Clusters, Assemble: Growth of Intermetallic Compounds from Metal

Dec 19, 2017 - Reactions in several common flux metals (Ga, In, Sn, Al) are amenable to DSC analysis, with melting and crystallization events observab...
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Article Cite This: Acc. Chem. Res. 2018, 51, 40−48

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Clusters, Assemble: Growth of Intermetallic Compounds from Metal Flux Reactions Published as part of the Accounts of Chemical Research special issue “Advancing Chemistry through Intermetallic Compounds”. Susan E. Latturner* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States CONSPECTUS: Metal flux synthesis involves the reaction of metals and metalloids in a large excess of a low-melting metal that acts as a solvent. This technique makes use of an unusual temperature regime (above the temperatures used for solvothermal methods and below the temperatures used in traditional solid state synthesis) and facilitates the growth of products as large crystals. It has proven to be a fruitful method to discover new intermetallic compounds. However, little is known about the chemistry occurring within a molten metal solvent; without an understanding of the nature of precursor formation and assembly, it is difficult to predict product structures and target properties. Organic chemists have a vast toolbox of well-known reagents and reaction mechanisms to use in directing their synthesis toward a desired molecular structure. This is not yet the case for the synthesis of inorganic extended structures. We have carried out extensive explorations of the growth of new magnetic intermetallic compounds from a variety of metal fluxes. This Account presents a review of some of our results and recent reports by other groups; this work indicates that products with common building blocks and homologous series with identical structural motifs are repeatedly seen in metal flux chemistry. For instance, fluorite-type layers comprised of transition metals coordinated by eight main group metal atoms are found in the Th2(AuxSi1−x)[AuAl2]nSi2 and R[AuAl2]nAl2(AuxSi1−x)2 series grown from aluminum flux, the CenPdIn3n+2 series grown from indium flux, and CePdGa6 and Ce2PdGa10 grown from gallium flux. Similarly, our investigations of reactions of heavy main group metals, M, in rare earth/transition metal eutectic fluxes reveal that the R/T/M/M′ products usually feature Mcentered rare earth clusters M@R8−12, which share faces to form layers and networks that surround transition metal building blocks. These structural trends, temperature dependence of products formed in the flux, and interconversions observed by differential scanning calorimetry support the idea that these clusters likely form in the melt, existing as precursors and assembling into different crystalline products depending on time, temperature, and reaction ratio. Proof of this mechanism will require future investigations using techniques such as pair distribution function analysis of flux melts to observe cluster formation and in situ diffraction during cooling to detect various phases as they crystallize and interconvert. These data will aid in understanding the parameters that control cluster formation and assembly in metal melts, allow for prediction of products of flux reactions, and will potentially enable the tailoring of reaction conditions to promote the formation of structures with desirable properties. The use of metal flux synthesis offers the possibility of coupling these targets. Metal flux reactions involve the use of a large excess of a low-melting metal (or metal mixture) that, upon melting, acts as a solvent for the other reactants present.6 This enables lowered reaction temperatures compared to traditional solid state synthesis and also facilitates crystal growth of products by means of slowly cooling the reaction mixture. The lowered reaction temperature allows for the formation of metastable or kinetically stabilized products, as compared to the more thermodynamically favored products formed at high temperatures. Evidence described in this Account−structural building blocks that are repeatedly seen, formation of homologous series, interconversions between

1. INTRODUCTION Multinary intermetallic compounds incorporating rare earth and transition metals are of great interest as potential magnetic materials. The combination of localized moments from felectrons with itinerant magnetism produced by transition metal networks produces the hard ferromagnetism seen for Nd2Fe14B and SmCo5, compounds used in applications ranging from hard drives to electric motors.1,2 Other magnetic properties found in RxTyMz (R = rare earth, T = transition metal, M = main group) materials include spin glass behavior (La21Fe8Sn7C12), Kondo effect and valence fluctuations (CeAg2−xCuxIn), and superconductivity (RNi2B2C).3−5 This wide variety of complex behaviors has inspired the search for new magnetic intermetallic compounds and the desire to design intermetallic compounds with structures likely to yield particular electronic characteristics. © 2017 American Chemical Society

Received: September 29, 2017 Published: December 19, 2017 40

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Accounts of Chemical Research related structures during synthesis−indicates that soluble building blocks form in the flux and then assemble into products, which crystallize out of the melt. However, the local energy minima may be similar for several structurally related products, leading to competition between multiple phases. Homologous series, sets of structures comprised of identical building blocks that expand by regular increments (expressed by a mathematical formula), are widely known in oxide chemistry. Layered perovskite systems such as the Ruddlesden−Popper (general series formula An−1A′2BnO3n+1) and Aurivillius phases (general formula Bi2An−1BnO3n+3) are extremely versatile with applications as catalysts, fuel cell electrodes, and superconductors and have been extensively reviewed.7,8 This versatility extends to the halide perovskite systems, which are of great recent interest for use in solar cells; the ability to vary the layer thickness and identity of ions in the different layers to manipulate optical properties is being heavily investigated.9 There are also many examples of homologous series in semiconducting metal chalcogenides, which have potential as thermoelectric materials (including the [(PbSe)5]n[(Bi2Se3)3]m and Am[M1+lSe2+l]2m[M2l+nSe2+3l+n] series where A = alkali metal and M = Sn, Pb, or Bi reported by Kanatzidis et al).10 Intermetallic homologous series are not quite as well-known, but the possibility of forming a series of RxTyMz compounds with building blocks of controllable size and composition offers the ability to tailor magnetic properties. To enable this control over the building blocks, it is necessary to understand how they form and what directs their assembly. This Account will discuss examples of recently explored families of structurally related intermetallic compounds and potential ways to investigate the nature of their growth.

