CuTa2O6 - crystal growth and characterization

M a t . R e s . Bull. Vol. I0, pp. Printed in the United States.

IZ73-1Z78,

1975.

Pergamon

Press,

Inc.

CuTa206 - CRYSTAL GROWTH AND CHARACTERIZATION John M. Longo Corporate Research Laboratories, Exxon Research and Engineering Company Linden, New Jersey 07036 and Arthur W. Sleight Central Research & Development Department, E. I. duPont deNemours & Co. Wilmington, Delaware 19898

(Received O c t o b e r 6, 1975; R e f e r e e d )

ABSTRACT The compound CuTa206 has been prepared as ~rystals fro~ a Cu/O melt and found to be tetragonal (~ = 7.510A, ! = 7.526A) rather than cubic as reported in the literature. The coefficient of thermal expansion between room temperature and 1000oc was found to be 8.0 x i0-6oc -I. Electrical resistivity measurements on a crystal showed semiconductor behavior b~tween room temperature (0 = 2 x 103 ~cm) and 140°K (0 = 7 x i0 ~cm) with an activation energy of E A = 0.2 eV. Magnetic measurements between 4.2°K and room temperature showed Curie-Weiss behavior with a change in ~eff at 120°K. For T>I20°K, ~eff = 1.76~B and @p = 0°K while for T
Introduction The Cu-Ta-O system has been investigated by various groups since 1960 with many of the results being contradictory. The initial work (1,2) on this system found a single compound thought to be CuTaO 3 with a simple cubicperovskite structure. In that structure, copper would have twelve-fold coordination and tantalum would have octahedral coordination. Both of the early groups believed that the compound could exist as the "valence isomers" Cul+Ta5+O 3 or Cu2+Ta4+O 3. During 1967, two independent groups (3,4) reported that the Cu-Ta-O system contained a single compound but with the composition CuTa206. From their x-ray data, it was clear that the new compound was in fact the same one as reported earlier. Both authors observed ordering lines in their x-ray data and reported a cubic unit cell which is double that described by reference 2. Since that time, others (5-10) have examined various properties of CuTa206 and confirmed the ~ 7 . 5 2 ~ cubic unit cell. Although CuTa03 cannot be prepared at atmospheric pressure, it has been prepared at high pressure and shown to have the LiNbO 3 structure (ii). In this paper on CuTa206 we report the preparation of single IZ73

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crystals, a tetragonal unit cell (a = 7.510~, ! = 7.526~), magnetic measurements to liquid He temperature, electrical resistivity data on single crystals and thermal expansion data obtained from x-ray diffraction. Preparation and Structure Bulk phase CuTa206 was prepared by the stoichiometric reaction of CuO and Ta205 in air at l~00oc for 20 hr. X-ray powder diffraction, using a Phillips diffractometer, showed the olive green product to be single phase and have essentially the powder pattern as reported earlier (3,4). However, by using a Guinier x-ray camera with monochromatic CuK~ and CrK~ radiation, it was clear that the symmetry of CuTa206 is not cubic as has been reported in all earlier work. The powder pattern could be completely indexed on the basis of a tetragonal unit cell (a = 7.510(2)A, ! = 7.526(2)A) which is a slight distortion of the cubic (~ = 7.52A) unit cell described in the literature. Table I lists the observed d values with a qualitative estimation of intensities for CuTa206. In order to determine the oxygen content of this phase (i.e. Cu2+/Cu I+) we used a DuPont Thermogravimentric Analyzer. A sample was heated at 10°C/min to 800°C in a flowing 15H2/85Ar gas stream to give Cu metal and Ta205. The observed 3.07(1) % weight loss is exactly the same, within experimental error, as the calculated loss for the composition CuTa206.00 and establishes the presence of only Cu 2+. Chemical analysis also confirms the composition (Cu-obs. 12.6%, calc. 12.2%; Ta-obs. 69.9%, calc. 69.4%; O-obs. 17.8%, calc. 18.4%). Although the x-ray data (size of unit cell and relative intensities of lines) strongly suggest the perovskite structure, AB03, with one-half of the A sites vacant, twelve-fold coordination Of an A cation in the ideal perovskite structure would not be suitable for Cu 2+. Since a single crystal x-ray study would be desirable to establish the details of the Cu coordination, we decided to prepare single crystals of CuTa206. Copper oxide was chosen as a flux since it would not introduce any foreign ions and there are no other Cu/Ta phases known at atmospheric pressure. An excess of CuO mixed with Ta205 (5CuO/Ta205) was packed into a platinum crucible, heated to above 1400oc and slow cooled at a few degrees per hour for five days. The excess copper oxide was removed with concentrated HCI leaving many black crystals which were rectangular plates. The shape of these crystals and those independently prepared by Felten and Fornwald (i0) under similar conditions supports our powder diffraction data which indicate that CuTa206 has lower than cubic symmetry. Unfortunately, all of the crystals we mounted for structure determination gave broad, multiple spots indicating twinning. Despite our failure to determine the structure of CuTa206, it would appear very likely that the Cu 2+ environment will closely resemble that recently found (12) for Cu 2+ in CaCu3Mn4012. This phase also has a perovskite related structure with Cu 2+ on A-sites; however the Cu 2+ coordination is close to square planar. Thermal expansion data were obtained on powdered samples of CuTa206 by x-ray diffraction using a Tempres high temperature attachment on a General Electric XRD-5 diffractometer. The resolution of our equipment was insufficient to observe the line splitting associated withlthe lower symmetry of CuTa206 and therefore we were not able to see any tetragonal to cubic transformation. The pseudocubic cell dimensions obtained at room temperature, 500°C and 1000oc, showed a linear relationship and gave a value of 8.0(5) x

