I. WARSHAW AND RusrruM R O Y Acknowledgments.-The authors are indebted to Dr. Jonathan Amy for his assistance in the preliminary design of the calorimeter bridge, and to Mr. Roy Hayes for his aid in electronic circuitry.
Vol. 65
Dr. J. C. Wallman and D. B. NcWhan of the Lawrence Radiation Laboratory were most generous in their aid and suggestions in the neptunium metal preparation.
POLYMORPHISM OF THE RARE EARTH SESQUIOXIDES' BY I. WARSHAW AND RUSTUM ROY Contribution N o . 60-98, College of Mineral Industries, The Pennsylvania State University, University Park, Pa. Received M a y $6, 1961
Under equilibrium conditions the polymorphic forms of the rare earth sesquioxides transform reversibly from one form to the other. The sesquioxides of the largest ions, La, Ce and Pr, exhibit the A-type structure, Ndz03 exists both m A- and Gdz03, Tbz03and DyzOs exist both as B- and C-type oxides. The remaining rare earth sesC-type, while Smz03,.Eu203, quioxides occur only in the C-type structure. The C to B transition temperature increases linearly with decreasing ionic radius of the rare earth cation.
Introduction The proper understanding of the polymorphic relationships among the rare earth sesquioxides is fundamental to any study involving these compounds and is of special importance when one considers the use or reaction of the rare earth oxides a t elevated temperatures. A number of investigators, starting with Goldschmidt, et C L ~ .in, ~ 1925, have tried to outline the polymorphic relationships among the rare earth sesquioxides, but there is wide disagreement among all the studies. The reasons for the widely differing results are readily apparent. Sufficiently pure rare earth oxides were not available to the earlier investigators, and thus it is to be expected that their work should contain many incorrect results. Purified rare earth sesquioxides were used for the more recent studies, but, with only minor exceptions, the experiments mere not carried out with a view to studying equilibrium assemblages. Obviously, unless equilibrium is attained, it is impossible to compare the results obtained in one laboratory with those obtained in another. Due to the fact that they are reconstructive transformations, the inversions are particularly sluggish and in some instances it is difficult to attain equilibrium. This, however, does not detract from the fact that with time the compounds may invert even at low temperaturw, and thus it is necessary to know what mill happen under equilibrium conditions. In view of the fact that the authors were undertaking a program on the crystal chemistry of rare earth compounds and phase equilibrium studies involving the rare earth oxides, the re-examination of the polymorphism of the rare earth sesquioxides under c.quilibrium conditions was of particular importance. The first studies of the polymorphism of the rare earth sesquioxides were carried out by GoldSchmidt, Ulrich and Barth2 in 1925, and followed by Goldschmidt, Barth and Lunde? in 1923. They found that the rare earth sesquioxides could exist in three polymorphic forms, which they denoted as A, (1) Disssrtatlon of I. War-haw in the Depsrtrneiit of Geophysics
and Geochemistry a t The Pennsylvania State University. (2) V. A t . Goldschmidt. F. Ulrich and T. Barth Skrz/ter Norvke Videnakapr-Aknd Oslo. I M a t . Natura. K l . , No. 5 (1925). ( 3 ) V. X . Goldschmidt, T. Barth and G . Lunde, abzd. No.7 (1925).
