Materials science of ternary metal boron nitrides

International Journal of Inorganic Materials 3 (2001) 201–209

Materials science of ternary metal boron nitrides Peter Rogl* ¨ Physikalische Chemie, Universitat ¨ Wien, A-1090 Wien, Wahringerstraße ¨ Institut f ur 42, Austria Received 21 November 2000

Abstract The present article intends to cover a brief review on the state of art of the materials science of ternary metal boron nitrides, comprising the stability of binary boron nitride BN, the formation of ternary compounds, their crystal structure(s) and the phase relations in ternary systems M–B–N. Furthermore the role of metal boron nitride compounds in the high-pressure conversion of hBN to cBN will be considered and physical properties of ternary metal borides will be briefly discussed from the viewpoint of band structure calculations.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Metal boron nitrides; Classification of phase equilibria; Classification of crystal structure types

1. Introduction Boron nitride in its low- and high-pressure form for many applications surpasses the properties of the corresponding carbon-based counterparts, graphite and diamond. Particularly the attractive behaviour of super hard cubic boron nitride, cBN, such as high temperature hardness, high fracture toughness and good oxidation resistance at low density, offers promising applications not only in modern multifunctional ceramic systems based on metal borides and nitrides but also as a radiation-resistant high temperature semiconductor component and wide band-gap material [1–3]. In all these applications and for efficient material design a profound knowledge on detailed phase relations concerning metal–BN interactions is required. Table 1 provides an overview on the correspondence among the crystal structures of the various Cand BN-modifications.

2. Structure and stability of binary boron nitride BN Since the pioneering high pressure synthesis of the cubic form of boron nitride, cBN, by R.H. Wentorf, Jr. [4–6], the

*Tel.: 143-1-4277-52456; fax: 143-1-4277-9524. E-mail address: [email protected] (P. Rogl).

p–T stability equilibrium diagram of BN has become the subject of continuous interest, in particular the equilibrium boundary hBN⇔cBN, which due to various experimental data [7–10,13], extrapolations [9,11,12], thermodynamic calculations [14–16] and ab initio computations [17] has undergone a series of modifications (for a comparison see Fig. 1). Whilst earlier versions of the p–T diagram reveal a larger region of existence for hBN [7–9,11], thermodynamic calculations favour a triple point at lower pressure and thus a significantly larger region for cBN [14–17]. An interesting aspect is the fact, that after Refs. [10,14–17] super hard cubic cBN should be the stable modification at normal pressure below about 1200 to 1500 K. Recent in-situ high pressure-high temperature synchrotron radiation data on the reversible hBN⇔cBN transformation [18,19], however, seem to some extent to reconfirm the early experimental diagram of Bundy and Wentorf [7,8] from 1963. A first evaluation of the kinetics of the hBN→cBN transformation as well as of the back-transformation allowed to redefine an equilibrium transformation boundary suggesting a significantly wider range of stability for the hBN phase extending at normal pressure as low as about 250 K (see also Fig. 1 and details in Refs. [18,19]) where hBN⇔cBN transformation kinetics become exceedingly slow. Thus at present all evidence points towards cBN as the stable phase at room temperature and normal pressure, in contrast to diamond being metastable under these conditions.

1466-6049 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00009-5

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Fig. 1. Pressure vs. temperature diagram for BN in various versions from literature (Refs. [5–19]; for details see text).

3. Ternary compounds and classification of crystal structures Since the early work of Goubeau and Anselment [20] and Gaude and Lang [21], dealing with the formation of alkaline earth and rare earth boron nitride compounds such as hCa, Sr, Baj 3 B 2 N 4 [20] and Ce 15 B 8 N 25 [21], a series of

