Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides

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Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides James T. Cahill, Michael Alberga, Joel Bahena, Christopher Pisano, Raul BorjaUrby, Victor R Vasquez, Doreen Edwards, Scott T. Misture, and Olivia A. Graeve Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00391 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 22, 2017

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Crystal Growth & Design

Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides James T. Cahill,1 Michael Alberga,2 Joel Bahena,1 Christopher Pisano,1 Raúl Borja-Urby,3 Victor R. Vasquez,4 Doreen Edwards,5 Scott T. Misture,2 and Olivia A. Graeve1,* 1

Department of Mechanical and Aerospace Engineering University of California, San Diego 9500 Gilman Drive - MC 0411 La Jolla, CA 92093-0411

2

Kazuo Inamori School of Engineering Alfred University 2 Pine Street Alfred, NY 14802

3

Centro de Nanociencias y Micro y Nanotecnologías Instituto Politécnico Nacional Av. Luis Enrique Erro S/N, C.P. 07738 Ciudad de México, México

4

Chemical and Materials Engineering Department University of Nevada, Reno 1644 N. Virginia Street - MS 388 Reno, NV 89557

5

Kate Gleason College of Engineering Rochester Institute of Technology 77 Lomb Memorial Drive Rochester, NY 14623-5604

Abstract We present the behavior of multiple solid solutions within ternary (BaxCa1-x)B6 and (BaxSr1-x)B6 compounds and demonstrate that nano-domain formation is preferred over uniform solid solutions under certain processing conditions. Instead of the expected single solid solution of M1 and/or M2 atoms within the MB6 phase, we note separation into nano-domain regions rich in either M1 or M2. This phase separation has been observed from detailed analyses of the shapes

*

Corresponding author (Tel: 858-246-0146; Email: [email protected]; URL: http://graeve.ucsd.edu)

ACS Paragon Plus Environment

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of the peaks in X-ray diffraction data, where peak splitting and asymmetry are the result of multiple solid solutions with lattice parameters differing by up to 1.4%.

High-resolution

transmission electron microscopy confirms the presence of these nano-domains, which are about 2-3 nm in size and reveals varying degrees of lattice misalignment. We also present X-ray diffraction analysis of (BaxCa1-x)B6 powders calcined from 1273 to 1973 K and document the enhancement in sample homogeneity as the separated phases merge into a uniform solid solution. As subsequent calcinations at lower temperatures do not result in a re-separation of phases, the nano-domains are deemed metastable. The greatest degree of phase separation is observed in the (BaxCa1-x)B6 system, which corresponds to the largest difference in cation radii (0.161 vs. 0.134 nm for Ba2+ and Ca2+, respectively). Analysis of the chemical reactions that occur during synthesis suggests that the decomposition of the metal precursors (nitrates and carbonates) to metal oxides may cause selective MB6 phase formation in mixed-cation hexaborides.

1.

Introduction Metal borides of the MB6 type are a class of refractory ceramics with a CsCl-type cubic

crystal structure, in which a cage of eight boron octahedra surround a single metal atom. The boron octahedra in the structure are centered at the vertices of the cubic unit cell and their strong covalent character impart a refractory nature to these compounds, resulting in materials with high melting temperatures and significant hardness as well as chemical stability, properties that have encouraged a variety of experimental and computational studies [1-2] and have made the materials viable for a wide variety of applications [3-6]. The metal cation at the center of the unit cell donates electrons to the boron sublattice, stabilizing the cubic structure and allowing for electronic conduction in some metal hexaborides [3, 7-8]. The valency of the metal determines


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the specific electronic behavior, where compounds with M3+ cations are metallic and compounds with M2+ cations exhibit semi-metallic behavior [9]. Hexaborides of a variety of compositions have very similar lattice parameters and therefore readily form solid solutions with one another, with the largest and smallest lattice parameters only differing by 4% (a = 0.4093 nm for ThB6 and a = 0.4262 nm for BaB6) [10]. Few studies have fully characterized the behavior of metal hexaboride solid solutions, although attempts have been made to synthesize and study some (M-N)B6 systems, namely (La-Y)B6 [11], (La-Ba)B6 [11], (La-Ce)B6 [12-13], (La-Na)B6 [14], (Eu-Y)B6 [11], (Eu-Ba)B6 [11], (Ca-Sr)B6 [15-16], (Ca-Ba)B6 [15], (Sr-Ba)B6 [15], (Ca-Yb)B6 [16], and (Ca-Sm)B6 [16], among others. Olsen and Cafiero [11] prepared single crystals of (La-Eu)B6, (La-Y)B6, (Eu-Y)B6, (La-Ba)B6, and (Eu-Ba)B6 from oxide precursors in an aluminum flux and found a negative deviation from Vegard’s law in their (La-Eu)B6 samples. They attributed this deviation to the compressibility or weakness of the larger EuB6 constituent, but noted solid solutions over the entire compositional range for all ternary materials, except those containing Y3+, which produced a mixture of MB6 and YB4 crystals. Gurin et al. [12] grew single crystals of ternary (La-Ce)B6 by solution-melt. From energy dispersive spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS), they found slight variations in the ratio of La:Ce across the length of the crystal and attributed the deviations to the error associated with the analytical technique and the low concentration of Ce3+ in the La1-xCexB6 (x = 0.01 to 0.05) samples, therefore concluding that there was an even distribution of Ce3+ across the crystal.

Takeda et al. [16] produced ternary (Sr-Ca)B6 by

borothermal reduction of metal oxides in increments of 25 at.% and subsequently consolidated the powders by spark plasma sintering (SPS) at 2073 K, noting that the ternary powders were held at the sintering temperature for 50 minutes longer than the binary counterparts to ensure


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uniformity of the specimens. Gürsoy et al. [15] also synthesized hexaborides of (Sr-Ca)B6, (BaSr)B6 and (Ba-Ca)B6 by heating high purity metals with amorphous boron in a high-frequency induction furnace. The powders were also sintered by SPS at temperatures between 1673 K and 2173 K. They reported that samples in all three compositions of MxN1-xB6 (x = 0.25, 0.5, 0.75) were monophasic with lattice parameters following Vegard’s Law. In this study, we prepared samples of binary and ternary alkaline-earth hexaborides of Ca2+, Sr2+ and Ba2+ using both combustion synthesis [17-21] and borothermal reduction of carbonates [22]. The existence of nano-domains with varying lattice parameters were observed in the (BaxSr1-x)B6 and (BaxCa1-x)B6 samples.

