New cobalt and rare earth metal tungstates CoRE<Subscript>2</Subscript>W<Subscript>2</Subscript>O<Subscript>10</Subscript>

A new family of cobalt and rare-earth metal tungstates (CoRE₂W₂O₁₀ where RE=Y, Dy, Ho and Er) were synthesized by heating in the solid-state equimolar CoWO₄/RE₂WO₆ mixtures. The obtained compounds are isostructural and crystallize in the monoclinic system. They melt incongruently in an inert atmosphere at 1494 K (CoY₂W₂O₁₀), 1523 K (CoDy₂W₂O₁₀), 1517 K (CoHo₂W₂O₁₀) and 1493 K (CoEr₂W₂O₁₀). For each CoRE₂W₂O₁₀ compound the solid product of melting is an adequate RE₂WO₆.     

Synthesis, characterization and thermal behaviour of new copper and rare-earth metal tungstates

Three series of new copper and rare-earth metal tungstates with the formulas: CuRE ₂W₂O₁₀ (RE = Nd, Sm, Eu) and Cu₃ RE ₂W₄O₁₈ (RE = Sm, Eu or RE = Dy, Ho, Er) were synthesized by the solid-state reaction method. The CuRE ₂W₂O₁₀ and Cu₃ RE ₂W₄O₁₈ (RE = Dy, Ho, Er) compounds crystallize in the monoclinic system. The Cu₃ RE ₂W₄O₁₈ phases with the large rare-earth ions crystallize in the triclinic system. All obtained compounds melt incongruently below 1273 K. The anion lattice of the Cu₃ RE ₂W₄O₁₈ phases is built from isolated groups of octahedra (W₄O₁₆)^8?, while CuRE ₂W₂O₁₀ from WO₆ octahedra forming structural elements [(W₂O₉)^6?]^∞. The EPR spectra of analyzed compounds consisted of an intense line originating generally from the rare-earth ions and a weak, narrow line from Cu²⁺ separate centers appearing only on the surface of the grains. The absence of bulk copper in the EPR spectrum is probably due to a very short relaxation time of the Cu²⁺ subsystem.     

Multianvil high-pressure/high-temperature preparation, crystal structure, and properties of the new oxoborates Dy4B6O14(OH)2 and Ho4B6O14(OH)2

This paper reports about two new hydrogen-containing rare-earth oxoborates RE₄B₆O₁₄(OH)₂ (RE=Dy, Ho) synthesized under high-pressure/high-temperature conditions from the corresponding rare-earth oxides, boron oxide, and water using a Walker-type multianvil equipment at 8 GPa and 880 °C. The single crystal structure determination of Dy₄B₆O₁₄(OH)₂ showed: Pbcn, a=1292.7(2), b=437.1(2), , Z=2, R1=0.0190, and wR2=0.0349 (all data). The isotypic holmium species revealed: Pbcn, a=1292.8(2), b=436.2(2), , Z=2, R1=0.0206, and wR2=0.0406 (all data). The compounds exhibit a new type of structure, which is built up from layers of condensed BO₄-tetrahedra. Between the layers, the rare-earth cations are coordinated by 7+2 oxygen atoms. Furthermore, we report about temperature-resolved in situ powder diffraction measurements, DTA/TG, and IR-spectroscopic investigations into RE₄B₆O₁₄(OH)₂ (RE=Dy, Ho).     

Role of crystal structure on the thermal expansion of Ln2W3O12 (Ln = La, Nd, Dy, Y, Er and Yb)

The negative thermal expansion material Y₂W₃O₁₂ belongs to Ln₂W₃O₁₂ family of compositions. The thermal expansion behavior of Ln₂W₃O₁₂ (Ln = La, Nd, Dy, Y, Er and Yb) members synthesized by the solid-state reaction have been studied and correlated to their crystal structure. The lighter rare earth tungstates (Ln = La, Nd and Dy) crystallize in monoclinic structure (C2/c) whereas the heavy rare earth tungstates (Ln = Y, Er and Yb) form the trihydrate orthorhombic Ln₂W₃O₁₂3H₂O at room temperature and above 400 K transforms to unhydrated orthorhombic structure (Pnca). The hot pressed (1273 K and 25 MPa) ceramic pellets have been studied for thermal expansion property by dilatometry and high temperature X-ray diffraction. The heavy rare earth tungstates show a large initial expansion up to 400 K, followed by a thermal contraction. The light rare earth tungstates, on the other hand, show thermal expansion. The difference in the thermal expansion behavior in Ln₂W₃O₁₂ series is attributed to the difference in the structural features. The heavy rare earth tungstates have corner sharing of LnO₆ octahedra with WO₄ tetrahedra, where the now well established mechanism of transverse vibrations operate. The light rare earth tungstates have edge sharing of LnO₈ polyhedra where in such a mechanism is absent.     

