A Raman spectroscopic study on the active site of sodium cations in the structure of Na2Ti3O7 during the adsorption of Sr2+ and Ba2+ cations

A detailed study on the structural deformation of trititanate nanofibers after the adsorption of divalent strontium (Sr) and barium (Ba) cations was conducted by using Raman spectroscopy, X‐ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the Raman bands at 309 cm⁻¹ corresponding to very long Ti O bonds (2.2 Å) and at 883 cm⁻¹ corresponding to the very short Ti O bonds (1.7 Å) decreased in intensity after the adsorption of Ba²⁺ and Sr²⁺ cations. Additionally, the band at 922 cm⁻¹ corresponding to an intermediate length Ti O bond was observed to weaken with the adsorption of divalent cations, indicating that the TiO₆ octahedra in Na₂Ti₃O₇ are more regular. These results suggest that the active sodium (Na⁺) cations in Na₂Ti₃O₇ should be located at the corner of the TiO₆ octahedral slabs, i.e. the plane (003). This was further confirmed by a large decrease of diffraction intensity of the plane (003) observed in the XRD pattern. 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.     

Transition of synthetic chromium oxide gel to crystalline chromium oxide: a hot‐stage Raman spectroscopic study

Chromium oxide gel material was synthesised and appeared to be amorphous in X‐ray diffraction study. The changes in the structure of the synthetic chromium oxide gel were investigated using hot‐stage Raman spectroscopy based upon the results of thermogravimetric analysis. The thermally decomposed product of the synthetic chromium oxide gel in nitrogen atmosphere was confirmed to be crystalline Cr₂O₃ as determined by the hot‐stage Raman spectra. Two bands were observed at 849 and 735 cm⁻¹ in the Raman spectrum at 25 °C, which were attributed to the symmetric stretching modes of O Cr^III OH and O Cr^III O. With temperature increase, the intensity of the band at 849 cm⁻¹ decreased, while that of the band at 735 cm⁻¹ increased. These changes in intensity are attributed to the loss of OH groups and formation of O Cr^III O units in the structure. A strongly hydrogen‐bonded water H O H bending band was found at 1704 cm⁻¹ in the Raman spectrum of the chromium oxide gel; however, this band shifted to around 1590 cm⁻¹ due to destruction of the hydrogen bonds upon thermal treatment. Six new Raman bands were observed at 578, 540, 513, 390, 342 and 303 cm⁻¹ attributed to the thermal decomposed product Cr₂O₃. The use of the hot‐stage Raman spectroscopy enabled low‐temperature phase changes brought about through dehydration and dehydroxylation to be studied. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

A Raman spectroscopic study of the antimony mineral klebelsbergite Sb4O4(OH)2(SO4)

Many minerals based upon antimonite and antimonate anions remain to be studied. Most of the bands occur in the low wavenumber region, making the use of infrared spectroscopy difficult. This problem can be overcome by using Raman spectroscopy. The Raman spectra of the mineral klebelsbergite Sb₄O₄(OH)₂(SO₄) were studied and related to the structure of the mineral. The Raman band observed at 971 cm⁻¹ and a series of overlapping bands are observed at 1029, 1074, 1089, 1139 and 1142 cm⁻¹ are assigned to the SO₄²⁻ν₁ symmetric and ν₃ antisymmetric stretching modes, respectively. Two Raman bands are observed at 662 and 723 cm⁻¹, which are assigned to the Sb O ν₃ antisymmetric and ν₁ symmetric stretching modes, respectively. The intense Raman bands at 581, 604 and 611 cm⁻¹ are assigned to the ν₄ SO₄²⁻ bending modes. Two overlapping bands at 481 and 489 cm⁻¹ are assigned to the ν₂ SO₄²⁻ bending mode. Low‐intensity bands at 410, 435 and 446 cm⁻¹ may be attributed to O Sb O bending modes. The Raman band at 3435 cm⁻¹ is attributed to the O H stretching vibration of the OH units. Multiple Raman bands for both SO₄²⁻ and Sb O stretching vibrations support the concept of the non‐equivalence of these units in the klebelsbergite structure. It is proposed that the two sulfate anions are distorted to different extents in the klebelsbergite structure. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the mineral gerstleyite Na2(Sb,As)8S13·2H2O and comparison with some heavy‐metal sulfides