AuAl2 formation was catalyzed by the presence of silicon. This fluorite structure compound was referred to as the “purple plague” due to its violet color and its brittle properties, which often led to contact failure.16 Accordingly, investigations of reactions of lanthanides or actinides, silicon, and gold in aluminum flux led to a large family of quaternary intermetallic compounds that feature AuAl2 slabs of varying sizes in their structures. The first three of these were a homologous series Th2(AuxSi1−x)[AuAl2]nSi2, with n = 1, 2, and 4 indicative of the thickness of the AuAl2 layers, and x = 0.6−0.9 representing the Au content in a Au/Si mixed site that caps this layer.17 These AuAl2 slabs are separated by a thorium/silicon layer featuring silicon zigzag chains bracketing thorium atoms. The structures, shown in Figure 1, can be viewed as intergrowths between AuAl2 blocks and CeNiSi2 type thorium/silicon slabs.

2. INTERMETALLIC COMPOUNDS FEATURING FLUORITE-TYPE LAYERS GROWN FROM GROUP 13 MELTS The fact that many complex intermetallic structures are built up from segments of simpler structures was recognized decades ago by some of the leading solid state chemistry groups in Europe.11−13 (We stand on the shoulders of giants who published in Russian, German, and French.) Many compounds with large unit cells and complex stoichiometries such as Ce4Ni2Ga17 and Ce5Pd2In19 feature layers that are comprised of transition metals in a cubic environment of 8 main group metal atoms (often group 13); these cubes share edges, resulting in a fluorite type (CaF2-type) layer.14,15 The stability of this structural building block may explain why it is so prevalent in compounds grown from group 13 fluxes and may indicate that precursor clusters likely exist in the melt (a supposition supported by pair distribution function experiments described in section 4).

Figure 1. Th2(AuxSi1−x)[AuAl2]nSi2 homologous series. Fluorite-type AuAl2 building blocks are shown as yellow polyhedra (Au and Al as yellow and black spheres), Th atoms as blue spheres, and silicon atoms as red spheres.

Another homologous series is produced from reactions of early rare earths with gold and silicon in aluminum flux. This family of tetragonal compounds features AuAl2 slabs capped by a Au/Si mixed site, stacked along the c-axis and separated by a layer of rare earth cations coordinated by the mixed site in BaAl4-type building blocks.18 The four members of this R[AuAl2]nAl2(AuxSi1−x)2 series that were isolated and characterized (n = 0−3) are shown in Figure 2. Analogs of each of these structures are found for the majority of the early rare earth elements (R = La−Eu) and Yb, with magnetic susceptibility measurements indicating that Eu is divalent and Yb exhibits fluctuating +2/+3 valence. The increasing size of the AuAl2 layers, regular symmetry changing from P- to Icentering of the unit cell, and consistently close to 50/50 ratio of Au and Si in the mixed site leads to the ability to predict the stoichiometry and structure of additional members of this series not yet isolated; for instance, RAu5Al10Si should have an I4/ mmm symmetry unit cell with a = 4.2 Å and c = 35 Å. This series of compounds represents the separation of 2-D layers of paramagnetic rare earth ions by nonmagnetic AuAl2 slabs of increasing size, possibly shedding light on the ability of rare earth moments to couple across large distances. Given their similar structures and building blocks, it is understandable that products of a typical R/Au/Si/Al flux reaction often contained more than one series member. While the RAu4Al8Si member was favored by increasing the amount of

2.1. Purple Plague Layers

Molten aluminum has proven to be an excellent growth medium for new aluminide phases. Syntheses of intermetallic compounds in aluminum flux may also serve as models for the growth of adventitious phases during the manufacture of industrial aluminum alloys and during formation of contacts to aluminum-based electronic components. This latter occurrence was especially problematic for gold−aluminum interfaces on silicon devices. The junction between the gold wire and aluminum pad was found to degrade rapidly, due to the formation of a number of binary Au/Al intermetallic compounds including Au4Al, Au2Al, and AuAl2. In particular, 41

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Figure 2. R[AuAl2]nAl2(AuxSi1−x)2 homologous series and intergrowth variant Gd2Au5Al10Si2. Fluorite-type AuAl2 building blocks are shown as yellow polyhedra (Au and Al as yellow and black spheres), rare earth atoms as blue spheres, and gold/silicon mixed site as red spheres.

Figure 3. Structures of CePdGa6, Ce2PdGa10, and Ce2PdGa12 grown from gallium flux, and heating profiles and centrifugation temperatures used to isolate each phase. Fluorite-type PdGa2 building blocks are shown as pink polyhedra (Pd and Ga as pink and black spheres), Ce atoms as blue spheres. Ce4Ni2Ga17 also shown for comparison (Ni as green spheres).

groups indicated that heating profile was an additional variable to exploit.19−22

gold used in the synthesis, mixtures of products were usually obtained. Another indication of the very small energy differences between stacking of different size AuAl2 slabs is the crystallization of intergrowth structures such as Gd2Au5Al10Si2 ,which is comprised of alternating RAu2Al4Si and RAu3Al6Si building blocks. This highlights the difficulty in separating these energetically similar products using reactant ratios alone. Subsequent work by the Chan and Kratochvilova

