The crystal structure of Ti3PD2.4

Journal of

ALLOTS

MqD COMPOL~DS Journal of Alloys and Compounds 236 (1996) 26-29

ELSEVIER

The crystal structure of Ti3PD2.4 T. Larsson, P.-J. Ahlz6n, Y. Andersson, S. Rundqvist, R. Tellgren Department of Inorganic Chemistry, University of Uppsala, Box 531, S-751 21 Uppsala, Sweden Received 1 September 1995; in final form 19 September 1995

Abstract The crystal structure of the deuteride phase of Ti3P has been refined from neutron powder diffraction data taken at 25°C and 250°C using the Rietveld method. The tetragonal fl-V3S-type structure, space group P42/nbc, involves three crystallographically independent deuterium sites. The room temperature cell parameters are a = 10.3466(4) ,~ and c = 5.0589(3) A, with a refined deuterium content of 2.4 D per formula unit. The 250°C structure is essentially the same as the room temperature structure. The structure of the deuterium solid solution of Ti3P at 300°C has also been studied using neutron powder diffraction. A new deuterium site was found but the structure was the same in other respects as that found at room temperature with only small changes in the cell parameters.

Keywords: Metal hydride; Neutron diffraction

1. Introduction The crystal structure of Ti3P has been investigated by Lundstr6m and Snell [1]. It crystallizes in a tetragonal structure and is a m e m b e r of the F e 3 P - T i 3 P VaS-Ta3As structure family. The structures possess various tetrahedral interstices which can accommodate hydrogen. H y d r o g e n solubility measurements for Ti3P have been reported by Halter et al. [2] in the temperature range 662-902°C and by Flanagan and co-workers [3] at temperatures between 170 and 350°C. A structure determination of the solid solution of deuterium in Ti3P has revealed an additional phase, the deuteride phase, which was assumed to be isostructural with the earlier investigated Zr3PD3_ x [4-6] This present investigation was therefore undertaken to locate the deuterium positions in the deuteride phase of Ti3P at 25 and 250°C, as well as of the deuterium solid solution at 300°C, to ascertain whether other sites become available at higher temperatures.

als as purchased were: Ti, 99.9%; P, 99.999%. The final sample was investigated by X-ray powder diffraction and, apart from the Ti3P phase, was found to contain an impurity phase later identified as Ti3POI_ x. The amount of Ti3POI_ x was estimated to less than 5% of the total amount of sample. The deuteride phase was formed by heating Ti3P powder in a deuterium atmosphere at 650°C for 2 h, and then cooling slowly to room temperature. The deuterium pressure was maintained at 1 bar.

2.2. X-ray diffraction R o o m temperature cell dimensions for undeuterated Ti3P and the deuteride phase of Ti3P were determined using a Guinier-H~igg focusing X-ray film camera using monochromatic Cu K a 1 radiation. Silicon (a = 5.431023 ,~ at 22°C) was used for calibration. The line positions on the films were determined with a film scanner, and the unit cell parameters refined using a local least squares program.

2. Experimental

2.3. Neutron diffraction

2.1. Sample preparation

Neutron powder diffraction data for the deuteride phase were recorded at the R2 medium flux reactor in Studsvik, Sweden: flux at sample approximately 10 6 cm 2 s-l; wavelength 1.470 A. A multidetector system

The preparation of Ti3P has been described elsewhere [4]. The purities claimed for the starting materi0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD! 0925-8388(95)02086-1

T. Larsson et al. I Journal of Alloys and Compounds 236 (1996) 26-29

consisting of ten independent detectors were used giving a total measuring time per point of 30 min. The step length used was 0.08 °. The 2 O-range covered was 0.20-128.04 °. The 20-range covered for the solid solution was 0.20-112.52 °. Absorption measurement made at 2 0 = 0 gave a /zR value of 0.22 for the hydride phase at 25°C. The sample was contained in a vanadiura tube for the 25°C measurement, and in an Inconel tube in an aluminium furnace for the 250°C study. A deuterium pressure of 2 bar was used for the deuteride phase at 250°C. The solid solution was prepared by heating the deuteride phase to 300°C under a deuterium atmosphere of 0.6 bar.