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I

X-RAY POWDER PATTERN FOR CuTe206 h

k

1

d cotc

0 1 l I 0 2 1 2 2 l 2 2 2 0 2 2 5 I 3 3 2 2 3 3 0 4 1 2 3 4 4 1 3 4 5 2 4 4 2 4 4 3 3 2 4

0 0 0 I 0 0 0 0 1 I 1 0 2 0 l 2 0 0 0 1 2 1 1 2 0 0 0 2 2 0 1 1 0 I 3 0 0 2 1 1 2 2 3 2 2

I 0 ] 0 2 0 2 1 0 2 1 2 0 3 2 1 0 3 1 0 2 3 2 1 4 0 4 3 2 1 0 4 3 1 0 4 2 0 4 2 1 3 2 4 2

7.526} 7.510 5.316

5.310} 3.763 3. 755 3. 3 6 4 " 3.360 3.359 3. 0 7 0 " 3.067 2.658 2. 655 2. 5 0 9 " 2. 5 0 6 2. 504 2. 5 0 3 .

TM

2.3794 2. 3754 } 2.3749 2.1695 2. 0 0 9 9 ) 2. 0 0 8 4 ~" 2.0074.,J 1.88t5 1. 8 7 7 5 1. 8251 - ] 1. 8 2 3 5 1. 8 2 2 4 1 8217 I 8214 I 7735 1 7720 1 7703 1 7701 I 6821 1 6800 1 6793 1 6415 1 6395 I 6390 1 6025 1 6018 1 5351 1 5335

dobs

Iobs

7. 516

W+

5.313

M-

3. 763

M+

3. 7 5 6

S

3. 361

W+ +

3. 0 6 8

M-

2. 6 5 8

SH "

2. 5 0 6

W-

2. 3 7 9 2

w+

2.3752

W++

2 . 1696

S

2. 0 0 8 5

W+

1 . 8816 1. 8 7 7 5

MM+

1. 8 2 2 6

W+

1

}

1 . 7709

W

1. 6817

M-

1. 6 7 9 5

M+

-~ 1 . 6391 }

}

W

1. 6019

W

1. 5 3 3 9

S

h

k

1

dcalc 1.5052 7

0

0

5

3

0

4

1.5040

4 5 4 1 3 4 5 4 5 2 3 4 5 4 5 2 5 5 4 4 3 3 5 4 5

0 0 3 0 1 1 0 3 1 0 2 2 0 3 2 1 1

3 0 0 5 4 3 1 1 0 5 4

1.5031 1.5020 1.5020 1.4758 '1 l. 4748 1.4739 1. 4 7 3 0 1. 4 7 3 0 1. 4 7 2 8 1. 3971 ] 1. 3 9 6 2 1.3955 1. 3 9 5 0 1. 3950 I. 3 9 4 6 1. 3 7 3 6 " ) 1.3715 ~ 1. 3712 1.3290 1.5276 1. 2 9 0 0 " ] 1. 2 8 9 2 1.2887 1. 2 8 8 7 1. 2 8 8 0

4 4 1 3 5 5 2 6 6 2 4 5 4

2 0 4 0 3 0

3 2 2 0 5 2 1 4 0 5

4 3

3

3

3

0

2 4 1 2 2 3 0 0 2 1

4 2 6 5 3 2 6 2 0 6

0

5 4

0

4

4

3 4

6 6 5

1 2 4

2 1 0

2

2

6

6 4

2 4

2 4

3

1 . 2 5 2 8 "~ 1.2520 J 1. 2 2 0 7 1. 2 2 0 0 1 I,2189 1. 2 1 8 6 1. 1897 1 1877 } 111874 _ l. 1751 -~ 1. 1744

dob s

Iob s

1.5032

W

1 4736 "

