Structural phase transitions of Mg3BN3 under high pressures and temperatures

Scripta METALLURGICA et MATERIALIA

Vol. 27, pp. 993-997, 1992 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

STRUCTURAL PHASE TRANSITIONS OF Mg3BN 3 UNDER HIGH PRESSURES AND TEMPERATURES H. Lorenz I), I. Orgzall I), E. Hinze 2) and J. Kremmler 2) i) High Pressure Laboratory at Potsdam University Telegrafenberg, O-1561 Potsdam, Germany 2) IGL University of Giessen, Senckenbergstrasse 3, W-6300 Giessen, Germany (Received June 25, 1992) (Revised August 3, 1992) Introduction As is well known the phase behaviour of a specific material influences its physical and chemical properties. Therefore, the knowledge of the phase diagram may support the directed search and generation of desired structures with special properties. Although magnesium boron nitride is often applied as a catalyst in the synthesis of cubic boron nitride (i), only little is known about its high pressure phase behaviour. So, in the following, some results are presented concerning phase changes in Mg3BN 3 at high pressures and temperatures. This compound also high pressures and diagram may lead to hBN to cubic BN and this process.

arises as an intermediate phase in the system Mg3N 2 - BN at temperatures (2). Therefore, the study of the Mg3BN 3 phase a better understanding of the catalytic transformation from result in valuable insights into the physical mechanisms of Experiments

Preparation of Mg3BN 3 A mixture of high purity Mg3N 2 and hexagonal BN with a slight excess of hBN is heated to about 1350 to 1450K at normal pressure in a nitrogen stream for 6 hours. The resulting product is powdered and homogenized. After this the heating procedure is repeated. These reaction conditions for the formation of Mg3BN3 are kept constant for approximately 20 hours to reach a full reaction of the nitride. This procedure follows the preparation scheme of Mg3BN 3 given in

(3). High pressure - high temperature experiments The high pressure experiments were carried out in a multi-anvil high pressure device. A general description of the experimental setup is given in (4). The individual hexaedrically positioned anvils slide in specially designed guiding blocks. The pressure is generated by shifting these blocks against each other with a hydraulic press. The X-ray spectra are measured in situ using the high energy synchrotron radiation at HASYLAB (DES¥ Hamburg). The beam enters through the slit between the anvils into the sample chamber. The scattered radiation is observed through the opposite slit under an angle of about 50 . The energy dispersive detecting system consists of a high purity Ge-detector mounted on a goniometer and an xy-z-positioning table. The spectrum is registered with a multichannel analyzer. Specially developed software is used for the evaluation of the spectra (5). 993 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.

994

PHASE TRANSITIONS OF Mg3BN 3

1

9

4

'

Fig.

6

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i. Sample chamber setup. 1 2 3 4 5 6 7 8 9

epoxy cube silver ring p y r o p h y l l i t e disk m o l y b d e n u m disk pyrophyllite container g r a p h i t e heater specimen NaCI thermocouple

