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Static high pressure studies on Nd and Sc
Physica 139 & 140B (1986) 285-288 North-Holland, Amsterdam
STATIC HIGH PRESSURE STUDIES ON Nd AND Sc* Jagannadham A K E L L A Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
J. XU** Carnegie Institution Geophysical Laboratory, Washington, DC 20008, USA
Gordon S. SMITH Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
We have investigated the crystal structural transformations in neodymium and scandium up to 4.0 GPa pressure and at room temperature, in a diamond-anvil high pressure apparatus. Nd has a double hexagonal-close packed (dhcp) structure at ambient pressure and temperature. Then it transforms to a face-centered cubic (fcc) structure at 3.8 GPa, which further transforms to a triple hexagonal-close packed structure (thcp) at about 18.0GPa. In scandium we observed only one transformation from the hexagonal-close packed (hcp) structure at room temperature to a tetragonal structure. This transformation occurs between 19.0 and 23.2 GPa pressure.
I. Introduction The lanthanide metals lanthanum (La) through lutetium (Lu) are trivalent at room temperature and pressure except europium (Eu) and ytterbium (Yb) which are divalent. The heavier trivalent rare-earth metals have hcp structure at ambient conditions and follow the crystal structure sequence hcp-Sm type-dhcp-fcc-thcp (or distorted fcc) as a function of increasing pressure. The trivalent transition metals scandium (Sc) and yttrium (Y) are also grouped along with the lanthanide metals, even though they are not strictly lanthanide group metals. Sc and Y are interesting contrasts to the rare-earths in behaving rare-earth-like in many aspects, but having no f electrons. Grosshans et al. have shown that Y indeed shows the complete rare-earth-like crystal structure sequence, whereas Sc fails to follow suit. We undertook a detailed study of Nd (rareearth) and Sc (rare-earth like) metals to under* Work performed under the auspices of the US Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. ** Originally from the People's Republic of China.
stand any systematics is their high pressure behavior, and in this preliminary report we present data up to 40.0 GPa pressure.
2. Apparatus and experimental procedure A diamond-anvil apparatus similar to that designed by Mao and Bell [2] was used. We loaded small ruby chips (10 micron) as pressure indicators in the cell along with the Sc sample. In the case of Nd, however, we loaded small ruby chips with and without gold to compare the pressures obtained. We believe the pressure determinations are accurate to 0.5 to 1.0 GPa. The scandium sample was in the form of 0.1290 mm thick chips of random sizes and 99.99+ purity. The Nd sample was a 0.076 mm foil, which was stored under mineral oil. The sample was mechanically cleaned under mineral oil to remove any surface oxidation or alteration products. A small sliver of the sample was then cut and loaded into a 120 micron pre-pressed stainless steel gasket-hole along with the pressure marker and silicone oil as pressure transmitting medium. We used 1:4 ethanol plus methanol mixture as the pressure
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
286
J. Akella et al. / Static high pressure studies on N d and Sc
Table I Impurities in the starting neodymium sample wt% al Ce Y La Sm Pr Si Fe Mg Ca AI Ta
<0.05 <0.05 <0.05 <0.05 <0.05 0.01 0.02 0.01 0.01
"~ Spectrographic analysis. Sample supplied by Research Chemicals Lot No. NO-M-3160.
transmitting medium with Sc. Some of the major impurities in the Nd sample are given in table I. A rotating anode X-ray generator with fine focus Mo target served as the X-ray source. Typical run times to collect diffraction data on film were around 48 hours. Values of the lattice constants were determined through least squares analysis.
3. Results
Nd has the hexagonal close packed (hcp) structure under ambient conditions and transforms to a face-centered cubic (fcc) structure at about 3.8 GPa. This pressure is slightly lower than the transformation pressure ( 5 . 0 G P a ) reported by Piermarini and Weir [3]. At about 18.0 GPa we found new X-ray diffraction lines compared to the lower pressure fcc phase. High pressure data between 1 8 . 0 - 3 8 . 0 G P a for Nd (Nd-III) were tested for a triple hexagonal close packed (thcp) structure, similar to that proposed for Pr-II| [4]. Nine out of twelve lines could be fitted to this structural form and three very weak lines ( I = 5 to 10) could not be fitted. Lattice parameters for Nd-III at --18.0 GPa are a -- 3.262(18)/~, c = 15.76(6),~, c / a = 4.831 and V/V o =0.709.