metal boron nitrogen compounds has been identified, synthesised in single phase form and most of their structure types have been evaluated: Sr 3 B 2 N 4 [22] (Sr 52x B 3 N 6 , x51) with an ordered variant LiCa 4 B 3 N 6 [23–27] and a defect variant bCa 52x B 3 N 6 [28], LiMgBN 2 [29,30], a-and b-Li 3 BN 2 [31–33], lP- and hP-Mg 3 BN 3 [34,35], Ce 15 B 8 N 25 [21], Ce 3 B 2 N 4 [36,37], La 5 B 2 N 6 [38,39], Ce 4 B 2 N 5 [39], PrBN 2 [40,41], LaBN 2 [42], La 5 B 4 N 9 [37], La 5 B 3 N 8 [39], La 6 B 4 N 10 [39], UBN [43] and Nb 2 BN 12x [44]. It is well known that small amounts of carbon and oxygen impurities may easily stabilise higher order compounds, a number of which has already been described: Ce 15 B 8 (N,O) 25 [21], Ca 910.5x (BN 2 ) 62x (CBN) x [45] and Ba 4 (BN 2 ) 2 O [30]. A listing of all hitherto reported structure types for ternary metal boron nitrides is presented in Table 2. Classification of structure types M x B y N z may be successfully performed as a function of boron–nitrogen aggregation as well as of the metal co-ordination around the non-metal atoms or as a function of the average valence electron concentration, avec [46], around the light atoms: avec5(mx13y15z) /( y1z). At present only a small variety of characteristic structural units B y N z have been encountered, which serve as building elements in about twenty different structure types. These structural units are shown in Fig. 2 comprising a set of four finite groups: [BN 2 ] 32 , [BN 3 ] 62 , [B 2 N 4 ] 82 , [B 3 N 6 ] 92 . Similar to UBC [47], the infinite boron–boron zigzag chains branched with nitrogen atoms in UBN may actually reduce to tightly bound groups [B 2 N 2 ] mx 2 , connected via weaker B–B bonds of 0.220 nm; the latter ones being about 15% longer than the covalent B–B bond length within the unit. Dependent on the boron–metal and nitrogen–metal bond formation, boron atoms tend to adopt a triangular prismatic metal co-ordination B[M 6 ] (sometimes enlarged to an Archimedian antiprism B[M 8 ] or to a tetrakaidekahedral surrounding B[M 9 ]), whilst nitrogen atoms usually prefer a typically octahedral metal co-ordination N[M 6 ], which is frequently replaced by a tetragonal bipyramidal co-ordina-

Table 1 Crystallographic correspondence among the various crystal structures of C- and BN-modifications (in nm) Carbon

Boron Nitride

Phase

Pearson symbol Space group

Lattice parameter

Phase

Pearson symbol Space group

Lattice parameter

cubic diamond hexag. diamond lonsdaleite hexagonal graphite trigonal graphite C–thin film –

cF8 Fd3¯ m hP4 P63 /mmc

a50.3567 a50.252 c50.412 a50.2464 c50.6711 a50.2476 c51.0026 a50.428

BN–sphalerite BN–wurtzite

¯ m cF8 F43 hP4 P63 mc

BN–hexagonal

hP4 P63 /mmc

BN–rhombohedral

hR6

a50.3615 a50.2551 c50.4210 a50.2504 c50.6661 a50.2504 c50.9991

– BN–compressed

mC4 C2 /c or Cc

hP4 P63 /mmc hR6 R3¯ m cI16 Im3¯ m

a50.433 b50.250 c50.310 to 0.330 b 592–958

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Table 2 Classification of crystal structures in the systems M–B–N with respect to characteristic structural units B x N y Units BN 2

Units BN 3

LiCa 4 (BN 2 ) 3 LiSr 4 (BN 2 ) 3 LiBa 4 (BN 2 ) 3 LiEu 4 (BN 2 ) 3 NaSr 4 (BN 2 ) 3 NaBa 4 (BN 2 ) 3 bCa 3 (BN 2 ) 2 5Ca 52x (BN 2 ) 3 Sr 3 (BN 2 ) 2 Ba 3 (BN 2 ) 2

