, the emission and absorption lines can be brought back into coincidence and absorption can occur. The isomer shift measures changes in the spacing of nuclear energy levels in terms of the velocity required to bring about absorption. For a source and
21.59
2160
T. P. ABELES, W. G. BOS and P. J. OUSEPH
calculations show that there can be small direct contributions from p electrons. It should be emphasized that the s electrons from all occupied levels contribute to 1$(0)12. Only s electrons from the valence level are gained or lost upon chemical reaction, but changes in J$(0)j2 due to indirect effects occur and are of impo~ance. in pa~icular the density distribution of s electrons in the atom core is affected by the gain or loss of valence electrons because of changes in shielding. The removal of valence electrons leads to slight increases in the contribution of inner s electrons to l~O)~2. The degree to which valence electrons shield inner s electrons depends on the kind of orbital they occupy - 3, p, d orf- as well as their principle quantum number. Further, the degree of ionicity of a chemical bond affects the isomer shift since \+(0)j2 will be sensitive to the extent to which electrons are gained or lost. In binary compounds of the rare earths with elements of relatively high electronegativity, the bonding is predominantly ionic. An estimate of the extent of covalent contributions can be made on the basis of electronegativity differences between the rare earth ion and the anion or on the basis of the polarizability of the anion. The measurement of isomer shifts provides an experimental approach to this question. Differences in rare earth isomer shifts for nominally ionic compounds are due to varying degrees of covalency which place electron density in the rare earth valence shell. The Massbauer effect has been observed for ten of the rare earth elements and is predicted for three others[2,3]. Apart from studies of metals and alloys, isomer shift studies of rare earth compounds have been limited in number[4,.5]. Only in the case of europium have a large number of compounds been studiedi&91. Because of the stability associated with 4f7 configuration of europium (II), the 4f level plays a prominent roIe in determining the isomer shifts of europium compounds. In compounds of other rare earth
elements, 4felectrons play a less important role in determining the isomer shift. Alloys of dysprosium with several other metals have been studied using Miissbauer spectroscopy 19-131. The isomer shift of Dy metal relative to Dy,O:, has been reported as 2.5 mm/set by Ofer[ IO]. The rare earth hydrides present an especially interesting problem with regard to bonding. A general review of the properties of rare earth hydrides is given in reference [ 141. The hydrogen species in transition metal hydrides has been variously described as protonic, hydridic or atomic in nature. The arguments for and against each model have been reviewed by Gibb[ 151, by Libowitz [ 161 and by Ebisuzaki and O’Keefe [ 171. The magnetic properties of yttrium and lanthanum hydrides favor either a protonic model [ 18, 191 or a hydridic model[20], rather than an atomic model. For the other members of the rare earth series which have been studied, the magnetic properties of the metals and their hydrides are dominated by unpaired 4f electrons and their interpretation is less straightforward. Kopp and Schreiber [2 1, 221 have interpreted hydrogen Knight shifts for the hydrides of cerium, praseodymium, neodymium and gadolinium in terms of a protonic model. The magnetic susceptibilities of several rare earth hydrides were interpreted in terms of a hydridic model by Wallace et a/.[231 though more recently Wallace and Mader[24] pointed out that the magnetic susceptibilities of praseodymium hydrides can be interpreted in terms of either a protonic or hydridic model. The reason for this ambiguity can be traced to the fact that the conduction band parameters involved are IV(&) and the product n(n’-nf. N(E,) is the density of states at the Fermi energy, n is the number of electrons in the band per atom and n’ is the number of electrons which the band can hold per atom. Both of A!(&.) and n(n’ - n) approach zero as a band is either filled or emptied 1201. Heckman [25] has recently reported that the
lelDy ISOMER SHIFTS
Hall coefficients for cerium hydrides are positive, increasing with increasing hydrogen content. These results favor the protonic model, but further study is needed to account for the lack of a linear relationship between the number of holes per metal atom and the hydrogenlcerium ratio and to account for the negative sign of the Seebeck coefficient measured by Heckman earlier[261. The results of a preliminary study of positron annihilation in cerium hydrides favor a hydridic model [27]. EXPERIMENTAL (A) Miissbauer
system
The Miissbauer system was a twin ‘speaker drive’ system, driven at constant acceleration by a triangular pulse. The trigger pulse for the start of each cycle was fed to a RIDL 54-6 time base generator, coupled with a RIDL 34-12B 400 channel analyzer. The mechanical drive system and the concomitant electronics are described by Bowers [28]. The detection system was assembled from commercially available components. A gas proportional tube powered by a Hamner N4305 high voltage power supply was utilized.. The output was amplified and gated through Austin Scientific Associates equipment and then sent to the RIDL multichannel analyzer which was operated in the time-base mode. The coupling of the drive system with the detector was that described by Wertheim [29]. The triangular pulse drives the source at constant acceleration. At one point in the cycle a trigger pulse trips the 400 channel analyzer. By adjusting the time base generator and the triangular pulse properly, the scale from channel 1 to 400 is linear in velocity. As the time base generator advances from one channel to the next, the analyzer stores the pulses sent by the detector in the appropriate location. To obtain the velocities necessary for dysprosium, it was necessary to remove the speaker cones and enlarge the gap in the speaker magnet surrounding the drive coils.
2161
The velocity was adjusted to a maximum of approximately + 3 cm/set and the instrument was calibrated utilizing an Fe-Pd source and an iron foil adsorber. The values for the magnetic splittings of the iron absorber were taken from the data of Wertheim and Herber [30] and Preston et a/.[311 and the IS for the iron foil absorber relative to the Fe-Pd source was taken from Herber’s data[32]. No drift was detectable in the systems during the course of the runs. The sample holders utilized were machined from lucite and allowed a circular sample of 3 cm rad. to be sandwiched between two pieces of &in. thick lucite. These were sealed with a solution of lucite dissolved in dichloromethane. A sample density of approximately 30 mg/cm2 was found to be adequate. The 15 mCi source used in this study was prepared as 161Tb in GdF, . jH,O by Nuclear Science (A division of International Chemical and Nuclear Corporation). 161Tb decays to lsl”Dy which in turn yields the 25.7 keV Mossbauer gamma ray whose absorption was measured. The spectra reported here were all obtained at room temperature. (B) Sample preparation
Dy metal was used in the form of filings obtained from an ingot (United Mineral and 99.9 per cent). Filing Chemical Corp., was carried out under mineral oil which was subsequently removed by washing with tetrahydrofuran and ether. Dysprosium hydrides were obtained as powders from the reaction of the metal with hydrogen from an Elhygen electrolytic generator. Freshly polished samples of the metal, approximately 0.5 g in weight, were placed in molybdenum pans suspended from a Cahn Gram electrobalance mounted in a vacuum chamber with a quartz hangdown tube. The initial reaction was carried out by heating each sample to 500°C and then introducing hydrogen to a pressure of 50 cm Hg. Compositions near DyH, resulted. Upon
2162
T. P. ABELES,
W. G. BOS and P. J. OUSEPH
cooling, additional hydrogen was taken up to a maximum composition of approximately DyH,.,. Each sample was thermally cycled between room temperature and 500°C several times to insure reaction by all portions of the metal. If a sample of the higher composition was desired, the sample was cooled to room temperature under hydrogen and the system was evacuated. No change in com~ition was observed for samples under vacuum at room temperature for periods up to 4 hr. To obtain samples of the dihydride, the trihydride was heated under vacuum until the desired composition was achieved, then cooled to room temperature. Samples were dumped from the pan and passed through a Whitey 43S4 ball valve into a glass sample tube. The valve was closed and the protected sample transferred to a dry box for grinding and mounting. DyN was prepared from the hydride at 900°C under a nitrogen atmosphere using the procedure described by Schumacher and Wallace[33]. The composition of the sample was monitored by measuring pressure changes in the reaction system whose volume was prec~ibrated. It is likely that the nitride thus obtained was slightly nitrogen deficient 1341. Dy,O, was obtained from the American Potash and Chemical Corp. and was of 99.9 per cent purity designation. This material was used as a Miissbauer absorber and as the starting material for the preparations described below. Anhydrous DyF, was prepared by the reaction of Dy,O, with prepurified ammonium bifluoride (MC & B) at 325°C in an Inconel reaction vessel using the procedure described by Spedding and Daane[35]. DYF, . BH,O was obtained using the method of Spedding and Daane[36]. The dried product was ground in a mortar and stored in a stoppered vial. Dy,(SO& .8&O was crystallized from a dilute sulfuric acid solution by cooling and evaporation as described by Wendlandt 1371.
