Formation mechanisms of Cr2AlB2, Cr3AlB4, and Fe2AlB2 MAB phases

Quenching studies of CrB-Al, CrB-Al-B and ß-FeB-Al mixtures revealed that all three undergo a topotactic transformation in the presence of Al to form Cr2AlB2, Cr3AlB4, and Fe2AlB2 MAB phases, respectively. The formation mechanism is intercalation for the Cr-based compounds, and reconstructive for the ß-FeB based mixture. GRAPHICAL ABSTRACT


Introduction
The MAB phases are a group of atomically laminated, transition metal borides (TMB), wherein M is a transition metal, A is typically Al, and B is boron. Their atomically laminated crystal structures are comprised of transition metal boride layers (M-B) interleaved with Al layer(s). Depending on the number of transition metal planes comprising the M-B layers and whether Al monolayers or bilayers separate them, the MAB phases take on the M 2 AlB 2 , M 3 AlB 4 , M 4 AlB 6 , M 4 AlB 4 or M 2 Al 2 B 2type structures ( Figure 1) [1]. One of the hallmarks of the MAB phases, and the leitmotiv of this work, is their close structural relationship to their binary borides. The germane binaries are the CrB, Cr 3 B 4 and ß-FeB-type structures, the other boride structures are not relevant to MAB phase formation. The former crystallizes in the Cmcm space group wherein the B chains are aligned along the a-axis (Figure 1(A)). The latter crystallizes in the Pnma space group, where the MB layers are comprised of singly-capped trigonal prisms [2] (Figure 1(B)). Furthermore, the two binaries are structurally related, as the ß-FeB-type structure can be obtained from the CrB-type by transposing the MB slabs of the latter by CONTACT Michel W. Barsoum barsoumw@drexel.edu 1/2 < 001 > [3]. The Cr 3 B 4 binary crystallizes in the Immm space group and is shown in Figure 1(C). Regardless of the structure type, the BM 6 trigonal prisms making up the M-B layers strongly resemble, and in most cases are identical to, those found in their corresponding binary TMBs with the same M:B ratio [4]. This is exemplified by comparing the crystal structures of CrB and Cr 3 B 4 with those of M 2 AlB 2 (Figure 1(D)) and M 3 AlB 4 ( Figure 1(E)), respectively.
A notable consequence of having isolated Al layer(s) is that they tend to be relatively more weakly bonded to the M-B layers than the strong covalent M-B bonds. This fact is demonstrated by their mechanical behavior, high-temperature thermal decomposition, and dissolution behavior in acids [4][5][6][7]. For example, the MAB phases are softer than their corresponding binaries [1]. At high temperatures, the Al atoms tend to de-intercalate and evaporate/sublimate from the MAB phases, precipitating the corresponding binary borides [6][7][8][9]. In the case of MoAlB, the Al atoms react with ambient air at high temperatures to form well-adhered alumina layers that are protective in air up to 1300°C. The same compound can oxidatively self-heal cracks and has excellent ablation resistance even at oxyacetylene flame temperatures [10,11]. Zhou et al. observed that Al selectively dissolved from Cr 2 AlB 2 particles at ambient temperatures when exposed to hydrochloric acid which in turn led to the precipitation of CrB particles [12]. Similarly, Alameda et al. [5] and Kim et al. [13] found that only one of the Al layers in the Mo 2 Al 2 B 2 crystal structure can be topochemically de-intercalated upon exposure to LiF and HCl solutions to from the metastable Mo 2 AlB 2 phase, whose structure had previously only been observed as stacking faults in MoAlB grains.
In this work, topotactical reactions are paramount. By definition, topotaxy requires that one crystalline phase transforms into another with a well-defined crystallographic orientational relationship between them [14]. Topotactic transformations can range from simple intercalation reactions to complex reconstructive ones [15]. As indicated above, the M-B bonding in the crystal structures show virtually unchanged bond lengths relative to their binary counterparts. For example, it has been noted that the Cr 2 AlB 2 crystal structure can be visualized as an intercalation of Al atoms into the CrB structure while causing each the Cr-B blocks in the latter to shift by 0.5 < a + c > along [101] [16]. Combined with the absence of binary Cr-Al and Al-B phases after CrB + Al or CrB + Al + B mixtures were heated to synthesize Cr 2 AlB 2 and Cr 3 AlB 4 , respectively [17], we hypothesized that these MAB phases can form by a topotactic intercalation of Al atoms into CrB and Cr 3 B 4 , respectively [16].
The purpose of this work was to understand how the binary borides transform into their respective MAB phases. Given their close structural resemblances, our working hypothesis was that there is a topotactic relationship exists between them. To test this hypothesis and to shed light on the reaction mechanisms, mixtures of CrB and Al or CrB, Al and B powder mixtures were heated and quenched from various temperatures. Using X-ray diffraction, XRD, we thus followed how the reactions developed at intermediate temperatures. Since Fe 2 AlB 2 is isostructural with Cr 2 AlB 2 and ß-FeB is considered a twinned, distorted form of CrB [3], we carried out analogous experiments with ß-FeB and Al mixtures to assess the effect of precursor structure on the reaction paths. Herein we show that for both CrB cases, the Al simply intercalates in between the Cr-B blocks [12] converting them to their MAB counterparts. However, in the FeB case, the Al first reacts with the Fe, forming an intermetallic before completing the reaction to form Fe 2 AlB 2 .

