Crystal structure and fluorescence lifetime of NdAl3(BO3)4, a promising laser material

The structure of NdAl₃(BO₃)₄, determined by single-crystal x-ray analysis, is rhombohedral with space group R32 and cell parameters $\text{a}\text{= 9.3416 (6)}\text{A}\text{?}$, $\text{c}\text{= 7.3066 (8)}\text{A}\text{?}$, Z = 3. A full-matrix, least-squares refinement gives R = 0.033 and R_w = 0.037. The Nd atoms, Al atoms and B atoms occupy trigonal prisms, octahedra, and triangles of oxygen, respectively. Edge-shared Al octahedra form helices along the c-axis. These helices are connected by isolated B triangles and isolated Nd trigonal prisms. The fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ transition of Nd⁺³ is reported for the system Nd_xGd_1?xAl₃(BO₃)₄. The lifetime of NdAl₃(BO₃)₄ is 19us, and concentration quenching is reduced in the series, as happens in NdNa₅(WO₄)₄ (85μs) and NdP₅O₁₄ (115μs). The shorter lifetime is attributed to noncentrosymmetry; the reduced concentration quenching to isolation of Nd polyhedra.     

Influence of local-site symmetry on fluorescence lifetime in high-Nd-concentration laser materials

We have correlated the radiative fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level of Nd³⁺ in laser crystals, including the recently developed high-Nd-concentration materials such as NdP₅O₁₄, with the local Nd³⁺ site symmetry. At Nd³⁺ concentrations low enough for non-radiative interactions to be negligible, the fluorescence decay rate is determined by the deviation from local inversion symmetry, which admixes even- and odd-parity electronic wavefunctions, allowing ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ radiative decay by electric-dipole transitions.     

Copper doped zinc oxide Y2BaZnO5: ERS and optical investigation

In copper doped Y₂BaZnO₅ oxides, copper exhibits a distorted square pyramidal coordination which is consistant with the values of g and A tensors obtained from O band ERS spectrum for a sample containing about 1 % Cu. Three values for g and A are observed, g₁ = 2.049₅, g₂ = 2.051₅, g₃ = 2.275, $\text{¦}\text{A}{}_1\text{¦ = 13 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$, $\text{¦}\text{A}{}_2\text{¦ = 10 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$ and $\text{¦}\text{A}{}_3\text{¦ = 147.5 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$. Since $\text{g}{}_1\text{?}\text{g}{}_2$ an approximate C_4v point symmetry can be assumed for copper. The electronic spectrum shows three bands at 11700, 14500 and 20500 cm^?1 which can be assigned to the transitions A₁ → B₁, B₂ → B₁ and E → B₁ respectively. The orbital reduction parameters are calculated and the bonding covalency is discussed.     

Crystal structure and fluorescence lifetime of potassium neodymium orthophosphate,K3Nd(PO4)2, a new laser material

Crystals of a new high-Nd-concentration laser material, K₃Nd(PO₄)₂, have been grown both from the melt and from a KF-KCl flux. Singlecrystal x-ray diffraction analysis shows the structure to be monoclinic with space group $\text{P}\text{2}{}_1\text{m}$, Z = 2, and cell parameters $\text{a}\text{= 9.532(2)}\text{A}\text{?}$, $\text{b}\text{= 5.631(3)}\text{A}\text{?}$, $\text{c}\text{= 7.444(3)}\text{A}\text{?}$, β = 90.95(2)°, giving a Nd concentration of 5.00 × 10²¹ cm^?3. The basic structural units are isolated PO₄ tetrahedra and isolated NdO₇ decahedra. The NdO₇ decahedra each connect six PO₄ tetrahedra to form two-dimensional Nd(PO₄)₂^3? sheets in the a-b plane, with K atoms inserted along the c-axis between these sheets. Each PO₄ tetrahedron is connected to three NdO₇ decahedra; half the tetrahedra share an edge with a decahedron, giving a short Nd-Nd separation of 4.873 Å. Of the seven O ions surrounding each Nd ion, six are near inversion positions. The radiative lifetime of Nd³⁺ ions in this structure is quite long because of the small deviation from inversion symmetry, but concentration quenching is much stronger than in other structures with isolated Nd-O polyhedra. The measured fluorescence lifetime decreases from 460 μsec in K₃Nd_0.005La_0.995(PO₄)₂ to 21 μsec in K₃Nd(PO₄)₂.     