W--

1 3967 "

W-

1,3709

W--

1.3285 1.3274

S M

1.2884

W

1 2524 "

M

1.2193

W. . . .

1. 1 9 0 0 1 1878 '

M M+

1.1740

W-

1.1345 1.1528

MM

1.0846

M-

1. 1738 1.1738 1. 1734 1.1731 1. 1729 I. 1729 I 1.1342 1.1324 1.0848

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i0-6°C -I for the coefficient of linear expansion. Electrical Resistivity Measurements Electrical resistivity measurements were made on a rectangular bar (l.70mm x .52m~ x .25mm) cut from a single crystal using the four-probe technique with indium soldered leads. Between room temperature (P = 2 x 103 ~cm) and 140°K (P = 7 x 106 ~cm), a plot of log P vs. I/T is linear. The activation energy over this temperature range is 0.20(2) eV as A

E[

determined from the formula P = 0o x e . Although resistivity measurements were made to 8°K, the experimental set^up used did not permit accurate measurement of resistivity greater than i0 ~ ~cm. Bazuev and Krylov (7) report resistivity measurements on compacted powder samples of CuTa206 (45% porosity) from room temperature to 650°C in vacuum. Their resistivity value of 108 ~cm at room temperature is five orders of magnitude greater than ours and reflects the difference usually observed for measurements on loose compacts and single crystals. Masnetic Measurements Magnetic measurements were made on powdered samples of CuTa206 using a vibrating sample magnetometer between 4.2°K and room temperature. A field of 10k0e was used with a 0.2089gm sample. A plot of i/xm vs. T(OK) is given in Figure i and shows two linear regions. The higher temperature portion of the data (T>I20°K) gives a ~eff. = 1.76~B and a Weiss constant 8p = 0OK.

800

,

I

l

I

,

• @

CuTa206

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600 @

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400

@ @

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@

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@@

200 • o " oO •°°°°°° •

0

I

I

I

I

I

50

100

150

200

250

300

TEMPERATURE (°K) FIG. i Inverse Molar Susceptibility versus Temperature for CuTa206 These values correspond to the spin only moment of one unpaired electron with

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widely separated (~5.2A) Cu 2+ ions. Below 120OK, Heff increases to 1.91HB and 8p = -15°K indicating some weak antiferromagnetic coupling at the lower temperatures. The magnetic susceptibility of CuTa206 was also examined as a function of applied magnetic field (H < 17kOe) at room temperature and 4.2°K. At room temperature a linear relationship was observed with a zero field intercept of zero. A linear relationship was also observed at 4.2°K but the zero field intercept showed a low spontaneous moment of 0.25 emu/g (130 emu/mole). The magnetic properties of CuTa206 have also been examined by Krylov et al. (6) using the Gouy method over the temperature interval 90300°K. They find a ~eff = 2.06~B and a gp = -15 which is in fair agreement with our low temperature data (T < 120°K) but is in poor agreement with our data that overlap the temperature range they studied. Acknowledsments We are grateful to J. L. Gillson for the electrical resistivity data. References i.

E. I. Krylov, V. A. Samarina and A. K. Shtolts, Dokl. Akad. Nauk SSSR 130, 556 (1960).

2.

L. Shick and K. S. Vorres, AEC Accession No. 46244, Report No. TID-22207, Paper D (1965).

3.

E. J. Felten, J. Inor~. Nucl. Chem.

4.

H. Kasper, Rev. Chim. Miner.

5.

G. V. Bazuev and E. I. Krylov, Russ. J. Inorg. Chem. (1969), Original Zh. Neor~. Khim 14, 3196 (1969).

6.

E. I. Krylov, G. V. Bazeuv and V. P. Khan, Izv. Akad. Nauk SSSR, Neorg. Mater. 5, 2029 (1969).

7.

G. V. Bazuev and E. I. Krylov, Tr. Ural. Nauch. Issled. Proekt. Medroi Prom. No. 12, 239 (1969).

8.

V. Propach and D. Reinen, Z. Anorz. and Allzem. Chem.

9.

D. Reinen and V. Propach, Inorg. Nucl. Chem. Lett.

29, 1168 (1967).

~, 759 (1967). 14, 1686

Inst.

369, 278 (1969).

~, 569 (1971).

i0.

E. Felten and D. Fornwald, private communication,

1973.

ii.

A. W. Sleight and C. T. Prewitt, Mat. Res. B,]I.

5, 207 (1970).

12.

J. Chenavas, J. C. Joubert, M. Marezio and B. Bochu, J. Solid State Chem. 14, 25 (1975).