X-ray diffraction and transmission electron

microscopy were used to characterize this phenomenon by analyzing X-ray diffraction peak splitting and observing the crystal lattice in terms of homogeneity and defects. The stability of these nano-domains was investigated at high temperatures. Detailed X-ray diffraction analysis of mixed-cation samples shows the homogenization of a multi-phase hexaboride system into one single solid solution.

2.

Methods Alkaline-earth hexaboride powders were prepared by solution combustion synthesis

according to the following reaction:

(

)

2M NO3 +12B + 3CH 6 N 4O → 2MB6 + 3CO2 + 8N 2 + 9H 2O 2

(1)

where the balanced reaction requires a fuel-to-oxidizer ratio (φ) of 3:2 (3 mol of carbohydrazide to 2 mol of metal nitrate). In order to reduce the intensity of the combustion reaction and increase powder yields, reduced φ values between 0.07 and 0.7 were implemented. Ca(NO3)2 (99.0% purity, Alfa Aesar), Sr(NO3)2 (99.0% purity, Alfa Aesar), and Ba(NO3)2 (99.0% purity,


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Alfa Aesar) were mixed with carbohydrazide (97% purity, Alfa Aesar) and amorphous boron (95% purity, Alfa Aesar) in 50 mL of type II purified water in 150 mL Pyrex crystallization dishes. While metal nitrates and carbohydrazide are soluble in water, boron is not soluble and requires agitation by stirring and boiling to properly disperse suspension of the boride particles within the water-nitrate-fuel solution. The solution was heated to 673 K until the majority of water was evaporated, and then cooled to 473 K to evaporate remaining water. Dried contents were then placed in a custom combustion furnace and heated at 50 K/min to 773 K or until the mixture combusted. The presence of several oxygen sources (nitrate, carbohydrazide, air) is known to lead to the formation of some metal oxides and borates. Because all of these oxide phases are soluble in HCl, they can be removed from the hexaboride powders with a concentrated solution of 80 vol.% HCl (36.5-38%, Alfa Aesar) and 20 vol.% water (type II purified). A mass of 2 g of as-synthesized combustion product was stirred (or “washed”) in 100 mL of this concentrated HCl solution for 10 minutes to remove any unwanted oxides, followed by two 10-minute purified water washes with 100 mL of water each. After each wash, the suspensions were separated in 50 mL polypropylene tubes and spun at 6000 rpm for 10 minutes in an Eppendorf model 5810 centrifuge and subsequently decanted.

Powders were dried

overnight in air at 350 K after the final water wash and appeared dark brown or black. Binary compositions (CaB6, SrB6, and BaB6) as well as ternary compositions of MxN1-xB6 (where x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) for all three composition sets were produced in this fashion. Alkaline-earth hexaboride powders were also prepared by borothermal reduction according to the following reaction utilizing metal carbonates and excess boron:

2MCO3 +14B → 2MB6 + B2O3 + CO 2


(2)

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CaCO3 (99.0% purity, Alfa Aesar), SrCO3 (99.0% purity, Alfa Aesar), and BaCO3 (99.0% purity, Alfa Aesar) were thoroughly mixed with amorphous boron (95% purity, Alfa Aesar) in a boron nitride crucible and heated in a Thermal Technologies LLC high-temperature graphite vacuum furnace at 8 × 10-10 MPa using a heating rate of 15 K/min to 1723 K and held for 180 minutes before being cooled to room temperature using a cooling rate of 15 K/min. Binary compositions (CaB6, SrB6, and BaB6) as well as ternary compositions of MxN1-xB6 (where x = 0.25, 0.5, 0.75) for all three compositions were produced in this fashion. Powders were subsequently calcined in two different ways: (1) using spark plasma sintering and (2) using a vacuum graphite furnace. Spark plasma sintering (SPS) was conducted under vacuum in an FCT Systeme D-25 spark plasma sintering unit [23-27] under 50 MPa at temperatures between 1173 and 1773 K, with a heating rate of 100 K/min and cooling rate of 50 K/min. Batches of powder (3 to 5 g) were placed in 18.75 mm graphite dies lined with graphite foil, producing sintered disks ~5 mm thick. Vacuum sintering was conducted in a Thermal Technologies LLC high-temperature graphite vacuum furnace at 8 × 10-10 MPa with temperatures between 1273 K and 1973 K using a heating and cooling rate of 15 K/min. A 2 g mass of powder was placed in a boron nitride crucible and placed in the center of the graphite furnace, beneath the main thermocouple.

Samples that underwent sintering were cut with

diamond blades and polished with diamond pads in preparation for characterization. As the SPS used in these experiments utilizes an optical pyrometer focused on the outer die wall, the reported temperature is lower than the actual inner temperature of the sample. This difference can be accounted for using the following relationship:

Ttop = 1.364Tside − 342.9

(3)


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which defines the temperature calibration of our SPS system, comparing the outer die wall temperature with the more accurate value measured by the top optical pyrometer [28]. The calibrated temperatures will be considered for the remainder of the discussion (i.e., the aforementioned maximum temperature of 1773 K is calibrated to 2075 K). X-ray diffraction (XRD) patterns were collected using a Bruker D2 Phaser X-ray Diffractometer (Bruker Co., Karlsruhe, Germany).