Verres fluores au tetrafluorure de zirconium proprietes optiques d'un verre dope au Nd3+

Glass formation has been observed in the ZrF₄BaF₂NaF system around the composition 50 % ZrF₄, 25 % BaF₂, 25 % NaF. A Nd³⁺ doped glass obtained by dissolving NdF₃ in the previous material has been studied. Absorption spectra in the region ${}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, I.R. fluorescence spectra and lifetime measurement of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level show a good protection of Nd³⁺ against the fluorescence quenching.     

Ionic conductivity in low carnegieite compositions based on NaAlSiO4

Carnegieite compositions of the type Na_1+xAl_1+xSi_1?xO₄ with x = 0 to ～0.7 were prepared. Na ion conductivities, measured with Na and Au electrodes at ～10³ Hz, range from $\text{4×10}{}_{\mathrm{?5}}\text{(Ω-}\text{cm}\text{)}{}^{\mathrm{?1}}$ for NaAlSiO₄ to $\text{5×10}{}^{\mathrm{?3}}\text{(Ω-}\text{cm}\text{)}{}^{\mathrm{?1}}$ at 300 C for Na_1.7Al_1.7Si_0.3O₄. Substitutions of Li, K, Ca, or Sr for Na lowered σ whereas substitution of Ti for Si raised σ. Na aluminum silicates with the nepheline structure had lower σ than carnegieite compositions.     

Structure determination of (3D) P2NbS8

Tridimensional P₂NbS₈ crystallizes in P4?n2 tetragonal space group, with $\text{a}\text{= 12.0483(4)}\text{A}\text{?}$, $\text{c}\text{= 7.2070(5)}\text{A}\text{?}$, $\text{V}\text{= 1046.2(1)}\text{A}\text{?}{}^3$ and Z = 4. The structure was anisotropically refined down to R = 2.3% from 468 reflexions and 53 variables. It is built from [Nb₂S₁₂] biprismatic bicapped units (average $\text{d}{}_{\mathrm{Nb?S}}\text{= 2.571}\text{A}\text{?}$) made of S^?II and S^?II₂ anions (dianionic distance of 2.014(3) Å). The niobium atoms are found as isolated Nb^IV ? Nb^IV pairs ($\text{d}{}_{\mathrm{Nb?Nb}}\text{= 2.859(1)}\text{A}\text{?}$) in these niobium group otherwise linked to each other through (PS₄) tetrahedral units (average $\text{d}{}_{\mathrm{P?S}}\text{= 2.051}\text{A}\text{?}$) themselves constituting interbonded [P₄S₁₂] rings. The P₂NbS₈ three-dimensional network thus obtained is compared to the (2D) P₂NbS₈, layered phase already described.     

A thin film electrochromic display based on the tungsten bronzes

The main features of a passive thin film display cell based on the electrochemically reversible formation of a tungsten bronze according to the reaction

$\text{(colourless) WO}{}_3\text{+ x}\text{M}{}^{\mathrm{+}}\text{+ x}\text{e}{}^{\mathrm{?}}\text{?}\text{M}{}_{\mathrm{x}}\text{WO}{}_3\text{(blue)}$

where 0 < x < 1 are considered. Chemical analysis of an electrochemically coloured WO₃ film has confirmed the presence of M. It is shown that a critical requirement of these cells is that D_τ(qC_m/Q)² ≈ 1, where the symbols are, in order, the M⁺ diffusion coefficient, the required device response time, the electronic charge, the maximum practical volume concentration of M in the WO₃ film and lastly the area colouring charge. Typical energy requirements might be about 10 mJ cm^?2 per complete cycle in a favourable case.Ionic injection overpotentials and ionic diffusion both appear to play a significant role in determining cell currents. Preliminary diffusion coefficient results for Li⁺ in r.f. sputtered WO₃ films are reported, and their predicted dependence of film structure is discussed. The optical absorption of coloured WO₃ films is presented, and it is interpreted as being predominantly due to free-electron intraband transitions.     