New cadmium and rare-earth metal molybdato–tungstates with scheelite-type structure

A new group of cadmium and rare-earth metal molybdato–tungstates with the formula Cd_0.25 RE _0.50□_0.25(MoO₄)_0.25(WO₄)_0.75 (RE = Pr, Nd, Sm–Dy, □—vacancies in cation sublattice) were synthesized by a high-temperature solid-state reaction between RE ₂MoO₆ and CdWO₄ mixed at the molar ratio of 1:3. Powder X-ray diffraction measurements showed that obtained phases adopt the scheelite-type structure. The phases melt congruently in the temperature range of 1382–1458 K.     

Ternary rare-earth titanium antimonides: Phase equilibria in the RE-Ti-Sb (RE=La, Er) systems and crystal structures of RE2Ti7Sb12 (RE=La, Ce, Pr, Nd) and RETi3(SnxSb1-x)4 (RE=Nd, Sm)

Investigations on phase relationships and crystal structures have been conducted on several ternary rare-earth titanium antimonide systems. The isothermal cross-sections of the ternary RE-Ti-Sb systems containing a representative early (RE=La) and late rare-earth element (RE=Er) have been constructed at 800 °C. In the La-Ti-Sb system, the previously known compound La₃TiSb₅ was confirmed and the new compound La₂Ti₇Sb₁₂ (own type, Cmmm, Z=2, a=10.5446(10) Å, b=20.768(2) Å, and c=4.4344(4) Å) was discovered. In the Er-Ti-Sb system, no ternary compounds were found. The structure of La₂Ti₇Sb₁₂ consists of a complex arrangement of TiSb₆ octahedra and disordered fragments of homoatomic Sb assemblies, generating a three-dimensional framework in which La atoms reside. Other early rare-earth elements (RE=Ce, Pr, Nd) can be substituted in this structure type. Attempts to prepare crystals in these systems through use of a tin flux resulted in the discovery of a new Sn-containing pseudoternary phase RETi₃(Sn_xSb_1−x)₄ for RE=Nd, Sm (own type, Fmmm, Z=8; a=5.7806(4) Å, b=10.0846(7) Å, and c=24.2260(16) Å for NdTi₃(Sn_0.1Sb_0.9)₄; a=5.7590(4) Å, b=10.0686(6) Å, and c=24.1167(14) Å for SmTi₃(Sn_0.1Sb_0.9)₄). Its structure consists of double-layer slabs of Ti-centred octahedra stacked alternately with nets of the RE atoms; the Ti atoms are arranged in kagome nets.     

High-pressure synthesis, crystal and electronic structures of a new scandium tungstate, Sc0.67WO4

Negative thermal expansion (NTE) materials possess a low-density, open structure that can respond to high pressure conditions, leading to new compounds and/or different physical properties. Here we report that one such NTE material - white, insulating, orthorhombic Sc₂W₃O₁₂ - transforms into a black compound when treated at 4 GPa and 1400 °C. The high pressure phase, Sc_0.67WO₄, crystallizes in a defect-rich wolframite-type structure, a dense, monoclinic structure (space group P2/c) containing 1-D chains of edge-sharing WO₆ octahedra. The chemical bonding of Sc_0.67WO₄ vis-à-vis the ambient pressure Sc₂W₃O₁₂ phase can be understood on the basis of the Sc defect structure. Magnetic susceptibility, resistivity, thermoelectric power and IR spectroscopic measurements suggest that the Sc_0.67WO₄ sample is a paramagnet whose conductivity is that of a metal in the presence of weak localization and electron-electron interactions. Oxygen vacancies are suggested as a potential mechanism for generating the carriers in this defective wolframite material.     