All measurements were taken at room

temperature using monochromatized CuKα radiation (λ = 0.154060 nm) and Rietveld refinements were performed using TOPAS. Powder XRD samples were prepared by tight packing of powders within a polymer holder against a flat surface, while solid sintered samples were cut with a diamond blade, polished with diamond pads and supported by a polymer holder. Scanning electron micrographs and energy dispersive spectroscopy maps were obtained on a FEI Quanta environmental scanning electron microscope (ESEM) and a FEI Sirion column ultra-high resolution scanning electron microscope (UHRSEM). Powder SEM samples were dispersed in acetone and a single drop was placed on a chip of single crystal silicon to dry, while solid sintered samples were cut with a diamond blade, polished with diamond pads and supported by an aluminum pin stub. Atomic resolution transmission electron micrographs were obtained with a JEM ARM-200CF transmission electron microscope (TEM). TEM samples were prepared by embedding the metal hexaboride powders in silicon and thinning the samples by polishing and focused ion beam milling. TEM images were analyzed with Gatan DigitalMicrograph® GMS 2 (Gatan, Pleasanton, CA) software to produce Fast Fourier Transforms (FFTs) and determine dspacings.

3.

Results and Discussion


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Figure 1 illustrates scanning electron micrographs of the samples prepared by combustion synthesis and borothermal reduction.

The samples produced by combustion

synthesis [Figure 1(a)] had submicron, cubic-shaped particles, whereas those produced by borothermal reduction [Figure 1(b)] had particles that approached tens of micrometers in size. The different morphologies resulting from the two methods can be attributed in part to the time spent at elevated temperatures. The entire combustion process is completed in under 10 minutes with just a few seconds spent at temperatures above 1200 K during the production of the adiabatic flame [29-30]. In contrast, the slow heating rate and long dwell of the borothermal reduction process provided the particles more time to grow with a total of 250 minutes spent above 1200 K. XRD patterns collected from the single-metal (binary) hexaboride powders, depicted in Figure 2, result in lattice parameters with minimal percent differences to those in the diffraction database (0.099% for CaB6, 0.095% for SrB6 and 0.187% for BaB6). Figure 3 illustrates the X-ray diffraction patterns of the samples with compositions (SrxCa1-x)B6, produced by both combustion synthesis [Figure 3(a)] and borothermal reduction [Figure 3(b)]. As determined previously [15], the X-ray diffraction patterns indicate a single solid solution phase. In general, the peak positions shift to lower 2θ angles, since SrB6 has a slightly larger lattice parameter compared to CaB6, owing to the slight differences between the ionic radii of Sr2+ and Ca2+ (with Sr2+ larger than Ca2+). The concentration of Sr2+ and Ca2+ on the metal-cation site of the materials was determined from the lattice parameters of the end members, assuming adherence to Vegard’s law [15], as follows:

 2+  a solid solution − aSrB6 Ca  = a CaB − a SrB 6

(4)

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where asolid

solution

is the lattice parameter of the solid solution and a CaB and a SrB are the 6 6

lattice parameters of the end compositions. Figures 4 and 5 illustrate the XRD patterns for samples prepared with compositions (BaxSr1-x)B6 and (BaxCa1-x)B6, respectively. Unlike the (SrxCa1-x)B6 system, many of the peaks in the XRD patterns collected from these powders appear broad and asymmetrical. In particular, powders in the (BaxCa1-x)B6 system produced by borothermal reduction [Figure 5(b)] and of composition x = 0.27, 0.34 and 0.59, displayed significant peak splitting, suggesting the formation of two distinct phases. For powders synthesized by both combustion synthesis and borothermal reduction, the (BaxCa1-x)B6 samples contained the greatest degree of phase separation as defined by peak broadening, asymmetry and splitting. Separation was less evident in the (BaxSr1-x)B6 system (Figure 4); however, the powders of x = 0.58 Ba in Figure 4(a) show some degree of both broadening and asymmetry. All patterns showing peak asymmetry were refined both as two phases and as a single phase, as illustrated in Figure 6(a) and Figure 6(b), respectively, for a sample of composition (Ba0.5Ca0.5)B6, to determine which method provided a better fit. In general, Rietveld refinement using two phases produced a better fit to the data, as illustrated by the difference curves in Figure 6. Refinements were then conducted to determine the lattice parameters of both phases and the composition (metal cation ratio) in each phase, calculated assuming Vegard’s Law. The composition of each phase was then fixed in further refinements of lattice parameter and phase fraction, with no constraints on either. The phase fractions determined by Rietveld refinement, along with phase composition determined by Vegard’s Law, were then used to calculate an overall sample composition. For the case of Ba:

      Ba total = f1⋅ Ba1 + f 2 ⋅ Ba2


(5)

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where f1 and f2 are fractions of each phase and [Ba1] and [Ba2] are the Ba content (cation basis) in each phase. The results of these refinements can be found in Tables 1, 2, and 3, which include the target as-prepared compositions for MxN1-xB6 (the intended composition based on the mixing of precursors), the lattice parameters, compositions of the two separate phases (if two phases are present), and microstrain. The tables also contain the agreement indices, or R values, for the refinement fit along with goodness of fit (GOF) values. Some of this data is represented graphically in Figure 7, which contains the lattice parameters for each sample as a function of cation composition. While there is no detectable phase separation in the (SrxCa1-x)B6 powders, some of the (BaxSr1-x)B6 powders display partial separation at a composition around 50% Ba, whereas the (BaxCa1-x)B6 powders display separation at nearly every composition synthesized (excluding the samples with x = 0 and 1). There are at least two possible reasons for the presence of two phases in (BaxSr1-x)B6 and (BaxCa1-x)B6. One possible explanation is the presence of an immiscibility gap. If this was the case, it should be thermodynamically possible to achieve homogenous single-phase samples at higher temperatures and stable two-phase mixtures at lower temperatures. Moreover, one might expect evidence of two phases to be most evident in samples with x near 0.5. A second possible explanation is that the samples were not reacted long enough at higher temperatures to ensure homogenization. In other words, the two-phase mixture would not be thermodynamically stable and the presence of the mixture is simply because of a kinetic effect. A series of heat treatments were performed on the BaxCa1-xB6 samples to test this concept. Two different heating methods were used to explore phase stability in the materials (spark plasma sintering and calcination in a graphite vacuum furnace). Both methods utilized a


