Single crystal preparation and characterization of the group VA metal hydrides and deuterides

23

Journal of the Less-Common Metals, 66 (1979) 23 - 32 0 Elsevier Sequoia S.A., Lausanne -Printed in the Netherlands

SINGLE CRYSTAL PREPARATION AND CHARACTERIZATION THE GROUP VA METAL HYDRIDES AND DEUTERIDES F. REIDINGER,

J. J. REILLY and R. W. STOENNER

Department of Energy and Environment and Department National Laboratories, Upton, New York 11973 (U.S.A.) (Received

OF

November

of Chemistry,

Brookhaven

7,1978)

Summary Single crystals of the Group VA (vanadium group) hydrides and deuterides were prepared. The quality of the specimens, as assessed by X-ray and neutron diffraction techniques, was usually sufficient for detailed structural analyses; these are currently in progress. It was noted in many cases that the single-crystal hydride (deuteride) could be cycled through the a’-6 (or 0) phase transition without a deterioration in crystal quality. In one case a single-domain single crystal of TaDa.,, (6 phase) was successfully produced.

1. Introduction Single crystals are preferred for structural studies of hydrogen-containing compounds by neutron diffraction. However, single crystals can be difficult, if not impossible, to prepare, in which case powder samples are used. In this event deuterium is generally substituted for protium because of the large incoherent scattering of the lighter isotope. To date most of the structural studies of the Group VA-hydrogen compounds have been carried out using powder samples of the corresponding deuterides [ 1 - 51. A notable exception is the work of Chervyakov et al. [ 31; these workers used a single crystal of (Y’ vanadium deuteride (VD,.,). The scarcity of singlecrystal studies of the Group VA-hydrogen systems is due to difficulties in sample preparation. This is evident upon examination of the phase diagrams [6], all of which exhibit wide miscibility gaps which persist to relatively high temperatures. A further serious problem is the multiple twinning which almost invariably occurs upon the transition from the (Y’ phase to phases of lower symmetry [ 71. This transition occurs above room temperature in all cases except in the vanadium-deuterium system which retains the (Y’ b.c.c. structure well below room temperature in the composition region VD,., _1.s; this property was exploited by Chervyakov et al. in the preparation of VDe.s. In this paper we shall describe our procedures for preparing single crystals of the Group VA metal hydrides and deuterides which were subse-

24

quently used as samples for neutron diffraction studies. Preliminary results of these studies, concerning the state of deuterium in VD0.038, VD,., and have already been reported [S] and more detailed discussions will VDO 79’ be presented in forthcoming papers. Here we shall discuss the preparation of single-crystal hydrides and deuterides and their characterization with respect to crystal quality. The preparation of a single-domain crystal of TaDo.7g (6 phase) will be especially noted.

2. Sample preparation Single crystals of V, Nb and Ta, with a typical volume of a few cubic millimetres and weighing up to 2 g, were cut from cylindrical specimens which had been obtained from one of three sources (Materials Research Corporation, C. V. Yang of Oak Ridge National Laboratory or F. A. Schmidt of Ames National Laboratory [9] ). Cold-worked surface layers were removed by repeated polishing with AlzO, and etching with a 1: 1 mixture of HF and HNO, until the sharpness of the reflections on back-reflection Laue photographs indicated that the surface was essentially strain-free. In order to prepare a sample having a hydrogen (deuterium)/metal ratio of about 0.8, a single crystal of the elemental metal was placed in a stainless steel cup l/2 in in diameter and 1.2 in deep. The cup was introduced into a high pressure stainless steel reactor which has been described previously [lo]. Situated just below the cup was about 5 g of powdered material of the same Group VA metal. This material, which had a relatively high surface area, served both as a getter of gaseous impurities and as ballast to absorb any excess hydrogen (deuterium) remaining in the gas phase after the sample had been equilibrated at a particular temperature and pressure (see below). After being sealed the reactor was attached to a gas-vacuum manifold and the sample was outgassed under a dynamic vacuum (10d5 Torr) and heated to 500 - 600 “C!. At this point hydrogen (deuterium) gas was admitted to the reactor until the pressure was 30 - 40 atm. The gas was derived from the decomposition of TiFeH(D), [ll, 121. The rate of pressure increase ranged from 1 to 60 atm h-l. The lower rate is preferred because, even at these temperatures, there is appreciable solution of H(D) in the metal which, if too rapid, causes appreciable distortion of the crystal. After the addition of H2 (Dz) the reactor was cooled at a uniform rate of about 2 “C h-l, resulting in a slow controlled dissolution of H(D) in the metal crystal. When the desired solid composition was reached, aa estimated from the phase diagram, the reactor valve was closed to isolate the sample and to minimize the amount of gas in contact with it. In all cases the hydride (deuteride) crystal at the end of the loading process was in the a’ phase and at a temperature appreciably above room temperature. The reactor was then cooled to room temperature; during this process most of the gas remaining in the reactor was absorbed by the ballast material while the crystal composition changed only slightly. The use of a ballast material to