The mineral gerstleyite is described as a sulfosalt as opposed to a sulfide. This study focuses on the Raman spectrum of gerstleyite Na₂(Sb,As)₈S₁₃·2H₂O and makes a comparison with the Raman spectra of other common sulfides including stibnite, cinnabar and realgar. The intense Raman bands of gerstleyite at 286 and 308 cm⁻¹ are assigned to the SbS₃E antisymmetric and A₁ symmetric stretching modes of the SbS₃ units. The band at 251 cm⁻¹ is assigned to the bending mode of the SbS₃ units. The mineral stibnite also has basic structural units of Sb₂S₃ and SbS₃ pyramids with C_3v symmetry. Raman bands of stibnite Sb₂S₃ at 250, 296, 372 and 448 cm⁻¹ are assigned to Sb S stretching vibrations and the bands at 145 and 188 cm⁻¹ to S Sb S bending modes. The Raman band for cinnabar HgS at 253 cm⁻¹ fits well with the assignment of the band for gerstleyite at 251 cm⁻¹ to the S Sb S bending mode. Raman bands in similar positions are observed for realgar AsS and orpiment As₂S₃. Copyright © 2010 John Wiley & Sons, Ltd.     

High‐pressure studies of the micro‐Raman spectra of iron cyanide complexes: Prussian blue (Fe4[Fe(CN)6]3), potassium ferricyanide (K3[Fe(CN)6]), and sodium nitroprusside (Na2[Fe(CN)5(NO)]·2H2O)

The pressure dependences of the peaks observed in the micro‐Raman spectra of Prussian blue (Fe₄[Fe(CN)₆]₃), potassium ferricyanide (K₃[Fe(CN)₆]), and sodium nitroprusside (Na₂[Fe(CN)₅(NO)]·2H₂O) have been measured up to 5.0 GPa. The vibrational modes of Prussian blue appearing at 201 and 365 cm⁻¹ show negative dν/dP values and Grüneisen parameters and are assigned to the transverse bending modes of the Fe C N Fe linkage which can contribute to a negative thermal expansion behavior. A phase transition occurring between 2.0 and 2.8 GPa in potassium ferricyanide is shown by changes in the spectral region 150–700 cm⁻¹. In the spectra of the nitroprusside ion, there are strong interactions between the FeN stretching mode and the FeNO bending and the axial CN stretching modes. The pressure dependence of the NO stretching vibration is positive, 5.6 cm⁻¹ GPa⁻¹, in contrast to the negative behavior in the iron(II)‐meso‐tetraphenyl porphyrinate complex. Copyright © 2011 John Wiley & Sons, Ltd.     

Temperature‐dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7

The temperature‐dependent Raman studies of A₂Ti₂O₇(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2–295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third‐order phonon–phonon anharmonic interactions. Moreover, the 312 and 330 cm⁻¹ modes of Er₂Ti₂O₇(Gd₂Ti₂O₇) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd.     

Single‐crystal Raman spectroscopy of brandholzite Mg[Sb(OH)6]2•6H2O and bottinoite Ni[Sb(OH)6]2• 6H2O and the polycrystalline Raman spectrum of mopungite Na[Sb(OH)6]