There seems to be little volume contraction in going from fcc to thcp and the 30% volume compression observed at 18.0 GPa is similar to that of other rare-earth elements. Using the unit cell constants proposed by Vohra et al. [5] for a distorted fcc phase, we calculated the possible reflections and d values for such a phase. Calculations done at L L N L (Smith and Akella) and at Penn State (D. Smith) show that for a distorted fcc phase there should be a strong reflection (with an intensity of =35) at about 5.25/~, which we failed to see in our films. Similar attempts to fit the data to an alpha-uranium structure also failed. We found that the thcp phase changes to another structural form (Nd-IV) at about 39.5 GPa. Grosshans and Holzapfel [6] also reported a structural change at 41.0 GPa. In both cases the crystal structure of the phase is not yet identified. Sc has the hexagonal close packed structure under ambient conditions. Between 19.0 and 23.2 GPa we observed new X-ray diffraction lines in the film. We took the data from our 26.0 GPa run (table II) where we think we have the diffraction pattern solely from the high-pressure phase for crystal structural determination. Grosshans et al. [1] have also reported a similar structural transformation in Sc at about 20.0 GPa and reported the structure for the high pressure phase as/3-neptunium type. Except for the energy dispersive X-ray diffraction pattern, they did not provide any d values in their paper for this structure (Sc-III), so a positive comparison could not be made with our data. We measured six X-ray films from runs at different pressures between 23.0 to 4 0 . 0 G P a pressure, and in table II have presented the data from one run at 26.0 GPa for further discussion. We used the indexing given by Grosshans et al. [1] for Sc-III and fitted our data (table II column 1) and we also incorporated the values for intensity from the powder diffraction file (PDF) for /3-Np. The following observations are important: a) reflections with h k l = 201,211 and 202 are not observed in the PDF data but are reported by Grosshans et al.; b) reflections with h k l =- 111 (•=80) and 311 ( ! = 100) are reported in the P D F but not
J. Akella et al. / Static high pressure studies on Nd and Sc
287
Table II Sc-III data at 26.0 GPa pressure* n
dobS (/~)
lobs
hkl
dc~,c (,~)
Io
1. 2.
2.651 2.382
VW VS
101 200 111
2.656 2.375 2.318
50 60 80
3. 4.
1.873 1.768
VVW VW
201 211
1.908 1.770
N.O. N.O.
5. 6.
1.676 1.600
VW VVW
7.
1.325
VVW
8.
1.285
VW
9.
1.249
VVW
220 1.679 002 1.602 102 1.518 112 1.446 311 1.360 202 1.325 212 1.279 N.I. fl-Np type Prim. tet; Z = 4 a = 4.750(12) ,~ c = 3.204(8) ,~ V/4 = 18.07(9) V/V~j = 0.734
60 5 30 30 100 N.O. 50 -
hkl
dc~,c (,~)
hkl
dca~c(•)
101 110 002
2.950 2.657 2.38l
200 112
1.879 1.773
211
N.I. 1.584
220
1.329
101 110 002 111 102 200 112 201 210 003 211 220
2.950 2.657 2.381 2.320 2.011 1.879 1.773 1.748 1.681 1.587 1.584 1.329
N.I.
221
1.280
N.I. b.c. tet; Z = 4
300 1.253 prim tet; Z = 4
a = 3.758(11) A c = 4.761(32) ,~ V/4 = 16.81(14)
~same ~sarne ~--same
V~V0 = 0.683
,-- same
* 10 = obs. int. f o r / 3 - N p : P D F 7-221, N.O. = not observed, N.I. = not indexed.
observed in their data; c) the d value calculated for the (201) reflection is a bad fit with our observed value; and d) a weak line with d = 1.299 is not indexable as a /3-Np type. We further tried to fit our data to a bodycentered tetragonal structure with Z = 4 (table II, column 2). We found a reasonable fit between the observed and calculated data. H o w e v e r , we found that three weak lines with d = 1.676, 1.285 and 1.249 could not be fitted and also we did not see a line (hkl = 101) at d = 2.950. Using the same lattice parameters as in column 3, we fitted our data to a primitive tetragonal cell (table II, column 3). Reflections with hkl = 101, 111, and 102 calculated with this cell were not observed in the experimental data, however there is excellent agreement between the rest of calculated and observed data with this cell. So far we have not found a prototype crystal to this cell as well as the one in column 2. Lattice parameters for Sc-III at P - - 2 6 . 0 GPa are
a = 3.758(11) A ; c : 4.761(32)/~, and V/Vo = 0 . 6 8 3 .
detailed elsewhere.
