Bismuth as reactive and non-reactive flux medium for the synthesis and crystal growth of intermetallics

3.1 Element crystals

The growth of black phosphorus crystals has been known for a long time; however, the reported experimental conditions were manifold, i.e., high-pressure application, reactions in the presence of mercury and also the bismuth flux. ^{107 , 108 , 109 , 110 , 111 , 112} An experimental hindrance is the non-solubility of red phosphorus in liquid bismuth. Baba et al. developed a completely closed bismuth flux setup which allows conversion of red to white phosphorus and finally the crystallization of black phosphorus. Crystals were obtained in needle-, film- and platelet-shape. The film-shaped ones had edge lengths of up to 500 µm. The availability of high-quality crystals of black phosphorus initiated broad studies on its physical properties and potential applications (transistors, photonics, optoelectronics, catalysis or batteries). ^{113 , 114}

3.2 Ternary bismuthides grown under self-flux conditions

The largest family of compounds discussed in the present micro review concerns ternary phases of the A-T-Bi systems (A = alkaline earth, rare earth or actinoid metal; T = transition metal) which were prepared under self-flux conditions, i.e., bismuth acts as a reactive flux and is incorporated in the product phase. The known phases and their experimental conditions for crystal growth are summarized in Table 1; some selected crystal sizes are given in Table 2. In the following we shortly summarize the most remarkable properties of the A _x T _y Bi_z phases.

Crystals of MgNi₂Bi₄ form as a kind of matted agglomerate. ³⁴ Selected needles form with 1 mm length. Resistivity measurements of MgNi₂Bi₄ show metallic behavior and this is corroborated by its Pauli paramagnetism.

Self-flux conditions were used for the ternary calcium bismuthides CaMnBi₂, ⁴⁴ CaMn₂Bi₂, ⁴⁷ equiatomic CaAuBi ⁶⁶ and Ca₃T₄Bi₈ with T = Pd and Pt. ⁵⁴ Of all flux experiments summarized herein, the growth of the Ca₃T₄Bi₈ crystals had the most complex temperature profile. A sawtooth-shaped profile with four steps was chosen for the cooling process with slow cooling rates of 3 K h⁻¹ and re-annealing by 50 K (at 50 K h⁻¹) after each 150 K cooling step (the complete annealing sequence is given in Table 1).

Titanium forms a broader series of bismuth-rich phases A₅Ti₁₂Bi_19+x with A = Sr, Ba and Eu. ³⁵ The x values range from 0.5 to 1.0. The most interesting phase in this family is the solid solution Sr_5−δEu _δ Ti₁₂Bi_19+x, where strontium is partially substituted by divalent europium. Samples were obtained for δ = 2.4 and 4.0. The divalent ground state of europium (4f ⁷ configuration) was confirmed by magnetic susceptibility measurements.

Mn₈Zn_1.8Bi_7.4 is a bit unique in the family of ternary bismuthides. ³⁶ It forms with two different transition metals and was discovered during explorative phase analytical work in the Mn–Zn–Bi system. Mn₈Zn_1.8Bi_7.4 crystallizes in the form of long (up to 1 mm) intergrown needles. The manganese substructure shows long-range ferromagnetic ordering with a sharp transition at T_C = 185 K.

Single crystals of EuMg₂Bi₂ were studied with respect to their magnetic properties. ^{37 , 40 , 41} The direction dependent susceptibility measurements show an antiferromagnetic transition at T_N = 8.2 K. The 1.9 K magnetization isotherms exhibit a notable anisotropy. Saturation of the europium magnetic moment with H parallel to the c axis deserves a larger external field. The saturation moment in both directions is close to the expected value of 7 µ_B for Eu²⁺.