25

absorb excess hydrogen (deuterium) is necessary with vanadium and niobium because in both systems it is possible to precipitate a d~yd~de (dideu~~de) phase at room temperature if the pressure is greater than several atmospheres. Since Ta does not form such a phase, ballast material is not strictly necessary although it serves as an effective getter. Upon removing the crystals from the reactor they come into contact with the air. Such contact deactivates the metal surface and prevents hydrogen (deuterium) loss even upon moderate heating. This deactivation phenomenon has been noted previously and has been exploited to obtain X-ray diffraction patterns of unstable metal hydrides at normal temperatures and pressures [11,131. For the preparation of a crystal with a specific concentration we employed a two-stage process. The crystal was loaded to a concentration higher than that desired, after which it was removed from the reactor and was cleaved into two unequal portions. The hydrogen (deute~um) content of the smaller fragment was determined by vacuum extraction at 1000 “C. The remaining fragment was introduced into a quartz reactor which was evacuated and gently heated. Eventually (above 130 “C) the crystal slowly began to evolve hydrogen (deuterium) which was pumped into a calibrated volume. Gas was removed in this fashion until the hydrogen (deu~~um) concentration in the crystal was at the desired level. At this point the pumping was stopped and the reactor was quickly cooled to room temperature. A number of a-phase single crystals were also prepared. In this case the synthesis is quite simple; the sample was heated to about 500 “C in a quartz reactor under a dynamic vacuum and the amount of hydrogen (deu~ri~m) required to give a specific concentration was introduced into the system. The reactor was then slowly cooled to room temperature. After preparation the crystals were examined by X-ray back-reflection techniques at room temperature and at various other temperatures by neutron diffraction on a four-circle diffractometer at the Brookhaven High Flux Beam Reactor. The (110) plane of a Be crystal provided quasi-monochromatic neutrons with a wavelength of 0.662 a. For studies at elevated temperatures a small aluminum furnace was used [14]. The hydrides (deuterides) of both V and Nb could be heated in air to about 200 “C with no loss of hydrogen or deuterium. However, as noted below, a tantalum deuteride crystal (TaD ee7a) lost a small amount of deuterium at 150 “C. In all cases no reaction with air was noted and the crystal surfaces remained shiny and bright with no evidence of tarnishing.

3. Results and discussion The quality of all the e-phase single crystals was unaffected by the hydrogen (deuterium) loading process and will not be discussed further here. In contrast, the high concentration crystals were generally heavily twinned at room temperature (Fig. 1). This is attributed to the passage through the

26

Fig. 1. Laue pattern of 6 -TaHo.,,

24 ‘C!, (loo),

face. Note multiple

twinning.

ru’-6 (or 0) phase boundary which occurs, with one exception, during the final cooling step in the synthesis procedure. The exception involves the vanadium deuteride crystals where the a’-& phase boundary, at the compositions of interest, lies well below room temperature. However, ‘upon reheating the crystals to a temperature above the phase boundary, they recovered sufficiently in the ty’ single-phase region to permit us to obtain neutron diffraction data suitable for detailed structural analyses. About 30 40 unique reflections could be obtained from which analyses could be made having R factors frequently less than 1%. Such results compare quite favorably with similar investigations using powder samples where a maximum of 6 reflections were obtained and in which subsequent structural analyses had R factors of about 4% [l - 53. We noticed that the degree of perfection of the crystals in the CY’ phase was affected by the rate of the pressure increase during the loading process. A rate of about 60 atm h-l, for instance, caused such distortion of a crystal of NbHosz that individual spots could barely be recognized on a backreflection photograph and the rocking curve in the ty’ phase was visibly broadened. At a rate of about 1 atm h-l, however, the crystals did not seem to be affected by the loading process; a small broadening and slight asymmetry of the rocking curve in the Q’ phase could be attributed to irreversible distortion caused by the twinned state of the crystal in the p phase. This effect is illustrated in Fig. 2 using a crystal of NbD0.,4. The increase of the

27

”

7.0

7.5

8.0

w (deg)

Fig. 2. Omega scan of (110) reflection of crystal of NbDo.,a: CJOO fl phase at 90 “C showing multiple twinning.