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low-pressure atmosphere to reduce oxygen contamination. Figure 8 illustrates the XRD patterns collected from the as-prepared (Ba0.5Ca0.5)B6 powders. Figure 8(a) and Figure 8(b) are for the combustion and reduction powders, respectively, calcined in the SPS, and Figure 8(c) is for the combustion powders calcined in the high temperature vacuum furnace. For the combustion powders [Figure 8(a) and Figure 8(c)], heating to increasingly higher temperatures ultimately resulted in a decrease in the full-width half maximum (FWHM), an increase in symmetry, and an increase in peak intensity. For the borothermal reduction powders, the distinct diffraction peaks in the as-synthesized samples appear to merge to a single more symmetric peak at 1973 K. While the sharpening of the peaks could be a result of crystallite growth, the merging of the two peaks suggests the formation of a uniform solid solution. In addition, the peak broadening is characteristic of crystallite microstrain and the microstrain decreases with temperature, as shown in Figure 9, to vanishingly small values. While the peaks are sharper in the patterns taken from the SPS calcined samples, the trend in both series of specimens is the same, suggesting that the preferred state of mixed-cation hexaborides at high temperatures is a homogenous solid solution. For the combustion and subsequently spark plasma sintered samples there does not seem to be any change in the XRD pattern of the powders treated at 1394 K when compared to the assynthesized powders, leading us to question whether two distinct phases were stable at these temperatures. To test this concept, alkaline-earth hexaboride powders produced by combustion synthesis were calcined to 1973 K, cooled to 1273 K, and held for 6 hours in the vacuum furnace. Powders from the same batch were heated to 1940 K in the SPS, cooled to 1257 K, and held for 30 minutes.

Figure 10 contains the XRD data collected from these powders in

comparison with the as-synthesized powders and the powders heated to 1973 K (vacuum furnace) and 1940 K (SPS), and subsequently cooled directly to room temperature. The patterns


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in Figure 10 show no sign of peak splitting after the secondary 1273 K (vacuum furnace) or 1257 K (SPS) calcinations and almost no discernable difference between the powders cooled directly from 1973 or 1940 K, confirming that a single solid solution is the preferred state at these temperatures. Although the data gathered from XRD is useful for examining the bulk behavior, it does not reveal specific information about the morphology of the phase separation seen in the mixed cation alkaline-earth hexaborides. Thus, one sample of (BaxSr1-x)B6 and one of (BaxCa1-x)B6, both produced by combustion synthesis, were selected for atomic-resolution TEM analysis to try and observe the phase separation in a more direct way. Figure 11(a)-(c) contain the TEM images taken from the (Ba0.5Sr0.5)B6 powder particles, while Figure 11(d)-(f) are from the (Ba0.5Ca0.5)B6 powder particles. The micrographs in Figure 11 portray crystal structure features such as lattice misalignment, dislocations, and changes in crystallite orientation.

Figures 11(a)-(c) are

micrographs from the as-prepared (Ba0.5Sr0.5)B6 powder and contain what appear to be clusters or nano-domains with various crystal orientations [Figure 11(a)], lattice misalignment between crystallographic planes [Figure 11(b)], and an example of a homogeneous (non-segregated) region [Figure 11(c)]. Figures 11(d)-(f) are micrographs from the as-prepared (Ba0.5Ca0.5)B6 powder, also displaying distinct changes in crystal orientation [Figures 11(d)-(e)] compared to that seen in images from the (Ba0.5Sr0.5)B6 sample, corresponding to the more clearly observed phase separation observed in the XRD data. Figure 11(f) is an example of a homogeneous (nonsegregated) region of the lattice similar to that in Figure 11(c) for the (Ba0.5Sr0.5)B6 sample. Thus, these high-resolution micrographs confirm that both (Ba0.5Sr0.5)B6 and (Ba0.5Ca0.5)B6 contain nano-domains [Figures 11(a)-(b), (d)-(e)], and that both contain regions where the


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lattice is homogenous [Figures 11(c) and (f)].

The solid solution regions can be further

examined by both EDS and FFT analysis. The (Ba0.5Ca0.5)B6 lattice shown in Figure 12(a) appears uniform and an EDS scan [Figure 12(b)] reveals the presence of both Ca2+ and Ba2+ cations. The ratio of Ba:Ca, as observed by EDS, is approximately 73:27 (Ca Kα, 3.691 keV and Ba Lα, 4.465 keV) and by using Vegard’s law from Equation 5 this equates to a lattice parameter of 0.424 nm. A micrograph with high resolution, such as the one in Figure 12(a), can also be used to extract structural information with FFT. Figure 12(c) contains an FFT of the same area with the corresponding d-spacing values for the {100} and {110} families of planes, which are then used to calculate a lattice parameter. The d-spacing value for the {110} family of planes yields a lattice parameter of 0.423 nm while the d-spacing value for the {100} family of planes yields a lattice parameter of 0.422 nm. Comparison of these results to those obtained from refinement of the XRD data from this sample reveals that this value matches the lattice parameter for the Sr-rich phase (0.424 nm), and the uniformity of the lattice seen in Figure 12(a) suggests that these phases are solid solutions. As seen in both the XRD data and TEM images, several of the alkaline-earth hexaboride ternary compositions display a degree of phase separation. The morphology of this separation, as observed in TEM, is represented by small crystallites (nano-domains). (BaxCa1-x)B6 powders produced by combustion synthesis possess two distinct regions with slightly different lattice parameters and (BaxSr1-x)B6 compositions near 50% Ba2+ display similar phase separation. (BaxCa1-x)B6 powders produced by reduction show a very distinct phase separation near 50% Ba2+, but there is no distinguishable separation in the (BaxSr1-x)B6 samples. Powders produced by both combustion synthesis and borothermal reduction demonstrate complete solid solutions for the (SrxCa1-x)B6 samples. This behavior coincides with the corresponding difference in cation


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radii, i.e., the larger the size difference between the two metal cations in ternary compounds, the larger degree of phase separation is observed. The ionic radii for Ca2+, Sr2+ and Ba2+ are 0.134, 0.144 and 0.161 nm, respectively, for 12-fold coordination. The difference in cation size is smallest for the (SrxCa1-x)B6 system and largest for the (BaxCa1-x)B6, which may explain why there is a larger degree of phase separation in this system. In addition to the difference in cation radii, there is also variance introduced by the synthesis method. Both synthesis methods utilize metal salt precursors (nitrates for combustion and carbonates for borothermal reduction) which decompose at elevated temperatures (reactions 6 and 7) into metal oxides and gas. The thermal stability of these salts increases with cation size, as seen in Table 4.

