A Raman spectroscopic and TEM study on the structural evolution of Na2Ti3O7 during the transition to Na2Ti6O13

The synthesis of sodium hexatitanate from sodium trititanate was characterized by Raman spectroscopy, X‐ray diffraction (XRD) and high‐resolution transmission electron microscopy (HRTEM). The structural evolution from trititanate to hexatitanate was studied using Raman spectra, XRD and HRTEM techniques. It was found that the Raman bands at 279 cm⁻¹ corresponding to very long Ti O bonds and at 883 cm⁻¹ corresponding to the very short Ti O bonds decrease in intensity and finally disappear during the transition from sodium trititanate to sodium hexatitanate. The band at 922 cm⁻¹ corresponding to an intermediate‐length Ti O bond was observed to become stronger with the increase in temperature, indicating that there is no terminal oxygen atom in the crystal structure of Na₂Ti₆O₁₃ and that all the oxygen atoms become linearly coordinated by two titanium atoms. Furthermore, the TiO₆ octahedron in Na₂Ti₆O₁₃ are more regular because the very long (2.2 Å) or very short (1.7 Å) Ti O bonds disappear. It is revealed that the phase transition from trititanate to hexatitanate is a step‐by‐step slipping process of the TiO₆ octahedral slabs with the loss of sodium cations, and a new phase with formula Na_1.5H_0.5Ti₃O₇ has been discovered as an intermediate phase to interlink Na₂Ti₃O₇ and Na₂Ti₆O₁₃. Copyright © 2010 John Wiley & Sons, Ltd.     

A Raman spectroscopic study on the allocation of ammonium adsorbing sites on H2Ti3O7 nanofibre and its structural derivation during calcination

The adsorption behaviour of ammonium ions and the structural features of layered proton trititanate were characterised by using Raman spectroscopy, X‐ray diffraction (XRD) and transmission electron microscopy. It revealed that the intensity of the Raman band at 309 cm⁻¹, assigned to very long Ti O bonds (0.22 nm), reduced, whereas the band at 890 cm⁻¹, assigned to very short Ti O bonds (0.17 nm), increased slightly after the adsorption of ammonium ions (NH₄⁺). The adsorption of ammonium ions enlarged the interlayer distance of the (200) plane. Ammonium ions were located at the corner of the TiO₆ octahedral slabs. This was further confirmed by XRD, with an increased intensity of the (201 ) plane being observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic study of layered quaternary ferrite Ba12Fe28Ti15O84

Ba–Fe–Ti oxides are nowadays attracting considerable interest for the production of permanent magnets and microwave devices, due to their high dielectric constant. Among these materials, recently the quaternary ferrite Ba₁₂Fe₂₈Ti₁₅O₈₄ (BFT) was discovered to possess ferrimagnetic properties at room temperature with a main magnetic transition at about 420?K and complete disappearance of magnetisation above 700?K. In this study, we report for the first time on the Raman spectrum of BFT samples prepared with different methods. Raman spectra were recorded in dependence of temperature and a preliminary assignment of modes was attempted. Coupling the Raman results with previous magnetic studies allowed gaining more insight on the structural mechanism at play in correspondence of the main magnetic transition.     

Crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 complex perovskites investigated by Raman scattering

This work investigates the crystal structures and phonon modes of Ba(Ca_1/2W_1/2)O₃, Ba(Ca_1/2Mo_1/2)O₃ and Ba(Sr_1/2W_1/2)O₃ perovskites by Raman spectroscopy. The samples were produced by conventional solid‐state processing at 1200 °C. X‐ray diffraction showed that single‐phase homogeneous materials were produced, which are cubic or pseudo‐cubic in symmetry. The existing controversies in the literature for these complex perovskites were investigated by comparing experimental Raman data with group‐theory analysis. Ceramics with Ca and W or Mo were found to be cubic, space group Fm3 m. For these materials, four Raman‐active bands were observed and the fitting parameters showed that the Ba(Ca_1/2Mo_1/2)O₃ ceramic presents bands at lower wavenumbers if compared with the Ba(Ca_1/2W_1/2)O₃ sample. For the Ba(Sr_1/2W_1/2)O₃ material, two hypotheses were investigated for monoclinic or triclinic structures. The experimental results showed 12 Raman‐active modes for this ceramic, which is in perfect agreement with the theoretical predictions for a monoclinic (I2/m) structure. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectroscopic study of Sr2ZnTeO6 double perovskites

