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A new iron titanyl phosphate Fe0.50Ti2(PO4)3 was synthesized by both solid-state reaction and Cu2+–Fe2+ ion exchange method. The material was then characterized by X-ray diffraction, Mossbauer, magnetic susceptibility measurements and optical absorption. The crystal structure of the compound was refined, using X-ray powder diffraction data, by the Rietveld profile method; it crystallizes in the rhombohedral system, space group R 3 ¯ , with a=8.511(1) A and c=20.985(3) A, V=1316.45(3) A3 and Z=6. The structure, which is compared to that of Mn0.50Ti2(PO4)3 is built up from [TiO6] octahedra and [PO4] tetrahedra which are linked by corner sharing along the c-axis. Fe2+ cations are located in half of the antiprism MI sites and are orderly distributed with vacancies within the two possible positions of the MI sites of R 3 ¯ . These results were supported by the Mossbauer studies that showed the presence of one Fe2+ site in the high spin state (t2g4eg2). The Curie–Weiss-type behavior is observed in the magnetic susceptibility. Diffuse reflectance spectrum indicates the presence of octahedrally coordinated Fe2+ ions.

The room-temperature crystal structure of a new Cu(II) oxyphosphate—α Cu0.50IITiO(PO4)—was determined from X-ray single crystals diffraction data, in the monoclinic system, space group P21/c. The refinement from 5561 independent reflections lead to the following parameters: a=7.5612(4)A, b=7.0919(4)A, c=7.4874(4)A, β=122.25(1)°, Z=4, with the final R=0.0198, wR=0.0510. The structure of α Cu0.50IITiO(PO4) can be described as a TiOPO4 framework constituted by chains of tilted corner-sharing [TiO6] octahedra running parallel to the c-axis and cross linked by phosphate [PO4] tetrahedra, where one-half of octahedral cavities created are occupied by Cu atoms. Ti atoms are displaced from the center of octahedra units in alternating long (2.308 A) and short (1.722 A) Ti–O(1) bonds along chains. Such O(1) atoms not linked to P atoms justify the oxyphosphate formulation α Cu0.50TiO(PO4). The divalent cations Cu2+ occupy a Jahn–Teller distorted octahedron sharing two faces with two [TiO6] octahedra. EPR and optical measurements are in good agreement with structural data. The X-ray diffraction results are supported by Raman and infrared spectroscopy studies that confirmed the existence of the infinite chains –Ti–O–Ti–O–Ti–. α Cu0.50TiO(PO4) shows a Curie–Weiss paramagnetic behavior in the temperature range 4–80 K.

The single crystals of lanthanum metaphosphate MLa(PO3)4 (M=Na, Ag) have been synthesized and studied by a combination of X-ray crystal diffraction and vibrational spectroscopy. The sodium and silver compounds crystallize in the same monoclinic P21/n space group ( C 2 5 h factor group) with the following respective unit cell dimensions: a = 7.255 ( 2 ) , b = 13.186 ( 3 ) , c = 10.076 ( 2 ) A , β = 90.40 ( 2 ) ° , V = 963.0 ( 4 ) A 3 , Z = 4 and a = 7.300 ( 5 ) , b = 13.211 ( 9 ) , c = 10.079 ( 7 ) A , β = 90.47 ( 4 ) ° , V = 972.09 ( 12 ) A 3 , Z = 4 . This three-dimensional framework is built of twisted zig-zag chains running along a direction and made up of PO4 tetrahedra sharing two corners, connected to the LaO8 and NaO7 or AgO7 polyhedra by common oxygen atoms to the chains. The infrared and Raman vibrational spectra have been investigated. A group factor analysis leads to the determination of internal modes of (PO3)− anion in the phosphate chain.

A new type of lithium rare earth oxyborate of formula Li2Ln5O4(BO3)3 (Ln=Yb, Lu) has been discovered in the ternary phase diagrams Li2O–Ln2O3–B2O3. Single crystals of Li2Yb5O4(BO3)3 were grown by the flux method. The structure of this oxyborate was solved from a data collection with a four circle automatic diffractometer and with MoKα radiation. The cell is monoclinic (space group P21/m, Z=2) with a=10.095(2) A, b=3.519(2) A, c=15.647(11) A, β=105.45(3)°, V=535.7(5) A−3. Refinement of 86 parameters using 3298 independent reflections having intensity I>2σ(I) led to R=0.037 (wR=0.091). The structure of Li2Yb5O4(BO3)3 is made up of a bidimensional framework of edge or corner-sharing YbO7 polyhedra parallel to the (10 1 ) plane and connected by three-coordinated boron atoms and lithium ions. It can also be described as a bidimensional assembly of OYb4 tetrahedra, ((Yb10O8)14+)n, separated by borate groups and lithium coordination polyhedra forming ribbons, ((Li4B6O18)14−)n, along the b direction. The presence of oxygen atoms sharing only Yb polyhedra justifies the oxyborate label.

