Fluorite related ordered compounds in the ZrO2CaO and ZrO2Y2O3 systems

ZrO₂:CaO and ZrO₂:Y₂O₃ systems have been examined for possible ordered compounds. Fluorite related ordered compounds CaZr₄O₉ and Zr₃Y₄O₁₂ have been successfully prepared and identified. While it is believed that the former is monoclinic with $\text{a}\text{= 17.813 ± 0.005}\text{A}\text{?}$, $\text{b}\text{= 14.612 ± .004}\text{A}\text{?}$, $\text{c}\text{= 12.065 ± .003}\text{A}\text{?}$ and β = 119.5 ± .02° and isostructural with CaHf₄O₉, the latter appears to be isostructural with M₇O₁₂ type of compounds with $\text{a}\text{= 9.723 ± .001}\text{A}\text{?}$ and $\text{c}\text{= 9.090 ± 0.002}\text{A}\text{?}$, which is formed commonly in compounds containing tri and tetravalent cations and is characterized by a chain oxygen vacancy along a 〈111〉 of the original fluorite. No other fluorite related ordered compound could be detected in these systems.     

High pressure synthesis of orthorhombic SnO2

A dense form of SnO₂ with an orthorhombic structure has been synthesized at 158 kbar and 800 °C. The boundary separating the normal pressure phase (tetragonal SnO₂-I) from the high pressure phase (orthorhombic SnO₂-II) is represented by P(kbar) = 140.0 + 0.022 T(°C). The lattice parameters of the quenched high pressure phase (SnO₂-II) are : $\text{a}\text{= 4.714 ± 0.001}\text{A}\text{?}$, $\text{b}\text{= 5.727 ± 0.001}\text{A}\text{?}$ and $\text{c}\text{= 5.214 ± 0.001}\text{A}\text{?}$ with the cell volume $\text{V}\text{= 140.8 ± 0.1}\text{A}\text{?}{}^3$. The volume decrease for the transition is calculated to be 1.8 %.     

High pressure synthesis of ilmenite type MnCrO3 and its magnetic properties

MnCrO₃ was synthesized by reaction of MnO with CrO₂ at 50 kbars and 1200°C. MnCrO₃ is isomorphous with triclinic vanadate ilmenites CoVO₃(I), CuVO₃(I) and MnVO₃(I). Unit cell dimensions are $\text{a}\text{= 5.113 ± 0.009 (}\text{A}\text{?}\text{)}$, $\text{b}\text{= 5.385 ± 0.007(}\text{A}\text{?}\text{)}$, $\text{c}\text{= 4.991 ± 0.007(}\text{A}\text{?}\text{)}$, α = 89° 57′ ± 9′, β = 118°12′ ± 7′ and γ = 62°33′ ± 6′. Magnetic properties of MnCrO₃ were also studied. Temperature variation of magnetic susceptibility obey the Curie-Weiss law.     

Comments on the system Ag2SIn2S3

The high temperature polymorph of AgInS₂ has been found to be orthorhombic with $\text{a}\text{=7.001}$, $\text{b}\text{=8.278}$, $\text{c}\text{=6.698}\text{A}\text{?}$, space group probably Pna2₁ with a distorted wurtzite structure. The phase transition temperature was found to be 620 ± 10°C and the melting point 880 ± 10°C. A new cubic spinel type phase was found at the composition AgIn₅S₈ with $\text{a}\text{=10.827}\text{A}\text{?}$.     

Structure NaCl des phases AgxSn1−xX (X = S,Se)

A cubic phase was observed at room pressure in the case of Ag_xSn_1?xSe (.43 < x < .48). Under 65 Kbars and 1000°C, this cubic domain is enlarged, reaching the upper limit x = .66, corresponding to Ag₂SnSe₃. The parameters $\text{a}\text{= 5.677 ± .003}\text{A}\text{?}$ and $\text{a}\text{= 5.640 ± .004}\text{A}\text{?}$ were found respectively for AgSnSe₂ and Ag₂SnSe₃. Cubic AgSnS₂ was formed at 65 Kbars and 1200°C with $\text{a}\text{= 5.506 ± .002}\text{A}\text{?}$. The crystal structures calculated for AgSnS₂ and AgSnSe₂ (high pressure compound) yield respectively to R = .06 and R = .04 for a rock salt structure hypothesis.     