Methods
All samples for quenching were prepared via a powder metallurgical route starting with CrB (H.C. Starck, 1-2 μm average particle size), ß-FeB (See [18], < 44 μm), amorphous B (Alfa Aesar, 98%, < 44 μm) and Al (Alfa Aesar, 99.5%, < 44 μm) in molar ratios of 2CrB:1.5Al, 3CrB:1B:1.5Al, or 2FeB:1.5Al. The large excess of Al was used to facilitate a more uniform reaction at the lower temperatures. The mixtures were mechanically ball-milled at 60 rpm in 8 g batches for 24 h in a plastic jar using an equal mass of 5 mm diameter yttria-stabilized zirconia media. Green bodies were cold pressed at a load corresponding to a stress of 200 MPa into 2 g pellets, with a diameter of 13 mm and a height of ≈ 10 mm. Pellets were heated one at a time in a horizontal tube furnace under a flowing Ar atmosphere. The pellets were heated at a heating rate of 5°C/min to the temperatures indicated in Table 1, and allowed to equilibrate for 3-5 min before quenching directly into distilled water at ambient temperature. The quenching temperatures were chosen to evaluate the status of the reaction as it progressed from reactants to the final products. The lowest temperature, 680°C, was chosen to evaluate the beginning of the liquid phase reaction between Al and the transition metal borides. The maximum temperature reflects typical reaction temperatures used for the synthesis of various MAB phase [1,16,17]. Lastly, one sample of each composition was reacted at the highest temperatures used for an extended time (see Table 1) in an attempt to achieve equilibrium and/or a fully reacted state.
The surfaces of all samples were lightly sanded to remove any surface oxides and drilled using a cobalt steel bit to produce a fine powder. This powder was characterized by using a X-ray diffractometer (Rigaku SmartLab) with a Cu K α source (λ = 0.0154 nm) and a detectorside graphite monochromator. The quantitative analysis was performed using MDI Jade 6.5 with the Easy Quant package using the RIR method [19].

Results & discussion
Cr 2 AlB 2 XRD patterns of the quenched samples, starting with CrB and 1.5 Al to produce Cr 2 AlB 2 , are shown in Figure 2. Quenching from 680°C, there is no evidence of the formation of the MAB phase. This is not too surprising since this temperature is just over Al's melting point and had insufficient time to react with the CrB (Figure 2A). Quenching from 850°C, a large quantity of Cr 2 AlB 2 forms, though unreacted CrB and Al still remain in the sample ( Figure 2B). Quenching from 970°C shows the continued consumption of the CrB and Al precursors, as shown by the decreased relative intensities of the CrB peaks relative to the 850°C quench ( Figure 2C). After reacting at 970°C for 15 h, a further reduction of the CrB peak intensities indicate that the Al was able to further intercalate between the CrB layers ( Figure 2D). The ratio of intensities between the (130) peak of Cr 2 AlB 2 and the (021) peak of CrB shows the evolution of the MAB phase with increasing temperatures and after reacting for 15 h ( Figure 2E). Based on these results, it is reasonable to conclude that the following single-step reaction is operative: The Al present after 15 h at 970°C is simply due to the large molar excess of Al present in the initial reagent mixture. Note that this result implies that Cr 2 AlB 2 is in most probably in equilibrium with molten Al.