Im3¯ m Im3¯ m Im3¯ m Im3¯ m Im3¯ m Im3¯ m

LiMgBN 2 aLi 3 BN 2 bLi 3 BN 2 Na 3 BN 2 lPMg 3 (BN 2 )N hPMg 3 (BN 2 )N

I4 /mmm P42 21 2 P21 /c P21 /c P63 /mmc Pmmm

La 15 (BN 3 ) 8 N Ce 15 (BN 3 ) 8 N

Im3¯ m Im3¯ m

Units B 2 N 4 R3¯ c R3¯ c

Units B 3 N 6

La 3 B 2 N 4 Ce 3 B 2 N 4 Pr 3 B 2 N 4 Nd 3 B 2 N 4

Immm Immm Immm Immm

La 5 (B 2 N 4 )N 2 Ce 4 (B 2 N 4 )N

C2 /m C2 /m

La 3 (B 3 N 6 ) Ce 3 (B 3 N 6 )

P1¯ P1¯

Pr 3 (B 3 N 6 ) Nd 3 (B 3 N 6 ) Sm 3 (B 3 N 6 ) Gd 3 (B 3 N 6 )

R3¯ c R3¯ c R3¯ c R3¯ c

La 5 (B 3 N 6 )(BN 3 ) La 5 (B 3 N 6 )N 2 La 6 (B 3 N 6 )(BN 3 )N

Pbcm – –

Nb 2 BN 12x ; Cmcm; B–B zig-zag chain and isolated N UBN; Cmcm; N-branched B–B zig-zag chain Carbon-, oxygen-stabilized compounds: La 15 (BN 3 ) 8 (N,O); Ca 910.5x (BN 2 ) 62x (CBN) x ; Ba 4 (BN 2 ) 2 O

tion N[BM 5 ] or N[B 2 M 4 ]. At low concentrations of B and N, isolated B,N atoms are encountered, which up to now are rather forming compounds at a defined composition such as Nb 2 BN than replacing each other in extended solid solutions. So far, no three-dimensional B–N framework structures have been found similar to those encountered for instance among aluminium boron carbides Al 3 B 48 C 2 , AlB 40 C 4 or Al 2 B 51 C 8 all deriving from icosahedral boron frameworks [47].

4. Ternary phase equilibria

4.1. Classification of phase equilibria in ternary metal– boron–nitrogen systems In a recent compilation [48], experimental data on about 50 ternary systems M–B–N, M being one of the metals in the Periodic Chart left of the Zintl-line, have been critically assessed with respect to phase relations, com-

Fig. 2. Characteristic structural units [B y N z ] mx 2 in metal boron nitride structure types; atom distances in nm.

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pound formation and crystal structures. Phase diagrams with a gaseous component such as nitrogen generally are strongly dependent on the nitrogen partial pressure and / or on the external pressure. All the phase equilibria observed at present can be classified with respect to the chemical ability of the metal element to form or not to form binary borides, binary nitrides and / or ternary boron nitride compounds. As a result of this classification four different groups (A to D) of distinct phase relations arise, which are summarised in Fig. 3. In group A (M5Cu, Ag, Au, Zn, Cd, Hg, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi (and Po)), the inability of the metal element to form binary borides (only binary nitrides of reduced stability may form) simply provides compatibility of metal and BN. These metal elements are located directly left of the Zintl-line in the Periodic Chart, where aluminium with its tendency towards binary boride and nitride compound formation is the only exception (see below). A metal affinity higher to boron than to nitrogen results in rather simple phase relations for the metal elements of group B, comprising all the platinum group elements as well as Mo, Tc, W, Re, Fe, Co, and Ni. At low temperatures and low nitrogen partial pressure stable two-phase equilibria between metal borides and boron nitride are formed, in all cases providing compatibility for M–BN. With formation of stable metal borides and metal nitrides, phase relations M–B–N are simply differentiated by the formation or non-formation of ternary compounds. Except for niobium, group C (no ternary compounds formed, M5Ti, Zr, Hf, V, Ta) comprises the most refractory metals, borides and nitrides. Accordingly, in these cases compatibility M–BN is subdued to the more stable combinations of the metal borides with metal nitrides, respectively. Although rather electronegative and thus

forming less stable binary nitrides, the 3d elements from chromium to iron have to be enlisted in group C. This group also comprises Sc, Y and all the small and heavy rare earth elements. Furthermore, aluminium as an exception from the homologue metametals in group A, forms simple phase equilibria Al–B–N, which resemble that of transition metal scandium, Sc–B–N. The fourth type of phase equilibria (group D) is dominated by the existence of ternary metal boron nitride compounds, usually interfering with metal / BN compatibility. Most of the metal elements that form group D phase equilibria are among the large electropositive elements from the first three main groups in the Periodic Chart including the (early) rare earth and the early actinoid elements. Also niobium belongs to this group forming stable ternary boron nitride compounds.