2.2. Further Insight from Flux Growth of Gallide and Indide Heavy Fermion Compounds

Due to the energy of cerium 4f orbitals being close to the Fermi level, cerium-containing intermetallic compounds can exhibit unusual behavior such as fluctuating valence, heavy fermion behavior, and superconductivity. These behaviors have been reported in many compounds containing cerium ions 42

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Accounts of Chemical Research surrounded by a nonmagnetic framework of late transition metals and main group metals.5 The Chan group carried out an exploration of gallium flux growth of heavy fermion compound CePdGa6, which competes in phase space with two structurally related ternary compounds.19,20 CePdGa6, Ce2PdGa10, and Ce2PdGa12 are all comprised of fluorite-type PdGa2 layers separated by Ce/Ga slabs with a BaAl4-related structure; see Figure 3. (It is notable that Grin et al. prepared nickel analogs CeNiGa6 and Ce2NiGa10 and furthered the series with Ce3NiGa15 and Ce4NiGa17; Pd analogs of the latter two compounds have not been isolated.)14,21 The three Ce/Pd/Ga phases can be grown from millimole ratio 1:1−1.5:20, with the product being dependent not on reaction stoichiometry but on the reaction cooling rate and centrifugation temperature (reactions were cooled to a specific temperature and removed from the furnace, and the excess gallium was removed by centrifugation, effectively quenching the reaction at that temperature). Optimized reaction heating profiles for each compound are shown in Figure 3. Similar temperature dependence was recently observed by Kratochvilova et al. for the indium flux synthesis of members of the CenMIn3n+2 (n = 1−3; M = Co, Rh, Ir, Pd, Pt) family.22 This homologous series of tetragonal structures is comprised of CeIn3-type slabs of varying thickness stacked along the c-axis and separated by one unit of fluorite-type MIn2 layers; see Figure 4. The parent compound CeIn3 exhibits superconductivity, as does the n = 1 member CeCoIn5. The n = 2 analogs Ce2MIn8 (M = Rh, Ir) exhibit heavy fermion behavior.23 The recently discovered n = 3 member of the series, Ce3PdIn11, shows a complex interplay of heavy fermion behavior, antiferromagnetism, and superconductivity.24 The Kratochvilova group carried out an exploration of indium flux growth of Ce/Pd/In and Ce/Pt/In compounds. Their work made use of DSC measurements on flux reactions to determine possible crystallization temperatures; these were then tested in bulk reactions with products monitored by microprobe analysis and crystallography. Similar to what was reported for the Ce/ Pd/Ga system, multiple phases were often observed, which were most effectively separated by varying the heating profile. For instance, heating a 2:1:25 mmol ratio of Ce/Pd/In to temperatures over 900 °C favored CeIn3; Ce3PdIn11 formed in reactions heated to 750 °C, and Ce2PdIn8 became predominant at lower temperatures.22

Figure 4. CenMIn3n+2 homologous series grown from indium flux. Fluorite-type MIn2 building blocks are shown as pink polyhedra (M, In, and Ce atoms represented by pink, black, and blue spheres, respectively).

nickel or cobalt of the eutectic mixture is typically inert if Fe or Mn reactants are present. A survey of the R/T/M/M′ products indicates that, as was seen for group 13 flux reactions, formation of common building blocks and temperature dependence of products is observed.3,26−34 While only one homologous series has been found thus far, two structural trends are apparent−the incorporation of clusters of rare earths centered by a heavy main group element, and the presence of transition metal building blocks capped by light elements. Heavy main group elements are relatively electronegative and are likely partially reduced upon dissolving in the rare-earth rich flux. It is probable that M@Rn clusters (anionic M surrounded by R cations) exist in the melt, possibly acting as templating agents for the formation of product structures. Clusters comprised of a main group element surrounded by 8−12 rare earth cations are found in many of the R/T/M/M′ products isolated from our flux reactions. These units are highlighted in Figure 5 in the structures of La14Sn(MnC6)3, La21Fe8Sn7C12, Nd6Co5Al2.3Ge1.7, Nd8Co4Ge2C3, La6(Mn/Ni/Al)13Sn, and La11(Mn/Ni/Al)13Sn4−d. In addition to electrostatic interaction, size effects (i.e., relative radii of the R and M ions) play a role in stabilizing certain M@Rn species. In our work, we have found Sn@La9 to be particularly stable; several compounds have been isolated containing this species that do not have analogs with different main group elements. When we switch to a Nd-based

3. INTERMETALLIC COMPOUNDS GROWN FROM RARE EARTH/TRANSITION METAL FLUX 3.1. R/T/M/M′ Phases: M@R8−12 Clusters and TxM′y Building Blocks

Switching from main group fluxes to rare-earth based fluxes offers the opportunity to discover more rare-earth rich intermetallic compounds. Rare earth metals themselves are not attractive as fluxes, due to their high melting points. However, examination of binary phase diagrams shows that when early rare earths (R = La−Nd) are combined with late first row transition metals (T = Co, Ni, Cu), eutectics are formed.25 These eutectic mixtures, such as La/Ni (67 mol % La/33 mol % Ni, mp 517 °C) and Ce/Co (76 mol % Ce, 24 mol % Co, mp 424 °C), are excellent solvents for the majority of elements in the periodic table. We have focused on reacting a potentially magnetic transition metal (T = Fe or Mn) with a heavy main group element (M = Ge, Sn, Pb, Sb, Bi, etc.) and a light metalloid (M′ = B, C, Al, Si) in R/Ni or R/Co fluxes. The 43