The single‐crystal Raman spectra of minerals brandholzite and bottinoite, formula M[Sb(OH)₆]₂•6H₂O, where M is Mg⁺² and Ni⁺², respectively, and the non‐aligned Raman spectrum of mopungite, formula Na[Sb(OH)₆], are presented for the first time. The mixed metal minerals comprise alternating layers of [Sb(OH)₆]⁻¹ octahedra and mixed [M(H₂O)₆]⁺²/[Sb(OH)₆]⁻¹ octahedra. Mopungite comprises hydrogen‐bonded layers of [Sb(OH)₆]⁻¹ octahedra linked within the layer by Na⁺ ions. The spectra of the three minerals were dominated by the Sb O symmetric stretch of the [Sb(OH)₆]⁻¹ octahedron, which occurs at approximately 620 cm⁻¹. The Raman spectrum of mopungite showed many similarities to spectra of the di‐octahedral minerals, supporting the view that the Sb octahedra give rise to most of the Raman bands observed, particularly below 1200 cm⁻¹. Assignments have been proposed on the basis of the spectral comparison between the minerals, prior literature and density functional theory (DFT) calculations of the vibrational spectra of the free [Sb(OH)₆]⁻¹ and [M(H₂O)₆]⁺² octahedra by a model chemistry of B3LYP/6‐31G(d) and lanl2dz for the Sb atom. The single‐crystal spectra showed good mode separation, allowing most of the bands to be assigned to the symmetry species A or E. Copyright © 2010 John Wiley & Sons, Ltd.     

DFT‐aided interpretation of the Raman spectra of the polymorphic forms of Y2Si2O7

Y₂Si₂O₇ is an intriguing material combining a complex structural polymorphism with several important technological applications. Raman spectra were experimentally determined for most of the seven known modifications of Y₂Si₂O₇ except the form ε, and in the case of β, γ, δ and ζ for the first time. The error‐prone procedure of mode assignment to the measured Raman bands, usually done by comparison with similar or related structures, has been replaced by quantum chemical calculations of the spectra of the polymorphs. Various functionals were evaluated considering the agreement of the calculated modes with the experimental data. The average and maximum deviations between calculated and experimental spectra are ± 8 cm⁻¹ and 20 cm⁻¹, respectively. Assignments of most of the observed bands to vibrational modes are given. The relationship between selected Raman bands, Si O and Y O polyhedra stretching and bending modes, and the crystal structures are discussed. Y₂Si₂O₇ offers the possibility to study the relationship between structural and spectral changes in a chemically fixed system. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman and infrared study of phyllosilicates containing heavy metals (Sb,Bi): bismutoferrite and chapmanite

The kaolinite‐like phyllosilicate minerals bismutoferrite BiFe³⁺₂Si₂O₈(OH) and chapmanite SbFe³⁺₂Si₂O₈(OH) have been studied by Raman spectroscopy and complemented with infrared spectra. Tentatively interpreted spectra were related to their molecular structure. The antisymmetric and symmetric stretching vibrations of the Si O Si bridges, δ SiOSi and δ OSiO bending vibrations, ν (Si O_terminal)⁻ stretching vibrations, ν OH stretching vibrations of hydroxyl ions, and δ OH bending vibrations were attributed to the observed bands. Infrared bands in the range 3289–3470 cm⁻¹ and Raman bands in the range 1590–1667 cm⁻¹ were assigned to adsorbed water. O H···O hydrogen‐bond lengths were calculated from the Raman and infrared spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Pressure effect on the Raman and photoluminescence spectra of Eu3+-doped Na2Ti6O13 nanorods

Eu³⁺-doped Na₂Ti₆O₁₃ (Na₂Ti₆O₁₃:Eu) nanorods with diameters of 30 nm and lengths 400 nm were synthesized by hydrothermal and heat treatment methods. Raman spectra at ambient conditions indicated a pure monoclinic phase (space group C2/m) of the nanorods. The relations between structural and optical properties of Na₂Ti₆O₁₃:Eu nanorods under high pressures were obtained by photoluminescence and Raman spectra. Two structural transition points at 1.39 and 15.48 GPa were observed when the samples were pressurized. The first transition point was attributed to the crystalline structural distortion. The later transition point was the result of pressure-induced amorphization, and the high-density amorphous (HDA) phase formed after 15.48 GPa was structurally related to the monoclinic baddeleyite structured TiO₂ (P2₁/c). However, the site symmetry of the local environment around the Eu³⁺ ions in Na₂Ti₆O₁₃ increased with the rising pressure. These above results indicate the occurrence of short-range order for the local asymmetry around the Eu³⁺ ions and long-range disorder for the crystalline structure of Na₂Ti₆O₁₃:Eu nanorods by applying pressure. After releasing the pressure from 22.74 GPa, the HDA phase is transformed to low-density amorphous form, which is attributed to be structurally related to the α-PbO₂-type TiO₂.     