BaMn₂Bi₂ crystals were grown as large platelets with up to 1 cm edge length. ³⁹ Resistivity measurements show semiconducting behavior with a small band gap of 6 meV. The magnetic substructure of BaMn₂Bi₂ exhibits antiferromagnetic ordering at the comparatively high Néel temperature of T_N ≈ 400 K. The c axis is the easy axis of magnetization. Single crystals were also grown for the potassium substituted solid solution Ba_1−xK _x Mn₂Bi₂. Due to the high volatility of potassium, the maximum reaction temperature was 100 K lower than for the parent compound. The potassium-for-barium substitution induces a semiconductor-metal transition.

The ternary compounds REBi _x Ge_2−x (RE = Y, Pr, Nd, Sm, Gd–Tm, Lu) structurally derive from the ZrSi₂ type structure. ⁴² The Bi/Ge mixed occupancies classify them as solid solutions. The bismuth content in these phases is only low. A typical composition refined from single crystal X-ray diffractometer data is PrBi_0.31Ge_1.63. The germanides REBi _x Ge_2−x with the paramagnetic rare earth elements all order antiferromagnetically at low temperatures with the highest Néel temperature of T_N = 23 K observed for the gadolinium compound.

TbTi₃Bi₄ is a really remarkable bismuthide. ⁴³ The titanium atoms form kagome networks. Precise direction dependent magnetic susceptibility measurements show antiferromagnetic ordering below 24 K. The Néel temperature is well expressed in the measurement parallel to [100]. The magnetization isotherm at 1.8 K fully corroborates the antiferromagnetic ground state. A pronounced metamagnetic transition with a first saturation plateau at around 1/3M_sat is observed for H parallel [100] along with substantial hysteresis. Rotation of the crystal to align the three crystallographic axes with the external magnetic field shows a shift of the critical field.

The bismuthides RET_1−xBi₂ with ZrCuSi₂ related structures have thoroughly been studied. ^{51 , 52} The cerium containing members are the most interesting ones in this series. The highest magnetic ordering temperature was observed for CeCuBi₂ (T_N = 11.3 K). ⁵² The magnetization isotherm at 2 K shows a pronounced anisotropy. The c axis is the easy axis of magnetization. Above a critical field of ∼27 kOe CeCuBi₂ shows multistep metamagnetism and reaches a saturation moment of 1.75 µ_B Ce atom⁻¹ above 43 kOe. CePd_1−xBi₂ and NdPd_1−xBi₂ order antiferromagnetically at the lower Néel temperatures of T_N = 6 and 4 K, respectively, but again with strongly anisotropic behavior. ^{56 , 57} LaPd_1−xBi₂ shows a superconducting transition below 2.1 K. ⁵⁵

Single crystals of the MgAgAs type phases (so-called half-Heusler phases) HoPdBi, ^{58 , 59} ErPdBi, ⁶⁰ ScPtBi, ⁶¹ NdPtBi, ⁶² GdPtBi, ⁶³ TbPtBi ⁶⁴ and LuPtBi ⁶⁵ were obtained by self-flux reactions and thoroughly studied with respect to their magnetic and transport properties. An interesting feature of these phases is the interplay of long-range magnetic ordering and superconductivity, e.g., T_N = 1.9 K and T_C = 0.7 K for HoPdBi. ⁵⁸ The magnetic structure of HoPdBi was determined from neutron diffraction data and revealed a propagation vector 1/2 1/2 1/2. Similar behavior was observed for ErPdBi (T_N = 1.06 K and T_C = 1.22 K). ⁶⁰ Magnetotransport studies of ScPtBi show signature of a chiral magnetic anomaly in agreement with the non-centrosymmetric crystal structure. ⁶¹

The NdPtBi magnetic structure was determined from single-crystal (2 × 1 × 1 mm³) neutron diffraction data. ⁶² The Néel temperature is T_N = 2.18 K and an ordered magnetic moment of 1.78 µ_B Nd atom⁻¹ was determined. The magnetic structure is of the AFM-I type with ferromagnetically ordered planes that are alternating antiferromagnetically along [100]. GdPtBi is a 9 K antiferromagnet which shows a sharp transition in the muon-spin resonance data. ⁶³ Contrarily, the antiferromagnetic transition for TbPtBi (T_N = 3.4 K) is broad, although single crystalline material was used. ⁶⁴