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Figure 5. Structures and crystals of intermetallic compounds grown from reactions of heavy main group metals (M = Ge, Sn, Pb, Sb, Bi) with other elements in rare earth/transition metal flux. Clusters of M surrounded by rare earth ions (M@R8−12) are shown as yellow/orange polyhedra (M and R as yellow and blue spheres). Transition metals shown as red spheres, carbon as black spheres, aluminum as purple spheres.

flux, reactions with tin are less productive, but reactions with the smaller Ge yielded several compounds containing Ge@ Nd9; Nd6Co5Ge2.5Al1.5 is an analog of La6SnNi3.7Ru0.7Al3.6 with the larger Sn@La9 clusters replaced by [email protected],34 On the other hand, R21T8M7C12 forms with different combinations of rare earth (R = La, Ce, Pr), transition metals (T = Fe or Mn), and main group elements (M = Si, Ge, Sn, Pb, Bi, Sb, Te), but the yield is higher for reactions involving larger M.3,33 The other building block in these flux-grown R/T/M/M′ intermetallic compounds is comprised of transition metal species capped by light elements (M′ = C, B, Al). These species are typically separated by a network of face-sharing M@Rn clusters. Several examples are depicted in Figure 6, ranging from monomeric MnC 6 units (in La 14 Mn 3 C 18 Sn) to borocarbide-capped Fe13 clusters (in Ce33Fe13B25C34) to aluminum-capped transition metal slabs in La11(Mn/Ni/ Al)13Sn4−δ.26,27,32 Carbon appears to stabilize small clusters (monomers, tetramers, chains); boron caps intermediate ones (squares, Fe13, Fe14), and aluminum caps sheets and layers (Nd6Co5Al2.3Ge1.7, La6Mn10Al3Sn, La11(Mn/Ni/Al)13Sn4−δ). The carbide products in particular find themselves between two established classifications. Jeitschko and Kniep categorized ternary RxTyCz structures into two types based on their metalto-carbon ratios.35 Those with high ratios ((x + y)/z ≥ 4) are metal-rich carbides, exemplified by Pr2Fe14C and ThFe11C2−x (1.5 ≤ x ≤ 2); they feature partially occupied interstitial carbon sites surrounded by 2-D or 3-D transition metal networks. RxTyCz compounds with low metal-to-carbon ratios ((x + y)/z < 2) are carbometalates, with transition metal atoms coordinated by carbide species, often in a trigonal planar or tetragonal fashion. These TCx units can be isolated from one

another by surrounding rare earth atoms or corner- or edgeshare through bridging carbons to form polyanionic networks. Several R/T/M/C intermetallic compounds grown from rare earth based fluxes exhibit a metal to carbon ratio between these classifications, or transition metal building blocks intermediate between the monomeric transition metal species of carbometalates and the transition metal 3-D networks of metal-rich carbides. These include the carbon-capped iron tetrahedra in La21Fe8Sn7C12, the borocarbide-capped cobalt squares in Ce10Co2.64B11.70C10, larger borocarbide-capped Fe13 clusters in Pr33Fe13B18C34, and the 1-D cobalt chains (with Co−Co bonds bridged by carbon) in Nd8Co4Ge2C3.3,29,31,32 Given the fact that transition metals in carbometalates are typically nonmagnetic, while the extended transition metal networks in metal-rich carbides often exhibit significant magnetic moments and ordering, the “intermediate” nature of the transition metal clusters and chains in R/T/M/C compounds offers potential insight into what conditions are necessary for a transition metal to become magnetic. For iron clusters, the threshold appears to be a cluster of 3 iron atoms. Compounds containing isolated Fe or iron dimers (capped with carbon) do not exhibit a magnetic moment on the iron. However, the carbon-capped iron tetrahedral clusters (Fe4C6) in La21Fe8Sn7C12 show geometrically frustrated spin glass behavior.3 3.2. Layered Structures Based on Transition Metal Slabs Capped by Aluminum

The presence of aluminum appears to stabilize the formation of layers of transition metals in R/T/M/Al phases, and it is often incorporated on a capping position on the surface of these layers. The higher dimensionality and increase in T−T bonding 44

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Ni/Al)13Sn4−δ compounds.27,30 These slabs are comprised of transition metal icosahedra linked to form sheets in the abplane; this building block is the same as that found in cubic NaZn 13 -type LaFe12Al. The aluminum is preferentially incorporated on the outermost capping sites of the transition metal sheets. These layers are separated by La/Sn slabs, which have the Cr5B3 structure (La and Sn atoms on the Cr and B positions, respectively). In the crystal structure of La6(T/Ni/ Al)13Sn (La6Co11Ga3-type), the transition metal layers are 7.5 Å apart; they are 14.7 Å apart in La11Mn13−x−yNixAlySn4−δ. These structures can be considered a homologous series of intergrowths of LaFe12Al layers and Cr5B3 type slabs, see Figure 7. As was the case with the homologous series described in section 2, the compounds are often found as mixed products; in syntheses targeting La11Mn13−x−yNixAlySn4−δ, byproducts La6(Mn/Ni/Al)13Sn and La5Sn3 are observed from flux reactions with low and high amounts of tin, respectively. Initial explorations indicate that reaction ratio and very likely temperature may both be useful variables to isolate them. LaT12Al was observed at high temps, and La11(T/Ni/ Al)13Sn4−d appears to be a low temperature product (not formed if reactions are centrifuged above 700 °C). The magnetic properties of the transition metal layers of La6Mn10Al3Sn and La11Mn13−x−yNixAlySn4−δ are particularly interesting. The former compound exhibits ferromagnetic ordering at 200 K.30 Mn-rich compositions of the latter compound (such as La11Mn10Al3Sn3.4) are spin glasses with a freezing transition of TC = 20 K, but substitution of Mn by nonmagnetic Ni or Al weakens the antiferromagnetic coupling interactions, eliminating the spin glass transition and leading to paramagnetic behavior for La11Mn6.7Ni2.9Al3.4Sn3.1.27