Raman spectroscopic study of the uranyl mineral pseudojohannite Cu6.5[(UO2)4O4(SO4)2]2(OH)5·25H2O

Raman spectra of pseudojohannite were studied and related to the structure of the mineral. Observed bands were assigned to the stretching and bending vibrations of (UO₂)²⁺ and (SO₄)²⁻ units and of water molecules. The published formula of pseudojohannite is Cu_6.5(UO₂)₈[O₈](OH)₅[(SO₄)₄]·25H₂O. Raman bands at 805 and 810 cm⁻¹ are assigned to (UO₂)²⁺ stretching modes. The Raman bands at 1017 and 1100 cm⁻¹ are assigned to the (SO₄)²⁻ symmetric and antisymmetric stretching vibrations. The three Raman bands at 423, 465 and 496 cm⁻¹ are assigned to the (SO₄)²⁻ν₂ bending modes. The bands at 210 and 279 cm⁻¹ are assigned to the doubly degenerate ν₂ bending vibration of the (UO₂)²⁺ units. 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.     

Raman spectroscopic study of the uranyl selenite mineral marthozite Cu[(UO2)3(SeO3)2O2]·8H2O

The mineral marthozite, a uranyl selenite, has been characterised by Raman spectroscopy at 298 K. The bands at 812 and 797 cm⁻¹ were assigned to the symmetric stretching modes of the (UO₂)²⁺ and (SeO₃)²⁻ units, respectively. These values gave the calculated U O bond lengths in uranyl of 1.799 and/or 1.814 Å. Average U O bond length in uranyl is 1.795 Å, inferred from the X‐ray single crystal structure analysis of marthozite by Cooper and Hawthorne. The broad band at 869 cm⁻¹ was assigned to the ν₃ antisymmetric stretching mode of the (UO₂)²⁺ (calculated U O bond length 1.808 Å). The band at 739 cm⁻¹ was attributed to the ν₃ antisymmetric stretching vibration of the (SeO₃)²⁻ units. The ν₄ and the ν₂ vibrational modes of the (SeO₃)²⁻ units were observed at 424 and 473 cm⁻¹. Bands observed at 257, and 199 and 139 cm⁻¹ were assigned to OUO bending vibrations and lattice vibrations, respectively. O H···O hydrogen bond lengths were inferred using Libowiztky's empirical relation. The infrared spectrum of marthozite was studied for complementation. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the antimonate mineral brandholzite Mg[Sb2(OH)12]·6H2O

Raman spectra of brandholzite Mg[Sb₂(OH)₁₂]·6H₂O were studied, complemented with infrared spectra, and related to the structure of the mineral. An intense Raman sharp band at 618 cm⁻¹ is attributed to the SbO symmetric stretching mode. The low‐intensity band at 730 cm⁻¹ is ascribed to the SbO antisymmetric stretching vibration. Low‐intensity Raman bands were found at 503, 526 and 578 cm⁻¹. Corresponding infrared bands were observed at 527, 600, 637, 693, 741 and 788 cm⁻¹. Four Raman bands observed at 1043, 1092, 1160 and 1189 cm⁻¹ and eight infrared bands at 963, 1027, 1055, 1075, 1108, 1128, 1156 and 1196 cm⁻¹ are assigned to δ SbOH deformation modes. A complex pattern resulting from the overlapping band of the water and hydroxyl units is observed. Raman bands are observed at 3240, 3383, 3466, 3483 and 3552 cm⁻¹; infrared bands at 3248, 3434 and 3565 cm⁻¹. The Raman bands at 3240 and 3383 cm⁻¹ and the infrared band at 3248 cm⁻¹ are assigned to water‐stretching vibrations. The two higher wavenumber Raman bands observed at 3466 and 3552 cm⁻¹ and two infrared bands at 3434 and 3565 cm⁻¹ are assigned to the stretching vibrations of the hydroxyl units. Observed Raman and infrared bands in the OH stretching region are associated with O‐H···O hydrogen bonds and their lengths 2.72, 2.79, 2.86, 2.88 and 3.0 Å (Raman) and 2.73, 2.83 and 3.07 Å (infrared). Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopy of the joaquinite minerals