Luminescence in the rare-earth ferroelectric-ferroelastic gadolinium molybdate

The present paper discusses a new result of photoluminescence in gadolinium molybdate and its behaviour in the temperatural region near the phase transition. The special temperature dependence of photoluminescence showed that the luminescence spectra are highly congested between $\text{λ = 4700}\text{A}\text{?}$ and 8000 Å which do not exhibit temperature dependence. Only the peak at $\text{λ = 5470}\text{A}\text{?}$ has a temperature dependence. We have also studied photoconductivity in this material. Our results indicate that the peak at $\text{λ = 5470}\text{A}\text{?}$ can only be explained as an optical transition for a free electron between bands. The electrons are captured at a recombination center. All experimental dependences are discussed and found to be in agreement with existing theories for such processes.     

Crystal data and thermal expansion of tricalciumborate

Single crystals of tricalciumborate were grown by the flux method. Ca₃B₂O₆ crystallizes in the rhombohedral system with the unit cell parameters $\text{a}{}_{\mathrm{R}}\text{= 6.3577(7)}\text{A}\text{?}$, α_R = 85.68(8)°, Z = 2. The equivalent hexagonal lattice parameters are a_H = 8.640(1), $\text{C}{}_{\mathrm{H}}\text{= 11.854(1)}\text{A}\text{?}$. The anisotropic thermal expansion parameters α_a = (1.00 ± 0.14) × 10^?5, α_c = (3.60 ± 0.47) × 10^?5 were determined by X-ray methods in the temperature interval 25 – 1000°C.     

High temperature form of Pb2WO5 and transformation phenomena to its low form

The phase transformation of Pb₂WO₅ was studied by means of high temperature x-ray diffractometry, differential scanning calorimetry ( DSC ) and visual observation of the crystal growth in the range between room temperature and to its melting point. Two forms were found to be stable in the range studied. The Pb₂WO₅, which had been reported by DeVries and Fleischer was confirmed to be low temperature form. The newly synthesized high temperature form is isomorphous with Pb₂MoO₅ and its cell parameters are; $\text{a}\text{=14.206 (2)}\text{A}\text{?}$, $\text{b}\text{=5.799 (1)}\text{A}\text{?}$, $\text{c}\text{=7.346 (1)}\text{A}\text{?}$ and β=113°54 (0.9)′. The transition temperature was determined by DSC measurements as 330±10°C. The transition from high to low form is remarkably affected by the crystal size of its high form. Large crystals easily transform to its low temperature form. However, small crystalline sample (?100 μm), the rate of transformation is quite low and the high form is easily retained to the room temperature. One of the previously reported x-ray diffraction pattern of Pb₂WO₅ was revealed to be the mixture of high and low form of this compound.     

Structure and interface oxidation state of ZnO r.f. sputtered onto silicon and SiO2/Si

The crystallographic structure and interface oxidation state of thin ZnO films r.f. sputtered onto silicon and SiO₂/Si substrates were investigated. The quality of the ZnO layers is characterized by a high optical transmission coefficient which is close to unity up to an optical band gap corresponding to that of single-crystal ZnO. The values of the transmission coefficients and band edge provide sensitive indicators of film quality. The deposited layers have a wurtzite crystal structure with lattice parameters $\text{a}{}_0\text{= 3.24±0.01}\text{A}\text{?}$ and $\text{c}{}_0\text{= 5.23±0.01}\text{A}\text{?}$, corresponding closely with bulk ZnO. The orientation of the ZnO grains with respect to the substrate depends critically on the interface oxidation state. An analysis of X-ray photoelectron spectra provides an assessment of this interface oxidation state.     

Alkali ion exchange reactions with RbA1SiO4: A new metastable polymorph of KA1SiO4

A new metastable polymorph of KA1SiO₄ has been prepared by K⁺ ion exchange of orthorhombic RbA1SiO₄ (a=9.215±.001, b=5.329±.001, $\text{c}\text{=8.742±.001}\text{A}\text{?}$). The new phase is pseudo-orthorhombic with a=18.151±.003, b=10.551±.002, $\text{c}\text{=8.490±.001}\text{A}\text{?}$ with apparent Cc★★ symmetry but may be monoclinic. This new polymorph is metastable at temperatures up to approximately 1100°C where it transforms to normal “low” orthorhombic KA1SiO₄.     