4. OBSERVATION OF GROWTH: CURRENT AND FUTURE WORK 4.1. In-Situ Detection of Intermediate/Competing Phases

Several metal flux growth systems have been identified in the previous sections that involve the formation of one intermetallic product at elevated temperatures and its conversion to different (but structurally related) phases at lower temperature. Those are just the systems we know about. Intermediate phases may occur in many flux reactions and are completely missed if the heating profile quenches at high temperature or cools to below the stability range of certain metastable intermediates. In order to discover these phases and optimize their synthesis, it is necessary to detect their formation and determine the temperature range at which they are stable. In-situ optical techniques (FTIR, Raman, UV/vis, visual inspection) can be used to monitor the progress of low temperature solvothermal syntheses but are not applicable to metal flux reactions (all of the intermetallic compounds mentioned herein were grown in alumina crucibles sealed in fused silica ampules and heated in closed furnaces). Two methods that are applicable are diffraction and calorimetry. Recent developments for in situ diffraction techniques have proven vital in detecting the growth of crystalline phases from solvothermal and salt flux reactions. The high energy X-ray scattering (HEXS) capabilities of modern synchrotrons, producing strongly penetrating X-rays at energies above 100 keV−an order of magnitude higher than conventional diffractometers−allow for diffraction experiments on large samples in containers at elevated temperatures and pressures. Such experiments provide data on phase changes of crystalline

Figure 6. Transition metal building blocks in R/T/M/M′ intermetallic compounds grown from reactions in rare earth/transition metal flux. Transition metals shown as red spheres; capping elements carbon, boron, and aluminum shown as black, green, and purple spheres, respectively.

compared to the smaller transition metal clusters described in the previous section results in the transition metals in these layers having a magnetic moment and often exhibiting magnetic ordering. A 2-D hexagonal net of cobalt is found in Nd6Co5Al2.3Ge1.7 (grown from the reaction of Al and Ge in Nd/Co eutectic). These cobalt sheets are bridged by an aluminum site and order ferromagnetically at 150 K.28 Thicker slabs of T = Fe or Mn (with some substitution by nickel and aluminum incorporated from the flux and the alumina crucible) are found in La6(T/Ni/Al)13Sn and La11(T/ 45

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Figure 7. LaM12Al−La6M10Al3Sn−La11M13Sn4−δ (M = (Mn or Fe)/Ni/Al) series grown from reactions of Mn, Sn, and Al in La/Ni metal flux. Transition metal layers shown as red icosahedra; La, Sn, and Al shown as blue, yellow, and purple spheres, respectively.

relating the DSC data to crystallization temperatures and phase diagrams.39 We explored the reaction of cobalt and zinc in molten tin using this technique. At elevated temperature (900 °C), the binary CoSn crystallized, but as the system cooled we were able to detect a small exotherm corresponding to a reaction with zinc and conversion to a new ternary phase at 550 °C.40 We were able to catch this crystallization in the act by quenching a normal-scale flux reaction by centrifuging at this temperature; Figure 8 shows the hexagonal rods of CoSn converting into the cuboid crystals of Co7.2Zn2.8Sn8. The structure of this ternary phase can be viewed as zinc atoms stuffed between building blocks found in CoSn. Similarly, the previously mentioned exploration of reactions of cerium and palladium in indium flux by the Kratochvilova group relied heavily on DSC studies to determine the optimal temperature range for the isolation of various CenPdmIn3n+2m series members. The temperature of exotherms and endotherms observed for a scaled down flux reaction were compared with known binary phase diagrams (DSC studies of each binary were also carried out, as controls) to isolate the transitions due to formation of Ce3PdIn11 and Ce2PdIn8.22

compounds, including dissolution/amorphization, crystallization, and structural transformations, essentially providing a “panoramic view” of what phases form at what temperature.36 This has not yet been applied to metal flux synthesis (surprisingly) but has yielded extremely useful results on several salt flux systems. A notable example is the growth of complex metal sulfides from reactions of metals in alkali polysulfide salt fluxes. Copper was reacted in K2S3 flux in a heated thin-wall capillary sample holder while diffraction data were collected at beamline 17-BM at the Advanced Photon Source (ANL); changes in diffraction pattern were monitored as a function of temperature and time.37 The melting of the flux and the dissolution of the reactant copper were easily observed, as was the formation and disappearance of KCu3S2 and the previously unknown phase K3Cu4S4 (while the capillary was still heating up to its maximum temperature!) and the formation of K3Cu8S6 upon cooling. The structures of the ternary phases contain identical building blocks, indicating a common precursor present in the melt. The intermediates that form as the reactant mixture heats up would be missed in a normal flux synthesis procedure, which focuses on what forms as the reaction cools. Another recent example is the in situ investigation of the growth of layered perovskites from reaction of Bi2O3, TiO2, Fe2O3, and Cr2O3 in molten Na2SO4.38 The IR heated reaction cell designed for use at beamline I12 at the Diamond Light Source (UK) allowed for the observation of the initial formation of the n = 3 Aurivillius phase Bi4Ti3O12 during heating and its conversion to the n = 4 phase Ba5Ti3Fe0.5Cr0.5O15. With suitable modifications of apparatus, these in situ diffraction techniques are likely applicable to metal flux reactions. Another in situ method that can be used to monitor crystallization events and other phase changes in flux mixtures is differential scanning calorimetry. This is feasible for relatively simple flux systems that can be scaled down to fit into the DSC sample holder. For many DSC instruments this is a small open alumina cup under a flow of inert gas; reaction systems involving volatile metals (or those that attack alumina) will require alternative sample holders. Reactions in several common flux metals (Ga, In, Sn, Al) are amenable to DSC analysis, with melting and crystallization events observable as endo- or exotherms. Canfield et al. used this method to investigate the flux growths of TbAl, YMn4Al8, and Pr7Ni2Si5,