AND DISCUSSION
RESULTS
Miissbauer absorption spectra were obtained for 16*Dyin Dy metal, DyHZ.80, DYH~+,~, DyN, Dy,O,, DyF3, DyF3. &H,O and Dy,(SO,), . SH,O. The isomer shifts for each spectrum were found by applying the method of centroids to the normalized curves. The values thus obtained are reported relative to DyF, in Table 1. The error limits given are maxima based on the estimated stability of the electronics system. Somewhat greater accuracy was probably realized; separate measurements of the isomer shift for the trihydride yielded values of -2.49 and -2.51 mm see-‘. The short half-life of the lslTb source prevented additional tests of reproducibility. The difference in isomer shifts between Dy metal and the Dy,O,, 2.51 mm set-‘, is in good agreement with the value reported by Ofer [ lo]. Table 1. Isomer shifts dysprosium compounds Compound
W DYN DY& DyHz.so DYHI.OLI DYFB DyWI-I,O Dy&O&. 8H,O
of
Isomer shift* (mm sec9) 3*05-t-o~08 o&5 rt 0.08 0~56-cO.08 0.55 3- 0.08 0.50 2 0.08 O~OOrtO~O8 -oGto% -0~46LtO.08
were shift values *Isomer measured relative to a source containing lalDy in a GdFs . t&O matrix but are reported here relative to spectrum.
the
measured
DyF,
Small uncertainties existed in the composition of the hydride and nitride samples. These compounds exhibited a tendency to desorb gas while under y-irradiation. For the hydrides, the change in com~sition due to desorption is estimated to have been less than HIDy = 0.05. X-ray powder patterns of the hydride and nitride samples
lGIDy ISOMER
were taken before and after their use in the Miissbauer spectrometer. No phase changes were detected. The dihydride is single phase, face-centered cubic, from DyH,+ to DYH~.~, and the trihydride single phase, hexagonal close packed, from DyH,., to compositions approaching DyH,[38]. Samples of compositions below DyH,+ and between DyH,., and DyH,., contain two phases. Of the compounds whose isomer shifts are reported here, DyF, is the most ionic. We take it to be completely ionic on the basis of an electronegativity difference of 2.9 and the Dailey and Townes[39] method of estimating ionic character. The isomer shifts of the other Dy compounds and of Dy metal provide a measure of the extent to which the Dy electron configurations in these materials differ from the Xe 4f” configuration of DY+~. The isomer shift of Dy metal relative to
SHIFTS
2163
DyF, can be attributed to the conduction electrons of the metal. On the basis of specific heat and Hall effect results, Gschneider [40] has proposed a simple model for the conduction band of Dy and several other rare earth metals. As shown in Fig. I(a), the 6s and 5d bands overlap to the extent that the Fermi level lies near the top of the 6s band. The 6s band is nearly filled by two electrons per metal atom and the d band is occupied to the extent of just over one electron per metal atom. While there may be some question as to whether distinct s or d character can be attributed to the electrons in the conduction band, Gschneider’s treatment, which depends explicitly on the number of holes in the 6s band and the number of electrons in the 5d band, gives a good explanation of conductivity in the metals to which it has been applied. In any case, this band picture provides a convenient method for recognizing
(a) Metal (n-l) d
N(E)
(b)
(cl
M (H;)
N(E)
M (Hi)
//fF~ ,/y E
fi(n)s fJ
N(E)
E
Fig. 1. Schematic band diagrams showing the location of the Fermi energy, _&, in yttrium and the rare earth metals and in their dihydrides; (a) metal; (b) dihydride according to the protonic model; (c) dihydride according to the hydridic model.
2164
T. P. ABELES.
W. G. BOS and P. J. OUSEPH
the relative contributions of the atomic orbitals to the character of the conduction band. For the 25.7 keV transition of 161Dy, AR/R is positive. Therefore, positive isomer shifts correspond to increased electron density at the Dy nucleus. The positive isomer shift for Dy metal can be attributed to the presence of two 6s electrons per Dy atom in the conduction band of the metal. There is, in addition, one 5d electron per Dy atom present in the metal. This latter should give rise to a negative contribution to the isomer shift for the metal since it shields the nucleus from the s electrons. Since a large positive shift is observed, the presence of s electrons must be the dominant effect on the isomer shift. To the extent that the d electron affects the isomer shift, the shift due to the s electrons must be larger than the measured shift. We will assume that the effect of the d electron is small and take the measured shift to be a good approximation of the shift due to the s electrons. This approximation is reasonable since only one d electron is present per Dy atom; the shielding of s electrons in the core by 5d electrons is expected to be small because most of the 5d electron density lies farther from the nucleus than that of the inner s electrons; and the d electron in the metal is delocalized and is therefore less effective in shielding core electrons than otherwise. Following Brix et a1.[9], we estimate the 6s electron density at the nucleus in the metal to be reduced to 0.5 of the free atom value due to delocalization. Then in our approximation, the isomer shift between Dy metal and DyF, is due to the presence of the equivalent of one localized 6s electron. On this basis we establish an approximate scale of 1.0 6s electrons per 3.0 mm set-’ for measuring the gain or loss of 6s electrons in terms of the isomer shift. The isomer shifts of DyF,, Dy,O, and DyN increase linearly with decreasing anion electronegativity. In these compounds the
direction of electron transfer via covalency effects must be from the anion to Dy. Since the isomer shift of these compounds are positive relative to DyFZ, the dominant effect on the isomer shift must be the transfer of electron density to the Dy 6s orbitals. Since the isomer shift is less sensitive to the indirect effect of 5d, 5p or 4forbital occupancy, some transfer of electron density to these orbitals is also possible. For these predominantly ionic compounds, the degree of covalency will be determined largely by the spatial extent of the unoccupied cation orbitals. For the free atom, the radius of the principal maximum of the 6s wave function is significantly larger than that of the 5d[41]. If the same were true in the +3 ion, the 6s orbital would be more readily available to participate in covalency than the 5d. Assuming no d orbital participation, we calculate from the isomer shifts that 0.20 and 0.27 electrons are transferred to the 6s orbitals of Dy in Dy,O, and DyN, respectively. To the extent that the 5d orbitals are occupied, greater occupancy of the 6s orbitals is required to account for the measured shift. The 6s occupancy derived here represents, therefore, an estimate of the minimum degree of covalency. Both hydrated salts reported here exhibit negative isomer shifts relative to DyF,, though that of DyF, . $H,O is small and within experimental error of zero. The isomer shift of Dy,(SO,), . 8H,O is relatively large and must be taken as real. In aqueous solution, Dyt3 is hydrated to the extent of eight or more water molecules[42]. In DYJSO&~ . 8Hz0 there is some possibility of coordination with sulfate ion as well as water. The rare earth ions frequently achieve coordination numbers of eight or more[43]. Covalent bond formation in these species involves some combination of 6s, 5d, 6p and, possibly, 4forbitals1441. In Dy,(SO.J, .8H,O coordination to DY+~ occurs through covalent bond formation in such a way as to decrease the electron density at the Dy nucleus and a negative isomer shift results.
