Pr3NbO4Cl6: A new praseodymium chloro niobate

Mat. Res. Bull., Vol. 18, p p . 1493-1498, 1983. P r i n t e d in the USA. 0025-5408/83 $3.00 + .00 C o p y r i g h t (c) 1983 Pergamon P r e s s Ltd.

Pr3NbO4CI6:

A NEW PRASEODYMIUM

CHLORO NIOBATE

L. H. Brixner, J. C. Calabrese, and C. M. Foris Central Research & Development Department* E. I. du Pont de Nemours & Company Wilmington, Delaware 19898

(Received A u g u s t 4, 1983; Communicated b y A. W. Sleight)

ABSTRACT Pr3NbOACI ~ crystallizes in the hexagonal space group P61/m WitM a = 12.714(3) and c = 3.962(2)K. The structure was refined using 420 diffractometer intensities to a final R value of 1.7%. The crystal structure consists of an infinite network of NbO_ and PrOgCl 7 polyhedra. The powder diffraction pattern is als~ r~ported. INTRODUCTION Recently I we described the preparation and structure of a novel lanthanum halo niobate, LaNb^O.CI. Contrary to the case with %ugg~ten, where several different halo tungstates exist ~'~'~, LaNb~O~CI appears to be the first reported chloro niobate composition. Efforts to prepare an isostructural compound with Pr failed, and a new low temperature form of PrNb.O^, as well as Pr~NbO~CI~, was obtained instead. The preparation and struct~re ~f ~he latter compound is the subject of the present communication. EXPERIMENTAL A.

AND RESULTS

Preparation

Pr~NbOACI ~ was first obtained by the interaction of PrOCI and NbgO ~ ~n a~se~led, evacuated quartz tube. PrOCI was made by dissolving Pr~O11 (99.99% purity, Research Chemical Corp.) in HCI, taking theVc1@ar green solution to dryness, and firing the resulting product in air at 600°C for 1-2 hrs. and subsequently at 850°C also in air for 8-14 hrs. Nb^O. was of optical quality (Kawecki/Berylco) and was heated at 1060~C in air before use. Stoichiometric quantities ( t o t a l i n g ~ 0 g ) of PrOCI and Nb205 were *Contribution

No. 3311

1493

1494

L.H.

BRIXNER, e t a l .

Vol. 18, No. 12

sealed under vacuum into a l-cm diameter, 20-cm long quartz tube and fired at 950 C for 8-14 hrs. Since PrOCl Nb20. In a I:i molar ratio does not yield single phase Pr3NbOaCl 6, ~e also obtained a new low temperature structural modification of PrNb309 as a byproduct. Single crystals of Pr~NbO~Cl~ were up to 1 mm long, bright green in color and had a deedle-like habit similar to that of La~WO~CI~(2). Despite the relatively high C1 content, these crystal~ w~re~completely stable in a normal laboratory atmosphere. After the single crystal x-ray study had established the composition of the needles as being Pr3NbO4CI6, the following reactions were run: •

O

•

~

4PrCl 3 + Nb205 + Pr203

~ 2Pr3NbO4CI 6

3PrOCI + 3PrCl 3 + Nb205

~ 2Pr3NbO4CI 6

.

and

Both were carried out in sealed quartz tubes at 900°C. PrCl~ was obtained by firing the hydrated chloride from the HCl solution in HCI gas at 700°C for 1 hr. The powder patterns of the products of both reactions could be indexed based on the Pr3NbO4Cl 6 parameters. B.

X-Ray Studies

I. Powder Examination The x-ray powder diffraction pattern of Pr~NbOdCl ~ was obtained with a Guinier-Hagg type focusing camera Cradius 40 mm). The radiation was monochromatic CuK~I (l =1.5405~), and Si (a=5.4305A) was used as an internal standard. Line positions on the film were determined to +5 ~m with a David Mann film reader (a precision screw, split-image comparator). Intensities were estimated by oscilloscopic comparison of film density with the strongest line of the pattern. Refined cell dimensions were obtained by a least squares procedure (local program). Calculated intensities were determined with a program supplied by the Molecular Structure Corp. The indexed powder pattern of Pr~NbO~Cl 6 is reported in Table I. The refined cell dimension~ ar~: a = 12.7301(4)A c = 2.