Sr₂ZnTeO₆ ceramics were prepared by the solid‐state route and their vibrational phonon modes were investigated using optical spectroscopic techniques, for the first time. X‐ray diffraction (XRD) and Raman and infrared spectroscopies were employed to investigate the structures of these perovskite materials and the results analysed together with group‐theoretical predictions. The number and behaviour of the first‐order modes observed in both spectroscopic techniques are in agreement with the calculations for a tetragonal I4/m space group. The complete set of the optical phonon modes was determined, and the intrinsic dielectric properties of the materials were evaluated, allowing us to discuss their potential application in microwave (MW) circuitry. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectra of mirabilite,Na2SO4·10H2O and the rediscovered metastable heptahydrate,Na2SO4·7H2O

Salt crystallisation in pores is known to cause serious damage to masonry. Sodium sulphate, often regarded as one of the most damaging salts, has a rich hydrate chemistry including one rediscovered metastable hydrate and a new high pressure octahydrate plus five known polymorphs of the anhydrous phase. The difficulty in working with these hydrates lies in their strong tendency to dehydrate or to convert to the stable phase, in the case of the heptahydrate. We present Raman spectra and a table of peak wavenumbers for randomly oriented crystals of mirabilite and the metastable heptahydrate, sufficient to distinguish between these phases that have SO₄ν₁ values of 989.3 and 987.6 cm⁻¹, respectively. Mirabilite has a Raman spectrum very similar to the free sulphate anion in solution, which is probably due to the mobility of oxygen atoms within the sulphate tetrahedron. The oxygen atoms in the heptahydrate sulphate groups have no partial occupancy, and predicted peak splitting is observed in the region 400–1200 cm⁻¹. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the antimonate mineral lewisite (Ca,Fe, Na)2(Sb,Ti)2O6(O,OH)7

The mineral lewisite, (Ca, Fe, Na)₂(Sb, Ti)₂O₆(O, OH)₇, an antimony-bearing mineral, has been studied by Raman spectroscopy. A comparison is made with the Raman spectra of other minerals, including bindheimite, stibiconite, and roméite. The mineral lewisite is characterised by an intense sharp band at 517 cm^?1 with a shoulder at 507 cm^?1 assigned to SbO stretching modes. Raman bands of medium intensity for lewisite are observed at 300, 356, and 400 cm^?1. These bands are attributed to OSbO bending vibrations. Raman bands in the OH stretching region are observed at 3200, 3328, 3471 cm^?1, with a distinct shoulder at 3542 cm^?1. The latter is assigned to the stretching vibration of OH units. The first three bands are attributed to water stretching vibrations. The observation of bands in the 3200–3500 cm^?1 region suggests that water is involved in the lewisite structure. If this is the case then the formula may be better written as (Ca, Fe²⁺, Na)₂(Sb, Ti)₂(O, OH)₇ xH₂O.     

Synthesis and Raman spectroscopic characterisation of the oxalate mineral wheatleyite Na2Cu2+(C2O4)2·2H2O

The mineral wheatleyite has been synthesised and characterised by Raman spectroscopy complimented with infrared spectroscopy. Two Raman bands at 1434 and 1470 cm⁻¹ are assigned to the ν_{(C O)} stretching mode and implies two independent oxalate anions. Two intense Raman bands observed at 904 and 860 cm⁻¹ are assigned to the ν(C C) stretching mode and support the concept of two non‐equivalent oxalate units in the wheatleyite structure. Two strong bands observed at 565 and 585 cm⁻¹ are assigned to the symmetric CCO in plane bending modes. The Raman band at 387 cm⁻¹ is attributed to the CuO stretching vibration and the bands at 127 and 173 cm⁻¹ to OCuO bending vibrations. A comparison is made with Raman spectra of selected natural oxalate bearing minerals. Oxalates are markers or indicators of environmental events. Oxalates are readily determined by Raman spectroscopy. Thus, deterioration of works of art, biogeochemical cycles, plant metal complexation, the presence of pigments and minerals formed in caves can be analysed. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the mixed anion mineral yecoraite Bi5Fe3O9(Te4+O3)(Te6+O4)2·9H2O