A
paper
on
Sc will
be
published
4. Summary In conclusion, we find that Nd transforms from hcp to fcc at a pressure of about 3.8 GPa and further transforms to a phase indexable as thcp at about 18.0 GPa. Both transformations occur with little atomic volume discontinuity. A fourth modification, as yet not identified, appears above - 3 9 . 5 GPa. Scandium remains in the hcp crystal structure from ambient pressure to about 19.0 GPa. A b o v e this pressure, the X-ray diffraction data seem to be best-fitted by a primitive tetragonal unit cell containing 4 atoms. This cell appears to provide a better fit to our observed data than tetragonal
288
J. Akella et al. / Static high pressure studies on Nd and Sc
s t r u c t u r e of t h e /3-Np t y p e p r o p o s e d by G r o s shans et al. O u r d a t a suggest that t h e r e a r e no similarities thus far, in the high p r e s s u r e s t r u c t u r a l b e h a v i o u r of b o t h t h e s e m e t a l s .
References [1] W.A.Grosshans, Y.K. Vohra and W.B. Holzapfel, J. Magn. and Magn. Mat. 29 (1982) 282.
[2] H. Mao and P.M. Bell, Carnegie Institution Washington, Geophys. Lab. Yr. Book. 74 (1975) 402. [3] C.J. Piermarini and C.E. Weir, Science 144 (1964) 69. [4] G.S. Smith and J. Akella, Phys. Lett. 105A (1984) 132. [5] Y.K. Vohra, V. Vijayakumar, B.K. Godwal and S.K. Sikka, Phys. Rev. B30 (1985) 6205. [6] W.A. Grosshans and W.B. Holzapfel, On Structural Systematics of the Lanthanides Under Pressure (preprint).
STATIC HIGH PRESSURE STUDIES ON Nd AND Sc* Jagannadham A K E L L A Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
J. XU** Carnegie Institution Geophysical Laboratory, Washington, DC 20008, USA
Gordon S. SMITH Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
We have investigated the crystal structural transformations in neodymium and scandium up to 4.0 GPa pressure and at room temperature, in a diamond-anvil high pressure apparatus. Nd has a double hexagonal-close packed (dhcp) structure at ambient pressure and temperature. Then it transforms to a face-centered cubic (fcc) structure at 3.8 GPa, which further transforms to a triple hexagonal-close packed structure (thcp) at about 18.0GPa. In scandium we observed only one transformation from the hexagonal-close packed (hcp) structure at room temperature to a tetragonal structure. This transformation occurs between 19.0 and 23.2 GPa pressure.
I. Introduction The lanthanide metals lanthanum (La) through lutetium (Lu) are trivalent at room temperature and pressure except europium (Eu) and ytterbium (Yb) which are divalent. The heavier trivalent rare-earth metals have hcp structure at ambient conditions and follow the crystal structure sequence hcp-Sm type-dhcp-fcc-thcp (or distorted fcc) as a function of increasing pressure. The trivalent transition metals scandium (Sc) and yttrium (Y) are also grouped along with the lanthanide metals, even though they are not strictly lanthanide group metals. Sc and Y are interesting contrasts to the rare-earths in behaving rare-earth-like in many aspects, but having no f electrons. Grosshans et al. have shown that Y indeed shows the complete rare-earth-like crystal structure sequence, whereas Sc fails to follow suit. We undertook a detailed study of Nd (rareearth) and Sc (rare-earth like) metals to under* Work performed under the auspices of the US Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. ** Originally from the People's Republic of China.
stand any systematics is their high pressure behavior, and in this preliminary report we present data up to 40.0 GPa pressure.