Crystals of the REAuBi₂ bismuthides were grown for the RE = La, Ce, Pr, Nd and Sm members. ⁶⁷ The paramagnetic ones order antiferromagnetically at low temperatures with the highest Néel temperature of T_N = 13.6 K observed for CeAuBi₂. The remarkable feature is the 2 K magnetization behavior of CeAuBi₂ which is highly anisotropic. The easy magnetization is along the c axis. In c direction CeAuBi₂ shows a pronounced metamagnetic transition which sets in at a critical field of ∼60 kOe. CeAuBi₂ exhibits a narrow hysteresis and saturates at the comparatively high value of 1.85 µ_B Ce atom⁻¹, comparable to CeCuBi₂ discussed above. Also, PrAuBi₂ and NdAuBi₂ show metamagnetic transitions, but less pronounced. UAuBi₂ orders ferromagnetically at T_C = 22.5 K. ⁶⁸ Again, the c axis is the easy axis of magnetization and UAuBi₂ shows a large magnetocrystalline anisotropy. Such details can only be deduced from precise single crystal data.

3.3 Ternary rare earth phosphides

Besides the use of iodine as mineralizer and salt fluxes, tin is the standard flux agent for the crystal growth of phosphides, especially for the large family of ternary rare earth transition metal phosphides. ^{3 , 11 , 115 , 116} A detailed summary of experimental details (starting compositions, annealing sequences etc.) is given in reference 3. Besides tin also lead is frequently used as flux. A change of the flux agent can pave the way to unusual compounds. A recent example is PbP₇. ^{117 , 118}

So far, only few examples of bismuth flux grown phosphides are known. ^{72 , 73 , 74 , 75 , 76 , 77 , 78 , 79} The examples (Table 1) all comprise ternary phosphides of the high melting inert transition metals rhodium and iridium. These are easily activated within the flux and well-shaped single crystals of many phosphides can be grown. ScRh₆P₄ forms larger blocks while isotypic YbRh₆P₄ forms large rods with up to 750 µm length. ^{72 , 73}

The probably most remarkable flux grown phosphide is La₄Rh₈P₉ which exhibits the unusual diphosphenide unit P₂²⁻ with 208 pm P–P distance besides P³⁻ phosphide anions. ⁷⁵ La₄Rh₈P₉ is Pauli paramagnetic. The differently charged phosphorus species were well resolved in the ³¹P solid state NMR spectrum. The P₂²⁻ units show side-on coordination to the rhodium atoms within the complex [Rh₈P₉]^δ− polyanionic network.

Three parameters need to be considered for the flux growth experiments of the phosphides: (i) due to the high reactivity and vapor pressure of phosphorus, the sample annealing proceeds in two steps (Table 1) in order to avoid a two vigorous reaction and bursting of the tubes, (ii) the samples with the less reactive iridium often have slightly higher maximal annealing temperatures and (iii) care should be taken, since the product sample can contain trace amounts of white phosphorus (complete workup in a hood is recommended).

3.4 Ternary rare earth and related arsenides

One of the early examples for the growth of ternary arsenide single crystals from liquid bismuth is DyNi₄As₂. ¹¹⁹ The starting sample composition was Dy:Ni:As:Bi = 2:6:2:10. The reaction was performed in a silica tube (RT→1,143 K at 10 K h⁻¹; 120 h at 1,143 K followed by cooling to 873 K at 2 K h⁻¹). The DyNi₄As₂ crystals were simply isolated mechanically from the bismuth flux. The detailed reaction conditions of many other arsenide flux growth experiments are summarized in Table 1.

The recent flux growth experiments on arsenides all focused on high crystal quality for physical property studies. The ternary system Mg–Si–As was unexplored. Bismuth flux growth experiments yielded the new arsenide Mg₃Si₆As₈ which crystallizes with a new non-centrosymmetric structure type. ⁸⁰ Mg₃Si₆As₈ is a semiconductor with a direct band gap around 2 eV. Unfortunately, Mg₃Si₆As₈ shows no nonlinear optical activity.