Tellurates are rare minerals as the tellurate anion is readily reduced to the tellurite ion. Often minerals with both tellurate and tellurite anions are found. An example of such a mineral containing tellurate and tellurite is yecoraite. Raman spectroscopy has been used to study this mineral, the exact structure of which is unknown. Two Raman bands at 796 and 808 cm⁻¹ are assigned to the ν₁(TeO₄)²⁻ symmetric and ν₃(TeO₃)²⁻ antisymmetric stretching modes and Raman bands at 699 cm⁻¹ are attributed to the ν₃(TeO₄)²⁻ antisymmetric stretching mode and the band at 690 cm⁻¹ to the ν₁(TeO₃)²⁻ symmetric stretching mode. The intense band at 465 cm⁻¹ with a shoulder at 470 cm⁻¹ is assigned the (TeO₄)²⁻ and (TeO₃)²⁻ bending modes. Prominent Raman bands are observed at 2878, 2936, 3180 and 3400 cm⁻¹. The band at 3936 cm⁻¹ appears quite distinct and the observation of multiple bands indicates the water molecules in the yecoraite structure are not equivalent. The values for the OH stretching vibrations listed provide hydrogen bond distances of 2.625 Å (2878 cm⁻¹), 2.636 Å (2936 cm⁻¹), 2.697 Å (3180 cm⁻¹) and 2.798 Å (3400 cm⁻¹). This range of hydrogen bonding contributes to the stability of the mineral. A comparison of the Raman spectra of yecoraite with that of tellurate containing minerals kuranakhite, tlapallite and xocomecatlite is made. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the selenite mineral mandarinoite Fe2Se3O9·6H2O

Selenites and tellurites may be subdivided according to formula and structure. There are five groups, based upon the formulae (a) A(XO₃), (b) A(XO₃·) xH₂O, (c) A₂(XO₃)₃·xH₂O, (d) A₂(X₂O₅) and (e) A(X₃O₈). Of the selenites, molybdomenite is an example of type (a); chalcomenite, clinochalcomenite, cobaltomenite and ahlfeldite are minerals of type (b); mandarinoite Fe₂Se₃O₉·6H₂O is an example of type (c). Raman spectroscopy has been used to characterise the mineral mandarinoite. The intense, sharp band at 814 cm⁻¹ is assigned to the symmetric stretching (Se₃O₉)⁶⁻ units. Three Raman bands observed at 695, 723 and 744 cm⁻¹ are attributed to the ν₃ (Se₃O₉)⁶⁻ anti‐symmetric stretching modes. Raman bands at 355, 398 and 474 cm⁻¹ are assigned to the ν₄ and ν₂ bending modes. Raman bands are observed at 2796, 2926, 3046, 3189 and 3507 cm⁻¹ and are assigned to OH stretching vibrations. The observation of multiple OH stretching vibrations suggests the non‐equivalence of water in the mandarinoite structure. The use of the Libowitzky empirical function provides hydrogen bond distances of 2.633(9) Å (2926 cm⁻¹), 2.660(0) Å (3046 cm⁻¹), 2.700(0) Å (3189 cm⁻¹) and 2.905(3) Å (3507 cm⁻¹). The sharp, intense band at 3507 cm⁻¹ may be due to hydroxyl units. It is probable that some of the selenite units have been replaced by hydroxyl units. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopic study on sodium hyaluronate: an effect of proton and γ irradiation

Raman spectroscopy was applied to the analysis of structural changes in lyophilised sodium hyaluronate after proton and γ irradiation (0.5, 5, 50, 100, 200 and 600 Gy). Characteristic Raman bands of the polysaccharide were sensitive to irradiation. Significant damage was observed at doses of 50 Gy or higher. The spectral changes confirmed radiation‐induced loss of native solution conformation, destruction of primary structure, fragmentation, cross‐linking and elimination of functional groups. Differences in the effects of proton and γ radiation on sodium hyaluronate are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural and crystallization behavior of (Ba,Sr)TiO3 borosilicate glasses