(

) ()

()

()

()

1 M NO3 s → MO s + 2NO2 g + O2 g (Nitrate decomposition) 2 2

()

()

()

MCO3 s → MO s + CO 2 g (Carbonate decomposition)

(6) (7)

In both cases, the synthesis method relies on the ability to separate the M-O bond to promote formation of an M-B bond. Although thermal stability of the alkaline-earth nitrates and carbonates increases with increasing cation size, the cohesion energy (Ea) of alkaline-earth oxides of the MO form follows the opposite trend such that CaO > SrO > BaO [31-32]. Serebryakova and Marek [22] used borothermal reductions to produce CaB6 and BaB6 and noted that the temperature of formation for these two compounds was approximately 1873 K and 1673 K, respectively. They also took into account the electronic structure of alkaline-earth metals, observing that the mobility of electrons due to differences in orbital shell transitions for Ca2+ (3d04s2) and Ba2+ (5d05f06s2) was vital for promoting M-B bonds, therefore affecting the hexaboride temperature of formation, again decreasing as CaB6 > SrB6 > BaB6 [22, 33]. Bao et


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al. [34] also synthesized binary alkaline-earth metal hexaborides using MO precursors and NaBH4 at a lower reaction temperature of 1423 K and demonstrated with XRD the formation of an MB6 phase at temperatures as low as 1223 K.

This supports the idea proposed by

Serebryakova and Marek [22] that the strength of the hexaboride bonds and the mobility of electrons promotes the M-B bonding at temperatures significantly lower than the melting point of the MO precursors. The variance introduced by synthesis is highest in the (BaxCa1-x)B6 system and lowest in the (SrxCa1-x)B6 system, similar to how the difference in cation size is largest in the (BaxCa1-x)B6 system and smallest in the (SrxCa1-x)B6 system. Considering these two factors, it is not surprising that the greatest degree of phase separation is observed in the (BaxCa1x)B6

samples and none is observed in the (SrxCa1-x)B6 samples. Although Gürsoy et al. [15] reported single solid solutions in their ternary alkaline-earth

hexaboride compounds, a close examination of their (110) peaks in the (BaxSr1-x)B6 samples reveals asymmetry similar to that in this work, especially in the (Ba0.5Sr0.5)B6 sample. If this peak asymmetry was a result of nano-domains, then it would likely be more pronounced in the peaks from the (BaxCa1-x)B6 samples (not shown in the report). Gürsoy et al. [15] utilized a synthesis reaction of elemental metals with boron, suggesting that the variance introduced from precursor reduction is not the only cause of phase separation and that cation size difference may play an important role. While the (SrxCa1-x)B6 powders produced by Takeda et al. [16] utilized metal oxides, the reported XRD patterns were those of their sintered specimens after an extended period at 2073 K to enhance homogeneity, which show no phase separation in the (SrxCa1-x)B6 samples. The oxide precursors used by Olsen and Cafiero [11] to produce ternary hexaboride crystals were held at 1773 K for 5 days, more than enough time to allow for homogenization of the metal cations within the hexaboride. The negative deviation from Vegard’s law reported in


15


Page 16 of 43

(La-Eu)B6 samples is curious, and without XRD patterns it is difficult to know if this is, in fact, due to compressibility of the EuB6 or an artifact of peak asymmetry similar to the results of this study. In summary, detailed XRD and TEM analysis has revealed that ternary alkaline-earth hexaboride compounds are susceptible to phase separation, present in the form of nano-domains with varying lattice parameters. The paucity of reports on ternary hexaborides has perhaps prevented detection of nano-domain formation in these systems, which is likely to yield misleading structure-property correlations. While combustion synthesis is a viable tool for producing hexaboride powders with sub-micron particles in a timely and efficient manner, attention needs to be given to the degree of homogenization when synthesizing ternary compounds. Calcining phase-separated hexaboride compounds has been shown to homogenize the separated phases into a single solid solution. On the other hand, the presence of nanodomains within ternary compounds may have desirable effects on the properties of these materials, and therefore it is critical we understand why and when they form.

4.

Conclusions In this study, detailed X-ray diffraction analysis of mixed-cation hexaboride powders

revealed the presence of two-phase systems. This was determined by examining asymmetry and broadening of the peaks in the X-ray diffraction data. The peak splitting was attributed to the existence of multiple solid solutions with varying compositions, present in the samples as nanodomains within the particles. High-resolution transmission electron microscopy revealed the presence of such nano-domains, lattice misalignment and varying crystal orientations within the mixed-cation hexaboride particles. The phenomenon of X-ray diffraction peak splitting was also


16

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present in the diffraction patterns of powders produced by borothermal reduction of metal oxides, even more pronounced for the case of the (BaxCa1-x)B6 system. The thermal stability of these separated phases was studied by calcining (Ba0.5Ca0.5)B6 powders, which exhibited peak splitting in both the combustion and borothermal reduction powders, in a high-vacuum graphite furnace and a spark plasma sintering furnace. Increasing calcination temperature corresponded with the sharpening and merging of separated peaks in all samples, indicating that a single solid solution is the preferred state at high temperatures. (Ba0.5Ca0.5)B6 samples were also calcined to 1973 K and then cooled and held at 1273 K before being returned to room temperature. The Xray diffraction peaks from these samples did not remain split or re-split upon cooling, suggesting that the single solid solution phase is stable over a wide range of temperatures and that the separated phases are metastable. The source of these nano-domains can be attributed to two main factors, the size difference between the two metal cations in each composition and the variance introduced by the chosen synthesis method. The greatest degree of phase separation occurs in the (BaxCa1-x)B6 system and corresponds to the largest difference in cation radii (0.161 vs. 0.134 nm for the Ba2+ and Ca2+, respectively) and the largest difference in precursor decomposition and formation enthalpy. As the mixing of metal cations in hexaboride compounds can be used to tailor their properties, understanding the formation of these nano-domains is crucial to obtaining consistent and desirable results.