Figure 1 gives the d e t a i l e d sample chamber setup. An outer cube of pressure t r a n s m i t t i n g material (epoxy with a small amount of boron) with an inner cylindrical hole of 4mm d i a m e t e r contains a p y r o p h y l l i t e cylinder. A carbon heater with outer diameter of 3mm is placed into this c o n t a i n e r and the upper half of the interior (diameter 2mm) is filled with the specimen (Mg3BN3) and the lower w i t h NaCl that serves as a s t a n d a r d to d e t e r m i n e pressure and t e m p e r a t u r e via a c o r r e s p o n d i n g equation of state as e.g. the DECKER equation. During the e x p e r i m e n t s the press p o s i t i o n is changed in such a way that at every p - T - s t e p a NaCI spectrum is first taken, followed by the sample observation. The setup is sealed at top and b o t t o m with p y r o p h y l l i t e discs. The electrical connections to the heater are realized by two small silver rings placed above a thin m o l y b d e n u m disc, both at the upper and lower sides of the cube and contacts at the c o r r e s p o n d i n g anvils. Additionally, a t h e r m o c o u p l e is placed d i r e c t l y into the sample chamber for the in-situ d e t e r m i n a t i o n of the temperature. Results and Discussion P r e s s u r e and t e m p e r a t u r e w i t h i n the sample chamber are increased stepwise. Two cuts are drawn across the p-T-plane. At first the p r e s s u r e is increased at c o n s t a n t ambient t e m p e r a t u r e up to a m a x i m u m of 5.SGPa. Then at this c o n s t a n t p r e s s u r e the t e m p e r a t u r e is raised to a m a x i m u m of a p p r o x i m a t e l y 1500K. A f t e r a fast quench the p r e s s u r e is r e l e a s e d to normal conditions. During this process several phase changes may be observed. Figure 2 gives some c h a r a c t e r i s t i c spectra for these d i f f e r e n t phase transitions. It must be noted that the strong peaks at low energy result from the carbon heater and the excess hBN due to the sample preparation. These remain nearly c o n s t a n t and are only shifted by p r e s s u r e and slightly by temperature, but do not undergo a phase change. The lowest s p e c t r u m c o r r e s p o n d s to normal conditions. Increasing the p r e s s u r e i s o t h e r m a l l y results in a first t r a n s i t i o n at about 4.4GPa. While the primary s t r u c t u r e is hexagonal with an initial cell volume of 0.174 nm 3 the new phase is c h a r a c t e r i z e d by an o r t h o r h o m b i c structure. The k i n e t i c s is slow so that also at higher pressures up to the m a x i m u m 5.SGPa the old phase is p e r s i s t e n t and coexists with the new one. A small t e m p e r a t u r e increase accelerates the t r a n s f o r m a t i o n so that the s p e c t r u m at 5.5GPa and 375K was chosen for the d e s c r i p t i o n of the o r t h o r h o m b i c phase first a p p e a r i n g at 4.4GPa and ambient temperature. Figure 3 shows the c o m p r e s s i o n a l b e h a v i o u r of the two d i f f e r e n t phases that are o b s e r v e d d u r i n g the p r e s s u r e increase. The p e r s i s t e n c e of the hexagonal phase up to m a x i m u m p r e s s u r e is clearly indicated. The volume jump due to the t r a n s i t i o n to the denser phase roughly amounts to 12% r e d u c e d to normal c o n d i t i o n s w h a t has to be c o m p a r e d w i t h the large initial cell volume. As the two upper spectra of Fig. 2 c l e a r l y d e s c r i b e t e m p e r a t u r e increase at constant pressure (=5.SGPa) results in further p h a s e transformations. The e v a l u a t i o n of the NaCl spectra showed that the p r e s s u r e is n e a r l y kept c o n s t a n t during the heating process with an e x p e r i m e n t a l error of 0.2 ... 0.4GPa.

8

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8

PHASE TRANSITIONS OF Mg3BN 3

995

5.5 GPa / 1175 K

I/775K ~ ! iIl ~I I5I .I 5 I GPa

,5.0

20.0

25.0

300

35.0

~o.o

45.0

50.0

Energy {keV]

Fig. 2. Characteristic energy dispersive spectra of Mg3BN 3 at different pressures and temperatures; the phase transitions are clearly marked by changes in the line structure. The different new phases are characterized by greater changes in the spectral structures. The second transition is observed in the range around 600K. A detailed numerical evaluation of the d-values shows that this is a transformation within the orthorhombic system. The atomic positions only change slightly and the resulting volume jump roughly amounts to i0 ... 15% compared with the unit cell dimensions at 5.SGPa and ambient temperature. This is also consistent with a parallel experiment at a constant pressure of 4.SGPa where this transition is observed at a correspondingly lower temperature with an analogous structure change. As already mentioned, the kinetics of this process is slow again so that old and new phases coexist in a larger region. The next transition to a further orthorhombic structure proceeds above approximately 1020K. This structure is described by the uppermost spectrum of Fig. 2. It is connected with a volume decrease in the same order of magnitude as determined for the first temperature transition. At the lower pressure of 4.SGPa this transformatioh is observed at temperatures near 800K. Rapid quench of the sample and stepwise pressure release show that this last under high pressure and temperature generated structure remains metastable also under normal conditions. So, during the experiments according to the chosen specific way across the p-T-plane, the initial hexagonal structure of Mg3BN 3 changes to a final orthorhombic metastable one.

996

PHASE TRANSITIONS OF Mg3BN 3

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v/v o 1.0, -

~

0.9

0.7

4

6

i

I

I

I

I

°'o.o

~o.o

20.0

30.0

40.0

60.0

Pressure [GPa] Fig. 3. Compressional behaviour of the initial hexagonal phase and the first pressure induced transition to an orthorhombic structure. Table i lists results for the structural data in more detail. The position (dvalue) corresponds to those lines marked in Fig. 2. These data are arranged according to increasing energy (from left to right in the figure). As already mentioned, unmarked peaks may either be assigned to unreacted starting material of the chemical preparation process, like Mg3N 2 and hBN, or belong to the carbon heater. Another possibility that cannot be ruled out entirely is the formation of a non-stochiometric magnesium-boron nitride compound what could result in additional smaller lines in the spectrum. During the experiments also smaller peaks of NaCl may occur due to compresion and deformation within the specimen. The lowest spectrum of Fig. 2 gives the structure of the initial hexagonal Mg3BN3-material. Position and index of each indicated peak correspond exactly to the spectrum of Mg3BN 3 given in (3). In the subsequent spectra taken under pressure and temperature all lines are marked that unambigouosly may be assigned to one phase. The corresponding positions and indices are determined. But always the limitations and uncertainties of the energy dispersive method like various influences on the line intensity etc., should be remembered, especially the energy dependent intensity of the beam,the detector sensitivity, the energy dependent beam absorption within the whole sample setup that is influenced by pressure and temperature, etc. Another accuracy limiting fact is the slow kinetics of the phase transitions so, that always rather a mixture of different structures with only slightly different spectra than the pure phase is observed and the lines mutually influence position and shape. So, the d-values and indices have to be seen within these experimental error limits. As above, besides the Mg3BN 3 structures lines of the additionl compounds also occur. In summary, it shall be stated that Mg3BN 3 undergoes a series of phase transformations in the course of pressure and temperature treatment. Under the action of pressure the structure changes from an initially hexagonal one to an orthorhombic. Further transitions occur within this structure type if temperature is applied. This orthorhombic structure is conserved after a rapid quench and thus metastable under normal conditions.