Various glass samples were prepared by melt quench technique in the glass system [(Ba_{1? x} Sr _x ) TiO₃]–[2SiO₂–B₂O₃]–[K₂O] doped with 1?mole% of La₂O₃. Infrared spectra show the number of absorption peaks with different spliting in the wave number range from 450 to 4000?cm^?1. Absorption peaks occurs due to asymetric vibrational streching of borate by relaxation of the bond B–O of trigonal BO₃. Raman spectra show the Raman bands due to ring-type metaborate anions, symmetric breathing vibrations BO₃ triangles replaced by BO₄ tetrahedra, and symmetric breathing vibrations of six-member rings. The differential thermal analysis of a glass sample corresponding to composition x?=?0.0 shows crystallization temperature at 847°C and glass transition temperature at 688°C. X-ray diffraction (XRD) pattern of glass ceramic samples shows the major crystalline phase of BaTiO₃ whereas pyrochlore phases of barium titanium silicate. Scanning electron micrographs confirm the results of XRD as barium titanate is major crystalline phase along with pyrochlore phase of barium titanium silicate.     

Raman spectroscopic study of the selenite mineral: ahlfeldite,NiSeO3·2H2O

Raman spectroscopy has been used to study the selenite mineral ahlfeldite. A comparison is made with the Raman spectra of chalcomenite, cobaltomenite and clinochalcomenite. Selenite minerals are characterised by the position of the symmetric stretching mode which is observed at higher wavenumbers than the anti‐symmetric stretching mode. The selenite ion has C_3v symmetry and four modes, 2A₁ and 2E. These modes are observed at 813, 472 cm⁻¹ (A₁) and 685, 710, 727 and 367 and 396 cm⁻¹ (E). Bands assigned to the water stretching vibrations are observed for ahlfeldite at 3385 cm⁻¹, for chalcomenite at 2953, 3184 and 3506 cm⁻¹ and for clinochalcomenite at 2909, 3193 and 3507 cm⁻¹. A comparison of the Raman spectra of chalcomenite, clinochalcomenite and cobaltomenite is made. The position of these bands enabled hydrogen bond distances in the selenite structure to be estimated. Hydrogen bond distances for ahlfeldite, chalcomenite and clinochalcomenite were determined to be similar. Copyright © 2008 John Wiley & Sons, Ltd.     

Crystal structure and phonon modes of ilmenite‐type NaBiO3 investigated by Raman and infrared spectroscopies

NaBiO₃ is an ilmenite‐type compound presenting a trigonal structure. In this work, we have performed optical spectroscopic investigations using Raman scattering and infrared reflectivity for this material. By using group‐theory calculations, it was possible to determine the number of optically active modes. Fitting procedures besides Kramers–Kronig (KK) procedures lead to a consistent collection of phonon modes. The Raman spectrum showed nine active first‐order modes, while the infrared one revealed eight polar phonons, in good agreement with the theoretical predictions. The results allowed us to confirm the R3 (#148) space group for this compound, and to establish a set of active phonons not yet reported in the literature. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the tellurite minerals: graemite CuTeO3·H2O and teineite CuTeO3·2H2O

Tellurites may be subdivided according to formula and structure. There are five groups based upon the formulae (a) A(XO₃), (b) A(XO₃)·xH₂O, (c) A₂(XO₃)₃·xH₂O, (d) A₂(X₂O₅) and (e) A(X₃O₈). Raman spectroscopy has been used to study the tellurite minerals teineite and graemite; both contain water as an essential element of their stability. The tellurite ion should show a maximum of six bands. The free tellurite ion will have C_3v symmetry and four modes, 2A₁ and 2 E. Raman bands for teineite at 739 and 778 cm⁻¹ and for graemite at 768 and 793 cm⁻¹ are assigned to the ν₁ (TeO₃)²⁻ symmetric stretching mode while bands at 667 and 701 cm⁻¹ for teineite and 676 and 708 cm⁻¹ for graemite are attributed to the ν₃ (TeO₃)²⁻ antisymmetric stretching mode. The intense Raman band at 509 cm⁻¹ for both teineite and graemite is assigned to the water librational mode. Raman bands for teineite at 318 and 347 cm⁻¹ are assigned to the (TeO₃)²⁻ν₂(A₁) bending mode and the two bands for teineite at 384 and 458 cm⁻¹ may be assigned to the (TeO₃)²⁻ν₄(E) bending mode. Prominent Raman bands, observed at 2286, 2854, 3040 and 3495 cm⁻¹, are attributed to OH stretching vibrations. The values for these OH stretching vibrations provide hydrogen bond distances of 2.550(6) Å (2341 cm⁻¹), 2.610(3) Å (2796 cm⁻¹) and 2.623(2) Å (2870 cm⁻¹) which are comparatively short for secondary minerals. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the tellurite minerals: emmonsite Fe23+Te34+O9·2H2O and zemannite Mg0.5[Zn2+Fe3+(TeO3)3]4.5H2O