The many phases of HUP, (UO2)[HPO4] · 4H2O. An electron-optical investigation

The prompt recrystallization of the tetragonal phyllophosphate HUP, (UO₂)[HPO₄] · 4H₂O, when rinsed with distilled water at ambient temperature, has been studied by TEM, SEM and SAD. In course of the rinsing process, to remove excess phosphorous acid, HUP underwent an orthorhombic phase change, which produced two orthorhombic phases O₁ and O₂. O₁ has $\text{a = 7.6(3)}\text{A}\text{?}$, $\text{c = 7.0(4)}\text{A}\text{?}$, and 001 absent for l odd. O₂ has $\text{a = 7.3(2)}\text{A}\text{?}$, $\text{c = 6.8(2)}\text{A}\text{?}$ and no systematic absent reflections. O₁ and O₂ are metastable and by spinodal decomposition modulate into monoclinic phases through a string of intermediate and disordered phases with the β-angle increasing from 90.0° (orthorhombic phases) to 91.3(3)°, 95(2)° and 97.7(2)° for M_2.2, M_2.1, and M_1.1 respectively, where M_1.1 is the monoclinic derivative ($\text{a = 6.9(4)}\text{A}\text{?}$, $\text{c = 6.4(4)}\text{A}\text{?}$, β = 97.7(3)°) of O₁ and M_2.1 ($\text{a = 7.3(4)}\text{A}\text{?}$, $\text{c = 12.1(3)}\text{A}\text{?}$, β = 95(2)°) together with M_2.2 ($\text{a = 14.22(4)}\text{A}\text{?}$, $\text{c = 13.08(4)}\text{A}\text{?}$, β = 91.3(3)°) are the derivatives of O₂. The crystallographic b axis could not be determined. All the abovementioned phases exhibit a striking epitaxial relationship which presumably gives the clue to the fact that these phase findings are at variance with X-ray findings given in literature. It is suggested that the phases are isomeric polymorphs of HUP.     

Crystal growth and magnetic properties of NdFeTiO5

Single crystals of NdFeTiO₅ have been grown for the first time by using the flux technique. The observed X-ray reflections were compatible with the space group Pbam and the lattice parameters deduced from both single crystal rotation photographs and powder diffraction patterns are $\text{a}\text{= 7.500}\text{A}\text{?}$, $\text{b}\text{= 8.746}\text{A}\text{?}$ and $\text{c}\text{= 5.847}\text{A}\text{?}$. The crystals were found to undergo a magnetic transition to an antiferromagnetic state at T_N = 18.5 ± 0.5 K. The calculated temperature dependence of the Fe³⁺ and Nd³⁺ sublattice magnetizations based on the results of magnetic susceptibility measurements is presented.     

The unit cell of YTa7O19, a new compound,and its isomorphism with the corresponding rare earth tantalates

A new yttrium tantalate, YTa₇O₁₉, has been prepared by the solid state reaction of yttrium oxide and tantalum pentoxide below 1450°C. The unit cell is hexagonal (or trigonal) with lattice parameters $\text{a}\text{= 6.2035±2}\text{A}\text{?}$ and $\text{c}\text{= 19.865±1}\text{A}\text{?}$. A critical reappraisal of published data for the rare earth tantalates RTa₇O₁₉, previously described as tetragonal, establishes that these are also hexagonal and isomorphous with the yttrium tantalate.     

Etude des transformations allotropiques de Mg2NiH4

Three allotropic varieties of Mg₂NiH₄ with monoclinic, orthorhombic and cubic symmetry have been identified. The unitcell dimensions are : $\text{a}\text{= 12.99}\text{A}\text{?}$, $\text{b}\text{= 6.390}\text{A}\text{?}$, $\text{c}\text{= 6.598}\text{A}\text{?}$ and $\text{β = 93.22}\text{A}\text{?}$ for the monoclinic form. The orthorhombic structure whose parameters are : $\text{a}\text{= 6.499}\text{A}\text{?}$, $\text{b}\text{= 6.415}\text{A}\text{?}$ et $\text{c}\text{= 6.589}\text{A}\text{?}$, represents a slight distortion from the cubic one. Under pressure, an irreversible transformation from the monoclinic to the cubic variety has been observed. The transition temperature is 245°C at 1 bar. On the other hand, the orthorhombic form transforms reversibly into the cubic one about 235°C under 1 bar pressure. The orthorhombic-monoclinic transition occurs only at very high pressure.     