B and C, and classified the structures as being hexagonal, probably monoclinic and cubic, respectively. They also noted slight differences within the B-type structures and classified them either as B, or €3,. In their earlier work they studied the reversibility of the transformations and concluded that the A to B transformations were reversible, while the B to C transitions were more complex, being monotropic for Sms03but enantiotropic for Dy203. While a single tentative stability diagram was presented in the first paper by GoldSchmidt, et al., eight generalized possible diagrams are given in the second paper. This larger number of diagrams is due to the inconclusive nature of the results and to the possibility that some of the polymorphs may be metastable. Lohberg4 prepared C-type La203 and Kd203 by gentle heating of the respective nitrate compounds. While Bommers was able to confirm the existence of C-type Kd203,he was not able to form C-type Ce203 or Prz03. Iandelli6 heated a number of the rare earth sesquioxides a t various temperatures to obtain the approximate transformation temperature. From his study he postulated that the transformation temperature varied almost linearly with the atomic number. He obtained C-type Laz03 and Prz03, but below 600" the former compound always consisted of a mixture of the C- and A-type structures. Shafer and Roy7 studied the transformation temperatures for Nd?O3, SmPOa and Gdz03 by hydrothermal techniques and as a result they chose what they considered the most probable of the eight diagrams proposed by Goldschmidt, et ~ l . , ~ indicating the relationships among the various polymorphic forms of the rare earth sesquioxides. Moreover, they confirmed the existence of B-type XdzO, which was found previously only by GoldSchmidt, et al. The polymorphism of the trivalent rare earth oxides was reinvestigated more recently by Roth and Schneider.8 They concluded that each oxide (4)
K. Lohbvrg, 2. physak. Chem., [B], 28, 402 (1935).
( 5 ) 11. Bommer, Z . anorg u. allgem. Chem , 241, 273 (1939). (6) A. Iandelli, Gazz. chmm. atal.. 77, 312 (1947). 17) RI. W. Sliafer and R. Roy, J . Am. Ceram. S O C ,42, 563 (1959). (8) R. S Roth and S. J Schneider, J . Research Natl Bur. Stand-
a r d p A . Phys. and Chem., 64A, 309 (1960).
POLYMORPHISM OF THE RAREEARTHSESQUIOXIDES
Nov., 1961
2049
RAREEARTHs E S Q U I O X I D E 8 La
Ce
Pr
Nd
Sm
Eu
Gd
B~
niS2
Tb
(800) (875)
c c --
_I--p___-
' a Shafer and Roy (1959)
a / a
I**l*j A
A (1030) B 915
c A
B 840
c
B
Iandelli
b
Dy
€10
Er T m
Yb
Lu Temp. indicated were determined from diagram. A11 4 t o B and some B t o C transforma-
C
B 1025
o c --
c
c
Temp. shown were determined from data and arr + 5 0 ° . Max. temp. attained was 1500'.
B 1350
C
A
a
A
A
B
Roth and Schneider (1960)
950
*
650
a
A
A
1I1 I 1 A
Narshaw and R o y
*
c
~~o
c
c
c
B 1075
1250
c
c
____
-__
R 875
c
c
C _ -
1200
c l c
.
-/Temp. shown are given by authors. Limits of error were not given. All transformations are considered t o be irreversible: all low-temp. forms believed to C be m&astable.
BIB
1100
c
...
c
I c
..
.
I C
..
111 transformations are revereible. 911 the polymorphs shown lare stahle within their respective ,temp. ranges.
,
. , , .. . ., L ne exiswnce 01CLIC umymorunic Iorms snown In small letters 1s indlcatea in tne aiagram or 1 x t of the paper; however, no experimental n ' x e : m, evidcnce is given in the paprr. 0 While the C'-typr polymorph is indicated in the diagram, the authors express doubt t h a t i t exists. ?.~.
1
Fig. 1.-comparison
? , I
1 .
I
I
of the data given by several investigators on the existence of the high (il), medium (B), and low (C) temperature forms of the rare earth oxides and their transformation temperatures.
exists stably in only one form, the A-type for La203 to Ndz03, the B-type for Sm203 to GdzO, and the C-type for all the remaining trivalent oxides. While they found that Kd203, Smz03, Eu203 and Gd203 could exist as C-type a t lower temperatures, they concluded that these polymorphs were metastable and that all low-temperature forms inverted irreversibly to the stable form at higher temperatures. All of the above work is summarized in Fig. 1. While most of the inversion temperatures shown in the figure are those specifically stated by the respective authors or taken from their tables of data, the temperatures given in parentheses were estimated by us from diagrams given in the respective papers.