In the context of a detailed investigation of the luminescent properties of the Na 2− x Ag x ZnP 2 O 7 compounds, the structure of the pure sodium phosphates was “revisited.” The crystal structure of Na 2 ZnP 2 O 7 has been determined by X-ray diffraction using single crystals. A new model explaining the actual large unit cell ( a = b =2177.1(3) pm, c =1028.5(2) pm, P4 2 / n , Z =32) was determined by assuming the existence of twins. The structure is made up of [ZnP 2 O 7 ] layers consisting of corners sharing [P 2 O 7 ] groups and [ZnO 4 ] tetrahedra, and the sodium atoms are located between the layers in 7- or 8-fold coordinated sites. This model is discussed with respect to the crystal chemistry of the monovalent cation zinc phosphate.

The luminescence of silver(I) ions was investigated in diphosphate of compositions Na 2− x Ag x ZnP 2 O 7 . The [P 2 O 7 ] groups share corners with [ZnO 4 ] tetrahedra in a layer arrangement. The Na/Ag atoms are located between layers in distorted oxygenated cubic sites sharing faces—with short intermetallic distances—or edges. Two luminescent centers were detected and attributed, respectively, to single Ag + and Ag + –Ag + pairs. The luminescent parameters are discussed with respect to the symmetry of the silver sites and of the ionocovalent character of the silver oxygen bonds

A new family of lithium rare earth oxyborates of formula LiLn6O5(BO3)3 (Ln=Pr–Tm) has been discovered in the ternary-phase diagrams Li2O–Ln2O3–B2O3. Single crystals of LiGd6O5(BO3)3 were grown by the flux method. The structure of the oxyborate was solved using a data collection with a four-circle automatic diffractometer (CAD-4) and with MoKα radiation. The cell is monoclinic (space group P21/c, Z=4) with a=8.489(4) A, b=15.706(3) A, c=12.117(6) A, β=132.27(2)°, and V=1195(1) A3. Refinement of 198 parameters using 4402 independent reflections having intensity I>3σ(I) led to R=0.037 (wR=0.087). The structure of LiGd6O5(BO3)3 can be described as a three-dimensional framework, (Gd6O14)10−, of Gd polyhedra connected by common edges and corners, forming cavities where Li and B atoms are inserted. Some oxygen atoms share only Gd polyhedra, justifying the oxyborate label.

The crystal structure of o-modification of diphosphorus pentaoxide was refined at 295 K. It consists of a network of [PO4] tetrahedra linked by three common apices. Moreover, this o-modification of P2O5is also found to crystallize in the orthorhombic system with the space groupFdd2 and the lattice parametersa= 16.314(2) A,b= 8.115(3) A,c= 5.265(9) A,Z= 8, andDcalc= 2.705 (g · cm−3). The main value of the P–O length is estimated to be 1.543 A. As expected, the nonbridging P–O bonds, equal to 1.445 A, are shorter than the bridging ones which average 1.582 A. The structure parameters are refined to a finalRof 0.0351 (wR= 0.0643) for 550 independent reflections [I> 3σ(I)]

Resume X-ray diffraction, differential thermal analysis, polarized light microscopy, and dielectric measurements have been used to study phase transitions and solid solutions in the K3MO3F3–A3MO3F3(A= Rb, Na;M= Mo, W) systems. K3MO3F3–Na3MO3F3(M= Mo, W) systems show two solid solutions; the first (SSI) nearx= 0 (K3−xNaxMO3F3) involves only one transition atTCwhich is practically independent of composition. The sharp transition atT1(