0.0 LY’phase at 140 ‘C;

full width of the (CY’phase) peak at half height (FWHH) is estimated to be about 0.06 f 0.02” over that of the same 110 reflection of a virgin Nb single crystal; the slight asymmetry is caused by distortion due to the heavy twinning apparent in the fl phase. Another example of the remarkable ability of the metal lattice tp recover from severe distortions was provided by a disc-shaped specimen of VH,.,, (r = 3 mm, h = 1.5 mm) which is monoclinic at room temperature [ 151. In this case the deviation from cubic symmetry is so great that the monoclinic phase was greatly distorted; the Laue photographs .did not show any distinguishable spots and no reflections could be found on the neutron diffractometer at 23 “C. At about 230 ‘C, however, reflections characteristic of the (Y’phase appeared and a satisfactory data set could be obtained. Unexpected difficulties were encountered during the preparation of several a’ vanadium deuteride crystals. Cylindrical specimens with axes parallel to the [ 1001 or [ 1111 directions frequently fractured along a (111) or (100) plane, respectively. Only one of these crystals (VDa.,a) was judged suitable for providing data of the required quality. It fractured a second time along a different (100) plane at about 200 “C! when it was being heated in preparation for data collection. Unlike our other specimens, the quality of this crystal appeared to deteriorate after it was cycled through the (r’-6 transition, which occurs at -60 “C for this composition. However, analysis of a data set taken at -52 “C indicated an increase in extinction even though considerable distortion was evident from the Laue diffraction pattern. It is possible that the anomalous behavior of these specimens was related to the shape and orientation of the starting vanadium crystals, because a specimen was recently prepared without fracture from a crystal with the of VDo.75 [ 1101 direction parallel to its cylinder axis.

28

(OIO),, (01 llc

flOOf$ (loo), I

, . r

Fig. 3. Diagram of single-domain crystal of TaD,.,,. Heavy lines delineate the singledomain region which has dimensions of 1.5, 2.0 and 4 mm. Twinned volumes at both ends were subsequently removed by cleaving.

Room ~mpe~ture Laue photo~aphs of a tantalum deuteride crystal, (TaDa.79, Fig. 3_)with dimensions 1.5, 2.0 and 5.0 mm along the [Oil],, [loo], and [Oil], directions, did not display any noticeable distortions. Deviations from cubic symmetry were so small that the (loo),, face still displayed the 4mm symmetry of the cubic phase. A 8 - 28 scan of the (022), reflection (Fig. 4(a)) showed that the volume amount of twinning was minimal and we concluded that the bulk of the crystal consisted of a single domain. Neutron scans with parts of the crystal covered with Cd foil confirmed tbat the twjnned volume was small and concentrated in unequal portions at the (Oil), faces of the crystal, as depicted in Fig. 3. At this point two data sets were collected at 110 and 150 “C in the (II’phase and a limited set in the o~orhombic phase at 23 “C. Analysis of the data collected at 150 “C indicated that a slight loss of deuterium had occurred which reduced the concentration in the crystal to TaD,.,,. Its quality did not suffer, however, and we do not consider that the small composition change affects our results or conclusions. The twinned portions of the crystal were then removed by cleaving and another limited data set was collected which proved that the salvaged specimen was indeed a single-domain crystal. From a subsequent full data set, using a Ge (331) monochromator to eliminate the h J2 component, we deduced that the Laue symmetry was mmm. A survey of the recorded reflections showed that their halfwidths depended on the orientation of the crystal. Figure 4(b) displays a fl - 20

F IOOOC

2 : d

b

;2 f

5ooc

8

C

I

39. 50

32.0 28

39.75

40.00

40.25

28 Cdegl

(deg)

(4

(b)

I

r’

,

600

500 7 c 5 400 d k i

300

z 2 ”

200

100

0

13.00

14.00 w

15.00

Cdeq)

(cl

Fig. 4. (a) 19- 28 scan of 6 tantalum deuteride (TaDu.79) at 24 “C; (004),, reflection. Note only slight indication of twinning. (b) 8 - 28 scan of 6 tantalum deuteride crystal (TaD0.n) after cleaving to remove twinned portions, 24 “C: 000 in [ 0101 zone; 000 in [ 1001 zone. (c) Omega scans of (220),, reflection of tantalum deuteride crystal (TaDo.77): 0.0 cr-6 mixed phase region at 97 “C; 000 6 phase at 94 “C.

scan of the (004), reflection which occurs near the focussing angle of the diffractometer. It will be noted that the reflection is split by about 0.12” when the [OlO], zone axis is perpendicular to the plane of diffraction, but careful examination of previously recorded high temperature data showed that the splitting had been present in the CY’phase as well. We are of the opinion that this splitting is not caused by monoclinic twinning because it