The arsenides RECo₂As₂ (RE = La, Ce, Pr, Nd, Eu) ^{81 , 82 , 83 , 84} were thoroughly studied with respect to their magnetic properties. Larger single crystals allowed for direction dependent measurements. The bismuth flux growth experiments revealed some remarkable features. The flux is not completely inert. Precise single-crystal X-ray diffraction experiments revealed the refined compositions La_0.97Bi_0.03Co_1.91As₂ and Ce_0.95Bi_0.05Co_1.83As₂ for the lanthanum and cerium containing crystals. ⁸² It seems that the rare earth sites in the ThCr₂Si₂ type structure for the larger rare earth elements can partially be substituted by bismuth. Thus, any flux grown crystal should be studied at least by EDX, in order to check for a potential inclusion of the flux. Another important feature of the RECo_2–xAs₂ arsenides concerns the formation of vacancies on the cobalt sites. ^{82 , 84}

LaCo₂As₂ is a good example for the importance of the correctly chosen flux medium for crystal growth. LaCo₂As₂ can easily be prepared in polycrystalline form from the elements by a diffusion-controlled synthesis at high temperature (1,250 K). Standard flux growth experiments with tin were not successful. ¹²⁰ The bismuth flux growth works, however, small quantities of bismuth were detected on the lanthanum site. ⁸³ The cobalt substructure of the bismuth flux grown samples shows ferromagnetic ordering at T_C = 200 K.

The whole series of RE₂Co₁₂As₇ arsenides is accessible by bismuth flux growth experiments. ⁸⁵ The striking representatives are Ce₂Co₁₂As₇, Eu₂Co₁₂As₇ and Yb₂Co₁₂As₇ which exhibit mixed valence behavior. The cobalt substructures of the RE₂Co₁₂As₇ arsenides show ferromagnetic ordering at comparatively high Curie temperatures in the range of 100–140 K, while the rare earth magnetic ordering proceeds in the low-temperature regime.

The standard flux growth synthesis of CeRh₂As₂ showed small compositional differences in the product crystals that influence the physical properties. Most recently a new horizontal configuration for a bismuth-based flux growth of CeRh₂As₂ crystals was proposed by Chajewski et al. ⁸⁶ This really remarkable experiment runs with a three-part ampoule in a two-zone horizontal furnace with three different temperature regimes: (i) high-temperature regime (1,423–1,293 K), (ii) medium-temperature regime (1,273–1,163 K) and (iii) low-temperature regime (1,133–1,043 K). The growth process lasts 15 days and produces the so far best quality CeRh₂As₂ crystals. For further examples of flux growth in horizontal configuration we refer to a review by Yan et al. ¹²¹

3.5 Rare earth transition metal tetrelides

The classical bismuth flux synthesis starting from the elements was successful for the germanides CeRh₆Ge₄, CeRh₂Ge₂ ⁸⁸ and Ce₂Rh₃Ge₅ ⁸⁹ (for the annealing sequence we refer to Table 1). Selected single crystals of Ce₂Rh₃Ge₅ ⁸⁹ and CeRh₆Ge₄ ⁸⁸ mounted on glass fibers are shown in Figure 1.

Figure 1:

Scanning electron micrographs of bismuth flux grown single crystals of Ce₂Rh₃Ge₅ ⁸⁹ and CeRh₆Ge₄ ⁸⁸ mounted on glass fibers.

All other tetrelides were obtained from arc-melted precursor compounds that were recrystallized from liquid bismuth. The flux growth experiments were performed in evacuated silica ampoules. Although bismuth acts as an inert flux during these experiments, severe attack of the silica ampoules was observed for some of the syntheses. Reacting powder of a precursor sample with the initial composition ‘HoRh₆Sn₄’ in liquid bismuth resulted in well-shaped crystals of Ho₃Rh₉Si₂Sn₃, a quaternary compound that adopts a new structure type. ¹²² Later on, the targeted syntheses from the elements via arc-melting led to the complete series of silicide stannides RE₃Rh₉Si₂Sn₃ with RE = Y, Sm and Gd–Lu in quantitative yield.