4.2. Detection of Building Blocks: Cluster Growth in Metal Melts

While in situ diffraction and DSC experiments are useful for observing the growth of crystalline phases, these techniques do not shed light on the mechanism of how their structures form. The majority of systems mentioned above involve structurally related compounds forming and interconverting at different temperatures, indicating that specific building blocks exist in the melt and are assembled in various time-, temperature-, and concentration-dependent ways. Identifying these precrystallized clusters is the first step to controlling their assembly, with the eventual goal of targeting structures with desirable properties. For this, methods that detect short-range order (and its dependence on temperature and composition) are needed. A particularly promising method is pair distribution function (PDF) analysis of melts, which is becoming more powerful with the advent of HEXS.36 Investigations of local structure of metallic melts and metallic glasses using PDF coupled with computational analysis indicate that these techniques may be a rich source of information about metal flux reactions. A study of Pd82Si18 eutectics carried out at APS reveals the existence Si@ 46

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mapped out through a large number of individual reactions. A number of methods (DSC, in situ diffraction, and PDF) may enable this information to be collected in the process of one reaction. It would be particularly convenient to carry out PDF studies on the melt at high temperature to observe cluster formation and diffraction studies as the reaction mixture is cooled to monitor how the clusters coalesce, the nucleation of product structures, and possible interconversion between products. This knowledge will contribute to understanding the growth mechanism and potentially to controlling it to target structures with desired properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Susan E. Latturner: 0000-0002-6146-5333 Notes

The author declares no competing financial interest. Biography Susan Latturner is a professor of materials chemistry at Florida State University. She received her undergraduate B.S. in Chemistry from the University of Virginia and her Ph.D. in Chemistry from UC Santa Barbara under the direction of Galen Stucky. She carried out postdoctoral work in the Kanatzidis group at Michigan State University before joining the faculty at FSU. Her research group investigates flux synthesis of intermetallic compounds and relationships between crystal structure and magnetic and electronic properties.

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ACKNOWLEDGMENTS Several graduate and undergraduate students, postdoctoral researchers, and collaborators have made major contributions to this project over the years (see references). The work was supported by the National Science Foundation (Awards DMR0547791, DMR-1106150, and DMR-1410214) and Florida State University.

Figure 8. Differential scanning calorimetry data of reaction of cobalt and zinc in tin flux, yielding CoSn below 900 °C and Co7.2Zn2.8Sn8 below 550 °C. Shared structural building blocks are circled. SEM image of products of a reaction quenched at 550 °C shows the CoSn crystals converting to the ternary phase.

Pd9 species in the melt (very similar to the M@R9 clusters observed in many R/T/M/M′ phases!).41 Similarly, investigations of Ba/Ge melts at SPring-8 in Japan indicated a prevalence of Ba@Gex clusters with x = 16−20; these may naturally lead to the Ba@Ge20 pentagonal dodecahedra that are found in clathrate phases such as Ba6Ge25, Ba8Ge43, and BaGe5.42,25 Advances in computational methods are leading to the possibility of determination of melt configurations without experimental PDF data. Ab-initio MD simulations of AuAl2 melts at 2000 K indicate that the most abundant clusters are those where gold is coordinated by 8 Al atoms, similar to the local environment of gold in fluorite-type AuAl2 (and all the intermetallic compounds in Figures 1 and 2!).43 These experiments support the existence of structure heredity between a melt and the phases that crystallize from it.

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REFERENCES

(1) Herbst, J. F. R2Fe14B materials: Intrinsic properties and technological aspects. Rev. Mod. Phys. 1991, 63, 819−898. (2) Stadelmaier, H. H.; Reinsch, B.; Petzow, G. Samarium cobalt phase equilibria revisited; relevance to permanent magnets. Z. fur Metallkunde 1998, 89, 114−118. (3) Benbow, E. M.; Dalal, N.; Latturner, S. E. Spin glass behavior of isolated geometrically frustrated tetrahedra of iron atoms in the intermetallic La21Fe8Sn7C12. J. Am. Chem. Soc. 2009, 131, 3349−3354. (4) Cava, R. J.; Takagi, H.; Zandbergen, H.; Krajewski, J.; Peck, W. F.; Siegrist, T.; Batlogg, B.; Vandover, R. B.; Felder, R. J.; Mizuhashi, K.; Lee, J.; Eisaki, H.; Uchida, S. Superconductivity in the quaternary intermetallic compounds LnNi2B2C. Nature 1994, 367, 252−253. (5) Szytula, A.; Leciejewicz, J. Handbook of Crystal Structures and Magnetic Properties of Rare Earth Intermetallics; CRC Press: Boca Raton, 1994. (6) Kanatzidis, M. G.; Pottgen, R.; Jeitschko, W. The metal flux: a preparative tool for the exploration of intermetallic compounds. Angew. Chem., Int. Ed. 2005, 44, 6996−7023. (7) Schaak, R. E.; Mallouk, T. E. Perovskites by design: A toolbox of solid-state reactions. Chem. Mater. 2002, 14, 1455−1471. (8) Anderson, M. T.; Vaughey, J. T.; Poeppelmeier, K. R. Structural similarities among oxygen-deficient perovskites. Chem. Mater. 1993, 5, 151−165.