Determination de la structure cristalline de la variete a des hydroxycarbonates de terres rares LnOHCO3 (Ln = Na)

A probable model for the structure of the orthorhombic A - type neodymium hydroxycarbonate is presented. It relies on an optical determination of the site symmetry of OH^?, CO^2?₃, and Nd³⁺ atoms from infrared and visible absorption spectra, together with a computation of 50 X-ray diffraction lines intensities of a high resolution Guinier orthorhombic powder pattern ($\text{a}\text{= 4,953}\text{A}\text{?}$, $\text{b}\text{= 8,477}\text{A}\text{?}$, $\text{c}\text{= 7,210}\text{A}\text{?}$; space group Pmcn (n^o62)). The CO^2?₃ groups lies flat between mettalic planes as in the aragonite type of structure and are linked to OH^? by hydrogen bond ($\text{d(CO}{}^{\mathrm{2?}}{}_3\text{?}\text{OH}{}^{\mathrm{?}}\text{) = 2,52}\text{A}\text{?}$). The rare earth coordination is nine fold: two OH^? groups being closer (2,58 Å) than the seven oxygen from the CO^2?₃ groups (2,58 to 2,70 Å).     

Electronic and spin configurations of Co3+ and Ni3+ ions in oxides of K2NiF4 structure: A magnetic susceptibility study

In La₄LiCoO₈, Li⁺ and Co³⁺ ions are ordered in two dimensions and Co³⁺ ions undergo transitions from the low-spin to the intermediate as well as the high-spin states. Both Sr₄TaCoO₈ and Sr₄NbCoO₈ exhibit low to intermediate-spin state transitions of Co³⁺ ions. In the system LaSr_1?xBa_xNiO₄, the e_g electrons are essentially in extended states forming a band. With increase in x, the band width decreases accompanying an increase in unit cell volume; high-spin Ni³⁺ ions are formed to a small extent with increasing x, but there is no spin-state transition. In LaSrAl_1?xNi_xO₄, at small x, there is a small proportion of high-spin Ni³⁺; when x ≈ 0.6, there is an abrupt decrease in the $\text{c}$/$\text{a}$ ratio, signalling the formation of the band. In LnSrNiO₄, the $\text{c}$/$\text{a}$ ratio decreases sharply between Ln = La and Nd; this is likely to be accompanied by a broadening of the band.     

Crystal chemistry of (Cd,Me)3(PO4)2 solid solutions

A number of solid solutions of $\text{Me}$₃(PO₄)₂ in Cd₃(PO₄)₂ have been prepared and equilibrated at 1070 K ($\text{Me}\text{=}\text{Mg, Mn, Fe, Co, Ni, Cu, Zn, or Ca}$). The respective ($\text{Cd}{}_{\mathrm{<mtext>1?</mtext><mtext>z</mtext>}}\text{Me}{}_{\mathrm{<mtext>z</mtext>}}\text{)}{}_3\text{(}\text{PO}{}_4\text{)}{}_2$ phases are either isotypic with β′-Cd₃(PO₄)₂ or with the mineral graftonite, both with five- and six- (or even seven)-coordinated cations. The monoclinic unit cell dimensions have been accurately determined from Guinier-Hägg photographs, and complete X-ray powder diffraction data are given for β′-Cd₃(PO₄)₂. The cell volumes are strongly correlated to the size and amount of incorporated $\text{Me}$²⁺ cation. The homogeneity regions and structures of the (Cd,$\text{Me}$)₃(PO₄)₂ phases, though, seem rather to be controlled by the $\text{Cd}{}^{\mathrm{2+}}\text{Me}{}^{\mathrm{2+}}$ cation ordering.     

Rate law versus oxygen pressure in spinel oxidation to γ lacunar spinels

In the controlled oxidation of spinels (Fe²⁺Fe³⁺_2?xM³⁺_x)O^2?₄ (M³⁺=Al³⁺, Cr³⁺ ; 0 < x < 2) and (Fe²⁺Al³⁺_2?xCr³⁺_x)O^2?₄ to the metastable phases γ(Fe³⁺_1?yM³⁺_y)₂O^2?₃ and $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{O}{}^{\mathrm{2?}}{}_{\mathrm{3?}}\text{(0 <}\text{y}\text{<}\text{2}\text{3}\text{)}$, over the temperature range 200–450°C, the rate law is usually v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{2}$. This law is well interpreted if we consider a total association vacancy-positive charge. However, for low oxidation extents a (a < 0.3) the law written as is in better agreement with a partial association of defects. For a total dissociation of defects exponent n of the pressure law is always smaller.     