4.2. Phase relations in the ternary systems M–B–N; M5 Ti, W, La, Ce High temperature applications of metal boride, metal nitride and cBN combinations require detailed knowledge on the experimental conditions i.e. the nitrogen partial pressure under operation. As an example for one of the most refractory metals, Fig. 4 summarizes the phase equilibria in the W–B–N ternary system at various temperatures and external pressures. Whilst at a rather low temperature T ,10008C tungsten and lower tungsten borides are in equilibrium with BN, we observe at slightly higher temperatures and in the absence of external nitrogen an increasing instability of W and lower tungsten borides with respect to BN. This fact is documented in Fig. 5, where the logarithm of the nitrogen partial pressure is shown as a function of inverse temperature for some of the

Fig. 3. Classification of phase equilibria in ternary systems M–B–N in four groups; elements in dark shaded boxes correspond to group A; medium dark boxes are group B; gray boxes are group C; lightly shaded boxes are group D. For explanation see text.

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equilibrium reactions such as 2W1BN⇔W2 B1]21 N 2 (Refs. [49–51]) and W2 B1BN⇔2WB1]21 N 2 (Refs. [49–51]). The enthalpy of reaction, R DH8(1600K)5159(610) kJ / mol (2W1BN⇔W2 B1]21 N 2 ), was successfully used to derive the free energy of formation of W2 B [50,51]. Although kinetics may thus be small, long term industrial use of W-heaters isolated by BN-sleeves appears to be rather limited. Similarly, the combination of TiN and BN may decompose at high temperature and low partial pressure of nitrogen according to the reaction TiN1 2BN⇔TiB 2 13 / 2N 2 . Fig. 6 shows the isothermal section Ti–B–N at 15008C as well as the percentage of the various reaction products calculated as a function of temperature starting from a mixture of 1 mole of TiN and BN under constant volume of argon under normal pressure [52]. The thermodynamic calculation [52] determines the onset of

Fig. 4. Phase relations in isothermal sections of the system W–B–N; (a) isothermal section at 12008C under 1 bar argon; (b) isothermal section at 14008C under 1 bar argon after Klesnar and Rogl [49]).

Fig. 5. Logarithm of nitrogen pressure versus reciprocal temperature for the reactions 2Mo1BN⇔Mo 2 B1 ]12 N 2 , 2W1BN⇔W2 B1 ]12 N 2 and W2 B1BN⇔2WB1 ]12 N 2 (after Refs. [49–51]).

Fig. 6. Phase relations in the ternary system Ti–B–N; (a) isothermal section calculated at 15008C; (b) predominance diagram (mass of constituents vs. temperature (in 10 3 K)) calculated for 1 mole of BN11 ¨ mole of TiN under 1 bar argon (after Grobner and Rogl [52]).

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the reaction at about 1600 K; the reaction is complete at 1952 K when all BN has reacted and where the tieline TiN–BN transforms to a TiB 2 1TiN1N 2 equilibrium (see Fig. 6) finally prevailing up to the melting range. Titanium and tungsten do not form ternary boron– nitrides. As discussed above, ternary compounds are essentially encountered with the electropositive elements such as the large rare earth metals. Accordingly, Figs. 7,8 portray the phase equilibria in the systems La–B–N and Ce–B–N, both at 14008C under argon and at 16508C (La–B–N) and 18008C (Ce–B–N), respectively under nitrogen [48,49,53]. With respect to the novel compounds found and crystal structures determined (see Section 3), phase equilibria reported earlier [48], were re-evaluated revealing the phase relations as shown in Figs. 7,8. It is interesting to note that, (1) the compounds La 15 B 8 N 25 and Ce 15 B 8 N 25 are only stable under nitrogen, and (2) equilibrium with BN is only formed with LaBN 2 , CeBN 2 and Ce 3 B 2 N 4 , respectively. Although the structure type of hLa, CejBN 2 differs from that of hPr, Nd, Sm, GdjBN 2 , the common structure units [B 3 N 6 ] 32 constitute subunits of hBN. It is interesting to mention, that under high pressure the hBN-unit in hPr, Nd, Sm, GdjBN 2 may achieve sp 2 to sp 3 bonding and we may thus expect formation of an ˚ A-size cBN-subunit.

