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A variety of processes of crystal growth proceeds via the gas phase. If the initial solid material is volatilized in a heterogeneous reaction under presence of a gaseous reactant, the transport agent, the term Chemical Vapor Transport Reaction (CVT) is applied. Crystallization processes by CVT are known for both elements, intermetallics, binary and complex oxides, halides, chalcogenides, and pnictides. Even if the formation of volatile halides is a common feature of almost all vapor transport reactions, significant differences are there concerning the choice of a suitable transport agent depending on the nature of the initial solid phase. Actually, the appropriateness of transport agents for the respective transport reaction can be described in a thermodynamic way. Besides some basic principles for systematic evaluation more practical recommendations for suitable experimental conditions are given for CVT of different classes of inorganic materials. [ABSTRACT FROM AUTHOR]

The standard entropy of compounds In2 M3O12 ( M = Mo, W) were determined to be S°298 = 390 ± 6 ( M = Mo] and S°298 = 404 ± 6 J ·mol-1 ·K-1 ( M = W). The entropies of formation from the binary oxides are Δ S°R, 298 = 52 J ·mol-1 ·K-1 ( M = Mo] and Δ S°R, 298 = 72 J ·mol-1 ·K-1 ( M = W). Both compounds possess positive enthalpies of formation from the oxides at 298 K and are entropically stabilized. Transition enthalpies of the monoclinic to orthorhombic phase transition were measured for In2M3O12 by differential scanning calorimetry. Crystals of In2M3O12 (size up to a few mm) were deposited by means of chemical vapor transport along a temperature gradient Δ T = 100 K using transport agent chlorine. Deposition rates are between 0.2 and about 15 mg ·h-1, depending on mean transport temperature and amount of chlorine. Experimental results were compared to predictions by means of thermodynamic modelling. A detailed analysis of the chemical vapor transport behavior was carried out regarding gaseous compounds involved in the transport process, dominant transport reactions and influence of moisture content. [ABSTRACT FROM AUTHOR]

The solid solutions (V1- xW x)OPO4 with β-VOPO4 structure type (0.0 ≤ x ≤ 0.01) and αII-VOPO4 structure type (0.04 ≤ x ≤ 0.26) were obtained from mixtures of VVOPO4 and WVOPO4 by conventional solid state reactions and by solution combustion synthesis. Single crystals of up to 3 mm edge length were obtained by chemical vapor transport (CVT) (800 → 700 °C, Cl2 as a transporting agent). Single crystal structure refinements of crystals at x = 0.10 [ a = 6.0503(2) Å, c = 4.3618(4) Å, R1 = 0.021, wR2 = 0.058, 21 parameters, 344 independent reflections] and x = 0.26 [ a = 6.0979(2) Å, c = 4.2995(1) Å, R1 = 0.030, wR2 = 0.081, 21 parameters, 346 independent reflections] confirm the αII-VOPO4 structure type ( P4/ n, Z = 2) with mixed occupancy V/W for the metal site. Due to the specific redox behavior of W5+ and V5+, solid solutions (V1- xW x)OPO4 should be formulated as (VIV xVV1-2 xWVI x)OPO4. The valence states of vanadium and tungsten are confirmed by XPS measurements. V4+ with d1 configuration was identified by EPR spectroscopy and magnetic measurements. Electronic spectra of the solid solutions show the IVCT(V4+ → V5+) and the LMCT(O2- → V5+). (V0.74W0.26)OPO4 powders exhibit semi-conducting behavior ( Eg = 0.7 eV). [ABSTRACT FROM AUTHOR]

Single crystals of tetragonal β-Fe1.00Se were grown from polycrystalline material by chemical vapor transport reaction at temperatures below 723 K using AlCl3 as transport additive. The plate-shaped single crystals have edge lengths up to 4 mm. The single crystals display complete diamagnetic shielding 4π χ ≈ -1 below the superconducting transition. [ABSTRACT FROM AUTHOR]

Two solid solution series exist in the system MgMoO4-NiMoO4. The α-Ni1- yMg yMoO4 solution series, isostructural to α-NiMoO4, is thermodynamically stable at ambient conditions for compositions between 0 % and about 75 % magnesium content. The solution series β-Mg1- xNi xMoO4, isostructural to MgMoO4 and the high temperature β modification of NiMoO4, is thermodynamically stable at ambient conditions for compositions with < 25 % nickel content. A complete solid solution series β-Mg1- xNi xMoO4 exists at higher temperatures (> 823 K). The transition temperature for the α → β transition decreases with increasing magnesium content. The coexistence of both polymorphs at room temperature in samples with a wide range of composition is a result of the kinetic inhibition of the phase transition β → α. The chemical vapor transport of β-Mg1- xNi xMoO4 solid solutions with chlorine was investigated. Crystals with a nickel content up to 25 % were synthesized in temperature gradients 1273 K → 1223 K or 1273 K → 1173 K. Deposited nickel richer crystals are destroyed during cooling down to room temperature due to the phase transition. The observed distinctive nickel enrichment during the transport process is in good agreement with predictions by thermodynamic modeling. [ABSTRACT FROM AUTHOR]