27

2.4.1. Ti3PD2. 4 at 25°C Earlier work had indicated that the hydride phase of TiaP was isostructural with Zr3PD3_ x [4]. Approximate deuterium positions could therefore be derived from Fourier difference maps based on the Ti3P host structure. A total of 34 parameters were refined in the final refinement cycles: scale factors (2), 2 0 zero-point (1), atomic coordinates (10), D-occupancies (3), isotropic temperature factors (3), background parameters (6), asymmetry parameter (1), cell parameters for two phases (5) and halfwidth parameters (3). The two phases, Ti3P-D and Ti3POl_x, were refined simultaneously. 2.4.2. Ti3PD2. 4 at 250°C A total of 39 parameters were refined in the final refinement cycles: scale factors (3), 2 0 zero-point (1), atomic coordinates (10), D-occupancies (3), isotropic temperature factors (3) (for Ti and D in the Ti3P-D phase, and one for the Inconel tube), background parameters (6), cell parameters (6), halfwidth parameters (6) and one preferred orientation parameter for the Inconel tube. The three phases Ti3P-D , Ti3POl_ x and Inconel were refined simultaneously.

2.4. Structure refinements

Structure refinements based on the Rietveld method [7] were made using the program LHPM1 [8,9]. A Gaussian peak-shape was assumed, and the neutron scattering lengths used were 3.438 fm, 5.130 fm, 6.672 fm and 5.805 fm for Ti, P, D and O respectively. The 20-range used in the refinements was from 9.96 ° to the end of the 20-range measured. The structure was not refined for the Ti3POI_ x impurity phase, but calculated in the refinements using the coordinates found for the Zr3PO~_ x phase [10]. The contribution from the Inconel tube was also included in the refinements of the higher temperature data.

3. Results and discussion

Table 1 summarizes the structure parameters for the deuteride phase. Selected distances are listed in Table

Table 1 Final struct~are parameters and some details of the refinement for Ti3PD24; estimated standard deviations within parentheses T (°C)

a (,A)

c (A)

Rp

R~p

n~

R~, v

22 250

10.3466(4) 10.353(1)

5.0589(3) 5.0822(8)

0.033 0.056

0.042 0.083

0.010 0.026

0.034 0.034

Atom

T(°C)

Position

x

y

z

B~so

Occupancy(%)

Til

25 250 25 250 25 250 25 250 25 250 25 250 25 250

~ ~ 8h 8h ~ ~ 8i 8i 4a 4a 16k 16k 16k 16k

0.1543(6) 0.155(2) 0.6109(6) 0.618(2) 0.5536(5) 0.551(2) 0.5426(5) 0.545(2) 1/4 1/4 0.0029(4) 0.0028(9) 0.174(1) 0.143(8)

x x 1/4 1/4 x x 1/4 1/4 1/4 1/4 0.1066(4) 0.1060(9) 0.322(1) 0.322(6)

1/4 1/4 0 0 1/4 1/4 l/2 1/2 0 0 0.0783(6) 0.074(2) 0.136(3) 0.11(1)

0.3(1) 0.5(2) 0.3(1) 0.5(2) 0.3(1) 0.5(2) 0.7(1) 1a 1.1(1) 1.5(2) 1.1(1) 1.5(2) 1.1(1) 1.5(2)

100 100 100 100 100 100 100 100 44(2) 44(2) 84(1) 85(1) 22(1) 22(1)

Ti2 Ti3 P D1 D2 D3

a Not refinec. Ti3PD24, space group P42/nbc (No. 133), origin at 1.