Similar phase analytical studies with a sample of the initial composition ‘SmRh₆Sn₄’ in liquid bismuth yielded the quaternary compound SmRh₄SiSn. ¹²³ Again, later targeted syntheses produced the series RERh₄SiSn with RE = Y, Nd, Sm and Gd–Lu. Attack of the ampoule under bismuth flux conditions led to the silicon incorporation. These quaternary phases can be considered as ordered version of the YRh₄Ge₂ type (space group Pnma). In extension to the work on the quaternary phases, also ScIr₄Si₂ and the series of RERh₄Si₂ silicides with RE = Sc, Y and Tb–Lu was synthesized. Summing up, experiments that went wrong due to ampoule attack led to the discovery of three larger series of new intermetallic compounds.

The initial crystal growth study on CeRh₆Ge₄ ⁸⁸ started from the elements. Further phase analytical work resulted in the isotypic compounds RERh₆Si₄ (RE = La, Nd, Tb, Dy, Er, Yb) ¹²⁴ and RERh₆Ge₄ (RE = Y, La, Pr, Nd, Sm–Lu). ¹²⁵ All these tetrelides were obtained as polycrystalline samples via arc-melting of the elements. The crystal growth procedure for the silicides (with RE = La, Nd and Yb ¹²⁴ ) was different from CeRh₆Ge₄. Here, arc-melted RERh₆Si₄ pellets were ground, mixed with 2 g of bismuth shot and sealed in evacuated silica ampoules. The latter were heated to 1,220 K within 2 h and the temperature was kept for 12 days, followed by cooling to room temperature at a rate of 3 K h⁻¹. The germanide crystals (with RE = La, Pr, Sm, Gd and Ho ¹²⁵ ) were grown similar to the cerium compound. The most interesting tetrelide of this family is CeRh₆Ge₄ ⁸⁸ which orders ferromagnetically below 2.5 K. This initiated further work, ^{126 , 127} classifying CeRh₆Ge₄ as a quantum critical heavy-fermion ferromagnet.

Ongoing crystal growth experiments also helped to crystallize quite complex silicides. The new phases RE₁₂Rh₆₀Si₃₉ (RE = Nd, Tb, Yb) ¹²⁸ are isotypic with the silicide Ce₆Rh₃₀Si_19.3 (space group P6₃/m, Pearson code hP112). ¹²⁹ The crystal growth process started with arc-melted ingots of the starting compositions ‘RERh₄Si₂’ (around 300 mg) which were powdered and mixed with 2 g of bismuth shot. The educts were sealed in carbonized silica tubes and subjected to the following annealing sequence: (i) RT→1,220 K within 2 h, (ii) 12 h at 1,220 K and (iii) cooling to room temperature at a rate of 3 K h⁻¹. The flux was finally dissolved in a 1:1 mixture of glacial acetic acid and H₂O₂ (35 %).

Nd₁₆Rh₇₃Ge₄₉ crystals (new structure type) ¹³⁰ were obtained from an arc-melted ‘NdRh₆Ge₄’ precursor sample that was powdered and mixed with a six-fold molar amount of bismuth. The experimental conditions were: (i) carbonized silica tube, (ii) RT→1,320 K within 2 h, (iii) 20 h at 1,320 K, (iv) cooling to 720 K at a rate of 3 K h⁻¹ and (v) 1:1 HOAc/H₂O₂. The complex germanide Ce₉Ir₃₇Ge₂₅ ¹³¹ was grown under comparable conditions: (i) ‘CeIr₄Ge₃’ precursor sample (∼500 mg + 3 g Bi), (ii) SiO₂ tube, (iii) RT→1,300 K within 2 h, (iv) 10 d at 1,300 K, (v) quenching and (vi) 1:1 HOAc/H₂O₂. These are first examples for a presumably large family of complex tetrelides.