5.

Acknowledgments This project was funded by grant #1360561 from the National Science Foundation.


17


6.

Page 18 of 43

Figure Captions

Figure 1.

Scanning electron micrographs of CaB6 powders produced by (a) combustion

synthesis and (b) borothermal reduction. Figure 2. X-ray diffraction patterns of (a) CaB6, (b) SrB6, and (c) BaB6 powders prepared by combustion synthesis. Figure 3. X-ray diffraction patterns for powders of (SrxCa1-x)B6 prepared by (a) combustion synthesis and (b) borothermal reduction. Figure 4. X-ray diffraction patterns for powders of (BaxSr1-x)B6 prepared by (a) combustion synthesis and (b) borothermal reduction. Figure 5. X-ray diffraction patterns for powders of (BaxCa1-x)B6 prepared by (a) combustion synthesis and (b) borothermal reduction. Figure 6.

X-ray diffraction patterns for (Ba0.5Ca0.5)B6 produced by combustion synthesis

showing the fit curves assuming (a) two-phases (with the two deconvolution curves) and (b) one phase. The difference curves show that the two-phase fit is better at describing the data. Figure 7. Lattice parameters from Reitveld refinement for (a) the SrxCa1-xB6 system, (b) the BaxSr1-xB6 system, and (c) the BaxCa1-xB6 system. Figure 8. X-ray diffraction patterns for (a) the combustion synthesized powders annealed in the spark plasma sintering unit, (b) the borothermal reduction powders annealed in the spark plasma sintering unit, and (c) the combustion synthesized powders annealed in the vacuum furnace. Figure 9. Microstrain results obtained from Rietveld refinement for the BaxCa1-xB6 powders annealed in the spark plasma sintering unit at 1394 K, 1803 K, 1940 K, and 2075 K. Figure 10. X-ray diffraction patterns from combustion synthesized powders annealed (a) in the vacuum furnace and (b) in the spark plasma sintering unit.


18

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Figure 11. Atomic-resolution transmission electron micrographs of combustion synthesized (Ba0.5Ca0.5)B6 (a-c) and (Ba0.5Sr0.5)B6 powders (d-f). Micrographs (a), (b), (d) and (e) contain evidence of phase separation in the form of nano-domains and distinct differences in crystal orientation, while micrographs (c) and (f) depict homogenous (non-segregated) solid solution regions in the corresponding lattices. Figure 12.

(a) High-resolution transmission electron micrograph of (Ba0.5Ca0.5)B6, (b)

corresponding EDS scan, and (c) FFT of micrograph with d-spacing measurements.

7.

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Page 22 of 43

Table 1(a). Lattice parameters, calculated Sr2+ and Ca2+ compositions, and microstrains, obtained from Reitveld refinements for the (SrxCa1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction. In these powders only one phase was formed (i.e., there is no nanodomain formation), thus we only present data for a phase that is constituted as a solid solution of Sr2+ and Ca2+ in the hexaboride lattice.

(SrxCa1-x)B6 Combustion Synthesis

(SrxCa1-x)B6 Borothermal Reduction

Target Experimental Composition, x 0.00 0.20 0.30 0.50 0.60 0.90 1.00 0.00 0.25 0.50 0.75 1.00

Lattice Parameter (Å)

Calculated [Sr2+]

Calculated [Ca2+]

ε

4.15553(7) 4.16967(20) 4.17464(15) 4.186138(81) 4.186789(68) 4.197592(76) 4.199297(49) 4.15371(2) 4.165317(89) 4.179582(62) 4.189715(27) 4.197624(23)

0.00 0.32 0.44 0.70 0.71 0.96 1.00 0.00 0.26 0.59 0.82 1.00

1.00 0.68 0.56 0.30 0.29 0.04 0.00 1.00 0.74 0.41 0.18 0.00

0.1162(52) 0.166(50) 0.208(40) 0.1399(23) 0.1413(21) 0.0627(28) 0.0633(19) 0.02204(56) 0.0938(15) 0.1049(10) 0.04985(56) 0.04232(47)


22

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Table 1(b). Calculated total compositions, R values, and goodness-of-fit (GOF) values obtained from Rietveld refinements for the (SrxCa1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction.

(SrxCa1-x)B6 Combustion Synthesis

(SrxCa1-x)B6 Borothermal Reduction

Calculated Total Composition, x 0.00 0.32 0.44 0.70 0.71 0.96 1.00 0.00 0.26 0.59 0.82 1.00

Rexp

Rwp

Rp

GOF

3.85 3.71 2.99 2.88 4.05 3.1 2.61 9.14 9.39 9.15 7.07 5.63

9.89 12.18 11.112 9.91 6.91 9.21 10.53 15.58 18.62 13.64 11.95 12.72

7.78 8.68 8.49 7.67 5.35 6.34 3.26 10.83 13.86 9.46 7.69 8.77

2.57 3.29 3.72 3.44 1.71 2.97 4.04 1.7 1.98 1.49 1.69 2.26


23


Page 24 of 43

Table 2(a). Lattice parameters, calculated Ba2+ and Sr2+ compositions, and microstrains for phase f1 obtained from Rietveld refinements for the (BaxSr1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction. In these powders two phases formed (i.e., there is nanodomain formation). The phase f1 in these powders is a Ba2+rich phase for the majority of the target compositions.