Hexatungstate subunit as building block in the hydrothermal synthesis of organic-inorganic hybrid materials: synthesis, structure and optical properties of Co2(bpy)6 (W6O19)2 (bpy=4,4′-bipyridine)

A hydrothermal reaction of WO₃, CoCl₂ and 4,4′-bipyridine, yields a novel organic-inorganic hybrid compound, Co₂(bpy)₆(W₆O₁₉)₂, at 170°C. X-ray single crystal structure determination reveals a two-dimensional covalent structure belonging to monoclinic crystal system, space group C2/c, with cell parameters a=19.971(4) Å, b=11.523(2) Å, c=16.138(3) Å, β=96.49(3)°, V=3690.0 Å³ and Z=2. The hexatungstate, [W₆O₁₉]²⁻, acts as a building block in bidentate fashion to bridge the Co(II) centers in the crystal structure. The title compound is found to have an optical energy gap of 2.2 eV from UV-Vis-NIR reflectance spectra.     

Phase relations in the K2W2O7-K2WO4-KPO3-Bi2O3 system and structure of K6.5Bi2.5W4P6O34

The phase relations in the cross-section of the K₂W₂O₇-K₂WO₄-KPO₃ containing 15 mol% Bi₂O₃ were undertaken using flux method. Crystallization fields of K_6.5Bi_2.5W₄P₆O₃₄, K₂Bi(PO₄)(WO₄), Bi₂WO₆, KBi(WO₄)₂ and their cocrystallization areas were identified. Novel phase K_6.5Bi_2.5W₄P₆O₃₄ was characterized by single-crystal X-ray diffraction: sp. gr. P−1, a=9.4170(5), b=9.7166(4), c=17.6050(7) Å, α=90.052(5)°, β=103.880(5)° and γ=90.125(5)°. It has a layered structure, which contains {K₇Bi₅W₈P₁₂O₆₈}_∞ layers stacked parallel to ab plane and sheets composed by potassium atoms separating these layers. Sandwich-like {K₇Bi₅W₈P₁₂O₆₈}_∞ layers are assembled from [W₂P₂O₁₃]_∞ and [BiPO₄]_∞ building units, and are penetrated by tunnels with K/Bi atoms inside. FTIR-spectra of K₂Bi(PO₄)(WO₄) and K_6.5Bi_2.5W₄P₆O₃₄ were discussed on the basis of factor group theory.     

La doping of the Sr2CoWO6 double perovskite: A structural and magnetic study

La-doped Sr₂CoWO₆ double perovskites have been prepared in air in polycrystalline form by solid-state reaction. These materials have been studied by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic susceptibility. The structural refinement was performed from combined XRPD and NPD data (D2B instrument, λ=1.594 Å). At room temperature, the replacement of Sr²⁺ by La³⁺ induces a change of the tetragonal structure, space group I4/m of the undoped Sr₂CoWO₆ into the distorted monoclinic crystal structure, space group P2₁/n, Z=2. The structure of La-doped phases contains alternating CoO₆ and (Co/W)O₆ octahedra, almost fully ordered. On the other hand, the replacement of Sr²⁺ by La³⁺ induces a partial replacement of W⁶⁺ by Co²⁺ into the B sites, i.e. Sr_2−xLa_xCoW_1−yCo_yO₆ (y=x/4) with segregation of SrWO₄. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below T_N=24 K independently of the La-substitution.     

Synthesis and crystal structure of new uranyl tungstates M2(UO2)(W2O8) (M=Na, K), M2(UO2)2(WO5)O (M=K, Rb), and Na10(UO2)8(W5O20)O8