T. Larsson et al. / Journal of Alloys and Compounds 236 (1996) 26-29

28

Table 2 Selected interatomic distances shorter than 3.05 A for Ti3PD2 4 at 25°C; estimated standard deviations within parentheses

D1-4D3 4Til 4D3 2D1 4D2 2P

1.28(1)" 1.887(7) 2.14(1) 2.529 2.982(4) 3.027(5)

1.826(3) 1.858(8) 1.935(5) 1.961(7) 2.20(1) 2.306(7) 2.344(8) 2.641(3) 2.728(4) 2.87(1) 2.96(1) 2.982(4) 2.994(4) 3.012(4)

D2-Ti3 Til

Ti2 Ti3

D3 D2 D2 P P D3 D3 D1 2D2 P

D3-D3 D1 Til Ti3 Til Til D3 D3 D1 D3 D2 D3 P D2 4D3 D2 P

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1.15(3)" 1.28(1)" 1.84(1) 1.88(1) 1.88(2) 1.98(1) 2.03(2) 2.09(3) 2.14(1) 2.17(3) 2.20(1) 2.46(3) 2.51(1) 2.87(1) 2.958(9) 2.96(1) 3.00(1)

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Counts

5000t 4000q 3000 20001000 Oi I

IIi I I

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IIII I I I I I I I I I I I I I I I I H I I I I I III

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IIIIIIIlllll

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IIIIIHllllllllll~

' 100

40

2o Fig. 1. Final observed (points) and calculated (full line) neutron diffraction profile for Ti3PDz4. The difference is plotted on the same scale. Vertical bars represent reflection positions for Ti3PDz4 (upper) and Ti3PO 1 x.

@

1/4

1/2

3/4

Ti O

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Cell parameters for the undeuterated Ti3P phase were a = 9.9621(2) ,& and c = 4.9883(2) ,&, which is in good agreement with earlier work [1,4]. For a discussion of the Ti3P structure, see Refs. [1,11]. On forming its deuteride, the Ti3P structure changes

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O

3.1. Ti3PD2.4: the deuteride

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0o

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~ 0

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oQ

P

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Q

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a Distances between partially occupied sites.

2 for the 25°C refinement. Fig. 1 shows agreement between the powder diffractogram and the model for Ti3PD2. 4 and TisPO1_ x at 25°C; a projection of the Ti3PD2. 4 structure along the c axis is shown in Fig. 2.

0

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D2

o

D3

o

Fig. 2. Projection of the crystal structure along the c-axis.

to the more symmetrical /3-V3S-type structure (from space group P42/n to P42/nbc); the same type of change was also observed in the Zr3P-D system. The room temperature cell parameters for the deuteride phase were considerably larger than for the undeuterated sample, see Table 1. The refined deuterium content corresponds to the formula TisPD24, which is in good agreement with absorption measurements [~11 The volume expansion per deuterium atom is 2.47 Three crystallographically independent deuterium positions were found in the structure. A 4a-site (D1, occupation 43%) is coordinated to four Til atoms at the corners of a regular tetrahedron. The T i l - D 1 distance (1.88 A) is the same as that found in the solid solution at room temperature [4]. The shortest P-D1 distance is 3.03 A, which can be compared with 2.98 in the solid solution. The other two independent Dsites are 16k-sites: D2, occupation 86%, coordinated to one Til, one Ti2 and two Ti3 at distances 1.82-1.97 ~,. It also has a close P - D 2 contact at 2.65 ,~. This is a similar environment to that for the D2 site in the solid solution, where the P - D 2 contact is 2.61 ,~. The second 16k-site (D3, occupation 23%) coordinates three Til and one Ti3 atom at distances 1.85-1.96 ,~.