3.9626(2)

Single Crystal Examination

A single, green, needle-shaped crystal with approximate dimensions 0.016 x 0.030 x 0.020 mm was placed on a Enraf-Nonius CAD4 x-ray diffractometer equipped with a graphite monochromator and a MoK~ source. The CAD4 search routines confirmed the hexagonal lattice constants observed from t~e precession camera study with a = 12.714(3)A, and b = 3.962(2)A. For Pr~NbO.CI. and Z = 2, the calculated density is 4.744 g/cc. 2777 reflections were collected from 4 < 28 < 55 using the 8-28 scan technique and a scan range of m ----I.i ~ .35(tan ) at 2 deg/min. The intensities were corrected for Lorentz and polarization effects

Vol. 18, No. 12

Pr3NbO4CI 6

and a small (5%) correction was also applied to account for the drop in the intensity of t h e two monitor ref l e c t i o n s during t h e data collection. The data were t h e n m e r g e d to y i e l d 4 4 2 independent reflections of w h i c h 420 with I>--2o(I) were u s e d for t h e s t r u c t u r e s o lution a n d refinement. A n examination of the data ind i c a t e d t h e space group choices as P63 or the centric e q u i v a l e n t P 6 3 / m . T h e s t r u c t u r e was d e t e r mined u s i n g the heavy-atom method in t h e centric choice. Anisotropic fullmatrix least-squares refinement converged with R=1.7%

and

Rw=2.2%.

1495

TABLE 1 POWDKR DIrPRACTIOH DATA fOR Pc3NbO4CI 6

2_. 21!1 8.018

13.801

75

d(ob*)

I:~

11.02,

11.0244

,~

6.3649 5.5084 4.1665 3.7277 3.6725 3.3634 3.2172

8.3631 5.5123 4.1009 3,7290 3.0749 3.3040 3.2175

3' I00 4 11 39 64

3.1023

3.1025

16.076 21.2,7 23.8~0 24.2~4 28.4"~8 27.704 20.015

30 05 15 1o 80 100 5

1 0 1 3 0 0 111 2 01

31.120

100

2 1 1 I~I

20.281

32,0,

37.110

55 60 ,o

37.350

go

3o.oz4

?o

44.o0? 45.751 40.553 48.004

I I

I:~

?5

33.228

40.004 42.358 43.30?

311

: ~:

20

5 0 0 ~I~

00 30

?o

411 002 51 0 10 2 2 0 3

85 lO2

1~ III

49.395

50.304

50.894 51.556

00 lO

51.741 52.415

10 10

54.530

1o

55.1,3'"°'°

1515 00 2

55.724 5?.0,

A

h k i

d(calc)

ZC/Xlc

2:

3.0566 2.8714 2.7555 2.8940 2.4200

3,0577 2.8715 2.7582 2.0943 2.4208

05 25 22 42

2.2049 2.1321 2.0833 2,0,59 1.981,

2.2049 2.1320 2.0835 2.0504 1.9813 1.9801 1.9'01 1.864,

53 10 23 29 -~2

2.40,8 2.2822

2.,0,, 2.2627

1,9492 1.8843

1.043, 1,0123

4

1.o441

:

1.853

21

1.0124

1.7.3

:i

5 2 0 3 0 2 ~ ~

1,7711 1,7653 1,7441 2,8014

1.7713 1.7654 1.7440 1.68121.6820

601 3 I 2 431 5 21 4 0 2 7 0 0 I ~

1.6667 1.6628 1.8482 1.6122 1.8087 1.5748 1.$'78 1.5295

1.6670 1.6628 1.6482 1.6126 1.0088 1.5750 1.5477 1.5294 1.5288

9 2 31 ? 7 21 2 B ' 12 34 0

final difference F o u r i e r 57.214 20 50.586 20 59.696 20 map revealed several us00.477 0o 6 2 0 ual large r e s i d u a l s (i. 1 ,3.010 ~I 1.,,0 1.4,37 2: 83.504 00S 1.4837 1.4636 e / ~ 3 ) near the Pr atom. 0,.,7 lO 1~ 1.,oo7 1.4,o3 : 1.4266 3.4284 T h e atomic s c a t t e r i n g 0,.35s 5 4 0 1.4118 1.4116 4 06.128 5 factors and anomalous 8?.348 s 1.3472 1.3473 69.745 10 7 2 0 1.3469 3 dispersion corrections ,o.31, ~ I 1.3376 1.3373 2 1.3182 1.3180 ? from t h e International 71.513 40 S 6 31 1.3100 1.3108 2 71.977 5 10 3 1.3115 Tables for X-ray C r y s ,.805 3o : ~ 1.2848 1.2845 4 1.1754 1.2752 8 t a l l o g r a p h y , Vol. IV 74.305 1o (19743, were used. The final r e f i n e d atom c o o r d i n a t e s , thermal p a r a m e t e r s , and interatomic d i s t a n c e s are g i v e n in T a b l e s II, III, and IV, r e s p e c t i v e l y .