lelDy ISOMER SHIFTS
Even if the Dy 6s orbital is one of those so used, the occupancy of the other orbitals which shield the nucleus and decrease the electron density there must be the dominant effect. The isomer shifts for dysprosium hydrides are of particular interest because they provide information concerning the electron distribution around Dy in these compounds. The isomer shifts of DyHz and DyH, are approximately equal to that of Dy,O,. The amount by which the shift for DyH, is less positive than that of DyH, is small and within the maximum experimental uncertainty. Based on an analysis of the magnetic properties of yttrium metal and yttrium hydrides, Parks and Bos [45] have formulated the simple band schemes for the protonic and hydridic models of rare earth hydrides shown in Fig. l(b) and Fig. l(c). Because yttrium has no unpaired electrons, the magnetic properties are due primarily to conduction band electrons. The great similarity of yttrium hydrides [46] to the rare earth hydrides [ 141 in properties which are less sensitive to the rare earth 4felectrons indicates that the general features of these band schemes should apply to the rare earth hydrides as well as to yttrium hydrides. The similarity is especially striking for yttrium and dysprosium. Both yttrium [47] and dysprosium [48] are hexagonal close packed (Y: a,, = 3.654 A, co = 5.750 A; Dy: a0 = 3.590, co = 5.647 A) and have very similar Fermi surfaces [47]. Both dihydrides are f.c.c. (YH,: a0 = 5.205 A; DyH,: a, = 5.201 A) and both trihydrides are hexagonal close packed[38] (YH,: a, = 3*674A, co= 6.595 A; DyH,: a0 = 3.671 A, co = 6.615 A). The heats of formation of the hydrogen deficient dihydrides are -44.4 and -45.4 kcal per mole of hydrogen for yttrium[46] and dysprosium [49], respectively. The protonic model requires that the conduction band in the hydrides be split in such a way as to accommodate a maximum of six electrons per metal atom in the lower portion of the band[ 181. The limiting composition of
2165
the hydrides, MHZ, results when three hydrogens per metal atom have donated their electrons to the conduction band. At this point the band is full and the metallic character of the dihydrides has disappeared. The dihydrides have the fluorite structure with hydrogen occupying the tetrahedral interstices around the metal atoms. In the crystal field of eight positive charges representing the ionized hydrogen in a cubic configuration surrounding the metal, the Tzg orbitals of the central atom would be lowered relative to the Eg orbitals. In Fig. 1, the 6s band is placed above the T,, band to preserve the role of the latter in determining the limiting composition. In addition, the long nuclear spin-lattice relaxation time for Y in YH, indicates that there is little s character at the Fermi energy. In the hydridic model the limiting composition is attributed to depopulation of the conduction band with the three electrons per metal being transferred to three hydrogens. Taking the eight cubic hydrogens of the dihydrides as being negatively charged, the d orbitals of the metal split such that the Eg band lies below the T,, band. The 6s band is placed above the Fermi energy of the dihydride to account for the long Y relaxation time in YH,. Predictions of the isomer shifts of Dy in Dy hydrides based on each of the two band schemes can be compared to the experimental results and used as one criterion of the validity of the schemes. According to the protonic model, the electron configurations of Dy in DyHp and DyH, are (Xe) 4f g 5d5 and (Xe) 4fy 5d6, respectively, with the 5d electrons being somewhat delocalized in DyH,. Isomer shifts at least slightly more negative than that of DyF, are predicted for both hydrides, with that of DyH, slightly more negative than that of DyH,. The observed isomer shifts are, of course, positive relative to DyF,. No covalency effects in the usual sense can be postulated here since the proton of this model has little or no electron density to contribute. Incomplete transfer of the hydro-
2166
T. P. ABELES,
W. G. BOS and P. J. OUSEPH
gen electrons to the conduction band, so long as the band is of d character, also fails to account for a more positive shift than that of DYFB. The hydridic model leads to (Xe)4fgW and (Xe)4fs5do configurations for Dy in DyHP and DyH,, respectively. If the bonding were purely ionic, DyH, would have a shift of zero and DyH, would have a slightly negative shift relative to DyF,. In the case of hydride ions, however, partial covalent bonding effects of some significance would be expected on the basis of hydrogen electronegativity and hydride ion polarizability. That these effects should result in a positive shift nearly equal to that of Dy,O, or even greater is not surprising. Further, a slightly less positive shift is predicted for DyH, than for DyH, due to the presence of a single, delocalized electron per metal atom in DyH,. The third hydrogen of DyH, occupies the octahedral interstices of the fluorite structure. The metal-hydrogen distance for the octahedral hydrogen is 2.58 A compared to 2.25 A for the tetrahedral hydrogens. Libowitz[50] has shown that the observed Dy-H distance in DyH, is near that to be expected from the ionic radii of Dyf3 and H-. Both the loss of ad electron and covalency contributions from the octahedral hydrogen would contribute to a positive shift from DyH, to DyH,. Since the observed shift is small, partial covalent bonding by the octahedral hydrogens must not occur to any significant extent. Hydrogen is less electronegative than either oxygen or nitrogen. The fact that a larger positive shift is not observed for DyH, may be attributed to the fact that only two-thirds of the hydrogen contribute to partial covalent bonding.
REFERENCES 1. SHIRLEY D. A., Rev. mod. Phys. 36,339 (1964). 2. GREENWOOD N. N:, Chem. Brit. 3,56 (1967). 3. KAINDL G. and MOSSBAUER R. L., Phys. Lett. 26B, 386 (1968). 4. CLIFFORD A. F., In The MGssbauer Eflect and its Application in chemistry (Edited by R. F. Gould), p. 113. American Chemical Society, Washington D.C. (1967). 5. HENNING W., HUFNER S., KIENLE P., QUITMANN D. and STEICHELE E., In Applicalions of the Miissbauer Efect in Chemistry and SolidState Physics, p. 173. IAEA, Vienna (1966).