2. Apparatus and experimental procedure A diamond-anvil apparatus similar to that designed by Mao and Bell [2] was used. We loaded small ruby chips (10 micron) as pressure indicators in the cell along with the Sc sample. In the case of Nd, however, we loaded small ruby chips with and without gold to compare the pressures obtained. We believe the pressure determinations are accurate to 0.5 to 1.0 GPa. The scandium sample was in the form of 0.1290 mm thick chips of random sizes and 99.99+ purity. The Nd sample was a 0.076 mm foil, which was stored under mineral oil. The sample was mechanically cleaned under mineral oil to remove any surface oxidation or alteration products. A small sliver of the sample was then cut and loaded into a 120 micron pre-pressed stainless steel gasket-hole along with the pressure marker and silicone oil as pressure transmitting medium. We used 1:4 ethanol plus methanol mixture as the pressure
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
286
J. Akella et al. / Static high pressure studies on N d and Sc
Table I Impurities in the starting neodymium sample wt% al Ce Y La Sm Pr Si Fe Mg Ca AI Ta
<0.05 <0.05 <0.05 <0.05 <0.05 0.01 0.02 0.01 0.01
"~ Spectrographic analysis. Sample supplied by Research Chemicals Lot No. NO-M-3160.
transmitting medium with Sc. Some of the major impurities in the Nd sample are given in table I. A rotating anode X-ray generator with fine focus Mo target served as the X-ray source. Typical run times to collect diffraction data on film were around 48 hours. Values of the lattice constants were determined through least squares analysis.
3. Results
Nd has the hexagonal close packed (hcp) structure under ambient conditions and transforms to a face-centered cubic (fcc) structure at about 3.8 GPa. This pressure is slightly lower than the transformation pressure ( 5 . 0 G P a ) reported by Piermarini and Weir [3]. At about 18.0 GPa we found new X-ray diffraction lines compared to the lower pressure fcc phase. High pressure data between 1 8 . 0 - 3 8 . 0 G P a for Nd (Nd-III) were tested for a triple hexagonal close packed (thcp) structure, similar to that proposed for Pr-II| [4]. Nine out of twelve lines could be fitted to this structural form and three very weak lines ( I = 5 to 10) could not be fitted. Lattice parameters for Nd-III at --18.0 GPa are a -- 3.262(18)/~, c = 15.76(6),~, c / a = 4.831 and V/V o =0.709.
There seems to be little volume contraction in going from fcc to thcp and the 30% volume compression observed at 18.0 GPa is similar to that of other rare-earth elements. Using the unit cell constants proposed by Vohra et al. [5] for a distorted fcc phase, we calculated the possible reflections and d values for such a phase. Calculations done at L L N L (Smith and Akella) and at Penn State (D. Smith) show that for a distorted fcc phase there should be a strong reflection (with an intensity of =35) at about 5.25/~, which we failed to see in our films. Similar attempts to fit the data to an alpha-uranium structure also failed. We found that the thcp phase changes to another structural form (Nd-IV) at about 39.5 GPa. Grosshans and Holzapfel [6] also reported a structural change at 41.0 GPa. In both cases the crystal structure of the phase is not yet identified. Sc has the hexagonal close packed structure under ambient conditions. Between 19.0 and 23.2 GPa we observed new X-ray diffraction lines in the film. We took the data from our 26.0 GPa run (table II) where we think we have the diffraction pattern solely from the high-pressure phase for crystal structural determination. Grosshans et al. [1] have also reported a similar structural transformation in Sc at about 20.0 GPa and reported the structure for the high pressure phase as/3-neptunium type. Except for the energy dispersive X-ray diffraction pattern, they did not provide any d values in their paper for this structure (Sc-III), so a positive comparison could not be made with our data. We measured six X-ray films from runs at different pressures between 23.0 to 4 0 . 0 G P a pressure, and in table II have presented the data from one run at 26.0 GPa for further discussion. We used the indexing given by Grosshans et al. [1] for Sc-III and fitted our data (table II column 1) and we also incorporated the values for intensity from the powder diffraction file (PDF) for /3-Np. The following observations are important: a) reflections with h k l = 201,211 and 202 are not observed in the PDF data but are reported by Grosshans et al.; b) reflections with h k l =- 111 (•=80) and 311 ( ! = 100) are reported in the P D F but not
J. Akella et al. / Static high pressure studies on Nd and Sc
287
Table II Sc-III data at 26.0 GPa pressure* n
dobS (/~)
lobs
hkl
dc~,c (,~)
Io
1. 2.