Structural, ferroelectric, pyroelectric, and optical properties of ceramics and crystals of BaAl2O4 have been investigated. A refinement of the atomic positions has been carried out from the X-ray powder data. BaAl2O4 shows anomalous behavior of the dielectric and pyroelectric properties: the dielectric constant e′r is very weak; its maximal value at Curie temperature for a crystal oriented along the hexagonal c-axis is about 15 and that of a ceramic of compactness 0.89 is around 7.5. The Curie-Weiss constant is relatively weak (C = 125). The behavior of the spontaneous polarization Ps is unexpected with respect to that of classical ferroelectric materials: Ps increases with decreasing temperature from Tc and decreases again after having reached a maximum at 200 K. A model based on ferrielectric behavior and supported by the Landau equation has been proposed. La structure et les proprietes ferroelectriques, pyroelectriques et optiques de ceramiques et de cristaux du compose BaAl2O4 ont ete etudiees. Un affinement des positions atomiques a ete entrepris a partir du spectre de diffraction X sur poudre. BaAl2O4 presente un comportement anormal des proprietes dielectriques et pyroelectriques: la constante dielectrique e′r est faible, sa valeur maximale a Tc est voisine de 15 pour les cristaux et de 7,5 pour des ceramiques de compacite 0,89. La constante de Curie est aussi relativement faible (C = 125). Le comportement de la polarisation spontanee Ps differe de celui des materiaux ferroelectriques "classiques": Ps augmente lorsque T decroit en-dessous de Tc pour atteindre un maximum a T = 200 K suivi d'une decroissance. Un modele de comportement ferrielectrique s'appuyant sur la relation de Landau pourrait justifier les proprietes observees.

The sodium titanium phosphate Na5Ti(PO4)3 crystallizes in the trigonal space group R32 (a = 9.0607(2) A, c = 21.734(1) A, V = 1545.3 A3, dm = 2.84(2), dc = 2.89 Mg · m-3, Z = 6). R = 0.022 and Rw = 0.023 for 831 reflections. The structure belongs to the Nasicon-type family and is characterized by a 2-2 titanium-sodium ordering along the c axis. The four crystallographically independent sodium atoms show coordination numbers 6 and 8. The cell parameters and the cationic distribution are consistent with the rules governing the structural evolution of such types of material.

The transport properties of undoped and Sn-doped In2O3 (ITO) single crystals prepared by a flux method are reported. Hall measurements are detailed: they show that the mobility increases as the Sn dopant concentration increases. A maximum value of 100 cm2V−1s−1 is measured with an electron concentration of about 1.6 × 1020 cm−3. However, at high dopant concentration the mobility decreases again and the presence of neutral entities such as (SnO)x is expected to be responsible for this behavior. Some results dealing with undoped and Sn- or Pb-doped In2O3 ceramics are also presented and discussed.

Optical absorption spectra of NdBO3 single crystals were measured between 3900 and 31,700 cm−1 at liquid helium to room temperature. The derived energy level scheme with 119 levels was simulated according to the crystal field theory including free ion effects yielding a rms deviation of 15 cm−1. The 14 free ion and 14 crystal field parameters allowed by the Cs symmetry of the neodymium site were calculated. The derived wavefunctions were used in calculating the paramagnetic susceptibility and its variation with the temperature between 4.2 and 500 K by the van Vleck formula. Experimental and calculated values are in good agreement.

The electrochemical oxidation of Sr 2 Fe 2 O 5 brownmillerite-type ferrite and La 2 M O 4 ( M = Ni, Cu) oxides is reviewed. The oxidation process appears as a very powerful method for preparing high oxidation states of transition-metal oxides. The characterization of the reaction products shows that they are well described as intercalation compounds of oxygenated anions. The fact that the intercalated species are the same as the host anions distinguishes these materials from most other intercalated compounds, in which guest species usually are chemically different from the host ions. The question of the nature and site of the intercalated species as well as that of their diffusion into the sample bulk is discussed on the basis of various characterizations and physical measurements as well as within the scope of recent considerations of the electronic distribution in oxides of transition elements in a high oxidation state.

Delafossite-type CuFeO2 single crystals have been prepared by a flux method: crystals obtained in a Cu crucible with LiBO2 as flux are n-type whereas those prepared in a Pt crucible with a Cu2O flux are p-type. Electrical measurements have revealed that n-type crystals exhibit weak anisotropic conductivities with large activation energies and small mobilities (r.t. values perpendicular and parallel to the c-axis: μ⊥ = 5 × 10−5 and μ‖ = 10−7 cm−2 V−1 sec−1). p-type crystals, less anisotropic, are characterized by low activation energies and higher mobilities (μ⊥ = 34 and μ‖ = 8.9 cm2 V−1 sec−1). A two -conduction-band model is proposed to account for the difference observed between the energy gap value deduced from photoelectrochemical measurements and the activation energy of the electrical conductivity in the intrinsic domain.