Experimental Equipment and Techniques.-For the most part the furnaces and cquipment used throughout this study have been described previously. This includes platinum-wound quenching furii;tces,Y the strip-furnacelO and hydrothermal equipment . l l Because of the temperatures involved, the strip-furnace was used extensively. During the course of this study the maximum temperaJure attainable with this furnace was extended from 1850 , the melting point of 60 platinum-40 rhodium, to 2400" by the use of iridium strips. Since it is preferable to heat iridium in an oxygen-free atmosphere, a plastic box utilizing an O-ring seal was constructed to maintain a purified nitrogen atmosphere around the sample. By attaching the optical pyrometer to the base of the furnace, it was possible to be only a few inches from the sample. As a result the sample could be observed more closely and the sample temperature could be determined with greater accuracy. At temperatures below IOOO', hydrothermal techniques were frequently used to aid diffusion of the ions and to establish equilibrium more rapidly. A variation in this -~
(9) E. 9. Shepard, G. A. Rankin and F. E. Wright, Amer. J . Sci., 178, 293 (1909). (10) M. L. Keith and R. Roy, Am. Minerdogist, 89, 1 (1954). (11) R. Roy and 0. F. Tuttle. "Physics a n d Chemistry of t h e Earth," Vol. I, edited by L. H. Ahrens, KalerVo Rankama. Frank Press and S. K. Runcorn, Pergamon Press, London and New York, (Ch. VI 1956).
technique, known as "leak quenching,"7 was employed so that the oxide being studied was in contact with water only a t the highest temperature of the experiment. In this method, the water is added after the material is up to temperature and removed (leak-quenched) before the temperature quench. After the water vapor is released, the sample is dried a t the temperature of the run for one-half to three hours before being quenched to room temperature. The longer drying times are required for low temperature runs. Leak-quenching was necessary to prevent the hydration of the rare earth oxides during cooling. With the oxides of all of the rare earths smaller than neodymium, the water vapor could act only as a catalyst because the temperatures of the experiments to determine oxide phase transitions were well above those a t which hydrates could form. Temperature Measurement.-The temperatures which are reported throughout this study were obtained by the use of Chromel-Alumel thermocouples if the temperature was below lOOO', of calibrated platinum-platinum-10 rhodium thermocouples when temperatures were in the range of 1000 to 1500' and of a calibrated optical pyrometer when the temperature exceeded 1500'. The melting points of the clcments or compounds used as calibration standards were Diopside Pseudowollastonite Platinum, C.P. Corundum
CaMgSizOe CaSiOr
Pt
2412Oa
1391 5' 1544" 1773' 2050'
Reagents and Preparation of Mixtures.-The purity of the rare earth sesquioxides is of special interest. For this study the rare earth compounds were a t least 99.8% ure, with most of the compounds being 99 9% pure. Whire no quantitative analyses of the starting materials were carried out, semi-quantitative X-ray fluorescence analysis of most of the compounds used indicated only traces of contaminating rare earth ions in any particular compound, generally within the manufacturer's stated limits. The nature of the starting materials used in this study varied with the particular experiment. Poorly crystallized, reactive hydroxides were used to avoid the metastable persistence of certain oxide starting materials. The former were prepared by dissolving the oxides in nitric acid followed by precipitation with ammonium hydroxide. The precipitates were separated and washed by centrifugation and then were dried at 105 In order to test reversibility, it was essential to start with the various polymorphic forms of the oxides. The determination of the equilibrium temperature was made by heating both polymorphs of a certain oxide side by side at various temperatures followed by examination of both to detect changes in either one. In this manner it was
.
I. WARSHAW AND RUSTUM ROY
2050
, 2200-
THERXIAL
Temp. (“C.)
2000-
18001600-
A
C
1400O
G. 1200-
560 600 600 600 600 600 696 715 715
Vol. 65
TABLE I DATAFOR OSE-COMPONENT RAREEARTH SESQUIOXIDE SYSTEMS Pros-
Time
22 da. 66 hr. 66 hr. 1 hr. 2 hr. 2 da. 24 hr. 20 hr. 20 hr.
sure (p.s.i.)