T. Larsson et al. / Journal o f Alloys and Compounds 236 (1996) 26-29

There is also a short P - D 3 contact at 2.50 .A. This type of site was also observed in the Zr3PD2. 6 structure. An empirical rule states that the hydrogen atoms prefer to occupy sites most distant from the p-elements. There is also the so-called '2 /k rule' which applies for D - D distances in metal hydrides containing transition elements and p-elements [12]. Together, lhese two rules help resolve the local structure. Short D1-D3 and D3-D3 distances (1.28 ,h, and 1.19 respectively) clearly do not arise in the structure; they ca~ be eliminated by the mutual avoidance of the partially occupied D1- and D3-sites. The Ti3P-D and Zr3P-D systems show many similarities: they adopt the same structure type, and undergo the same structural change on going to the solid solution and to the hydride phase [4,5]. The same hydrogen positions are also found in both structures. They se,em to differ only in the amount of hydrogen they cart absorb: Zr3P can absorb up to 3 hydrogen atoms per formula unit, while Ti3P can take up a maximmn of only about 2.4 hydrogen atoms per formula unit [3]. This is because short D3-D3 distances can be avoided in the Zr3P-D system if every second site is vacant; while in the Ti3P-D system, short D3-D3 distances would still remain, since the distance,; are here close to 2 A. The second empirical rule can be invoked in discussing the stability of the different D sites. The shortest P - D distance for the three sites D1, D2 and D3 are 3.03 A, 2.65 ,~, and 2.50 ,~ respectively. The stability of these sites would then be in the reverse order. The most efficient way of filling these sites would, therefore, be to fill D1 and D2 completely, and to leave D3 empty. From strictly crystallographic considerations, this would give a maximum deuterium content corresponding to the formula Ti3PD: 5, which is in good agreement with the absorption measurements which give Ti3PD24 [3]. Band structure calculations for the Ti3P-H system are underway to gain a clearer picture of the bonding in this system. 3.2. Ti3P-D: the solid solution

The refinements made of the higher temperature solid solution data indicated a deuterium site not

29

previously found in the structure. Therefore, a reinvestigation of the solid solution system is underway. A comparison of the calculated diffraction pattern based on the structure found at room temperature [4] and the measured pattern showed that there is no significant change in the structure at higher temperatures, except the changes observed in the cell parameters. 3.3. TisPO l x

The Ti3POI_ x phase appears to absorb deuterium. Further investigations of the Ti3PO l_x-D system are therefore underway.

Acknowledgements Financial support from the Swedish Natural Science Research Council is gratefully acknowledged. Also much appreciated is the help with corrections and comments on the manuscript by Professor John O. Thomas, Uppsala University, Sweden.

References [1] T. LundstrOm and E-O. Snell, Acta Chem. Scand., 21 (1967) 1343. [2] U. Halter, M. Mrowietz and A. Weiss, J. Less-Comm. Met., 118 (1986) 343. [3] H. Noh, T.B. Flanagan, E-J. Ahlz6n, Y. Andersson and S. Rundqvist, Z. Phys. Chem., 179 (1993) 139. [4] E-J. Ahlz6n, Y. Andersson, S. Rundqvist and R. Tellgren, J. Less-Comm. Met., 170 (1991) 263. [5] E-J. Ahlz6n, Y. Andersson, S. Rundqvist and R. Tellgren, J. Less-Comm. Met., 172-174 (1991) 206. [6] P.-J. Ahlz6n, Y. Andersson, S. Rundqvist and R. Tellgren, J. Less-Comm. Met., 161 (1990) 269. [7] H.M. Rietveld, J. Appl. Crystallogr., 2 (1969) 65. [8] D.B. Wiles and R.A. Young, J. Appl. Crystallogr., 14 (1981) 149. [9] C.J. Howard and R.J. Hill, Rep. No. Ml12, 1986 (Australian Atomic Energy Commission). [10] E-J. Ahlz6n and S. Rundqvist, J. Less-Comm. Met., 167 (1990) 21. [11] S. Rundqvist, Y. Andersson and S. Pramatus, J. Solid State Chem., 28 (1979) 41. [12] S. Rundqvist, R. Tellgren and Y. Andersson, J. Less-Comm. Met., 101 (1984) 145.