Fig. 8. Phase relations in the isothermal sections of the system Ce–B–N; (a) isothermal section at 18008C under 1 bar nitrogen; (b) isothermal section at 14008C under 1 bar argon after Klesnar and Rogl [48,49,53]).

5. Role of metal boron nitrogen compounds in the high pressure conversion of hBN to cBN

Fig. 7. Phase relations in the isothermal section of the system La–B–N; (a) isothermal section at 16508C under 1 bar nitrogen; (b) isothermal section at 14008C under 1 bar argon after Klesnar and Rogl [48,49,53]).

Although the pioneering experiments of DeVries and Fleischer in 1972 [54], defined the stability region for cBN using various alkaline and alkaline earth elements or compounds as catalyst / solvent in the hBN⇔cBN conversion, the mechanisms and kinetics of the materials chemistry processes involved in the cBN synthesis hitherto remained essentially unsolved. Mg 3 N 2 is the commonly used flux precursor in the cBN synthesis process; therefore the system BN–Mg 3 N 2 has attracted most attention [55– 61]. The kinetics of cBN formation was monitored by recent in-situ high-temperature and high-pressure synchrotron experiments [61] revealing two distinct temperature regions with different mechanisms of cBN formation: region I (at 16006140 K and 5.5 GPa) with a fast transformation process and high nucleation rate rendering but small cBN crystals (,100 mm), and region II (1700– 2300 K at 5.5 GPa) with a slow transformation to few but large cBN crystals. The synchrotron study [61] furthermore served to reconstruct the BN–Mg 3 N 2 phase diagram, explaining the prominent features of the hBN⇔cBN conversion: (1) starting from a mixture BN–Mg 3 N 2 at constant pressure of 5.5 GPa Mg 3 BN 3 is formed in a solid state reaction above 1170 K; (2) at T51500 K a meta-

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stable eutectic L⇔Mg 3 BN 3 1BN is reached and cBN is formed in the two-phase region L1BN up to 1690 K in a fast transformation, where Mg 3 BN 3 is the active agent strongly reducing the critical radius for the formation of cBN nuclei by thermal fluctuations. This process (cBN precipitation from the eutectic melt) provides the high nucleation rate via nearly instantaneous nucleation, but growth processes in turn are limited to small particle size by the large number of nuclei impinging on each other. For T .1690 K a metastable peritectic reaction is reached; (3) changing the mechanism to slow cBN formation on further heating by enhanced solubility of cBN in the melt. Cooling from 1690 K leads to the peritectic formation of a new compound Mg 3 B 2 N 4 , which was claimed to be unstable at lower temperatures where it is said to decompose into Mg 3 BN 3 1BN [61]. Figs. 9a,b compare the vertical sections of the phase diagram Mg–Mg 3 BN 3 and Mg 3 BN 3 – BN at 2.5 GPa (after Ref. [55]) and the section Mg 3 N 2 – BN at 5.5 GPa (as derived in Ref. [61]). Crystallographic data of the ternary Mg–B–N compounds are listed in Table 3 revealing also the various compound labels used in early attempts to construct the Mg–B–N phase diagram and to explain the cBN formation processes. Fig. 10 compares the structural details of the low- and highpressure modification of Mg 3 BN 3 : linear BN 2 -groups besides isolated nitrogen atoms are the typical structural units common to both modifications — the denser highpressure form accordingly reveals the denser coordination figures [Mg 10 ]BN 2 and [Mg 6 ]N with respect to [Mg 8 ]BN 2 and [Mg 5 ]N in the low pressure form. Since the early high pressure experiments by Corrigan and Bundy [9], the high activation energy Q|1000 kJ / mole (at 6.5 MPa) associated with the hBN⇔cBN conversion [9] was taken as an indication for the high covalent bonding among B–N atoms: the simpler the B–N aggregation of the catalyst / solvent the lower the p,T parameters in the high pressure conversion. This would explain the higher efficiency of alkaline and earth alkaline nitrides or metal boron nitride ‘catalysts’ (all with [BN 2 ]-type structural units) with respect to corresponding materials with higher BN-aggregation (for a comparison of structural units and representative M x B y N z structure types see also Fig. 2 and Table 2).