5. CONCLUSIONS AND OUTLOOK Reactions in metal flux have yielded a wide variety of intermetallic compounds with structures that can be viewed as an assembly of common building blocks. An increasing amount of experimental data indicates that these clusters are formed in the flux and assembled into the final structure(s). The parameters that may control this assembly−temperature, heating/cooling rate, reactant concentration−can be laboriously 47

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Accounts of Chemical Research

(28) Zhou, S.; Latturner, S. E. Flux growth and magnetic properties of rare earth cobalt germanide, RE6Co5Ge1+xAl3‑x (RE = Pr, Nd; x = 0.8). J. Solid State Chem. 2016, 238, 189−194. (29) Zhou, S.; Latturner, S. E. Nd8Co4‑xAlxGe2C3: A case study in flux growth of lanthanide-rich intermetallics. J. Solid State Chem. 2016, 236, 159−165. (30) Benbow, E. M.; Dalal, N. S.; Latturner, S. E. Crystal growth and magnetic behavior of R6T13‑xAlxMy phases (R = La, Nd; T = Mn, Fe; M = main group) grown from lanthanide-rich eutectic fluxes. J. Solid State Chem. 2009, 182, 3055−3062. (31) Zhou, S.; Mishra, T.; Lyman, D.; Tucker, P.; Latturner, S. E. New cerium cobalt borocarbide synthesized from eutectic metal flux mixture. Inorg. Chem. Front. 2017, 4, 450−455. (32) Tucker, P. C.; Nyffeler, J.; Chen, B.; Ozarowski, A.; Stillwell, R.; Latturner, S. E. A tale of two metals: New cerium iron borocarbide intermetallics grown from rare-earth/transition metal eutectic fluxes. J. Am. Chem. Soc. 2012, 134, 12138−12148. (33) Zaikina, J. V.; Schellenberg, I.; Benbow, E. M.; Pöttgen, R.; Latturner, S. E. The influence of the La/M network on magnetic properties of Mn 4 tetrahedra in intermetallic compounds La21‑δMn8M7C12 (M = Ge, Sn, Sb, Te, Bi). Chem. Mater. 2011, 23, 1768−1778. (34) Zaikina, J. V.; Jo, Y. J.; Latturner, S. E. Ruthenium Intermetallics grown from La-Ni flux: Synthesis, structure, and physical properties. Inorg. Chem. 2010, 49, 2773−2781. (35) Dashjav, E.; Kreiner, G.; Schnelle, W.; Wagner, F. R.; Kniep, R.; Jeitschko, W. Ternary rare earth and actinoid transition metal carbides viewed as carbometalates. J. Solid State Chem. 2007, 180, 636−653. (36) Soderholm, L.; Mitchell, J. F. Perspective: Toward “synthesis by design”: Exploring atomic correlations during inorganic materials synthesis. APL Mater. 2016, 4, 053212. (37) Shoemaker, D. P.; Hu, Y. J.; Chung, D. Y.; Halder, G. J.; Chupas, P. J.; Soderholm, L.; Mitchell, J. F.; Kanatzidis, M. G. In situ studies of a platform for metastable inorganic crystal growth and materials discovery. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10922− 10927. (38) Moorhouse, S. J.; Wu, Y.; Buckley, H. C.; O’Hare, D. Timeresolved in situ powder X-ray diffraction reveals the mechanisms of molten salt synthesis. Chem. Commun. 2016, 52, 13865. (39) Janssen, Y.; Angst, M.; Dennis, K. W.; McCallum, R. W.; Canfield, P. C. Differential thermal analysis and solution growth of intermetallic compounds. J. Cryst. Growth 2005, 285, 670−680. (40) Reynolds, P. C.; Stojanovic, M.; Latturner, S. E. Flux growth of a new cobalt−zinc tin ternary phase Co7+xZn3‑xSn8 and its relationship to CoSn. J. Solid State Chem. 2011, 184, 1875−1881. (41) Dong, F.; Yue, G. Q.; Guo, Y. R.; Qiao, C.; Kramer, M. J.; Wang, S. Y.; Ho, K. M.; Chen, L. Y.; et al. Si-centered capped trigonal prism ordering in liquid Pd82Si18 alloy study by first-principles calculations. RSC Adv. 2017, 7, 18093−18098. (42) Ishikura, A.; Mizuno, A.; Watanabe, M.; Masaki, T.; Ishikawa, T.; Kohara, S. Structure Analysis of Molten Ba−Ge Alloys Using Electrostatic Levitation Technique Combined With High-Energy XRay Diffraction. J. Am. Ceram. Soc. 2007, 90, 738−741. (43) Xiong, L. H.; Lou, H. B.; Wang, X. D.; Debela, T. T.; Cao, Q. P.; Zhang, D. X.; Wang, S. Y.; Wang, C. Z.; Jiang, J. Z. Evolution of local atomic structure during solidification of Al2Au liquid: An ab initio study. Acta Mater. 2014, 68, 1−8.