The solid-state reactions of UO₃ and WO₃ with M₂CO₃ (M=Na, K, Rb) at 650°C for 5 days result, accordingly the starting stoichiometry, in the formation of M₂(UO₂)(W₂O₈) (M=Na (1), K (2)), M₂(UO₂)₂(WO₅)O (M=K (3), Rb (4)), and Na₁₀(UO₂)₈(W₅O₂₀)O₈ (5). The crystal structures of compounds 2, 3, 4, and 5 have been determined by single-crystal X-ray diffraction using Mo(Kα) radiation and a charge-coupled device detector. The crystal structures were solved by direct methods and Fourier difference techniques, and refined by a least-squares method on the basis of F² for all unique reflections. For (1), unit-cell parameters were determined from powder X-ray diffraction data. Crystallographic data: 1, monoclinic, a=12.736(4) Å, b=7.531(3) Å, c=8.493(3) Å, β=93.96(2)°, ρ_cal=6.62(2) g/cm³, ρ_mes=6.64(1) g/cm³, Z=4; 2, orthorhombic, space group Pmcn, a=7.5884(16) Å, b=8.6157(18) Å, c=13.946(3) Å, ρ_cal=6.15(2) g/cm³, ρ_mes=6.22(1) g/cm³, Z=8, R1=0.029 for 80 parameters with 1069 independent reflections; 3, monoclinic, space group P2₁/n, a=8.083(4) Å, b=28.724(5) Å, c=9.012(4) Å, β=102.14(1)°, ρ_cal=5.83(2) g/cm³, ρ_mes=5.90(2) g/cm³, Z=8, R1=0.037 for 171 parameters with 1471 reflections; 4, monoclinic, space group P2₁/n, a=8.234(1) Å, b=28.740(3) Å, c=9.378(1) Å, β=104.59(1)°, ρ_cal=6.13(2) g/cm³,  g/cm³, Z=8, R1=0.037 for 171 parameters with 1452 reflections; 5, monoclinic, space group C2/c, a=24.359(5) Å, b=23.506(5) Å, c=6.8068(14) Å, β=94.85(3)°, ρ_cal=6.42(2) g/cm³,  g/cm³, Z=8, R1=0.036 for 306 parameters with 5190 independent reflections. The crystal structure of 2 contains linear one-dimensional chains formed from edge-sharing UO₇ pentagonal bipyramids connected by two octahedra wide (W₂O₈) ribbons formed from two edge-sharing WO₆ octahedra connected together by corners. This arrangement leads to [UW₂O₁₀]²⁻ corrugated layers parallel to (001). Owing to the unit-cell parameters, compound 1 probably contains similar sheets parallel to (100). Compounds 3 and 4 are isostructural and the structure consists of bi-dimensional networks built from the edge- and corner-sharing UO₇ pentagonal bipyramids. This arrangement creates square sites occupied by W atoms, a fifth oxygen atom completes the coordination of W atoms to form WO₅ distorted square pyramids. The interspaces between the resulting [U₂WO₁₀]²⁻ layers parallel to plane are occupied by K or Rb atoms. The crystal structure of compound 5 is particularly original. It is based upon layers formed from UO₇ pentagonal bipyramids and two edge-shared octahedra units, W₂O₁₀, by the sharing of edges and corners. Two successive layers stacked along the [100] direction are pillared by WO₄ tetrahedra resulting in sheets of double layers. The sheets are separated by Na⁺ ions. The other Na⁺ ions occupy the rectangular tunnels created within the sheets. In fact complex anions W₅O₂₀¹⁰⁻ are built by the sharing of the four corners of a WO₄ tetrahedron with two W₂O₁₀ dimmers, so, the formula of compound 5 can be written Na₁₀(UO₂)₈(W₅O₂₀)O₈.     

Bimetallic single-source precursors [M(NH3)4][Co(C2O4)2(H2O)2]·2H2O (M = Pd, Pt) for the one run synthesis of CoPd and CoPt magnetic nanoalloys

All the steps of the proposed technique, from the synthesis of single-source precursors to the preparation of CoPd and CoPt nanoalloys, are described. The double complex salts (DCS) [M(NH₃)₄][Co(C₂O₄)₂(H₂O)₂]·2H₂O (M = Pd, Pt), which were synthesized by mixing solutions containing [M(NH₃)₄]²⁺ cations and [Co(C₂O₄)₂(H₂O)₂]²⁻ anions, have been used as precursors. The salts obtained were characterized by IR spectroscopy, thermal analysis, XRD and single crystal X-ray diffraction. The prepared compounds crystallize in the monoclinic (space group I2/m, M = Pd) and orthorhombic (space group I222, M = Pt) crystal systems. Thermal decomposition of the salts in helium or hydrogen atmosphere at 200-600 °C results in the formation of nanoalloys powders (random solid solution Co_0.50Pd_0.50 and chemically ordered CoPt). The size of the bimetallic particles varied from 5 to 20 nm. Order-disorder structural transformations in Co_0.50Pt_0.50 nanoalloys were studied. The magnetic properties of both chemically disordered Co_0.50Pd_0.50 and ordered CoPt clusters have also been measured.     

Mn5Si3-type host-interstitial boron rare-earth metal silicide compounds RE5Si3: Crystal structures, physical properties and theoretical considerations

A series of binary rare-earth metal silicides RE₅Si₃ and ternary boron-interstitial phases RE₅Si₃B_x (RE=Gd, Dy, Ho, Lu, and Y) adopting the Mn₅Si₃-type structure, have been prepared from the elemental components by arc melting. Boron “stuffed” phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6₃/mcm, Z=2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6-1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and θ_p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er₅Si₃B_x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl-Klemm concept would predict a limiting composition RE₅Si₃B_0.6 for a valence compound and should then preclude the stoichiometric formula RE₅Si₃B. Density functional theory calculations carried out on some RE₅Si₃Z_x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.     

