I. M. Shmyt’ko

Fine sructure of Na5Lu9F32 nanocrystallites formed at the initial stages of crystallization

X-ray diffraction investigations have demonstrated that the initial stages of the formation and growth of Na5Lu9F32 nanocrystallites from a powder mixture of lutetium fluoride, ammonium fluoride, and sodium fluoride during the solid-phase synthesis are accompanied by complex structural transformations, among which a cyclic transformation of the type single-phase → two-phase → single-phase state can be distinguished. It is shown that the two-phase state consists of two isomorphous phases with a small misfit of the lattice parameters. The isomorphous phases have common extended interphase boundaries, along which a continuous change of the lattice parameters occurs as one phase transforms into the other. A long-term annealing of the two-phase state leads to the final single-phase state, which is characterized by the growth of nanocrystallites to the scale of microcrystallites.

Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu2(MoO4)3 during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ∼550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.

Anomalous structural states of rare-earth oxides in solid-phase synthesis during continuous heating

The kinetics of solid-phase synthesis of complex rare-earth (RE) oxides from an amorphous precursor state during continuous heating is studied by X-ray diffraction. Using the synthesis of RE borates (ReBO3) as an example, it is shown that a continuous heating of such precursors stimulates the formation of new phases which are unknown in an equilibrium state of one rare-earth element but known for other rare-earth borates. It is established that the regime of continuous heating during solid-phase synthesis of garnets (Re3M5O12) exhibiting the only crystal phase state for all rare-earth atoms results in an accelerated growth of crystallites and that the higher the heating rate the higher the growth rate.

“Isomorphous” phases in nanodispersed powders of rare-earth oxides

Using x-ray diffraction, a structural state consisting of two “isomorphous” phases was revealed in nanocrystalline powders of simple oxides Re2O3 (Re = Eu, Gd, La, Lu) and Y3Ga5O12 prepared by solvent thermolysis from simple-oxide solutions followed by annealing of obtained precursors at elevated temperatures. A model is proposed explaining such two-phase states in terms of the excess energy of nanograin surface layers which causes the formation of a surface phase isomorphous to the core phase having a smaller lattice parameter. Analogous two-phase states were also obtained in microcrystalline LuBO3 and Eu2(MoO4)3 powders subjected to long-term grinding.

Features of the doping effect on YAlO3 phase sequences during synthesis from an amorphous precursor state

Using X-ray diffraction methods, it has been shown that small lanthanum (La) dopants during solid-phase synthesis of YAlO3 from an amorphous precursor state initiate the formation of the high-temperature hexagonal modification of yttrium aluminate in the first crystallization stages, whereas the equilibrium perovskite phase has an orthorhombic cell. At the same time, during YAlO3 synthesis without dopants, yttrium-aluminum garnet Y3Al5O12 is formed in the first crystallization stages, rather than the perovskite modification, as it follows from the precursor stoichiometry. The effect of lanthanum dopants on YAlO3 phase sequences during synthesis from the amorphous precursor state is qualitatively explained.

Structure of precursors of complex oxides of rare-earth elements prepared by solvent thermolysis

X-ray diffraction and electron microscopy were used to study the structural states of precursors of borates LuBO3, YBO3, and EuBO3, garnets Lu3Al5O12 and Y3Al5O12, and perovskites LuAlO3 and LaAlO3 prepared by solvent thermolysis from a nitric-acid solution or an ammonium nitrate melt solution of simple oxides and nitrates of rare-earth elements. The precursors are mainly found to be amorphous. In some cases, small amounts of non-reacted initial oxides and nitrates are observed to be in an amorphous matrix.

Spectral and structural features of Lu1 − xRExBO3 compounds

The luminescence spectra, luminescence excitation spectra, IR absorption spectra, and crystal structure of orthoborates Lu1 − xRExBO3 (RE = Eu, Gd, Tb, Y, Dy) have been investigated. It has been found that the solid solution consisting of a LuBO3 orthoborate, which has two stable structural modifications (calcite and vaterite), and an REBO3 orthoborate, which has one structural modification (vaterite), crystallizes only in the vaterite structure when the concentration of a rare-earth ion substituting for lutetium exceeds 15–20 at %. The investigation of the photoluminescence spectra has demonstrated that, for rare-earth ions Lu3+, Eu3+, Y3+, and Gd3+ in the vaterite modification of Lu1 − xRExBO3 orthoborates, there are at least two positions that are not equivalent in the symmetry of the local environment. It has been established that the maximum intensity of the luminescence of the vaterite modification of Lu1 − xTbxBO3 compounds synthesized at 970°C, which is observed at a terbium concentration of 15 at %, is several times higher than the maximum intensity of the luminescence of the calcite modification.

X-ray diffraction by polydomain crystals modulated by transverse waves of atomic displacements. 1. Single-wave modulation in crystals

The intensity distributions of diffuse X-ray scattering caused by domains characterized by the transverse wave of atomic displacements polarized in the (010) plane and propagating along the [001] direction are calculated by numerical methods analyzed in the kinematical approximation. The regular changes in the intensity and half-width of the satellite peaks are revealed as functions of the wave amplitude and the average domain size. The characteristic features of diffraction are considered in the case where the defect-density wave in the crystal affects the parameters of the displacement wave.

Features of LaBO3 phase formation during solid-phase synthesis from the amorphous precursor state

The features of the phase formation of pure lanthanum borate during solid-phase synthesis from an amorphous precursor state and from a homogenized mixture of initial oxide micropowders have been investigated using X-ray diffraction methods. It has been shown that LaBO3 phase sequences obtained in both cases significantly differ from each other. The role of cross doping with Y, Sc, and La on the La0.95Sc0.05BO3, La0.95Y0.05BO3, Y0.95La0.05BO3, and Sc0.95La0.05BO3 phase sequence during synthesis from the amorphous precursor state has been clarified.

Structural hierarchy and transformations in crystals of the perovskite family

The specific features of structural transformations in lanthanum manganite and lithium niobate are analyzed in terms of the macrosymmetry conservation principle discussed in the papers by L.A. Shuvalov. The possible mechanisms of phase transformations are proposed reasoning from a sequence of structural transitions, including the restoration of a virtual configuration of cubic perovskite with double the lattice parameter. The basic scheme of mutual transformations of two orthorhombic modifications is described for lanthanum manganite. A similar scheme proposed for lithium niobate is used to elucidate the nature of the mechanical twinning observed upon mutual switching of orientational domains.