3.6 Binaries under inert flux conditions

Single crystals of semiconducting GeP can be grown from liquid bismuth (Table 1). ⁶⁹ After the crystal growth procedure a final annealing step for 24 h at 573 K is important in order to convert the remaining white phosphorus to red phosphorus (!). While residual red phosphorus sublimes from the crystal surfaces, remaining bismuth traces can be dissolved by a treatment with a dilute HCl/H₂O₂ mixture. GeP crystals form on the larger µm scale allowing high-quality data collections for structure determination.

Liquid bismuth also served as inert flux for the growth of GaSe crystals (Table 1). ⁷⁰ The sizes of the obtained crystals are strongly depending on the temperature profile. While most studies use a linear decrease of temperature in the final crystal growth step, for GaSe, an oscillating temperature profile (similar to the temperature profile discussed above for the Ca₃T₄Bi₈ phases ⁵⁴ ) helped to strongly increase the crystal size. The decisive parameter influencing the number and size of crystals is the number of crystal nuclei formed during the cooling process. It seems that the oscillating temperature profile can minimize the nucleus number. Thus, many crystal growth procedures might be optimized using modified temperature profiles in the cooling regime.

Single crystals of antiferromagnetic Mn₂Au were grown from a starting composition Mn: Au: Bi = 7:1:12 (Table 1). ⁷¹ Larger block-shaped crystals with edge lengths up to 3 mm were obtained. Mn₂Au shows hydrolysis resistance. Bismuth flux residues on the crystal surface were acid etched with a 2:1 mixture of HNO₃/HCl for less than 2 min.

3.7 Suboxides

Besides pure bismuth intermetallics, also some suboxides (still with metal-metal bonding) were grown from reactions in a bismuth flux. The ɛ-TiO polymorph, ¹³² which is isotypic with ɛ-TaN is one of the striking examples. For single crystal growth Bi, Bi₂O₃ and Ti were mixed with a Ti:O molar ratio of 1.0:0.9 and elemental bismuth was used in excess as fluxing agent. The elements were placed in a h-BN crucible which was subsequently sealed in a stainless-steel tube. The sample was heated to 1,173 K at a rate of 7.5 K min⁻¹ and kept at that temperature for 2 h followed by cooling to 773 K at a rate of 10 K h⁻¹. After furnace cooling, the excess bismuth flux was removed by treatment with 4 M HNO₃. The needle-shaped ɛ-TiO crystals had edge lengths up to 900 µm.

Ti₈Bi₉O_0.25 ¹³³ was obtained from Ti (0.85 mmol), TiO₂ (0.125 mmol) and Bi (1.5 mmol) using a tantalum crucible that was inserted into a stainless-steel ampoule (sealed with a stainless-steel cap). After rapid heating (400 K h⁻¹) to 1,073 K, the temperature was kept for 10 h and subsequently lowered to 723 K at a rate of 10 K h⁻¹, followed by quenching. Ti₈Bi₉O_0.25 single crystals could be isolated from the crushed sample. The oxygen is located in a tetrahedral Ti₄ site.

Comparable reaction conditions were applied for the syntheses of Ti₃Zn₃O _x with x = 1.07 and 1.23. ¹³⁴ The two different starting compositions Ti: Zn: ZnO: Bi of 0.8:0.4:0.8:2.0 and 0.45:0.3:0.75:1.95 and a slightly different temperature profile for the crystal growth procedure led to the crystals with different oxygen content. The obtained crystals have edge lengths around 60 µm. Further flux growth experiments led to incorporation of small amounts of bismuth into the product crystals, Ti₈BiO₇, ¹³⁵ and the solid solutions Ti_12−δGa _x Bi_3–xO₁₀, ¹³⁶ Ti₈(Sn _x Bi_1–x)O₇ and Ti_11.17(Sn_0.85Bi_0.15)₃O₁₀. ¹³⁷