(BaxSr1-x)B6 Combustion Synthesis

(BaxSr1-x)B6 Borothermal Reduction

Target Experimental Composition, x 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 1.00 0.00 0.25 0.50 0.75 1.00

f1, x


1.00 1.00 1.00 1.00 0.61 0.59 0.34 0.42 0.56 1.00 1.00 1.00 1.00 1.00 1.00

4.199297(49) 4.202849(48) 4.205872(62) 4.207124(49) 4.20774(21) 4.20988(24) 4.21489(47) 4.21645(32) 4.23086(68) 4.269803(32) 4.197624(23) 4.209383(55) 4.227032(84) 4.245116(61) 4.267936(91)

Calculated Calculated [Ba2+] [Sr2+] 0.00 0.05 0.09 0.11 0.12 0.15 0.22 0.24 0.45 1.00 0.00 0.17 0.42 0.68 1.00


1.00 0.95 0.91 0.89 0.88 0.85 0.78 0.76 0.55 0.00 1.00 0.83 0.58 0.32 0.00

ε (in phase f1) 0.0633(19) 0.0692(18) 0.0936(22) 0.1034(16) 0.1166(35) 0.1347(32) 0.1512(54) 0.162(58) 0.2284(79) 0.0552(12) 0.04232(47) 0.13983(99) 0.1786(13) 0.4095(37) 0.04780(71)

24

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Table 2(b). Lattice parameters, calculated Ba2+ and Sr2+ compositions, and microstrains for phase f2 obtained from Rietveld refinements for the (BaxSr1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction. The phase f2 in these powders is a Sr2+-rich phase for the majority of the target compositions.



Target Experimental Composition, x 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 1.00 0.00 0.25 0.50 0.75 1.00

f2, x 0.00 0.00 0.00 0.00 0.39 0.41 0.66 0.58 0.44 0.00 0.00 0.00 0.00 0.00 0.00


4.2246(12) 4.22651(97) 4.23242(71) 4.24219(23) 4.2517(21)

Calculated Calculated [Ba2+] for [Sr2+] for f2 f2

0.36 0.39 0.47 0.61 0.74


0.64 0.61 0.53 0.39 0.26

ε (in phase f2)

0.17(13) 0.161(11) 0.1942(77) 0.1718(43) 0.1379(63)

25


Page 26 of 43

Table 2(c). Calculated total compositions, R values, and goodness-of-fit (GOF) values obtained from Rietveld refinements for the (BaxSr1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction.



Target Calculated Experimental Total Composition, Composition, x x 0.00 0.00 0.10 0.05 0.20 0.09 0.30 0.11 0.40 0.21 0.50 0.25 0.60 0.38 0.70 0.45 0.80 0.58 1.00 1.00 0.00 0.00 0.25 0.17 0.50 0.42 0.75 0.68 1.00 1.00

Rexp

Rwp

Rp

GOF

2.61 2.69 2.76 2.86 2.95 3.03 3.18 3.31 3.43 3.93 5.63 7.08 9.53 12.02 11.18

10.53 9.68 10.15 8.97 6.93 7.65 7.2 7.12 7.68 9.84 12.72 12.42 15.08 16.04 18.29

3.26 7.2 7.82 6.89 5.19 5.76 5.37 5.38 5.67 7.04 8.77 9.46 11.53 10.45 11.74

4.04 3.6 3.68 3.14 2.35 2.52 2.26 2.15 2.24 2.5 2.26 1.75 1.58 1.33 1.64


26

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Table 3(a). Lattice parameters, calculated Ba2+ and Ca2+ compositions, and microstrains for phase f1 obtained from Rietveld refinements for the (BaxCa1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction. In these powders two phases formed (i.e., there is nanodomain formation).

(BaxCa1-x)B6 Combustion Synthesis

(BaxCa1-x)B6 Borothermal Reduction

Target Experimental Composition, x 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.00 0.15 0.20 0.25 0.50 0.75 0.87 1.00

f1, x


1.00 0.27 0.33 0.34 0.34 0.46 0.56 0.52 0.66 0.60 1.00 1.00 1.00 0.12 0.16 0.32 1.00 1.00 1.00

4.15553(6) 4.187493(57) 4.202202(69) 4.214123(42) 4.227277(35) 4.237924(21) 4.245755(13) 4.252800(45) 4.260372(90) 4.265301(61) 4.269803(32) 4.15371(2) 4.173285(68) 4.23696(24) 4.24233(17) 4.24473(18) 4.250965(52) 4.261164(47) 4.267936(99)

Calculated Calculated [Ba2+] for [Ca2+] for f1 f1 0.00 0.28 0.41 0.51 0.63 0.72 0.79 0.85 0.92 0.96 1.00 0.00 0.17 0.73 0.78 0.80 0.85 0.94 1.00


1.00 0.72 0.59 0.49 0.37 0.28 0.21 0.15 0.08 0.04 0.00 1.00 0.83 0.27 0.22 0.20 0.15 0.06 0.00

ε (in phase f1) 0.0708(23) 0.283(35) 0.409(13) 0.391(10) 0.364(8) 0.3053(58) 0.2301(45) 0.1767(33) 0.1369(36) 0.1044(61) 0.0552(12) 0.02204(56) 0.0862(17) 0.267(15) 0.2365(84) 0.1707(49) 0.1042(13) 0.0645(11) 0.04780(71)

27


Page 28 of 43

Table 3(b). Lattice parameters, calculated Ba2+ and Ca2+ compositions, and microstrains for phase f2 obtained from Rietveld refinements for the (BaxCa1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction.

