Single
crystals of a new zinc oxysulfide SrZn2S2O were
grown in a eutectic KF–KCl flux, and the structure
was determined by single-crystal X-ray diffraction. SrZn2S2O crystallizes in the noncentrosymmetric polar space
group Pmn21 with lattice parameters of a = 3.87440(10) Å, b = 9.9847(3) Å,
and c = 6.0916(2) Å. In the crystal structure,
close-packed corrugated double layers of ZnS3O tetrahedra,
which are derived from the wurtzite structure, are vertically separated
by Sr2+ ions. In addition, the O/S anion ordered arrangement
in each close-packed layer yields two distinct orientations of the
Zn-centered tetrahedra. The crystals of SrZn2S2O are colorless and transparent, and the oxysulfide has a band gap
of 3.86 eV, based on UV–vis–NIR diffuse reflectance
measurements. Thermogravimetric measurements showed that SrZn2S2O is stable up to 650 °C in O2 gas atmosphere. First-principle calculations indicate that the valence
band maximum is mainly composed of O-2p and S-3p states, whereas the
conduction band minimum is derived from Zn-4s, Zn-4p, and Sr-4d states.
The calculated band dispersion reveals a direct band gap corresponding
to a transition between S-3p and Zn-4s energy levels. Second harmonic
generation (SHG) measurements determined that SrZn2S2O is phase matchable with twice the SHG intensity of potassium
dihydrogen phosphate (KDP) in contrast to CaZnSO with similar ZnS3O building units, which exhibits non-phase matching behavior.
Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: CsK[(UO)O(PO)], CsK[(UO)O(PO)], RbK[(UO)O(PO)], K[(UO)O(PO)], KNaRb[(UO)O(PO)], KNaRb[(UO)O(PO)], CsK[(UO)O(PO)], and RbK[(UO)O(PO)]. All structures crystallize in the monoclinic space group, P2/ c and contain uranyl phosphate layers with alkali metals located between the layers for charge balance. Ion exchange experiments on CsK[(UO)O(PO)], RbK[(UO)O(PO)], and K[(UO)O(PO)] demonstrated that Cs and Rb cations cannot be exchanged for K cations; however, K cations can be readily exchanged for Na, Rb, and Cs. Enthalpies of formation were calculated from density functional theory (DFT) and volume-based thermodynamics (VBT) for all six structures. A value for the enthalpy of formation of the phosphuranylite sheet, [(UO)O(PO)], was derived using single-ion additive methods coupled with VBT. DFT and VBT calculations were used to justify results of the ion exchange experiments. CsK[(UO)O(PO)], RbK[(UO)O(PO)], and K[(UO)O(PO)] exhibit typical luminescence of the uranyl group.
In this review we discuss general trends in the use of alkali bromide and iodide (ABI) fluxes for exploratory crystal growth. The ABI fluxes are ionic solution fluxes at moderate to high temperatures, 207 to ∼1,300 • C, which offer a good degree of flexibility in the selection of the temperature profile and solubility. Although their main use is to dissolve and recrystallize "soft" species such as chalcogenides, many compositions with "hard" anions, including oxides and nitrides, have been obtained from the ABI fluxes, highlighting their unique versatility. ABI fluxes can serve to provide a reaction and crystallization medium for different types of starting materials, mostly the elemental and binary compounds. As the use of alkali halide fluxes creates an excess of the alkali cations, these fluxes are often reactive, incorporating one of its components to the final compositions, although some examples of non-reactive ABI fluxes are known.
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