TABLE II. Fractional C o o r d i n a t e s (XI0000) and Estimated Standard D e v i a t i o n s Atom Pr(1) Nb(1) CI(1) CI(2) O(i) 0(2)

X 6024.8(3) 3333 7516(1) 6268(1) 333J 4905(4)

Y 8841.1(3) 6667 8575(1) 10704(1) 6667 7984(4)

Z 7500 2500 2500 12500 7500 2500

1496

L.H.

BRIXNER, et al.

Vol. 18, No. 12

TABLE III Anisotropic Thermal Parameters (XI000)* exp[-19.739(Ullhha*a*...+2(U12hka*b*...))] Atom Pr(1) Nb(1) CI(1) CI(2) O(I) 0(2)

UII 24.7(2) 7.1(3) 14.7(6) 13.8(6) 28(3) 13(2)

U22

U33

16.5(2) 7.1 15.6(6) 10.8(6) 28 14(2)

6.7(2) 16.6(4) 15.9(7) 15.6(7) 13(4) 15(2)

UI2 14.0(2) 3.5 8.6(5) 4.6(5) 14 6(2)

*All UI3 and U23 terms are constrained to zero by symmetry.

TABLE IV. Interatomic Distances (A)* Pr(1)-Nb(1) Pr(1)-Cl(1)a Pr(1)-CI(1) Pr(1)-Cl(2)b

3.7171(8) 3.054(1) 2.876(1) 3.021(2)

Pr(1)-CI(2) Pr(1)-O(2) Nb(1)-O(1) Nb(1)-O(2)

2.983(1) 2.364(2) 1.981(1) 1.858(4)

*The subscripts a and b refer to equivalent symmetry operators, respectively, (a) y, l+y-x, I/2+z, (b) l+x-y, x, -I/2+z. DISCUSSION The crystal structure of Pr~NbOACl ~ is shown in Figure I. All atoms are located in speciaI po~itIons in the unit cell, with Pr, two Cl's and one oxygen located on mirror planes (m) and Nb and 0(I) on different 6 sites. This gives rise to an unusual

FIGURE

i.

Cell View along [001] showing the PrO^C1. and NbO. con~ec~ivitles. 3

Vol. 18, No. 12

Pr3NbO4CI 6

trigonal-bipyramidal coordination of Nb in NbO. units as seen in Fig~ 2. As observed in previous studies, the Nb-O equatorial distances are slightly shorter (0.12~) than the axial distances. Pr is coordinated to seven chlorine and two oxygen atoms in a Cl face-capped trigonal prismatic arrangement in which two of the apices are oxygens (Fig. 3). The face-capping Cl's yield slightly longer PrCl distances. The crystal structure contains infinite chains of corner-sharing NbO~ trigonal bipyramids w~th the equatorial oxygens corner-sharing the oxygens of the PrCl_O^ polyhedra. This res61~s in a column of NbPr_ units in which the niobium atoms are surrounded by six Pr atom in a trigonal prismatic arrangement. The columns are joined through edge-sharing of the chloride atoms of PrCI_O_. Each C1 and O(2)/t~iply bridge three metals, while the apical O(I) doubly bridges two niobiums.

1497

<

01

N81

01

FIGURE

2.

Coordination of NbO 5 shown at 50% thermal ellipsoids.

L2

CLA

C

R! CL2

CL2

It is interesting to compare Pr~NbOaCl ~ with the recentIy d~sc~ibed La~WO~CI~ (2). While the nu~be~ add distribution ~,,JCL2 of anions (mainly due to cL~ ~ the valence difference of the transition metal) are 62 not exactly the same, both the habit (needles) F I G U R E 3. and the symmetry (P6~/m) Coordination Sphere of PrO2CI 2 are identical for th~se at 50% thermal ellipsoids. two compounds. While W is six-coordinated by the oxygen in the form of an unusual trigonal prism, Nb is only 5-coordinated and thereby closer to the environment of W in LaWO4CI.

)

1498

L.H.

BRIXNER, et al.

Vol. 18, No. 12

REFERENCES i.

J. C. Calabrese, L. H. Brixner and C. M. Foris, J. Solid State chem., 48, (1983).

2.

L. H. Brixner, H. Y. Chen, and C. M. Foris, J. Solid State Chem., 44, 99, (1982).

3.

L. H. Brixner, H. Y. Chen and C. M. Foris, J. Solid State Chem., 45, 80 (1982).

4.

Z. Zikmund, Acta Crystaliogr.

Sect. B, 30,

2587

(1974).