(BaxCa1-x)B6 Combustion Synthesis

(BaxCa1-x)B6 Borothermal Reduction

Target Experimental Composition, x 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.00 0.15 0.20 0.25 0.50 0.75 0.87 1.00

f2, x 0.00 0.73 0.67 0.66 0.66 0.54 0.44 0.48 0.34 0.40 0.00 0.00 0.00 0.88 0.84 0.68 0.00 0.00 0.00


Calculated Calculated [Ba2+] for [Ca2+] for f2 f2

ε (in phase f2)

4.1613124(74) 4.168133(11) 4.1760661(12) 4.185595(26) 4.1938747(37) 4.2083908(85) 4.2278207(11) 4.2425161(50) 4.2546941(63)

0.05 0.11 0.18 0.26 0.34 0.46 0.63 0.76 0.87

0.95 0.89 0.82 0.74 0.66 0.54 0.37 0.24 0.13

0.1448(29) 0.1986(44) 0.2365(53) 0.2692(71) 0.3043(92) 0.352(16) 0.431(17) 0.274(17) 0.301(18)

4.17702(12) 4.18321(12) 4.21088(17)

0.20 0.26 0.50

0.80 0.74 0.50

0.1734(28) 0.2134(30) 0.1966(39)


28

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Table 3(c). Calculated total compositions, R values, and goodness-of-fit (GOF) values obtained from Rietveld refinements for the (BaxCa1-x)B6 alkaline-earth hexaboride powders produced by combustion synthesis and borothermal reduction. Target Calculated Experimental Total Composition, Composition, x x (BaxCa1-x)B6 0.00 0.00 Combustion 0.10 0.11 Synthesis 0.20 0.21 0.30 0.29 0.40 0.39 0.50 0.51 0.60 0.65 0.70 0.75 0.80 0.86 0.90 0.92 1.00 1.00 (BaxCa1-x)B6 0.00 0.00 Borothermal 0.15 0.17 Reduction 0.20 0.27 0.25 0.34 0.50 0.59 0.75 0.85 0.87 0.94 1.00 1.00

Rexp

Rwp

Rp

GOF

3.85 4.01 4.02 3.97 3.99 3.94 4.05 3.98 4 3.73 3.93 9.14 11.27 11.73 9.31 9.03 9.4 10.94 11.18

9.89 7.69 7.28 6.13 6.34 6.36 6.91 6.82 7.38 8.92 9.84 15.58 13.9 13.73 11.32 10.63 13.69 17.22 18.29

7.78 6.12 5.69 4.78 4.9 4.92 5.35 5.35 5.6 6.35 7.04 10.83 9.54 9.6 8.41 7.28 9.32 11.91 11.74

2.57 1.92 1.81 1.54 1.59 1.61 1.71 1.71 1.84 2.39 2.5 1.7 1.23 1.17 1.22 1.18 1.46 1.57 1.64


29


Page 30 of 43

Table 4. Enthalpies and decomposition temperatures for alkaline-earth metal nitrates and carbonates

Nitrate

∆H (kJ mol-1)

Tdecomp, air (K)

Ca(NO3)2

+24

834

Sr(NO3)2

+36

843

Ba(NO3)2

+42

865

Carbonate

∆H (kJ mol-1)

Tdecomp, vacuum (K)

CaCO3

+178

1247

SrCO3

+235

1256

BaCO3

+267

1458


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For Table of Contents Use Only Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides James T. Cahill, Michael Alberga, Joel Bahena, Christopher Pisano, Raúl Borja-Urby, Victor R. Vasquez, Doreen Edwards, Scott T. Misture, and Olivia A. Graeve

We present the behavior of multiple solid solutions within ternary (BaxCa1-x)B6 and (BaxSr1-x)B6 compounds and demonstrate that nano-domain formation is preferred over uniform solid solutions. Instead of the expected single solid solution of M1 and/or M2 atoms within the MB6 phase, we note separation into nano-domain regions rich in either M1 or M2 of about 2-3 nm in size.


31


Figure 1. Scanning electron micrographs of CaB6 powders produced by (a) combustion synthesis and (b) borothermal reduction. 82x124mm (300 x 300 DPI)


Page 32 of 43

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Figure 2. X-ray diffraction patterns of (a) CaB6, (b) SrB6, and (c) BaB6 powders prepared by combustion synthesis. 82x93mm (300 x 300 DPI)



Figure 3. X-ray diffraction patterns for powders of (SrxCa1-x)B6 prepared by (a) combustion synthesis and (b) borothermal reduction. 82x175mm (300 x 300 DPI)


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Figure 4. X-ray diffraction patterns for powders of (BaxSr1-x)B6 prepared by (a) combustion synthesis and (b) borothermal reduction. 83x175mm (300 x 300 DPI)



Figure 5. X-ray diffraction patterns for powders of (BaxCa1-x)B6 prepared by (a) combustion synthesis and (b) borothermal reduction. 83x175mm (300 x 300 DPI)


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Figure 6. X-ray diffraction patterns for (Ba0.5Ca0.5)B6 produced by combustion synthesis showing the fit curves assuming (a) two-phases (with the two deconvolution curves) and (b) one phase. The difference curves show that the two-phase fit is better at describing the data. 82x86mm (300 x 300 DPI)



Figure 7. Lattice parameters from Reitveld refinement for (a) the SrxCa1-xB6 system, (b) the BaxSr1-xB6 system, and (c) the BaxCa1-xB6 system. 82x247mm (300 x 300 DPI)


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Figure 8. X-ray diffraction patterns for (a) the combustion synthesized powders annealed in the spark plasma sintering unit, (b) the borothermal reduction powders annealed in the spark plasma sintering unit, and (c) the combustion synthesized powders annealed in the vacuum furnace. 174x97mm (300 x 300 DPI)



Figure 9. Microstrain results obtained from Rietveld refinement for the BaxCa1-xB6 powders annealed in the spark plasma sintering unit at 1394 K, 1803 K, 1940 K, and 2075 K. 81x65mm (300 x 300 DPI)


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Figure 10. X-ray diffraction patterns from combustion synthesized powders annealed (a) in the vacuum furnace and (b) in the spark plasma sintering unit. 82x177mm (300 x 300 DPI)



Figure 11. Atomic-resolution transmission electron micrographs of combustion synthesized (a) (Ba0.5Ca0.5)B6 and (b) (Ba0.5Sr0.5)B6 powders. 177x101mm (300 x 300 DPI)


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Figure 12. (a) High-resolution transmission electron micrograph of (Ba0.5Ca0.5)B6, (b) corresponding EDS scan, and (c) FFT of micrograph with d-spacing measurements. 83x167mm (300 x 300 DPI)


Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides

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