6. DEENEY F. A., DELANEY J. A. and RUDDY V.P.,.l.inorg.nucl.Chem.30,117.5(1968). 7. DEENEY F. A., DELANEY J. A. and RUDDY V. P., Phys. Lett. 25A, 370 (1967). 8. WICKMAN H. H., NOWIK I. and WERNICK J. H.,J. app/. Phys. 37 1246 (1966). 9. BRIX P., HUFNER S., KIENLE P. and QUITMANN D., Phys. Left. 13,140 (1964). 10. OFER S. and SEGAL E., Phys. Rev. 141,448 (1966). 11. OFER S., RAKAVY M., SEGAL E. and KHURGIN B., Phys. Rev. 138, A241 (1965). 12. STONE J. A., NICOL M., JURA G. and RASUSSEN J. O., Rep. No. UCRL-1063, Rev. (1963). 13. KHURGIN B., OFER S. and RAKAVY M., Nucl. Phys. AllO, 577(1968). 14. BOS W. G. and GAYER K. H., _r. nucl. Marer. 18, 1 (1966). 15. GIBB T. R. P., In Progress in Inorganic Chemisfry (Edited by F. A. Cotton), Vol. 3, p. 315. Interscience, New York (1962). 16. LIBOWITZ G. G., The Solid State Chemistry of Binarv Metal Hvdrides. Beniamin, New York (1965). 17. EBISUZAKI Y. and O’KEEFFE M., In Progress in Solid State Chemistry (Edited by H. Reiss), Vol. 4, p. 187. Pergamon Press, Oxford (1967). 18. SCHREIBER D. S., Phys. Rev. 137, A860(1965). 19. SCHREIBER D. S. and COTTS R. M., Phys. Rev. 131,1118(1963). 20. BOS W. G. and GUTOWSKY H. S., Inorg. Chem. 6,552 (1967). 21. KOPP J. P. and SCHREIBER D. S., J. appl. Phys. 38,1373 (1967). 22. KOPP J. P. and SCHREIBER D. S., Phys. Lett. 24A, 323 (1967). 23. WALLACE W. E., KUBOTA Y. andZANOWICK R. L., In Nonstoichiometric Compounds (Edited by R. Ward), p. 122. American Chemical Society, Washington, D.C. (1963). 24. WALLACE W. E. and MADER K. H., J. them. Phvs. 48.84 (1968).
Acknowledgements-This work was supported, in part, by a National Science Foundation Grant and, in part, by a Sustaining University Program Grant to the University of Louisville from the National Aeronautics and Space Administration. T.P.A. held a National Defense Education Act Title IV Fellowship from 1966 to 1968.
25. HECKMAN R. C.,J. them. Phys. 462158 (1967). 26. HECKMAN R. C.J. them. Phys. 40,2958 (1964). 27. GUSTAFSON D: R., Rep. No. SC-CR-67-2572, to Sandia Corporation (1966). 28. BOWERS K. R., Msc. University of Louisville (1968). 29. WERTHEIM G. K., The Miissbauer Effect. Academic Press, New York (1964).
la’Dy
ISOMER
30. WERTHEIM G. K. and HERBER R. H., J. them. Phys.38,2106(1963). 31. PRESTON R. S., HANNA S. S. and HEBERLE J., Phys. Rev. 12J3,2207 (1962). 32. HERBER R. H., In Applicafions of fhe Miissbauer Effect in Chemistry and Solid-State Physics, p. 121. IAEA, Vienna (1966). 33. SCHUMACHER D. P. and WALLACE W. E., Inorg. Chem. 5,1563 (1966). 34. SCLAR N.,J. uppl. Phys. 35,1534 (1964). F. H. and DAANE A. H., Prog. nucl. 35. SPEDDING Energy&413 (1956). 36. DAANE A. H. and SPEDDING F. K., J. electrothem. Sot. 100,442 (1953). W. W., J. inorg. nucl. Chem. 7, 37. WENDLANDT 51 (1958). 38. PEBLER A., and WALLACE W. E., J.phys. Chem. 66, 148 (1962). 39. DAILEY B. P., and TOWNES C. H., J. them. Phys. 23. 118 (19551. K. A., In Rare Earth Research III 40. GSCHNEIdER
41. 42. 43. 44.
45. 46. 47. 48. 49. 50.
SHIFTS
2167
(Edited by L. Eyring), p. 153. Gordon and Breach, New York (1965). WABER J. T. and GROMER D. T., J. them. Phys. 42,4116(1966). HOARD J. L., LEE B. and LIND M. D., J. Am. Chem. Sot. 87,1612 (1965). MUETTERTIES E. L. and WRIGHT C. M., Q. Rev. them. Sot. 21, 109 (1967). LIPPARD S. J., In Progress in Inorganic Chemistry (Edited by F. A. Cotton), Vol. 8, p. 109. Interscience, New York (1962). PARKS C. D. and BOS W. G., To be published. LUNDIN C. E. and BLACKLEDGE J. P., J. electrochem. Sot. 109,838 (1962). SPEDDING F. H., DAANE A. H. and HERRMANN K. W.,Actu crystullogr. 9,559 (1956). KEETON S. C. and LOUCKS T., Phys. Rev. 168, 672 (1968). BECKR.L.,Rep.No. LAR-10(1962). LIBOWITZ G. G.. In The Solid State Chemistrv of Binary Metal Hydrides, pp. 7-9. Benjamin, New York (1965).
Solids
Pergamon Press 1969.Vol. 30, pp. 2159-2167.
lslDy ISOMER
SHIFTS
Printed inGreat Britain.
IN DYSPROSIUM
COMPOUNDS*
TOM P. ABELESt and WILLIAM G. BOS Department of Chemistry and P. J. OUSEPH Engineering Physics Department, University of Louisville, Louisville, Ky 40208, U.S.A. (Received
6 January
1969; in revisedform
17 April 1969)
Abstract-MMiissbauer absorption spectra were obtained for the 25.7 keV transitions of 161Dy in dysprosium metal and several compounds at room temperature. Isomer shifts relative to DyF, are: Dy (metal), 3.05 mm set-I; DyN, 0.85 mm set-‘; Dy,O,, 0.56 mm set-‘; DYH,,~, 0.55 mm set-‘; DyH2.,, 0.50 mm set-‘; DyF, . *HH,O,-0.04; and Dy.JSO& . 8H,O, -046 mm set-I. The relative isomer shifts of these compounds are attributed to varying degrees of covalency. The isomer shifts for the hydrides are compared to the predictions of the protonic and hydridic models for these compounds id appear to favor the hydrid; model. INTRODUCTION MEASUREMENTS
absorber of different chemical composition, the isomer shift, 6, is given by [l]
*Based in part, on a Ph.D. dissertation submitted by T. P. Abeles to the Graduate School, University of Louisville ( 1968). tPresent address: Department of Chemistry; Virginia Polytechnic Institute; Blacksburg, Va 2406 1, U.S.A.