3.8 Binaries as flux medium

The use of binary Ni₂Bi is at first sight an unusual flux medium, however, it is bismuth based. The Ni₂Bi has repeatedly been used for crystal growth experiments of the rare earth-based quaternary boride carbides RENi₂B₂C. ^{138 , 139 , 140 , 141 , 142 , 143} This flux has several advantages: (i) its melting temperature of ∼1,373 K is well below the decomposition temperature of the RENi₂B₂C phases and (ii) Ni₂Bi is a self-flux and does not introduce further elements into the synthesis. In a typical experiment for crystal growth of YNi₂B₂C, ¹³⁸ a polycrystalline sample is prepared by arc-melting. The product button is then placed into an alumina crucible and covered with an equal mass of Ni₂Bi. The flux growth proceeds under flowing argon: (i) fast heating to 1,763 K and subsequent slow cooling to 1473 K at a rate of 10 K h⁻¹. Platelets of YNi₂B₂C can finally be broken out of the solidified melt. The family of RENi₂B₂C boride carbides has intensively been studied with respect to the interplay of superconductivity and long-range magnetic ordering.

3.9 Miscellaneous

The bismuthides Ca₁₁Ga _x Bi_10−x (x ≈ 0.8), Ca₁₁Ga _x Bi_10−x (x ≈ 1.1), Yb₁₁Ga _x Bi_10−x (x ≈ 0.7) and Yb₁₁In _x Bi_10−x (x ≈ 1.8) ¹⁴⁴ derive from the tetragonal Ho₁₁Ge₁₀ type structure ¹⁴⁵ and can be considered as solid solutions with partially mixed occupied sites. The crystal growth experiments start with Ca(Yb):Bi:In(Ga) atomic ratios of 1:1:4 using alumina as crucible material. The heating sequence was: (i) RT→1,073 K at 100 K h⁻¹, (ii) 24 h at 1,073 K and (iii) 1,073 K→773 K at 5 K h⁻¹. Although gallium (303 K) and indium (429 K) both have lower melting points than bismuth (544 K), ¹² during the first heating step, all three elements melt and one can assume a mixed flux situation.

The metastable fibrous phase Bi_21.1Mn_20–xCo _x was crystallized under bismuth self-flux conditions. ¹⁴⁶ Crystal growth attempts with starting compositions Mn:Co:Bi 10–x:x:90 with x values of 0, 0.5, 1.0 and 2.0 were performed using alumina as crucible material. The mixtures were rapidly heated to 1,423 K, kept at 1,423 K for 5 h and then cooled to 663 K at a rate of 152 K h⁻¹. After dwelling for 1 h at 663 K, the samples were cooled to 553 K at a rate of 1 K h⁻¹ followed by centrifugation. The product was an agglomerate of matted needles with up to 2.5 mm length. The striking crystal chemical feature of Bi_21.1Mn_20–xCo _x is the pseudo-pentagonal symmetry. This is structurally closely related to Ni₂₅Bi₂₈ ¹⁴⁷ and Ni₈Bi₈S. ¹⁴⁸

The bismuth self-flux technique is not limited to intermetallics. An interesting example concerns the topological insulator Bi₂TeI. ¹⁴⁹ Although broad property studies were performed for this telluride iodide, high-quality single crystals were not available. Ryu et al. systematically investigated the crystal growth conditions for Bi₂TeI and obtained good results with the bismuth self-flux technique. The starting composition for flux growth was Bi_2+xTeI with x ≥ 0.5. The elements were sealed in a quartz tube under 2 mbar argon pressure. The ampoule was heated to 1,123 K, kept at that temperature for 72 h followed by cooling to 823 K at rates between 0.5 and 2 K h⁻¹. Single crystals up to 5 mm edge lengths could be obtained. The grown crystals were exfoliated with 3 M Scotch^® tape to remove surface contaminations prior to the property studies.

A final example concerns indium arsenide quantum dots. ¹⁵⁰ This is not a classical crystal growth experiment. For those systems bismuth can serve as surfactant for modifying the morphology and optical properties. Such surface layers are grown using bismuth fluxes.