30

is not affected by the cu’-S transition, unlike the larger orthorhombic splitting of about 0.50”, as shown in Fig. 2, which disappears above the transition temperature. This hypothesis is supported by a satisfactory (R = 1%) analysis of the data using a model which incorporates the symmetry elements of the orthorhombic space group Cccm, No. 66 [ 161. Further, no comparable splitting could be found in any of the other hydride or metal single crystals we prepared. It is quite possible that this small distortion, which may have arisen from long-range elastic interactions [ 171, was instrumental in preserving the single-domain nature of this crystal during the (~‘-6 transition. The above-mentioned crystal (TaDe.,,) was heated to 97 ‘C, which proved to be within the a’-6 two-phase region as predicted by the phase diagram [ 181. As can be seen from Fig. 4(c), a wide peak and three sharp ones appear in the vicinity of the (200),, reflection. When the temperature was lowered to 94 “C only the presence of the 6 phase was indicated and the crystal displayed a surprising recovery. At 23 “C the low angle tail (o = 13.5”) and the shoulder at o = 14.5” were completely absent and the crystal had regained its original singledomain state. Two crystals of specific intermediate compositions were prepared, the p phases of VDe.&s and TaHo.502. In both cases twinning was limited. A Laue photograph of the (loo), face of the vanadium crystal taken at 24 “C (Fig. 5) shows a distinct reduction of symmetry from 4mm to m. There

Fig. 5. Laue pattern of fl-VD0.498, (loo), from 4mm to m.

face, 24 OC, showing reduction of symmetry

31

no evidence of the presence of the cyphase. A similar but less pronounced reduction in symme~ was noted in a Laue photograph of the same face of the tantalum hydride crystal. Some twinning, however, was evident in photographs of the (Oil), and (Oil), faces of both crystals. These crystals will be examined by neutron diffraction in order to determine whether sufficiently large single domains are present for satisfactory data collection and subsequent structural analyses. was

4. Conclusions 1. Multiply twinned crystals of Group VA metal hydrides and deuterides recover sufficiently in the (Y’ phase to allow acquisition of the high precision data required for detailed structural analysis. 2. The degree of twinning in the low temperature phases is frequently less than expected and probably depends on the shape and size of the original metal crystal. 3. Multiple twinning does not invariably accompany a transition from a higher symmetry to a lower symmetry hydride or deuteride phase.

Acknowledgments This work was supported by the Division of Chemical Sciences, U.S. Department of Energy, W~hing~n, D.C., under Contract No. EY-76-C-020016. We wish to thank M. Eison, J. B. Hughes and A. Cendrowski for their assistance and Dr. Paul Thompson for his helpful suggestions.

References 1 V. A. Somenkov, A. V. Gurskaya, M. G. Zemlyanov, M. E. Kost, N. A. Chernoplekov

and A. A. Chertkov, Sou. Phys. Solid State, 10 (1968)1076. 2 V. A. Somekov, A. V. Gurskaya, M. G. Zemlyamov, M. E. Kost, N. A. Chernoplekov and A. A. Chertkov, Sou. Phys. Solid State, 10 (1969)2123. 3 A, Yu. Chervyakov, I. R. Entin, V. A. Somenkov, S. Sh. Shil’stein and A. A. Chertkov, Sov. Phys. Solid State, 13 (1972)2172. 4 D. G. Westlake, M. H. Mueller and H. W. Knott, J. Appl. C~~taZ~ogr., 6 (1973) 206. 5 H. Asano and M. Hirabayashi, Phys. Status Solidi A, 15 (1973) 267. 6 T. Schober and H. Wenzl, The Systems NbH(D), TaH(D), VH(D): Structures, Phase Diagrams, Morphologies, Methods of Preparation. In G. Alefeld and J. VSlkl (eds.), Hydrogen in Metals II, Springer, Berlin, 1978. 7 T. Schober, Phys. Status Solidi A, 29 (1975) 395. 8 F. Reidinger, J. J. Reilly and R. W. Stoenner, abstract in G. D. Mahan and W. L. Roth (eds.), Superionic Conductors, Plenum Press, New York, 1976, p. 427. 9 0. N. Carlson, F. A. Schmidt and W. M. Paulson, Trans. Am. Sot. Met., 57 (1964)

536. 10 J. J. Reilly and R. H. Wiswall, Znorg. Chem., 6 (1967) 2220. 13 J. J. Reilly and R. H. Wiswall, Znorg. Chem., 13 (1974) 218.

32 12 13 14 15 16 17 18

M. A. Pick and H. Wenzl, Znt. J. Hydrogen Energy, 1 (1977) 413 - 420. J. J. Reilly and R. H. Wiswall, Znorg. Chem., 9 (1970) 1678. F. J. Hollander, D. Semmingsen and T. F. Koetzle, J. Chem. Phys., 67 (1977) 4825. J. Wanagel, S. L. Sass and B. W. Batterman, Phys. Status Solidi A, 10 (1972) 49. International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1969. H. Wagner and H. Horner, Adu. Phys., 73 (1974) 587. H. Asano, Y. I&no, R. Yamada and M. Hirabayashi, J. Solid State Chem., 15 (1975) 45.