Syntheses, structures, and second-harmonic generating properties in new quaternary tellurites: A2TeW3O12 (A=K, Rb, or Cs)

The syntheses, structures, and characterization of a new family of quaternary alkali tungsten tellurites, A₂TeW₃O₁₂ (A=K, Rb, or Cs), are reported. Crystals of the materials were synthesized by supercritical hydrothermal methods using 1 M AOH (A=K, Rb, or Cs), TeO₂, and WO₃ as reagents. Bulk, polycrystalline phases were synthesized by standard solid-state methods combining stoichiometric amounts of A₂CO₃, TeO₂, and WO₃. Although the three materials are not iso-structural, each exhibits a hexagonal tungsten oxide layer comprised of corner-sharing W⁶⁺O₆ octahedra. Te⁴⁺O₃ groups connect the WO₆ layers in K₂TeW₃O₁₂, whereas the same groups cap the WO₆ layers in Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂. This capping results in non-centrosymmetric structures for Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂. Powder second-harmonic generation measurements on Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂ revealed strong SHG efficiencies of 200 and 400×SiO₂, respectively. These values indicate an average non-linear optical susceptibility, 〈d_eff〉_exp of 16 and 23 pm/V for Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂, respectively. Crystallographic information: K₂TeW₃O₁₂, monoclinic, space group P2₁/n (No. 14), a=7.3224(13) Å, b=11.669(2) Å, c=12.708(2) Å, β=90.421(3)°, Z=4; Rb₂TeW₃O₁₂, trigonal, space group P31c (No. 159), a=b=7.2980(2) Å, c=12.0640(2) Å, Z=2.     

Luminescence properties of efficient X-ray phosphors of YBa3B9O18, LuBa3(BO3)3, α-YBa3(BO3)3 and LuBO3

The samples of YBa₃B₉O₁₈, LuBa₃(BO₃)₃, α-YBa₃(BO₃)₃ and LuBO₃ powders have been synthesized by the solid-state reaction methods at high temperature and their X-ray excited luminescent properties were investigated. All the studied materials show a broad emission band in the wavelength range of 300-550 nm with the peak centers at about 385 nm for YBa₃B₉O₁₈ and LuBa₃(BO₃)₃, 415 nm for α-YBa₃(BO₃)₃ and 360 nm for LuBO₃ powders, respectively. Even though those compounds have the different atomic structures, they have the common structural feature of each yttrium or lutetium ion bonded to six separate BO₃ groups, i.e., octahedral RE(BO₃)₆ (RE=Lu or Y) moiety. This octahedral RE(BO₃)₆(RE=Lu or Y) moiety seems to be an important structural element for efficient X-ray excited luminescence of those compounds, as are the edge-sharing octahedral TaO₆ chains for tantalate emission.     

Enthalpies of formation of rare earth orthovanadates, REVO4

Rare earth orthovanadates, REVO₄, having the zircon structure, form a series of materials interesting for magnetic, optical, sensor, and electronic applications. Enthalpies of formation of REVO₄ compounds (RE=Sc, Y, Ce-Nd, Sm-Tm, Lu) were determined by oxide melt solution calorimetry in lead borate (2PbO·2B₂O₃) solvent at 1075 K. The enthalpies of formation from oxide components become more negative with increasing RE ionic radius. This trend is similar to that obtained for the rare earth phosphates.     

High-pressure synthesis, crystal structure, and structural relationship of the first ytterbium fluoride borate Yb5(BO3)2F9

Yb₅(BO₃)₂F₉ was synthesized under high-pressure/high-temperature conditions in a Walker-type multianvil apparatus at 7.5 GPa and 1100 °C, representing the first known ytterbium fluoride borate. The compound exhibits isolated BO₃-groups next to ytterbium cations and fluoride anions, showing a structure closely related to the other known rare-earth fluoride borates RE₃(BO₃)₂F₃ (RE=Sm, Eu, Gd) and Gd₂(BO₃)F₃. Monoclinic Yb₅(BO₃)₂F₉ crystallizes in space group C2/c with the lattice parameters a=2028.2(4) pm, b=602.5(2) pm, c=820.4(2) pm, and β=100.63(3)° (Z=4). Three different ytterbium cations can be identified in the crystal structure, each coordinated by nine fluoride and oxygen anions. None of the five crystallographically independent fluoride ions is coordinated by boron atoms, solely by trigonally-planar arranged ytterbium cations. In close proximity to the above mentioned compounds RE₃(BO₃)₂F₃ (RE=Sm, Eu, Gd) and Gd₂(BO₃)F₃, Yb₅(BO₃)₂F₉ can be described via alternating layers with the formal compositions “YbBO₃” and “YbF₃” in the bc-plane.     

