Asimetričan oblik potpuno simetrične vrpce istezanja u Ramanovom spektru bis(trimetilsilil)acetilena

The asymmetric band profile of the triple C = C stretching Raman band in bis(trimethylsilyl)acetylene was decomposed into a higher and a lower wavenumber part, each described as one half of a Lorentzian bandshape with its own halfwidth (Γhigh and Γlow), but with common band centre and maximum point. Γhigh and Γlow were fitted at seventeen different temperatures ranging from 110 K to 323 K (the melting point occurring roughly at 290 K). It was found that lower wavenumber side of the band was considerably broader than the higher wavenumber side, throughout the whole temperature interval chosen. This unexpected crystal-band asymmetry, present together with the ”normal” phonon spectrum, seems to indicate a freezing of orientational disorder of methyl groups, while molecular centres of mass remain positioned on the Bravais lattice – like in the case of the so called glassy crystals. The asymmetry of the band above 290 K suggests the existence of a nonisotropic liquid state.Asimetričan oblik Ramanove vrpce trostrukog C = C istezanja bis(trimetilsilil)acetilena rastavili smo u visoko- i niskofrekventni dio, i svaki dio opisali polovicom Lorentzovog profila vlastite poluširine (Γhigh i Γlow), zajedničkog središta i jednakog maksimalnog intenziteta. Točke Ramanovog spektra u području trostrukog istezanja zabilježene su u intervalu temperature od 110 K do 323 K (do taljenja dolazi oko 290 K). Proveli smo postupak prilagodbe parametara (Γhigh i Γlow) na opažene točke. Našli smo da je niskofrekventna strana vrpce značajno šira od visokofrekventne strane i to u čitavom istraživanom području temperature. Čini se da je ova neočekivana asimetrija vrpce kristala, uz istovremeno opažen fononski spektar, znak zamrzavanja orijentacijskog nereda metilnih grupa, dok su središta masa molekula raspoređena pravilno na Bravaisovoj rešetci, kao u slučaju staklastih kristala. Asimetrija vrpce iznad 290 K ukazuje na postojanje neizotropne tekućine

Bose peak and vibrational bands in Raman spectra of sodium borosilicate glass

The existence of an asymmetric broad band called boson (Bose) peak in VV as well as in VH polarized Raman spectra of sodium borosilicate glass with various amounts of SnO is shown and its bandshape fitted according to the formula I_VH ~ C(w) g(w) (n(w)+1)/w, where n(w) stands for Bose-Einstein occupation mnumber and C(w)g(w) is given by C(w) g(w) = w^5 / ( w^2 + (w_0)^2)^2. Parameter w_0 seems to depend on the chemical composition and the tzpe of glass. The low frequency band of our sodium borosilicate glass (1:1:4 molar ratio of Na_2O : B_2 O_3 : SiO_2) obeys this formula well, with w_0 = 52 cm-1, which is approximately the peak position of the band. On addition of SnO (1.5 to 10 wt %) the shape of boson peak in VH spectra changes on addition of 1.5 wt%, but remains unchanged on further addidtion of SnO. In the VV - 4/3 VH spectrum, a new peak around 530 cm-1 appears, indicating that tin ions behave as network formers

Istraživanje termičkih svojstava titan dioksida rentgenskom difrakcijom

Temperature dependence of microstructure of titanium dioxide, TiO2, and the phase transition of anatase (A) to rutile (R) were studied by in situ X-ray powder diffraction. The as-synthesized TiO2 p.a. showed a gradual transition A→R during the temperature increase from ≈ 1200 K to ≈ 1570 K and during the temperature decrease to ≈ 600 K. High-energy ball milling at room temperature induced a partial transition A→R. The transition continued during the temperature increase to ≈ 1370 K and during the temperature decrease, and is accompanied by sharpening of diffraction lines. Anisotropy of thermal expansion was noticed for both A and R. In the transition A→R, the nuclei of R are formed either throughout the A crystallites (in the case of as-synthesized TiO2 p.a.) or mainly in the interior of the A crystallites (in the case of the milled TiO2 p.a.). These nuclei grow in number and size with a prolonged time of thermal agitation.Pomoću in situ rentgenske difrakcije u prahu, istraživali smo ovisnost mikrostrukture titan dioksida, TiO2 o temperaturi i fazne pretvorbe anatasa (A) u rutil (R). Za polazni TiO2 p.a. opaža se postepena pretvorba A→R pri porastu temperature od ≈ 1200 K do ≈ 1570 K, te pri smanjenju temperature do ≈ 600 K. Mljevenjem TiO2 p.a. pri sobnoj temperaturi dolazi do djelomične pretvorbe A→R. Ta se pretvorba nastavlja pri porastu temperature do ≈ 1370 K, kao i pri smanjenju temperature, a prati je izoštravanje difrakcijskih linija. Uočena je anizotropija temperaturnog rastezanja za obje faze TiO2, A i R. Pri pretvorbi A→R, jezgre R nastaju ili u cijelom volumenu kristalita A (za polazni TiO2 p.a.), ili uglavnom u unutarnjem dijelu kristalita A (za mljeveni TiO2 p.a.). Veličina i broj jezgri R raste tijekom produljene termičke obrade uzorka

X-ray diffraction study of thermal properties of titanium dioxide

Temperature dependence of microstructure of titanium dioxide, TiO_2, and the phase transition of anatase (A) to rutile (R) were studied by in situ X-ray powder diffraction. The as-synthesized TiO_2 p.a. showed a gradual transition A→R during the temperature increase from ≈ 1200 K to ≈ 1570 K and during the temperature decrease to ≈ 600 K. High-energy ball –milling at room temperature induced a partial transition A→R. The transition continued during the temperature increase to » 1370 K and during the temperature decrease, and is accompanied by sharpening of diffraction lines. Anisotropy of thermal expansion was noticed for both A and R. In the transition A→R, the nuclei of R are formed either throughout the A crystallites (in the case of as-synthesized TiO_2 p.a.) or mainly in the interior of the A crystallites (in the case of the milled TiO_2 p.a.). These nuclei grow in number and size with a prolonged time of thermal agitation