Reactants
The system Nd,03 .. A 5000 A 5000 Hydroxide gel . . C hydroxide . . Hydroxide gel . . Hydroxide gel 8000 C A 4000 C 4000 Hydroxide gel
+
Products
+ +
A tr. C C ( hydrate) C ( + hydrate) C tr. A
+
C
c+A x
+
A ( A(
+ hydrate) + hydrate)
The system Sm2O3 860 5 hr. 5000 Hydroxide gel 5000 872 48 hr. B 880 20 hr. 5000 C 890 4 hr. 5000 Hydroxide gel 2350 30 min. .. C
1000-
000600-
400-
IONIC
974 1096 1105 1145
22 hr. 24 hr. 30 hr. 18 hr.
980 1182 1190 1215 1235 1309
24 hr. 35 hr. 24 hr. 30 hr. 20 hr. 18 hr.
1800 1860 1870 1880 1945 2200
30 min. 30 min. 30 inin. 30 min. 30 min. 15 niin.
RADIUS
Fig. 2.-Temperature stability relationships of the rare earth sesquioxide polymorphs. readily possible to determine which polymorph was the stable form a t any given temperature. Identification of Phases.-The phases formed under given conditions were identified in every case by powder X-ray diffractometry using Sorelco and GE instruments. In this study the distinctions are simple and unequivocal.
Results
The system Eu203 3000 B .. C .. C C
..
The system Gd203 3000 Hydroxide gel .. B B . . Hydroxide gel . . Hydroxide gel .. C C , . Hydroxide gel The system Tb& C B B .. C .. C .. C
.. .. ..
+
C
C C small amt. B B C
+ small amt. C +
C B small amt. B B C
B B C
+ small amt. C + ti-. C + small amt. B
The results of the critical experiments are listed in Table I and are plotted in Fig. 2. In Fig. 2 , one can readily observe the relationships B+C 13 of the various polymorphs to each other and the dependence of the C to B inversion temperature on the The system Dy& radii of the rare earth ions. The linear relationship . . C 2050 15 min. C between the ionic radius and the transformation B C+B temperature for those compounds which involve 2080 50 min. . . C B + C both C- and B-type polymorphs is very striking. 2190 15 inin. . . C u + small amt. c The second feature of the diagram is the fact that 2300 10 min. . . the boundary between the A- and B-type polymorphs is almost vertical so that ;luTd2O3does not forms in which the various oxides can exist and exhibit any B-type structure and Sme03does not their stability relationships. The results found in this investigation are almost exhibit the A-type structure even near its melting point. While the boundary is drawn in such a a composite of certain statements by all the premanner that it appears that promethium sesyui- vious workers who haye studied this subject. To oxide would exhibit all three polymorphs there is no begin with, all the transformations, cspecially the experimental evidence for this. The problem of C to B, are reversible. While Goldschmidt, et U Z . , ~ what forms Pmz03would exhibit is strictly hypo- and Shafer and Roy7 found this to be true for some thetical and is not likely to be solved readily since of the sesyuioxides, Roth and Schneide? claim that all the trailsformations are monotropic and that the promethium-147 has a very short half-life. low temperature forms are always metastable. Discussion The linear relationship between the transformaThe diagram shown in Fig. 2 most nearly cor- tion temperature arid the ionic radius of thobe responds to that previously determined by Shafer oxides which exhibited both €3- and C-type strucand ROY.? However, there are considerable dif- tures was mentioned in a general seiise by Iandelli.6 ferences between this work and all previous studies Hornever, his ideas were based on his inaccuratc regarding the inversion temperatures, the various data and, moreover, he includcd the C to A as w l l
Nov., 1961
DIPOLEMOMENTS OF PHOSPHITE ESTERS AND THEIR DERIVATIVES
as the C t,o B transformations in his statement. The almost vertical boundary between the Aand B-type polymorphs also was postulated by Roth and Schneider,* while both Goldschmidt, et a1.,2 and Shafer and Roy7 show the boundary to have a moderate slope. These moderate slopes are due to the fact that Goldschmidt, et al., obtained A-type Smz03and B-type NdzOa while Shafer and Roy also found B-type KdZO3. I t is believed that the availability of purer samples accounts for some of the differences between the present and previous studies. It is especially likely that the sesquioxides which Goldschmidt, et al., used were impure since B ~ m m e rwho , ~ claimed that he used the purest rare earth oxides which were available in 1939, points out that his samarium oxide contained 0.8% europium and 0.3% gadolinium. The