A neutron diffraction study of the d and d lithium garnets Li3Nd3W2O12 and Li5La3Sb2O12

The garnets Li₃Nd₃W₂O₁₂ and Li₅La₃Sb₂O₁₂ have been prepared by heating the component oxides and hydroxides in air at temperatures up to 950 °C. Neutron powder diffraction has been used to examine the lithium distribution in these phases. Both compounds crystallise in the space group with lattice parameters a=12.46869(9) Å (Li₃Nd₃W₂O₁₂) and a=12.8518(3) Å (Li₅La₃Sb₂O₁₂). Li₃Nd₃W₂O₁₂ contains lithium on a filled, tetrahedrally coordinated 24d site that is occupied in the conventional garnet structure. Li₅La₃Sb₂O₁₂ contains partial occupation of lithium over two crystallographic sites. The conventional tetrahedrally coordinated 24d site is 79.3(8)% occupied. The remaining lithium is found in oxide octahedra which are linked via a shared face to the tetrahedron. This lithium shows positional disorder and is split over two positions within the octahedron and occupies 43.6(4)% of the octahedra. Comparison of these compounds with related d⁰ and d¹⁰ phases shows that replacement of a d⁰ cation with d¹⁰ cation of the same charge leads to an increase in the lattice parameter due to polarisation effects.     

Synthesis and thermal stability of rare-earths molybdates and tungstates with fluorite- and scheelite-type structure

Polycrystalline samples of rare-earths molybdates and tungstates, i.e., CdRE₄Mo₃O₁₆ (RE = Eu, Gd, Y, Ho) and Pb_1–3x□_xEu_2xWO₄ (0 < x ≤ 0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO₄/RE₂MoO₆ and PbWO₄/Eu₂(WO₄)₃ mixtures, respectively. According to the X-ray diffraction analysis, the CdRE₄Mo₃O₁₆ compounds crystallize in a cubic, fluorite-related-type structure with space group \(Pn\bar{3}n\). In turn, new Pb_1–3x□_xEu_2xWO₄ phases crystallize in the scheelite-type, tetragonal symmetry, space group I4₁/a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., RE₂MoO₆ and RE₂(MoO₄)₃. Thermal stability of CdRE₄Mo₃O₁₆ decreases with decreasing of RE³⁺ radius. The melting point of each sample of Pb_1–3x□_xEu_2xWO₄ solid solution is lower than melting point of pure matrix, i.e., PbWO₄ (1116 °C), and it decreases with increasing in Eu content. Both CdRE₄Mo₃O₁₆ as well as Pb_1–3x□_xEu_2xWO₄ samples are insulators, and their optical band gap (E _g) is bigger than 3 eV.     

Preparation and photoluminescence properties of RE:Na3La9O3(BO3)8 (RE=Er, Yb) crystals

Using Na₂CO₃-H₃BO₃-NaF as fluxes, transparent RE:Na₃La₉O₃(BO₃)₈ (abbr. RE:NLBO, RE=Er, Yb) crystals have been grown by the top seed solution growth (TSSG) method. The X-ray powder diffraction analysis shows that the RE:NLBO crystals have the same structure with NLBO. The element contents were determined by molar to be 0.64% Er³⁺ in Er:NLBO, 2.70% Yb³⁺ in Yb:NLBO, respectively. The polarized absorption spectra of RE:NLBO have been measured at room temperature and show that both Er:NLBO and Yb:NLBO have a strong absorption bands near 980 nm with wide FWHM (Full Wave at Half Maximum) (21 nm for Er:NLBO and 25 nm for Yb:NLBO). Fluorescence spectra have been recorded. Yb:NLBO has the emission peaks at 985 nm, 1028 nm and 1079 nm and the emission peak of Er:NLBO is at 1536 nm. Spectral parameters have been calculated by the Judd-Ofelt theory for Er:NLBO and the reciprocity method for Yb:NLBO, respectively. The calculated values show that Er:NLBO is a candidate of 1.55 μm laser crystals and Yb:NLBO is a candidate for self-frequency doubling crystal.