Structure cristalline et proprietes magnetiques de la variete basse temperature TlFeBr3β

DTA and X-Ray studies of TlFeBr₃ show a transition at 384°C. The low temperature phase β-TlFeBr₃ is related to NH₄CdCl₃ and crystallizes in the orthorhombic system with $\text{a}\text{= 9.279(4)}\text{A}\text{?}$, $\text{b}\text{= 3.984(3)}\text{A}\text{?}$, $\text{c}\text{= 15.070(7)}\text{A}\text{?}$, Z = 4. The crystal structure has been determined from 564 independent reflexions. The structure contains FeBr₆ octahedra in which each Fe atom is coordinated to six Br atoms. The FeBr₆ octahedra share vertices to form infinite double chains along the b? axis. This compound behaves paramagnetically above 40 K, with a Curie-Weiss temperature of θ = 12 K and a magnetic moment of μ = 5.59 μB, but becomes antiferromagnetic below 14 K.     

Investigation of the electroluminescence and electroabsorption of GaS single crystals

Electroluminescence and electroabsorption in GaS single crystals have been studied at 77°K. Four maxima at energies of 1675±2, 1992±2, 2194±2 meV have been observed. Three of them were caused by recombination through local levels in the forbidden, and the high-energy maximum 2684±2 meV correspondes to the energy of the indirect transition GG₄^? ? M₃⁺ (E_q = 2591±2meV) accompanied by the radiation of two phonons with an energy of $\text{h}\text{?}\text{ω ? 46.5±2}\text{meV}$. The frequency dependence of brightness waves has been studied. The appropriate exciton parameters of GaS have been caculated.     

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.     

CuTa2O6 - crystal growth and characterization

The compound CuTa₂O₆ has been prepared as crystals from a Cu/O melt and found to be tetragonal ($\text{a}\text{= 7.510}\text{A}\text{?}$, $\text{c}\text{= 7.526}\text{A}\text{?}$) rather than cubic as reported in the literature. The coefficient of thermal expansion between room temperature and 1000°C was found to be 8.0 × 10^?6°C^?1. Electrical resistivity measurements on a crystal showed semiconductor behavior between room temperature ($\text{? = 2 × 10}{}^3\text{Ω}\text{cm}$) and 140°K ($\text{? = 7 × 10}{}^6\text{Ω}\text{cm}$) with an activation energy of E_A = 0.2 eV. Magnetic measurements between 4.2°K and room temperature showed Curie-Weiss behavior with a change in μeff at 120°K. For T>120°K, μeff = 1.76μ_B and θ_p = 0°K while for T<120°K μeff = 1.91 μB and θ_p = ?15°K.     

Electron diffraction study of phase transformations of the compound CuSe

The transformations of the compound CuSe were investigated. At temperatures below 350°C the consecutive phase transitions CuSe→Cu_2?xSe→Cu₂Se were observed. Above 350°C decomposition of the CuSe into two phases, Cu₂Se and an f.c.c. phase with $\text{a=5.63}\text{A}\text{?}$, takes place. It was determined that the range of the lattice parameter a of the f.c.c. phase Cu_2?xSe is from 5.68 to 5.75Å and the existence ranges of the phases observed were evaluated. It was shown that the f.c.c. phase with $\text{a=5.63}\text{A}\text{?}$ is an independent phase.     

The preparation,crystal structures and properties of the potassium vanadium sulfides KXV5S8 (0.5≦x≦0.7)

The crystal structures of the potassium vanadium sulphides K_0.7V₅S₈ and K_0.5V₅S₈ (=KV₁₀S₁₆) have been determined. K_0.7V₅S₈ has a C-centered monoclinic unit cell of dimensions $\text{a}\text{=17.499(3)}\text{A}\text{?}$, $\text{b}\text{=3.2986(6)}\text{A}\text{?}$, $\text{c}\text{=8.489(1)}\text{A}\text{?}$, ß=103.98(1)°, spacegroup C2/m; isomorphous with TIV₅S₈; K_0.5V₅S₈ has essentially the same structure, but due to ordering of the K atoms, the monoclinic b-axis is doubled, thus forming a superstructure. The cell parameters are: $\text{a}\text{=17.462(4)}\text{A}\text{?}$, $\text{b}\text{=6.556(2)}\text{A}\text{?}$, $\text{c}\text{=8.4595(9)}\text{A}\text{?}$, ß=103.86(1)°, spacegroup P2. The structures are characterized by a three-dimensional framework of VS₆ octahedra with channels in which the K atoms are situated. Both compounds exhibit metallic behaviour.