Selected joaquinite minerals have been studied by Raman spectroscopy. The minerals are categorised into two groups depending upon whether bands occur in the 3250 to 3450 cm⁻¹ region and in the 3450 to 3600 cm⁻¹ region, or in the latter region only. The first set of bands is attributed to water stretching vibrations and the second set to OH stretching bands. In the literature, X‐ray diffraction could not identify the presence of OH units in the structure of joaquinite. Raman spectroscopy proves that the joaquinite mineral group contains OH units in their structure, and in some cases both water and OH units. A series of bands at 1123, 1062, 1031, 971, 912 and 892 cm⁻¹ are assigned to SiO stretching vibrations. Bands above 1000 cm⁻¹ are attributable to the ν_as modes of the (SiO₄)⁴⁻ and (Si₂O₇)⁶⁻ units. Bands that are observed at 738, around 700, 682 and around 668, 621 and 602 cm⁻¹ are attributed to O Si O bending modes. The patterns do not appear to match the published infrared spectral patterns of either (SiO₄)⁴⁻ or (Si₂O₇)⁶⁻ units. The reason is attributed to the actual formulation of the joaquinite mineral, in which significant amounts of Ti or Nb and Fe are found. Copyright © 2007 John Wiley & Sons, Ltd.     

Mesoporous CdS-pillared H2Ti3O7 nanohybrids with efficient photocatalytic activity

Heterostructured CdS-pillared H₂Ti₃O₇ nanohybrids were prepared by the self-assembly of exfoliated trititanate nanosheets and CdS nanosol particles under the electrostatic interactions. It was revealed that the present nanohybrids were mesoporous with specific surface areas of about 90 m² g⁻¹. The nanohybrids exhibited high photocatalytic activity and good recurrence stability in the H₂ evolution from water splitting. When the preparation molar ratio of H₂Ti₃O₇/CdS was 2:1, the nanohybrid reached a high H₂-evolution rate of 1523 μmol h⁻¹ g⁻¹ under a 300 W Xe lamp irradiation, which was 13 times higher than the bare CdS. Apart from the wider spectral responsive range, the superior photocatalytic performance of the nanohybrids was predominantly attributed to the efficient photogenerated charge separation between the trititanate nanosheets and the encapsulated CdS nanoparticles.     

Characterization of sodium alginate and its block fractions by surface‐enhanced Raman spectroscopy

The surface‐enhanced Raman scattering (SERS) of sodium alginates and their hetero‐ and homopolymeric fractions obtained from four seaweeds of the Chilean coast was studied. Alginic acid is a copolymer of β‐D ‐mannuronic acid (M) and α‐L guluronic acid (G), linked 1 → 4, forming two homopolymeric fractions (MM and GG) and a heteropolymeric fraction (MG). The SERS spectra were registered on silver colloid with the 632.8 nm line of a He Ne laser. The SERS spectra of sodium alginate and the polyguluronate fraction present various carboxylate bands which are probably due to the coexistence of different molecular conformations. SERS allows to differentiate the hetero‐ and homopolymeric fractions of alginic acid by characteristic bands. In the fingerprint region, all the poly‐D ‐mannuronate samples present a band around 946 cm⁻¹ assigned to C O stretching, and C C H and C O H deformation vibrations, a band at 863 cm⁻¹ assigned to deformation vibration of β‐C₁ H group, and one at 799–788 cm⁻¹ due to the contributions of various vibration modes. Poly‐L ‐guluronate spectra show three characteristic bands, at 928–913 cm⁻¹ assigned to symmetric stretching vibration of C O C group, at 890–889 cm⁻¹ due to C C H, skeletal C C, and C O vibrations, and at 797 cm⁻¹ assigned to α C₁ H deformation vibration. The heteropolymeric fractions present two characteristic bands in the region with the more important one being an intense band at 730 cm⁻¹ due to ring breathing vibration mode. Copyright © 2009 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.     

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.