where 2 is the atomic number of the nucleus, e is the charge of the electron, R is the radius of the nucleus in the ground state AR = R,, RYd is the difference in the radii of the excited and ground states, S’(Z) is a relativistic correction factor and [$J(O)/,~and lt,!~(O)l,~ are the electron densities at the nucleus in the absorber and source, respectively. For a given isomeric transition the isomer shift is directly proportional to the difference in electron density at the nucleus in the absorber and in the source with a proportionality constant determined by the characteristics of the nucleus. If AR/R is positive, a positive 6 indicates a greater electron density at the nucleus in the absorber than in the source. Since the isomer shift is a linear function of IW)la2comparisons of electron densities in different absorbers can be made without explicit reference to the source. The dominant direct contribution to 1$(0)12 is from s electrons, though relativistic
of the isomer shifts of Miissbauer absorption lines afford a means for the study of the electronic structure of solids. The spacing of nuclear energy levels is sensitive to the chemical environment of the nucleus because of hyperfine interactions between the nucleus and the electrons surrounding it. Thus the energy of the y-radiation emitted or absorbed in a transition between the nuclear ground state and an excited state will in general differ from one compound to another. The y-radiation emitted by the nuclei in a source will not be of the energy required for absorption by nuclei in a sample of different chemical composition. If, however, the source is moved at an appropriate velocity relative to the absorber, the emission and absorption lines can be brought back into coincidence and absorption can occur. The isomer shift measures changes in the spacing of nuclear energy levels in terms of the velocity required to bring about absorption. For a source and
21.59
2160
T. P. ABELES, W. G. BOS and P. J. OUSEPH
calculations show that there can be small direct contributions from p electrons. It should be emphasized that the s electrons from all occupied levels contribute to 1$(0)12. Only s electrons from the valence level are gained or lost upon chemical reaction, but changes in J$(0)j2 due to indirect effects occur and are of impo~ance. in pa~icular the density distribution of s electrons in the atom core is affected by the gain or loss of valence electrons because of changes in shielding. The removal of valence electrons leads to slight increases in the contribution of inner s electrons to l~O)~2. The degree to which valence electrons shield inner s electrons depends on the kind of orbital they occupy - 3, p, d orf- as well as their principle quantum number. Further, the degree of ionicity of a chemical bond affects the isomer shift since \+(0)j2 will be sensitive to the extent to which electrons are gained or lost. In binary compounds of the rare earths with elements of relatively high electronegativity, the bonding is predominantly ionic. An estimate of the extent of covalent contributions can be made on the basis of electronegativity differences between the rare earth ion and the anion or on the basis of the polarizability of the anion. The measurement of isomer shifts provides an experimental approach to this question. Differences in rare earth isomer shifts for nominally ionic compounds are due to varying degrees of covalency which place electron density in the rare earth valence shell. The Massbauer effect has been observed for ten of the rare earth elements and is predicted for three others[2,3]. Apart from studies of metals and alloys, isomer shift studies of rare earth compounds have been limited in number[4,.5]. Only in the case of europium have a large number of compounds been studiedi&91. Because of the stability associated with 4f7 configuration of europium (II), the 4f level plays a prominent roIe in determining the isomer shifts of europium compounds. In compounds of other rare earth
elements, 4felectrons play a less important role in determining the isomer shift. Alloys of dysprosium with several other metals have been studied using Miissbauer spectroscopy 19-131. The isomer shift of Dy metal relative to Dy,O:, has been reported as 2.5 mm/set by Ofer[ IO]. The rare earth hydrides present an especially interesting problem with regard to bonding. A general review of the properties of rare earth hydrides is given in reference [ 141. The hydrogen species in transition metal hydrides has been variously described as protonic, hydridic or atomic in nature. The arguments for and against each model have been reviewed by Gibb[ 151, by Libowitz [ 161 and by Ebisuzaki and O’Keefe [ 171. The magnetic properties of yttrium and lanthanum hydrides favor either a protonic model [ 18, 191 or a hydridic model[20], rather than an atomic model. For the other members of the rare earth series which have been studied, the magnetic properties of the metals and their hydrides are dominated by unpaired 4f electrons and their interpretation is less straightforward. Kopp and Schreiber [2 1, 221 have interpreted hydrogen Knight shifts for the hydrides of cerium, praseodymium, neodymium and gadolinium in terms of a protonic model. The magnetic susceptibilities of several rare earth hydrides were interpreted in terms of a hydridic model by Wallace et a/.[231 though more recently Wallace and Mader[24] pointed out that the magnetic susceptibilities of praseodymium hydrides can be interpreted in terms of either a protonic or hydridic model. The reason for this ambiguity can be traced to the fact that the conduction band parameters involved are IV(&) and the product n(n’-nf. N(E,) is the density of states at the Fermi energy, n is the number of electrons in the band per atom and n’ is the number of electrons which the band can hold per atom. Both of A!(&.) and n(n’ - n) approach zero as a band is either filled or emptied 1201. Heckman [25] has recently reported that the
lelDy ISOMER SHIFTS
Hall coefficients for cerium hydrides are positive, increasing with increasing hydrogen content. These results favor the protonic model, but further study is needed to account for the lack of a linear relationship between the number of holes per metal atom and the hydrogenlcerium ratio and to account for the negative sign of the Seebeck coefficient measured by Heckman earlier[261. The results of a preliminary study of positron annihilation in cerium hydrides favor a hydridic model [27]. EXPERIMENTAL (A) Miissbauer
system
The Miissbauer system was a twin ‘speaker drive’ system, driven at constant acceleration by a triangular pulse. The trigger pulse for the start of each cycle was fed to a RIDL 54-6 time base generator, coupled with a RIDL 34-12B 400 channel analyzer. The mechanical drive system and the concomitant electronics are described by Bowers [28]. The detection system was assembled from commercially available components. A gas proportional tube powered by a Hamner N4305 high voltage power supply was utilized.. The output was amplified and gated through Austin Scientific Associates equipment and then sent to the RIDL multichannel analyzer which was operated in the time-base mode. The coupling of the drive system with the detector was that described by Wertheim [29]. The triangular pulse drives the source at constant acceleration. At one point in the cycle a trigger pulse trips the 400 channel analyzer. By adjusting the time base generator and the triangular pulse properly, the scale from channel 1 to 400 is linear in velocity. As the time base generator advances from one channel to the next, the analyzer stores the pulses sent by the detector in the appropriate location. To obtain the velocities necessary for dysprosium, it was necessary to remove the speaker cones and enlarge the gap in the speaker magnet surrounding the drive coils.