Raman spectroscopy has been used to study zemannite Mg_0.5[Zn²⁺Fe³⁺(TeO₃)₃]4.5H₂O and emmonsite Fe₂³⁺Te₃⁴⁺O₉·2H₂O. Raman bands for zemannite and emmonsite, observed at 740 and 650 cm⁻¹ and at 764 and 788 cm⁻¹, respectively, are attributed to the ν₁ (TeO₃)²⁻ symmetric stretching mode. The splitting of the symmetric stretching mode for emmonsite is in harmony with the results of X‐ray crystallography which shows three non‐equivalent TeO₃ units in the crystal structure. Two bands at 658 and 688 cm⁻¹ are assigned to ν₃ (TeO₃)²⁻ anti‐symmetric stretching modes. Raman bands observed at 372 and 408 cm⁻¹ for zemannite and 397 and 414 cm⁻¹ for emmonsite are attributed to the (TeO₃)²⁻ν₂(A₁) bending mode. The two Raman bands at 400 and 440 cm⁻¹ for emmonsite are ascribed to the ν₄(E) bending modes, while the band at 326 cm⁻¹ is due to the ν₂(A₁) bending vibration. Copyright © 2008 John Wiley & Sons, Ltd.     

一氧化碳合成金刚石薄膜的形貌和结构分析

利用微波等离子体化学气相沉积(MPCVD)技术，采用偏压增加成核(BEN)、两步生长的方法在一氧化碳(CO)和氢气(H₂)的环境下制备了金刚石薄膜. 利用扫描电子显微镜(SEM)、Raman光谱仪和透射电子显微镜(TEM)对金刚石薄膜的形貌和结构进行了分析. 研究发现金刚石晶粒在第一步成核及生长的过程中产生了层错和孪晶，而在第二步的生长过程中产生的层错和孪晶很少，最终形成的金刚石晶粒外表面比较光滑，包含有近五次对称或者平行的片状的孪晶，并可以观察到少量的位错. 而在样品的边缘由于等离子体的不均匀产生了比样品中心成核密度低的区域. 在这个区域中，发现了一个新的非金刚石的碳结构.     

Raman spectroscopy study of Na2MoO4·2H2O and Na2MoO4 under hydrostatic pressure

Raman spectroscopy measurements of polycrystalline Na₂MoO₄·2H₂O (NMHO) and Na₂MoO₄ (NM) under hydrostatic pressure (from 0 to 10 GPa) were performed. This study allowed us to monitor the stretching and bending vibrations of MoO₄ ions as well as the translational modes as a function of pressure. The pressure dependence of the wavenumbers of the modes indicates that the Na₂MoO₄·2H₂O undergoes two phase transitions at about ∼3 and ∼4 GPa. When releasing pressure, we have observed that the original spectrum is recovered, thereby pointing to a reversible process. The Na₂MoO₄ (NM) starting phase was found to be stable up to 10 GPa. The pressure‐dependent Raman data for NM did not reveal any structural modification. The influence of the pressure‐transmitting medium on the phase transitions is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the uranyl mineral natrouranospinite (Na2,Ca)[(UO2)(AsO4)]2· 5H2O

Raman spectra of natrouranospinite complemented with infrared spectra were studied and related to the structure of the mineral. Observed bands were assigned to the stretching and bending vibrations of (UO₂)²⁺ and (AsO₄)³⁻ units and of water molecules. U O bond lengths in uranyl and O H···O hydrogen bond lengths were calculated from the Raman and infrared spectra. Copyright © 2009 John Wiley & Sons, Ltd.