The structure of LiW 3 O 9 F was determined from 972 single crystal reflections and refined by least squares to an R factor of 0.065. It has orthorhombic symmetry with space group Fdd 2 and parameters a = 12.716(2)A˚, b = 15.230(2)A˚, c = 7.288(1)A˚, and Z = 8. The structure is related to the HTB structure and can be described as a complex stacking of HTB layers perpendicular to the b axis. The lithium atoms are found in the hexagonal cavities of the HTB layers.

The crystal structure of sodium tantalum oxyfluoride, β-Na2Ta2O5F2 has been determined from single crystal diffraction data. The symmetry is monoclinic, space group C2 m with a cell having dimensions: a = 12.855 ± 0.008, b = 7.349 ± 0.005, c = 12.833 ± 0.003 A, β = 108.97 ± 0.05° and containing eight formula units. The structure was refined by full-matrix least-squares to a final R value of 0.072. The structure consists of two interpenetrating sublattices: the first, which has the overall formula Ta16X52, is made up of TaX6 octahedra and the second with an overall formula Na14X4 is composed of Na4X tetrahedra. The two remaining sodium atoms occupy the center of a hexagonal bipyramid. This arrangement can be described as a succession of weberite and pyrochlore type slabs parallel to the [100] direction.

Resume Deux series de phases de structure “bronzes oxygenes de tungstene quadratiques” et de composition BaxLi5−2xT5O15(T = Nb, Ta) ont ete mises en evidence au sein des systemes BaNb2O6LiNbO3 et BaTa2O6LiTaO3. Toutes ces phases possedent des transitions ferroelectriques-paraelectriques. La temperature de Curie TC augmente avec le taux en lithium. La valeur de TC correspondant a la phase Ba2.03Li0.94Nb5O15 est la valeur la plus elevee obtenue jusqu'a present pour le type structural envisage. Ces phases se comportent comme des doubleurs de frequence. Le niobate Ba2.14Li0.71 Nb5O15 presente un rendement harmonique tres eleve, de l'ordre de 250 fois celui du K.D.P., 2.5 fois superieur donc a celui du “banana” Ba2NaNb5O15. Les etudes radiocristallographique et dielectrique du systeme Ba2.14Li0.71Nb5O15Ba2.14Li0.71Ta5O15 ont permis la mise en evidence de trois domaines respectivement antiferroelectrique, ferroelectrique et paraelectrique. La temperature de Curie ferroelectrique et le rendement harmonique en optique non lineaire diminuent lorsque le taux en tantale augmente.

Crystal structures of complex oxyfluorides Na 2 WO 2 F 4 , LiW 3 O 9 F, and NaMoO 3 F have been previously determined by X-ray diffraction but without assigning the specific position of oxygen or fluorine. Using various techniques such as 19 F nuclear magnetic resonance, Raman spectroscopy, electrostatic energy and site potential calculations, analysis of the local balance between charge of neighboring ions, clear evidence has been given of an ordered oxygen-fluorine distribution within the anionic sublattices which is proposed. The accuracy of each method used has been discussed for each phase.

The magnetic properties of RbVCl3, CsVCl3, CsVBr3, and CsVI3 have been investigated in the temperature range 4.2–800°K. In agreement with the 2H structure of these phases, the obtained results have been interpreted on the basis of a monodimensional Heisenberg model. The values of the exchange integrals have been determined by means of Smith and Friedberg [Phys. Rev. 176, 660 (1968)] calculations. They involve strong antiferromagnetic interactions, which decrease from the chloride to the iodide. The nature of the coupling between vanadium atoms is discussed.

A precise structural determination of KPbCr 2 F 9 was carried out. The symmetry is orthorhombic with a = 9.81(5), b = 5.412(3), c = 13.93(1) A, and Pnma . The structure was refined from 1285 X-ray reflections by full-matrix least squares to an R = 0.041. The lattice is made up of double chains (Cr 2 F 9 ) 3 n − n running along the b axis with Pb and K atoms ensuring its cohesion. The results of the magnetic studies are reported and discussed.

The value of the ferroelectric Curie temperature in compounds with the “tetragonal tungsten bronze structure” is related to the steric effect of large alkali or alkaline-earth cations in the lattice tunnels, to the covalency of the transition element-oxygen bonds, and to the introduction of Li + ions, which due to their small size are able to occupy the nine coordination sites of the structure anisotropically.