6. Physical properties of metal boron nitrides

Fig. 9. (a) Temperature–composition diagram at 2.5 GPa for the sections Mg–Mg 3 BN 3 and Mg 3 BN 3 –BN after [55], modified for correct composition of compound ‘Mg 3 B 2 N 4 ’ (now M 3 BN 3 ). (b) Temperature– composition diagram at 5.5 GPa for the section Mg 3 BN 3 –BN, after [61].

Although many of the ternary boron nitride compounds have been characterised with respect to their crystal structure, there is still a lack of detailed knowledge on their physico–chemical properties. This is essentially due to a general sensitivity of these materials to moisture; a condition that tends towards worse, the more electropositive the metal element involved. Secondly for most crystallographic studies tiny crystal specimens are sufficient, which, however, were too small for transport property measurements. At present the only information available are

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Table 3 Crystallographic data for ternary phases in the system Mg–B–N Phase / Temperature Range(8C)

Pearson Symbol / Space Group / Prototype

Lattice Parameters (pm)

Comments

t1 , lP–Mg 3 BN 3 ,1200 at RT stable up to 5.8GPa Ref. [65]

hP14 P63 /mmc I–Mg 3 BN 3

a5354.453 c51603.536 a5303.7 c51601.4

Ref. [34] earlier ‘Mg 3 B 2 N 4 ’ Ref. [55] stable up to 4.4 GPa at RT Ref. [56]

t1 , hP–Mg 3 BN 3 ,1489 at 2.5 GPa ,1685 at 6.5 GPa

oP7 Pmmm II-Mg 3 BN 3

a5309.33 b5313.36 c5770.05

Ref. [35], earlier ‘Mg 3 B 2 N 4 ’ Ref. [64] earlier ‘aMg 3 B 2 N 4 ’ Ref. [63]

t2 ?

unknown

–

X-phase Refs. [31,58] hP–Mg 3 BN 3 of Ref. [64]

t3 , Mg 3 B 2 N 4

unknown

–

at 5.5 GPa, .12278C Refs. [59,61]

susceptibility data [37] for La 3 B 2 N 4 , which infer temperature independent paramagnetism, whilst susceptibility data [37] for isotypic Ce 3 B 2 N 4 reveal a strong temperature dependency corresponding to a paramagnetic moment on the cerium atoms significantly smaller than for a Ce 31 ground state. These findings are in agreement with the unit

cell volume within the isotypic series RE 3 B 2 N 4 [36] as well as with band structure calculations [46,37,62]. Both, extended Hueckel calculations in tight binding approximation [46,37] and recent calculations employing the FLAWP-method [62], suggest a metallic behaviour for hLa, Cej 3 B 2 N 4 characterised with a rather small DOS (density of states) at the Fermi-level. Extended Hueckel calculations are also available for Ce 15 B 8 N 25 [46], PrBN 2 [46] and for superconducting Nb 2 BN [46] (T C 52.0 K [44]). From the DOS Ce 15 B 8 N 25 will be metallic assuming a Ce 41 ground state, whilst a band gap of about 3.5 eV would result for Ce 31 in contradiction to the metallic lustre of the material. PrBN 2 is a semiconductor or isolator with a band gap of about 4 eV. As a conclusion we may expect a metallic behaviour for rare earth- and boron-rich compounds M x B y N z , but with increasing nitrogen content the compounds will generally develop semi conducting to insulating behaviour. A verification of this trend is to be expected from the various new compounds synthesised, that await detailed physical property inspection in the future.

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Fig. 10. Comparison of the crystal structures of the low- [34] and the high [35]-pressure form of Mg 3 BN 3. .

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