2161
The velocity was adjusted to a maximum of approximately + 3 cm/set and the instrument was calibrated utilizing an Fe-Pd source and an iron foil adsorber. The values for the magnetic splittings of the iron absorber were taken from the data of Wertheim and Herber [30] and Preston et a/.[311 and the IS for the iron foil absorber relative to the Fe-Pd source was taken from Herber’s data[32]. No drift was detectable in the systems during the course of the runs. The sample holders utilized were machined from lucite and allowed a circular sample of 3 cm rad. to be sandwiched between two pieces of &in. thick lucite. These were sealed with a solution of lucite dissolved in dichloromethane. A sample density of approximately 30 mg/cm2 was found to be adequate. The 15 mCi source used in this study was prepared as 161Tb in GdF, . jH,O by Nuclear Science (A division of International Chemical and Nuclear Corporation). 161Tb decays to lsl”Dy which in turn yields the 25.7 keV Mossbauer gamma ray whose absorption was measured. The spectra reported here were all obtained at room temperature. (B) Sample preparation
Dy metal was used in the form of filings obtained from an ingot (United Mineral and 99.9 per cent). Filing Chemical Corp., was carried out under mineral oil which was subsequently removed by washing with tetrahydrofuran and ether. Dysprosium hydrides were obtained as powders from the reaction of the metal with hydrogen from an Elhygen electrolytic generator. Freshly polished samples of the metal, approximately 0.5 g in weight, were placed in molybdenum pans suspended from a Cahn Gram electrobalance mounted in a vacuum chamber with a quartz hangdown tube. The initial reaction was carried out by heating each sample to 500°C and then introducing hydrogen to a pressure of 50 cm Hg. Compositions near DyH, resulted. Upon
2162
T. P. ABELES,
W. G. BOS and P. J. OUSEPH
cooling, additional hydrogen was taken up to a maximum composition of approximately DyH,.,. Each sample was thermally cycled between room temperature and 500°C several times to insure reaction by all portions of the metal. If a sample of the higher composition was desired, the sample was cooled to room temperature under hydrogen and the system was evacuated. No change in com~ition was observed for samples under vacuum at room temperature for periods up to 4 hr. To obtain samples of the dihydride, the trihydride was heated under vacuum until the desired composition was achieved, then cooled to room temperature. Samples were dumped from the pan and passed through a Whitey 43S4 ball valve into a glass sample tube. The valve was closed and the protected sample transferred to a dry box for grinding and mounting. DyN was prepared from the hydride at 900°C under a nitrogen atmosphere using the procedure described by Schumacher and Wallace[33]. The composition of the sample was monitored by measuring pressure changes in the reaction system whose volume was prec~ibrated. It is likely that the nitride thus obtained was slightly nitrogen deficient 1341. Dy,O, was obtained from the American Potash and Chemical Corp. and was of 99.9 per cent purity designation. This material was used as a Miissbauer absorber and as the starting material for the preparations described below. Anhydrous DyF, was prepared by the reaction of Dy,O, with prepurified ammonium bifluoride (MC & B) at 325°C in an Inconel reaction vessel using the procedure described by Spedding and Daane[35]. DYF, . BH,O was obtained using the method of Spedding and Daane[36]. The dried product was ground in a mortar and stored in a stoppered vial. Dy,(SO& .8&O was crystallized from a dilute sulfuric acid solution by cooling and evaporation as described by Wendlandt 1371.
AND DISCUSSION
RESULTS
Miissbauer absorption spectra were obtained for 16*Dyin Dy metal, DyHZ.80, DYH~+,~, DyN, Dy,O,, DyF3, DyF3. &H,O and Dy,(SO,), . SH,O. The isomer shifts for each spectrum were found by applying the method of centroids to the normalized curves. The values thus obtained are reported relative to DyF, in Table 1. The error limits given are maxima based on the estimated stability of the electronics system. Somewhat greater accuracy was probably realized; separate measurements of the isomer shift for the trihydride yielded values of -2.49 and -2.51 mm see-‘. The short half-life of the lslTb source prevented additional tests of reproducibility. The difference in isomer shifts between Dy metal and the Dy,O,, 2.51 mm set-‘, is in good agreement with the value reported by Ofer [ lo]. Table 1. Isomer shifts dysprosium compounds Compound
W DYN DY& DyHz.so DYHI.OLI DYFB DyWI-I,O Dy&O&. 8H,O
of
Isomer shift* (mm sec9) 3*05-t-o~08 o&5 rt 0.08 0~56-cO.08 0.55 3- 0.08 0.50 2 0.08 O~OOrtO~O8 -oGto% -0~46LtO.08
were shift values *Isomer measured relative to a source containing lalDy in a GdFs . t&O matrix but are reported here relative to spectrum.
the
measured
DyF,
Small uncertainties existed in the composition of the hydride and nitride samples. These compounds exhibited a tendency to desorb gas while under y-irradiation. For the hydrides, the change in com~sition due to desorption is estimated to have been less than HIDy = 0.05. X-ray powder patterns of the hydride and nitride samples
lGIDy ISOMER
were taken before and after their use in the Miissbauer spectrometer. No phase changes were detected. The dihydride is single phase, face-centered cubic, from DyH,+ to DYH~.~, and the trihydride single phase, hexagonal close packed, from DyH,., to compositions approaching DyH,[38]. Samples of compositions below DyH,+ and between DyH,., and DyH,., contain two phases. Of the compounds whose isomer shifts are reported here, DyF, is the most ionic. We take it to be completely ionic on the basis of an electronegativity difference of 2.9 and the Dailey and Townes[39] method of estimating ionic character. The isomer shifts of the other Dy compounds and of Dy metal provide a measure of the extent to which the Dy electron configurations in these materials differ from the Xe 4f” configuration of DY+~. The isomer shift of Dy metal relative to
SHIFTS
2163
DyF, can be attributed to the conduction electrons of the metal. On the basis of specific heat and Hall effect results, Gschneider [40] has proposed a simple model for the conduction band of Dy and several other rare earth metals. As shown in Fig. I(a), the 6s and 5d bands overlap to the extent that the Fermi level lies near the top of the 6s band. The 6s band is nearly filled by two electrons per metal atom and the d band is occupied to the extent of just over one electron per metal atom. While there may be some question as to whether distinct s or d character can be attributed to the electrons in the conduction band, Gschneider’s treatment, which depends explicitly on the number of holes in the 6s band and the number of electrons in the 5d band, gives a good explanation of conductivity in the metals to which it has been applied. In any case, this band picture provides a convenient method for recognizing
(a) Metal (n-l) d
N(E)
(b)
(cl
M (H;)
N(E)
M (Hi)
//fF~ ,/y E
fi(n)s fJ
N(E)
E
Fig. 1. Schematic band diagrams showing the location of the Fermi energy, _&, in yttrium and the rare earth metals and in their dihydrides; (a) metal; (b) dihydride according to the protonic model; (c) dihydride according to the hydridic model.
