Yu. Prots

Structure, luminescence and scintillation properties of the MgWO4–MgMoO4 system

The importance of luminescent tungstates and molybdates in several technological applications motivated the study of the structural, luminescence and scintillation properties of the MgWO4–MgMoO4 system. X-ray diffraction studies allowed the identification of three main types of structures in the pseudo-binary MgWO4–MgMoO4 system (sanmartinite β-MgMoO4, cuprosheelite α-MgMoO4, and wolframite MgWO4) and the refinement of the parameters of the crystal lattice. It is found that the single-phase solid solution MgMo1−xWxO4 with a β-MgMoO4 structure is created only at x<0.10, while for a higher tungsten content a mixture of different phases is formed. The x-ray luminescence spectra of a series of samples of the MgWO4–MgMoO4 system are measured at T = 8 K. The principal emission bands are assigned to the main structural phases as follows: β-MgMoO4, 520 nm; α-MgMoO4, 590 nm; MgWO4 (wolframite), 480 nm. The phase composition of the sample determines the actual shape of the observed spectra. Possible relations between the crystal structure and luminescence properties of different phases are discussed in terms of a configuration coordinate model. Of all the compounds under test, MgWO4 is found to have the best scintillation response for particle excitation (0.90 ± 0.15 that of ZnWO4 at T = 295 K). Further, the light yield also remains high with decreasing temperature, which makes this material potentially useful for cryogenic applications.

A case study of complex metallic alloy phases: structure and disorder phenomena of Mg-Pd compounds

In order to achieve a deeper insight into the relation between the homogeneity range, crystal structure and disorder phenomena of complex metallic alloy phases (CMAs) we have redetermined the Mg–Pd phase diagram in the range 60–100 at.% Mg. The existence of the intermediate phases Mg6Pd (β), Mg57Pd13 (γ), Mg56.4Pd13.5 (δ), Mg306Pd77 (ε), Mg78.5Pd21.5 (ζ), Mg3Pd (η), Mg5Pd2 (θ) and Mg2Pd (ι) has been reconfirmed. The first five of these are CMAs. The β phase melts congruently whereas the CMAs γ, δ, ε and ζ form in a cascade of peritectoid and peritectic reactions in a narrow window of approximately 30°C and 3 at.%. It is assumed that the stability of the Mackay icosahedron plays an important role in the phase formation of these CMAs. However, the β phase reveals an intricate pattern of disorder at the atomic positions of the Mackay icosahedron. Therefore, the concept of the stability of the Mackay cluster should be used only as a rule of thumb.

First-order structural transition in the magnetically ordered phase of Fe1.13Te

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and high-resolution synchrotron x-ray powder diffraction investigations of single crystals Fe(1+y) Te (0.06 = 0.13. Most strikingly, all measurements on identical samples Fe(1.13)Te consistently indicate that, upon cooling, the magnetic transition at T(N) precedes the first-order structural transition at a lower temperature T(s). The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic c axis display a small distortion close to T(N) due to a lattice striction as a consequence of magnetic ordering, and a much larger change at T(s). The lattice symmetry changes, however, only below T(s) as indicated by powder x-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.

The phase equilibria in the quasi-ternary Ag2SeGa2Se3GeSe2 system

Abstract The seven polythermal sections of the quasi-ternary HgSeGa 2 Se 3 GeSe 2 system were investigated using differential thermal, X-ray phase and microstructural analyses and microhardness measurement. New quaternary compounds were not found. Extended solid solutions on the basis of Ga 2 Se 3 and HgSe have been established. The projection of the liquidus surface and the isothermal section of the HgSeGa 2 Se 3 GeSe 2 phase diagram at 770 K over the whole concentration region have been constructed.

The neodymium-(vanadium, chromium, manganese)-silicon systems

Abstract The isothermal sections of the Nd{V,Cr,Mn}Si phase diagrams at 870 K over the whole concentration region have been constructed using X-ray phase analysis. Ternary compounds are not formed in the NdVSi system. We have confirmed the existence of the previously reported ternary compounds NdMn 2 Si 2 with the CeGa 2 Al 2 -type structure, and NdMnSi with the PbFCl-type structure. A new ternary compound, i.e. NdCr 2 Si 2 , with the CeGa 2 Al 2 -type structure has been prepared for the first time and its crystal structure has been determined using X-ray powder diffraction data. The limits of solid solution of the binary compound NdSi 1.8 have been established.

The X-ray investigation of the ternary NdAgSi system

Abstract The isothermal section of the NdAgSi phase diagram at 600°C over the whole concentration region has been constructed using X-ray phase analysis. Four ternary compounds are formed: NdAg 2 Si 2 (CeGa 2 Al 2 -type), NdAgSi (AlB 2 -type), NdAg 0.2–0.75 Si 1.8–1.25 (α-ThSi 2 -type) and Nd 5 Ag 0.5 Si 3.5 (Sm 5 Ge 4 -type). The crystal structure of the Nd 5 Ag 0.5 Si 3.5 compound has been determined by single-crystal analysis; other structures have been established using X-ray powder diffraction data.