(9) Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden−Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28, 2852−2867. (10) Kanatzidis, M. G. Structural evolution and phase homologies for “design” and predictions of solid state compounds. Acc. Chem. Res. 2005, 38, 359−368. (11) Parthe, E.; Chabot, B. A.; Cenzual, K. Complex structures of intermetallic compounds interpreted as intergrowth of segments of simple structures. Chimia 1985, 39, 164−174. (12) Pani, M.; Fornasini, M. L. Examples of linear structures of intermetallic compounds described as intergrowths of segments of simple basic structures. Z. Kristallogr. 1990, 190, 127−133. (13) Grin, Y. N.; Yarmolyuk, Y. P.; Gladyshevskii, E. I. Crystal chemistry of series of inhomogeneous linear structures. 1. Symmetry and numerical symbols of structures composed of fragments of structural types BaAl4, CaF2, AlB2, AuCu3, Cu, α-Fe, and α -Po. Krystallografiya 1982, 27, 686−692; Sov. Phys. Crystallogr. 1982, 27, 413−417. (14) Grin, Y.; Yarmolyuk, Y. P.; Usov, O. A.; Kuz’min, A. M.; Bruskov, V. A. The crystal chemistry of series of inhomogeneous structure. The crystal structure of the compounds Ce4Ga17Ni2 and Nd4Ga17Ni2. Sov. Phys. Crystallogr. 1983, 28, 710−711. (15) Tursina, A.; Nesterenko, S.; Seropegin, Y.; Noel, H.; Kaczorowski, D. Ce2PdIn8, Ce3PdIn11 and Ce5Pd2In19members of homological series based on AuCu3- and PtHg2-type structural units. J. Solid State Chem. 2013, 200, 7−12. (16) Philofsky, E. Intermetallic formation in gold-aluminum systems. Solid-State Electron. 1970, 13, 1391. (17) Latturner, S. E.; Bilc, D.; Mahanti, S. D.; Kanatzidis, M. G. Quaternary Intermetallics Grown from Molten Aluminum: The Homologous Series Th2(AuxSi1‑x)[AuAl2]nSi2 (n = 1, 2, 4). Chem. Mater. 2002, 14, 1695−1705. (18) Latturner, S. E.; Kanatzidis, M. G. RE(AuAl2)nAl2(AuxSi1‑x)2: A New Homologous Series of Quaternary Intermetallics Grown from Aluminum Flux. Inorg. Chem. 2008, 47, 2089−2097. (19) Macaluso, R. T.; Millican, J. N.; Nakatsuji, S.; Lee, H. O.; Carter, B.; Moreno, N. O.; Fisk, Z.; Chan, J. Y. A comparison of the structure and localized magnetism in Ce2PdGa12 with the heavy fermion CePdGa6. J. Solid State Chem. 2005, 178, 3547−3553. (20) Phelan, W. A.; Menard, M. C.; Kangas, M. J.; McCandless, G. T.; Drake, B. L.; Chan, J. Y. Adventures in Crystal Growth: Synthesis and Characterization of Single Crystals of Complex Intermetallic Compounds. Chem. Mater. 2012, 24, 409−420. (21) Grin, Y.; Yarmolyuk, Y. P.; Zavodnik, V. E. Crystal chemistry of series of inhomogeneous linear structures. The crystal stucture of the compound Ce3Ga15Ni2. Sov. Phys. Crystallogr. 1984, 29, 135−137. (22) Kratochvilova, M.; Dusek, M.; Uhlirova, K.; Rudajevova, A.; Prokleska, J.; Vondrackova, B.; Custers, J.; Sechovsky, V. Single crystal study of the layered heavy fermion compounds Ce2PdIn8, Ce3PdIn11, Ce2PtIn8 and Ce3PtIn11. J. Cryst. Growth 2014, 397, 47−52. (23) Macaluso, R. T.; Sarrao, J. L.; Moreno, M. O.; Pagliuso, P. G.; Thompson, J. D.; Fronczek, F. R.; Hundley, M. F.; Malinowski, A.; Chan, J. Y. Single-Crystal Growth of Ln2MIn8 (Ln = La, Ce; M = Rh, Ir): Implications for the Heavy-Fermion Ground State. Chem. Mater. 2003, 15, 1394−1398. (24) Kratochvilova, M.; Prokleska, J.; Uhlirova, K.; Tkac, V.; Dusek, M.; Sechovsky, V.; Custers, J. Coexistence of Antiferromagnetism and Superconductivity in Heavy Fermion Cerium Compound Ce3PdIn11. Sci. Rep. 2015, 5, 15904. (25) Villars, P.; Okamoto, H.; Cenzual, K. ASM Alloy Phase Diagram Database; ASM International: Materials Park, OH, 2006−2016. (26) Zaikina, J. V.; Zhou, H.; Latturner, S. E. Structural relationships between new carbide La14Sn(MnC6)3 and fully ordered La11(MnC6)3. J. Solid State Chem. 2010, 183, 2987−2994. (27) Zaikina, J. V.; Griffin, V.; Latturner, S. E. Switching on a spin glass: Flux growth, structure, and magnetism of La11Mn13‑x‑yNixAlySn4‑δ intermetallics. Inorg. Chem. 2017, DOI: 10.1021/acs.inorgchem.7b02555. 48

DOI: 10.1021/acs.accounts.7b00483 Acc. Chem. Res. 2018, 51, 40−48