Cr 3 AlB 4
XRD patterns of the 3 CrB, 1.5 Al and 1.0 B mixture after quenching from different temperatures and after holding at the maximum temperatures for 10 h are shown in Figure 3. XRD of the samples evidences a threestep evolution. First, the elemental B reacts with CrB to form CrB 2 , followed by the formation of Cr 3 B 4 at higher temperatures. The last reaction is one with Al to form Cr 3 AlB 4 . At 680°C, there is some indication that CrB 2 has started to form, but a majority of the  CrB and all the Al remains unreacted ( Figure 3A). It should be noted that B is not detected by XRD because it remains amorphous at temperatures below 1100°C [20]. At 750°C, the CrB reacts further with the B forming more CrB 2 ( Figure 3B). However, no peaks corresponding to Cr 3 AlB 4 or Cr 2 AlB 2 are present in the pattern, despite the fact that Cr 2 AlB 2 showed signs of formation between 680-820°C in Figure 1. Once the sample reaches 820°C, the CrB and CrB 2 react and form Cr 3 B 4 , which in turn reacts with Al and the formation of Cr 3 AlB 4 begins ( Figure 3C). At this temperature, Cr 3 B 4 and Cr 3 AlB 4 are the majority phases present, with some CrB and Cr 2 AlB 2 impurities present. At 1020°C, only small amounts of CrB and Cr 3 B 4 impurities exist with almost all the binary borides reacted to form Cr 3 AlB 4 ( Figure 3D).
After reacting for 10 h, there is a negligible amount of CrB present, yet Cr 3 B 4 remains at the same levels ( Figure 3E). As the reaction temperature increases, the ratio of relative intensities of the (102) peak of Cr 3 AlB 4 to the (021) peak of CrB illustrates the progression of the reaction ( Figure 3F). Between 820°C and 1020°C, the Cr 3 AlB 4 :CrB ratio more than doubles, and after reacting for 10 h there is a fourfold increase in Cr 3 AlB 4 . The near absence of Cr 2 AlB 2 at higher temperatures implies that it is most likely an impurity phase, not a step along the reaction pathway. Once the Cr 3 B 4 has formed, the Al intercalates between the layers forming the 314 MAB phase structure. This process can be represented by the following reactions:  Unlike the Cr based MABs, the Fe 2 AlB 2 XRD patterns show a different reaction mechanism, as the Al does not directly intercalate into the FeB structure ( Figure 4). Even at 680°C, the Al reacts with the ß-FeB to form the intermetallic Fe 4 Al 13 ( Figure 4A). This low temperature formation of Fe 4 Al 13 is described by Li et al. [21] as taking place at temperatures as low as 650°C and would result in the formation of free B. As in the Cr 3 AlB 4 , this free B required to mass balance the reaction (see Equation 3) is not detected by XRD due to its amorphous nature. At 850°C the reaction shifts to the third step as the ß-FeB peak intensities decrease, and Fe 2 AlB 2 begins to form ( Figure 4B). However, the intensities of the intermetallic peaks remain constant over this temperature range. At 1050°C, all of the ß-FeB has reacted, forming a majority phase of Fe 2 AlB 2 with some remnant intermetallic impurities ( Figure 4C). After reacting for 10 h, there is no discernible presence of the intermetallic phase as the reaction goes to completion ( Figure 4D). This reaction is sequence can be represented by the equations:

Discussion
Based on these results, the formation of both Cr 2 AlB 2 and Cr 3 AlB 4 both seem to be a product of a hightemperature intercalation of Al atoms between atomic layers of CrB and Cr 3 B 4 , respectively. In this reaction, the CrB and Cr 3 B 4 structures were retained in each M-B layer during this process, as noted by the lack of additional phases. As discussed previously, a topotactic transformation is solely concerned with the relationship between the initial and final crystal structure. Though the formation of Fe 2 AlB 2 from FeB and Al does not follow the same classic intercalation mechanism as occurs during the formation of Cr 2 AlB 2 and Cr 3 AlB 4 , it still undergoes a topotactic transformation. The ß-FeB structure can be considered to be a deformed CrB structure and is therefore related to the final Fe 2 AlB 2 structure. The decomposition of the ß-FeB and subsequent intermediate formation of the Fe 3 Al 14 intermetallic are steps along the reaction pathway.

Summary and conclusions
To investigate the formation mechanisms of Cr 2 AlB 2, Cr 3 AlB 4 , and Fe 2 AlB 2 , high temperature quenching experiments were carried out. The Cr 2 AlB 2 reaction path was single step as the Al began reacting with the CrB to form the Cr 2 AlB 2 between 680°C and 820°C. The Cr 3 AlB 4 reaction path was three steps: formation of CrB 2 from CrB and B, formation of Cr 3 B 4 from CrB and CrB 2 , and finally the reaction with Al between 750°C and 820°C. Both systems undergo a similar topotactic transformation, as the Cr-B structure is retained after the Al reaction. The final products of the reactions are dependent on the initial stoichiometry, as no Cr 3 AlB 4 formed from 2CrB + 1.5Al case and an insignificant amount of Cr 2 AlB 2 formed when the initial mixture was 3CrB + 1B + 1.5Al.
Unlike the Cr based MABs, the Al did not simply intercalate between the binary boride layers; the system underwent a reconstructive reaction instead. The Al first reacted with the Fe to form an intermetallic at low temperatures. As the temperature increased, the ß-FeB and intermetallic reacted to form Fe 2 AlB 2 .

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was funded by National Science Foundation (NSF) [grant number DMREF 1729335].