2164
T. P. ABELES.
W. G. BOS and P. J. OUSEPH
the relative contributions of the atomic orbitals to the character of the conduction band. For the 25.7 keV transition of 161Dy, AR/R is positive. Therefore, positive isomer shifts correspond to increased electron density at the Dy nucleus. The positive isomer shift for Dy metal can be attributed to the presence of two 6s electrons per Dy atom in the conduction band of the metal. There is, in addition, one 5d electron per Dy atom present in the metal. This latter should give rise to a negative contribution to the isomer shift for the metal since it shields the nucleus from the s electrons. Since a large positive shift is observed, the presence of s electrons must be the dominant effect on the isomer shift. To the extent that the d electron affects the isomer shift, the shift due to the s electrons must be larger than the measured shift. We will assume that the effect of the d electron is small and take the measured shift to be a good approximation of the shift due to the s electrons. This approximation is reasonable since only one d electron is present per Dy atom; the shielding of s electrons in the core by 5d electrons is expected to be small because most of the 5d electron density lies farther from the nucleus than that of the inner s electrons; and the d electron in the metal is delocalized and is therefore less effective in shielding core electrons than otherwise. Following Brix et a1.[9], we estimate the 6s electron density at the nucleus in the metal to be reduced to 0.5 of the free atom value due to delocalization. Then in our approximation, the isomer shift between Dy metal and DyF, is due to the presence of the equivalent of one localized 6s electron. On this basis we establish an approximate scale of 1.0 6s electrons per 3.0 mm set-’ for measuring the gain or loss of 6s electrons in terms of the isomer shift. The isomer shifts of DyF,, Dy,O, and DyN increase linearly with decreasing anion electronegativity. In these compounds the
direction of electron transfer via covalency effects must be from the anion to Dy. Since the isomer shift of these compounds are positive relative to DyFZ, the dominant effect on the isomer shift must be the transfer of electron density to the Dy 6s orbitals. Since the isomer shift is less sensitive to the indirect effect of 5d, 5p or 4forbital occupancy, some transfer of electron density to these orbitals is also possible. For these predominantly ionic compounds, the degree of covalency will be determined largely by the spatial extent of the unoccupied cation orbitals. For the free atom, the radius of the principal maximum of the 6s wave function is significantly larger than that of the 5d[41]. If the same were true in the +3 ion, the 6s orbital would be more readily available to participate in covalency than the 5d. Assuming no d orbital participation, we calculate from the isomer shifts that 0.20 and 0.27 electrons are transferred to the 6s orbitals of Dy in Dy,O, and DyN, respectively. To the extent that the 5d orbitals are occupied, greater occupancy of the 6s orbitals is required to account for the measured shift. The 6s occupancy derived here represents, therefore, an estimate of the minimum degree of covalency. Both hydrated salts reported here exhibit negative isomer shifts relative to DyF,, though that of DyF, . $H,O is small and within experimental error of zero. The isomer shift of Dy,(SO,), . 8H,O is relatively large and must be taken as real. In aqueous solution, Dyt3 is hydrated to the extent of eight or more water molecules[42]. In DYJSO&~ . 8Hz0 there is some possibility of coordination with sulfate ion as well as water. The rare earth ions frequently achieve coordination numbers of eight or more[43]. Covalent bond formation in these species involves some combination of 6s, 5d, 6p and, possibly, 4forbitals1441. In Dy,(SO.J, .8H,O coordination to DY+~ occurs through covalent bond formation in such a way as to decrease the electron density at the Dy nucleus and a negative isomer shift results.
lelDy ISOMER SHIFTS
Even if the Dy 6s orbital is one of those so used, the occupancy of the other orbitals which shield the nucleus and decrease the electron density there must be the dominant effect. The isomer shifts for dysprosium hydrides are of particular interest because they provide information concerning the electron distribution around Dy in these compounds. The isomer shifts of DyHz and DyH, are approximately equal to that of Dy,O,. The amount by which the shift for DyH, is less positive than that of DyH, is small and within the maximum experimental uncertainty. Based on an analysis of the magnetic properties of yttrium metal and yttrium hydrides, Parks and Bos [45] have formulated the simple band schemes for the protonic and hydridic models of rare earth hydrides shown in Fig. l(b) and Fig. l(c). Because yttrium has no unpaired electrons, the magnetic properties are due primarily to conduction band electrons. The great similarity of yttrium hydrides [46] to the rare earth hydrides [ 141 in properties which are less sensitive to the rare earth 4felectrons indicates that the general features of these band schemes should apply to the rare earth hydrides as well as to yttrium hydrides. The similarity is especially striking for yttrium and dysprosium. Both yttrium [47] and dysprosium [48] are hexagonal close packed (Y: a,, = 3.654 A, co = 5.750 A; Dy: a0 = 3.590, co = 5.647 A) and have very similar Fermi surfaces [47]. Both dihydrides are f.c.c. (YH,: a0 = 5.205 A; DyH,: a, = 5.201 A) and both trihydrides are hexagonal close packed[38] (YH,: a, = 3*674A, co= 6.595 A; DyH,: a0 = 3.671 A, co = 6.615 A). The heats of formation of the hydrogen deficient dihydrides are -44.4 and -45.4 kcal per mole of hydrogen for yttrium[46] and dysprosium [49], respectively. The protonic model requires that the conduction band in the hydrides be split in such a way as to accommodate a maximum of six electrons per metal atom in the lower portion of the band[ 181. The limiting composition of
2165
the hydrides, MHZ, results when three hydrogens per metal atom have donated their electrons to the conduction band. At this point the band is full and the metallic character of the dihydrides has disappeared. The dihydrides have the fluorite structure with hydrogen occupying the tetrahedral interstices around the metal atoms. In the crystal field of eight positive charges representing the ionized hydrogen in a cubic configuration surrounding the metal, the Tzg orbitals of the central atom would be lowered relative to the Eg orbitals. In Fig. 1, the 6s band is placed above the T,, band to preserve the role of the latter in determining the limiting composition. In addition, the long nuclear spin-lattice relaxation time for Y in YH, indicates that there is little s character at the Fermi energy. In the hydridic model the limiting composition is attributed to depopulation of the conduction band with the three electrons per metal being transferred to three hydrogens. Taking the eight cubic hydrogens of the dihydrides as being negatively charged, the d orbitals of the metal split such that the Eg band lies below the T,, band. The 6s band is placed above the Fermi energy of the dihydride to account for the long Y relaxation time in YH,. Predictions of the isomer shifts of Dy in Dy hydrides based on each of the two band schemes can be compared to the experimental results and used as one criterion of the validity of the schemes. According to the protonic model, the electron configurations of Dy in DyHp and DyH, are (Xe) 4f g 5d5 and (Xe) 4fy 5d6, respectively, with the 5d electrons being somewhat delocalized in DyH,. Isomer shifts at least slightly more negative than that of DyF, are predicted for both hydrides, with that of DyH, slightly more negative than that of DyH,. The observed isomer shifts are, of course, positive relative to DyF,. No covalency effects in the usual sense can be postulated here since the proton of this model has little or no electron density to contribute. Incomplete transfer of the hydro-
2166
T. P. ABELES,
W. G. BOS and P. J. OUSEPH
gen electrons to the conduction band, so long as the band is of d character, also fails to account for a more positive shift than that of DYFB. The hydridic model leads to (Xe)4fgW and (Xe)4fs5do configurations for Dy in DyHP and DyH,, respectively. If the bonding were purely ionic, DyH, would have a shift of zero and DyH, would have a slightly negative shift relative to DyF,. In the case of hydride ions, however, partial covalent bonding effects of some significance would be expected on the basis of hydrogen electronegativity and hydride ion polarizability. That these effects should result in a positive shift nearly equal to that of Dy,O, or even greater is not surprising. Further, a slightly less positive shift is predicted for DyH, than for DyH, due to the presence of a single, delocalized electron per metal atom in DyH,. The third hydrogen of DyH, occupies the octahedral interstices of the fluorite structure. The metal-hydrogen distance for the octahedral hydrogen is 2.58 A compared to 2.25 A for the tetrahedral hydrogens. Libowitz[50] has shown that the observed Dy-H distance in DyH, is near that to be expected from the ionic radii of Dyf3 and H-. Both the loss of ad electron and covalency contributions from the octahedral hydrogen would contribute to a positive shift from DyH, to DyH,. Since the observed shift is small, partial covalent bonding by the octahedral hydrogens must not occur to any significant extent. Hydrogen is less electronegative than either oxygen or nitrogen. The fact that a larger positive shift is not observed for DyH, may be attributed to the fact that only two-thirds of the hydrogen contribute to partial covalent bonding.