New ternary holmium-ruthenium-germanides: HoRuGe and Ho3Ru2Ge3

Abstract The crystal structures of the compounds HoRuGe and Ho 3 Ru 2 Ge 3 were determined using X-ray single crystal analysis (Nicolett R3 automated diffractometer, MoKα radiation). HoRuGe has an orthorhombic structure of the TiNiSi-type, space group Pnma , a = 6.980(2) A , b = 4.351(1) A , c = 7.281(2) A , V = 221.2(2) A 3 , z = 4 . The final R value was 0.027 in the anisotropic approximation for 278 unique reflections. Ho 3 Ru 2 Ge 3 has an orthorhombic structure of the Hf 3 Ni 2 Si 3 -type space group Cmcm , a = 4.242(2) A , b = 10.731(6) A , c = 13.840(9) A , V = 630.01(1) A 3 , z= 4 . The final R value was R = 0.038 in the anisotropic approximation for 440 observing reflections. These compounds are first representatives of TiNiSi and Hf 3 Ni 2 Si 3 structure types in rare-earth metal-ruthenium-germanium systems. HoRuGe and Ho 3 Ru 2 Ge 3 can be interpreted as being built up only from layers consisting of linked prisms: Ho 4 Ru 2 for the HoRuGe compound, Ho 6 and Ho 4 Ru 2 for Ho 3 Ru 2 Ge 3 . Both kinds of prisms are centered by germanium atoms.

Thermo-chemical properties and electrical resistivity of Zr-based arsenide chalcogenides

abstract Ternary phases in the systems Zr–As–Se and Zr–As–Te were studied using single crystals of ZrAs1.40(1)Se0.50(1) and ZrAs1.60(2)Te0.40(1) (PbFCl-type of structure, space group P4/nmm) as well as ZrAs0.70(1)Se1.30(1) and ZrAs0.75(1)Te1.25(1) (NbPS-type of structure, space group Immm). The characterization covers chemical compositions, crystal structures, homogeneity ranges and electrical resistivities. At 1223 K, the Te-containing phases can be described with the general formula ZrAsxTe2–x, with 1.53(1)≤x≤1.65(1) (As-rich) and 0.58(1)≤x≤0.75(1) (Te-rich). Both phases are located directly on the tie-line between ZrAs2 and ZrTe2, with no indication for any deviation. Similar is true for the Se-rich phase ZrAsx–ySe2–x with 0.70(1)≤x≤0.75(1). However, the compositional range of the respective As-rich phase ZrAsx–ySe2–x (0.03(1)≤y≤0.10(1); 1.42(1)≤x≤1.70(1)) is not located on the tie-line ZrAs2–ZrSe2, and exhibits a triangular region of existence with intrinsic deviation of the composition towards lower non-metal contents. Except for ZrAs0.75Se1.25, from the homogeneity range of the Se-rich phase, all compounds under investigation show metallic characteristics of electrical resistivity at temperatures >20 K. Related uranium and thorium arsenide selenides display a typical magnetic field-independent rise of the resistivity towards lower temperatures, which has been explained by a non-magnetic Kondo effect. However, a similar observation has been made for ZrAs1.40Se0.50, which, among the Zr-based arsenide chalcogenides, is the only system with a large concentration of intrinsic defects in the anionic substructure.

Crystallographic disorder and electron scattering on structural two-level systems in ZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl-type structure) were grown by chemical vapour transport. While their thermodynamic and transport properties are typical for ordinary metals, the electrical resistivity exhibits a shallow minimum at low temperatures. Application of strong magnetic fields does not influence this anomaly. The minimum of the resistivity in ZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structural two-level systems. Significant disorder in the As–Se substructure is inferred from x-ray diffraction and electron microprobe studies.

Crystal structure of the compound Ho4Ir13Ge9

Abstract The crystal structure of the compound Ho 4 Ir 13 Ge 9 was determined using X-ray single-crystal analysis (DARTCH-1 automated diffractometer, Mo Kα radiation and a total of 925 independent observed reflections; in the anisotropic approximation the final R value was 0.0296). The compound represents a new type of crystal structure: space group Pmmn, a = 3.954(3) A , b = 11.186(3) A , c = 19.301(6) A , Z = 2 . The coordination polyhedra of the holmium atoms have 20 apexes. Those of iridium are tetragonal prisms with five additional atoms and distorted cubo octahedra. The germanium atoms are characterized by trigonal prismatic arrangements with three or four additional atoms.