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Table

8

PHASE

TRANSITIONS

OF M g 3 B N 3

i. S t r u c t u r a l data of the d i f f e r e n t p h a s e s The d - v a l u e s and indices are d e t e r m i n e d a p p r o p r i a t e p r e s s u r e and t e m p e r a t u r e .

5.5 GPa

/ 375

K

5,5

GPa

997

in Mg3BN 3. for the

/ 775

K

5.5

GPa

/ I175K

d (nm)

index

d (nm)

index

d (nm)

index

d (nm)

index

0.3037 0.3014 0.2669 0.2433 0.2216 0.2011 0.1836 0.1771

1 1 0 1 1 1 1 1

0.2985 0.2905 0.2626 0.2480 0.2398 0.2372 0.2159 0.2078 0.1973

0 2 0 0 3 0 1 1 3 1 2 2 2 1 0 2 3 2 O3O

0.2909 0.2576 0.2418 0.2312 0.2177 0.2094 0.1736 0.1546

0 3 0 1 0 1 0 5

0.2657 0.2478 0.2298 0.2216 0.2165 0.2101 0.1720 0.1661 0.1483

0 1 3 0 1 2 4 1 1

0 0 0 0 0 0 0 1

0 1 6 4 5 6 7 0

0 1 0 1 0 0 1 1 O/

0 0 1 1 2 2 2 0

1 0 1 1 0 0 1 0

0 0 0 2 1 0 0 2 2

1 1 0 0 1 1 0 1 1

The r e s u l t s thus o b t a i n e d here s h o u l d give the first h i n t s for a d e t a i l e d construction of the m a g n e s i u m boron nitride phase diagram. This k n o w l e d g e s h o u l d also be t a k e n into a c c o u n t in the i n t e r p r e t a t i o n of c a t a l y t i c cBN s y n t h e s i s r e s u l t s w h e r e Mg3N 2 is used as a c a t a l y s t and Mg3BN 3 forms as an intermediate phase under high pressure-high temperature conditions. So, e s p e c i a l l y in the e v a l u a t i o n of i n - s i t u m e a s u r e m e n t s of this process, these r e s u l t s on the p h a s e b e h a v i o u r of m a g n e s i u m b o r o n n i t r i d e s h o u l d be t a k e n into a c c o u n t b e c a u s e a n o t h e r i n t e r m e d i a t e h i g h p r e s s u r e - h i g h t e m p e r a t u r e phase in the s y s t e m Mg3N 2 - BN is Mg3B2N 4 (6,7) and its s p e c t r u m c l e a r l y has to be d i s t i n g u i s h e d from t h a t of the new p h a s e s of Mg3BN 3 d e s c r i b e d above.

References i. O. Fukunaga, S. Nakano, J. Maki, H. V o l l s t ~ d t and H. Lorenz, High Press. Res. ~ , 9 4 7 (1990) 2. I.S. Gladkaya, G.N. Kremkova, N.A. Bendeliani, H. Lorenz and U. Kuehne, J. Mater. Sci. (1992), in p r e s s 3. T. Sato, T. Endo and O. Fukunaga, U.S.Pat. 4,409,193 (1983) 4. O. Shimomura, S. Yamaoka, T. ¥agi, M. Wakatsuki, K. Tsuji, H. Kawamura, N. Hamaya, O. Fukunaga, K. Aoki and S. Akimoto, in: S o l i d S t a t e Physics u n d e r Pressure, S. M i n o m u r a (Ed.),D. Reidel Publ. Comp., D o r d r e c h t (1985), p. 351 5. J. L a u t e r j u n g , G. Will and E. Hinze, Nucl. Instr. and Meth. A 239(1985), 6. T. Endo, O. F u k u n a g a and M. Iwata, J. Mater. Sci., 14(1979), 1375 7. T. Endo, O. F u k u n a g a and M. Iwata, J. Mater. Sci., 14(1979), 1676

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