differences between this work and that of Roth and Schneider8 are mostly due to their not using hydrothermal techniques to attain equilibrium a t the lower temperatures and to the fact that, with only three exceptions, their study was limited to a maximum temperature of 1500". In one instance they used an arc image furnace to melt DyZO3, but they were not able to quench the sample rapidly and did not observe the B-type polymorph. The sesquioxides of the largest ions (La, Ce, Pr and Kd) most commonly exhibit the A-type structure. Laiithana has been reported4 to exist in the C form, but it has not been possible to repeat this preparation of C-type LazOa. This polymorph has been prepared hydrothermally from the appropriate A- or B-type oxides of S d , Sm and Eu. It should be emphasized here that, in the method used, the oxides were in contact with water only at the highest temperature of the run. It is not possible to obtain C-type oxides for the still larger rare earths hydrothermally since the A-type polymorph is stable down to the upper temperature limit for the oxyhydrosides. The inversion of A to C by dry heating has not been ef-fectedexcept in the instance
2051
of neodymia, which indicates either that C is not a stable form of Laz03,Cez03 or Prz03or that the inversion temperature is too lorn for it to occur in a reasonable length of time. C transition of NdzO,, is about 600" The A whereas Roth and Schneider8stated that a temperature of 650" was required to change C to A. While Shafer and Roy7 found that Nd203 could exist as the B-type polymorph as well as in the X and C forms, it was not possible to reproduce their results even though the same conditions and hydrothermal equipment were used in this study. This is probably due to a sample of higher purity being used in the present study. The intermediate rare earth oxides exist in both the B- and C-type polymorphs and these transfoimations are reversible, as shown in Table I. These include Sm203, Eus03, Gd203. Tb2O3 and Dy203. While B-type Dy,03 was reported preneither they nor viously by Goldschmidt, et any other previous investigators mentioned B-type Tb208 even though this oxide transforms at a lower temperature than Dyz03. As for Dy203,it should be noted that it has not been possible to obtain pure B-type. As stated previously, the transformations are very sluggish and thus a considerable length of time is required to transform completely a particular sample. With our equipment it was not possible to maintain approximately 2400' for more than 15 to 30 minutes and thus the samples of Dy,Oa consist of both the B- and C-type polymorphs. The interpretation of such data js based on the fact that one polymorph will not grow a t the expense of a second unless the first is the stable form under the conditions of the experiment. The sesquioxides of the smallest ions, 1'. Ho, Er, Tm, Yb and Lu, exist only as the C-type polymorph. All previous studies agree on this point. Acknowledgment.-This work forms part of a research program in crystal chemistry supported by the Chemical Physics Branch of the U. S. Army Signal Corps.
THE DIPOLE MOMENTS OF SOME PHOSPHITE ESTERS AND THEIR DERIVATIVES BY THEODORE L. BROWS,J. G. VERKADE llXD T. S. PIPER Soyes Cheirizcal Laboratory, University of Illinois, rrbana, Ill. Rcceized June 6, 1951
The dipole moments of the constrainrd phosphite esters l-methyl-4-phospha-3,5,8- trioxabicyclo [2.2.2]octane (I) and l-phospha-2,8,9-trioxa-adamantine (VI) have been determined in dioxane solution. In addition the moments of the phosphate and thiophosphate of I have been determined. The constraints imposed by the bonding in I preclude free rotation about the P-0-R bonds, thus permitting a better estimate of the apparent P=O and P=S group moments. These are 2.95 and 2.62 D, respectively.
Introduction
lated comDounds have been determined. Rotation of the alkdxy groups about the P-0 bond is not possi-
This paper reports the measurements of the di- ble in either 1or 1 7 1 . The interpretation of the dipole pole moments of the constrained phosphite esters, moments of these COmpOUndS, and more particul-methyl-4-phospha-3,5,8-trioxab~cyclo[2.~.~l-~clarly of the difference in moment between phosphite tane (I) and 1- phospha - 2,8,9 - trioxa - adamantine and phosphate or thiophosphate, is more straight(VI). In addition the moments of a number of re- forward 6han in ordinary phosphite compounds.