Pressure induced phase transformation in BaWO4

A new dense form of BaWO₄(BaWO₄-II) was prepared under high pressure. The phase boundary between the normal pressure form (BaWO₄-I, scheelite structure) and BaWO₄-II was determined as P(kb) = 26.₇+0.26₅T(°C), (T=600–1000 °(C). Crystallographic data were obtained from the single crystal and powder X-ray analyses. BaWO₄-II is monoclinic with 8 formula units in the unit cell. The possible space group is $\text{P}\text{2}{}_{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$ and the cell parameters are; $\text{a}\text{= 13.159}\text{A}\text{?}\text{,}\text{b}\text{= 7.161}\text{A}\text{?}\text{,}\text{c}\text{= 7.499}\text{A}\text{?}\text{, β = 93.76°}$ and the cell $\text{volume}\text{= 705}\text{A}\text{?}{}^3$. The volume decrease upon transformation is estimated to be 12.1%.     

ESR evidence for the existence of rutile-type,nonstoichiometric vanadium antimonate

A nonstoichiometric vanadium antimonate has been synthesized from a high temperature solid state reaction of V₂O₅ and Sb₂O₃. Powder-X-ray diffraction data of this phase could be indexed on a tetragonal basis with $\text{a}\text{= 4.60}\text{A}\text{?}$, $\text{c}\text{= 3.02}\text{A}\text{?}$. The phase exhibits a well resolved hyperfine ESR spectrum at 298 K due to V(4+)(3d′) ions interacting with ${}^{51}\text{V}\text{(}\text{I}\text{=}\text{7}\text{2}\text{)}$, thus establishing its identity as a rutile type phase postulated earlier. The ESR parameters are $\text{g}{}_{\mathrm{}}\text{= 1.933±0.002}$, g_⊥ = 1.984±0.002, $\text{A}{}_{\mathrm{}}\text{= 193±3}\text{G}$, A_⊥ = 75±3G. Present results are discussed in relation to ESR of V(4+) in other rutile-type hosts.     

Spectres vibrationnels,transitions electroniques et structures a courtes distances dans les verres: Sulfures de terres rares - sulfure de gallium

Absorption spectra of chalcogenide glasses of general formula Ln₂S₃-1,8 Ga₂S₃ (Ln = La⁺³, Ce⁺³, Pr⁺³, Nd⁺³, Gd⁺³ were investigated continously from far infrared to ultra-violet region. Raman spectra were recorded in the 5cm^?1–600 cm^?1 range. Vibrational spectra of glasses have been compared to those of polycrystalline compounds $\text{Ln}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{Ga}{}_6\text{S}{}_{14}$ (Ln = La⁺³, Ce⁺³) and $\text{Ln}{}_6\text{Ga}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{S}{}_{14}\text{(}\text{Ln = La}{}^{\mathrm{+3}}\text{to Gd}{}^{\mathrm{+3}}\text{)}$.From the observed spectra it could be concluded that the short distance order around gallium atom is strongly modified in going from crystalline to glassy state.Electronic transitions of Ce⁺³, Pr⁺³ and Nd⁺³ ions have been also identified in the visible region and in the mid infrared absorption spectra.     

Etude par spectrometrie d'absorption infra-rouge des facteurs influencant la transformation ordre-desordre de ferrites γ lacunaires derivant de l'oxydation de spinelles ferreux

The order-disorder phenomenon in defect γ-spinels of type $\text{(Fe}{}_{\mathrm{<mtext>(8?8y)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{<mtext>8y</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ and $\text{Fe}{}_{\mathrm{<mtext>(8?2x)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{x}}{}^{\mathrm{2+}}\text{□}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ with M³⁺ = A1³⁺, Cr³⁺ and M²⁺ = Zn²⁺, Co²⁺, Mn²⁺ obtained from ferrous spinels was investigated using IR spectrometry. These compounds possess a vacancy ordering on octahedral sites for substitution extents of less than 0.30 and no ordering has been observed for substitution extent > 0.40. The ordering process is also influenced by the nature and position of the trivalent or divalent cation, particle size, heating temperature and oxidation time.     