2.651 2.382
VW VS
101 200 111
2.656 2.375 2.318
50 60 80
3. 4.
1.873 1.768
VVW VW
201 211
1.908 1.770
N.O. N.O.
5. 6.
1.676 1.600
VW VVW
7.
1.325
VVW
8.
1.285
VW
9.
1.249
VVW
220 1.679 002 1.602 102 1.518 112 1.446 311 1.360 202 1.325 212 1.279 N.I. fl-Np type Prim. tet; Z = 4 a = 4.750(12) ,~ c = 3.204(8) ,~ V/4 = 18.07(9) V/V~j = 0.734
60 5 30 30 100 N.O. 50 -
hkl
dc~,c (,~)
hkl
dca~c(•)
101 110 002
2.950 2.657 2.38l
200 112
1.879 1.773
211
N.I. 1.584
220
1.329
101 110 002 111 102 200 112 201 210 003 211 220
2.950 2.657 2.381 2.320 2.011 1.879 1.773 1.748 1.681 1.587 1.584 1.329
N.I.
221
1.280
N.I. b.c. tet; Z = 4
300 1.253 prim tet; Z = 4
a = 3.758(11) A c = 4.761(32) ,~ V/4 = 16.81(14)
~same ~sarne ~--same
V~V0 = 0.683
,-- same
* 10 = obs. int. f o r / 3 - N p : P D F 7-221, N.O. = not observed, N.I. = not indexed.
observed in their data; c) the d value calculated for the (201) reflection is a bad fit with our observed value; and d) a weak line with d = 1.299 is not indexable as a /3-Np type. We further tried to fit our data to a bodycentered tetragonal structure with Z = 4 (table II, column 2). We found a reasonable fit between the observed and calculated data. H o w e v e r , we found that three weak lines with d = 1.676, 1.285 and 1.249 could not be fitted and also we did not see a line (hkl = 101) at d = 2.950. Using the same lattice parameters as in column 3, we fitted our data to a primitive tetragonal cell (table II, column 3). Reflections with hkl = 101, 111, and 102 calculated with this cell were not observed in the experimental data, however there is excellent agreement between the rest of calculated and observed data with this cell. So far we have not found a prototype crystal to this cell as well as the one in column 2. Lattice parameters for Sc-III at P - - 2 6 . 0 GPa are
a = 3.758(11) A ; c : 4.761(32)/~, and V/Vo = 0 . 6 8 3 .
detailed elsewhere.
A
paper
on
Sc will
be
published
4. Summary In conclusion, we find that Nd transforms from hcp to fcc at a pressure of about 3.8 GPa and further transforms to a phase indexable as thcp at about 18.0 GPa. Both transformations occur with little atomic volume discontinuity. A fourth modification, as yet not identified, appears above - 3 9 . 5 GPa. Scandium remains in the hcp crystal structure from ambient pressure to about 19.0 GPa. A b o v e this pressure, the X-ray diffraction data seem to be best-fitted by a primitive tetragonal unit cell containing 4 atoms. This cell appears to provide a better fit to our observed data than tetragonal
288
J. Akella et al. / Static high pressure studies on Nd and Sc
s t r u c t u r e of t h e /3-Np t y p e p r o p o s e d by G r o s shans et al. O u r d a t a suggest that t h e r e a r e no similarities thus far, in the high p r e s s u r e s t r u c t u r a l b e h a v i o u r of b o t h t h e s e m e t a l s .
References [1] W.A.Grosshans, Y.K. Vohra and W.B. Holzapfel, J. Magn. and Magn. Mat. 29 (1982) 282.
[2] H. Mao and P.M. Bell, Carnegie Institution Washington, Geophys. Lab. Yr. Book. 74 (1975) 402. [3] C.J. Piermarini and C.E. Weir, Science 144 (1964) 69. [4] G.S. Smith and J. Akella, Phys. Lett. 105A (1984) 132. [5] Y.K. Vohra, V. Vijayakumar, B.K. Godwal and S.K. Sikka, Phys. Rev. B30 (1985) 6205. [6] W.A. Grosshans and W.B. Holzapfel, On Structural Systematics of the Lanthanides Under Pressure (preprint).