REFERENCES 1. SHIRLEY D. A., Rev. mod. Phys. 36,339 (1964). 2. GREENWOOD N. N:, Chem. Brit. 3,56 (1967). 3. KAINDL G. and MOSSBAUER R. L., Phys. Lett. 26B, 386 (1968). 4. CLIFFORD A. F., In The MGssbauer Eflect and its Application in chemistry (Edited by R. F. Gould), p. 113. American Chemical Society, Washington D.C. (1967). 5. HENNING W., HUFNER S., KIENLE P., QUITMANN D. and STEICHELE E., In Applicalions of the Miissbauer Efect in Chemistry and SolidState Physics, p. 173. IAEA, Vienna (1966).
6. DEENEY F. A., DELANEY J. A. and RUDDY V.P.,.l.inorg.nucl.Chem.30,117.5(1968). 7. DEENEY F. A., DELANEY J. A. and RUDDY V. P., Phys. Lett. 25A, 370 (1967). 8. WICKMAN H. H., NOWIK I. and WERNICK J. H.,J. app/. Phys. 37 1246 (1966). 9. BRIX P., HUFNER S., KIENLE P. and QUITMANN D., Phys. Left. 13,140 (1964). 10. OFER S. and SEGAL E., Phys. Rev. 141,448 (1966). 11. OFER S., RAKAVY M., SEGAL E. and KHURGIN B., Phys. Rev. 138, A241 (1965). 12. STONE J. A., NICOL M., JURA G. and RASUSSEN J. O., Rep. No. UCRL-1063, Rev. (1963). 13. KHURGIN B., OFER S. and RAKAVY M., Nucl. Phys. AllO, 577(1968). 14. BOS W. G. and GAYER K. H., _r. nucl. Marer. 18, 1 (1966). 15. GIBB T. R. P., In Progress in Inorganic Chemisfry (Edited by F. A. Cotton), Vol. 3, p. 315. Interscience, New York (1962). 16. LIBOWITZ G. G., The Solid State Chemistry of Binarv Metal Hvdrides. Beniamin, New York (1965). 17. EBISUZAKI Y. and O’KEEFFE M., In Progress in Solid State Chemistry (Edited by H. Reiss), Vol. 4, p. 187. Pergamon Press, Oxford (1967). 18. SCHREIBER D. S., Phys. Rev. 137, A860(1965). 19. SCHREIBER D. S. and COTTS R. M., Phys. Rev. 131,1118(1963). 20. BOS W. G. and GUTOWSKY H. S., Inorg. Chem. 6,552 (1967). 21. KOPP J. P. and SCHREIBER D. S., J. appl. Phys. 38,1373 (1967). 22. KOPP J. P. and SCHREIBER D. S., Phys. Lett. 24A, 323 (1967). 23. WALLACE W. E., KUBOTA Y. andZANOWICK R. L., In Nonstoichiometric Compounds (Edited by R. Ward), p. 122. American Chemical Society, Washington, D.C. (1963). 24. WALLACE W. E. and MADER K. H., J. them. Phvs. 48.84 (1968).
Acknowledgements-This work was supported, in part, by a National Science Foundation Grant and, in part, by a Sustaining University Program Grant to the University of Louisville from the National Aeronautics and Space Administration. T.P.A. held a National Defense Education Act Title IV Fellowship from 1966 to 1968.
25. HECKMAN R. C.,J. them. Phys. 462158 (1967). 26. HECKMAN R. C.J. them. Phys. 40,2958 (1964). 27. GUSTAFSON D: R., Rep. No. SC-CR-67-2572, to Sandia Corporation (1966). 28. BOWERS K. R., Msc. University of Louisville (1968). 29. WERTHEIM G. K., The Miissbauer Effect. Academic Press, New York (1964).
la’Dy
ISOMER
30. WERTHEIM G. K. and HERBER R. H., J. them. Phys.38,2106(1963). 31. PRESTON R. S., HANNA S. S. and HEBERLE J., Phys. Rev. 12J3,2207 (1962). 32. HERBER R. H., In Applicafions of fhe Miissbauer Effect in Chemistry and Solid-State Physics, p. 121. IAEA, Vienna (1966). 33. SCHUMACHER D. P. and WALLACE W. E., Inorg. Chem. 5,1563 (1966). 34. SCLAR N.,J. uppl. Phys. 35,1534 (1964). F. H. and DAANE A. H., Prog. nucl. 35. SPEDDING Energy&413 (1956). 36. DAANE A. H. and SPEDDING F. K., J. electrothem. Sot. 100,442 (1953). W. W., J. inorg. nucl. Chem. 7, 37. WENDLANDT 51 (1958). 38. PEBLER A., and WALLACE W. E., J.phys. Chem. 66, 148 (1962). 39. DAILEY B. P., and TOWNES C. H., J. them. Phys. 23. 118 (19551. K. A., In Rare Earth Research III 40. GSCHNEIdER
41. 42. 43. 44.
45. 46. 47. 48. 49. 50.
SHIFTS
2167
(Edited by L. Eyring), p. 153. Gordon and Breach, New York (1965). WABER J. T. and GROMER D. T., J. them. Phys. 42,4116(1966). HOARD J. L., LEE B. and LIND M. D., J. Am. Chem. Sot. 87,1612 (1965). MUETTERTIES E. L. and WRIGHT C. M., Q. Rev. them. Sot. 21, 109 (1967). LIPPARD S. J., In Progress in Inorganic Chemistry (Edited by F. A. Cotton), Vol. 8, p. 109. Interscience, New York (1962). PARKS C. D. and BOS W. G., To be published. LUNDIN C. E. and BLACKLEDGE J. P., J. electrochem. Sot. 109,838 (1962). SPEDDING F. H., DAANE A. H. and HERRMANN K. W.,Actu crystullogr. 9,559 (1956). KEETON S. C. and LOUCKS T., Phys. Rev. 168, 672 (1968). BECKR.L.,Rep.No. LAR-10(1962). LIBOWITZ G. G.. In The Solid State Chemistrv of Binary Metal Hydrides, pp. 7-9. Benjamin, New York (1965).