TlxTi6Se8 and TlxNb6Se8, two phases with filled up Nb3Te4 type of structure showing pronounced cation disorder

The new ternary chalcogenides Tl_xTi₆Se₈ (x= 0.76) and Tl_xNb₆Se₈ (x= 0.70) crystallize with a partially filled up Nb₃Te₄-structure: space group $\text{P}\text{6}{}_3\text{m}$, z= 1 with

$\text{a= 9.882(1) c= 3.590(1)}\text{A}\text{?}\text{(Tl}{}_{0.76}\text{Ti}{}_6\text{Se}{}_8\text{)}$

$\text{a= 10.033(2) c= 3.475(1)}\text{A}\text{?}\text{(Tl}{}_{0.70}\text{TNb}{}_6\text{Se}{}_8\text{)}$

The crystal structures were determined from single crystal diffractometer data and refined to a conventional R-value of 0.077 and 0.055 respectively. The T1 atoms in the octahedral channels cannot be localized. In Tl_xTi₆Se₈ diffuse scattering indicates one-dimensional short range order. A model of the cation distribution in Tl_0.76Ti₆Se₈ is discussed.     

Alkaline earth scandates

In the CaOSrOSc₂O₃ and BaOSrOSc₂O₃ systems, complete series of mixed crystals were found for (Ca,Sr)Sc₂O₄ and (Sr,Ba)₃Sc₄O₉ respectively. In the BaOCaOSc₂O₃ system a new ternary phas?e with composition Ba_x(Ca_2xSc_2?2x)Sc₆O₁₂ was detected having an incommensurate structure which may be described with a basic hexagonal unit cell ($\text{a}\text{=9.7039}\text{A}\text{?}$, $\text{c}\text{=3.1274}\text{A}\text{?}$) and a second hexagonal cell having the same basic a-axis. The c-axis of the second cell is x-dependent: $\text{c}{}^{\mathrm{x}}\text{= 3.13/}\text{x}\text{A}\text{?}$, x varying from 0.782 to 0.736, corresponding to the region from BaCa₂Sc₈O₁₅ with $\text{c}{}^{\mathrm{x}}\text{= 4.00}\text{A}\text{?}$, to BaCa₂Sc₉O_16.5 with $\text{c}{}^{\mathrm{x}}\text{= 4.25}\text{A}\text{?}$.     

The spinels CoRh2O4 and Co2RhO4

Polycrystalline CoRh₂O₄ and Co₂RhO₄ are cubic, spinel-type double oxides, S.G. Fd3m (No. 227), Z = 8. For CoRh₂O₄, $\text{a}\text{= 8.4992(1)}\text{A}\text{?}$, $\text{U = 613.95(3)}\text{A}\text{?}{}^3$, D_X = 7.11 Mgm^?3, $\text{x}\text{= 0.257}$, Co in 8(a) positions, R = 0.043. For Co₂RhO₄, $\text{a}\text{= 8.299(2)}\text{A}\text{?}$, $\text{U = 517.6(5)}\text{A}\text{?}{}^3$. D_X = 6.62 Mgm^?3, $\text{x}\text{= 0.261}$, half the Co content in 8(a) positions, R = 0.041. IR absorption band frequencies for both spinels are included.     

Nature de la fluorescence de l'europium divalent dans les fluorures

The levels of the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ (4f⁷) and 4f⁶5d¹ states of europium (+II) have been determined from fluorescence spectra in a series of fluorides M_xB_yF_z (M = alkaline-earth element, B = Li, Be, Mg, Y, Si). As in BaY₂F₈, SrSiF₆ and BaSiF₆, the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ level lies even at room temperature far below the 4f⁶5d¹ band, only the f → f emission is observed. The influence of the choice of the alkaline-earth and the B cations and of the coordination of europium on the relative positions of the energy levels is discussed.